Why so few flyback PFC IC's?

[eem2am]

hello,

I cannot understand why there are so few specific Offline Flyback PFC IC's?

The only ones in the world are the NCP1651 and the L6561.


When the PFC regulations were announced recently for all offline SMPS's of 50W and above, it seemed obvious that the 50W PFC's would be implemented with single stage flyback PFC's.....

..since flyback , single stage PFC , is the simplest and cheapest way to get PFC for low power converters.

However, there are only two flyback PFC IC's in the world.

Does any reader know why this is?

...i am wondering if the PFC laws are perhaps less stringent at the 50W level, and maybe engineers are just using 'normal' current mode control IC's in flyback topology, and achieving a decent power factor by simply removing the primary high voltage bus capacitor, and using a slow voltage feedback loop to achieve reasonably high power factor?

 

[mtwieg]

Well flyback is economical for low power converters, but active PFC isn't really economical for low power. So they don't overlap in most applications. If you wanted a medium power, off line PFC with isolation, then a push pull topology would probably better.

Also I guarantee that while dedicated flyback PFC chips are rare, it's not hard to jury rig a generic PFC chip to work in flyback.

And for low power converters, passive PFC is often more economical than active.

 

[eem2am]

the thing is that 50W is low power but it is too high power for a passive PFC circuit.

50-100W SMPS's account for 50% of the worlds offline SMPS's, and yet so few flyback PFC chips occur.

-I think you must be right that people are just rigging up standard boost PFC chips to do flyback?

 

[mtwieg]

I wouldn't discount passive PFC so easily... there are lots of techniques beyond just having a giant LC filter. In order to develop low power, high PF LED lighting, people have come up with some amazing stuff.

Check out this document:
http://www.irf.com/technical-info/designtp/temp004.pdf

International rectifier and National semiconductor both have good app notes on this. It may not turn out to fit your application, but it's probably worth looking into.

 

[FvM]

I cannot understand why there are so few specific Offline Flyback PFC IC's?
The only ones in the world are the NCP1651 and the L6561.

I don't exactly understand the statement. L6561 is an industry standard PFC controller, many parts from other manufacturers are pin compatible to it. And it's usually operated in boost mode.

 

[eem2am]

yes indeed L6561 (+L6562) is usually used in transition boost mode...but st.com produce an App Note (AN1059), stating how to use L6561 in flyback PFC.

AN1059
https://www.st.com/stonline/books/pdf/docs/5956.pdf

-No other single semiconductor company produces literature on how to use their transition mode PFC chips in flyback mode.

...this is amazing given that single stage Flyback PFC is the method of choice (simpler and cheaper) for PFC between 50W and 75W.

also, do you believe that at 50W-75W, a simple current mode flyback with a valley fill front-end can meet EMC regulations on power factor?

-i do believe that PFC regulations are less strict at the lower power levels?

 

[FvM]

O.K., I see. I would expect, that other PFC chips could be used in a similar configuration, too. But I didn't check in detail.

I agree, that a flyback configuration can be particularly interesting, if low output ripple isn't an objective.