You need to feed your +15V to the +pin of each of your two high side drivers through two diodes. When each side of the h-bridge is at ground, the capacitor will charge to +15, and then carry the 15 volts with respect to the emitter pin voltage as the emitter climbs up to +36V, at which time the gate will be 51V. Depending of the power needs of the driver and the IGBT gate capacitance you are driving, you may need a larger cap, as it needs to maintain most of the 15V charge for the duration of the high side ON pulse.