I have some components here to make an SMPS from mains for a CV/CC power supply for my workshop, and I decided to add a linear post-regulator to smooth the output. To avoid big heatsinks I wanted the SMPS voltage to follow the post-regulator voltage when I adjust it, and I've found a possible solution from this question. However my simulations showed better results while using an OpAmp (image below).

As you can see, I set up the comparator of the SMPS in a way that the output voltage of the post-regulator will determine the output voltage of the SMPS (keeping it always 5V more then the post-regulator output for example, precision is not a problem). The circuit works fine on simulation, however I wanted to know if ripple, noises or imperfections of hardware that doesn't exist in my simulation might make this a bad idea, and what filters should I study to put on the output of this OpAmp (if any) to avoid any problems with this "follower" circuit before I buy the components I need.

I have another comparator not shown in the image that is doing the current sensing. enter image description here

  • \$\begingroup\$ I've seen this post, I even added an Hiperlink to your answer Dave, but before finding your aswer I had success with this OpAmp implementation, does it work as well as yours? By success I mean in simulations. \$\endgroup\$ Aug 21, 2020 at 22:18
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    \$\begingroup\$ Your circuit makes the input voltage proportional to the output voltage, while mine gives it a constant offset. \$\endgroup\$
    – Dave Tweed
    Aug 21, 2020 at 23:56
  • \$\begingroup\$ Impossible to explain it better. Dave, what happens if the post regulator voltage goes below 0.7V? In my simulation the current stops flowing from the power transistors and start flowing through the current mirror, keeping a constant 1V at the output. \$\endgroup\$ Aug 22, 2020 at 2:05

1 Answer 1


Your simulation has more feedback gain but is overkill if you know the dropout max for your LDO. It may reduce your load regulation error which is not much, but will also reduce phase margin and overshoot of pre-reg since Vout1 has a reactive load with delay and any surges on C1 add more delay.

  • \$\begingroup\$ Hmmm so in real life I'll probably face a bigger phase margin and bigger overshoot from the SMPS then, makes sense. \$\endgroup\$ Aug 21, 2020 at 22:56

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