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Design Idea DI-26

Broadcast-Terrestrial (DVB-T) Supply

September 2002

DI-26

®

Application

DVB-T

TOP242P

2.5 V / 3.3 V / 6.2 V / 30 V

Power Output

7 W

Input Voltage

Output Voltage

Topology

Device

195 to 265 VAC

Flyback

Figure 1. TOPSwitch-GX 7 W Multiple Output Supply for DVB-T.

Design Highlights

•Meets CISPR22B/EN55022B conducted EMI limits with

output return grounded

• <0.5 W input power at zero load

• 132 kHz operation and programmable current limit allows

small, low cost EF16 transformer for 7 W output

• Low component count design occupies 80 x 30 x 16 mm

• Integrated soft-start reduces start-up component stresses

Operation

The TOPSwitch-GX flyback supply provides 4 outputs, delivering

7 W from a 230 VAC

±15% input. The TOP242P was selected

for low cost, the DIP-8 package removing the need for an

external heat sink. Resistor R7 programs the internal TOP242P

current limit to 78% of nominal, just above the level needed for

full load at low line. This feature allows a more continuous

transformer design for better efficiency and cross-regulation,

without requiring a larger core size.

Resistor R12, C10 and L2 filter conducted EMI; R12 is a

flame-proof fusible type, also functioning as a fuse. For

lower cost, if the supply does not have to meet conducted

EMI with the output connected to earth ground, the common

mode choke can be replaced with a

π filter. A Zener clamp

(D11 and VR1) was selected over an RCD clamp to minimize

zero load consumption. Secondary side feedback is taken

from the 3.3 V

±5% output since this has the tightest tolerance

requirement. The 2.5 V

±5% output is derived directly from

the 3.3 V output using D4. A 60 V Schottky was selected for

D1, since the slightly higher forward drop centers the 6.2 V

and 30 V outputs.

Post-filters (L1/C3, L2/C12 and R1/C5) reduce output noise

and ripple to <

±1% of the respective output voltage. A soft-

finish capacitor (C7) eliminates output turn-on overshoot.

Key Design Points

• The transformer is designed to operate in continuous

mode for tight secondary cross-regulation.

®

PI-3023-0916102

T1

D

S

C

CONTROL

M

C9

6.8 µF

400 V

C13

100 nF

C8

1 µF

50 V

C6

0.1 µF

C14

47 µF

10 V

C4

470 µF

10 V

C12

47 µF

35 V

C5

10 µF

50 V

C3

47 µF

35 V

C7

10 µF

16 V

C15 2.2 nF

Y1

8

7

6

5

3

4

1

2

C1, C2

220 µF

35 V

D6

1N4148

D4

1N5400

D3

UF4002

D2

SB140

D1

SR506

U2

LMV431

U3

PC817

C10

47 nF

X2

D7-D10

1N4005

195-265

VAC

VR1

P6KE200

D11

BYW95C

R12

8.2 Ω

Fusible

R7

10 kΩ

R11

6.8 Ω

R6

75 Ω

R5

1 kΩ

R2

15 kΩ

1%

R1

150 Ω

R3

9.1 kΩ

1%

R4

3.3 kΩ

L1

4.7 µH

1.8 A

L2

4.7 µH

1.8 A

L3

10 mH

0.1 A

RTN

2.5 V, 1 A

3.3 V, 0.5 A

6.2 V, 0.4 A

30 V, 5 mA

U1

TOP242P

TOPSwitch-GX