# Programmable power supply

I have been assigned to design a kind of programmable power supply. Can't decide on a way to make it. Need help with this.

This is a high level block diagram of the required system:

How it works (How it should work)


The system will be given 0 to 10 Volt variable input, which decide output voltage. The output should be proportional to the input. So that when input is Min, output is also Min (i.e. 0v), when input goes to Max, output also should be Max (i.e.24V). Simply, if the input voltage is V_I, then the output voltage should be (24/10)*V_I

The output is drawn from the systems power supply, not from the input voltage. (Actually input and output should be isolated.)

I hope that part is clear.

Then there is another input (input 2). Let’s think about it as a Potentiometer. So that its value can vary between two values, for the sake of explanation, lets say it changes between -10 and +10.

Its role is like this. (I’ve mentioned it as tolerance.)

Output value is decided from the input 1. That value is changed a bit around the original value by this input 2. For example, if output is 10V now, I can increase it a bit by rotating my preset clockwise. Or decrease it a bit by rotating it counterclockwise. Maximum change, lets say, will be 10% of the set value.

That’s it. I tried to explain the thing to the best I can. If any clarifications are needed please ask.

This is the requirement. I’m very good at microcontrollers, But not much experienced in electrical side. I thought a lot to make this using microcontroller. May be there is, but can’t figure out.

But it doesn’t need to be microcontroller-based, this is an actual requirement. So if there is any way to achieve this, then I’ll move to that.

Please share your knowledge. If anyone can point me in an exact direction that will be much appreciated.

• Sumudu - Comment on whether the following makes enough sense to allow you to proceed. || A "high side switch Q1 (probably a P Channel MOSFET provides power from Vin (Input Power supply) to output. Q1 can be driven on and off as required by a uC (microcontroller). This can be eg by a npn transistor Q2 driving the gate of Q1 so that as Q2 is increasingly turned on then Q2 is also turned on. The voltage Vout is measured by the uC with an ADC and Vcontrol is measured with an ADC and the two are compared. The uC changes the drive to Q2 to maintain Vout = 2.4 x Vin. – Russell McMahon Jul 24 '12 at 10:53
• The same job can be done very very very simply with a single opamp and a hi side P Channel MOSFET and a few resistors. Vout is scaled by 2.4:1 by resistors and fed to an opamp input. Vin is fed to the other opamp input. The two are thus "compared" and the opamp output drives the P Channel FET to control Vout. A comparator might be used instead of an opamp but linear rather than on off control may be desired, making the opamp probqbly the best choice. – Russell McMahon Jul 24 '12 at 10:57
• Thank you Russell for your quick reply. To be honest, I don't have much practical experiences on FETs. Of course I've learnt about them, but haven't used. So please be kind enough to explain in a bit more details. Please consider me as a novice. I'm learning things through these projects. Also I think you haven't considered about the second input, to do fine tuning to the output. (change the output value by (+/-)10% ). – Anubis Jul 24 '12 at 12:01
• Actually, I’m a third year undergraduate (UOM, Sri Lanka) and currently I’m having my industrial training (Part of the course). At the location where I’m training (Brandix Textiles), they've faced a problem which needs this kind of solution. There is no one here with relevant knowledge. So they've asked me whether i can help. It won’t help, but just to tell about the background. – Anubis Jul 24 '12 at 12:02
• @Russell: Your simple opamp and high side linear pass element would work fine if he didn't need the output to be isolated. That makes things rather more difficult. – Olin Lathrop Jul 24 '12 at 12:59

What you want is called a power supply amplifier or PSA in the business. You are essentially looking for a power amplifier with a gain of 2.4, although it only needs to drive in one direction but has to be isolated.

Think of how a power supply works. At some point there is a feedback signal indicating how high or low the output is and a reference that indicates what that feedback signal should be. The controller looks at the difference and tweaks the output up or down. You can control the output by changing the reference signal.

Isolation makes things more difficult. Sometimes this comparison to the reference is performed on the isolated side with a simple digital high/low indication transmitted back thru a opto to the hot side. The system still works as I described, but the reference is on the isolated side which is not where you want it.

This leaves two possibilities. Communicating the reference to the isolated side and doing the comparison there, or communicating the actual output level to the hot side and comparing it to the reference there. Both schemes have some merit, but I'd probably pick the first in a DIY project.

The best solution is to simply buy a PSA if you can find one with the right characteristics.

• Thank you Olin for the great info. You made me aware of some important points. I'll try your ideas. Of course there will be many problems, sooner or later, I'll post them too.Thanx a lot..(PS. This is a great forum!!!) – Anubis Jul 25 '12 at 5:13