DC-DC isolating converter

How 'bout an opto-isolator? Transformers just don't work at DC.

George Herold

In my work (biomedical research instruments) I've had to design my own, including custom-made transformers. This is tough when your quantities are really low.

If you find anything (particularly with low capacitances and low leakage currents) I'd sure be interested.

-f

Err.... it might not be practical, but in high voltage installations, it's not uncommon to have a motor/insulated coupling-belt-shaft/generator to get a really well isolated power supply. 10 MV isolated power is almost always done this way.

"Interwinding capacitance" can be reduced by using C-cores and keeping the windings for the primary and secondary distant one from another, of course, or by putting a foil shield into the recipe for winding.

Thanks to everybody! I have two more issues:

1. Projected volume is 1-5k/year (off-the-shelf solution is very desirable)
2. It is hand-held instrument (it will be difficult to "sell" motor- generator isolator to our mechanical guys) I am still looking. Most of the converters I've found so far have hundreds pF in-out capacitance. I need something in the order of single pFs :o(

What you need is a shield between the primary and secondary windings. You should be able to get the capacitance easily down to a couple pF that way. I once made a small DC-DC converter transformer with quadrifilar wire to make it easier to wind, as I was going to use a 1:1:1:1 winding ratio. It started blowing the power driver chip, and I discovered to my horror that the interwinding capacitance was something like 400 pF! I re-did it with 4 separate wires on a pot core with a split bobbin, separating the 2 agressor windings from the sensitive winding, and got the capacitance down to tens of pF with no shield, just the space built into the bobbin. I know the ultra-isolation transformer guys get down to single pF on multi-KVA transformers at 60 Hz through the use of shields. You then have capacitance between shields, and that may also be a problem in your application.

I have no knowledge of specific models, you may have to make your own converter. Analog Devices has some "chips" that provide small amounts of isolated power for sensor excitation and signal conditioning. You might check these out, I believe they do have low capacitance at the expense of a lot of leakage inductance. I saw some "x-ray" pictures of what is in them, and I would expect the capacitance to be down to a couple pF.

Jon

No voltage was specified. Look for "medical supplies", some have less than 15 pf.

greg

e.

Check Analog Devices, Agilent and Linear Tech if you haven't already done so. Also, there was a recent article in Power Technology magazine at