JAJSER7D November 2018 – January 2019 LMG1210
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 Qrr as the bootstrap diode, it decreases the Qrr 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:
VHS 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 (IL is positive) the bootstrap voltage can be charged up to a voltage higher than VDD if IL × RDSON is greater than VF. Take care not to overcharge the bootstrap too much in this application by choosing a diode with a larger VF or limiting the IL × RDSON product.
In an application where IL is negative, the IL × RDSON product subtracts from the available bootstrap cap voltage. In this case using a smaller VF diode is recommended if IL × RDSON is large.