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Design Idea DI-23

High Speed Modem Power Supply

December 2004

DI-23

®

Application

High Speed Modem

TOP243P

3.3 V / 5 V / 30 V

Power Output

10 W

Input Voltage

Output Voltage

Topology

Device

85-264 VAC

Flyback

Figure 1. TOP243P – 10 W, 3.3 V at 1.5 A, 5 V at 0.9 A, 30 V at 0.03 A, High Speed Modem Power Supply.

Design Highlights

• Compact low cost design (113 mm

× 39 mm × 25 mm) with

only 46 parts

• Meets Blue Angel no-load power consumption

• Line undervoltage protection prevents turn-off output

glitches and overvoltage protection provides extended line

surge protection

• Hysteretic thermal shutdown provides automatic fault

recovery

• Frequency jittering dramatically reduces EMI and meets

EN550022 Class B without Y1 capacitor

• Ultra low leakage current (<1

µA at 265 VAC) eliminates

audio hum in voice applications

• Surge immunity to 4 kV (EN61000-4-5)

• High efficiency >70%

Operation

TOP243P (DIP package) is shown in Figure 1. The power

supply utilizes many TOPSwitch-GX features such as frequency

jittering, input voltage sense, thermal shutdown, internal current

limit, soft-start, etc. without adding any extra components.

Typical applications are wall mount adapters, set-top box, high-

speed modem, standby power supplies, and other applications

requiring

very

low

cost,

small

size,

and

low

no-load input power consumption.

The example discussed here passes EMI conducted and radiated

emissions tests without using a Y1 capacitor between primary

and secondary. This is possible with the frequency jittering

feature of TOPSwitch-GX and careful transformer construction.

Shield windings are used in the transformer to reduce common

mode EMI currents. Transformer specifications are given in

Table 1.

Good cross-regulation of output voltages is achieved by careful

transformer construction and using a TL431 voltage regulator

instead of a Zener regulator.

The power supply uses a low cost RCD clamp with a slow

1N4007GP diode, which reduces EMI emissions and recycles

leakage energy, increasing efficiency. Input line sensing prevents

reverse conduction of current in TOPSwitch-GX at very low

input voltages due to the 1N4007GP in the RCD clamp.

PI-3366-120704

C1

33 µF

400V

R6

100 Ω

D5

1N4007GP

D

S

C

CONTROL

M

D7

UF4003

D9

SB540

L4

10 µH

L2

3.3 µH

L3

3.3 µH

C6

820 µF

16 V

C14

1 µF

50 V

R6

75 Ω

U3

LM431BCZ

1%

C10

0.1 µF

R12

10 kΩ

1%

R10

15.4 kΩ

1%

C10

10 µF

50 V

C7

820 µF

16 V

C9

150 µF

6.3 V

0.03 A, 30 V

0.9 A, 5 V

1.5 A, 3.3 V

RTN

R4

6.8 Ω

C3

47 µF

10 V

TOPSwitch-GX

R2

2 MΩ

R1

150 kΩ

0.5 W

U1

TOP243P

T1

EE25

1

2

D6

1N4148

C13

2.2 nF

C11

0.1 µF

D8

SB360

R8

10 Ω

5

4

7

6

9

10

R5

10 Ω

C5

820 µF

16 V

C4

47 µF

50 V

C8

100 µF

10 V

U2

LTV817A

R7

1 kΩ

R11

10 kΩ

1%

R13

12 kΩ

D1-D4

1N4007

1 A, 1000 V

F1

3.15 A

L1

20 mH

0.8 A

L

N

C15

0.1 µF

250 VAC

X2

85 - 265

VAC

C2

1 nF

1 kV