Role of Bootstrap capacitor

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emie

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Hello,

I'm looking for what's exactly the role of a bootstrap capacitor placed in the ate of a N-Mosfet?

Thank you,

Emie
 

in order to drive high side switch in h bridge or half bridge ckt

in mosfet
vgs should greater than 10 or 15 v.

in igbt

Vge >10 or 15 v

see when high side switch is on, dc link voltage will com to the source.

if dc link voltage is 100v

in order to turn on the high side switch

Vge or Vgs should greater than 100 + 10v =110v,
 

Heh, good analogy.

Yeah, an Nfet needs a positive Vgs so it needs a voltage above the S pin. The "high side" mosfet is often switching the highest voltage in the system so some sort of voltage boost circuit is needed to generate something ~10V higher than that. The bootstrap cap charges up to around 10V above the S when S is at ground and is used as a voltage reservoir to maintain that +10V even as the mosfet turns on and S rapidly rises.
 

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