This power supply circuit uses chip TL494. Similar circuit is used in the most power supplies with output power about 200W.Device use push-pull transistor circuit with regulation of output voltage.
In stand-by mode is main power supply blocked by positive voltage on the PS-ON pin through resistor R23 from secondary power supply. Because of this voltage is opened transistor Q10, which opens Q1, which applies reference voltage +5V from pin 14 IO1 to pin 4 IO1. Switched circuit is totally blocked. Tranzistors Q3 and Q4 are both opened and short-circuit winding of auxiliary transformer T2.Due to short-circuit is no voltage on the power circuit. By voltage on pin 4 we can drive maximum pulse-width on the IO1 output. Zero voltage means the highest pulse-width. +5V means that pulse disappear.
Mainboard needs “PowerGood” signal. When all output voltages goes to stable, then PowerGood signal goes to +5V (logical one). PowerGood signal is usually connected to the RESET signal.
+3.3V Voltage stabilisation
Look at circuit connected to output voltage +3.3V. This circuit makes additional voltage stabilisation due to loss of voltage on cables. There are one auxiliary wire from connector for measure 3.3V voltage on motherboard.
For example when I by mistake short-circuit -5V with +5V, then positive voltage goes across D10, R28, D9 to the base Q6. This transistor is now opened and opens Q5. +5V from pin 14 IC1 comes across diode D11 to the pin 4 IC1 and power supply is blocked. Beyond that goes voltage again to base Q6. Power supply is still blocked, until he is disconnected from power line input.
ATX Power Connector
|Pin||Signal||Color 1||Color 2||Pin||Signal||Color 1||Color 2|
Source :: http://www.pavouk.org/hw/en_atxps.html