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]

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?

[Moffeltje]

On another note, my chief powertrain was wondering why it was not okay to use fire retardant ABS for the prints. Since I have no experience in 3D-printing I hoped you could explain this a bit further?

[Rootthecause]

It's not the material. FR ABS is fine in general. The problem is the FDM printing technology (ABS can only be FDM printed). It's prone to gaps in the coil former, rendering the insulation properties useless. On the other hand SLA (Resin) printing is very gap-less. Even single pixel failures on LCD machines is not likeley to leave a gap due to surface tension and blooming.
I did some tests with a spark generator (~ 2 cm sparks / roughly 20 kV) and a 1 mm thick SLA printed sheet. The arcs did never penetrate, also not on the actual coil formers. It's a really good insulator.
However, there is one caveat: The resin used must be high temperature resistant, otherwise it can get brittle and crack. I tested this with some standard resin (Phrozen Aqua Gray 4K) and some Phrozen TR300 in a oven at 250°C. With that I simulated 100h at max. expected temperature in the DC/DC transformer. The stadard resin was quickly flaking. The TR300 was still perfectly in shape and also didn't get brittle after a few minutes. The TR300 is not UL94-0 so I changed to the Licreate FR HDT (back then I though UL94-V0 is be required due to rules, but it is not).

It's not impossible to use FDM for this - I've used this in the past with specificly designed coil formes and much larger distances between coils, but that resulted in other design issues.

[Rootthecause]

Oh, and I totally forgot to say:
ABS is thermoplastic, whereas SLA-printed parts are usually thermosetting.
Depending on composition, ABS can get flexible above 90°C to 120°C. So not a good idea at all, if the transformer can reach 120°C inside.

[Moffeltje]

Ahh all seems very logical, thanks for the quick response as always. Do you think the inside of the transformer can reach 120°C? On the thermal image in the documentation only the core temp can be seen iirc. Btw, I have just tested the Lp = 150uH and Lr = 50uH transformer with an extension and it comes to Lp = 159.6uH and Lr = 53.7 uH, which falls into the 5% tolerance you gave. I guess I missed a winding somewhere afterall then, so sorry for doubting you. Thank you for helping me so extensively, it would have taken me way longer if you didn't.
Regarding something we have spoken about before, the voltage rating of the litz wire and the rules compliancy of it, you said the following:

About EV 4.5.2: The DCDC is placed inside the TSAC. So the TSAC is afaik the isolator, otherwise every pcb with TS would require a housing inside the TSAC.

I think the problem in rule compliancy lies in this rule (if a scrutineer sees the HF litz as a wire, which was one of the problems in the issue we on the git iirc):
'EV5.4.7 Every wire used in a TSAC, regardless of whether it is part of the LVS or TS, must follow
EV4.5.2, EV4.5.3, and EV4.5.5, the exception being wires used in the intent of EV5.8.11.'

I am curious to know what you think, I am going to try to recreate the transformer with a higher rated litz, which we are in the process of obtaining. So either I will let you know the result.

[Rootthecause]

Well ... not so quick this time - but finally got the first draft of the v10 schematic published ^^

Do you think the inside of the transformer can reach 120°C?

It did for sure in older versions, but I just had to check my data on the v9-3.
At 450 W load it reached 101°C without a fan (600V, Z562). It might reach 120°C under full load if the fan does not work. With fan its rather around max. 70°C with 25°C air temperarture. If it was 60°C (inside the TSAC) it probably would reach at least 105°C. There might be scenarios with worse performance, increasing the temps even more. Also all measured temps are only surface temps. So I think 120°C inside is a valid guess, worth considering.

Glad to hear!
But please never stop doupting me! :D
I'm confident that I do make mistakes, and I totally do not rule out, that there might be an error in my documentation or that I accidentally wound 29 Windings (Note: I did just check the pictures again - I almost did count 29 because it's confusing, but there are 28).

Here's the rundown on what can lead to confusion:

First of all, we have the oppotunity to assume a wrong cross sectional view, seen from below at the dashed line.
If you assume that, then winding 14 will confuse a lot, as it is in the cross sectional view right to winding 5.
However, on the picture winding 14 is right to winding 13. But with the correct cross sectional view, it's in the correct place.
On that note, windings 11, 12 and 13 should move one place to the left in the cross sectional view (I'll fix that in the next push).

The next confusion is the number of windings in the right top layer. If you count at the bottom, it's four. But if you count on the top, it's five. But the fifth winding is not a closed winding, but just the end bend over the "ear" of the coil former for convenience.

Now the last confusion is the right picture. Its upside down compared to all others in the build gudide, but it is actually oriented as in the actual transformer (secondaries go to the right of it)(I'll also fix that in the next push). Now if you count the windings on the right top layer on this picture, it's 4 windings, which can conflict with the previously seen 5 windings - an invitation to errors.

Could you please check, if anything of this might have been the source of the missing turn?

I think the problem in rule compliancy lies in this rule (if a scrutineer sees the HF litz as a wire, which was one of the problems in the issue we on the git iirc):
'EV5.4.7 Every wire used in a TSAC, regardless of whether it is part of the LVS or TS, must follow
EV4.5.2, EV4.5.3, and EV4.5.5, the exception being wires used in the intent of EV5.8.11.'

Haha, what a coincidence!
I did not read your message before I posted the update on the v10 (which includes also this exact topic about the v9-3).
You can read it here.
I do think that a higher rated litz isolation is a solution to this issue. Can you tell me, where you try to order it from?
Not that Elektrisola is doing the same work twice :D

[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!
I would not use passive rectification here. The power losses are hard to manage with passive rectification on the PCB unless you're using some wire extention (caution! larger ringing due to higher leakage inductance) to THT diodes mounted on heatsinks. In older 48V versions I used these, which worked quite well without heatsinks - but that was lower current due to the 48V.
I would rather use different SR-MOSFETs like the ISC015N06NM5LF2ATMA1 , which seem from my power loss calculations 30% lower/better. But I have not confirmed this in tests. The gate charge is rather high on these, so I worry the turn on losses are high.

For the resonant tank, as far as I have tested, Lr and Lm stay the same, so there are no changes required other than one secondary winding instaed of two. But I've not confirmed this on the DCDC in operation. If you - or somebody sucessfully built a 12V version, please let me know.
For the gauge - if you referr to the secondary wire - yes, if you can fit a larger litz wire there, this would decrease your losses.
But since you're drawing "only" 30 A - I don't see a change required. The converter is designed to handle 31.25 A for 60 seconds. The limit is due to the SR heating, and not the transformer heating. Mounting a heatsink to the SR FETs might be the only change required to allow 30A continously with the current SR FETs.

The changes required for a 12V version is getting asked quite often. I might need to write a Q&A on this someday - but the v10 solves that entirely, as it has dual rail output (24V and 1V to 24V adjustable).

[CalebRezmer]

Thanks for the clarification. Is there a simulation for the synchronous rectifier, or was this thought of as trivial? I was wondering if it is worth building a simulation for it; I may do so with the new FETs. Is switching to the new SR FETs a one-to-one switch, or are there many changes that need to be made for increased gate charge and increased gate capacitance (ringing)?
This may be a dumb question, but when you say mount a heat sink on the SR FETs, you just mean mount them on the top right since they are SMD? Also, would we need to change the bobbin dimensions for the 1 winding on the secondary side, or can it be left the same?

I am quite new to this. I apologize for all of the questions. I will definitely keep you informed on the project; looking to build it in November.

[Rootthecause]

Hi there!
Sorry for the late answer - I was quite busy with my thesis.
Currently I've not done a simulation on acrive rectification - but I think it would be a good idea to help better understand the losses for optimisation.
I've used two different gate drivers and tested maybe 6 different FETs and even some GaN HEMTs.
In my tests, I do not recall any mayor issue with gate ringing - but that might be due to my learnings. When I uesed the previous SR driver, which was the UCC24612, I didn't even use gate resistors - but because I thought you would not need it because it was not shown in the schematic - unless you look at page 23 and beyond :D 
Later I noticed that the DCDC got slightly more efficient when I implemented 4.7 Ω or similar as gate resistor.
What I want to say with this: You're probably fine testing FETs with different Ciss by using the 4.7 Ω on the board. Adjusting the gate resisor is always possible and imho the only thing really needed to be adjusted for efficiency. It‘s also rather easy to measure with an oscilloscope if ringing is a problem or not.
I don't recall testing the optimal gate resisor on the v9-3 (which would probably a good thing in hindsight). So there might be some efficiency left on the table.
The other thing which can be optimized is the Cs pin lowpass. Due to the lowpass nater, it causes a input delay, thus turning the gate on/off delayed and with a differend theshold voltage - but for what I've tested it seems sufficiently good at preventing false turn-ons. Better loosing some efficiency than loosing the whole FET :D

you just mean mount them on the top right since they are SMD?

yes. You might be able to mount it on the top side of the PCB too as the thermal conductivity is high due to the vias, but the space there is limited. Putting the heatsink with some thermal interface material directly on the FETs (bottom side of the PCB) is the easiest option. The heatsink probably does not need to be large, 20 x 20 x 5 would be already rather much. Everything will be better than no heatsink to keep them below 100°C.

Also, would we need to change the bobbin dimensions for the 1 winding on the secondary side, or can it be left the same?

Depens on what litz size you are using. You should avoid having a gap between the litz and the bobbin, otherwise it could move and change the Lr. If you use a litz large enough to fill the current bobbin, this would be the best (also regarding efficiency).
You should avoid having both secondary windings next to each other (side by side), as both will have rather large differences in Lr. So they sould be on top of each other like this:

For the position I think keeping it horizontally centered or on the side where it is closest to the primary would be the best option to keep it similar to two turns. It also might be, that the change due to position is rather small. You can test it by measuring L on the
primary while positioning the secondary with shortened leads - this is your Lr (only short one secondary at the time when trying to determine Lr when comparing to my datasheet).

I am quite new to this. I apologize for all of the questions. I will definitely keep you informed on the project; looking to build it in November.

Nice! I hope it works out well. I might have some more time then to answer :)