HV isolation DC/DC conversion - please help

Yeah, this is for a more permanent installation. The conventional transformer/bridge/cap was the next option. But I'm going to need 30. That's the problem. I was thinking small ferrite toroidal cores, each wound with a bunch of turns for their secondaries. Then I could loop a

30kV insulated wire through them all and charge pump it. But who supplies cores and winding services?

Unless I can find off-the-shelf small form factor isolation transformers. Any ideas?

The project is an IGBT switched Marx-generator for plasma ion implantation. It charges caps in parallel and switches them in series. Hence, you get a HV step-up only during the pulse.

I do infact use fiber optics to control the switching. It's driving the IGBT's that need energy and the drivers need to float in between the caps during the pulse.

If its rated at 6kV, I woudl not attempt to run it beyond that even for short periods. Things like resistors and transformers have ratings based on thermal limits (although resistors sometimes voltage too). But here, the situation is very different.

Can't use just get a transformer wound and use a conventioal bridge/capacitor/regulator? I don't think getting a transformer woulnd to do that would be hard. Transformers are regularly made to produce much hgiher than 30kV, so one could be wound. But it would not be cheap.

Is it a one-off experiement, or something expected to last? If the former, and the consequences of failure are not large, then perhaps just taking a chance might be sensible. But 30 kV is a lot more than 6kV, so I would not have too much hopes.

Are you sure you need to transfer energy across the isolation interface, or only information? If the latter one, you could feed the receiver with a battery, and use fiber optic at the information channel, or just a homemade optical link.

I find it curious that you need isolation only during 10 % of the time. Maybe with more information we could find a solution not by answering your question, but by approaching the problem in some other way.

How about a solar cell and a headlight bulb? Crap efficiency, but isolation off the charts - you've got a few inches of vacuum (and/or gas fill) and at least an eighth of an inch of glass working for you, and then you can put the solar cell further away for more isolation (at a reduced power output).

Matt Roberds

For a measly 30, DIY winding is far cheaper than paying a service.

Nominally, a DC supply is used to do the charging; high voltage resistors (or resistor chains) are used to connect the plates in parallel. Then at the ground electrode, a small arc is produced to break down the series capacitive chain. This small arc is usually configured as a coaxial insulated wore in the ground electrode; the tip of the wire at the "tip" of this main electrode. Light pipes may be used to transport the UV from the initiating spark to the other gaps to speed up and help initiate ionization. No isolation is needed. Check out how a standard Marks generator is configured.

Replace the sparkgaps with IGBT's then look at IGBT switching requirements and you'll see what I mean.

Good idea, I considered this too. Unfortunately, when considering spacial constraints, number of solar cells needed, power requirement per unit, I think this one's out the window.

Thanks Joerg,

I'll look into it.

JT

Hello JT,

If this needs to look professional and you really have to make 30 looks at series resonant conversion. The "air gap" could be 1/4", 1/2", or whatever you need. But there doesn't have to be air in the gap, you could use a nice insulating material.

Probably pot core halves would be easiest here. For ease of winding you can often buy split bobbins for these. Another hint: De-burr everything.

Regards, Joerg

How about these:

- Use a battery in series with a phototransistor as the "output" side, and an LED as the "input" side. Perhaps use rechargeable batteries and recharge them when the equipment is otherwise idle, then open the recharge circuit with a high-isolation switch when the equipment is in use.

- Use a "flying capacitor" - charge up a cap from the "input" side and then use a DPDT relay or equivalent transistors to switch it over to the "output" side.

Matt Roberds

Thanks for the suggestions Matt. In the short term, I think batteries are the way to go. As for the flying cap - I like it, but I ran some numbers, and for a 1h experiment, I'm going to need a

1.68F, 18V cap for every switch. I found some at Vishay, physically not too big. I'll have to think a little bit about the most convenient charging method...But a 1.68F cap gives me the creeps. Isn't this all a little unorthodox?

Stpid Question:

Why is it not possible to run the IGBT drivers off the not-so-HV voltage present across each IGBT switch? Presumably, you need bleeders.

If not, you can stack transformers with lower isolation ratings and feed the one on ground potential. The advantage is that the ground capacitance decreases up the stack, which is probably what you want, and losses/regulation becomes progressively worse.

This is easiest done by toroids and HV-Wire - the supply is taken off the toroid, one loop of HV wire goes throught the toroid for the next stage and on loop comes from the lower stage. You end up with two loops of wire through each core except the top one (and you just might want to connect the conductor of each HV cable loop to a potential betweent the two stages, to deal with voltage distribution and providing a convenient place to split the primary when something blows up).

Making the secondaries resonant can help to improve regulation.

You Understand??

Alternatively, use a flyback topology with a primary winding made out of HV wire and the many secondaries being ring cores. Good regulation, but capacitance will hurt more and more as you go up the stack.

In any case, I would buy some ready-made toroid inductors and design around those for a first shot.

Hello JT,

Unitrode App Notes are IMHO the best in that respect. Now via Texas Instruments.

Regards, Joerg

If you're going to run it for a long time, rechargeables are probably best, but an alkaline D-cell is something like 15 Ah @ 1.5 V at low load currents. Since you need 0.1 Ah, you could run your experiment for over six days straight on a set of batteries. A set would be 300 batteries at around $1 each. Or, you could use carbon-zinc cells for about half the capacity and half the cost.

See your local car stereo shop. Theirs may only be rated to 15 V, though. If you spend a few more bucks you can get one with a l33+ bar-graph voltmeter on top.

Cooper Powerstor capacitors might also work, except that you'll have to stack them up as they are only 2.5 V each.

If you don't like the cap, I just had another goofy idea. How about a small permanent magnet motor being run as a generator. Couple the motors to a common shaft and use a small AC induction motor to drive the lot.

Probably. I think the only other ways to go are to wind your own transformers, as has been discussed, or change the design.

Matt Roberds

You can drive IGBTs directly from a tramsformer, some experimenting is of course needed. Have a look at how a conventional current transformer works, with luck a dazzling flash of inspiration will hit you.

As the doctor might say, take two Farads a day...

In article , Robert Baer wrote: [...]

You use the *wrong* coil winder or you aren't nearly well enough paid.

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