Current Limiting/Control in power supply

[jamessinghal]

Hey eevblog,

My main question is How do mainstream (BK precision/ Rigol) bench power supplies allow the operator to adjust the current without just using a potentiometer?
I am designing a power supply for my lab, and have the voltage control side of things mostly figured out, but the current side of things is where I cannot find a way to regulate it.
(My minimum requirements are 0-36v and 0-5a)
Also, is there a good circuit that incorporates both voltage and current control into one, instead of two separate parts.

Thanks,
James
 

[Cerebus]

Here's a quick simulation of the classic CV/CC design. The op amps are ideal ones, which can lead to some crazy looking values like that 49kV at the output of the current error amp. This is not a practical circuit, just a demonstration, and there is much more to be done to get to a practical power supply design including better choice of component values, but it is enough to see how both Constant Current and Constant Voltage control can co-exist. The secret is the analogue 'OR gate' formed by those two diodes. Have a look, think about how it works, download the simulation and try it in LTSpice.

Sorry, for some reason I can't get the usual "insert the image full size" trick to work, you'll have to click on the image below to enlarge it.

[T3sl4co1l]

^^

Alternately, you design the circuit to deliver one quantity primarily, and, using an error amplifier (an opamp), vary that quantity to control the other quantity.

Okay, that's pretty vague, but usually, you'll be delivering current, and adjusting that current to control voltage.  It works the other way around, too.  That's all.


The textbook switching supply operates this way, as does any linear or switching regulator that can be ganged in parallel (without problems with current sharing).

Practical example:
https://www.eevblog.com/forum/projects/building-a-simple-switching-circuit/msg1252706/

The output current is controlled, based on a reference input ISET, and sensed by the feedback IBATT.  Battery current will never be higher than the maximum value of ISET.

To adjust output voltage (or, in this case, LED current actually), all we need to do is adjust ISET to get the voltage we want.

So, ISET is driven by the voltage error amp.  ISET can never be higher than the maximum output voltage range of that amp (i.e., VCC).

Thus, even if the voltage error amp demands 100% full throttle, the current error amplifier simply sees -- oh, I need to set this current, yup, here we go.  And that's it -- it's fully linear, always under control!

To make an adjustable current limit, all we need to do is limit the maximum ISET value.  This can be done with a diode, or more precisely with an active rectifier circuit.

This applies to linear regulators where the pass device has a current output signal: LDOs (common emitter/source) are typical, but it is a good design method for followers (common collector/drain) too.

By the way, the tidbit about connecting supplies in parallel: current sources can be connected in parallel with no problems whatsoever; voltage sources cannot.  If we build our power supply out of current source blocks, we can wire as many as we want, in parallel.  They're all commanded by the same ISET, and we're done!

Tim

[cowasaki]

There is a EEVblog video explaining this in great detail......

Here is part 1 -

[lukasz.kostka]

Hi.

I ve build such a circuit and voltage control works great. I have a problem with current sensing. I am using LM358 op amps. Configured them as a differential amps with 10x gain. Matched resistors as close as I could and op amp output stays ~ 740 mV. As far as I know LM358 is not a rail to rail op amp. Which means that it can't output e.g. 10 mV. I was looking for a MAX4080F but no one sells them in my country.

I am building a linear power supply (controlled by atmega with DAC, 10mV 10mA resolution for me is fine) and I can't find a good IC that can measure voltage drop across a resistor (in my case 100 mOhm). I want to sense current and read it with a ADC.

Can you point me to a good IC or just say what parameters are important for this IC and I will try to find one ?

[Cerebus]

Hi.

I ve build such a circuit and voltage control works great. I have a problem with current sensing. I am using LM358 op amps. Configured them as a differential amps with 10x gain. Matched resistors as close as I could and op amp output stays ~ 740 mV. As far as I know LM358 is not a rail to rail op amp. Which means that it can't output e.g. 10 mV. I was looking for a MAX4080F but no one sells them in my country.

The LM358 will go to its negative rail, but it has highly asymmetric current sourcing/sinking ability. The sinking side is just an internal constant current sink, and it's quite small (-50uA from memory). So, to get the LM358 down to the rail with any significant load you need to supply a pull down yourself - easy if you've access to a lower voltage, not quite so easy if you haven't.

I am building a linear power supply (controlled by atmega with DAC, 10mV 10mA resolution for me is fine) and I can't find a good IC that can measure voltage drop across a resistor (in my case 100 mOhm). I want to sense current and read it with a ADC.

Can you point me to a good IC or just say what parameters are important for this IC and I will try to find one ?

You'd be better off starting a new thread for this. It's probably going to be unhelpful to make suggestions without seeing a full-ish schematic for what you have so far. Once thing that is certain, you need something with relatively low input bias currents, simply to avoid injecting current into the circuit you're trying to measure.

[lukasz.kostka]

The LM358 will go to its negative rail, but it has highly asymmetric current sourcing/sinking ability. The sinking side is just an internal constant current sink, and it's quite small (-50uA from memory). So, to get the LM358 down to the rail with any significant load you need to supply a pull down yourself - easy if you've access to a lower voltage, not quite so easy if you haven't.

So I need a negative voltage for this. I have a transformer with multiple taps and I can get a negative voltage out. Does this approach with diff amp good enough for my requirements ?

You'd be better off starting a new thread for this. It's probably going to be unhelpful to make suggestions without seeing a full-ish schematic for what you have so far. Once thing that is certain, you need something with relatively low input bias currents, simply to avoid injecting current into the circuit you're trying to measure.

Shall I start a new thread in beginners section ?

[Cerebus]

Shall I start a new thread in beginners section ?

Best to, it's generally considered 'bad form' to hijack someone else's thread with a question that's more than incidental to the original question, especially on a board like this one where there's no restriction stopping you starting your own thread.