Circuit protection problem driving high side MOSFET continuously
VB is a power source for the gate of the IGBT, and must always be higher than the emitter by enough to turn the gate on.  The type of bootstrap you have drawn in depends on VS going to zero periodically so that your +15V can charge CBoot.  Otherwise Dboot is always back biased so you will turn off the IGBT after CBoot discharges.  You can make a simple power supply for VB with a transformer.  Set the turns ratio so that you get at least 10-12 volts or so on CBoot.  This can be a free-running oscillator because VB can always be present; the gate is always controlled by the input. 







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