Hi,

I am an electronics student designing my own lab power supply, and I need some help with a couple of things. My main concern is: does the concept look good enough to start building a prototype on a protoboard, and how do I calculate or estimate the capacitors so that the supply doesn't oscillate?

Main power supply components
For my main DC power source, I will use a 220V-24V 108W LM100-20B24 switching power supply. I know that a toroidal transformer would be better, but I don't have the money to spend on one so i chouse this option. I want my power supply to have constant current (CC) and constant voltage (CV) modes of operation. I took inspiration from R. Lawrence's High Performance POWER SUPPLY UNIT and incorporated similar CC and CV regulation plus a couple of other things. The main difference in my design is that I will use a REF02 for my voltage reference and an ICL7660 for the negative voltage rail. The power transistor that I chose is the TIP120

Power supply design



My power supply will have two potentiometers: one to set the current limit and one to set the voltage. A diode from the output of U4 to the
non-inverting input of U1 is there to switch between CC and CV modes. I added capacitors to the feedback loop (known as an integrator) on the op-amps so that if the operator changes the potentiometer value, the op-amp output will change linearly instead of instantaneously and start resonating.

The output stage consists of a TIP120 power transistor and an NPN transistor that is meant to prevent faults from happening when the negative voltage rail decays (don't quote me on that one — I added this part from R. Lawrence’s circuit). Voltage is measured by a simple voltage divider, and current is differentially measured through a shunt resistor. The measurement signals are fed back to the inverting inputs of the op-amps.

Lastly, there is a CC mode indicator that consists of a simple PNP transistor and an LED.

If you have any advice or ideas to improve my design, feel free to drop a comment. If there are any questions, I will try to answer them as best as I can. I am thankful in advance for any advice.

This is actually my first post anywhere, so if there’s anything I could improve about the format or how I wrote it, I’d love to hear your feedback. Thanks!

Have a great day!
Domen

 

Is the output filter capacitor the one shown in the load?

How are you generating the negative voltage, as your switching supply has only one output.
The Lawrence circuit uses a tap and rectifier circuit off the AC transformer for that.

If you use Rail-Rail type op amps you may be able to avoid the requirement for a negative voltage.
That would also give you a higher max output voltage by a volt or two.

What were the results of your circuit simulation?

 

Yes the output capacitor is for filtering and not part of the load

I generate the negative voltage with the ICL7660 IC.

When I run the simulation the regulation runs fine (CC and CV regulation works).
I will add ltpspice schematic. For the schematic to work you will need to add the potentiometer. To set the value of the potentiometer you must set the val parameter in a range from 1-99.

Do you have any recommendations for a rail to rail op-amp?

Is the output filter capacitor the one shown in the load?

How are you generating the negative voltage, as your switching supply has only one output.
The Lawrence circuit uses a tap and rectifier circuit off the AC transformer for that.

If you use Rail-Rail type op amps you may be able to avoid the requirement for a negative voltage.
That would also give you a higher max output voltage by a volt or two.

What were the results of your circuit simulation?

 

I generate the negative voltage with the ICL7660 IC.

Sorry, I missed that in your post.

Do you have any recommendations for a rail to rail op-amp?

Looking more closely at the circuit, I see that the current-limit won't work properly without a negative supply, so there's no reason to go with a RR op amp.

One relatively inexpensive 36V RR op amp is the OPAx197.

 

In my simulation I did find an oscillation when the current-limit engages.
You need a 10kΩ resistor from U5 out to U4 in for stabilization, as the original circuit had (R21).