This nonisolated, unregulated, minimum component converter fills the void between low-power zener regulation and the higher power use of a 60-Hz input transformer. It is intended for use where)`er a nonisolated supply can be used safely. The circuit operates~by conducting only during the low-voltage portion of the rectified sine wave.
Rl and D2 charge Cl to approximately 20 V, which is maintained by Ql. This voltage is applied to the gate of Q2, turning it on. When the rectified output voltage exceeds the zener voltage ofD4, Ql turns on, shunting the gate of Q2 to ground, turning it off.