I've made a simple AC to DC chopping/switching power supply circuit (I'm not sure if it's called that) as shown below. in the left part of circuit (which is the gating/chopping part) when the rectified input voltage gets higher than 20 volts, the Zener diode conducts, transistor turns on and the MOSFET turns off consequently, therefore the maximum voltage on 100uF C3 cap remains at about 20V. the right side of the circuit is a buck converter based on MC34063 based on its typical application.
the circuit works fine when tested on 25v AC input (with no load). I get 5.2volts output capable of providing up to 300mA; but when I connect it to 220v AC line the 33ohm R4 resistor burns and the PCB track along that resistor snaps off immediately, breaking the circuit. any Idea about that??
and here are some information and test results :
- the MOSFET Vds-max is 600v, the 100uF C3 Cap is 25v tolerant.
- R4 resistor is 1W, Zener diode is 20v 0.5W, the other parts are 1206/0805
- in 25v AC input test, the voltage across the C3 remains at 20.5v which is fine. no part got hot
- the parts are cheap Chinese ones. but the problem remained through various tests, various PCBs