Actually I have just found a very good reason why Dr Ridley might be very correct about full bridge transformer imbalance.......when you have a full bridge with a duty cycle of above 0.5, then during start-up, the adjacent primary current pulses can be vastly different in length, (subharmonic oscillation) and this can be so for many many cycles.........ie, going one way through the primary, the current pulse is wide, then going the other way, it is always narrow......BANG!.....the following LTspice simulation, as attached, demonstrates this.......I fear now that the full bridge is indeed not safe for duty cycles above 0.5, due to subharmonic oscillation, because even with slope compensation, during start up, subharmonic oscillation still happens....just take a look yourself, -this could consign the full bridge to the history books for duty cycles above 0.5.....