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0-70V, 0-5A Lab Power Supply Design

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graybeard:
I see a couple of fundamental flaws in your schematic.

The emitter of Q6 is directly driving of M1 through M4.  There is no current path to keep that transistor turned on.  You need to add some sort of current path to keep Q6 in it's active region.  Perhaps a resistor from gate to source across the MOSFETs.

You have the four MOSFETs in parallel.  In SPICE where they match ideally this will work fine, but in real life the MOSFETs will not match.  You will need to include a small source degeneration resistor series with the source of each MOSFET, and you will need to do your best match the devices on a curve tracer.  You might have better luck replacing the MOS series pass devices with NPNs, each with a emitter degeneration resistor to help mismatches and prevent thermal runaway.

There may be other flaws, but those two stuck out at me.

H713:
The simulation file is a rough topology. No doubt there will be numerous compensation capacitors that need to be added as well, and I suspect that I will need grid stoppers to stop this thing from going ballistic into capacitive loads. The need for a gate-source resistor is a good point- I had initially drawn the schematic using NPN transistors and neglected to add this when I decided to switch to FETs.

I couldn't be bothered to add the source resistors to the LTSpice simulation since its main purpose was to verify the voltage and current controls. Source resistors will be employed in order to minimize issues with current sharing. I was initially planning on using rather generous source resistors of about 1 ohm, which would then be mounted to the heatsink.

I also considered using BJTs for the series pass transistors, but the performance wasn't as good. They are still on the table, the MJL21194 being the most likely candidate. Suitable BJTs will be more costly than FETs, require significant current to drive the base (most BJTs of this power level have a relatively low hfe), and have a smaller SOA than the FETs that I have been looking at. As mentioned previously, in testing the FQA8N90C, I was unsuccessful in killing it, even with 6 amps at 45 volts (or something like that). Numerous times I dumped the entire 65 volt, 8800 uF capacitor bank into the FET under test, with a peak current of something like seven amps, and it survived. More testing still needs to be done on the FETs to ensure they can take what I want to throw at them, however, they performed better than the BJTs in the testing I have done thus far.

I'll probably test a 2SC5200 and an MJ21194 just to see what they can realistically take, but as of right now the FETs look far more interesting. Many linear audio amplifiers using paralleled HEXFETs have been built, so I expect I should be able to work out the current sharing, possibly using fairly large source resistors mounted to the main heatsink.

I'm quite busy at the moment, with numerous other projects that take priority, so testing for this power supply has progressed rather slowly.

H713:
Welp, I should have listened. Even with huge 1 ohm source resistors, the FQA8N90C-F109 FETs won't share current well at all, in some cases one FET will be handling 3A while another will take less than an amp. I'm hesitant to go above 1 ohm for the source resistors, since anything above that will have to be at least of the 10W variety, ideally 25W. This starts to get pretty big and inefficient. 

With some reasonable (if not particularly tight) matching I believe one could get a pair of these FETs to share current reasonably well, and it could make for an excellent rugged and low-cost bench supply in the 2-3A range.

I'm planning to use NJW21194G BJTs ($3.55 in 10 qty at DigiKey) and I'll just have to live with using BJTs in a darlington configuration. I'm going to try really, really hard to avoid having to use a triple. Triple EFs are a pain and they can be a b!tch to stabilize, so hopefully I can get away without the second driver transistor.

I did some preliminary testing using a pair of 2SC3281s with an MJE15032 driver. My stock of 2SC3281s are pulls, and using a DMM on diode check to "kind of sort of" match the vbe, I picked the two worst. They still current share just fine with 0.22R emitter resistors. This isn't  really surprising, since BJT power amps rarely bother to match the output devices too closely, and they usually us 0.22R emitter resistors.

I still have to test my current limiting circuit, but the voltage regulation part works fine and seems to be reasonably easy to stabilize, mostly by borrowing techniques from power amp design including base stoppers, output inductor (which only helped a tiny bit) and an output zobel. This is in addition to the compensation cap, of course.

I'm going to design this power supply such that the transistor SOA can take the full 80V across them at 6A. I don't want them to ever fail. The tap switching will only be to minimize power dissipation. My plan is to control tap switching, fan speed and crowbar range through a microcontroller. I am aware that this can raise issues if the microcontroller crashes, so extra care will be needed to make sure that it goes into a safe fault mode rather than blowing up. Since I'm "digitally challenged", I'll be using training wheels on the microcontroller- an Atmega328 programmed using the Arduino IDE.

Kleinstein:
Current sharing with fets is tricky. There is a way to use 1 extra bjt per fet for feedback to ensure current sharing. The bjts form a kind of differential amp from the current sharing resistors. This is used in some supplies. See here:
https://www.eevblog.com/forum/projects/why-mosfet-is-used-as-a-power-transistor-on-the-aim-tti-ql355tp/msg3137964/#msg3137964

The bjts may still be the easier choice.

magic:
I have heard of some crazy people using FETs to drive power BJTs to increase "beta" and "fT" of the driver...
No clue how it turns out in practice. A readily apparent problem is the need for higher supplies.

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