for half wave rectifier for 1 half of the input cycle the transformer secondary is unused .but for centre tap full wave rectifier for both halves 1 half of the 2ndary is used. for the bridge rectifier however the entire 2ndary is used for both the halves . so T.U.F of half wave < full wave < bridge rectifier . am i correct ? but i think TUF of primary winding in all the 3 cases should be same .is it ?
You should sketch the primary waveform of the half-wave rectifier and estimate it's RMS current to answer the question. No, it's not the same utilization factor.
An additional important reason why a half wave rectifier can't work with good efficiency is the core DC magnetization.
Utilisation factor highest for single winding o/p with 4 diodes doing the rectifying, for a CT rectifier (2 diodes) the current in each winding is 0.707 of that in the single winding (and there are two f them) - so there is a penalty for CT o/p's...
Single winding o/p can also have voltage doubler with 2 diodes and two caps...
and current doubler with 2 diodes, 2 inductors...
All other config's are lower utilisation than CT o/p...