HF step up transformer

Can't use a simple boost topology?

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Never heard of a step up transformer with a DC input.

I should think that a square-wave forward converter would be most efficient. With a single primary winding (h-bridge drive) and a single secondary (bridge rectifier, driving L-C filter), duty-cycle modulated to handle the input range thing.

John

Just be careful with really small loads at high input voltage. Then the PWM behaves more like an idling Harley engine with old spark plugs.

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Not many DC step up transformers around, so your choices will be limited.

Naa. Flyback ist only good below 100W. Go for a transformer with a 7:1 winding

Rene

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I've done 400W flybacks (24V output), but its kinda brutal; 350W @ 60V =

5.833A so a 50% duty cycle DCM flyback has 23.3A peak primary current, 9.5Arms. thats not too bad at 60V, until you realise you need AT LEAST 650V FETs.

A diagonal half-bridge means you can use 500V FETs, but its still kinda nasty. CCM helps a fair bit, until you realise how big the damned transformer gets :(

I'd pick a diagonal half-bridge forward converter - again so you can use

500V FETs. the power level really isnt high enough to justify full-bridge, and using a moderate sized output inductor will give you a primary RMS current of about 2.5Arms. a lot better.

But his input is only 125V. If he keeps leakage inductance low and uses good snubbers/clamps he should get by with lower voltage FETs. In most of my half-bridges the FETs don't see much in terms of spikes.

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Do you mean this one?

+125-------------+-----------------+ | | | | | | cap fet | | | | +----primary------+ | | | | | | cap fet | | | | gnd--------------+-----------------+

That should be happy with 200 volt fets.

John

Hi Joerg & John,

I cant read. well, I can, but I didnt :)

125V in, not 420V like I mis-read. without a super-low leakage design, he'll need 500V FETs.

It'll be hard (but achievable) to keep the leakage low enough to use

200V FETs, and at this power level he's gonna have a *lot* of snubber loss.

a clamp winding forward converter will have lower leakage than a flyback xfmr, as the turns are the same as Np & it can be bifilar wound, but he'll still need > 300V FETs.

indeed. but I meant this one:

drive both FETs from same signal. hence the diagonal half-bridge nomenclature. works for forward & flyback, and returns all mag/leakage to the primary supply.

Cheers Terry

Ok Terry, now we know you drink, a lot! Was it just wine or something better? While Terry is recouping he wanted me to tell you all to replace the caps with fast diodes, cathodes up. There are some neat IC drivers from IRF to drive both FETs from one 8 pin pack. Good night Terry.

Doh! pulled most of an all-nighter last night, and am busy helping my tech break a pre-production sample of something expensive, by doing dumb things, before I fly to China tonight. aaargh.

So it looks like this:

+125-------------+-----------------+ | | | | _ | ^ fet | | | | +----primary------+ | | | | | _ fet ^ | | | | gnd--------------+-----------------+

oops.

Cheers The engineer formerly known as terry

Your circuit would resemble a standard PC power supply. The differences would be that instead of low voltages at the output (+5,-5,+12), you would need a 400 volt (or so) secondary. For efficiency, you would probably want to run synchronous rectifiers (using power FET's). Getting passive shottky rectifiers at this voltage would be a pain, anyhow. You would need another winding to control the synchronous switching of the FET's. As a protoype, you could probably scavenge a few PC power supplies, rewind the transformers (make sure that you follow safety standards with the 400 volts). The little experience I've had with these circuits tells me that going for >90% efficiency will require a lot of expertise. I would recommend you check out the various engineering rags (EDN, Electonics), and application notes from the various semiconductor houses. Check out Apr 3, 2008, EDN, p47, "Boost efficiency for lowcost flyback converters".

-Paul