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Please comment: 10A 13.8V linear power supply
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thinkfat:

--- Quote from: duak on August 26, 2019, 05:29:47 pm ---
I'm not quite sure how stable the current limit will be.  I suppose the voltage between VCC and SENS is a function of output current but it will also be affected by temperature.  I expect that the current limit will increase as the temperature of the pass transistors increases because their betas increase.  On the other hand, their VBEs decrease at the same time somewhat compensating for the increase.  Have you run temperature tests on the model?


--- End quote ---

I've meanwhile redone the design in KiCad and I've run a temperature sweep with ngspice and the current limiting drifts a lot with temperature. I've since then moved the "SENS" net to one of the load sharing resistors which are now at the emitters of the pass transistors, but I found that by far the worst problem is the crude instrumentation amplifier I implemented with the TLC2272. I guess it's the input leakage current that's causing the problem. Also, the fact that the inputs are not symmetric is definitely not helping either.

Updated schematic attached.
thinkfat:

--- Quote from: David Hess on August 26, 2019, 04:54:07 pm ---You might get some ideas from checking the service manual schematics for the older Astron linear power supplies which are available online.  They also use the 723 but with an all NPN output stage.

--- End quote ---

I've looked at some schematics now for the Astron supplies, currently looking at the RS-12A (http://www.repeater-builder.com/astron/pix/rs12a-1983-11-02.jpg).

Interestingly, they're not using any shunt resistor for the current limiting, instead they're using the voltage at the base of the power transistors referenced to the regulated output voltage. What do you think about that? Shouldn't their Vbe also be quite temperature dependent? Do you see any compensation for that?
duak:
In 1974 I built and still have a power supply based on the attached circuit.  It was from the Popular Electronics January 1974  issue: https://www.americanradiohistory.com/Archive-Poptronics/70s/1974/Poptronics-1974-01.pdf

It uses a current sense resistor in the unregulated side of the pass transistor Q4.  Q1 detects overcurrent and reduces drive to the pass element by turning the 723 current limit transistor on (pin 2) .  The threshold is determined by the VBE of Q1 so it is temperature dependant.  A good thing is that it reduces current at high temperatues.

Back to the OP's new circuit - the function that Q1 performs in the Poptronics schematic could be improved by closing the loop around it with an opamp.  The opamp will have to operate with its inputs at VCC so its V+ is connected to VCC and its V- is connected to GND through a dropping resistor.  The zener diode is stil connected across the opamp to limit the voltage.

The stability of the current limit can be improved by averaging the voltages across the sense resistors.

David Hess:

--- Quote from: thinkfat on September 02, 2019, 12:37:10 pm ---
--- Quote from: David Hess on August 26, 2019, 04:54:07 pm ---You might get some ideas from checking the service manual schematics for the older Astron linear power supplies which are available online.  They also use the 723 but with an all NPN output stage.
--- End quote ---

I've looked at some schematics now for the Astron supplies, currently looking at the RS-12A (http://www.repeater-builder.com/astron/pix/rs12a-1983-11-02.jpg).

Interestingly, they're not using any shunt resistor for the current limiting, instead they're using the voltage at the base of the power transistors referenced to the regulated output voltage. What do you think about that? Shouldn't their Vbe also be quite temperature dependent? Do you see any compensation for that?
--- End quote ---

The emitter series resistors add to that voltage so the temperature coefficient of Vbe is reduced.  The current limit is for fault protection and not particularly accurate.

I think that circuit also protects the driver transistor from excessive current.

Note that as the output votlage decreases, the voltage across R4 also decreases so the current limits earlier.  This is foldback current limiting.  So with an output short, the current limit lowest.
thinkfat:

--- Quote from: duak on September 02, 2019, 10:07:15 pm ---In 1974 I built and still have a power supply based on the attached circuit.  It was from the Popular Electronics January 1974  issue: https://www.americanradiohistory.com/Archive-Poptronics/70s/1974/Poptronics-1974-01.pdf

It uses a current sense resistor in the unregulated side of the pass transistor Q4.  Q1 detects overcurrent and reduces drive to the pass element by turning the 723 current limit transistor on (pin 2) .  The threshold is determined by the VBE of Q1 so it is temperature dependant.  A good thing is that it reduces current at high temperatues.

--- End quote ---

The idea to set the current limit through R15 is a bit crude, though. The wiper contact has to bear the full load current.
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