DCDCv9-3r Q&A

[Rootthecause]

Ask me anything about it or older designs :)

Edit May 16th 2026: I've now locked this discussion because of it's hardly navigable size. Please create new discussions instead!
Read more about here: #15

[Jamesteng0214]

Hello I was wondering if you could share some more of your simulation result in LTspice? I tried your LTspice circuit and UCC25600 is not generating PWM signals..... I am not sure what went wrong, I also imported models the same way you mentioned in the documentation.

My second question is what does this 9v do? I tried looking for the answer in the documentation several times but still could not figure it out.

Lastly, why is there a buck and a boost converter?

[Jamesteng0214]

This is the simulation circuit by the way, I don't think I've altered anything.

Also did you end up simulating the entire design??

Thank you!

[Rootthecause]

Hi James!
Regarding the LTspice Simulation, I'm not sure why the UCC25600 Model does not work for you.
Did you correclty place the whole Folder "ValVol" in \lib\sym\ ?
Normally if there was a misconfiguration with the model, there should be a error message appear. But if the simulation just starts fine (simulation window with waveforms appears), then I assume there might be something wrong with the SPICE settings. Have you tried the settings from page 54? If your simulation barely makes it beyond 2 µs, then this might be the solution. Setting "abstol" from 1E-12 to 1E-10 usually does the trick for me. Let me know, if that solved it.

Doc Changelog:
I've added "Folder ValVol - Place in \lib\sym" to the LTspice chapter. Translated chapter title (was still German 😅)

My second question is what does this 9v do?

U2 is providing clean 9V from the 12V rail. The reason for that lays in the nature of the regulation loop. If the feeding voltage of the regulation loop changes, some parameters e.g. optocoupler- and bias-current will change as well. However, there will be no sigificant change of output voltage, as this is detemined by the TL431 with it's internal 2.5V reference.
So the reason for the extra 9V rail has to do with fast voltage changes on the 12V rail, as the output capacitors and cpacitors present in the feedback loop will dampen/buffer/delay any voltage change to it, which changes the feedback value by a slight amount until it reached a static value again.
When I tested older versions with a 12V fan hooked up to the 12V rail, the current ripple caused voltage changes, thus affecting the whole feedback loop 😵‍💫.
You can find some slight ripple on the primary transformer current (cyan) seen best at the bottom of the picture.

But when I hooked up a 5V fan to the 3.8V start capacitor, there was no influence, as the source is relively low impedance and the step-up will counteract voltage changes.
The 9V LDO solves this issue for loads to the 12V rail entirely and also makes it more robust against internal ripple/noises.
I didn't add the 9V source to the signal and power path diagram, because it's not that relevant imho.

Also did you end up simulating the entire design??

No, for several reasons:

• It would be quite challanging to implement/find all functions/ICs and also very time consuming to redo the whole schematic in LTspice
• Even if you where able to implement all of it, it would take forever to simulate, as the capaciors will take long to charge up
• there is no need to do this, as there is no real benefit from simulating something simple as a voltage divider or step-up converter, which should just work
• debugging of issues will be much harder/slower to find for the whole system

You can assume, that some circuits do not influence each other, e.g. the Precharge and the rest of the DCDC, so you can simulate them seperately.
Speaking of Precharge: I forgot to include the LTspice Precharge Circuit. It's now included in the latest push :)

Lastly, why is there a buck and a boost converter?

The buck converter is mainly there to provide a constant output voltage for the internal systems, which would be 12 V.
However, if you wanted to store 12V for a startup-source, you would need to put either super capacitors or battery cells in series, adding up in space but also complexity (balancing, costs, self discharge). This pretty much limites you to only one cell. Normal super caps are often limited to around 2.5V which would result in relatively high currents (= losses). This is one of the main reasons, why I've choosen the Lithium Super-Cap Hybrid, as it has very good long term storage properties, high energy density and relatively high output voltage (2.5 to 3.8V). Now if your storage runs at 3.8V then you somehow need to increase the voltage to 12V, thus the boost converter.
You could use the step-down to feed the 12V rail directly and use an additional LDO for charge up the start-capacitor, which uses the boost converter just at start-up. This could probably save you 0.2 to 0.4 W at normal operation, but requires at least one additional component. Also stopping the converter would be not as easy with one switch, as you would need to stop the 12V rail as well as the 3.8V Rail.

[Jamesteng0214]

Hello Rootthecauuse, I downloaded UCC25600 model from the link and it was a zip file. Am I meant to move the ValVol.lib file in the zip in lib.sym ? Thanks

[Rootthecause]

Btw, how was FSA? Did you guys get on the podium? hahahah

Haha, nope 🥲 Although we made it trough endurance (and not a single DCDC failure since the change from 3 Weeks ago), it was not enough to compete against the others. But I think my team has now a solid base to work with and improve.
However, one day my colleagues told me, that during an electrical inspection the head of Electrical Scruti came to them and asked for "the guy, who made the DCDC converter".
The next day I meet by chance Eugen Hoffelner and he said "cooles Teil" ["cool device"]. Apperantly he was aware of my DCDC through TUG Racing (my beta tester team). That was kinda cool getting aknowledged by someone who works with power converters job related ^^

Now getting back to your other questions:

I see why you said a gain smaller than 1 is not achievable now! However, from the resource I've read online, I plotted the gain versus normalised frequency graph and it shows that the gain goes below 1 at a frequency around 150kHz.

Uhmm, I'm kind of confused because your chart basically shows a similar thing compared to my answer before.
Maybe I should reword my previous statement: You can achive a gain smaller than 1, however it is not reasonable due to the high switching frequency required for this. Also your chart is not showing, that happens with no (or very light) load applied: Your grain graph will decrease much less with rising frequency. So for a gain of 0.8 you'll need even higher switching frequency than shown.

Why is the 20% load curve used? If full load represents 35A, I think the converter will always be running at 20A at least as long as it's switched on.

If you can make somehow sure, that your converter has always load applied, then this low-gain approach would be a tiny bit more reasonable. Ignoring the low efficiency for a sec, there would be still the question how you want to make sure that your load is ALWAYS applied at all possible states of operation.
Let's think about our LV Load: It probably consists of some microcontollers, fans, water pump and maybe actuators (if you're doing driveless). How do those loads behave during startup?
The micocontrollers are probably using a LDO or stepdown. For an LDO the input current would rise with rising input voltage until the desired output voltage is reached and will be constant after that. So e.g. 3.3V and 100 mA are required, then it will still draw 100 mA at 24V.
If it uses a stepdown converter instead, it will be different: you'll draw e.g. 15 mA at 24V. This does not sound like a problem at first, but if you have 10 of them in your LV system, it will draw 1 A in the start up phase of the DCDC converter, and later on just 150 mA.
This becomes especially problematic, if you would use a step-down from 24V to 12V to supply power for the pump and fans.
Even when you're direcly supplying 12V from the DCDC to the fans and pump, their current consumption will rise with the DCDC's output voltage during start up. They may use some integrated soft-start as well, so their current consumption might be almost 0 at startup. Also what happens, if you blow the output fuse of the DCDC? It would run without load then, and might damage itself due to overvoltage on the output.
What I want to say with this: You would need to be very careful in operating your DCDC to always apply enough load. It's a nice way to make it unreliabe 🙃
I won't repeat all the issues correlated with this < 1 gain design approach, but if you want to figure it out the hard way, I suggest you to try it out for yourself 😉 Imho it's really not worth the struggle compared to a few more windings on the primary.

in your design of 166uH magnetising and 56 uH leakage, does 166 uH include the leakage inductance? (Since measuring with secondary open gets you Lm + Lr)

no, the 166 µH are Lp (primary inductance) and the 56 µH are Lr, so Lm is 110 µH.
With secondary leads open you'll measure on the primary side: Lp = Lm + Lr
With secondary leads shorted you'll measure on the primary side: Lr = 0 + Lr

So this flux density parameter is something you decide, not a characteristic of the ETD core used?

Yep. The core's datasheet will tell you the maximum flux density, which you should avoid going beyond. Going lower in flux density will decrease core losses.

Is this secondary side OCP value important? It affects the max voltage cross the resonant capacitor but I am not aware of a OCP on the secondary side for the DC-DC9 converter.

Not really important. I was not able to find a direct match for the secondary OCP in the NCP4390's Datasheet, but in this calculation tool I think it's just the maximum designed output current. You could say, the output fuse is the OCP :D

why is the min gain constantly being adjusted?

You don't need to do it, but it will help you to get closer to your desired design values. It's just a parameter you can adjust to your needs, so why then not use it?!

I had a read at the onsemi LLC paper and I think this virtual gain is an adjustment for the secondary leakage inductance's influence on the resonant gain for integrated transformer design.

Hm, interesting. Might have a read on that later.

[Jamesteng0214]

The next day I meet by chance Eugen Hoffelner and he said "cooles Teil" ["cool device"]. Apperantly he was aware of my DCDC through TUG Racing (my beta tester team). That was kinda cool getting aknowledged by someone who works with power converters job related ^^

Woah, I totally get how nice of a feeling that must've been. Well my copy of DCDC is pretty much assembled and I'm just waiting for the equipment (HV power supply etc) to come so I can start testing. So you probably won't hear from me for a few weeks, hopefully the next time I post on this, it'll be "I got it working!!!" hahaha.

Thanks for your help again!

[Rootthecause]

Very nice! I hope it works as expected!
Just in case you've missed it: Be sure to download the latest build guide, because the f_min setting is different.

You're welcome ;)

[Jamesteng0214]

Hello Rootthecause, how are you doing?

I was wondering if you have any suggestions on the specs of the hipot tester used for transformer testing? Or as long as it is able to generate 3 times the voltage it's fine?

Thanks!

[Rootthecause]

Hi!
I'm quite busy with my thesis ;)

3x TS Voltage is okay (note: 1800 V AC RMS, so 2546 Vp), but I would recommend just a little bit higher than that just to be very sure, like 2000 V AC RMS.

Via my survey I received an anonymous rumor that there might be a increase of TS Voltage to 800V, so 2400 V for testing.
But I was not able to confirm any of this, so either it's a hoax/Joke or a real rumoring rumor which could unfold at some point. Either way, if it were to become true, I doubt that any team would switch within the next season, as the needed changes are rather substential and the gains probably not really worth the changes. But in the long term it would make sense as the automotive indursties seem to steer towards 800V topologies. So if you want to future prove your hi-pot tester, I think 2500V would be a good choice. For the DCDC it would't be too hard getting it work for 800V. It's "just" redesigning the magnetic circuit and maybe some better clearances.

Since I'm developing the transformer, my test voltage is usually way higher, as I test it until isolation fails. The stated voltages in the datasheet is around 100 V to 200 V below the failure voltage.

[salalkhann]

Hi Rootthecause, this may be a dumb question but I'm trying to order the pcb from JLC using the bom and position.csv provided in the kicad folder, yet the when it comes to the component assembly page, there's an error for redundant information in the position page. I'll briefly attach these below:
U5,C20,C22,U8,C21,D12,C24,C35,D11,C23,C34,D14,C37,C36,NT1,NT2,F3,Q11,Q10,U13,U12,U3,TH2
These aren't present in the bom_jlc page. Additionally the component assembly page is only assembling the bottom side of the pcb and not at all anything on the top side. Any ideas as to why this is happening or fixes would be greatly appreciated.

[Rootthecause]

Hi, did you check, that "assemble both sides" was checked?

It should be only the following error appear, which can be ignored then: The below parts won't be assembled due to data missing. NT1,NT2 designators don't exist in the BOM file. JP1 designator don't exist in the CPL file.

[Rootthecause]

Quick update: There might be a change in the JLC ordering process. I'm working on it.

[Rootthecause]

Okay, found the fix. Please pull the latest bom_for_JLC_PCBA.
The issue is somewhat funny: I've got this BOM from JLC themselves (you can download it from their page after adding the order to the chart). They have two collumns for top and bottom designators. However, they are unable to correctly process their own BOM, as only a single collumns for both sides is expected 🤣
So I combined both collumns and it seems to work now:

[Ciel-MC]

Hi, I'm making an electric kart though as a hobby project instead of for FSAE/Student, so I don't have that high voltage battery pack. I was wondering if it's possible to lower the input voltage down further? Or maybe it's better to use an off-the-shelf part at this point? Though I haven't been very good at looking for them.

On a side note, do you know of any good place to ask questions about building an e-kart? Is the r/FSAE reddit(or their discord) fine? I'm quite clueless rn as I'm sure you're supposed to have some seniors teach you the ways when you're doing FSAE XD

[Rootthecause]

I'm in contact with a battery seller rn and I'm looking at a 40140 cell, 15Ah capacity and 8C continuous discharge and 30C for 15s. Each Na cell is 1.5V dead 3V nominal and 3.95V Max, so I plan on doing 192V nominal, which means 96 dead and 252 max. The motor I think I will get 144V because current~=voltage is nice or smth(20kw cont.), just felt like it.

Ah, I see.
Current propotional to voltage? Afaik RPMVoltage and CurrentTorque.

You might get mad but... maybe 4? Dirtbikes seem interesting too but my current plan is 2 motors rwd with funny torque vectoring

nope, why should I? ^^ I was just wondering.

or maybe just use a 12V(well, LV) battery...

actually, I should suggest that to you in the first place, as this is the simplest and safest option.

I personally wondered quite a few times why EVs(Like the ones on the road) still have 12V bats, I guess I see the struggle now lol.
um, nope. They still have it, because it makes jump starting and powering the LV system without HV much easier. You will be suprised, that they not only have a multiple kW DC/DC converter (ever wondered how the 12V battery is recharged?), but also that it is bidirectional. You can actually recharge the HV battery by getting 12V on the LV battery.

I'm vaguely aware of them from learning about motor controllers, it's so that capacitors don't draw one morbillion(that meme is still alive right?) amps when you connect the battery but instead it's trickle charged by some big resistor without connecting the main contactor, something like that right. This is another point of knowledge to investigate

Sorry, that meme didn't ring a bell even when I googled it.
But yep, correct.

I saw those while strolling the FSAE Reddit, I looked but the voltage range might be a little niche, the closest I found it 120-420V but I don't know if I want to run it at the minimum rating, I also just looked at the price and it made my heart skip a beat, and it seems I can't even order less than 25 from mouser and/or digikey XD, so I think I might skip on that, what makes them better than a DC/DC converter like this or just a regular power supply anyway? Just size? 🤔 since I'm not that tight on space(uhh if you ignore the 80kg battery that I'm trying to have 2 of) the money-saving options seem more enticing to me :p

haha, yep. They are pretty expensive despite their specs beeing not that impressive, especially for the costs. They are DC/DC converters as well (normal PSU's are too!). The Vicor modules are - well - ready made and tested. Still, some teams managed to blow them up :D

P.S. how are you supposed to power the logic level stuff like on the motor controller or driving the big boy mosfets? Is it just an on-board buck converter since it's very low power? I don't think they need extra 5V input right?

Depends on your controller. The ones for higher voltages that I know require extra LV power. Depending on controller, it could be anywhere from 5W to 20W. But a 60V VESC controller has a step down included.

[Ciel-MC]

Current propotional to voltage

Approximately equal 😅 I have no idea how to type those outside of a mac and didn't want to copy/past it. The motor is 20kw and they offer rewinds to any voltage, and I for some reason decided that 144V140A seems like a good place to be, with some headroom on the bats for stall current and safety and voltage sag and such. Now I'm thinking about it maybe I should just go for even higher to avoid having to deal with high voltage and high current, though that may make the battery setup a little more difficult since my cells are in increments of 15Ah(120A)

as this is the simplest and safest option

Yeah I know, but dc/dc sounds so much cooler :p

Sorry, that meme didn't ring a bell even when I googled it.

The morbius movie from a while back, maybe you missed it lol

They are pretty expensive despite their specs beeing not that impressive

Yeah I think I can go for something a bit more shottily made, since it's much smaller of a deal if it blows up :p

Depends on your controller

Well uhm here's the thing, I'm trying to make my own controller(yeah ik), I don't think it's a good idea but I just wanna do it, and I'm not sure how well two off-the-shelf controllers will work for this double rwd setup, plus it's a good chance for me to learn electronics(well it's not gonna be my first pcb cause that sounds like heck of a deathwish, I'll probably make something for, say, the dashboard first).

[Rootthecause]

Oh, lol, I forgot that when I type two of those ~ then it gets italic xD

I for some reason decided that 144V140A seems like a good place to be

What type of motor are you using? PSM ?

Yeah I know, but dc/dc sounds so much cooler :p

haha, true. I mean, you also only need to recharge one battery and your HV battery can supply the LV system much longer.

Well uhm here's the thing, I'm trying to make my own controller(yeah ik), I don't think it's a good idea but I just wanna do it, and I'm not sure how well two off-the-shelf controllers will work for this double rwd setup, plus it's a good chance for me to learn electronics(well it's not gonna be my first pcb cause that sounds like heck of a deathwish, I'll probably make something for, say, the dashboard first).

Umm... even as a second or third PCB, a motor controller (inverter) is be a deathwish. A friend and I are building our teams inverter since last year (600V 80kW) based on the VESC design and we made it sucessfully trough the FSG endurance.
In your case, I would go with VESC controllers too. They are open source and versions up to 100V 250A available for purchase. For software differential you just need to tick a box in the settings. That said, it would require you to use a different battery config.

But how I read it, it sounds like this would be your first electronics project or project with motos and stuff?
I really would advise you to start muuuuch smaller, e.g. an E-Skateboard. Otherwise it could be very frustrating if you invest a lot for your said setup, and fail at some point or loose interest because it was too complicated or prone fo failure at the start.
That reminds me, when I build a wooden water wheel to generate electricity. Despite beeing 1 m in diameter and approx. 30 kg in weight, I used wood glue instead of screws to put it together. So on the first test it flew apart.
At very least, get someone on board who did a project similar to yours before :)

[Ciel-MC]

What type of motor are you using?

Yep, PMSM with interior mounted magnets, rated at 15kw continuous but seller said it can do 20cont with decent water

Umm... even as a second or third PCB, a motor controller (inverter) is be a deathwish

Absolutely agree, there will be billions of steps before that. I ordered both a high voltage and a high current controller to copy their designs from, and will be making one with much much lower power than what I'm going for, baby steps.

VESC controllers

They're a bit underpowered for the motor but I may use them as an intermediary before I make my own since they're pretty cheap.

E-Skateboard

We'll see about that, not sure how much I want to spend time on a separate rabbit hole compared to getting stuck in my goal rabbit hole, but maybe...

get someone on board who did a project similar to yours before

I wish I could but I have no idea where to find such a person, lmao.

tick a box in the settings

Hmmm, but how does it control two motors? Two vescs and CAN?

different battery config

I'm planning on building a smaller pack to begin with, plenty of things to try out screw up. I also wanted to try out laser welding so not testing in prod is probably a good idea.

I'm pretty clueless on everything but I'm well aware of that, start small and dream big. We'll get there someday.

Edit: I think I'll stick with the car plan but work on other components first before I touch the motors with anything not off-the-shelf. I personally prefer struggling on a big project and learn along the way vs learning from increasingly harder projects, which is also how I learn programming, so I figure that's probably the model that fits me better, that plus my gut feeling not liking anything but working on something "meaningful".

[Rootthecause]

Okay, I see.

Hmmm, but how does it control two motors? Two vescs and CAN?

correct. I've sucessfully connected 4 together via CAN. It's pretty much plug and play.

I personally prefer struggling on a big project and learn along the way vs learning from increasingly harder projects

yeah, does not sound too bad. I mean, it's more fun this way ^^

[FlamingTiger1785]

Hi, I was wondering if you could share the part number of the thermistor you used on the transformer. I can vaguely see this "392F" marking on the surface which might indicate a 3.9k at 25 degrees C (room temp) but I am not sure and would appreciate clarification.

[FlamingTiger1785]

Awesome Thank you!
Sorry I am not familiar with KiCad, I am using Altium, but now that I know that in the symbol properties you also included the mouser link this will be incredibly useful for adding other components.
Amazing Job

[Rootthecause]

Ah I see ^^ np mate :)
You can also have a look at the BOM and import it directly into mouser, as all Part numbers and URLs are included.
Btw. if you highlight a component (click on it) in KiCAD and then press "D" will open the included link.
Have fun!

[williamjtadeu]

Hi @Rootthecause, speaking about thermistors, I am unable to find the location of the TH2 for the OTP on the KiCad PCB doc since the U6 pin 2 net trace leads to a via on the the top layer that is not connected. Would you be able to help please ?

[Rootthecause]

Hi @williamjtadeu, the via is connecting to the inner layer 2 (maybe you've disabled the display of the inner layers).
If you are using KiCAD, you can search for TH2. I've highlighted the trace and the component below.

[williamjtadeu]

awesome thanks! :)

[momen-exe]

Hi,
I have a quick question about the Litz wire for the transformer's secondary windings. The wire we are supposed to use is 600 x 0.071 mm litz wire, but we are only able to find 615 x 0.071 mm wire. Would the increase in threads affect anything?

[Rootthecause]

I see. What a bummer :/ 
I wish I had the time and capacity to resell the litz wire from my source to other teams worldwide...
(If soemebody can help me building a PCB transformer in the future, we could get rit of it alltogether ^^).

[FlamingTiger1785]

We are trying to get it off Ebay but the seller from your source doesn't deliver to Australia, we found a Ukranian seller and they deliver to us but the only size we could find was 615 threads

Hi @momenali1 we have the same issue where i am from, are you able to share the link for this ukranian source please?

[FlamingTiger1785]

@Rootthecause, silly question here, but why can we not use the same litz wire used in the primary for the secondary?

[Rootthecause]

The primary litz wire is effective 0.92 mm^2, whereas the secondary is 2.38 mm^2. If there is not a lot output power needed, the primary litz wire could indeed be used. However, the resonant inductance might be slightly different, due to different geometry, but this should be doable.
Note, that more effective surface area might not reduce resistance as much as for DC currents, as the high frequencies will add skin- and proximity effects. There is even a small chance, that the primary litz wire used as secondary actually has lower losses. However, due to the smaller surface area (around the litz wire) it will be harder to dissipate heat, causing higher local temperatures despite lower losses. Also note, that the secondary is used as heat sink for the active rectification. I think 200 W output power should be feasable, maybe even 300W or more - proper testing with thermal cam required!

Using 2 or 3 pirmary wires in parallel as secondary litz must be done with caution and proper testing for sure, as the secondaries will have different inductances just by their different locations (e.g. when wound on top of each other). This can create load imbalance to the point, where only one of the litz wires carries the whole current 🔥
Load imbalance can be also an issue between both secondary windings, causing more stress on one of the SR FETs. With the current design I was able to minimise those imbalances, resulting in almost equal power losses over the SR FETs.

tl;dr: I would recommend using primary litz as secondary if less output power is ok. Otherwise better don't parallel the litzes and go for a larger one.

[FlamingTiger1785]

thanks that very much makes.
In the meantime i think i found this mentioned litz wire 615 x 0.071mm if anyone else needs it here is the link

[wenjinwuwu]

Hi!
I reached out to you on Reddit earlier regarding the transformer design. I have attached the Onsemi design calculator with my parameters also I used TI calculator sheet for the UCC25640x controller IC.( Where I got Cr = 20nF, Lm = 375uH, Lr = 88uH, Lp = 375 + 88 = 463uH)
Onsemi sheet say 20:2:2 ratio will keep my flux density below 0.2T, but I will also do a 30:3:3 for optional testing.

ETD39_of UCC25640x Design Calculator Rev4.0 - Copy.xlsx
LLC-RESONANT-HALF-BRIDGE-CONVERTER-DESIGN-TOOL-FOR-FAN7688.XLSX

Below is the response I sent to you on Reddit but I should paste it here:
ETD39_of UCC25640x Design Calculator Rev4.0 - Copy.xlsx
LLC-RESONANT-HALF-BRIDGE-CONVERTER-DESIGN-TOOL-FOR-FAN7688.XLSX

[Rootthecause]

Hey!
Thanks for sending your calc sheet!
I think the files are linked twice in your message - was confused, but sorted it out :)
I had a look at the UCC25640x Design Calculator - it's very impressive to see the loop compensation and response waveforms calculated in excel. However, it's quite bulky (I mean the layout), so iterating on the parameters seems a bit troublesome to me 😅
But so far, the values are similar to the FAN7688 sheet, so I will use it instead :)

Regarding the Flux density:
I did some calculations with different formulas to calculate the flux density and also compared it with the onsemi sheet (see page 33 ff in the documentation). The onsemi sheet resulted in the lowest flux density. Their result seems odd to me, so I won't trust that. Even if: There is a rule of thumb for inductors (and transformers) to keep the ripple current between 15% and 40% of the max. ripple current, which is limited by saturation. So keeping the flux density between 15% and 40% of your saturation point will keep your magnetics cooler. For the N87 material it saturates at 0.32 T so you should stay between 0.048 and 0.128 T. If you can get lower for the same switching frequency, you'll run them cooler. You can got higher ofc, but the heat (= losses) will increase. Just for reference: my v9-3 design is according to the onsemi sheet at 0.139 T and my cores are reaching a temperature of around 90°C with no load - causing the most losses at no load.
You can read more about that design rule here:
slup384.pdf
Just from my guts, I would suggest you rather going with 30:3:3 winding ratio if you are caring about efficieny.

Now looking at your design parameters, there is something off with the minimum gain at max. input voltage. As written in a comment above you should have your minimum gain above 1, better slightly above the gain at resonant frequency. In your case it should be 1.14 or higher.
Just on a side note on the onsemi calc sheet: If you're integrating the resonant inductor, then "Integrated Tx" must be set to 1 otherwise the min. required gain is 1.0.
Also the minimum switching frequency is rather low (around 53 kHz). This will increase the need for more windings, as the switching frequency it is inversely propotional to the flux density. If using something higher, then the 20:2:2 windings might be feasable. For the M-Value: This is where you can tweak the ratio of Lm and Lr. Your M was set to 4.25. If you want to use my transformer design without changing a lot, then it would need to be 3. However, despite requireing wide input range from 300V to 600V your power draw is rather low at 200W, so a higher M is possible. I will try to keep the Lr and the winding ratio constant to my 56 µH and 28:2:2 for your ease. So in the end, the only thing to change from my design will be the air gap and Cr.

I did a iteration on the design like explained here.
Those are my suggested parameters:

LLC-RESONANT Tool 250811 wenjinwuwu.xlsx

I was able to decrease the flux density to below 0.12 T (lower RMS primary Winding current) which increases efficieny. The max. switching frequency is higher (180 kHz), but still very managable with SiC MOSFETs and N87 cores. Your required air gap for Lm = 361 µH is then 0.31 mm (make sure to take measurements of Lp while approaching the suggested air gap - the claculated air gap might be not very accurate). Cr is 10.8nF which is perfect for a design with 4 Caps in total, each of them is 2.7 nF.

Regarding the input voltage of your design: If used for a 400V and 600V battery, then the lower end at 400V_max would be at 240V (if 96 Cells are used @2.5V min). If your cutoff is at 300V, each cell has 3.125V - not going below this is helathy for the cells, but I want to stress, that the DCDC is the very last thing you want to turn off on your LV system if the battery is empty. Especially if the current control of the inverter is not perfectly tuned, it might get below 300V - and turn off the whole car. This is why I designed my DCDCv9-3 for an input range of 200V to 600V to fully support both 400V and 600V systems. That said, you could actually use my design - but gathering experience with your own design is nice ;)
I also had a look at the UCC25640 because of the higher max. switching frequency, but in the end, it couldn't win against a STM32 feature wise for the future DCDCv10.

[momen-exe]

Secondary Winding's Inductance:

We were able to get the primary inductance within the +- 5% requirement. The secondary winding's measurements are around 1.48 to 1.5 microhenries for both W2 and W3. Is this fine, or do we need to get it as close to 1 microhenry as possible?

[Rootthecause]

hm, yeah, thats not running. Did you have a look at the gate driver singnals? Maybe the UCC21520 is not working in your sim.

[momen-exe]

Will have a look, but just to make sure so far and to eliminate any issues, I am not supposed to have a -1.1kv reading at the TL431?

[Rootthecause]

nope 😅

I just checked the TL431 model I've linked in the doc. It's still the same as the model I use.

[CalebRezmer]

I've also been having the same issue. I have the same outputs as momen-exe and -1 kv on the TL431's cathode. Changing the line in the TL431.sub file, I1 CATHODE N001 5ľ, to, I1 CATHODE N001 5u, fixed my voltage measurement at the cathode.

[Rootthecause]

ohhhhh! I see!
So the weird symbol from your comment is a "µ" (micron), but it must be a "u" to be understood correrctly by LTspice.

Edit: I looked into my model - it's a "u" there as well. I tested the "µ" and saw the -1 kV too. So this is the fix.
Will update the documentation with the next push.

[jsugijanto3]

Hi Rootthecause!

I was wondering where I could find the footprint file for the transformer? I couldn't find it anywhere in the github repo.

Thank you so much and this is an amazing board that you have designed!

[Rootthecause]

Yep, LTspice.
There are general formulas for calculating the required capacity, but these often assume a sinusoidal supply/load (e.g. from a full-bridge rectifier and a mains input). With LLC converters, this is not the case in most operating points, and the transformer stray inductance causes secondary oscillations. Therefore, it is much easier and more reliable to simply perform the simulation for the lowest input voltage and the highest output load. It is also easier to insert all parasitic components from the data sheet (or use the manufacturer's model if you're feeling fancy) without having to worry about formulas. This is especially true if you want to try out different filter types or find out how they behave at start-up.

Regarding the values I have determined: There is a wide range that works technically. Higher capacity reduces ripple (when used correctly – pay attention to ESR/ESL!), but requires more space and money. But also the question arises: what happens if we don't use capacitors? On the input side, EMI is transmitted through your cables (galvanic EMI) to the traction system, inverter, BMS, etc. But the inverter also transmits its EMI to the DC/DC converter, causing current and voltage spikes.
For the output, there are other factors relevant: reducing ripple is one thing, but sufficient capacity to keep the loop control stable against large current peaks from connecting uncharged capacitors at the output is another factor.

You can look up the actual measured Vpp and RMS ripple voltages for the input and output in the datasheet page 3.

[jsugijanto3]

Thank you so much for the reply!

I recently purchased the components needed to build the transformer and had some questions that I want to ask:

1. In the build guide it states, "However, when connecting the secondary windings, the windings need to have opposite winding directions or generate opposite voltages seen from the center tap (pin 4)". Could you clarify what this means? To achieve this center tap configuration, could I short the "negative" of one of the secondary windings with the "positive" end of the other secondary winding?

2. How did you join the two ferrite cores together? The build guide states, "Coat the outer legs of the ferrite core with a thin layer of 2K adhesive". If the ETD39 core is the letter E, should I apply adhesive to the top and bottom legs that are in contact with the other core? Or do I apply adhesive on the bottom leg that contacts the SLA isolator 3D print?

3. How do I test that I have built the transformer correctly? I tried to supply the primary coil with a sinusoid using a function generator and probed the secondary winding using an oscilloscope. However, I did not read any signal/sinusoid on the secondary winding with the oscilloscope.

4. Since I am in the US, I had to buy litz wire from a different vendor. Here is the link:
   https://www.remingtonindustries.com/magnet-wire/litz-wire-18-awg-unserved-single-build-5-20-38-stranding-ideal-for-100-khz-applications-6-spool-sizes/
   It does not come with the nylon insulation that your litz wire has. Could these red litz wires work with no nylon insulation?

5. The documentation uses L_p when measuring the W1 inductance (W2 and W3 open). To clarify, L_p is not equal to L_m (magnetizing inductance) right? I am confused because in the KiCad schematic, "L_m = 166uH" is written. Is L_p = L_m + L_r?

Sorry for the long questions, but thank you, and I appreciate all of your help!

[Rootthecause]

1. In the build guide it states, "However, when connecting the secondary windings, the windings need to have opposite winding directions or generate opposite voltages seen from the center tap (pin 4)". Could you clarify what this means?

There are two approaches to achieve the correct polarity:
A) Both windings are wound in the same direction (CW or CCW). Both windings are in series, so they look like one continuous winding, but in the middle there is the center tap.
B) The windings are wound in the opposite direction (CW and CCW). Both windings look like in parallel, but are connected at one side.

In the end, the positive ending of the first secondary needs to be connected to the negative ending of the second secondary. One could say that this is the electrical view and the one mentioned earlier is the mechanical view. The benefit from the mechanical view is, that you don't need an oscilloscope to figure it out; and this is why I prefer the mechanical view, although both result in the same electrical behaviour.

I ended up using B) as I though it would safe some space and requires a few mm less litz wire. But I think the actual difference to leap back to the center tap is neglectable. Side note: B) does not really work for less than 2 Windings per secondary, otherwise the crossing due to the opposite winding directions will look very ugly and requires more space than A).

1: A) in the schematic, 2: A) as wound, 3: B) as wound, 4: (Don't!)

To achieve this center tap configuration, could I short the "negative" of one of the secondary windings with the "positive" end of the other secondary winding?

Correct :)

2. How did you join the two ferrite cores together? The build guide states, "Coat the outer legs of the ferrite core with a thin layer of 2K adhesive". If the ETD39 core is the letter E, should I apply adhesive to the top and bottom legs that are in contact with the other core? Or do I apply adhesive on the bottom leg that contacts the SLA isolator 3D print?

→ Apply adhesive to the top and bottom legs that are in contact with the other core is correct. See the red lines in the picture.

Just a heads up notice: It might be required to glue down the transformer at the bottom to the isolator and then glue the whole transfomer to the PCB. See issue #10

3. How do I test that I have built the transformer correctly? I tried to supply the primary coil with a sinusoid using a function generator and probed the secondary winding using an oscilloscope. However, I did not read any signal/sinusoid on the secondary winding with the oscilloscope.

Using a signal generator might not work depending on the output strenght and frequency. Try a high frequency like 300 kHz. This will put less load on the generator and thus increase voltage over the primary. However, in theory you should be able to measure something at the output. The turns ratio is 28:2 so 1 Vpp on the primary will be 71 mVpp on the secondary (ignoring coupling factor and resonance). I would rather use the mechanical view to get the connection right. When you do the first test, just hook up the scope with two channels. Connect the grounds to the center tap and the probe tips to the outer connections. You should see the opposite polarity if done right.

4. Since I am in the US, I had to buy litz wire from a different vendor. Here is the link:
   https://www.remingtonindustries.com/magnet-wire/litz-wire-18-awg-unserved-single-build-5-20-38-stranding-ideal-for-100-khz-applications-6-spool-sizes/
   It does not come with the nylon insulation that your litz wire has. Could these red litz wires work with no nylon insulation?

Other then increasing spacing between the windings, the Nylon does not isolate. It just keeps the strands together and might protect them from scatches and kinks. It should work the same without nylon 👍🏻

5. The documentation uses L_p when measuring the W1 inductance (W2 and W3 open). To clarify, L_p is not equal to L_m (magnetizing inductance) right? I am confused because in the KiCad schematic, "L_m = 166uH" is written. Is L_p = L_m + L_r?

Ouh - my bad! You're totally right! It should be L_p in KiCAD. Will push that fix with the next revision. Thanks for spotting that!

[jsugijanto3]

Hi Rootthecause!

I wanted to ask additional questions about the converter:

1. The Datasheet states that the converter is rated for 750W for 1 minute. How was the power and time value achieved? Will the converter (or maybe the FETs) break if we use it at 750W for longer than 1 minute?
2. How was the ETD39 chosen instead of the ETD29, for example. Was 39 chosen solely due to its ability to withstand more power? I couldn't find the power values on their datasheets.
3. In section 6.3.12.3, it is stated that the leakage inductance was used as an energy storage. Could you clarify why a larger leakage inductance in the transformer is required for the LLC design?

Thank you for your help and support!!

[Rootthecause]

Hi!

1. This test was performed at 25°C ambient temperature with the small fan attached (as per build guide). The converter was running until it reached stable temps. Then A load drawing 31.25 A (750W) was attached to the output and the time was measured until the conveter turned of due to the measured SR overtemperature. I could not find the exact measurments on the fly, but iirc it took something like 70 seconds. So 60s is achievable, with good cooling even longer.
   It shouldn't break anything. If you apply the load longer, the OTP will kick in or the 20A output fuse will melt (it will conduct 30A for 100s). If your load is higher, the fuse will melt faster, which is somewhat in accordance with the thermal mass of the converter. Some details are in the SES document in the doc folder.

2. The power transformed through the cores depends roughly on the switching frequency and the flux density. If you double the frequency at the same flux density, you'll get double the power. However, your core losses will roughly more than double. To compensate, you can lower the flux density by increasing the core size. Now depending on core Material, there is a limit to the flux density (magnetic saturation). If you'r flux density is above that, your inductor (and transformer) will not limit the current change anymore like a inductor would and behave like a resistor instead. For an LLC converter, this can lead to operation in the capacitive region (very high power losses or even destruction of the half brigde).
   In summary, you should lower your flux density as much as feasable - to make it safe and reduce losses. A rules of thumb is 15% to 30% of the saturation flux density. For N97 Cores B_sat is 410 mT, so 123 mT would yield low losses (cooler running cores). But note, that some cores will have lower losses at higher temperatures. I think N97 was optimal at 90°C. You can actually measure that the efficiency goes down by 0.1 to 0.2 % if you cool the converter with a strong fan.
   Now ETD29 has only 76 mm^2 cross section and ETD39 has 125 mm^2, thats 1.6 times more. The same design would be 202 mT for a ETD29 core instead of 123 mT.
   Those are the most basic considerations to start with, but there is much more to it: How are you gonna fit 28 Windings considering that you can't make the wire too thin (resistive losses)? Isolation spacings are trickier too.
   If you go up with the switching frequency to increase the power, you could on first glance lower the flux density to a reasonable point. However if you go in the MHz range, you will encounter high skin- and proximity losses not just in the wires, but also in the magnetic cores (magnetic skin effect). The choice of the right core material and coil design (and number of turns) becomes quite challanging then.

3. Traditional low frequency (50/60 Hz) transformers try to have high coupling (low leakage) in order to achieve stable output voltage under load and higher power. If the coupling weak (higher leakage inductance), the output voltage would decrease a lot when a load is applied. Now for HF transformers in traditional SMPS which use a PWM half-bridge control the same is still true, but those have a problem with varying input voltage: the output voltage will change accordingly and can't really be changed with the PWM duty-cycle. This is one of the key benefits of LLC: Depending on switching frequency, you can change the resonant gain factor, allowing for adjustable voltage control. However, you need resonance for this to work. The transformer does not need to be lossy, but you need some kind of leakage inductance in the resonant cicuit. This can be an external inductor in series to a low leakage transformer (this is also the reason, why you sometimes see two inductors in the L-L-C topology :D). The key benifits of having a lossy transformer instead of low loss transformer + external inductor are lower space requirements and higher efficiency.

I hope, that helps :) 

[momen-exe]

Hi @Rootthecause, hope everything is well.

Had a quick question about testing, is it possible to test the operation of the LV part of the circuit without the transformer installed?
Also in the initial operation part of testing, are we supposed to see the 0.1 mA current being drawn from the Power Supply?

[momen-exe]

Sorry, it has been a long day, I meant to say 0.1 A.

the 5 V LED did not turn on at 23 V, and the build guide said not to go over 24V, I have tested this on 2 different PCBs and I am not getting any current drawn from the power supply at all.

[Rootthecause]

Hm. The fuse F2 is not blown by chance?
You can check the expected currents in the Table on the "Checklist v9-3" sheet: https://github.com/Rootthecause/DCDC/blob/main/documentation/DCDCv9-3_Table.xlsx (need to download, won't be displayed in browser). If all is on, it is around 22 mA (+-5mA).
Sounds like something is wrong with the stepdown. Check IC polarity.
(Might not answer futher questions untill tomorrow.)

[momen-exe]

Do we need the fan and it's header installed?, we have all other components except the fan and the transformer? I am genuinely a little lost on how to debug this?

[Rootthecause]

No, the fan is not needed in this case. It will work without it.

From my experience you should be able to debug most of it just with a multimeter. Open the schematic and start measuring the voltage all the way from source to sink.
Apply 23 V to the output (you wont harm it with up to 35 V (capacitor limit) but it might generate no PWM on the halfbridge due to the control loop regulating to 24 V).
You can connect the negative side of the multimeter permanently to ground while measuring this. Make sure to not accidentally short something with the tip due to slipping. Start measuring with the positive tip on the following spots and compare it with the voltages:

• U3 pin 8 (23 V)
• U3 pin 2 (5 V)
• Q7 Pin 2 (around 3.7 V)
• Q7 Pin 3 (around 3.7 V)

Since you mentioned, that the 5V LED (D7) is not lighting up, check the polarity of the LED is correct. Also measure the resistance between Q7 and GND (remove 23 V before that). In case you've shortened something, the IC U3 will go into "hiccup mode". It will draw around 11 mA in this case.

Please let me know, what voltages you've measured at those pins and the resistance. Also, how much current is going into the output connector (J5) currently? In case it is close to < 1 mA, switch to µA range and tell me the reading - because it can't be absolutely zero, unless the main output use F3 is blown or defective (in some cases soldering it lead to a defect for unknown reasons). Also please let me know, if you've made any changes to my desing and if you've assembled the PCB yourself of used the PCBA (with automatic assembly).

[momen-exe]

Hi, sorry for the late reply. We had to put DC DC testing off because of midterm exams, which we just finished, so I will start testing again and document the values. Thank you so much for your help so far.

[R-E-Nielsen]

Hello,
I had a quick question about EMI coming from the DCDC. We are housing it close to our bms and LV communication lines. Would you reccomend any shielding on the wires and creating a housing for it?

[Rootthecause]

Hi!
This is really hard to estimate, because there are so many factors involved (frequency, voltages, distance, filtering).

I would recommend to just test/measure it. Hook up a oscilloscope to the lines (especially to the analog ones) and measure the voltages when the DCDC is turned on vs. turned off. If you discover changes which are influencing your signals too much, then shielding of the cables would be an option. You could shield the converter too (caution about ventilation), but that sound like much more work for the same effect.
Another thing that I would recommend is using proper lowpass (RC) filtering on analog inputs. Sometimes even 1 nF parallel to the inputs do wonders - check the impedance of the capacitors! 100 nF can have less effect than 1 nF on HF-EMI because of parasitic inductance. Choose a small package like 0603.
As the converters EMI is mostly in the range of 100-400 kHz, it is easy to filter out with small capacitors on affected lines.

Depending on inverter, the DCDC's EMI might be not your main concern. Definetely check your signals with a running Motor as well! In that case, shielding the converter won't help, but shielding the cables will.

Edit: Make sure to understand the different kinds of EMI (galvanic, electric, magnetic :)

[Moffeltje]

Hi, first of all, this project is awesome! I had a question regarding the ETD44 transformer. You say this in the documentation: 'ETD39 is recommended for the DCDCv9 versions, even if there is theoretically enough space for the ETD44 transformer from v9-2 onwards (Y-cap soldered on the underside), which offers approx. 1% better efficiency (compared to ETD39) and more continuous power
without a fan.' Why is it recommended to use a ETD39? Would a lot need to change to make the converter compatible for the ETD44?

[Rootthecause]

It is very hard to say with that limited info, what the issue is. But it could be the F_min issue. What is your minimum input voltage, and which voltage did you use to set F_min according to the build guide?

For many test you can actually set the deadtime to minimum, so turn it clockwise as much as possible. I don't remember how I measured the 3 kΩ 😅
But if you want to do it accurately, you surely should lift at least one of the pins off the board. The Deadtime does not need to be very accurate in general, +- 1kΩ might be totally fine. It's just, that you slightly loose some efficiency.

I am unsure if I tuned the dead time correctly, so could I try placing a 3kOhm resistor in the position of R3 as in the PCB files?

Yes, that what I implemented R3.

[Moffeltje]

I set the F_min using only 10V at the input and 23V at the output since any time when I used HV the last few times the board would break and I was also thinking of an F_min issue. I set it to around 95 kHz with that input, is this correct or is it very different if I set it at high voltage and could this be an issue?

[Moffeltje]

I probably found what the repeating problem was, one of the resonant capacitors was shorted. Now about the F_min setting in 8.8 I have a similar question to someone else in the discussions. With a 15V/1.5A my board dies after a while and cant sustain an output voltage of 5V+. I have checked if there is voltage over C1 and there is. Could the issue then be my transformer?

[Moffeltje]

Just to update, I have found why it couldnt sustain itself. Since I damaged C34 I substituted the resonant capacitors with C3 but did not see the text saying you should connect pin 1 with pin 2 of the transformer with a wire lol. It works now!

[Rootthecause]

Sorry for the late reply!

I set it to around 95 kHz with that input, is this correct or is it very different if I set it at high voltage and could this be an issue?

This is fine. As I've written it in the build guide: In the author's setup, this frequency was approx. 94 kHz

Just to update, I have found why it couldnt sustain itself. Since I damaged C34 I substituted the resonant capacitors with C3 but did not see the text saying you should connect pin 1 with pin 2 of the transformer with a wire lol. It works now!

:D alright, glad you found it out yourself and great that it works now! :)

[CalebRezmer]

Hello Rootthecause,
I am currently adapting your design to a 600-volt input and 12-volt output system. I was wondering if it would be possible to use passive rectification instead. I know this would significantly increase power dissipation at the rectifier, but do you think it is manageable if we are looking to draw around 30 amps?
Additionally, to convert to 12 volts, we plan to reduce the number of windings on the secondary; however, we are concerned about changes in resonant tank characteristics and the resulting change in the gain curve. I recall that you reported no change in leakage inductance in a previous post, but does this still impact the resonant tank? From your gut instinct, how drastically would we need to change the design to fit this modification? Also, should we need to change the wire gauge in order to compensate for an increase in output current?

Thank you,

I really appreciate your thorough documentation and quick response time; it's truly astounding.

[Rootthecause]

Hi!
Thanks for adding a picture - this helps a lot in your case.
First of all, your secondary leads are way too long for a proper Lr measurement. Even a few 100 nH on the secondary will translate to a few µH on the primary side. Trim those leads, so that it fits for the PCB mount like shown in the build guide (Pages 19 and 20) and measure again. Also please remove the metal clamp and use something made from plastic, just to be sure, your measurement is not influenced by it. The green and blue adapter leads are also too long. Remove their insulation and trim them so you can fully insert it into the clamps of the device (so that the thicker leads are almost directly inserted at the clamps).
Secondly, doing a measurement of the voltage ratio is tricky, because even slight loads like probe capacitance will decrease the amplitude, as the stray inductance acts like a large impedance. At what frequency did you do that measurement? Doing the Inductance measurement is easier for LLC Transformers.

Regarding the different numbers of secondary turns: Your Lp and Lr will stay almost constant. I've confirmed this here in the comments somewhere before with actual measurements. If Lr increases by a lot, then you've might not wound it tightly around the coil former. You should definetely avoid large air gaps like seen with the secondary winding in your picture.

[swift-wastaken]

Hello again!
We were finally able to get some numbers consistent with what you had in the build guide, but before we glue everything and finalize the transformer we want to make sure that we aren't making any errors.
Our main question is if we're doing the secondary winding turns correctly? The way we currently have it built is detailed in the photos below:

We are going for a 28:1 winding ratio, but intuitively it seems like we have a 28:1.5 winding ratio.

Removing the supposed 0.5 turn(s) yielded a coupling coefficient of K = 0.541 & uncharacteristic inductance values

Are we getting bad measurements or is our initial 28:1 (28:1.5) winding ratio correct?
Thanks for the help so far by the way!

[Rootthecause]

Actually, from your drawing, you've got a 28:2 winding. As long as bot wires of the secondary exit on the same "side" of the ferrite core, you can't really have a .5 winding.
From your pictures I assume the primary winding is measured wrong - you can't just bend wires around the core to the other side and hope the measurement is not affected by the magnetic flux from the outer core legs. I think the way you did it will result in one winding virtually compensated, so 27:2. Your W1 (Lm) measurement does confirm this. As this creates a lot of stray flux, your stray induactance (Lr) will be higher - which is confirmed by your measurement as well - but I'm a bit suprised that it is this high.

Removing the supposed 0.5 turn(s) yielded a coupling coefficient of K = 0.541 & uncharacteristic inductance values

How did you try to remove the 0.5 winding on the secondary? Or was this the bent winding on the primary I've pointed out before?

If I understand your scondary drawing correctly, it's also not correctly wound. One winding is supposed to be wound on the other and not side by side. If you do it side by side, the winding further away from the primary will have much more stray flux (Lr).
Here you can see how the two windings are wound:

winding.mp4

When you do the 28:1 winding, it might be tricky to get both windings ontop of each other, as the current coil former is too wide. Either make one wall of the coil former thicker or add something to the side to support the winding from not slipping past the other.

[swift-wastaken]

I went ahead and made a revision to the ETD39_sek to help support the 28:1 winding pattern, since you voiced concerns about the wires slipping without a spacer.


Do you think the channel I made for the wires to follow will interfere with the fields? I imagine it would only interfere slightly, but I want to hear your thoughts just in-case you've already tried something similar?

[Rootthecause]

umm, I think your design is too complicated. You just would need to thicken the wall next to the E-Core like this:

In these files the remaining space is 2.5 mm.
ETD39_sek_single_winding.zip

[CalebRezmer]

Hello RootTheCause,

We are currently working on transformer using your suggestions (we ended up getting a leakage inductance close to what you measured in the build guide). However, I'm posting seperately to discuss an issue we had with board testing today. First I want to note that we are trying to create a 12 volt version of the board, so we have been simply applying 12 volts rather than 24 volts and haven't changed anything in the power supply circuit. The only changes we made to the board were to the voltage divider to the TL431 reference as well as some changes to the output filtering. When testing the power supply circuit, it went very smoothly at first until we noticed some strange behavior once shorting the DC enable (note this is without the startup capacitor on). Basically, in order for the circuit to function properly, we have to first supply 12 volts and then short the DC-enable. In this scenario, all of the voltage measurements are correct at the pads of the supercap, pads of U1, 9 volts supply, etc. and we draw about 84 mA of current from the supply. However, if we try to first short the DC-enable and then slowly raise the supply voltage, we end up hitting our current limit on the supply of 100 mA at about 3.8 volts (suspicious since its the voltage of the supercap). We also raised this limit to 200 mA and had the same result. Despite this issue, we then decided to put the start-up cap on and see what the results were. This time we essentially had the same issue with a current spike at 3.8 volts, but this time it happens no matter the order. All of the lights have been turning on as expected, its just the large current spike being the problem right now. I am a bit confused as to what may be causing the issue, but I feel like the fact that we see the spike at 3.8 volts is significant. Do you have any ideas as to how to troubleshoot this?

EDIT: Perhaps the cap is not charged and drawing a ton of current? (Cap measured around 2.5 volts before inserting, but we never charged it before putting it on, are we supposed to charge the cap beforehand?)

[CalebRezmer]

Thanks for your feedback! So we did some more testing. Yes, those 2200 uF caps have been on. The startup capacitor turned out to be one of the main issues. It was not fully charged (only charged to about 2.5 volts), which is why we were getting a relatively large current rush without the DC enable on and with 12 volts at the output. Now that is fixed. However, when shorting the DC enable with an ammeter, we measured an unstable current of around 300-400 mA that was increasing relatively quickly and draining the supercap. However, when the power supply was connected with the cap charged and the DC enable on, the current again increased rapidly, but then dropped to around 140 mA after some time, drawing around 84 mA from the power supply as seen before. Might be irrelevant, but after doing these tests, I then took off R61 to deactivate UVP/OVP, and the current stopped dropping back down to 140 mA and just kept increasing. We also noticed that the reference pin on the control loop is now 1.8 volts rather than 2.49 as it was before we put the startup cap on. This definitely points to the control loop as an issue. I'm looking to do some more troubleshooting on this and trying to find a thermal camera to better spot the problem. We greatly appreciate your assistance on this!

[Rootthecause]

If the power supply is not connected and the enable is shorted, then the voltage decreases on the super cap as load is drawm. The load is constant power, of course the current must increase if the voltage drops. The current I stated is when the cap is fully charged at 3.70 V. It stays at this voltage in continous operation. What is your voltage on the supercap if the powersupply is connected in continous operation?
When you disable the UVP/OVP by removing R61 the circuit draws more power (instantly), as the ocillator and the gate driver become active. However, it should not increase over several seconds. If this happens, this could hint to two things
a) a component gets hot and starts to draw more curent over time
b) the voltage on the supercap drops, as the buck converter is malfunctioning or Q7 is not conducting properly.

the reference pin on the control loop is now 1.8 volts rather than 2.49 as it was before we put the startup cap on

Whats your voltage on the HV input and LV output? If your voltage divider is correctly done, and 12 V equals 2.49 V then your output voltage should be around 8.67 V if you measure 1.8 V on the ref pin. If the voltage dropped when you put the discharged super cap on, this just means, that it is putting load on the output. But if your HV input voltage is too low, then the loop can't compensate, even when f_min would allow it without load.

I think I will add your cenario to the build guide, requiring to charge the cap (it is technically not required, but it seems to confuse you if you do the measurements at different super cap voltages).

[CalebRezmer]

We do not have a voltage applied to the HV input, just 12 volts at the output from a DC power supply. We are only doing the initial tests listed in the build guide. We draw a consistent 84 mA from the 12 volt power supply with 3.7 volts at the supercap. We also measured 233 mA at the DC enable. When the 12V supply is off and the DC enable is on (startup condition), the current measured at the enable pins steadily increases from 100 mA to 300 mA to maintain constant power and compensate for the decreasing cap voltage when discharging. It then eventually drops to a constant 100 mA. I hope these details make sense. (One issue from before was that we were using current limits on our power supply which was just confusing us more than anything)

However, my main point of concern is that despite measuring 12 volts at the rail, we only measure 1.8 volts at the reference of the TL431 when DC enable is on. When DC enable is off, we measure 2.49 volts as expected. This has left us in a bit confusion. Getting a thermal camera right now to do some further tests.

[Rootthecause]

3.7 volts at the supercap. We also measured 233 mA at the DC enable.

Thats way too high in this condition! It should be around 100 mA. This sounds like a component overheating or something shorted

When the 12V supply is off and the DC enable is on (startup condition), the current measured at the enable pins steadily increases from 100 mA to 300 mA to maintain constant power and compensate for the decreasing cap voltage when discharging. It then eventually drops to a constant 100 mA.

I don't understand your measurements. Why is it minimal 100 mA now, not 233 mA?

However, my main point of concern is that despite measuring 12 volts at the rail, we only measure 1.8 volts at the reference of the TL431 when DC enable is on. When DC enable is off, we measure 2.49 volts as expected. This has left us in a bit confusion. Getting a thermal camera right now to do some further tests.

Sounds like your TL431 is either broken or you've got the version with the other pin conficuration (happened to me years ago :D). Otherwise something might be not soldered properly.
Which resistor values are you using for the voltage divider?

[CalebRezmer]

Hello RootTheCause,

Thank you so much for your help! We have made some more progress on troubleshooting the boards. You were right about the TL431. Apparently, JLC's TL431 has a different pin configuration than the standard TL431, but luckily, I bought some backup TL431s from Mouser with the correct pinout! We made that change and now see 2.49 volts instead of 1.8 at the reference pin as expected. However, we still draw around 80 mA from the supply and measure around 220 mA across the DC enable (disabled UVP/OVP). We took an image of our measurements below. To find the source of the excessive current draw, we probed around the board, and everything seemed fine; all VCCs of any ICs were at the expected voltages. We used a thermal camera, but all we found was that U1 was around 45 C. Since we were stumped on this issue, we went ahead to the next step. We got a correct square wave from the oscillator and the gates of the FETs. This is our progress at the moment. We appreciate all of your help!

[williamjtadeu]

Hello,

We have built the transformer and done the W1 indunctance of the primary winding test and measured it using the Analog Discovery 3 with the Impedence Analyzer extension board as reccomended in the documentation. The resultant graph shape resembles the expected outcome seen in the documentation but the value at 100kHz is wrong, sitting at 130uH. Do you have any reccomendation on how to fix this issue?
We also followed the latest primary winding video you recently posted.
Below is our setup and the results.

[Rootthecause]

okay. Still low, but you can proceed

[williamjtadeu]

Hello,

Update:
We managed to succesfully finish all of the remaining transformer tests on W2/W3 with values within 5%.
We also conducted all the tests succusfully from the Build guide section 8.2, where we checked all voltage regs and measured the UCC25600 square wave outputs, and it was all correct.
We then soldered the transformer to the board and attempted to do the minimum switching frequency test in section 8.8. However with 15V / 1.5A at the HV input, there is 0A current draw from the power supply and so i did some investigation.
Turns out that after removing the power supply that keeps the LV alive at 23V, after about 1 minute the 12V_SW goes from 12V to 3.5V and keeps dropping.
Any advice or procedure that i am missing ?

[Rootthecause]

Hi!
Sorry for the late answer.
If your 23V supply is removed, and the DCDC is unable to provide more than 5V to the output to recharge it's C_start, it will be discharged depending on load after 1 to 2 minutes. Because of this, the 12V_SW will drop when C_start capacitor is empty.

If you can't generate any voltage on the output despite following section 8.8, then please check if there is any voltage on C1. If not, then the issue is somewhere around the precharge or the fuse F1 is blown. Can you check this please?

Also just to be sure: Did you change anything on the design (or imported it into Altium), or did you use my files?

[williamjtadeu]

Hi, turns out D12 was soldered the wrong orientation which explains what we were observing.
It is now fully working thank you !!!

[Rootthecause]

Ah, perfect!
Yeah, if D12 is oriented wrong, then the pre-charge does not close, thus no voltage at the DC-Link cap.
Glad it works now! :)

[M11453012]

Hello, I have recently been using LTspice for simulations, but I’ve encountered some issues.

At the moment, I have successfully imported all the components. However, the simulation does not seem to start properly, as shown in the figure. After I removed the "IC=23" setting, the simulation was able to run, but the resulting behavior/trend does not seem correct.

Could this be a problem related to the component models?

Thank you.

[Rootthecause]

Hi!
Have you tried the spice settings shown in the documentation (Step size and so on)?
It happend very often to me, that the simulations with complex models freezes in LTspcie. Its annoying, but there hardly seems a clean fix.
Also you need to run the model for quite a while before anything happens, as the controller/regulation loop has some internals which delay the start. I've just run my the DCDCv9-3r model I've published here - you won't see anything happen before around 600 µs simulation time.
If you remove IC = 23 (initial condition sets the capacitor voltage to 23V) you will need to wait longer until the output is stable - I would not remove it unless you want to figure out inrush currents at start up.

Edit: Your syntax error starts with the comma. Maybe some language/OS settings do interpret this differently than on my OS (MacOS). Which OS are you using? Did you try google it or ask an AI?

[M11453012]

Dear Author,

Hello, I referred to your previous replies and suggestions, and now the simulation appears to be running normally. I am currently using a Windows-based computer for the simulation, and I have made the following modifications:

I changed the TL431 model description from
I1 CATHODE N001 5ľ to I1 CATHODE N001 5u.
I modified the output filter capacitor syntax from
2200u, IC=23 to 2200u IC=23 (replacing the comma with a space), which resolved the error.
I have currently set the simulation parameters as shown in the figure. Would this setup be considered insufficiently rigorous?

Thank you.

[Rootthecause]

Glad it works now!
I tested without the comma and it works also fine on my side, so I just pushed the change.

I have currently set the simulation parameters as shown in the figure. Would this setup be considered insufficiently rigorous?

I'm not sure what you mean by that, but your settings seem fine to me.

[CalebRezmer]

Hello RootTheCause,

It appears you are gaining more and more popularity lol. Anyway, we ended up getting the 12-volt converter working! We drew up to 32 amps through it at around 95% efficiency. We will be sure to update you more about this later. Now, we are just looking to actually implement it into our car. I had a couple of questions. Our team is still planning to install a small 12-volt battery in the car and connect it in parallel with the converter, along with some droop control resistance. I'm thinking this would then eliminate the need for a supercap, as the output would constantly see 12 volts. Would this be possible? However, you also note that a load switch is necessary due to the possibility of overload at high loads. Is it possible to get away with not using a load switch?

[Rootthecause]

Hi Rez (?),
by the way, just call me Liv :D

It appears you are gaining more and more popularity lol.

Yeah, seems like it :D I wish people would start new threads instead of everyone writing in this thread - the almost 300 comments cause my browser to not respond for a few seconds, until I can start writing a reply 😆
So just or future questions, please read this first: #15
I might lock this thread in the future, because it got way to cluttered.

we ended up getting the 12-volt converter working! We drew up to 32 amps through it at around 95% efficiency

Wow, this is great news! Glad somebody proved in real life that the 12V version works well 🥳

Our team is still planning to install a small 12-volt battery in the car and connect it in parallel with the converter, along with some droop control resistance. I'm thinking this would then eliminate the need for a supercap, as the output would constantly see 12 volts. Would this be possible?

Possible yes, but it would be still good to install at least a electrolytic cap (e.g. 6.3V) and 1000 uF (at least 220 uF would be good, not sure what fits there physically) instead of the super cap due to ripple and inrush currents.
Doing current limiting/sharing with a droop control resistance in series between the converter output and 12V battery sounds ok if done well, but might be an issue in some cases. So imagine you want to limit the current to 10A per Volt of droop, then you would need a 0.1 Ohm resistor. This would result in 10 W of heat from the resistor - so it would be adequately sized and cooled. Be aware of scenarios, where the battery might be discharged to e.g. 9V → 30A → 90 W resistive loss!

However, you also note that a load switch is necessary due to the possibility of overload at high loads. Is it possible to get away with not using a load switch?

This highly depends on the load. If your load behaves like a resistor (voltage goes up → current also goes up), then it's fine (e.g. a 12V battery). However, if you have large buck- or boost-converters with large capacitors, then they will draw at start-up (=low voltage) much more current, than at higher voltages. This could cause the current to be too high at the DCDC converter's start-up and trip the primary side overcurrent protection (you'll see an ocillation on the output of a few Hz). In the worst case you might be able to blow up the primary HB FETs or the SR FETs - but I think they are adequately sized in the v9-3, and it did not happen so far.
So I would suggest to just test it and measure the inrush current on the DCDCs output. If it's below 32 A it's safe for sure. If it's below 100A for just a few micro seconds, it might be ok as well. I might do a simulation if those currents are reached.

In any way, you need some sort of load switch at least behind the battery to disconnect it (as required per rules), as you need some way to turn off the LVS. Or how does that work otherwise in your case?

[CalebRezmer]

That's funny, Rez is actually my nickname. Sounds good Liv. Will do for future discussions, my apologies. Our team is currently planning to put the DC-DC after the AIRs. The 12-volt battery stays in the car at all times, but the tractive battery will have the DC-DC converter and connect in parallel with the low voltage battery when placed in the car. All of the testing I did before to confirm that the converter worked was with a 600 volt power supply. However, today we tested it with the tractive battery in the setup that I explained above. For some reason the voltage of the tractive battery lowered from 500 volts to 330 volts when we tested it connected directly to small test load (300 mA fan). We measured this both at the input and battery terminals. This also happened with a larger load in parallel with the battery and droop control resistor. However, we also did not use a load switch which may be the issue. The output regulated to only about 11.4 volts which makes sense because our minimum input voltage was set to about 350 volts (we set our fmin to 110 kHz). I'm still trying to troubleshoot this, but wanted to see if you had any insight.

[Rootthecause]

Yeah, I wasn't sure because it was in capslock in your GH profile ^^

Ouh! Thats veeeery bad! If you TS battery drops like this, there is surely something not right with it. I would think that some connection is resulting in a large resistive drop. Just a tought: If the converter draws around 8W (just some guess) at no load and you've applied 300 mA at 12V then this is in total 11.6W on the TS. At 330V thats 35.2 mA. So your voltage drop is 170V and thus the internal resistance is 4830 Ohm.
You're not using by chance a precharge resistor in this range somewhere?
You can check the voltage drop inbetween the batteries to find out, where the bad connection is.
Caution when doing this. Better do not apply the DCDC while trying to measure this and use some resistive load, as poking around with the probe might move connections and connects/disconnects the DC-Link capacitor (very high inrush currents!).

The output regulated to only about 11.4 volts which makes sense because our minimum input voltage was set to about 350 volts (we set our fmin to 110 kHz).

Yep, that thought is correct :)

[CalebRezmer]

Yes, you are correct. There is 4.7 kohm precharge resistor in series with the converter at startup. This was originally designed for the inverter only, before we added the DC converter in parallel. Once the IR+ output charges fully to the battery voltage, the precharge resistor is bypassed. I'm thinking the problem is that the voltage never charges up to the battery voltage because the load from the DC converter creates a divider. As a temporary solution, we will test a relay at the DC-converter input that turns on only after the precharge fully charges. Is this a good method? Or is it better that we wait to enable the DC-DC until the precharge is complete? I was a little worried about having high voltage present without powering the DC supply on the converter first. How pertinent is the order of operations?

Another concern is that with only the power supply on, we measured around 300 mV at the high-voltage input. I don't recall if I measured this before, but it seems to be concerning, and I haven't been able to track where it's coming from. Let me know if you have any ideas.

[Rootthecause]

Yay :D
I would just wait with the enabeling of the DCDC until precharge is completed.
It might be easiest to use a relay to externally close the enable pins of the DCDC when precharge is completed.

I was a little worried about having high voltage present without powering the DC supply on the converter first.

I'm not sure if I understood your worries correctly, but the DCDCv9-3 is designed to be permanently connected to high voltage while not having any power at the LV side. Turning it on after precharging is totally fine. It's like connecting it to a live battery (as you usually would) and then turning it on.

Another concern is that with only the power supply on, we measured around 300 mV at the high-voltage input. I don't recall if I measured this before, but it seems to be concerning, and I haven't been able to track where it's coming from. Let me know if you have any ideas.

This is totally normal, even when it is a few volts :) In fact, I often see even the HV LED slightly glowing orange without any HV connected. I've not fully traced the source, but I think it might be due to the sense current from D12. If you want to further investigate, connect a 10 kΩ resistor across the HV input terminals and tell me the newly measured voltage.

[AissaBouzaied]

Issue: Precharge does not work above 16 V

Hello @Rootthecause,

We really like your open source project! : )
But sadly we are having issues with the Precharge of the DCDC. The board completes precharge successfully with voltages from 0-16 V on the HV input, but after that the precharge doesn't work. I have used the RP-1212S instead of RP-1515S for the isolated DCDC could that cause the issue?

I have troubleshooted as you suggest in the build guide looking at the Halfbridge mosfets, precharge fets, comparators, R6 and they seem fine. Any ideas?

[Rootthecause]

Hi!

I have used the RP-1212S instead of RP-1515S

Huh! I'm not sure why the RP-1515S is in the schematic... it should be the RP-1212S.
I must have copied it from another project when updating the insulated DCDCs. Glad you got the 12V version. However, the 15V version might work as well, but I've not tested that. Will fix that asap!

The board completes precharge successfully with voltages from 0-16 V on the HV input, but after that the precharge doesn't work.

Could you please check R1, R71 being 100 MOhm and not 100mOhm?
See Build notes 260208

Edit: BOM and other files are updated now :)

[AissaBouzaied]

Thanks for the reply, yeah that seemed to be the issue missed that issues thread! We managed to switch at 400 V no load at f=114k hz, but it stopped working at 500 V no load. Highside mosfet Q11 is shorted between gate and source and the resonance capacitors measured 50k across them : /. I have a bunch of questions if that is okay : ).

I have desoldered Q11 and measured on it physically and it seems broken, 0,2ohms between gates and source, is the gate driver giving too high voltage? We are using IMBG120R220M1HXTMA1 and the upper threshold is 20V while tvs breakdown voltage 13 but clamping voltage is 28V so it can still technically break, D16 CDSOD323-T12.

Additonally we have 50k ohm over Cr_b resonance capacitors indicating that they got damaged by over voltage?
Do you think it could be voltage transients causing the damage to the MOSFET and resonance capacitors? I mean it is during no load so it has to be?

Now after replacing the High side mosfet and resonance capacitors the gate switching looks like this, (the high side mosfet did not switch at all before).

Low side.

High side.

We are using ETD44 28 turns, Lprimary= 210uH, Lleakage= 72uH Cr = 22.4nF, Q factor = 0.18.

Deadtime = 500ns, fmin= 95khz

We are not sure if it might be a smart idea to change Cr so that our Q factor is 0.16 and switching to higher voltage rated cog capacitors. Any ideas?

[Rootthecause]

is the gate driver giving too high voltage? We are using IMBG120R220M1HXTMA1 and the upper threshold is 20V while tvs breakdown voltage 13 but clamping voltage is 28V so it can still technically break, D16 CDSOD323-T12.

First of all, the IMBG120R220M1HXTMA1 is not a good choice, as in the DCDCv9-3 the most losses are from conduction. With this FET having 294 mΩ, the total losses might be around 9.2 W per FET at high load (simple calculations, not simulated). If you want a 1200V type, I would recommend the AIMBG120R080M1XTMA1 with only 80 mΩ, which would result in 6.3W losses. For comaprison, the recommended IMBG65R050M2HXTMA1 is at 5.5W losses.

The CDSOD323-T12 has a clamping Voltage of 28.3V @ 11 A, tested at a "8/20 μs" surge test. This is something from e.g. a lightning strike or switching action in the power grid.
When you consider what you want to clamp, you should be aware of its currents. My intention of those TVS diodes is to clamp the Vgate(ON) in case of a power supply / bootstrap failure.
The maximum clamping voltage for this TVS is 19V at 1A. Now if I wanted to have 1 A across the diode, the Gate resistor of 10 Ohms would need to drop 10V. So even at 29V of static gate drive voltage would not do harm to the gate. But as my power source can only deliver 84 mA (static), it's not that likeley to see 1A there.
Another source of destroying FETs in the half bridge might be from parasitic capacitances and inductances, but in case of very fast spikes (ns range) a TVS might not help at all due to the stray inductances inside the FET package and PCB traces.
Now just a further thought on this 12V model and not e.g. the 8V version: The 8V version will bleed 1 mA at 8.5V. So at 12V (normal operation) it might be at 20 mA (or something like this). Due to the duty cycle of around 50% you would constantly bleed half that current. And as you have two FETs, it’s 20 mA. The PSU for the isolated side can only provide 84 mA. Just with that choice you would draw 24% from your PSU. But the worst thing is the drop of highside voltage due to R46.
Using TVS to protect gates is a controversial topic - in my design they do not harm, but it's true that they can't protect from everything. I did remove them in the V10 design, because of their limited use.
What are your thoughts on this?

Additonally we have 50k ohm over Cr_b resonance capacitors indicating that they got damaged by over voltage?
Do you think it could be voltage transients causing the damage to the MOSFET and resonance capacitors? I mean it is during no load so it has to be?

If you've measured across Cr_b on the PCB, you need to be aware, that there's also the lowside FET connected to it. Depending on test voltage and polarity, you could measure Q10's body diode. If you really want to know their resistance, you should desolder them for measuring them :) 
About the voltage transients: As your IMBG120R220M1HXTMA1 has only 9.4 nC gate charge, it might be that without any adjustments of Rgate the rise- and fall-times are too fast, causing ringing / transient overvoltage. Transients from the power supply might be possible too, but are tolerated to some extent. If you have e.g. 700V transients, these 650V FETs will go into avalanche breakdown and clamp the voltage like zener diodes. For longer transients, the OVP will turn off the converter.
My guess is the max. current of 33A for this FET is not enough for handeling regulation transients on the output. But if your FET blew while turn on, then it was most likely the charging current of the output capacitors.
The primary transformer current while turn-on looks something like this (just for reference) as I couldn't find a up to date picture:

We are using ETD44 28 turns, Lprimary= 210uH, Lleakage= 72uH Cr = 22.4nF, Q factor = 0.18.
Deadtime = 500ns, fmin= 95khz
We are not sure if it might be a smart idea to change Cr so that our Q factor is 0.16 and switching to higher voltage rated cog capacitors. Any ideas?

Those values for Lp, Lr and Cr seem fine to me. I would not alter Cr.

Also higher voltage might be not the solution, as the issue might be something else, like thermal stress or overcurrent from the HS FET permanently conducting.

So tl;dr: Your FETs are probably not suited. Use something that can handle at least 30 A continuously. Also, just to be safe: replace all Cr capacitors.

[AissaBouzaied]

First of all, the IMBG120R220M1HXTMA1 is not a good choice, as in the DCDCv9-3 the most losses are from conduction. With this FET having 294 mΩ, the total losses might be around 9.2 W per FET at high load (simple calculations, not simulated). If you want a 1200V type, I would recommend the AIMBG120R080M1XTMA1 with only 80 mΩ, which would result in 6.3W losses. For comaprison, the recommended IMBG65R050M2HXTMA1 is at 5.5W losses.

Okay makes sense.

The CDSOD323-T12 has a clamping Voltage of 28.3V @ 11 A, tested at a "8/20 μs" surge test. This is something from e.g. a lightning strike or switching action in the power grid. When you consider what you want to clamp, you should be aware of its currents. My intention of those TVS diodes is to clamp the Vgate(ON) in case of a power supply / bootstrap failure. The maximum clamping voltage for this TVS is 19V at 1A. Now if I wanted to have 1 A across the diode, the Gate resistor of 10 Ohms would need to drop 10V. So even at 29V of static gate drive voltage would not do harm to the gate. But as my power source can only deliver 84 mA (static), it's not that likeley to see 1A there. Another source of destroying FETs in the half bridge might be from parasitic capacitances and inductances, but in case of very fast spikes (ns range) a TVS might not help at all due to the stray inductances inside the FET package and PCB traces. Now just a further thought on this 12V model and not e.g. the 8V version: The 8V version will bleed 1 mA at 8.5V. So at 12V (normal operation) it might be at 20 mA (or something like this). Due to the duty cycle of around 50% you would constantly bleed half that current. And as you have two FETs, it’s 20 mA. The PSU for the isolated side can only provide 84 mA. Just with that choice you would draw 24% from your PSU. But the worst thing is the drop of highside voltage due to R46. Using TVS to protect gates is a controversial topic - in my design they do not harm, but it's true that they can't protect from everything. I did remove them in the V10 design, because of their limited use. What are your thoughts on this?

Interesting, very good point about it being very unlikely to damage the gate even at 29V static gate drive. Still, I am personally not fully convinced that the TVS will not help against transients caused by parasitics or at least extend the lifetime of the FETs. Excluding them in the V10 design will not be a problem I think, you sound like you have thought about the different scenarios quite well : ).

Those values for Lp, Lr and Cr seem fine to me. I would not alter Cr.

Okay, nice.

About the voltage transients: As your IMBG120R220M1HXTMA1 has only 9.4 nC gate charge, it might be that without any adjustments of Rgate the rise- and fall-times are too fast, causing ringing / transient overvoltage. Transients from the power supply might be possible too, but are tolerated to some extent. If you have e.g. 700V transients, these 650V FETs will go into avalanche breakdown and clamp the voltage like zener diodes. For longer transients, the OVP will turn off the converter. My guess is the max. current of 33A for this FET is not enough for handeling regulation transients on the output. But if your FET blew while turn on, then it was most likely the charging current of the output capacitors. The primary transformer current while turn-on looks something like this (just for reference) as I couldn't find a up to date picture:

Yeah it was during turn on without load so will probably be even worse with load, I will measure the primary current during turn could definitely be an issue with these FETs very useful insight thanks.

So tl;dr: Your FETs are probably not suited. Use something that can handle at least 30 A continuously. Also, just to be safe: replace all Cr capacitors.

Okay I will follow your recommendation, I agree with you that I choose a bad very badly suited FET : /. Thanks again for the help! :D

[Rootthecause]

Glad I could help.
Let me know how it works out with the other FETs :)

Just FIY: I'll lock this thread, because it got too long. Feel free to open up a new one.