As [Sam Ben-Yaakov] points out in a recent video, you don’t often see flyback converters these days. That’s because there are smarter ways to get the same effect, which is to convert between two voltages. If you work on old gear, you’ll see plenty of these, and going through the analysis is educational, even if you’ll never actually work with the circuit. That’s what the video below shows: [Sam’s] analysis of why this circuit works.

The circuit in question uses a bridge rectifier to get a high-voltage DC voltage directly from the wall. Of course,  you could just use a transformer to convert the AC to a lower AC voltage first, but then you probably need a regulator afterwards to get a stable voltage.

The converter operates as an oscillator. The duty cycle of the oscillator varies depending on the difference between the output voltage and a zener diode reference. These circuits are often difficult to model in a simulator, but [Ben] shows an LTSpice simulation that did take a few tweaks.

As he mentions, today you’d get a switching regulator on a chip and be done with it. But it is still interesting to understand how the design works. Another common flyback circuit used an oscillator driving a CRT for the primary, more or less. If you want to learn more, we can help with that, too.