Is a 250W Flyback practical?

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Right. Although I prefer pat2pdf.org because it results in a full screen PDF. Plus I have a bone to pick with Google because of their IMHO less than ethical handling of newsgroup spam flooding.

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Here in America we call that "Cutting off one's nose to spite one's face" ;-)

...Jim Thompson

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No, the world does not revolve around Google, no matter what its founders may believe. It is possible to live without it. Just like it is possible to live sans cable TV and sat TV. Or in good old American, one can "stick it to the man" :-)

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This one does regulation and PFC in one go.

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Theoretically you can but the question is whether it's good enough.

Then it's the wrong transformer for this app note. There's no fudge factor at 250W in a flyback.

Dropping the original power train components and topology is hardly a minor tweak, but it does address the original issue in a straitforward manner.

Congrats. You've just exhibited your reading and comprehension skills.

RL

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I tried it and it was good enough over the whole load range to power an amplifier (40V). A PSU like this usually gets my OK if the ripple is within 100mV p-p.

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What power factor values did you get out of it?

Let's see coupled inductors phasing inverted. Looks like a flyback to me. The first one is a two switch forward the next example is a flyback (two switch).

Congratulation's you've just exhibited your lack of reading and comprehension.

I'm not above reading other peoples suggestion and or application notes to find a solution to something. I think they call it research.

The controller choice TOP Switch isn't the best external FET's would be better. The dual switch is a good solution for getting extra power from the flyback; any generic PWM controller could be used controlling a HS driver. If you have any practical experience that says otherwise chime in.

The reason for my interest is the fact that a ready made transformer is available for 5 bucks, and it's designed for the output voltage and power I want.

Wasn't trying to raz you. The dual switch topology is useful, in that it allows recovery of leakage energy directly to the power rails, if the correct turns ratio is applied, without extra windings or elaborate intermediate storage. As leakage energy is one of the prime goblins of the higher power flyback, this is a keeper, if use of flyback is carved in stone. You should, of course, investigate the suitability of the same '5 buck' transformer in the new arrangement, as use of the dual switch does not change design considerations in the flyback circuit, if all other inputs and outputs remain the same.

Top-switches are extremely difficult to use anywhere other than the original application design , as their performance otherwise is undocumented, uncharacterized and unpredictable. If your application requires a micropower continuous housekeeper, you might consider using one for that purpose and deriving from it a dithering clock for the larger interruptible converter, to assist in EMI control. It could also serve as an overtemperature protection device, low-power start-up circuit etc etc etc.

RL

The use of a flyback is the only practical solution for me because I have to use off the shelf magnetic's. The pickings are pretty slim for anything but the flyback topology. This does seem less complex then a half bridge though and I'm pretty familiar now with flybacks.

I have some 70W flyback transformers I used for a 12V @ 4.5A with the NCP1203. I'll try out the dual switch approach with them. It should actually be cheaper or the same price and smaller more efficent I can use 400V FETS, and I can omit the RCD snubber (heater). The FAN7371 is a good candidate for the driver 4A/3A with a couple of FQP17N40 should do the job, that and I have lots on hand.

If this is the main design criteria, you could consider multiple phased converters. Most of the work may have already been done.

RL

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Good question. I don't recall. It sure isn't 1. I remember doing some simulations and measurements before final design so I guess it was good enough.

How about a resonant converter? You can incorporate the inductor turns in the PCB and mount an E-I core through the PCB. Probably even cheaper than the transformer.

links to that directory expire soon after the PDFs are generated.

Yeah, probably on account of storage space, but it takes only seconds to have one re-generated. Intl' patents are a whole 'nother matter though.