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DRV8323R: IC Fails occasionally with gate drivers shorted

Part Number: DRV8323R

We have a design using the DRV8323 that has been working great but every once in a while the DRV8323 fails with all the fet gate drives shorted.  The Mosfets are fine!  The etches to pins 7 (VDRAIN), 8(GHA) and 9 (SHA) get blown off the PCB making the board un-repairable.  The PCB layout was taken from the App notes and Data sheet.  The pcb copper is 2oz.  The board temperature by the DRV8323 is 50C. We use it in PWM x 1 mode, supplying a 15KHz signal.  The supply voltage is 48Volts and has a large snubber to keep the supply to less than 52 volts (measured) during a back EMF event.  The motor is rated for 500 watts. The internal switching power supply continues to operate properly!  Please advise

  • In reply to David Cassidy:

    David,

    Before talk about solution, we have to find the root cause first. The waveform is same after you added 10uF ceramic capacitor. That is a little bit surprise to me. From the schematic, only one FET is between SHA and that capacitor (please help me confirm that). But, if it is true, that tells us:
    1. 10uF is much less than what you have there. This is not true, right?
    2. The FET path has a very high parasitic inductance.

    Please also double check your probe ground connection. Don't let ground current di/dt give a measurement error here.

    Best Regards,

    Wang Li

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  • In reply to David Cassidy:

    Answers to your questions:
    1. Each 1/2 bridge of fets has 0.1uF & 2.2uF from +48V to gnd right at the fets. See C6 and C26 above. The ground is 2 layers of 2oz copper that is right under the fets. There is also 3 x 6800uF bulk storage 40-50mm from the 1/2 bridges. So my guess would be that placing a 10uF cap across the 2.2uF/0.1uF pair would not do much which agrees with what I measured.
    2. The fet case package is a TO263-7. Infineon pn IPB180N08. They don't give an package inductance.

    I have used a probe from a diff amplifer for making the measurement. It is soldered in place right at the pins with less than 12mm unsheilded. With such an extremely fast spike I can see your point here. Do you have another sugestion? The scope is a 2Gs/s 100MHz.

    Thanks
  • In reply to David Cassidy:

    David,

    If the measurement gives us the true info, next step we check: does TO263-7 package inductance or lead parasitic inductance cause the issue:
    a. can you slow down the low side FET turn-on speed? It can slow down the di/dt transfer from low side FET to high side FET path.
    b. can we change the FET to a small package (such as TO220) and short the lead wire to verify the parasitic inductance factor.

    I would think the item a could be a potential solution.

    Best Regards,

    Wang Li

    Motor Drive Solutions

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  • In reply to David Cassidy:

    Hello

    I used anther scope and probes to verify the waveforms I was measuring.  Both scopes have similar wave forms but the second scope the spike voltage was 2-3 volts higher.  The BW limit was on in the first scope.

    I don't see how to just change the low side mosfet turn on time (I'm not using the spi bus) with out adding a gate resistor/diode. I tried lowering the gate drive current from an iDrive Resistor value of 18K to infinite.  (570/1140ma down to 120/240ma)  The spike drops 4-5 volts so it has gone from low 60s to high 50s volts.  The board runs a few degrees warmer.  What is a safe limit?  Should I try a gate resistor and diode?

    Thank you

  • In reply to David Cassidy:

    David,

    Yes. Change gate drive current setting can change the low side FET turn-off speed which can reduce the voltage spike.
    But, we just want to eliminate the spike. The spike energy should not bring the whole board few degree warmer. If it does bring few degree warmer, we may slow down too much.
    If the TO263-7 FET current path has a very high parasitic inductance, can we add a schottky diode parallel with it (directly from the load to the 2.2uF or 10uF ceramic capacitor?

    Best Regards,

    Wang Li

    Motor Drive Solutions

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  • In reply to David Cassidy:

    Hello

    I ordered some schottky didoes.  I also looked for a negative spike below ground and see a -5V spike in the 20ns wide range.  Should I also put diodes to ground?

    Thank you!

  • In reply to David Cassidy:

    David,

    Yes. Please also put a schottky diodes from switching node to ground. This is on debugging stage. It won't hurt to find the root cause.

    Best Regards,

    Wang Li

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  • In reply to David Cassidy:

    I added in the schottky diodes as you suggested.  The spike dropped again but was not eliminated.  I lowered the gate drive current again, which seems to have the biggest impact.  I have been running the system now for days starting and stopping the motor quickly and changing speeds with no failures.   I removed the schottky diodes and have again have run the motor for days monitoring for spikes and have not seen any spikes greater than 2 volts over the power supply.  Comments?

    Thanks

  • In reply to David Cassidy:

    If the gate drive current control can eliminate the switching spike, you may not need the schottky. Adding schottky just tries to provide another current path besides the high parasitic inductance switching FET path.

    Best Regards,

    Wang Li

    Motor Drive Solutions

    Battery Charger Solutions