1. There is a small mistake in the circuit, but it is probably just in the drawing, not the actual circuit:
This circuit looks good, if 112V_P, 2V_Q1, and 12V_Q3 are isolated power supplies. How often does the MOSFET fail, and which MOSFET is it?
2. The bootstrap diode will see the bus voltage + 12V + overshoot across it. For a 400V bus, I would guess a 500V rate diode should be good. A common choice for bootstrap diodes is the MUR series, such as the MUR150.
Another cause of the bootstrap capacitor failing could be from overcurrent. If the highside MOSFET fails and causes damage to the A channel of a driver, a short could be formed between VDDA and GNDA. Once that happens, if the lowside channel brings the midpoint voltage to GNDB, the bootstrap diode would see high current and fail.
The cause of the MOSFET failing could be many things. Here are a few:
1. Temperature overstress
2. Repeated peak voltage stress due to ringing
3. Timing error on input side of gate driver
4. Saturation of a load inductor
5. Short circuit on load being driven
Is there any noise on VIA and VIB? Sometimes with a resistor divider network VIA and VIB can be more susceptible to noise from high currents. This is because with a resistor there, a high impedance is seen from the input nodes.
The bootstrap capacitor size is independent of the bus voltage. It depends on the switching frequency, duty-cycle, and quiescent current of the high-side driver. The circuit note http://www.analog.com/static/imported-files/circuit_notes/CN0196.pdf covers sizing the bootstrap capacitor.
Without know the minimum switching frequency and duty cycle of a particular design, I can't suggest a bootstrap capacitance. I suggest looking at the bootstrap capacitor sections in the circuit note first.
