Are you still going for a single PWM transistor and a relay H-bridge? IMHO, that's doomed to failure unless you can find an absolutely massive relay. What was the reasoning behind selecting that IGBT rather than a FET? IGBTs are generally used when you need to switch high voltages and FETs do a better job switching high current at lower voltages. That particular IGBT is not going to switch much more current than a TZ85a in a real-world situation; the large current figures are for 1 millisecond at 25 degrees.