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13.8V 20A power supply-Looking to add adjustable current limit?

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floobydust:
OP, so you've changed over from PNP Darlington to NPN power transistors and op-amps to LM723?

not1xor1, that looks reasonable and I use an aux. winding on the power transformer or a small transformer to make the control op-amp's power. There needs to be an undervoltage lockout to prevent the main output spiking as the aux. rails come up or power down at a different rate than the main raw DC.

not1xor1:

--- Quote from: floobydust on January 28, 2019, 06:51:27 pm ---OP, so you've changed over from PNP Darlington to NPN power transistors and op-amps to LM723?

not1xor1, that looks reasonable and I use an aux. winding on the power transformer or a small transformer to make the control op-amp's power. There needs to be an undervoltage lockout to prevent the main output spiking as the aux. rails come up or power down at a different rate than the main raw DC.

--- End quote ---

C17 allows slow rise of output voltage at turn-on
I fully simulated a variation of that circuit on turn-on/off via a voltage controlled switch and it looked like it behaved nicely.
I'm slowly building a first low voltage-low power (18V-1A) prototype and will soon check how it behaves in the real world.

spec:
Hi Xnke,

I see you have changed tack and abandoned the original circuit- a shame because it would have been interesting to see the development of that architecture which is great for DC performance, with two reservations about your implementation, but more difficult to compensate, as has been discussed.

I started drawing up an outline circuit illustrating one method to fit a precision (apart from a small standing current) and variable current limit to your original circuit, which may also be of of interest, even for your 723 approach, so I thought I would post it anyway (frequency compensation is not included):

By the way, with a few other mods, you could fit NPN output Darlington transistors to the circuit shown and get a conventional PSU, with an emitter follower output, which has no voltage gain and is thus easier to compensate.

UPDATE #1 2019_01_30  The transformer voltage and current indicated on the schematic are a hang-over from a previous design and are not correct for this application.

floobydust:
Bench lab power supplies frequently fail or misbehave. I've repaired so many and I'll play my harp here about the design issues that get missed. Spice sims need to include several scenarios.

Because your pass-transistors are normally full on, the op-amps needs to be up and running to back off the output.
When the power supply is switched on, there is a delay for the op-amp rails to come up, so the op-amps are late controlling the pass transistors. This causes a nasty overshoot.

When the power supply is switched off, there is a delay for the op-amp rails to falloff.
If, at the moment, the power suppy was running a heavy load, the main filter caps drop off fast and no drama.
But a light load can mean the op-amp rails falloff before the main filter caps are discharged and you get a nasty upsurge because the op-amps are powered down before the main filter caps are empty.

On the day OP needs to charge a car battery, the power supply dies:

If the power supply is off when a battery is connected, you get backfeed through the pass transistor's E-B junction. Same if the load has a large capacitance.
If the power supply is on when a battery is connected, but at a lower setpoint (say 5V with 12V battery connecting), you also can get backfeed through the pass transistor's E-B junction.

These scenarios can cause the pass transistors and/or the op-amps to fail, or your load to get damaged.

Doctorandus_P:
Power suplies as drawn with PNP transistors in this configuration are inherently less stable than with (usuallly) NPN transistors in emitter follower configuration.

If the base voltage of an emitter follower is held at a constant voltage, the output will not change much regardless of the current.

You've drawn your PNP transistors as a current source and are using the controll circuit to maintain a constant voltage on the output. This can be done, and is used in "low drop" voltage regulators, but it's more difficult to tame.

You have 3 emitter resistors to stabilize the current through your power transistors.
The voltage over these resistors varies lineairly with the output current.
You can use 3 resistors(100 ohm to 1k or so) to average the voltages over these resistors.
Then you can use this voltage for your current limiting.
The easiest way  is to use an PNP transitor with the Emitter connected to V+ and the base to the common point of these 3 resistors. This transistor wil be opened when there is >600mV over your power resistors.

But your LM324 probably can not deliver enough base current for your power transistors. Usually the power transistors are used in a darlington or shizalsky configuration.

For stabilisation you have used a simple zener + resistor. Stabilisation of this ciruit is not so good. A very nice (and well known) circuit is to not provide the current through that resistor from the input, but from the output of an opamp that buffers the zener voltage. This opamp has a constant voltate output, which results in a constant current through the restor, which stabilizes the voltage over the zener diode. You've got plenty of opamps in a lm324, so with the addition of 2 resistors for a bit of amplification you're done.

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