I am designing a BUCK converter 24V/12V, 80A, 300kHz switching freq. It will be used for stepping down battery voltage, so it can be expected for input voltage to vary from 22 to 30 volts.
I am stuck trying to design a viable bootstrap circuit. The problem is that if I just put a diode from 24V rail to charge the bootstrap capacitor, it will charge to 24 volts which is too much for mosfet gate (irf3206, Vg=20V, Qg=170nC).
If I were to use a 10V zener in series with said diode, the capacitor would be charged to acceptable 24-10 = 14V, BUT in case where input voltage was 29V, it would be charged to 19V which is dangerously close to gate's absolute maximum 20V. Besides, turn-on times, and thus switching losses would vary wildly depending on input voltage.