Im trying to design 36W constant voltage SMPS circuit that has an 24V DC mean output voltage and it will be adjusted 22V to 28V by potentiometer. I have a limited experiment about it. And Im using circuit design tool to design smps. I created 3 different design for 22V, 24V and 28V output. Feedback circuits of them are below.

22V 1.63A Circuit 22V 1.63A

24V 1.5A Circuit 24V 1.5A

28V 1.28A Circuit 28V 1.28A

As scheme seen that feedback resistor(Rfbt) change between 32 kohm to 42 kohm. At this point ;

  • I removed regulation circuit that is at feedback block(I don't know why I need it or not)
  • I added 32K resistor and 10k variable resitor

Variabled feedback circuit

Is this a proper way to adjust output voltage? Or is there any good way to do it?

There isn't enough article about variabled smps or I couldnt found. I need an advice from experts for this feedback circuit


No, you want to hook up the pot as a variable resistor, not a potentiometer to ground. Remove the connection from the end of the variable resistor that you have going to ground, and hook that to the wiper of the variable resistor like this:


The way you have it drawn puts the "bottom" resistance of the pot in parallel with the lower feedback resistor, which will definitely cause unwanted behavior.

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  • \$\begingroup\$ Good catch! These supplies have little voltage tracking range (if they use a boost winding), so errors in the feedback loop can be fatal to the MOSFET. \$\endgroup\$ – VTNCaGNtdDVNalUy Aug 2 at 1:19
  • \$\begingroup\$ @VTNCaGNtdDVNalUy Exactly- The OP seems to have demonstrated that the circuit works over the range he needs, but with the pot hooked up as shown it would definitely have caused problems. \$\endgroup\$ – John D Aug 2 at 18:09

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