2
\$\begingroup\$

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?

\$\endgroup\$
1
  • \$\begingroup\$ Startup behavior would be of primary concern. \$\endgroup\$
    – winny
    Jun 12, 2018 at 12:26

1 Answer 1

3
\$\begingroup\$

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.

\$\endgroup\$
2
  • \$\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, 2018 at 12:37
  • 1
    \$\begingroup\$ @Ata likely what's happening is since the bootstrap capacitor is taking a while to charge, in the beginning the IGBT may be partially turning on (operating in the linear region) and this is not good. This will generate large losses across the IGBT and cause it to fail. \$\endgroup\$ Aug 12, 2021 at 15:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.