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Please comment: 10A 13.8V linear power supply

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thinkfat:
Hi,

this was about the first project I did when I started going into Electronics as a hobby (again), I had an old 10A power supply that I used for HAM radio stuff previously but it failed, and since I wasn't happy with it anyway (very susceptible to RFI) I decided to do something with the case and the transformer. The original PSU used that standard-issue LM723+2N3055 design, but wiring was a mess and I figured some of the RFI problems came from that. I did the schematics, simulation and PCB with EasyEDA.

I don't quite remember if I had an underlying idea for the design, I think I cobbled it together from bits I found online and from application notes on the LM723.

The board is in 2oz copper, nowadays I'd probably just tin-coat the traces that need higher carrying capacity.

I only recently got around to assembling the PCB after having it laying around for months (years, even) and it seems to work, but I could not test it fully because the heatsink is not yet mounted. Anyway, it basically works, as far as I can see, but still I'm thinking about possible improvements.

Looking forward to your comments.

Kleinstein:
Current sharing just with the base resistors is not as accurate as the more normal emitter resistors. The resistors at the collector side do not make much sense.

The OP amplifier part for the current sensing is also odd - the amplifier should work with gain larger 1. This way the resistors are less critical in effecting the CMRR.
I think one should rethink the current sensing together with the position of the 0.1 Ohms resistors in the power stage.

David Hess:
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.


--- Quote from: Kleinstein on August 26, 2019, 03:56:20 pm ---Current sharing just with the base resistors is not as accurate as the more normal emitter resistors. The resistors at the collector side do not make much sense.
--- End quote ---

This is the first thing I noticed.  Move the collector resistors to the emitter side.

duak:
I agree with Kleinstein about R23 to R26 - they should be in the emitter leads instead.  I think you will find that the PNP pass transistors will not share current equally and some will get much hotter than the rest.  Worst case, one or more will probably fail shorted.

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?

One of neat things about the 723 regulator is that when the integrated current limiter is used with the sense resistor between the pass transistor(s) and the output connection it responds very quickly and protects the pass transistors and sometime the load too.  In this circuit, a delay is imposed by the opamp.  It should't be a big problem, but an accidental direct short can damage a pass transistor faster than the current limiter can respond.

I would add a high current diode across the output connections to prevent the supply from seeing negative output voltages.  These can occur when connecting supplies in series.

If you put an LED between the two CLIM  tags it should light when the supply is in current limit.  You might need a high efficiency LED because the current will probably be quite low.  It's probably a good idea to also add a 1K resistor from U2-2 (CL) to ground to handle any leakage current and also provide more current to the LED when in current limit.

I'd also add a thermal switch on the heat sink to shut off the supply if it gets too hot.

R18 has to dissipate at least 200 mW.  Is it sized correctly, say 1/2 W?

thinkfat:
Thanks for the comments!

I already noticed an imbalance in heat dissipation, the misplaced load sharing resistors explains that. I wasn't able to do a real temperature simulation, the transistor model I found for the power transistors doesn't have that (or I didn't notice).

Funny thing, those base resistors were not really meant for load sharing, I just put them to lower the power dissipation of the TIP31C a bit. Ah well, I didn't know better.

The opamp circuit for the current limiting was added because I couldn't figure out a way to use the built-in limiting circuit of the LM723 without it. It seems it's just a transistor that pulls down the input of the internal output stage, the limit pin being the base of that transistor.

The current limiting is not very clever altogether, but that's probably in the nature of the LM723, it will just reduce the output voltage once you get beyond the set-point to keep the current stable, and that will of course increase power dissipated in the pass transistors. I'll have to think about a protection there - temperature sensor on the heat sink probably, as you suggested.

I guess it's a good opportunity to redo the design, I wanted to move it to Kicad anyway and to figure out how simulation works there. I'm also thinking about the mechanical construction, I think I'll flip the pass transistors around so that I can mount the PCB onto the heatsink, with the transistors under the PCB. Would make for a more compact design.
 

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