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

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H713:

--- Quote from: magic on July 18, 2020, 01:28:28 pm ---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.

--- End quote ---

Kyocera did this in one of their receivers back in the day... sounded good, but it had an annoying tendency to blow up. I considered it, but I my experience with doing this in power amps has been that stability is even more tricky than with the EF triple.

At this point, I think the BJTs will be an easier option. I suspect that I can get away without using a triple, if am willing to pass more current through Q8. With the MJE15032 I am using here, I anticipate this will be just fine.

David Hess:

--- Quote from: magic on July 18, 2020, 01:28:28 pm ---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.
--- End quote ---

Bipolar transistor beta and fT drop at high currents so that sort of makes sense although I cannot say that I have seen that configuration.  A composite transistor with power MOSFET driven by a bipolar transistor is common though to boost the transconductance of the MOSFET.

Pawelr98:

--- Quote from: magic on July 18, 2020, 01:28:28 pm ---A readily apparent problem is the need for higher supplies.

--- End quote ---

Not really a problem.



The pump charge can produce the required extra voltage without any extra winding, using existing transformer.
No special elements required, just plain 50Hz charge pump.
Zener may be added  for limiting the max voltage though.

With this extra voltage the Vdrop is at maximum the Vcesat of BJT which is ~0.6-1V or almost nothing for mosfet transistors.
During normal operation the unregulated voltage can be barely higher than the output voltage, providing good efficiency.

Tap switching is one thing to consider.
With simple comparators and maybe some logic gates it shouldn't be a problem.
SCR preregulator can be used but has to be filtered well.
There's also main capacitor switching.
Lower capacitance means lower mean voltage which in turn means lower average power dissapation.
Ripple will go up and this has to be accounted for when testing and when setting rules for the switching.

Series resistors somewhere within the design is not efficient but does a decent job at protecting the pass element.
I like to use those when testing "rough" loads which can be unpredicible.
But this I only used for HV 150-400V DC supply. 0-300ohm 25W wirewound potentiometer that goes in series with the transformer secondary.
If the load is too great then the main capacitor voltage will sag, along with OCP limiting the current.
By adjusting the resistance I can allow higher voltage at certain currents.

If one searches through some military surplus parts then finding 1ohm adjustable high power resistors is possible.
But the same can be implemented by a combination of relays/transistors and normal high power resistors.

H713:
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. I should be able to get it quite a bit better than it is now, and I suspect I can lower the value of C4 considerably, which is always nice since the current limiting is a bit more effective.

Stabilizing the current limiting circuit was not easy. I had initially thought that I could get it to behave with a small cap from the output of the op-amp to the inverting input, but all this did was lower the frequency of the saw-tooth oscillation. Ultimately the only thing that did anything useful was the series RC circuit consisting of C1 and R18. This of course made the voltage regulator part of the circuit a little squirrelly. A 470 pF from the output to the inverting input of the voltage regulator op-amp seemed to make things worse, so this may come down to careful tweaking of values. I've never had much success with simulating or calculating compensation capacitors, as it is difficult to account for parasitic properties of components. I'm sure some of the real analog geniuses (who are a whole lot smarter than me) are able to calculate / simulate values and have them work perfectly.

A need for higher supplies would not have been an issue. An easier solution to the charge pump is to simply spec a slightly higher voltage power transformer. One of the goals here is to try and minimize how much noise and ripple the regulator circuit has to reject. I don't care how efficient this thing is as long as it can still happen in a 2U enclosure (probably from a Crest CA9). Switching converters are efficient, but the noise they introduce can be difficult to shut up. I thought really hard about using a buck converter as a preregualtor, but I decided that I'd rather deal with slightly reduced efficiency by using tap switching than have to worry about that noise making its way to the output.

magic:

--- Quote from: H713 on July 18, 2020, 07:53:28 pm ---Kyocera did this in one of their receivers back in the day... sounded good, but it had an annoying tendency to blow up. I considered it, but I my experience with doing this in power amps has been that stability is even more tricky than with the EF triple.

At this point, I think the BJTs will be an easier option. I suspect that I can get away without using a triple, if am willing to pass more current through Q8. With the MJE15032 I am using here, I anticipate this will be just fine.

--- End quote ---

--- Quote from: David Hess on July 18, 2020, 10:47:15 pm ---Bipolar transistor beta and fT drop at high currents so that sort of makes sense although I cannot say that I have seen that configuration.  A composite transistor with power MOSFET driven by a bipolar transistor is common though to boost the transconductance of the MOSFET.

--- End quote ---

As usual, lots of creative analog technology is found on DIYAudio.
For example, John Curl from Parasound expressed preference for that kind of topology in the first few pages of this thread; it looks like the motivation in his case is just a general preference for FETs combined with the difficulty of protecting FET outputs from shorts. So that's another commercial product which uses such output stage and they make claims of good reliability. What kind of sorcery goes into making it work, we don't know.

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