inductor for HV flyback supply

Why futz around with flyback supplies when you can use a Baxandall Class-D oscillator?

Peter Baxandall invented it to deal with the problem of getting a kV or so for photomultiplier supplies, and Jim Williams used it to generate even higher voltages for the cold cathode back-lights that used to be popular in lap-top computers.

It's two pot cores - transformer and inductor - rather than just one, but it's a lot tidier, and you should be able to get away with smaller pot cores.

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Bill Sloman, Sydney

No photo; took a long time to "load"...

Single stage..?

Yeah, that's gonna cost you dearly if you want any kind of efficiency, or compact size. And cost, but I'm guessing most of your circuits are lab use, so that one's not too important, at least.

Have you considered a mains inverter transformer? They're largely around $5, and about as big as the RM core, or the multilayer rod inductor, but... they are much easier to use, at least.

Figure that, since they're made for 2.5kV reinforced isolation, and 320VDC primary supplies, there should be no problem getting 500V or more (without even adding a doubler).

The one I made is resonant (sort of, but not really sinusoidal.. so bug off, Bill :^) ), and delivers up to 2kV and 10mA. Here's the circuit:

and a picture of the build:

There's very little winding area used on that ~200W capacity transformer (EE33 size). I wound it poorly on purpose, but it still had less leakage than I was wanting/expecting. (As a result, the operating frequency swings over quite a range, between unloaded and short-circuit conditions.) It would be quite reasonable to get 50, maybe 100W from a transformer this size.

Of course, UC384x are quite capable, and a good choice. Just beware of leakage inductance and parasitic capacitance, if you look into transformers at all. :)

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:

Parastic - parallel - capacitance in high turns ratio transformers can be a real problem. If you can bank-wind the secondary, you can cut it down quite a bit, but multi-section formers for even big RM cores seem to have become hard to get hold of.

The charm of the Baxandal class-D oscillator is that handles the parallel capacitance more elegantly than the Royer inverter, but it does set an upper limit on the oscillation frequency.

Leakage inductance is also less of a problem in the Baxandall set-up - you aren't trying to change the currents through the transformer as fast as you need to in a Royer oscillator.

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Bill Sloman, Sydney

Hmm, I clicked the dropbox link in a second browser window, and it popped right up. The link is spread across two lines, maybe your newsreader / browser only paid attention to the first line, not good. Try this one, with a shortened filename:

--
 Thanks, 
    - Win

Thanks for the suggestion Bill, I know you love that circuit, and resonance is good. Indeed for higher power and voltages it'd be the right idea.

But the uc3843 and other parts are small, SO-8, etc., mounted on the underside of the PCB, and in the beginning I had hoped to use a single simple commercial off-the-shelf inductor, and avoid any custom magnetics. May still do so.

Anyway, even with having to wind 22 turns on an RM8, the result is still simple, small and cheap.

BTW, some of your web article links are broken.

--
 Thanks, 
    - Win

ator1.htm

Too many people make a habit of avoiding custom magnetics. The Rowland Inst itute ought to have a simple coil winder - there are cheap ones available f rom China if it doesn't - and you don't need much of a range of RM cores, f ormers, mylar tape and enamelled wire to do useful stuff.

Printed windings are neater, but you've got to talk to a specialist printed circuit shop to get useful copper fill factors. I talked to one in the Net herlands, but didn't get to the point of having anything made - that would have been for a better electric piano keyboard, but I couldn't get the Linu x mechanical drafting program to do anything useful, and tossed it all into the too-hard basket.

That's been on the to-do list for a while. I seem to be coming out of jet-l ag today, and my wife won't be back until Monday, so I might get it done ov er the weekend.

I've had a some help with the Baxandall article and it should look a little better in the next few hours.

--
Bill Sloman, Sydney

We have a winder, but need an fixture for small bobbins. I have the full range of materials, including my favorite Kapton tape, in various widths. It's the painful magnet- wire enamel removal and soldering to tiny bobbin pins that slows me down. That'd be an even worse issue with extra windings. But I heard a rumor about a special magnet wire with disappearing enamel at soldering temps.

It'd be nice if you'd add a downloadable pdf version.

--
 Thanks, 
    - Win

The professional coil winders at Cambridge Instruments had "solder pots" - little iron pots of molten solder with a tight-fitting lid with a very smal l - wire-sized - hole in the top. You poked the end of the enamel wire into the hole and it came out tinned. I was decidedly envious, since I had to c lean off the enamel with a sharp knife or a chunk of emery paper. If I'd be en closer to their benches, I'd have walked my prototypes across to one of the pots, but scraping was quicker.

That's exactly what it is. On my machine Firebox calls up Adobe Acrobat to display it and that offers you a down-load option. When I checked just now, it picked up the new - tidier, if marginally bigger - version.

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Bill Sloman, Sydney

I have posted this before, but...

I've found that high-ratio boosts can have problems, so I like to use dual-winding inductors as autotransformer flybacks. Lots of people make these things in various sizes. They are cheap enough that you could even parallel some.

I pushed that inductor some, and did get a little warm, so I added some copper to heat sink it.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Perhaps a half-bridge forward converter. The 12V will be easy to drive into the primary, and turns ratio of 1:40 is not a problem in a forward

Cheers

Klaus

John beat me to suggesting coupled inductors ... I got distracted simulating one.

An boost inductor charged for 8uS @ 12V, will discharge into 500V in

8uS * 12V/(500V-12V) = 200nS. That's pretty fast! And quite a whack for your rectifier and filter components.

Driving the tap of two coupled inductors in series lets you double the discharge time, halve the discharge current, and halve the switch's voltage rating.

It's kind of a nice option for low-power high-ratio dc-dc converters, short of resorting to a forward converter with a custom transformer.

They're jelly-bean, e.g.:

Cheers, James Arthur

I think Win is charging a capacitor. Forward converters are very inefficient when heavily loaded, namely when the capacitor voltage is low. Most of the voltage drop is in the mosfets or the transformer copper.

An ideal flyback can be 100% efficient over the entire charging curve.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Those dual-winding coils are good for all sorts of things. I should test a few for breakdown voltage.

Oops, I posted a pre-release version of that HV supply. Here's the real thing:

I did the PCB layout myself! The HV clearances and thermal issues added some interesting complexities.

I have the Spice file for that one if anybody is interested.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Forward converters are not inefficient. It's just a transformer fed buck converter, so in the beginning of Wins charge cycle, he of course needs to start with 0 duty cycle

The halfbridge MOSFETs never see more than the supplied voltage, so they can be selected to have low RDSon, whereas the flyback has a reflected voltage so the primary side MOSFET must be selected for a 2-3 times higher voltage

The flyback is a power per cycle topology, so boosting more current is not possible without changing the transformer

Cheers

Klaus

Beldsol, thermal strip. I got a high-temp Metcal clone tip from Amazon,

that solders this stuff great.

I hate custom magnetics. Expensive, boring hassle. The technical challenge of designing transformers is to NOT design transformers.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Well, even my poor inductors can do that well, the issue is high-frequency core loss. I'd rather use commercial inductors than a custom. Looking over available stuff, I've been discouraged by a lack of the right info. But newly-designed parts tend to work better, maybe because the newer smps ICs are running faster, to over 1MHz.

Anyway, yes, coupled inductors are awesome. I've used a few in recent designs and have been accumulating a collection of candidate parts for experimenting.

Right. My test universal-supply PCB layout predated my interest in coupled inductors, but I'll shoehorn one in there to see how it works.

--
 Thanks, 
    - Win

The forward converter needs an additional load-side inductor, and input current limiting, to be efficient. And two mosfets. A current-mode flyback switcher is simpler. Forwards are usually used for higher-power apps.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Search ebay for Beldsol wire

I got my #28 cheap from that antique dealer in Virginia.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com