Is a 250W Flyback practical?

The size and the cost of the transformer would probably be impractical. Also, they claim the efficiency of 85% which is not great, and I also have some doubts about it.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

For modest values of "practical" it is likely ok. The inductor would need to be a darn good design. The input current is in shortish pulses so a power factor correction stage would have to go in front of this stage. The result wouldn't likely end up very effecient.

I have some experience with the 120W version and I must say that 250W is pushing the limits very very hard. 120W is already on the limits of a flyback power supply.

You'll need a transformer with an extraordinary low leakage inductance and you'll lose a lot of power in the snubber circuit. In other words: I recommend to look into another topology if you need 250W continuously.

No mention of power factor in there? In many areas of the world this is mandatory for power levels starting around 70W. So you couldn't sell it there. Hmm ...

Anyhow, while 250W in flyback is certainly possible one has to still pass EMC and that's no small feat.

CWS has the coupled inductors ETD44H ; 650uH primary 35uH max leakage.

XT datasheet ETD44H

I ran the variables through here

I do agree a flyback isn't the best choice which is why I had to look twice at the datasheet. I've only built Flybacks to 70W I've read of interleaving to extend the power range but never tried it. The reason I prefer flybacks is because it's simple to get low volume ready made cores. The 250W mark is pushing it though definitely.

There is a simple way to get passive PFC with a centre tapped primary winding and an extra inductor.

That is also an expensive and space-consuming way :-)

Yep. We definitely want to keep it CHEAP ;-)

...Jim Thompson

I doubt that unless I'm missing something. For active PFC you basically need an extra switcher, inductor and 450V input capacitors (more expensive). The 400V input voltage eats away headroom for the flyback switching mosfet and reduces efficiency because the snubber dissipates more.

All I can tell you from my past designs is that an inductor serious enough for decent PFC barges into the BOM budget at $3 or more. The inductor for active PFC is miniscule and usually the grand total comes to much less.

Although there are many reasons why you should be shy of any Power Integrations app circuit for a 250W flyback, the fact that the output voltage is 48V, in this instance, is one point in it's favor. The nearer-unity turns ratio of the higher voltage output assists in making lower leakage inductance isolation transformers.

You should consult the most recent app note, in any event; yours is dated 2001 and there has been at least one revision (July'03)since it's publication for the TOP242-250 series.

RL

I used a 100uH inductor rated for the peak primary current. The rectified AC is fed into centre tap of the primary turn. The input capacitors are connected to one end of the primary, the switcher to the other.

The transformer at coilws sugessted output is 33VDC at 7.5A.

This would be the way to do it.

Two-switch topology boosts forward, flyback designs

For that 250W flyback a Choke for a 100kHz APFC is going to be around

Yep, that's why active PFC is needed. Ok, copper prices have come down a bit lately but probably passive PFC is still more expensive than active. And usually not as good.

Apparently it's practical; somebody bothered to design and build it. I probably wouldn't want to try it at home, but that's just me - I've never had any luck designing or building switchers. )-;

Cheers! Rich

This patent describes the operation (IIRC this is one of the 4 or 5 patents describing the circuit):

Well, quote: "A secondary winding of the transformer connects to DC-conversion output circuitry, which may be of substantially any known conventional type, such as boost, buck, buck-boost, Cuk, flyback, forward, SEPIC, or Zeta, for example."

At quick glance that seem to mean you need the usual, a switcher that does the PFC and another that does the voltage regulation. So what's the big deal?

BTW, for those who aren't "members" you can see the patent in full (meaning with drawings) here: