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DRV8305-Q1: Short negative spikes on GHx and SHx, is this a problem?

Jan Lewerentz
Prodigy 30 points
Part Number: DRV8305-Q1
Other Parts Discussed in Thread: DRV8305

We use DRV8503 to drive three half-bridges using six Infinon auirfsa8409-7P MOSFETs in a PMSM application. I measure GHx and SHx using a probe without tip-clip using a 1/2" ground loop connected to groundplane near DRV8305. When top MOSFETs switch off I get a negative spike that, for about 30ns, goes below maximum rating of -5V for GHx and SHx. Is this a problem?

I have played around with IDRIVE ranging from 8 to B without significant change. I have also tried, as suggested on e2e, adding 1nF and 4.7nF capacitors gate-source on both low and high side. Not much change here either...

Image below shows SHA (blue) and motor phase C current (green), motor current is +/-40A. IDRIVE is BB for both high and low MOSFET.

Setting IDRIVE to 88 for both low and high MOSFET reduces negative spike to 5.8V that is still out of spec. And this point high side MOSFET gate rise time is so long that it hardly turns on during short PWM.

  • 0
    TI__Guru** 114905 points
    Hi Jan,

    Exceeding the absolute maximum or min voltage may affect device reliability. Please see footnote (1) in the Absolute Maximum ratings section.

    What is the voltage on the SLx pin during this negative spike? Is it less than -3 V?
  • 0 in reply to Rick Duncan
    Prodigy 30 points
    Hi Rick,

    Voltage level at SLx pins are within +/- 1V. It is only high-side MOSFET signals that show negative spikes.

    Should I interprete your answer that even very short spikes must be inside maximum ratings?

    Regards,
    Jan
  • 0 in reply to Jan Lewerentz
    TI__Guru** 114905 points
    Hi Jan,

    Yes, short spikes exceeding the absolute limits may affect reliability. The voltages should remain inside the max ratings.
  • 0 in reply to Rick Duncan
    TI__Guru** 114905 points

    Hi Jan,

    A possible solution to minimize the negative spikes at the device SHx pins is:

    placing a 5 to 10 Ohm resistor between the SHx pin of the device and the source/drain connection of the high side/low side FETs. In addition to the resistor, place a properly rated diode at the SHx pin with the anode to gnd and the cathode to SHx.

    We typically recommend a schottky, but with the abs max of -5V a standard diode may work.

    The resistor is used to reduce the current rating of the diode. 

  • 0 in reply to Rick Duncan
    Prodigy 30 points
    Hi Rick,

    I have put fast switching diode (BAW56) with 1ohm resistor in series from SHx to GND placed close to DRV8305. This reduces the negative spikes to within ratings when HS IDRIVE is set to 0x99 and LS IDRIVE to 0xAA. I also tested just BAW56 diode without series resistor but then some additional ringing turned up. I get 1.8V over 1 ohm during the negative spike and that is within pulse spec of BAW56.

    I don't feel happy about resisor put in series with SHx since this would decrease gate drive. We need IDRIVE setting for high side at 0.75A sink and 0.5A source to get decent switching.