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I'm designing a flyback power supply and I need to use an optocoupler for zero-crossing detection. The reference design I was given put the optocoupler on the MCU/low voltage side and I find that to be problematic.

I believe you should group them together with high voltage components on one side of creepage gap? I could be wrong, and I hope to get some input from experts and experienced engineers, thanks.

I have already searched and no one had asked this specific question or anything close.

Edit: by "creepage gap" I actually was referring loosely to creepage clearance and air gap, the deliberate blank area on PCB boards to separate high and low voltage components.

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    \$\begingroup\$ In you terminology is "creepage gap" synonymous to "isolation barrier" ? A sketch could help make this question clearer. \$\endgroup\$ Commented Mar 20, 2017 at 2:47
  • \$\begingroup\$ Um, actually I was referring to the board cut-outs between high voltage components and low voltage components. i.sstatic.net/a4oGJ.jpg \$\endgroup\$
    – Matt Cox
    Commented Mar 20, 2017 at 3:12
  • \$\begingroup\$ The optocoupler should be straddling the gap (slot). Otherwise you compromise the entire purpose of the gap. \$\endgroup\$
    – DerStrom8
    Commented Mar 20, 2017 at 13:27

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Generally you will have a few critical components that straddle the gap between supply and output voltage- a feedback optocoupler, a Y safety rated capacitor, the 'transformer', and, in your case apparently, another opto.

If you are having the board maker mill slots to increase the creepage distance, the slot should go under the optoisolator. See, for example, this PCB layout:

enter image description here

If that means putting the optocoupler "backwards" with respect to other chips on the board, that's just too bad.

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Generally the whole point of opto-coupler (also known as opto-isolator) is to put them at the boundary between the logic side and the high voltage side.

Further, if there is a split in the power planes or ground planes at that point they should straddle that.

From your definition of creepage slot, the opto-couplers should straddle the gap with the low side closest to the low voltage area.

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    \$\begingroup\$ Hello, thank you for your answer, I think I know what you mean, I don't have to care about where to put them relative to air gaps, even on the same side as MCUs, but I do have to make sure the optocoupler straddles the ground polygon pour and the non-ground (high voltage polygon pour, if there is one at all) polygon pour (or a lack there off). \$\endgroup\$
    – Matt Cox
    Commented Mar 20, 2017 at 3:19
  • \$\begingroup\$ Yup, but you don't want to run your high voltage traces on the output of the opto-coupler over to the logic side either. (or vica-versa) Hence they should be at the crossover point. \$\endgroup\$
    – Trevor_G
    Commented Mar 20, 2017 at 3:21
  • \$\begingroup\$ No problemo! I had that in mind when doing the designing, the low V side to digital, high V side to power source. \$\endgroup\$
    – Matt Cox
    Commented Mar 20, 2017 at 5:18

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