DCDCv9-3r Q&A

[Rootthecause]

Ask me anything about it or older designs :)

[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]

Hi James,
I'm back from FSA - will try to answer your questions over the next days.

Regarding the VMF256M3R8 Super capacitor ordering issue:
There are multiple Vendors to choose from, which deliver in single quantities:

• Digikey
• LCSC
  Note: LCSC is a sister company of JLCPCB. I think some componentes for PCBA are sourced from them. I'm saying this, because their prices seem like sketchy low, but they're dealing with high order quantities and local business. So the only downside I can see is the probably longer shipping time.

Other alternatives which seem to use the same Footprint ( 8 mm diameter, 3.5 mm lead spacing) from Mouser:

Manufacturer Number	Capacity	ESR	Leakage current	Height	Link	Note
VMF256M3R8 (REFERENCE)	25 F	300 mOhm	2.5 µA	20 mm	Mouser	just for reference, order quantity 100 :/
HSH0820-3R8406-R	40 F	550 mOhm	3.2 µA	20 mm	Mouser	ESR is rather high, not recommended
HSH0825-3R8556-R	55 F	450 mOhm	5 µA	25 mm	Mouser	ESR is medium high, leakage seems high
AHCR-S04R0SA306Q	30 F	500 mOhm	3 µA	16 mm	Mouser	ESR is medium high, seems ok
AHCR-S04R0SA406Q	40 F	400 mOhm	3 µA	20 mm	Mouser	good option, recommended
AHCR-S04R0SA506Q (new recommendation)	50 F	350 mOhm	3 µA	25 mm	Mouser	also good, rather large (but height is not an issue), recommended

So I would probably go with the last two options. Both have way more capacity than the original, so a hold-up time with no HV of 3 minutes (40F) or almost 4 minutes (50F) could be reached compared to the ~2 minutes (25F) with the original. The higher capacity leaves more room for leakage losses and probably less wear, so I would recommend the AHCR-S04R0SA506Q over the original VMF256M3R8. Leakage seems fine for most of them - but I havn't conducted any long term test for them.
It's really amazing to see, what progresss has been made over recent years when I decided to use the VMF256M3R8.

[Jamesteng0214]

Hello Rootthecause, I have managed to order from Digikey, will try other capacitors when testing begins!
Btw, how was FSA? Did you guys get on the podium? hahahah

[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 ;)

[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.

[Rootthecause]

Hi, TH1 is also in the Schematic/BOM :)
https://www.mouser.de/ProductDetail/Vishay-Dale/TFPTL15L3901FL2B?qs=EYbNv5wfoHs1U3qiZRAw9A%3D%3D

[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!

[momenali1]

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.