Need input on high voltage regulator design

I'm really not sure. There's still a doubler after the secondary winding that's going to introduce some loss. The bigger problem is finding the right transformer. Winding a custom one isn't an option. I'm currently using a $3 CCFL driver transformer - it's compact and works pretty well.

Scott

How would you set one of these up in a high voltage circuit? And wouldn't any shunt regulator still be pretty inefficient?

Scott

Yeah, that's why I'm trying to get rid of it. I know the GM tube doesn't draw much current - it spends most of its time not conducting at all. And the polypropylene capacitors seem to hold a charge pretty well - I've been zapped by one that had been charged and accidentally left lying on my bench for at least half an hour.

For this application, 500 volts. Some tubes take 900 volts. I'm sticking with 500 volts because it requires fewer components, most of the tubes I'm considering are 500 volts, and Electronic Goldmine's selling tiny 500 volt tubes (I'm using one in testing right now) for $35, which is a decent price for single-unit quantities.

Scott

I might leave in a shunt regulator to keep it from exceeding 550 or 600 volts in the event of a processor lockup, as long as the leakage below that threshold isn't much. The idea is to charge the output capacitor to the tube's operating voltage, then scale the duty cycle WAY back to just make up for leakage and whatever the tube uses. It can tolerate a fair amount of ripple, the load is relatively constant, and I think a simple loop running a few hundred times a second should be adequate to maintain regulation. Not sure of the exact algorithm I should use, though.

Even without a regulator, I don't think this circuit is capable of generating more than about 600 volts from a 9 volt input. Not enough to cause any damage. Of course, I've been wrong before - a previous, higher-voltage version of this circuit managed to arc through an FR4 circuit board, and as far as I can tell it shouldn't have been able to do that.

Scott

The neon lamp tubes? I've seen that done. I'm using 250v varistors instead. Doesn't seem like it would be any more efficient - it's still a shunt regulator, and will put a constant drain on the PSU even when the GM tube's not conducting, right?

Scott

Hey, that's pretty slick. I'll have to try that. I know the A/D will work with 10K impedance. I was also thinking of trying a comparator, but the buffered A/D scheme only uses the one input pin, and at < 50 cents for the TL081 that sounds like a cheap, flexible solution. Though it'd be nice to find some cheap carbon film resistors in > 10 megohms to reduce the parts count. The high-value thick film resistors go for about $4 at Digi-Key.

I'll do a full writeup on this project when I get a chance. Most of the hobby kits and published designs I've seen have been pretty simplistic, with just a speaker and maybe a meter, and the kits sell for about $150 and up. Mine's got an LCD display and serial interface, and can be scrounged together for under $50 in its current form.

Thanks for the input...

Scott

I don't know how tight your voltage spec is, but would a separate sense winding do it?

One of the circuits I based this on had a similar setup, but it just kept the primary driver disabled while it was on. I tested it and it didn't seem to do much good. I may experiment with this some more, I'm just a bit worried that having only the single reference might make it hard to maintain regulation.

Seems like the shunt would still be burning off power even in that setup - though I guess it would depend on what kind of current limiting resistor I put inline. Is that right?

There's still the matter of the limited lifespan on the varistors... not sure they should really be there at all.

Right. The version I built for myself ran off two AAs with a charge pump to drive the LCD and MCU, but going with 9 volts lets me replace that with a much cheaper linear regulator and reduce the number of multiplier stages.

Thanks,

Scott

I'll take a look.

Isn't it? It's using a transformer with isolated windings. Doesn't that make it an isolated flyback converter? Or does that not count because it feeds back into the +9v rail through the shunt?

Scott

Gah... I think that went a bit over my head. You're saying to just use an appropriate transformer to get the required output voltage without the doubler? Not something I really want to do for cost and time reasons - the CCFL transformer is a cheap, off-the-shelf part.

As for the primary driver, I replaced the one originally specified in the design (no longer made) with a BS170 MOSFET set up as a low-side switch. Not sure how that would affect what you're suggesting.

Scott

Indeed it does... draws about 40 or 50 ma, with < 10 of that being the MCU and display. Didn't know that about MOVs - one more reason to find another design.

Thanks,

Scott

I'm luck if my local RS has ANY parts. No problem, I order from Digi-Key almost every week, and I have 10Meg 1/4W resistors by the hundreds. Use 'em for feedback in MCU clock oscillators.

I'll probably try out the direct version and see how much noise it's got. Would a simple RC network make an acceptable filter if there's a lot of noise on the op amp output?

Thanks,

Scott

The MCU's supply, which is provided by a 5-volt linear regulator in this case. It's fixed internally, can't select a different reference.

Sounds kind of tricky. With the voltage doubler stage after the secondary, how sure can I be that what I see on the primary will reflect the actual output voltage?

This is more like what I was looking for. Fred Bloggs posted something similar. Between the two examples, I ought to have enough to keep me busy experimenting for a bit. Just need to check my parts bins for an appropriate op amp, or go pick one up.

Thanks...

Scott

Yeah, it's way high. I can reduce that by 2 ma by replacing the regulator, and drop the remaining draw by a huge amount just by setting it up to use low-power wait modes and reducing the system clock to something more reasonable. The only reason it's so high is that I wrote the code in a hurry as a proof-of-concept and never got around to making it efficient. Didn't seem worth bothering with when the rest of the circuit was drawing far more juice.

The LCD draws around 1 ma I think, though it originally had its backlight on permanently, drawing waaaay more. Had to desolder a couple of resistors to turn it off. I've got more appropriate displays, I just happened to have a drawer full of these that I picked up for like $1.50 each.

If I can find an appropriate case, I'd prefer to go with 6 AAs. Or even 3 CR123As, but that gets expensive. I don't like 9v batteries.

Scott

Ok, so I checked my parts bin and found a TL082. And now I see that it's a DUAL op-amp, so I get the part about using the other one as a filter. Duh. Ok, time to go experiment a bit.

Scott

Not serial, standard HD44780 parallel. I've got it running in 4-bit mode, write-only, so it takes 6 I/O lines. I just checked my old emails - it was a special from Electronic Goldmine, part number G14023, $1.49 each, back in January. No such deals at the moment that I can see. I think I bought 10 - should have gotten a few more.

Scott

That sounds about right. My first design used a tube from an old Civil Defense meter. Those things usually took B or lots of D batteries and didn't last too long. I see counters today running for thousands of hours on a single battery. Just wish I could find a schematic for one of those supplies.

I've got a couple suggestions and an op amp to play with, so I'll probably see what I can do tonight.

Scott

Ok, thanks... I'll do some more research.

BTW, it turns out that using a 40Meg / 330K voltage divider, I was able to get feedback directly to my MCU's ADC. Now there's the little matter of figuring out the algorithm to best control the PWM. The point at which you take the A/D sample does seem to be really important - I'll have to figure out how to sync it up. Fortunately, I've got a dual-trace DSO that does a pretty good job of showing me what's going on between the PWM input and the voltage output.

A thoroughly educational experience all around...

Scott

Hi Scott,

This is frequently done with a Texas TLV431 or similar from other sources such as National. Cheap, small, available at almost every street corner but best of all, well under a uA on the ref pin. You wouldn't really need the ADC here.

As for resistors, it is customary to connect some in series in HV applications. It would be hard to obtain anything below 5% when you are above the 10M region.

Regards, Joerg