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Глянь AN-978 от IR. И также AN-1123. Цитата из главы HOW TO SELECT THE BOOTSTRAP COMPONENTS: The bootstrap diode must be able to block the full voltage seen in the specific circuit; in the circuits of Figures 25, 28 and 29 this occurs when the top device is on and is about equal to the voltage across the power rail. The current rating of the diode is the product of gate charge times switching frequency. For an IRF450 HEXFET power MOSFET operating at 100 kHz it is approximately 12 mA. The high temperature reverse leakage characteristic of this diode can be an important parameter in those applications where the capacitor has to hold the charge for a prolonged period of time. For the same reason it is important that this diode have an ultra-fast recovery to reduce the amount of charge that is fed back from the bootstrap capacitor into the supply.