7.3.1 Bootstrap Diode Operation

An internal low impedance switch enables the bootstrap only when the low-side GaN FET is on. If used in a converter where the low-side FET operates in third quadrant conduction during the dead times, this provides two main benefits. First, it stops the bootstrap diode from overcharging the high-side bootstrap rail. Second, if using a p-n junction diode with Q_rr as the bootstrap diode, it decreases the Q_rr losses of the diode. There is a 1 kΩ resistor connected between the drain and source of this internal bootstrap switch to allow the bootstrap capacitor to slowly charge at start-up before the low-side FET is turned on.

The part does not have an actual clamp on the high-side bootstrap supply. The bootstrap switch disables conduction during the dead times, and the actual bootstrap capacitor voltage is set by the operating conditions of the circuit during the low-side on-time. The bootstrap voltage can be approximately calculated in Equation 1 through Equation 3.

The bootstrap voltage is given by Equation 1:

V_HS is calculated in Equation 2:

Substituting (2) into (1) gives the expression for the bootstrap voltage as Equation 3:

From (3) one can determine that in an application where the current flows out of the half-bridge (I_L is positive) the bootstrap voltage can be charged up to a voltage higher than V_DD if I_L × R_DSON is greater than V_F. Take care not to overcharge the bootstrap too much in this application by choosing a diode with a larger V_F or limiting the I_L × R_DSON product.

In an application where I_L is negative, the I_L × R_DSON product subtracts from the available bootstrap cap voltage. In this case using a smaller V_F diode is recommended if I_L × R_DSON is large.