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UPDATE

Thanks Aaron and jp314 for your comments. We just probed some more and I think jp314 is right.

We found that the high side driver was skipping some pulses. The pink pulse train is from the mosfet driver on the high side.

Does anyone have any idea why this could be and what I can do to solve it?

Thanks Again

harsha

enter image description here

Original Question

Im using the below inverter with 3 parallel STP150N10F7 mosfets to control my PMSM motor

Mosfet Inverter phase

enter image description here

When trying to understand why the current waveform was non-sinusoidal at high currents I noticed that some of the mosfets seemed to be turning on without a gate signal - As seen in the picture below.

enter image description here

Here the blue and green waveforms are the V phase lower side MOSFETs gate-source (green) and drain-source (blue). When the gate goes high, the switch turns on and the voltage across the switch is supposed to go low. However in 3 places, without the gate going high the drain-source voltage has gone low. The pink and yellow waveforms are waveforms of the lower side MOSFET of the U phase. The pink waveform which is the gate has been inverted, so it might be a little confusing. Here you can see the drain-source (yellow) only going low when the gate (pink waveform which is inverted) goes high.

Would appreciate anyone explaining why this is happening. I did a search and came up with 'parasitic turn on off MOSFETs'. To be honest I didnt understand it too well and so am not very clear how to ensure this doesnt happen also. Any comments are very welcome.

Thank you, harsha

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  • \$\begingroup\$ There is also dv/dt turn on issues with mosfets. If the drain voltage dv/dt is too high, it'll cause the FET to turn on. toshiba.semicon-storage.com/info/docget.jsp?did=59473 \$\endgroup\$ Commented Oct 7, 2021 at 13:35
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    \$\begingroup\$ MOSFETs can have a parasitic turn on where th G-D capacitance couples to the gate node and if dV/dt on the drain is very high, it will couple to the gate and pull it high. If this occurs, you would see it on the gate pin. However it would be brief (because the driver would quickly overcome this coupling). It can also be caused by bad layout or parasitics. However your malfunctions seem to be of a long duration, and may be more likely from a logic or controller error. \$\endgroup\$ Commented Oct 7, 2021 at 13:56
  • \$\begingroup\$ @jp314, thanks for your comments. I think your right. Have updated my question above with some new findings. \$\endgroup\$ Commented Oct 7, 2021 at 15:37

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If there is a Hall sensor or there is a non-overlap control in the drivers (to avoid shoot through), then possibly some of the signals that allow the HS to turn on are being missed ?

Is the fault only on one phase ? Does it occur when the motor is loaded differently ? Does it occur on every cycle ?

Sometimes if it is a parasitic issue, you can modulate it by changing (removing or adding capacitance) to determine the sensitive nodes.

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