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Design Idea DI-20

TOPSwitch-GX

145 W PC Main ATX Supply

December 2002

DI-20

®

Application

PC Main

TOP247

3.3 V / 5 V / 12 V

Power Output

145 W

Input Voltage

Output Voltage

Topology

Device

90-130 VAC / 180-265 VAC

Forward

Design Highlights

• 145 W cont. (160 W peak) in PC ATX form factor

• TOPSwitch-GX integrated features enable extremely low

component count

•Meets 1 W standby spec. (0.91 W input 0.5 W output)

• High efficiency (71% minimum)

• Integrated line undervoltage and overvoltage detection

• Low EMI due to frequency jitter

• SOURCE referenced TO-220 tab lowers conducted EMI

•Maximum duty cycle (DC

saturation for fault and transient loads

• Uses conventional magamp for 3.3 V output

•Meets CISPR22B/EN55022B conducted EMI

Operation

TOPSwitch-GX integrates many features designed for use with

forward converters. Transistors Q4, Q6, R1, R2, R3, R5 and R6

form an active capacitor (C2, C3) balancing circuit, operating

Figure 1. 145 W (160 W pk.) PC Main Power Supply Schematic (Note: Schematic does not include transformer Y-capacitor).

only as needed to minimize zero-load power consumption.

Resistors R3, R5 and R6 implement start-up undervoltage

lockout, which prevents the supply from starting below

180 VDC. Components R4, R14, Q1 and R30 implement an

independent undervoltage using the X pin, which allows the

supply to continue delivering power all the way down to

140 VDC (increasing hold-up time). Resistor R7 provides

additional hysteresis.

The primary side components D1, VR3-5 and C4, along with

secondary side C9 and R30, implement the Zener/capacitor

reset/clamp circuit. This circuit provides reset voltage for the

transformer and clamps the DRAIN pin voltage to a safe level

(<~600 V) under all conditions.

The reset circuit works in conjunction with the DC

MAX

reduction circuit (R8, R36, C22, VR19 and D18) to limit the

maximum duty cycle and prevent transformer saturation

under fault and transient conditions.

®

C13

220 µF

63 V

C3

330 µF

200 V

C2

330 µF

200 V

LG

N

RTN

+5 V

+12 V

R20

270 kΩ

R16

1 kΩ

U5

TL431

D

S

C

L

F

X

CONTROL

R19

4.12 kΩ

1%

F1

4 A

RV1

275 V, 14 mm

R10

560 K

1/2 W

CX2

0.33 µF

250V

CY4

2.2 nF

(Safety)

RT1

10 Ω

CY3

2.2 nF

(Safety)

JP9

CX1

0.047 µF

250 VAC

L7

8.2 mH

BR1

KBL06

To AC Selector

Switch

R1

330 kΩ

R2

330 kΩ

R3

2.2 MΩ

R5

180 kΩ

R6

2.2 MΩ

R8

130 kΩ

1%

R14

75 kΩ

R12

12 kΩ

R30

3.3 kΩ

Q1

2N3908

C22

100 pF

50 V

C10

1 nF

50 V

C20

330 pF

50 V

D7

MBR3045

R36

43.2 kΩ

1%

R13

10 Ω

D18

BAV20

VR19

1N5229

R4

2.2 MΩ

R7

560 kΩ

R11

300 Ω

Coupled

choke

L1

13 µH

R30

1 Ω

1 W

D8

MBR6045

C9

47 nF

50 V

C12

2200 µF

6.3 V

C11

1000 µF

16 V

L2

0.5 µH

U2

SFH615A-2

R17

15 kΩ

C16

100 nF

50 V

D6

BAV20

C8

50 V

R9

47 Ω

C6

16 V

C7

100 nF

50 V

D105

1N4148

Remote

ON/OFF

R38

5.1 kΩ

U3

LTV817

R37

10 kΩ

R106

27 kΩ

+12 V STBY

C23

33 nF

50 V

VR3

BZY97C-

200

C4

2.2 nF

1 kV

VR4

BZY97C-

180

VR5

BZY97C-

180

C5

100 V

TO

MAGAMP

(3.3 V)

R18

4.74 kΩ

1%

R15

1.8 kΩ

C17

100 nF

50 V

Q7

2N3904

PI-3165-120902

TOPSwitch-GX

U1

TOP247

1

5

13

14

11,12

8, 9

3

7

3

4

1, 2

5, 6

D1

1N5407

Q4

MPSA42

Q6

MPSA92

R21

270 Ω