current limiting for power supply

[akis]

Hello

My bench power supply (over 35 years old) has died and I have decided to build a new one.

I am trying to implement a current limiting device based on a sense resistor. In addition I would like to be able to select the current limit level with a rotary switch, eg 50mA, 100mA, 250mA, 1A, 2A, 3A.

I am trying to use a very low value sense resistor to keep the voltage drop low. Suppose that I use 0.05R, then at 3A I will have 150mV.

I would like the circuit to limit as soon as the voltage drop across the sense resistor is 150mV and I would like the limit to be quite rapid. In other words I do not want to see the output voltage starting to drop as I approach 150mV, I want it to be sudden.

So I tried to use a comparator and an op-amp, just to experiment. I do not know how to wire up a comparator so that it conducts at 150mV (or at any other arbitrary voltage difference). It seems the comparator will conduct at the most tiny voltage difference.

I also tried an op-amp but that also fails because my circuit is absolute voltage dependent, eg it behaves differently depending on the absolute voltages present at + and - inputs of the op-amp, even though the differential voltage is the same.

Can someone please post any simple circuit I can experiment with.

Thanks
Akis

[Andy F]

Hi:

http://www.diodes.com/_files/product...zetex/an39.pdf

On page 23 you can see a good and simple example.

[AndrewT]

you can insert the adjustable current limit upstream of the output voltage regulator.
If the +ve supply uses a PNP or Pch device then the control voltage moves to the upstream side and is ideal for integrating a current limiter.

[akis]

Now that you have shown me that there are such things as "current monitors" I have experimented in simulation with the chip INA170 buffered by an OPA340.

The problem however still remains with the current limiting action being too progressive and affecting the output voltage well before the limit is reached or well after it has been exceeded. I use an NPN transistor to short the voltage reference when the current limit is reached.

Is there a way to achieve a swifter current limitng action?

[AndrewT]

yes,
comparator (393) as you mentioned earlier.
An opamp can be made to operate as a reasonable comparator.
You must set up a ref voltage at one of the input pins.
Then when the monitor goes above or below ref voltage the comp swings to the opposite polarity triggering the limiter. A little positive feedback (big value resistor) can insert some hysteresis to remove the tendency to flutter.

[cliffforrest]

"Is there a way to achieve a swifter current limitng action? "

Do on search on "fold-back current limiting".

[Aucosticraft]

Would you mind if the design is Switching voltage controller? If you dont mind with switching supply. you can easily design one using LM3524.

[akis]

Originally Posted by AndrewT

yes,
comparator (393) as you mentioned earlier.
An opamp can be made to operate as a reasonable comparator.
You must set up a ref voltage at one of the input pins.
Then when the monitor goes above or below ref voltage the comp swings to the opposite polarity triggering the limiter. A little positive feedback (big value resistor) can insert some hysteresis to remove the tendency to flutter.

In simulation still, I removed the negative feedback from the op-amp / buffer that follows the monitor (OPA340), used a reference voltage on its negative pin, and now it works very nicely.

Except it oscillates / flutters. Say the current limit is at 100mA. As soon as the limit is breached, the op-amp goes full positive, the limiter cuts the driving voltage and the output drops. That makes the current fall well below 100mA, so the monitor drops and the op-amp goes to the ground, causing the limiter to stop limiting and so on.

Basically the act of limiting causes the error condition to go away and we go into a vicious circle.

I have tried the positive feedback trick but that does not help as the underlying condition (output 100mA) is being completely removed when the output voltage drops.

Any way I can make it more stable ?

[tomchr]

Some suggested using a comparator for the current limiter. I would advise against that. The gain of a comparator is rather high at the transition point and the supply will chatter as you're describing.

I suggest lowering the gain by using the op-amp as an amplifier rather than a comparator. You may also have to make the feedback frequency dependent to avoid oscillation.

Franco Chapter 8 would be a good place to start.

~Tom

[cliffforrest]

Originally Posted by tomchr

Some suggested using a comparator for the current limiter. I would advise against that. The gain of a comparator is rather high at the transition point and the supply will chatter as you're describing.

I suggest lowering the gain by using the op-amp as an amplifier rather than a comparator. You may also have to make the feedback frequency dependent to avoid oscillation.

........
~Tom

I would beg to differ! This is precisely what comparators are FOR; never being operated in linear mode.

Hysteresis from +ve feedback is the usual remedy, and, as I suggested earlier, check out fold-back limiting which has its own useful characteristics.