200W ATX PC POWER SUPPLY

Czech version

Here I bring you wiring diagram of PCs power supply of DTK company. This
power supply has ATX design and 200W performance. I was drawed diagram, when I
repaired this power supply.

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.

Line voltage goes through input filter circuit (C1, R1, T1, C4, T5) to the
bridge rectifier. When voltage is switched from 230V to 115V, then rectifier
works like a doubler. Varistors Z1 and Z2 have overvoltage protect function on
the line input.
Thermistor NTCR1 limits input current until capacitors C5
and C6 are charged. R2 and R3 are only for discharge capacitors after
disconnecting power supply. When power supply is connected to the line voltage,
then at first are charged capacitors C5 and C6 together for about 300V.
Then
take a run secondary power supply controlled by transistor Q12 and on his
output will be voltage. Behind the voltage regulator IC3 will be voltage 5V,
which goes in to the motherboard and it is necessary for turn-on logic and for
"Wake on something" functions.
Next unstabilized voltage goes through diode
D30 to the main control chip IC1 and control transistors Q3 and Q4. When main
power supply is running, then this voltage goes from +12V output through diode
D.

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.

Now we can explain function of running power supply.

Somebody pushes the power button on computer. Motheboard logic put to ground
input pin PS-ON. Transistor Q10 closes and next Q1 closes. Capacitor C15 begins
his charging through R15 and on the pin 4 IC1 begins decrease voltage to zero
thanks to R17. Due to this voltage is maximum pulse-width continuosly increased
and main power supply smoothly goes run.

In a normal operation is power supply controlled by IC1. When transistors Q1
and Q2 are closed, then Q3 and Q4 are opened. When we want to open one from
power transistors (Q1, Q2), then we have to close his exciting transistor (Q3,
Q4). Current goes via R46 and D14 and one winding T2. This current excite
voltage on base of power transistor and due to positive feedback transistor
goes quickly to saturation. When the impulse is finished, then both exciting
transistors goes to open. Positive feedback dissapears and overshoot on the
exciting winding quickly closes power transistor. After it is process repetead
with second transistor. Transistors Q1 and Q2 alternately connects one end of
primary winding to positive or negative voltage. Power branch goes from emitor
of Q1 (collector Q2) through the third winding of exciting transformer T2. Next
throug primary winding of main transformer T3 and capacitor C7 to the virtual
center of supply voltage.

Output voltage stabilisation

Output voltages +5V and +12V are measured by R25 and R26 and their output
goes to the IC1. Other voltages are not stabilised and they are justified by
winding number and diode polarity. On the output is necessary reactance coil
due to high frequency interference.
This voltage is rated from voltage
before coil, pulse-width and duration cycle. On the output behind the rectifier
diodes is a common coil for all voltages. When we keep direction of windings
and winding number corresponding to output voltages, then coil works like a
transformer and we have compensation for irregular load of individual
voltages.
In a common practise are voltage deviations to 10% from rated
value. From the internal 5-V reference regulator (pin 14 IC1) goes reference
voltage through the voltage divider R24/R19 to inverting input(pin 2) of error
amplifier. From the output of power supply comes voltage through divider
R25,R26/R20,R21 to the non inverting input (pin 1). Feedback C1, R18 provides
stability of regulator. Voltage from error amplifier is compared to the ramp
voltage across capacitor C11.
When the output voltage is decreased, then
voltage on the error amplifier is toodecreased. Exciting pulse is longer, power
transistors Q1 and Q2 are longer opened, width of pulse before output coil is
grater and output power is increased. The second error amplifier is blocked by
voltage on the pin 15 IC1.

PowerGood

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

This circuit is composed from Q5, Q6 and many discrete components. Circuit
guards all of output voltages and when the some limit is exceeded, power supply
is stopped.
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.

Links

http://www.belza.cz/swmodeps/compow1.htm
(Czech language only)

http://www.belza.cz/swmodeps/compow2.htm
(Czech language only)

ATX Power Connector

PIN

SIGNAL

PIN

SIGNAL

1

3.3V

11

3.3V

2

3.3V

12

-12V

3

GND

13

GND

4

5V

14

PS_ON

5

GND

15

GND

6

5V

16

GND

7

GND

17

GND

8

PW_OK

18

-5V

9

5V_SB

19

5V

10

12V

20

5V

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