 When the low-side N-FET turns off, the low side of the bootstrap capacitor remains connected to the source of the high-side N-FET, and the capacitor discharges some of its energy driving the gate of the high-side N-FET to a voltage sufficiently above V+ to turn the high-side N-FET fully on; while the bootstrap diode blocks that above-V+ voltage from leaking back to the power rail V+.A MOSFET/IGBT is a voltage-controlled device which, in theory, will not have any gate current.This makes it possible to utilize the charge inside the capacitor for control purposes.