high voltage boost DC/DC converters (5V->250V)

Hi there - at work I'm probably going to have to design a high voltage DC/DC converter sometime in the near future. It looks like it'll be something along the lines of 5V input and 250V output. Current will be under 10ma. Probably more like 1-2ma. The more regulated the better, but I will probably just regulate it down a bit with a FET to clean it up.

My understanding is that once you start to get to something like an output voltage 10x or more of the input voltage the standard boost circuit (cap + diode + inductor) no longer really cuts it. Instead, I've read that step-up transformers are a better way to go. However, when I look through Digi-Key, all I see are step down transformers for stepping down wall power, as well as various specialty transformers.

Any suggestions? Thanks!

-Michael

Reply to
Michael
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Why would you say that? What's your switching frequency?

I hacked something like this

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with a boost converter in place of the motor, and got up to 100V out: enough to light up a 110V night light with about 10V (eight NiMH AAs in series).

The inductor I used was pretty beefy, though: from a throw-away blender. Thick gauge wire.

- another Michael

Reply to
mrdarrett

Wurth

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has solutions for these sort of 'problems'. They have standard transformers which can be connected in several ways to make all kind of ratios. Getting some samples and support from these guys is never a problem.

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Reply to
Nico Coesel

On a sunny day (Thu, 29 Nov 2007 14:14:52 -0800 (PST)) it happened Michael wrote in :

This always worked for me: +5V | | | e b------------------------------------------------------------------------------------------- c any PNP | | | | | ---------------------------------- B Vout | | _---------A | A---------------a D k---------------------------------------------------------- + 250V | R2 2k7 |( | diode 1kV | | B +5 | | |(300turns | | R1 +5 | | | |-------- |( | | | | | R 2k7 | | 1/2turn) |(------------ | | c c | | | -- |( 5turns C 10uF 300V |--- b b ----| | | | |------------- | | e e | | | | c | | | |______| | |+ |- ---- b any NPN C1 22nF poly | | | Z zener 3.3V C3 10uF C2 10uF e | | R 10k R | |- |+ | | | | | | /// /// /// /// /// /// /// /// small potcore 2x any NPN calculate R1 for Vout Waveform is sine! select C1 for frequency (few kHz). connect A with A,m and also B with B. Note C2 is reverse biased. C1 must be poly (ceramics may not work right). C3 and C2 best be tantalum, the transistors Si. Typos reserved. Copyright Jan Panteltje 2007 All right reserved. Usenet patent 0x754 Released under the hardware GPL.

The only thing is that you will have to wind 300 turns or so thin wire on a coil former. Impregnate it too.

Reply to
Jan Panteltje

Small smps transformers are very easy to wind.

Here's what I designed for a similar need about 12 years ago. I used an LT1172 switcher IC in a miniDIP package.

My technician wound the transformer bobbin.

I used an RM8 core with 3B7 material, gapped (on one side) for A160 (A_L = 160nH/N^2). The primary was 10 turns of #24 magnet wire (for L = 25uH), then a layer of tape, and the secondary was 100 turns of #30 wire. So you can see it was easy to wind.

Reply to
Winfield Hill

b--------------------------------------------------------------------------------------------

|
|
|

B Vout

k---------------------------------------------------------- + 250V

| B +5

R1 +5 | |

| | | R 2k7

| c c |

|--- b b ----|

| e e |

| |______| |

| | Z zener 3.3V

R 10k R |

| | |

/// /// ///

2x any NPN

calculate R1

for Vout

coil former.

You might want to look at the articles in the "Circuits" section at

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for a few ways you can build your power supply.

Reply to
fred

I went well past 10:1 on bucks. But when the factor nears 20 it does get old. You need a ferrite transformer like those used in very small switcher wall warts. Except that you'd operate this transformer in reverse. The problem is the usual: Very difficult to obtain at reasonable prices when outside China. Sure, you could get the for a few dimes from China but only if you buy a whole pallet of them. Also, keep in mind that the flyback versions are air-gapped.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

^^^^^^^^^^^^^^^^^^^

Aha! I always have to put lots of aloe creme on my index fingers after doing too much of this winding. Else they'll split open in cold weather.

I mostly go non-gapped. Push pull or AC-coupled half-bridge. And don't mis-count. One-Mississippi, two-Mississippi, ...

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

If you really want to do it on the cheap, get a 60 Hz 120:6VCT transformer. Apply push pull drive to the 6V windings at about 100 Hz, and take the HV off the 120 winding with a full wave doubler circuit. You will want to take the drive off a Dff to insure 50% duty cycle, otherwise there will be a DC component in the transformer. There should be a reasonable looking square wave on the 120 winding.

Tam

Reply to
Tam/WB2TT

You're boosting about 10:1. He needs 50:1. High boost ratios are tough. The energy in the inductor is 0.5*L*I^2. When the fet turns off and things fly up, the winding, fet, and diode capacitance have to be charged, and that energy is 0.5*C*V^2. As the boost voltage goes up, the energy lost in those capacitances goes up as V^2. At typical numbers like 50 or 100:1, there's just nothing left for the load. The "fix" is less L, which requires a beefier, higher-voltage fet, which adds drain capacitance, and the whole thing bites you in the butt.

Using a transformer, or an autotransformer, helps a lot. Mouser and Digikey sell lots of dual-winding inductors, which can be configured as a center-tapped autotransformer: +5 on one end, fet drain in the middle, flyback from the other end.

It could be that a spiffy bipolar, like one of the super-saturating Zetex parts maybe, would have a better ratio of saturation current to output capacitance than a fet.

John

John

Reply to
John Larkin

Along the lines of page 10 in the LT1172 datasheet, top image?

robert

Reply to
Robert Latest

I think a few people do standard magnetics for Power Integrations' Topswitch family - these may be suitable. Also look at transformers for photo flash/strobe charging - you may be able to find these off the shelf

Reply to
Mike Harrison

Close, that's a 20-30V to 5V step-down, whereas mine was a 5V to 220V step-up. BTW, I see they specify a 1:3 transformer, whereas wouldn't turning it around, making 3:1, be more appropriate?

I used a 1:10 ratio, so the 220V output corresponded to a 22+5 = 27V flyback swing, plus a little leakage inductance spike. Looking at my old RIS-177 drawing, I don't see a clamp or snubber. Hmm, seems to have worked well all these years, but I do know better. Note, they suggest a 25V zener plus a fast diode.

Reply to
Winfield

Well, of course, modulo that. I need some 1kV/10uA, preferably from a single AA (for an insulation tester; I'm tired of lugging around the mains-powered one), but I'm so swamped with other things that I'll probably never get around to it.

robert

Reply to
Robert Latest

1000:1 is a bit of a challenge for the transformer, all kinds of painful issues, like excess winding capacitance, etc., may come up, especially at low power. Not to say it can't be done, or hasn't been done, but I'd be inclined to first step up from 1V to 5 or 10V, then go the rest of the way with a flyback transformer.
Reply to
Winfield

Hi Winfield - so is the standard path to wind your own for prototypes, then have a custom transformer wound when you hit production? I had been hoping to find a readily available transformer (as my quantities will never be high enough to warrant a full custom run of transformers, but will be high enough to make me scared at the thought of hand winding all of the buggers). Thanks!

-Michael

Reply to
Michael

There are hordes of custom transformer shops out there that'll make quantities as low as 5 pieces, at affordable prices, hoping you'll come back for more later. Who can say, your product may take off.

Reply to
Winfield Hill

Electronic photo flash units commonly work off 3 or 6 volts, and deliver several hundred V DC. You should be able to find a schematic for such, or even fix a junked flash unit; maybe from a disposable single use camera.

Tam

Reply to
Tam/WB2TT

family - these may be

A bit too pricey for my taste, and single-sourced. I'd rather roll my own.

to find these off the

And those for CFL backlights since those have HV windings.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Whenever I looked I found them to be rather expensive, even at 1k qties. Then when my clients went into production and we sourced it all in China the cost of a custom made dropped to 10% or so. Why is that? Somewhere in between there must be somebody trying to make oodles of money. So I never buy more than a handful.

Zetex makes really nice transistors. Unfortunately mostly single-sourced, but good.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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