Whats the deal with only 6 mosfets for 250A? Generally higher current boards have more mosfets, but these 6 are BIG, is it like a size vs space vs cost triangle? Im used to seeing people just slapping more and more mosfets on.
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Has anyone gotten encoders to work with these? I can’t seem to get mine to work.
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There’s a tradeoff between just adding more mosfets or going with a bigger sized mosfet. Adding more mosfets in parallel can be less electrically stable and require a lot of attention in the design so that current is shared evenly. Simply going to a larger mosfet doesn’t have as much risk. In my case, I wouldn’t be able to jam in more TO-220 mosfets without making the PCB much bigger but I saw a way to go with the larger TO-247 package size.
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For the v1 units? Could you take that over to the correct thread and provide more details on exactly what you’re doing and how it’s not working?
This one
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Can you tell me about how you determined the current limit? Is it just by experimentation, thats all you can get out of the fets before they overheat? Would better heatsinking allow higher current? Or is it avalanche related? I ask because i am looking to do a similar design on a different mcu platform and want to know how to pick a fet that wont blow when i run @58v, 100A
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Thats the power i can pull from the DC system, that is. I also have a very large aluminum heatsink at my disposal
That will depend on a lot more than current limits.
Your FET driver (on/off speeds), gate capacitance, heat spreading/removal, switching node inductance, length/inductance of the wiring to the ESC, and input capacitance/ESR will all affect how robust the design is and the ratings you can give it.
To prevent overheating you will need to deal with the switching and ohmic losses in the FETs as well as the total junction-to-ambient thermal resistance. Those will help to determine how high a current rating you’ll be able to give the device. A larger heat sink can only help if the thermal resistance from the FET junctions to the heat sink are low enough to prevent FET overheating.
The current rating(s) for the FET is just a small part of the decisions to be made. 
Good luck with your design though! I hope you will post about it once it’s up and running.
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Thanks! Yep im doing what i can to support all of those other facets, my question is rooted in the fact that the original vesc4s mosfets were rated at 240A, despite the vesc only being 50-60A capable. Didn’t know if that was to minimise rds and therefore heating or if there was some other effect or equation i didnt know about that created a massive peak current for the fets to deal with
That 240A rating is a theoretical max for microsecond-long pulses. It is based on a case temperature near room temp and that is impossible once the FET starts conducting current. That rating can’t be used for anything we might do with the FET.
Those high current specs are so often quoted (we love big numbers!) but they are useless and any company bragging about their “240A-rated FETs” should be looked at with a very suspicious eye in my opinion. It shows a lack of understanding what that rating means and that does not inspire confidence in such a company’s designs.
There are other continuous current ratings that are more useful but the most useful thing is to calculate the junction temp from the Rds-on number (when hot), thermal resistances, and current amount you want the FET to handle. That is just for the ohmic heating. The switching heating calculations are more complex and often prototype testing a couple of different versions is easier.
A low RDs-on spec certainly helps to decrease the ohmic heating but it can come at the expense of higher switching heating due to the higher gate capacitance of the lower resistance FETs (which slow down switching through the heat-producing linear region).
For an ESC, or other device switching FETs on/off constantly, you can actually end up with hotter FETs by using lower resistance ones without having robust enough FET drivers to keep the on/off times short enough. For devices like BMS’ and AS switches the ohmic resistance is much more important since the FETs are only occasionally switching.
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Big numbers sell shit.
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Thank you that was super helpful! Feel much better about my fet selection now that i learned up a little bit on how to do some of those calculations. Can you tell me how i might find the thermal resistance of the heatsink i own? I know its aluminum and its big and old. Its cast i think, the fins are maybe a pencil width thick. I dont have an intuitive feel for thermal conductivity. My total resistance is around 2 including a mica sheet, needs to stay below 6 for worst case scenario.
You can go to the web sites for Aavid/Thermalloy and Wakefield or check vendors like Mouser, Digi-Key, Allied-Elec, and others to see what the thermal resistance is for any sinks you can find near the size/configuration of yours.
The other way is to put a known heat load on the sink and measure the sink’s temp rise after it “soaks” for a while. A little bit of algebra and you have its thermal resistance. This makes it easy to test with/without a fan, in/out of an enclosure, etc.
For any temp measurements forget using ever-popular thermistors. Use thermocouples.
Thermistors take about a minute to come to the the temp of what they’re connected to. Their round shape also makes it hard to achieve good thermal contact with a flat or concave surface.
Type-k thermocouple beads are tiny and very low mass (fast reacting). A dab of thermal paste on the bead and Kapton tape it down along with at least 0.5” of its wire.
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Big thanks!!!
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Any plans to make your own BMS sort of like Ennoid XLITE format? Because I don’t think that charge bms should cost 180usd so looking for more reasonably priced alternative - flexiBMS but it is 12s only. Would nice to get some competition on that market, demand is huge!
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Yeah I did have ambitions for designing a stackable BMS system but the chip shortage put a halt to that. Looks like I’ll have to wait until next year for when the chip set I want to use comes back into stock
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Focening, you have me interested because for the EUC community and E-moto I see them running 24S and 28S setups alot, would be amazing if we could get a VESC that had all four things
- High voltage support
- Screw terminals for power and motor connections
- DRV free
- JST connectors with those latches (looks neat and has less hot glue on them)
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That kind of power goes to the Tronic series I design. It’s close to what you’ve specified but with even stronger connectors. Still uses cables though. First batch should be here next month sometime
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Very exciting
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