®
Design Idea DI-20
®
TOPSwitch-GX
145 W PC Main ATX Supply
Topology
Device
Power Output
Input Voltage
Output Voltage
Application
Forward
PC Main
145 W
90-130 VAC / 180-265 VAC
TOP247
3.3 V / 5 V / 12 V
Design Highlights
only as needed to minimize zero-load power consumption.
Resistors R3, R5 and R6 implement start-up undervoltage
lockout, which prevents the supply from starting below
180 VDC. Components R4, R14, Q1 and R30 implement an
independent undervoltage using the X pin, which allows the
supply to continue delivering power all the way down to
140 VDC (increasing hold-up time). Resistor R7 provides
additional hysteresis.
• 145 W cont. (160 W peak) in PC ATX form factor
• TOPSwitch-GX integrated features enable extremely low
component count
• Meets 1 W standby spec. (0.91 W input 0.5 W output)
• High efficiency (71% minimum)
• Integrated line undervoltage and overvoltage detection
• Low EMI due to frequency jitter
• SOURCE referenced TO-220 tab lowers conducted EMI
• Maximumdutycycle(DCMAX)reductionpreventstransformer
saturation for fault and transient loads
• Uses conventional magamp for 3.3 V output
• Meets CISPR22B/EN55022B conducted EMI
The primary side components D1, VR3-5 and C4, along with
secondary side C9 and R30, implement the Zener/capacitor
reset/clamp circuit. This circuit provides reset voltage for the
transformerandclampstheDRAINpinvoltagetoasafelevel
(<~600 V) under all conditions.
Operation
The reset circuit works in conjunction with the DCMAX
reduction circuit (R8, R36, C22, VR19 and D18) to limit the
maximum duty cycle and prevent transformer saturation
under fault and transient conditions.
TOPSwitch-GX integrates many features designed for use with
forwardconverters. TransistorsQ4, Q6, R1, R2, R3, R5andR6
form an active capacitor (C2, C3) balancing circuit, operating
C10
1 nF
50 V
Coupled
choke
L1
13 µH
4
3
+12 V
+5 V
14
1
C4
1, 2
5, 6
D7
C11
1000 µF
16 V
R1
R3
2.2 nF
1 kV
MBR3045
330 kΩ
2.2 MΩ
C9
47 nF
50 V
L2
0.5 µH
R4
13
2.2 MΩ
D1
1N5407
R5
180 kΩ
R30
1 Ω
1 W
BR1
KBL06
Q4
MPSA42
5
C13
C2
C12
220 µF
330 µF
200 V
2200 µF
8, 9
R21
270 Ω
63 V
D8
MBR6045
6.3 V
CY3
2.2 nF
(Safety)
TO
MAGAMP
(3.3 V)
C20
330 pF
50 V
R6
2.2 MΩ
11,12
RTN
CY4
JP9
TOPSwitch-GX
D6
2.2 nF
7
R13
10 Ω
U1
R18
4.74 kΩ
1%
C5
BAV20
(Safety)
TOP247
1 µF
CX1
D18
BAV20
100 V
Q6
MPSA92
R36
R8
0.047 µF
3
C17
To AC Selector
Switch
43.2 kΩ
130 kΩ
250 VAC
100 nF
50 V
1%
1%
L7
8.2 mH
VR19
1N5229
R7
560 kΩ
D
S
L
C22
VR3
BZY97C-
200
U2
R15
1.8 kΩ
R20
270 kΩ
CX2
0.33 µF
250V
100 pF
50 V
SFH615A-2
CONTROL
C3
330 µF
200 V
R16
1 kΩ
C
VR4
BZY97C-
180
R11
R10
560 K
1/2 W
300 Ω
RT1
10 Ω
C16
C8
X
F
R9
47 Ω
100 nF
50 V
0.033 µF
R17
15 kΩ
Q1
2N3908
50 V
VR5
BZY97C-
180
R2
330 kΩ
RV1
275 V, 14 mm
D105
R30
3.3 kΩ
1N4148
C6
47 µF
16 V
R14
75 kΩ
U5
TL431
R19
4.12 kΩ
1%
PI-3165-120902
+12 V STBY
R12
12 kΩ
R106
27 kΩ
R38
5.1 kΩ
Remote
ON/OFF
U3
LTV817
F1
4 A
C7
100 nF
50 V
Q7
2N3904
C23
33 nF
50 V
R37
10 kΩ
L
G
N
Figure 1. 145 W (160 W pk.) PC Main Power Supply Schematic (Note: Schematic does not include transformer Y-capacitor).
December 2002
DI-20
www.powerint.com
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