Does the bootstrap capacitor store energy

What does a bootstrap capacitor do?

The bootstrap capacitor provides power (VBS) to the high-side circuitry. The first parameter to consider is the maximum voltage drop across the bootstrap capacitor when the high-side switch is on. The maximum allowable voltage drop (Vbs) depends on the minimum gate drive voltage to be maintained.

When does a bootstrap capacitor discharge?

The bootstrap capacitor discharges only when the high-side switch S1 is turned on. The bootstrap capacitor provides power (VBS) to the high-side circuitry. The first parameter to consider is the maximum voltage drop across the bootstrap capacitor when the high-side switch is on.

What size bootstrap capacitor should a high-side MOSFET have?

As a general rule of thumb, the bootstrap capacitor should be at least 10 times greater than the gate capacitance of the high-side FET to have enough energy to drive the gate without being depleted by more than 10%.

What happens if a bootstrap capacitor is high?

On the flip side, higher values of the bootstrap capacitor lead to lower ripple voltage and longer reverse recovery time in some conditions (when initially charging the bootstrap cap or with a narrow bootstrap charging period) as well as higher peak current through the bootstrap diode.

Where does the charge to replenish the bootstrap capacitor come from?

The charge to replenish the bootstrap capacitor must come from some larger bypass capacitor, usually the VDD bypass capacitor. It is generally recommended to use low ESR and ESL surface mount multi-layer ceramic capacitors (MLCC) with good voltage ratings (2xVDD), temperature coefficients and capacitance tolerances.

Why should bootstrap cap be more than gate capacitance?

The bootstrap capacitor should be at least 10 times greater than the gate capacitance of the high-side FET. This is to account for capacitance shift due to DC bias and temperature changes, as well as skipped cycles during load transients. The gate capacitance can be determined using Equation 1: Qg (1)