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High Voltage Bench Power Supply Design

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duak:
Fair enough AD.  No reason comes to mind other than different current ballasting resistors for different ranges.

I remembered a design idea in EDN for a wide range load where paralleled FETs were used and the gate drive switched in and out for high/low ranges.


H713:
I also found that there is a "sweat spot" range for FET voltage rating. Too low, (100V like the IRF510), and if obviously has minimal SOA, but too high (like the FQA8N90C and the 2SK3675), and they seem to perform rather poorly at 120V in the linear region compared to the IRFP450s. Some of this may be due to the age and product family of the transistors, but I think that 900V parts are not designed to handle as much current as parts in the 500V range, and I suspect that this is hurting their performance here.

Really, I think parts like the IRFP460 and probably IRFP360 like Duak mentioned are probably the best option. A lower power version could be built using IRF640s, they performed reasonably well (they don't have DC specs, but as we have seen that means little), but the TO-220 package is a limitation. They were used by BSS Audio in the EPC 760 and EPC 780 power amplifiers from the early 90s as output devices, and they are a good part. For a 0-500V 0-100mA supply, they would be pretty comfortable with about 6 in parallel. The FQA8N90C performed considerably worse in this application, blowing up at or before the IRF640s did, and costing over twice as much.

IMO, range switching drastically improves this situation, as a 200V @500mA output (perfectly reasonable for many applications) would result in a total dissipation of 200 watts. That's a lot of heat and comes out to about 32W per transistor. That doesn't seem bad, but that makes for a big heatsink, noisy fan and and efficiency of just 33%. Using something with a pair of 120V primaries would solve this issue, at least in North America. In the low range, the primaries would be wired in series (as if wired for 240 operation), while in the high range they would be wired in parallel. This transformer doesn't have to cost of a fortune as a result. In civilized parts of the world, you would want something with a pair of 240V windings on each side.

While we're on the subject of the power transformer, I know there may be people who worry about the use of a control transformer wired backwards to get a pair of 120 windings on one side and a pair of 240 windings on the other. It is fine in most cases to use a transformer backwards (secondary as primary) so long as the voltage is correct. That is, applying 240V to a 120V secondary will saturate the core. When this happens, the inductance drops like a rock and the winding will pull heavy current and burn up in short order. By contrast, applying 120 to a 240 winding is fine, though you need to keep your copper losses in mind.

The really nice way of doing this is to use a variac driven by a servo motor for this. I've seen this done on some really big magnet power supplies from the 70s. With a smaller variac (rather than about 500 pounds worth of them), it could be pretty fast-tracking. Something for someone a little more ambitious than me. That could, in theory, drastically increase the possible output current and make this quite an efficient power supply.

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