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Strange BLDC wave form and failed drivers

Discussion in 'Power Electronics' started by Lord_grezington, Apr 30, 2020.

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  1. Lord_grezington

    Lord_grezington

    117
    2
    May 3, 2013
    Hello All

    I have this BLDC project and have chosen to use the Silocon Labs SI8233BB isolated drivers.

    I am getting this strange stepped waveform on the rising edge of the PWM, has any one seen this before and know what might be causing it?

    The drive is bootstrapped with 3 fets in parralell, each with its own gate resistors and diodes. The waveforms were taken with the motor running no load with only around 0.3A current from the PSU (about right for this motor). The fets are FDP61N20 and the bootstrap caps are 470nF each.

    I had all three SI8233BB fail (Now repaired), do you thin this could be causing the problem?

    Thanks

    upload_2020-4-30_15-7-25.png
     
  2. Alec_t

    Alec_t

    2,788
    742
    Jul 7, 2015
    At which point in the circuit? A schematic would be helpful.
     
  3. Lord_grezington

    Lord_grezington

    117
    2
    May 3, 2013
    Hi

    I have since noted that the PWM step goes away once I load the motor. The graphs above were measured between the motor phase and GND. I have since doing the schematic also added additional 100R gate resistors in parallel (to take total to 50R), schottky diodes between the motor phases and GND (to prevent negetive voltage transients) and 10nF caps parrallel with the diodes in attempt to neaten the waveforms up a bit.

    Although the PWM step has now gone, I would like to know why they were there in the first place and if it could have caused the drivers to fail.

    upload_2020-5-1_8-4-59.png
     
  4. Alec_t

    Alec_t

    2,788
    742
    Jul 7, 2015
    This App Note is well worth a read regarding failed drivers. See Section 2.3 onwards.
     
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