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DRV8302: GVDD undervolts during switching

Tim Yao
Prodigy 30 points
Part Number: DRV8302

I'm implementing a BLDC controller using the DRV8302 chip and IRFS7530 fets. I am debugging a problem where GVDD drops rapidly and causes a GVDD undervolt fault. So far, I have done some preliminary analysis. I have the system running at 12V and have a 1000uF capacitor near the FETs. From probing, I can see that PVDD, DVDD, and AVDD are all stable at the correct voltages. GVDD is stable at about 11V when there is no switching, but when it tries to switch the FETs, the voltage drops to 8V in a few uS (I will try to get a scope capture). Weirdly, this only happens when I have a motor connected and does not happen when there is no motor. Because I am using FETs with large gate charge (236nC), I have the switching frequency limited to max of 20kHz to stay under the 30mA gate driver current limit. 

My question is what are some main causes for GVDD to drop so rapidly? Is it possible that the internal charge pump has failed? 

I have attached my schematic for the DRV8302 and power fets for reference.

  • 0
    TI__Guru** 114905 points
    Hi Tim,

    If there is a problem in a high side FET, connecting the motor provides a path through the windings.
    Please double check the high side FETs for proper operation.

    You could also try placing a resistive load on each phase of approximately 1k. Then switch the FETs on and off, comparing the gate and source voltages of the FETS.
  • 0 in reply to Rick Duncan
    Prodigy 30 points

    It seems that the high sides FETs are operating correctly from the resistive load tests.

    I used an 1.5k resistor as load between each of the phases and that setup did not cause GVDD undervoltage fault. With a 12V PVDD, the source of the high side FETs are 12V when on and the gates are driven at about 21V. I do not see any noticeable voltage dips on the GVDD pin if I only have 1 phase connected. If I connect all three phases, I see about a 2V dip when the FETs are switched. This is different from when I have a motor connected as the load. With the motor, the gates are at the same voltage as the source (12V) when turned on. It looks as if the gate and source are shorted, but this only occurs only when the motor is connected and not with the resistive load. Additionally (with the motor as the load), I can see that the DRV8302 is trying to drive the gate at about 20-21V, but because the gate won't go above 12V, there is a 9V drop across the series gate resistors (4.7Ω). Is this a sign of FET failure or something else?

    I also measured CP1 and CP2, about 12V across it with periodic switching of 5kHz. During the switching, there's some ringing and CP1 is briefly at 12V. Is this correct? I have attached a scope capture of the switching of CP1 and CP2 (channel 1 is CP1, channel 2 is CP2). 

  • 0 in reply to Tim Yao
    Prodigy 30 points

    I have figured out the problem. It turned out to be a poor soldering job and the GH_C and SH_C pins were shorted. After removing the short, everything is working perfectly now. Thanks!