More methods to salvage leakage energy in a flyback?

Is this a transformer-coupled flyback PFC converter?

Yes. It provides both PFC and the isolation. The regulation is unorthodox in our case and rather loose, another converter afterwards does the finer regulation. But we are looking at other architectures now, like classic PFC plus LLC. The challenge is that we need a combined overall efficiency of around 90% and also low standby consumption when there is very little load.

Hi Joerg,

Yes air gap has a contribution on leakage inductance (% of primary magnetizing inductance) but i would say that windings strategy brings the major influence on the overall magnetization leakage, that is the mutual inductance coefficient K. Afterall a xformer is just a coupled inductance with K = 0.9... Usually a leakage inductance is almost 3% of the primary magnetizing inductance, 5% is the best xformer i've ever built myself.

Ok. I see Coss as it is drawn in the schematics to steers the energy back to the rail ... as it has been said ... i really doubt of that ! believe me i consider myself as an experienced designer on flyback topologies. BTW i can see a Y capacitor from the output to the input. Many designers seem ignore how this Y capa is usefull ...

Ok, although i did not really understand what you mean. PFC is generally required when power is high (> 250W) to improve power factor (power utilities are sensitive on maintain cos(phi) = 1 ...). Is your design focused on high power ?

I agree with you, a PFC remains "delicat" with Flyback topologies.

Habib.

The classic PFC is a single-inductor non-isolated boost switcher, 400 volts typ output, so there's no leakage inductance as such. Then there's an isolated forward converter or something. All you need is

MeanWell has some supplies like that, claiming 90% overall efficiency at 100 watts. Buy one and snoop it!

On the bright side, it doesn't have to carry nearly as much power as the main supply, nor does it have to be super-efficient to still have a positive impact on the overall efficiency.

I was going to suggest that there would be something off the shelf, but you really need something that either regulates the snubber rail to some desirable voltage, or that drains the snubber rail at a constant current down to some desirable voltage (probably equal to whatever your supply rail is). Neither of those are needed in laptops, so there's probably not a $0.25 part in a SOT-23-5 package that'll do it with a pea-sized inductor.

Why can't you just use one of the other PWM outputs from whatever microprocessor you're sensibly using to run the main power supply? Heh heh.

Coss? It's the recuperating winding leftmost in T1 that recycles the otherwise unused energy. And it does work. It's just that you then become duty cycle limited in the converter and tat doesn't work with PFC.

The cap across the barrier is often a problem with agency approvals. For example medical where AC leakage must be samll. Even if there is a nice piece of chocolate right in front of you that doesn't always mean that you can have it :-)

No, but nowadays the PFC requirement is already mandatory at much lower power levels. Even for very low levels if you have to assume a mass installation at each location. It's like a law, nothing I can do anything about.

It works great if you don't need 90+ percent efficiency. But this time we do.

Yeah, that's "all" I need :-)

It's not that easy. Our application has much stricter isolation and EMC requirements than consumer or industrial.

Why don't you go for the 2 switch flyback? It's simple and has no fancy control scheme and the reduced voltage stresses on the MOSFETs allows for lower rated parts.

Cheers

Klaus

I would never leave it "up" all the time either. the mere generation of

I think Coss/Lprimary are reponsible of oscillations in primary side when current in secondary windings becomes 0, although it helps for reducing L leakage ringing. Bad news for me when you say PFC are always mandatory even in low power flyback's ... i will see.

Thx, Habib.

Below 70W (or 50W) there is no requirement for PFC, at least for industrial/Consumer products

Cheers

Klaus

Input voltage is 115VAC/60Hz, power is around 70W and the output voltage can be anywhere between 30V and 45V.

Ok. I see Coss as it is drawn in the schematics to steers the energy

I'll definitely simulate that but in real live it isn't trivial. The flyback PFC ICs have no output to drive it, needs a transformer, some timing massaging and so on. Haven't thought that through yet but I believe that two-switch also may have the 50% duty cycle limitation which would be a problem here.

True, but it has to work in a somewhat orderly fashion or it messes with the PFC because we are still talking a few percent here.

Power Integrations actually might have a cheap enough device for this job in their LNK series.

That woud be another regulator loop in there and they'd throw the coffee mugs at me.

Joerg, I'm clueless as to your problem, but I totally love your thread. Thanks. Do you have to maintain low loss over the entire range, or could you separate the on and off state and then have high efficiency mostly at the high power end. (did you say how much power?)

George H.

Hello Klaus,

Please. What is that 2 switch flyback ?

Habib.

Supposed to be different grounds but the transformer has drawing errors.

piglet