Electronic – Why do people hand wind flyback transformers

switch-mode-power-supplytransformer

300V Flyback converter schematic

I'm intending on building this circuit, but re-specified for an output of about 150V @ 5mA. Enough to light two neon bulbs from a 12V supply.

Why do flyback transformers such as the Wuther's seem to be either specially made for this purpose, or hand wound in the DIY world? There are numerous web examples of people hand winding these things with spooled wire onto various formers.

This circuit comes with an LTSpice model. LTSpice treats all transformers and inductors similarly, and the model does not mention things like "magnetic saturation, eddy current, flux density". The simulation works very well (it's what LTSpice was written for) so there seems to be no modelled difference to using a standard mains step down transformer. Indeed, this item available on a well known auction site clearly shows a 12-0-12 mains transformer-

SMPS driver board with mains transformer

There is a 1:10 transformer in my original above example. That's very similar to a standard 230v – 24v transformer backwards. Why hand wind or look for the elusive Wurth?

Best Answer

Flyback transformers require a very small air (or other dielectric) gap within the magnetic core structure. This helps prevent core saturation but lowers inductance. Most transformers that you're familiar with aren't the flyback type so they would saturate early and fail to function well as a flyback so you can't just swap in a random transformer from some old, no longer used electronics usually. Between difficulty finding the exact right type of transformer for your application, cost, availability, and ease of finding magnet wire, it's not terribly surprising that many choose to make their own for one-offs.

Lastly, although using old transformers from other electronics may work, it likely won't work well because there's engineering tradeoffs that are made that probably aren't anywhere near ideal for your application. In total, not only are you looking for the correct ratio of turns, you want wire thickness that will keep your resistive losses to a minimum, inductance that's suited to your circuit, and in this case, an airgap in the magnetic core.

See here for more info on why flyback transformers need the special air-gap.