PFC Offline Converters -- SEPIC, Cuk, Boundary Conduction Mode Flybacks

[berkeman]

I'm upgrading a non-PFC (power factor corrected) offline power supply design to include PFC for European deployment. The total output power is less than 25W, and the two output windings are around 20V.

I'm familiar with boost-flyback topologies for isolated PFC supplies, but that seems to have been passed up recently by other topologies to limit the peak intermediate storage capacitor voltages. SEPIC and Cuk topologies would seem to be much more efficient in terms of any capacitor voltages, but they involve extra complexity in terms of switch FETs.

Have any of you worked with offline PFC supplies in the 20W range that use one of these topologies? Can you let me know about the tradeoffs and why you chose the topology that you went with in the end? The Cuk topology seems to have some technical advantages (for the isolated version which I obviously need), but it seems to be a bit overly complicated compared to a SEPIC topology version.

The topology that I'm being recommended by the IC vendors is a boundary-conduction mode flyback followed by multiple buck down-converters, but that seems a bit clumsy compared to the SEPIC and Cuk converter topologies.

On a related question, have you had any experience with tapping multiple voltages off of a tapped-inductor buck topology DC-DC converter? This project is the first time I've heard of this option, and although it sounds okay at first suggestion by the IC vendors, I'm having a bit of a hard time with my initial visualization of the concept. The regulation loop is obviously only closed on one of the output voltages, but it seems like the cross-regulation issues would be pretty significant.

Thanks for your thoughts!

 

[unplebeian]

Hi Berkeman,

It is surprising that you need PFC for something as low as 25W. As far as I know, EU mandates PFC for powers >= 70W (or was it 75W)? The other question is do you need isolation or not. If not, then use a standard Boost PFC in DCM which will be well suited for this application and then use a second stage that is a DC-DC however, it seems to be a bit cumbersome just by intuition for only 25W.

If the application is something like an LED driver you can use a PFC+isolated flyback all in one. Power Integrations, TI have many solutions and the former may even have a reference design online.

If it is not an LED driver you can also try the buck PFC stage. http://www.ti.com/tool/ucc29910aevm-730

HTH.

 

[Jeff Rosenbury]

EU regulations seem to only apply under the 39th harmonic (of 50 HZ). Is it possible to go with a higher frequency?

I don't know, but it's a thought.