How do you calculate Bootstrap Capacitor?

Use the formula Cboot = Qg / ΔV. In a high-side gate drive, a bootstrap capacitor stores the charge that turns the high-side MOSFET on. Enter the values you know in the calculator above and it solves for the remaining variable.

What is the formula for Bootstrap Capacitor?

The Bootstrap Capacitor formula is Cboot = Qg / ΔV, where Cboot—Bootstrap capacitance (F); Qg—Total gate charge of the high-side FET (C); ΔV—Allowed bootstrap voltage droop (V).

What does Cboot mean in the Bootstrap Capacitor formula?

Cboot is Bootstrap capacitance (F).

Bootstrap Capacitor

Cboot = Qg / ΔV

Calculator

Bootstrap Capacitance

Gate Charge

Allowed Droop

Result

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Formula

Cboot = Qg / ΔV

Description

In a high-side gate drive, a bootstrap capacitor stores the charge that turns the high-side MOSFET on. It must be large enough that delivering the gate charge does not droop its voltage below the level needed to keep the MOSFET fully enhanced. This formula gives the minimum bootstrap capacitance for an allowed droop; designers typically use 10–20× this value for margin.

Variables

Cboot — Bootstrap capacitance (F)
Qg — Total gate charge of the high-side FET (C)
ΔV — Allowed bootstrap voltage droop (V)

Practical Notes

Add the high-side quiescent and leakage charge over the on-time for a complete estimate. A factor of 10–20× the minimum keeps the gate drive stiff; too large slows bootstrap refresh at startup.

Related Concepts

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