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High Voltage Bench Power Supply Design
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Wolfgang:
The crowbar is OK, maybe it needs a small cap across the resistor so it does not fire from just a small spike.
Manuall switching transformer ranges is possible, but unelegant. You could do this electronically.

Look here:

https://electronicprojectsforfun.wordpress.com/power-supplies/preregulator-circuits-for-linear-power-supplies/

for some ideas.
H713:
Thanks, I didn't even think of switching transformer taps. If I use the Tek power transformer which has a lot of windings, that could work quite well. The voltage doubler on the 2.5 ohm 155V winding (which would be put in series with two of the seven 6.3V taps this thing has) was just the easiest way to get around 500 volts- but this is definitely a better way to do it.

Electronic switching would be great, but it wouldn't solve the potentiometer issue. I could, of course, just use a fine and coarse adjustment system, which would be pretty easy to implement.

Regardless of whether I go for the electronic switching or doing it manually, I do like the idea since it reduces the stress on the FETs.

One thing that should be considered with this design is that if the wiper on the potentiometer were to go open, the power supply would jump to it's highest output setting. It wouldn't be high enough to trigger the crowbar circuit, however, because it would still be within the power supply's acceptable output range. This is a problem I've seen on most of these designs (including many tube ones). Potentiometer failures aren't super common, but they do happen and I would hazard a guess that it's more likely than having the FETs blow without triggering the crowbar. The dangers of this could be mitigated however by having the crowbar circuit change whenever the transformer taps are switched. While an open pot wiper would still be a bad situation, at least the power supply wouldn't dump 500 volts into a load intended for 100.

A current limit fixed at 750 mA would help mitigate the damage done if something were to fail. 750mA is still way more than enough to kill you, but a short probably wouldn't be quite as spectacular.

Edit: Forgot to attach schematic.
duak:
Background: I have a Kepco ABC-425 power supply (400 V @ 50 mA) that has the difficult to get 8068 regulator tube.  It was built in 1973 and except for the tube is all solid state.  I've been toying with the idea of replacing the tube with FETs should it expire.  I'd sort of known that switching FETs weren't the best choice for linear applications even though in the late 70s and 80s the SOA graphs said they should work well.  Apparently, the problem is Electro-Thermal Instability.  I've attached some files showing this a bit better.  NASA plus the FET manufacturers have more info on this phenomenon.

To get back to the OP's design:  When I look at the attached graph, I see that ETI is more likely at low current and high voltage.  This tells me that it's probably better to put the pass FETs in series rather than in parallel.  It's not quite as easy as paralleling them but it can be done with a voltage divider.

As others have said, a short circuit is bad news.  This can happen if a capacitor fails.  When I look at the latest schematic, I don't see any current limiting.  If the output is shorted or the crowbar is fired, C6 will discharge through the 15 V zener and will probably short it out. Depending on the zener's series resistance, it may momentarily increase the drive to the FETs, causing them to dump the DC bus into the load as well.

I work with Variable Frequency Drives using IGBT modules that incorporate cycle by cycle current limiting, quite often by monitoring the IGBT's source current.  Most of these drives can handle a direct short to ground without damage.  Applying this to the OP's design, I would add a fast current limiter that would monitor the voltage across the FET source resistors, and reduce gate drive if too high.  This could be done with an NPN transistor and a few resistors.  If the pass FETs were put in series as I suggest above, I would put the current limiter on the upper FET and have it latch off when there was gross overcurrent.  This could be done with a PNP-NPN latching circuit.  A more precise variable current limit could be implemented around the lower FET if desired.

For what it's worth,
001:

--- Quote from: duak on January 27, 2019, 12:46:26 am ---I have a Kepco ABC-425 power supply (400 V @ 50 mA) that has the difficult to get 8068 regulator tube.  It was built in 1973 and except for the tube is all solid state.  I've been toying with the idea of replacing the tube with FETs should it expire.  I'd sort of known that switching FETs weren't the best choice for linear applications even though in the late 70s and 80s the SOA graphs said they should work well.  Apparently, the problem is Electro-Thermal Instability.  I've attached some files showing this a bit better.  NASA plus the FET manufacturers have more info on this phenomenon.

When I look at the graph, I see that ETI is more likely at low current and high voltage.  This tells me that it's probably better to put the pass FETs in series rather than in parallel.  It's not quite as easy as paralleling them but it can be done with a voltage divider.

As others have said, a short circuit is bad news.  This can happen if a capacitor fails.  When I look at the latest schematic, I don't see any current limiting.  If the output is shorted or the crowbar is fired, C6 will discharge through the 15 V zener and will probably short it out. Depending on the zener's series resistance, it may momentarily increase the drive to the FETs, causing them to dump the DC bus into the load as well.

I work with Variable Frequency Drives using IGBT modules that incorporate cycle by cycle current limiting, quite often by monitoring the IGBT's source current.  Most of these drives can handle a direct short to ground without damage.  I would add a fast current limiter that would monitor the voltage across the FET source resistors, and reduce gate drive.  This could be done with an NPN transistor and a few resistors.  If the pass FETs were put in series as I suggest above, I would put the current limiter on the upper FET and have it latch off when there was gross overcurrent.  This could be done with a PNP-NPN latching circuit.  A more precise variable current limit could be implemented around the lower FET if desired.

For what it's worth,

--- End quote ---

I need  Kepco schematic to help You
I googled it but no results
Can You post Yours circuit diagram here?
duak:
001, I don't have a schematic for the Kepco supply.  It is working fine so I don't need it.  If the vacuum tube fails the circuit is simple enough to trace out.  I mentioned it because I have tubes of IRFP360 MOSFETs (rated for 400 V) and I wondered if I could use them for something.  I looked into using them for regulators and loads and found the research on Electro-thermal Instability.  Thank you for your offer which made be realize that my post was confusing.  I've edited it to hopefully make it clearer.

As an aside, I was thinking about the size of a vacuum tube versus a transistor.  The active volume of the tube is a few cc and the plate area is a few square cm.  The transistor die is a few square mm in area with the actual active volume just the top tens of microns.

Cheers,
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