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High Voltage Bench Power Supply Design

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H713:
So I simulated and redrew the schematic for series operation (this what you had in mind for the configuration?)- seems to simulate well. In addition to stability, the other big advantage of this is that I can use slightly lower voltage FETs- which means that IRFP460s or other old MOSFETs will work for this design- even in a shorted output, no device will have to drop more than 170 volts. I think I will add one or two more FETs just to improve my safety factor. Tomorrow I'll try to test series operation to see how it behaves before redrawing the board. Not thrilled about needing to do so, but a bit of reading suggests that other people have had lots of trouble paralleling HV FETs. Oh well, the old boards will be useful for something or other.

duak:
H713, may I suggest adding a 10K resistor between the junction of U4-2 & the 1n capacitor and the wiper of RV1?  This will present a more constant resistance to the RC network formed with the 1n capacitor and make the feedback more predictable.

You may also find that Q1 may not completely shut off when U4-6 goes to maximum -Vout.  The bottom end of R3 should probably go to GND.

H713:
I spent some time today testing IRFP450s at the voltages they would see when used in series. Even with 175 volts across it I was able to draw about 850mA.  I can only imagine that IRFP460s will perform even better. Interestingly, the 2SK3675 performed quite poorly. Good thing I only payed $1.50 each. I think that the 500V Hexfets are probably going to be the ticket for this, though I still have more testing to do. If I figure that the FETs are good to ~800mA, that gives me a safety factor of around 1.7. That goes up if I go from using four series devices to five or six. Even with four pass elements, the safety factor of 1.7 is a worst-case-scenario number for a power supply set to 500V with the output shorted and with perfect load regulation on the DC bus- in practice it will be better. I still have to do a bit more testing before I feel confident in the design, but so far things look good. 

Again, no FET will have to drop more than 150V across in in the worst case scenario with 4 devices. In fact, they will never see even that because with a shorted output the supply will current limit at 500mA (or whatever it ends up being), and at that current the DC bus will drop by about 100V- that's another issue I need to address is the load regulation, and that may involve trying to find a transformer with lower copper losses. I've dumped the idea of a voltage doubler (not necessary with the control transformer I have now), and I'm going to implement range switching on the primary side so as to avoid the need for expensive high-voltage relays.

One other thing to consider- the only time the FETs should ever see 120V across them is in the case of a shorted output when the supply is set to its maximum voltage. This is (or should be) a rare occurrence. If the output is shorted while the supply is set to ~250V or so, then the FETs won't have to drop more than about 75 volts each.

I may also move the crowbar circuit off this board and onto the control PCB to save some space.

I sure would like to see the test setup where they came up with the SOA graph for some of these transistors. :bullshit: In the case of the FQA8N90C, the DC curves are optimistic by about 200%, and I had them clamped (with very good thermal compound) to a large block of aluminum- when they failed, they were gently warm. This is quite a shift from the tube world where many tubes can be pushed to 200% their absolute maximum ratings without much ill-effect.

I do hope that I can come up with a viable solution, since I'm not the first one to want a solid-state high-voltage linear bench power supply. I've read through the few threads that actually got passed the brainstorming phase. In many cases, everything simulated well, but once testing ensued the optimistic SOA ratings, current sharing and oscillation issues rendered the design impractical. Hopefully the series operation will fix these issues.

duak:
Back in my design days, some parts would require 50% derating for reliability.  The corollary is that the parts' datasheet promise 100% more than you'll ever get. eg., Motorola used to make the MJE802/4502 complementary power transistors for audio amplifiers and servos.  They were in aluminum TO-3 packages and if the environment was quiet enough you could hear them sing at the input signal frequency - very much like "needle talk" with records.  I was integrating systems using tape drives that used these transistors in the reel and capstan servos.  These transistors were failing in the field after a couple of years even though they weren't anywhere near their limits.  The common heat sink never got above 50 C.  The tape drive manufacturer changed to different devices of similar ratings but in steel cases and the failure rate dropped to nothing.  Turned out the transistor dice would partially detach from the aluminum case because of the different thermal expansion rates.

I'd like to see your final schematic when done.  I have an old Kepco DC supply with one vacuum tube in it for the pass device.  If it expires, it'd be nice to replace it with semiconductors of some sort.  I have tubes of IRFP360s (400 V Vdsmax) so putting them in series may work well.

H713:
Here's the current schematic. There are a few improvements I'd like to make and some more stability testing to do. I'd like to make the current limiting more adjustable (maybe without requiring an expensive 20 ohm rheostat?) and I have yet to run a test with 6 IRFP460s in series- the most I've done is 4 FQA8N90Cs in series, but I have no reason to believe it won't work. Oscillation is not a big issue with this circuit. It would be nice to improve the ripple rejection a little, though with a well-filtered DC bus it shouldn't be a huge issue.

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