How does this non-isolated buck converter work?
Very inefficiently: The 0.22uF cap and 100 ohm resistor create an RC oscillator, periodically turning on BUX85. The 4.7mH inductor smoothes out the voltage somewhat, and the zener tries to make it a regulated 15ish volts. Altogether it's an inefficient and dangerous design. Good luck getting it to pass UL.