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
DI-26
• Safety Y1 capacitor C15 is connected between secondary
return and primary DC rail to minimize noise coupling
during AC common mode line transients.
• Good layout practices should be followed:
- Locate C13, R11 and C14 close to U1, with grounds
returned to the SOURCE pin.
- Minimize the primary and secondary loop areas to
reduce parasitic leakage inductance, improve EMI and
cross-regulation.
TRANSFORMER PARAMETERS
Core Material
Bobbin
EF16 gapped for 190
nH/T
2
EF16-8 pin
Primary: 105T, 35 AWG
Bias: 17T, 35 AWG
3.3 V Secondary: 4T, 4 x 26 AWG T.I.W.
6.2 V Secondary: 3T, 26 AWG T.I.W.
30 V Secondary: 29T, 30 AWG T.I.W.
(T.I.W. = Triple Insulated Wire)
Primary (1-2), Tape, Bias (3-4),
Tape, 3.3 V (5-6), 5 V (6-7), 30 V (7-8)
4
Line Regulation (%)
3
2
1
0
-1
-2
-3
-4
-5
190
210
230
2.5 V
3.3 V
6.2 V
3.0 V
250
270
Line Voltage (VAC)
Figure 2. Full Power Line Regulation.
PI-3022-080202
Winding Details
80
70
60
QP Limit Line
AV Limit Line
Winding Order
(Pin Numbers)
Primary: 2.1 mH
±
10%, Leakge: 50
µH
(max.)
Inductance
Primary Resonant
650 kHz (minimum)
Frequency
Amplitude (dBµV)
50
40
30
20
-10
0
-10
-20
0.15
Quasi-Peak Scan
Average Scan
Table 1. Transformer Construction Information.
Load
Range
(%)
-10 -7
10-100
10-100
10-100
100
Voltage
(V)
2.5
3.3
6.2
30
Regulation (%)
-4
-3
-2
-1
0
1
2
3
4
7
EN_V_QP
EN_V_AV
1
10
PI-3021-080202
5
30
Table 2. Worst Case Output Cross-Regulation-all Outputs Taken
from Minimum to Maximum Load.
Frequency (MHz)
Figure 3. Conducted EMI, 230 VAC, Full Power, Output Earth
Grounded.
For the latest updates, visit our Web site:
www.powerint.com
Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability.
Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any
license under its patent rights or the rights of others. The products and applications illustrated herein may be covered by one or more U.S.
and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power
Integrations’ patents may be found at www.powerint.com.
The PI Logo,
TOPSwitch
,
TinySwitch
and
EcoSmart
are registered trademarks of Power Integrations, Inc.
PI Expert
is a trademark of Power Integrations, Inc. ©Copyright 2002, Power Integrations, Inc.
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