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

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Kleinstein:
For the high power level one can hardly get around tap switching or some preregulator (e.g. SCR or switched mode).
Tap switching can be electronic with the pass-elements in series. So the transistors would only see a reduced voltage. This can be easier on the soa.

It depends on the topology if the extra drive voltage is  problem. For the usual floating regulator it is not.

H713:
Agreed, DiyAudio is the playground for some very talented analog designers. For an audio amplifier, I wouldn't hesitate to play with such a topology. The big issue is probably less about oscillation and more an issue of thermal stability, but it's probably no worse than the EF triple in that regard.

For any sort of lab power supply below 37 volts, it's really hard to justify doing this kind of thing these days when you could use a uA723, LM317 or other IC to control it. Sure there are some challenges to work around (like the high reference voltage in the 723), but these are easier problems to work around than the challenges that come from doing it with op-amps and transistors.

I still don't love my current limiting circuit, I'm sure there's a more elegant way to do it. The traditional way (which is how the 723 does it) is great, except for that the vbe of the current limiting transistor changes with temperature. As a result, I find that it's both touchy and drifts like crazy. If anyone knows of a more elegant way to do what I'm trying to achieve here (and maybe one that's easier to stabilize), I'd love to hear about it.


magic:
TLV431 is sort of transistor with more thermally stable (but also higher) Vbe.

Kleinstein:
The usual topology for a lab supply (especially with more than some 25 V) is the floating regulator. Here current regulation is naturally with an OP.
The main positive aspect of the simple Vbe type current limit is the simplicity and speed - so it could still be used as an additional fast emergency limit.

The lm317 is not really helping for lab supply and the lm723 is not much more than a possibly (some versions) good reference and a poor OP with access to the compensation. Using OPs and a separate reference is not more complicated. One has to think about the compensation anyway.

David Hess:

--- Quote from: H713 on July 19, 2020, 05:12:19 am ---Here's the test setup so far. Seems to work okay, there are a few changes that I need to make in order to optimize stability and ripple rejection.
--- End quote ---

A base-emitter shunt resistor should be added across the 2N3055 output transistors.  Otherwise there is no way to remove charge turning the transistor off which implicates the closed loop stability.  Darlington transistors have this shunt resistor built in for exactly this reason.  If emitter ballast resistors are used, which should be done also for current sharing, then the shunt resistor can encompass the emitter ballast resistors also.

The alternative is to drive the base of the output transistor with a class-ab driver instead of class-a like you have.

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