After a bit of reading I sort of understand the start up procedure of a typical SMPS. A start up capacitor is charged from rectified mains, usually via a power resistor. This cap is connected to the controller circuit supply rail. When the psu is turned on and the caps is charged enough, the controller is turned on and starts driving the main switching transformer via a power MOSFET. On the main switch transformer, a small secondary winding output is rectified and fed to the controller circuit, this will now power the controller instead of the small start up cap.
I managed to ID the startup cap, which measured fine.
I then probe 2 points, the PWM controller supply pin(fed from a LT1121 regulator) and the secondary winding output.
The scope shows that the startup cap circuit works fine. But the output of the secondary winding isn’t. It only shows oscillation for about 50ms before disappearing, causing the circuit to shut down and restart. I’ll do more probing around the controller, driver section tonight.
