Blew up my Unity

Took my board out for a ride after some successful in-garage tests, and about 30 meters into my ride the board stopped responding, and as I picked it up got a bit of smoke and sizzle coming out of my unity. I took it inside and there’s a part in front of the battery-connector-side hall sensor plug that’s got a hole burned in it. The DRV chip on the same side also has some blisters suggesting that it’s toast as well. Powering up the unit caused more smoke to come out of the first part.



Now this is likely 100% my fault, but I’d like to know which of my mistakes to avoid to not burn through any more of these controllers. In no particular order:

  • Took the thing apart, i.e. removed the silicone cover but also the heatsinks. I’m pretty sure everything went back together correctly.
  • Scratched a little hole in the conformal coating from the shunts to try and diagnose what turned out to be a software problem (reinstalling firmware fixed it).
  • Installed VESC 5.3 beta firmware on the thing (ran through the full setup and everything seemed normal, with the possible exception that the default switching frequency was 25kHz and I left it there).
  • Possibly engaged the handbrake feature at 10A while moving at a brisk walking pace (this feature should be disabled over 0.1 m/s but my math might be off).
  • Set a battery current limit that might have made the batteries’ internal protection circuit kick in

Any insight into what the fried part does? This is totally my fault, right?


Bad antispark? You wanna get rid of your fried unity?

The DRV chip blowing out like that indicates that one of your MOSFETs failed (most likely). This chip drives the FETs (which drive your motors)

If you have a failed MOSFET it would be likely due to the Motor current being too high for this little FOCer to handle, or a Voltage spike that exceeds the Maximum, either of those could cause a FET to fail which can cause the DRV to melt, and you will likely measure a short-circuit between the Motor connections (with motor is disconnected)

Possible you had a short in the motor cables or some physical problem with the installation (i.e. Voltage Spike on E-Brake). But this only really equates to a failed MOSFET causing the Driver to Blow, so if the MOSFETs look good, it’s possible the fault was more obscure.

One concern you mentioned in the configuration after your firmware uploading that could have strained this device in an unintended way. Too high switching frequency will heat the DRV chip more, I don’t know what’s normal, so not sure if 25kHz is high enough to be a catastrophic difference, switching MOSFETs too fast will create excessive heat.

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The resistances are definitely off on the one side of the controller. 115 ohms between the two outer phase leads.

The motor currents were set a good deal lower than the rated capability, so I don’t think it was over current on the motor side.

Would a battery over current protection kicking in potentially cause a voltage spike?

How much throttle were you giving it? Are you using the discharge, on BMS? What battery?

There was a long discussion about switching frequency a few years ago. It seems as though people were pushing the default 20kHz up to 25-30kHz, and getting significant noise reduction, obviously getting very excited, and then finding that it causes lots of wasted energy in the VESC.

Doing this massively increasing the energy required to switch the MOSFETS (which is the job of the DRV chip). That Awful blue line below shows a Flipksy 6.6 @25kHz (vs ~20 in yellow/orange), we can see a 5% system drop, all that energy is likely in the MOSFET switching, so the VESC will get much hotter and waste more energy by switching faster.


Since your MOSFETs are not reading 0 Ohm, It’s likely they are okay. The fault could have been overheating in the DRV, and once failed burning out the inductor supplying it. Replacing the two failed components may bring that device back to life, but it’s a gamble that other components haven’t fried in the melt-down, and you would need a re-work station and to know how to use it, if you want to make that repair. Where are you based, and do you know anyone that can solder SMT components?

The FOCBOX shouldn’t have fried this easily, if you opened it and found improperly fitted heat-pads around the failed components, then that is a worthy discussion for the manufacturer. But also keep in mind that this likely worked when it was shipped to you, and through firmware updating you may have set something out of range causing the issue.

My advice for next time is ride it before you update the firmware, obviously need to do the Motor/Battery setup, but that is all. My friend also did an update for a Flipsky6.6 and lost so much default configuration, it took a long time and several FW versions to get it spinning again.

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