Two switch flyback not suitable for single stage PFC design

[treez]

This relates to this thread...
https://www.edaboard.com/threads/348884/

The flyback energy will distribute very well without blowing anything up.
It can feed both the bulk input capacitor (via the clamp diodes) or the output capacitor (via the rectifier).

Your absolutely right, however, i looked into the two switch flyback for a single stage PFC LED driver, and the fact that the two switch flyback shovels energy back to the input when v(referred to primary) is greater than vin means that its not suitable...damn!
As you know, the single stage flyback PFC input bus goes up and down from zero volts.

Unless there is some way round this...hmmm
(Looking into Single Stage PFC'd Offline (100-265VAC) LED drivers for 100W+ and NO electrolytic capacitors allowed)

 

[mtwieg]

I don't see how this is possible. The rectifier diode on the secondary should prevent bidirectional operation.

 

[treez]

whilst sec diode is conducting, if v(reflected to pri) is greater than vin, then current will also be reflected to the pri and go into the pri dc bus capacitor.

 

[mtwieg]

Okay I see what you mean. Thought that "shovels energy back to the input" meant from the secondary side to the primary side, which isn't correct. A better way of saying it is that the flyback stops absorbing average power from the source at all.

 

[treez]

A better way of saying it is that the flyback stops absorbing average power from the source at all.

Thanks but sorry i dont understand....as long as the fet is on, and current flows through the primary , then energy gets stored into the core..surely

 

[mtwieg]

Thanks but sorry i dont understand....as long as the fet is on, and current flows through the primary , then energy gets stored into the core..surely

During the on period of a full cycle, the core absorbs power, and during the off period it is all returned to the source (assuming there's no flux walking). I am referring to the average energy transferred over a full cycle, which is zero (aside from dissipation in the components).