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.


Stand-By mode

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.

Overvoltage circuit

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
1 3.3V orange violet 11 3.3V orange violet
2 3.3V orange violet 12 -12V blue blue
3 GND black black 13 GND black black
4 5V red red 14 PS_ON green grey
5 GND black black 15 GND black black
6 5V red red 16 GND black black
7 GND black black 17 GND black black
8 PW_OK grey orange 18 -5V white white
9 5V_SB violet brown 19 5V red red
10 12V yellow yellow 20 5V red red

Source :: http://www.pavouk.org/hw/en_atxps.html

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