Flyback primary clamp voltage vs efficiency

[grizedale]

In a flyback smps, the higher we make the primary clamp voltage, the faster the transformer leakage inductance is "discharged".......this means the secondary can begin fully conducting sooner.

It also makes for higher efficiency, but how is this?

 

[mtwieg]

In a flyback smps, the higher we make the primary clamp voltage, the faster the transformer leakage inductance is "discharged"

Okay this makes sense.

.......this means the secondary can begin fully conducting sooner.

Huh, I had never realized this. I just threw together a simulation and verified it. The clamp voltage affects the fall time of the primary current, which is proportional to the rise time of the secondary current.

It also makes for higher efficiency, but how is this?

I was also able to see this in the simulation, though I'm not sure as to the exact cause. Maybe lower switching losses in the rectifier diode?

 

[grizedale]

OK Thanks,

.....i am not sure, but i think that the action of the leakage inductance flowing current through the core after the switch has turned off.........means that some of the stored field inside the core is "used up"......i.e. the flux produced by the flow of current in the leakage inductance opposes the flux already in the core and cancels some of it out....such that there is then less energy in the core available to flow into the secondary.......that is, a loss of efficiency.

is this right?

..Basically, the leakage term delays onset of secondary current, and this gives rise to inefficiency, but where is the energy getting dissipated to give this inefficiency?

p591 - 596 of Basso book explains it but doesnt say where the energy is being dissipated.......it cant be in resistance of tracks because thats negligible.......is it dissipated as a switching loss in the fet?

It seems that the conservation of energy has been violated here........its unknown where the energy has gone.