80kHz CCM PFC followed by 80kHz Two transistor forward...beat with each other?

[treez]

Hi

Our contractror has done a schem for an 80kHz, 200W CCM PFC followed by a 80kHz Two transistor Forward. (they are not in any way sync’d with each other).

We are requesting he changes the PFC F(sw) to 70kHz in order to reduce the severity of beat frequencies. Would you say this is reasonable?
(230VAC input)

 

[mrinalmani]

I think the beat effect will be even more severe at 80KHz followed by 70KHz, beating at 10KHz. If both the stages are at 80KHz then beat will be in order of a few 100Hz considering slight mismatch. But to beat at such low frequency the system should probably have a very low damping factor. It's like two frequencies in the MHz range trying to beat in a few Hz, the beat should smudge out due to damping.

 

[Easy peasy]

Depends on the quality of the electro's on the o/p of the boost stage - if they are good you won't have a beat freq problem - there are some chips out there that turn on the fwd mosfets just as the booster turns off - thus some of the current that would have gone into the caps - goes into the 2nd power stage instead - it is claimed that this greatly reduces effective ripple current in the caps and lowers temp rise.

For a very slight freq diff @ 80kHz you may hear or see a low value diff freq beat - but for good quality caps LOW ESR - it will be OK ...