I am researching about SMPS, specifically AC/DC flyback converters.

In the datasheet for the switching controller (Fairchild FSCQ0765), it refers to an auxiliary winding. What is that? All the current transformers I've looked at only have a primary and secondary winding.

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1 Answer 1


The auxiliary winding is the one that is not a primary or secondary.

In this case it provides power and a timing signal (synchronization) to the controller- since it's a quasi-resonant controller it needs to be in tune with the transformer.

  • \$\begingroup\$ What kind of transformer provides an auxiliary winding? I have looked at isolation transformers, as well as high frequency current transformers, and both these only have a primary and secondary coil. \$\endgroup\$
    – tgun926
    Jul 6, 2014 at 2:16
  • \$\begingroup\$ Usually these are custom transformers, not off-the-shelf, unless someone happens to offer a transformer for this particular chip. \$\endgroup\$ Jul 6, 2014 at 2:20
  • \$\begingroup\$ Most of the flyback controllers I've looked at have some sort of auxiliary winding. Does this mean homebrew SMPS's are almost impossible to make? \$\endgroup\$
    – tgun926
    Jul 6, 2014 at 2:34
  • \$\begingroup\$ You can wind your own transformer. The number of turns is usually reasonable, and you can fake the gapping with a spacer at the cost of a bit more EMI. Small quantities are more of a problem than one-offs IME. \$\endgroup\$ Jul 6, 2014 at 2:41
  • 1
    \$\begingroup\$ @tgun926 Here's a thread, which deals with finding transformers for SMPS. In general, it's safe to assume that off-the-shelf transformers for SMPS are... scarce. \$\endgroup\$ Jul 6, 2014 at 3:57

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