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

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Xnke:
So normally in this situation, a pair of 723's would be used to get adjustable voltage and current limiting. I don't have any, AND I have PNP transistors for pass elements. I had a bit of time to sit down and play with this tonight and came up with the following:



Still need to figure the bleeder resistor on the filter capacitor, but it's not terribly important at the moment.

Input voltage is nominally 16V at 20A load, and output voltage is adjustable around 12 volts.

But, in this situation how should I add adjustable current limiting?

Yes, I know, use a pair of 723's. And ditch the PNP pass elements. Sure, sure. But, I have everything in the existing circuit to hand, and it's nice to be able to get by with what's in the parts bin.

So, how would YOU add adjustable current limiting to this? I'd like to be able to dial the current down to less than an amp, or up as high as 20A, with overcurrent situations behaving gracefully instead of crowbarring the output to ground and blowing a fuse, which is what I see commonly.

Mazo:
You should add frequency compensation or this thing will oscillate.On the current sense problem you should decide first on the type of current sensor(a shunt or smth else?)If a shunt is okay then you should choose low or high side placement.If it were to me I would regulate the "negative" to still use the PNPs that you have on hand but getting the stability of emitter follower topology.If you insist on that schematic you should derive a current signal somehow and then probably just pull down the vref node to gnd based on the Isense signal and the setting of the current(a two op-amp solution is possible probably) that you want (another potentiometer)

Xnke:
Frequency compensation will be needed-that is for sure. The 2N6051's have a minimum Hfe of 750, so some significant gain is available in the error amp loop for oscillations to occur.

I like the notion of pulling Vref down to ground based on Isense, a transistor in parallel with the Vref diode, (I will probably use a green LED or something, I've got a few zeners in appropriate values but the LED works fine.) and use the output of the Isense error amp to turn on the transistor, shunting current away from the diode and once 3-4mA is shunted off of it, it will shut off and the transistor's collector would become the voltage reference. This would tend to limit the lowest output current available to whatever the output voltage would be when Vref is a Vce when the transistor is at saturation. This may be high enough still to cause a problem-I don't know yet.

I have thought about using one of the .33 ohm emitter ballast resistors on the pass elements as my current sense element-as the current through the resistor rises, the voltage drop across the resistor rises. So measuring the voltage drop across the ballast resistor and comparing it to the potentiometer setting would generate a current "error" that could be applied to the Vref node.

Using the Vref node in this manner I probably should drop the green LED and use a 6v or 9v zener diode, as this would give the current regulator more headroom to pull the current limit around. The again, the more room I give it to move in, the less likely it is to behave the way I expect it to...

I think by implementing the current limiter in this manner, any oscillation that developed would be current limited too, so if some outside influence or a deteriorating component in the frequency compensation network were to cause an oscillation, the current limiter should be able to cut the output current down enough to prevent damage to the pass elements. Maybe.

Going back to frequency compensation, I thought I had a good start on it, with the 10uF cap across the feedback loop to ground. There may need to be a miller compensation cap from the collector of the pass transistors to the positive-input of the op amp, but I will need some help figuring out how to figure that value.

iMo:
This may work. You have to play with compensation, if required.
Simulation only.
The input voltage is an example, you need at least 18V thus it works.
I would add a transil (15-16V) to protect the output against peaks.
Also set the R_SHUNT as you need (now it limits at about 28A).

Adjustable current limit: to create a precise setting is not easy, you would need an additional circuitry. That would involve a propagation delay, you have to deal with then.
Or use a 1k trimmer (instead of the R12=100ohm) in the base of the current limiting transistor. Calibrate.
See another threads on the efforts in that regard. You may copy THIS as the current limiting transistor in LM723 is wired the same way as the Q5 in my schematics below.

No warranties of any kind are provided, use at your own risk :)

Kleinstein:
The circuits with the output from the collector side usually  need a rather large capacitor at the output so more like >1000 µF than 10 µF. Compensation can still be a little tricky if fast response is wanted.
The problem is not the current gain (Hfe), but the voltage gain added by the output stage. The output stage is more like setting the current.

The circuit from imo might want a different OP, that gives a higher output voltage to get away with less input voltage.

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