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

[Jamesteng0214]

Hello Rootthecause, you mentioned that the leakage inductance I have might be too large for LLC? May I know what leakage inductance you used? From LTSPICE I calculated around 52 uH from the k factor but I might be wrong ……. Mine is currently set at 42.9 uH, thanks !!

[Rootthecause]

you mentioned that the leakage inductance I have might be too large for LLC?

Are you referring to my comment on the picture from your comment before?
As you increase spacing, you increase isolation. But also increase leakage inductance. In your shown design, the leakage inductance might be way too high for LLC operation.

Yeah, this design would pretty sure lead to a high leakage inductance. Use mine, and you're fine.
Please provide your calculation, because I still get 56.27 µH with the values from LTspice.
Btw. you can get the real world measurements for the transformer either from the DCDC datasheet or from the exel sheet.

FYI: I've added a coupling factor calculator to the excel sheet in the calculations tab.

[Jamesteng0214]

Yeah, this design would pretty sure lead to a high leakage inductance. Use mine, and you're fine.

woof that's some nice assurance haha, and yes the inductance is 56.27, didn't do the calc accurately.

Btw. you can get the real world measurements for the transformer either from the DCDC datasheet or from the exel sheet.

FYI: I've added a coupling factor calculator to the excel sheet in the calculations tab.

That's perfect! Thanks so much for your help!

[Jamesteng0214]

It's rather the power dissipation, that sets the limit on my DCDC design. You can some feeling of the max. permittable power losses by using a thermal camera and knowing the power losses over the whole DCDC.

Hello Rootthecause, it's been a while haha and I've come back with another question on this topic. You mentioned this a while ago when I was selecting the SR FET. After looking over a few datasheets of FETs and the excel table you made for fet's power losses, is it right to say: In the datasheet, the "power dissipation" is usually around 100 something watts which is a lot, but as the fets dissipates energy the temperature rises (this is given as thermal resistance), thus despite it says it can dissipate 100 watts, it is in fact way less?

If I used the datasheet of ISC015N06NM5LF2 as example, for every watt it dissipates without heat sink, the temperature will rise by 50 degrees, thus if it originally was 25 degrees, and it can only operate below 175 degrees, thus the actual power dissipation we could have across that FET is (175-25)/50 = 3 watts?

You also considered only the conduction loss, switching loss, and RR loss in you calc so I assume that these are the main ones to consider?

May I also know how you came to these specs for the SR FET? (especially for I_gate = 4Arms, it is used to estimate the Tr and Tf time right?)

How should I calculate the RMS current used in the calculation for the power loss across FETs? (My output current is 40A and the peak across the FET is 40*pi/2 = 62A. I couldn't figure it out from the excel table).

I think I've found a replacement of the SR FETs - BSC007N04LS6 is able to handle my desired current output and has the same footprint as the current one used on v9-3.

It handles 48A without a heatsink, I suppose I should look at the R_DS when V_DS is 10V as that is the voltage NCP4305DMTTWG provides?

Sorry for the amount of questions.........

Thanks a lot!

[Jamesteng0214]

Hello Rootthecause, thought I'd separate this as another post since this is about NCP4305DMTTWG.

How did you set the resistor values used to configure the min_ton and min_toff time? Am I correct to think that - the min_toff time is used to prevent the ringing effect of the transformer from turning on the MOSFET thus it is set to several cycles (2~3) * the time constant of the ringing behaviour from the leakage inductance of the secondary side transformer (L_leakage) and the C_oss of the SR FETs?

The min_toff is set to around a small percentage of the conduction time? (This is highly speculative...... please kindly correct this).

Last but not least, the Driv pin is connected to the other FET's trigger pin, which I suppose is to turn the other FET off when one is on? Could you explain the rationale on this?

Thank you!🙏🏻

[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 ^^