Design Idea DI-26
®
TOPSwitch-GX
7 W Digital Video
Broadcast-Terrestrial (DVB-T) Supply
Application
DVB-T
Device
TOP242P
Power Output
7W
Input Voltage
195 to 265 VAC
Output Voltage
2.5 V / 3.3 V / 6.2 V / 30 V
Flyback
®
Topology
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.
C15 2.2 nF
Y1
D3
UF4002
T1
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.
R1
150
Ω
L2
4.7
µH
1.8 A
C4
470
µF
10 V
C1, C2
220
µF
35 V
C5
10
µF
50 V
C12
L1
47
µF
4.7
µH
35 V
1.8 A
C3
47
µF
35 V
R6
75
Ω
R5
1 kΩ
R2
15 kΩ
1%
30 V, 5 mA
8
1
D7-D10
1N4005
VR1
P6KE200
7
D2
SB140
6.2 V, 0.4 A
D11
BYW95C
C9
6.8
µF
400 V
2
D1
SR506
6
3.3 V, 0.5 A
2.5 V, 1 A
D4
1N5400
RTN
5
L3
10 mH
0.1 A
3
TOPSwitch-GX
U1
TOP242P
C10
47 nF
X2
R12
8.2
Ω
Fusible
195-265
VAC
D
M
CONTROL
C
4
D6
1N4148 C8
1
µF
50 V
U3
PC817
R7
10 kΩ
C6
0.1
µF
R4
3.3 kΩ
U2
LMV431
R3
9.1 kΩ
1%
PI-3023-0916102
S
C13
100 nF
R11
6.8
Ω
C14
47
µF
10 V
C7
10
µF
16 V
Figure 1. TOPSwitch-GX 7 W Multiple Output Supply for DVB-T.
DI-26
www.powerint.com
September 2002
POWERINT [ POWER INTEGRATIONS, INC. ]
