®
Design Idea DI-43
TOPSwitch®-GX 30 W, Universal Input,
12 V Supply with <250 mW No-load
Topology
Flyback
Device
Power Output
Input Voltage
Output Voltage
Application
Adapter
30 W
85-265 VAC
TOP244Y
12 V
Design Highlights
limiting the maximum overload power of the supply. The
reduced current limit allows continuous conduction mode
operation with a small transformer, reducing primary and
secondarypeakcurrents,optimizingefficiency,andreducing
component stress.
• 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
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
someoftheleakageenergyfromVR1,reducingitstemperature
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.
Operation
TheTOPSwitch-GXintegratesmanyfeaturestoimplementlow
cost, switched mode power supplies.
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.
A single resistor (R1) implements input UV/OV protection
usingtheLpin(UVtyp.100VDC;OVtyp.450VDC). Resistor
R4 programs the U1 current limit to 85% of nominal, and
resistor R2 reduces this current limit as input voltage increases,
C7
2.2 nF
Y1
C12
R11
470 pF 33 Ω
100 V 1/4 W
D3
MBR10100
L2
3.3 µH
T1
12 V, 2.5 A
6,7
1
3
C2
4.7 nF
1 kV
C6
680 µF
35 V
C7
680 µF
35 V
C8
100 µF
35 V
BR1
2 A, 600 V
VR1
P6KE
200
R3
47 Ω
1/2 W
RTN
9,10
D2
1N4148
D1
UF4005
5
4
C3
1 µF
50 V
L1
22 mH
R7
15 kΩ
R1
2 MΩ
1/2 W
C1
100 µF
400 V
R2
8.2 MΩ
U2
ISP817C
R9
38.3 kΩ
1%
1/2 W
CX1
TOPSwitch-GX
D
S
L
220 nF
D4
250 VAC
R6
CONTROL
1N4148
U1
TOP244Y
C
1 kΩ
R8
4.7 kΩ
F1
3.15 A
C11
10 µF
35 V
R5
X
F
C10
47 nF
6.8 Ω
C4
100 nF
L
R4
8.25 kΩ
1%
C5
47 µF
10 V
U3
TL431
85-265 VAC
N
R10
10 kΩ
1%
PI-3405-112702
Figure 1. TOPSwitch-GX 12 V, 30 W Universal Power Supply.
November 2002
DI-43
www.powerint.com
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