www.powerint.com

Design Idea DI-43

12 V Supply with <250 mW No-load

November 2002

DI-43

®

Application

Adapter

TOP244Y

12 V

Power Output

30 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Flyback

Figure 1. TOPSwitch-GX 12 V, 30 W Universal Power Supply.

Design Highlights

• High efficiency (79% minimum)

• Low component count

• Excellent input no-load consumption <200 mW at 115 VAC

and <250 mW at 230 VAC input

• Rugged supply includes UV/OV shutdown, thermal and

short circuit protection with auto-recovery

• Comfortably meets EN55022B and CISPR22B conducted

EMI standards

• Overload power delivery at 265 VAC limited to 160% of

rated load

Operation

The TOPSwitch-GX integrates many features to implement low

cost, switched mode power supplies.

A single resistor (R1) implements input UV/OV protection

using the L pin (UV typ. 100 VDC; OV typ. 450 VDC). Resistor

R4 programs the U1 current limit to 85% of nominal, and

resistor R2 reduces this current limit as input voltage increases,

limiting the maximum overload power of the supply. The

reduced current limit allows continuous conduction mode

operation with a small transformer, reducing primary and

secondary peak currents, optimizing efficiency, and reducing

component stress.

Diode D1 and VR1 form a clamp circuit that absorbs leakage

inductance energy during normal operation, with Zener VR1

clamping the voltage to a safe level. Capacitor C2 diverts

some of the leakage energy from VR1, reducing its temperature

and increasing overall efficiency. After clamping, reverse

current flows through D1, recovering some of the clamp

energy. R3 limits the reverse current in D1 and improves

EMI by limiting drain voltage ringing.

Resistors R9 and R10 set the output voltage. Components

C10 and R8 provide compensation with R6 setting DC loop

gain. Using a TL431 error amplifier gives better regulation

and output voltage tolerance than a Zener reference and also

provides better no-load performance due to lower (1 mA)

bias current.

®

PI-3405-112702

D1

UF4005

D3

MBR10100

D2

1N4148

VR1

P6KE

200

C5

47

µF

10 V

C1

100

µF

400 V

U2

ISP817C

C10

47 nF

U1

TOP244Y

TOPSwitch-GX

3

R1

2 M

Ω

1/2 W

R5

6.8

Ω

R10

10 k

Ω

1%

R7

15 k

Ω

U3

TL431

R8

4.7 k

Ω

RTN

12 V, 2.5 A

1

6,7

9,10

5

4

L1

22 mH

F1

3.15 A

85-265 VAC

BR1

2 A, 600 V

CX1

220 nF

250 VAC

L

N

C6

680

µF

35 V

L2

3.3

µH

C3

1

µF

50 V

C7

2.2 nF

Y1

C12

470 pF

100 V

R11

33

Ω

1/4 W

C8

100

µF

35 V

D

S

C

L

F

X

CONTROL

R2

8.2 M

Ω

1/2 W

R4

8.25 k

Ω

1%

C4

100 nF

T1

C2

4.7 nF

1 kV

R3

47

Ω

1/2 W

C7

680

µF

35 V

D4

1N4148

R6

1 k

Ω

C11

10

µF

35 V

R9

38.3 k

Ω

1%