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Look at my amazing BLDC, see something weird? (help plz)

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javivi's picture
Last seen: 1 year 3 months ago
Joined: 2019-10-09 12:15
Posts: 4
Look at my amazing BLDC, see something weird? (help plz)

I am having issues driving big motors with VESC firmware and IGBT (they explode when driven above 90% duty)

is that spike normal for trapezoidal modulation?

In the oszi capture the motor is running at low speed and no load, yes i am respecting the deadtime specs for this IGBT 2,4us

Signals explanation:

Yellow: Phase-Ground Voltage

Blue: high side Gate signal

Pink: low side Gate signal

Green: 0.5mA shunt detecting shortcircuits



Closer look



Last seen: 2 weeks 5 days ago
Joined: 2019-02-20 19:57
Posts: 4

Hi, Show pls Your DC link voltage. Is there any spikes?? Spikes appear after low sie IGBT turn off. U can try increase ded time and watch what happend.  Shunt measures  may be affected by EMC. Try differential using two probes. Which IGBT explode?? Low or high side?? IGBT usually can withstand fiev us of permanent short circuit.

Regards Marcin

Last seen: 1 day 2 hours ago
Joined: 2017-09-22 01:27
Posts: 571

Your spike might be caused by self turn on from coupling of the low side gate signal, but I would expect that to be happening on the low side not high side. 

So in that case the spike is probably back emf inductive spike from turning off the current quickly. 

Which igbt blows?

javivi's picture
Last seen: 1 year 3 months ago
Joined: 2019-10-09 12:15
Posts: 4

Hello and thanks for your interest.

Answering your questions:

  • Im using a "system on chip"? IGBT ( FNB3560T ) , everything is integrated inside the same casing, it just blew up, and with it all the ceramic caps i had in the DC side filtering transformed into fine black myst.
  • if i raise the dead time i get some nasty false triggers  !!?!??! ( i just noticed i swapped the pink and blue probes sorry, pink is high side gate signal now)

    false trigger igbt automotive high dead time.png
  • if i lower the dead time i get the usual shorts and the IGBT starts to heat up.
  • Ill snap the DC bus also in the next test round.

Im finding references of this weird initial pulse even in STM application notes! they just dont mind it, maybe is a normal thing?

page 16

stm aplication note snap.png


Last seen: 2 years 5 months ago
Joined: 2019-11-19 20:50
Posts: 1

Hi Javivi,

I think the spikes you see is the demagnetisation of the motor phase. I am not a motor expert, but it looks like that the demagnetisation results in a current in the direction of phase B (the green signal). As soon as both the bottom and top switch of phase B are turned off, the current of phase B runs through the body diode of the top switch to +Vbus. The higher the load current, the longer that this demagnetisation period will become. (off course this occurs in all phases subsequently)

This demagnetisation effect in case of outward current from the inverter will cause the (body) diode of the bottom mosfet conducting. You can see that in phase A just after turning off the PWMing.

These effects are normal and kind of belong to 'six step' BLDC control.

I hope this helps you to some extend.

Kind regards,