I see your point, ...though its just to kind of get a ball-park idea of how close the reset zener value is to what's required...i mean, i dont want to overdo the zener voltage, even though it would still work.
For this UCC28070A PFC application, the idea is to set the UCC28070A max duty cycle to the max level that could still always ensure reset of the CST....but its practically impossible, over the entire mains half cycle, ,to calc what is the exact reset zener voltage required at whatever exact max duty cycle setting.....i mean, you can overdo it and get it working, but we want to maximise duty cycle and minimise zener reset voltage....we were hoping the sim could help in this optimisation process?
woops sorry, attached is the LTspice sim with reset zener in the right place.
Its amazing UCC28070A even bothers to offer max duty cycle setting facility...i mean, in the above sim, even if you set the zener reset to 4V7, (ridiculously too low), the txfmr still resets fine........if you view the magnetising current ( I(L9)+I(L8)/48 ) then it never builds up high at all.....the CST i am using uses E20/10/6 and Isat is over 150mA....and imag, even with 4v7 zener, only goes up to 10mA.......i mean, is resetting a CST the easiest job in power electronics?...almost whatever you do and it resets fine.