I almost finish with my Half-Bridge SMPS design. I'm now designing the output voltage feedback to the controller stage.

The controller stage drives both MOSFET transistor through an isolation transformer and the power supply for the controller stage is a separate 12V-50Hz transformer so there is a complete isolation between the controller stage and 220V power line.

Now my question is: Do I need to isolate the output voltage feedback using opto-isolators? I know I have to isolate from the 220V power line because of it's high voltage but do I need to do it for my output voltage? The output is 35V 10A max.

The book Power Supply Cookbook from Marty Brown explains I need isolation from the output stage to the control stage but doesn't explain why.

Here is a simple block diagram where you can see the control stage is completely isolated. In my case I can't power the control circuit from the same secondary because voltage goes from 1V to 35V.

enter image description here

  • \$\begingroup\$ This is a good question. If I understand you correctly, the controller is on the secondary side. But, could you post a schematic of what you're trying to do or a block diagram (even if it's partial)? \$\endgroup\$ Aug 21, 2014 at 3:34
  • \$\begingroup\$ The controller is powered from a separated simple 50Hz transformer so is completely isolated from the primary and secondary of the switching transformer. \$\endgroup\$
    – Andres
    Aug 21, 2014 at 3:38

1 Answer 1


Completely isolating the controller from both primary and secondary sounds strange.
Here are the common approaches.

Controller on the primary side. The secondary side is isolated. The output power flows through the transformer. The feedback signal is brought back to the controller on the primary side. Typically, an opto-isolator used to bring the feedback signal back to the controller across the isolation barrier.

enter image description here

Controller on the secondary side. The half-bridge remains, of course, on the primary side. Since both output voltage and controller are on the secondary side, the feedback signal doesn't need to cross the isolation barrier. However, the gate control signals need to cross the isolation barrier from the controller to the Half-bridge. Gate-drive transformers or opto-couplers are used for that. The controller also needs an auxiliary power supply be able to start up.

enter image description here

Block diagrams taken from: http://powerelectronics.com . The block diagrams show flyback converters, rather than half-bridge. But, they still represent the isolation schemes.


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