FS Rules Implementation

[Rootthecause]

This thread is for discussion of Formula Student Rules applied to the DC/DC converter.

[Hazelnutter03]

Hi, FSUK team here - just a few quick questions if you don't mind.

As far as I can see, the rules are practically identical to FSG re DC/DC converters. We run a much lower draw system, about 80W continuous at 12V (Accy is 250-430V), if you could point us in the right direction on how we can adapt the design to our use case.

Does the DC/DC connect to the accumulator before or after the AIRs - I would be concered with an insulation fault in the DC/DC and not being able to isolate it.

[Rootthecause]

Hi,
I've not read the FSUK rules, so I cannot tell if there are changes needed. If it is simular to the FSG rules, then it might applicable.
You can check the rules that I've covered in the Documentation.

For the input voltage there needs nothing to be changed, as the converter also runs at 200V (but 300W max), except you want to modify the UVP/OVP threshold, covered on page 39 in the build guide.

If you want a 12V version, you should be good by using one turn on the secondaries instead of two and adjust the feedback loop to 12V (see voltage divider R17 and R12 + R10 in the Schematic). However, I've not tested this - so please properly test it on your own and let me know if it worked well. If you're only drawing 80 W then this is much less of a hassle in terms of thermals :)

The converter connects before the AIRs. You could put it behind the AIRs, if you add some batteries to power your system until TS is turned on, but this would be quite a hassle.

About isolation:
It is mandatory to test the insulation before use, as described in the build guide and scrutineering support presentation.
Of course this is not a 100% guarantee that the insulation will last forever due to thermal stress and vibration.
If there is a single isolation fault, the IMD will trigger or the Insulation test on the event would not pass. However, a single insulation fault is not a hazard per se as no current will flow. Only if a second fault closes the loop, it would pose a problem.
I'm aware, that having AIRs as a layer of protection is comforting, but other components like the precharge and BMS are directly connected between TS and LV as well.

[Hazelnutter03]

Perfect thank you. What would the efficiency be at the lower power draw? Is there a way to improve the efficiency at lower power, potentially at the cost of peak power?

Yes thats true, I hadn't considered the BMS connecting to TS before the AIRs, that does give me a bit more confidence!

[Rootthecause]

What would the efficiency be at the lower power draw?

Very hard to say. There are a lot of factors affecting the efficiency. If you just decrease the the secondary winding to just one turn, then you might reach 87% at 80W. Here's the Plot from the Datasheet, but for lower efficiency ranges:

Is there a way to improve the efficiency at lower power, potentially at the cost of peak power?

Jupp, definetely! If you decrease the Airgap between the cores, your magnetizing inductance will increase (decreasing switching frequency and resonant current). The resonant current cycling around in the resonant circuit actually is the main cause of power loss in idle. In order to increase the frequency in order to not saturate the transfomer, the resonant capacitors need to have smaller capacity. If you do the resonant circuit just right, I belive you could hit 95% (4W loss at 80W output).
But let me tell you this: Designing your own resonant circuit is a massive rabbit hole with many hurdles.
If you value your time, just build the original design with one secondary turn. At the end of the day, the efficiency will not matter, unless your converter overheats due to the power loss. In your case, you will look at around 10W of power loss, which is far away from any thermal issues, even in a enclosure. Also driving for 1 Hour will leave 10 Wh on the table - your battery is probably in the order of kWh ;)

Yes thats true, I hadn't considered the BMS connecting to TS before the AIRs, that does give me a bit more confidence!

It also gives you a way of closing the loop 😄 It is as always: "Caution! Life can lead to death."

[Hazelnutter03]

Thanks for the help! We will keep you posted