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Design Idea DI-17

TOPSwitch-GX

17 W, PC Standby

Design Highlights

•Meets Blue Angel efficiency requirements providing output

power of 3.9 W with 5 W input power

• Input undervoltage (UV) detect eliminates power-up/down

output glitches

• 132 kHz switching frequency allows small, low cost EEL19

based transformer to deliver 17 W

• Regulation derived from 3.3 V and 5 V outputs ensuring

±5%

regulation on both outputs

• 15 V output for primary side circuitry

• Primary soft-start minimizes start-up component stresses

Operation

The design in Figure 1 utilizes the TOP242Y and takes advantage

of many of the device’s features. Input UV is set at 195 VDC

using a single 3.9 M

Ω resistor (R1). On increasing input

voltage, the supply is disabled until 195 VDC is reached (this

threshold is typically the lowest operating DC input voltage

for a PC main power supply with a doubler input

configuration). On decreasing input voltage the supply

continues to operate until regulation is lost, even if this is

below the UV threshold. Once regulation is lost the input

voltage has to exceed the UV threshold again before the

supply is enabled.

During the first 10 ms of operation internal soft-start is

enabled. The duty cycle is linearly increased from 0% to

78% and the current limit from 70% to 100%, lowering

stresses on the internal power MOSFET, clamp and output

rectifier. Diode D1 and Zener VR1 are used to clamp the

leading-edge drain-voltage spikes caused by transformer

leakage inductance. At light load, a Zener clamp provides

November 2002

DI-17

Figure 1. TOPSwitch-GX 17 W PC Standby.

Application

PC Standby

TOP242Y

3.3 V / 5 V

Power Output

17 W

Input Voltage

Output Voltage

Topology

Device

200-375 VDC

Flyback



®

D

S

C

L

F

X

CONTROL

3.3 V, 2 A

D2

BAV21

T1

C8

4.7 µF

10 V

C11

100 µF

10 V

C5

47 µF

* Optional

U2

LTV817

U3

TL431

C7

0.33 µF

R6

300 Ω

R9

16.2 kΩ

1%

D1

UF4005

VR1

BZY97C-200

U1

TOP242Y

TOPSwitch-GX

R7

510 Ω

C1*

0.01 µF

1 kV

C6

1 µF

50 V

D4

SB540

D3

SB540

5 V, 2 A

C12

1000 µF

10 V

C10

1000 µF

10 V

CY1

1 nF

RTN

15 V, 30 mA

R5

6.8 Ω

R1

3.9

MΩ

L2

10 µH

L1

10 µH

R10

12.1 kΩ

1%

R11

10 kΩ

1%

C13

100 µF

10 V

C2

0.1 µF

PI-2963-092402

+V

IN

200 - 375 VDC

(primary

referenced)

1

2

5

7,8

6

3

4