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

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duak:
The hp 6002A is a supply of similar rating that uses thyristors to switch the transformer secondary windings.  The general design is written up in this hp Journal issue: https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1977-06.pdf  The authors go into the algorithm of how the windings are selected.  Some older hp supplies have two voltage ranges that the user can manually select, kind of like a transmission in a vehicle.  Automatic range switching is nice, but can sometimes be fooled close to the transition point(s).

I've come to appreciate the design of hp and Lambda lab power supplies.  Most use a floating regulator where everything is referenced to the positive output terminal.  This makes the voltage and current feedback loops much easier to stabilize as the error amplifier does not need to swing over the full range of the raw DC input.  There are a lot of bells and whistles in the 6002A that aren't needed here.

Foldback Current limiting can protect the pass transistors by reducing the current limit as the voltage across them increases.  Not much could be worse than to have a pass transistor in the supply fail shorted while powering a test circuit.  You could easily have two corpses afterwards.

H713:

--- Quote from: Neomys Sapiens on April 08, 2020, 09:57:48 pm ---I would refrain from digital control other than providing the set values via D/A converters.

I think that those PS that use multiple MosFets in a linear mode must have had the benefit of a builder who could ask everything from a supplier, like very very tight selections. The only appliance which I have personal experience with was a custom unit which was used to weld hermetic cases for hybrids. If one of the Mosfets did break ranks even a bit, mass extinction ensued. While it is certainly feasible, you would have to do the selection yourself, which could be costly. If you want to try anyway, APT, IH (now Renesas) and IR had appnotes on the topic.

Where I see problems is the 2HE requirement. Even assuming that you use fans with tunnel-type heatsinks, the heatsink has to have some capacity for heat which means mass, and therefore size.

--- End quote ---

I'm hoping to mitigate the issues with the FETs in a few ways. For one, I'll be using 900V rated parts with a fairly high Rdson. Additionally, I will be using quite a few of them in parallel to have as much overhead as possible. The idea is that there will be enough beef in the series pass transistors that they will survive even if the tap-switching fails. They will also have some rather generous source resistors in order to help with the current sharing.

Heatsinks are not a problem either- I currently have an Ashly FTX-1000 on my bench that is beyond repair (No seriously... it's cooked. https://www.diyaudio.com/forums/solid-state/352468-ashly-ftx-1000-failure-analysis.html#post6154245). The heatsinks and chassis may end up being used for this project, unless I come up with something better.

Also, I have plans of running the heatsinks live and not trying to isolate the MOSFETs from the heatsink, so the thermal interface should be quite good.

magic:
Maybe you really need a fancy high-spec 24V supply and another more relaxed one for 70V? ;)

As for MOSFETs, whole threads are written on DIYAudio on how to use them for amplifier output stages. It seems the most "interesting" problem is their very high fT and resulting proclivity to crazy fast oscillations.

I have actually had a surprising encounter with this problem when a simple ON/OFF rail switching FET went into tens-of-MHz oscillation during transition and coupled noise into the control circuitry which promptly turned it off. The rail never came up even though everything seemed right :wtf:

H713:
When I was testing the FETs for this project, I initially had about 1 meter of 20 AWG wire connecting it to the filter caps, leading it it to oscillate so violently that it was picked up on an oscilloscope almost 10 feet away. 1 single MOSFET, 1 meter of wire = violent oscillator. Silly me had thought "It's just one FET bolted to a heatsink. I don't need no stinkin' bypass cap!"

Yes, a separate power supply for 24V is ideal (and I do have one), however, I would like to design this to perform well at 24V.

magic:
One Bob Cordell built quite a few FET power amps and wrote a bunch about it over the years on DIYA and in his articles and books. He recommended tight layout, base stoppers and local Miller compensation of the outputs IIRC.

Also on short circuits and overload protection, there is a big MOSFET vs BJT thread on DIYA where people tested some jellybean power FETs for pulsed SOA and found them to withstand quite a lot.

The question of SOA of switching power MOSFETs recurs on this forum every now and then but the discussion usually revolves around quoting app notes because it seems few people here are crazy enough to actually have tried to push the limits of jellybean parts ;D

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