I couldn't post a circuit.... it's confidential. I got to the bottom of it eventually.The (faulty) IGBTs had horrible leakage- a range of up to
850uA. Good (new) ones measured 0.5uA @ 600V to 1uA @ 900V. Turn on/off times were ok. 1uA drops around 0.6V down each stage. 850uA = >50V No wonder they didn't work.