What happen if we choose bootstrap capacitor value too high in high side gate driver applications? Can it be harmful for MOSFET/IGBT or it is just not important?

  • \$\begingroup\$ Startup behavior would be of primary concern. \$\endgroup\$ – winny Jun 12 '18 at 12:26

The only drawbacks which came into my mind:

  • it takes longer to charge it up after startup
  • You need a capacitor with low ESR if you need to utilize the whole voltage for the switch drive. The ESR will have a voltage drop in it during charging up the high-side gate, plus it slows down the process. You need a cap where it will not be the bottleneck in your design.
  • a large cap has lower self resonance frequency. You want that your capacitor behaves as a capacitor at the switching frequency and its first few harmonics.
  • more expensive

Increasing the bootstrap cap well beyond the gate cap - it might be better to think in sum of charges, but saying cap is easier - will not give you much. As long as the voltage after the charge sharing process is close enough to the supply you will gain nothing from a bigger cap.

  • \$\begingroup\$ Thanks for your reply. In my BLDC application, VBus=420V and carrier frequency is 16KHz. I chose 220uF/25V bootstrap capacitor for high side gate driving. Usually driver and motor work fine. But sometimes when i power on, before rotating motor, one of the IGBT legs fails and damages. Is it possible that it is because of high value of bootstrap capacitor? \$\endgroup\$ – Ata Jun 12 '18 at 12:37

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