Power semiconductors

Which circuit uses a diode and capacitor to create a gate voltage above the supply to drive the upper (high-side) n-channel MOSFET of a half bridge?

  1. ABootstrap circuit
  2. BCurrent mirror
  3. CSample-and-hold circuit
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Answer: A. Bootstrap circuit

To turn on a high-side nMOSFET, its gate must be driven above the potential of its source (the output node). A bootstrap circuit charges a capacitor while the low side is on, then adds that voltage on top of the output node's potential to drive the gate when the high side turns on.

  • A. Correct. It drives the high side with the voltage stored on a capacitor.
  • B. A current mirror produces copies of a current.
  • C. A sample-and-hold temporarily holds a voltage.
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