Need input on high voltage regulator design

"Scott Miller" schreef in bericht news: snipped-for-privacy@corp.supernews.com...

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I have no idea about the varistor, how sharp their knee is. In your current setup you must have given the PWM a safe (too large) value to make sure you are getting your 500V and have the varistors do their job. Well past the knee. Adding the transistor, you could sit somewhere halfway the knees.

Perhaps software can help a bit. As soon as you reach the ~500V, and the transistor is on, turn off the drive. Turn on the drive again, not before a long period of idle time has passed, or when the tube has given a couple of pulses.

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Did you try a simple single inductor, with just one diode and cap? It seems that a small 330uH inductor and pulsing it with 25uS would also give you 500V in no time. You need a better fet of course. Or use more windings on the transformer and get rid of the multiplier ;)

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Thanks, Frank.
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...and what is wrong with using a number of cheap SMD 1% 10 meggers?

MOVs, varistors, TVSes, etc all seem to be roughly exponential, so there is no "knee". A temperature compensated 500V zener would be far better, especially if one could get one that was not noizy from nA to a few mA or so. Using such a beast as a "reference" or the high voltage drop component in a divider in the feedback might do the trick. Below 500V the drain would be zero, and one could work in the 100nA to

10uA region for rather low power loading, making it act as a shunt ergulator. If you do not need a 185C part, then I could make some for you - not cheap but not expensive either (cheaper by the thousands).

Hi Frank,

That would be a pretty scary circuit. Leakage might keep it turned on and at the other end even the slightest overshoot could fry the transistor.

Most voltage dependent devices these days are MOVs which are not at all suitable in any situation where they have to permanently conduct. Even if it's just a little. They are similar to brake pads and wear out.

Yes, but I probably wouldn't waste an ADC input on that unless it's there anyways. A comparator input should be fine. uCs with ADC on board tend to be expensive.

Regards, Joerg

Just to add to Graham's comments about doing this as an inverter application without feedback: If you were to scrap the regulator and, say, operate the whole thing from three AA cells (assuming a uC that would be happy with that) you could run this transformer bipolar. One side switched to VCC, the other to GND, then reverse.

This is off the cuff and you have to check that nothing gets fried: Since it is such a light load, maybe two uC pins could provide enough drive for that? You may need a couple of Schottky pairs to prevent spikes above VCC and below GND if the uC doesn't have enough protection. And maybe a cap or a routine that makes sure that 'rest' is either both port pins high or both low.

If the uC can't do that, maybe a stiff 74HC driver chip could.

Regards, Joerg

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My recommendation would be to look at some of the recent portable electronic photoflash designs. The application is similar and there's a great emphasis on economy and long battery life. Unless you need a very closely controlled voltage, I'd probably look at a flyback supply rather than a square wave with voltage doubler.

Norm Strong

Hi Frank,

You are right, the transistor won't fry in this application. But it would still be a mushy regulator because you are relying quite a bit on hfe. Voltage control is a lot less mushy.

$2.41 is a lot of money for a uC. But it seems that Microchip isn't charging such a premium for the privilege of having an ADC. That is different with other brands. The MSP, for example, goes from $1 to $2 if you want a converter.

Regards, Joerg

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transistor.

The resistor in the collector has to be low enough so that hfe * leakage is of no importance - and there was a 470K resistor in series with the voltage doubler ouput, so I can't imagine frying anything, really.

That rules them out, then, if it happens sooner than the life expectance of the entire device.

A comparator is okay, but sometimes they are giving away analog inputs for free, almost. A few weeks ago I selected a PIC16F88 (7 multiplexed adc inputs, 10 bit). Another version, the 16F87 is identical but only has digital inputs. The 16F88 is $2.41, the 16F87 is $2.26. That's 15 cents for the adc, not bad ;) I didn't need the adc, but since I only need to build 50 pieces, I choosed the 16F88, in case I get a crazy idea.

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Thanks, Frank.
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I use a dual 8V unit giving 17V after (some) TC.

Its not isolated because the primary and secondary circuits share a common "ground"

Cheers Terry

little dutch timebomb, tick-tock-boom.

Cheers Terry

#define zener avalanche diode

I confess, I do it too.

Actually, I once debugged some code that contained: #define x a #define y b #define d a #define q b

It had some interesting side effects. (three co-ordinate systems - xy, ab, dq. And a programmer who didnt know about unions)

Mind you, that code also had a giant static array of constants which was generated by macros (ie at compile time), including generating the variable names (it was a table of variable parameters - max, min, default etc). So a text search on the variable name cannot find the table entry. That was incredibly frustrating - so much so we ended up running the pre-compiler, keeping the output as our new source code, and throwing away all the macros.

Cheers Terry

Hi Terry,

They might not go boom in this app because there is little energy available but they'll just eat themselves away.

Question: Why is this one 'Dutch'?

Regards, Joerg

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Why not a Bang-Bang controller? How god does this have to be, anyway?

Basically, one uses a comparator with hysteresis to switch the HV Supply on/off, between two set limits. The filter capacitor will make sure that the total duty-cycle of the power supply is very low, thus power consumption will be low too. Easy.

Hi Joerg,

I have no idea where I first heard that saying, but thats how I heard it. Working for 10 years on high power electronics with a dutch engineer made it very appropriate :)

I saw a guy use a MOV as a leakage clamp in a 50W smps once. He ignored my sage advice, and consequently suffered a 100% failure rate - catastrophic failures at that, enough to write the whole PCB off (a dozen units). The re-design used an avalanche diode (which ran cool).

Cheers Terry