Design Idea DI-17
TOPSwitch-GX
17 W, PC Standby
Application
PC Standby
Device
TOP242Y
Power Output
17 W
Input Voltage
200-375 VDC
Output Voltage
3.3 V
/
5 V
Topology
Flyback
®
Design Highlights
• Meets Blue Angel efficiency requirements providing output
power of 3.9 W with 5 W input power
• Input undervoltage (UV) detect eliminates power-up/down
output glitches
• 132 kHz switching frequency allows small, low cost EEL19
based transformer to deliver 17 W
• Regulation derived from 3.3 V and 5 V outputs ensuring
±5%
regulation on both outputs
• 15 V output for primary side circuitry
• Primary soft-start minimizes start-up component stresses
voltage, the supply is disabled until 195 VDC is reached (this
threshold is typically the lowest operating DC input voltage
for a PC main power supply with a doubler input
configuration). On decreasing input voltage the supply
continues to operate until regulation is lost, even if this is
below the UV threshold. Once regulation is lost the input
voltage has to exceed the UV threshold again before the
supply is enabled.
During the first 10 ms of operation internal soft-start is
enabled. The duty cycle is linearly increased from 0% to
78% and the current limit from 70% to 100%, lowering
stresses on the internal power MOSFET, clamp and output
rectifier. Diode D1 and Zener VR1 are used to clamp the
leading-edge drain-voltage spikes caused by transformer
leakage inductance. At light load, a Zener clamp provides
L1
10
µH
5
2
VR1
BZY97C-200
R1
3.9
MΩ
D1
UF4005
7,8
D3
SB540
C10
1000
µF
10 V
C12
1000
µF
10 V
L2
10
µH
C11
100
µF
10 V
C13
100
µF
10 V
Operation
The design in Figure 1 utilizes the TOP242Y and takes advantage
of many of the device’s features. Input UV is set at 195 VDC
using a single 3.9 MΩ resistor (R1). On increasing input
CY1
1 nF
+V
IN
200 - 375 VDC
5 V, 2 A
3.3 V, 2 A
D4
SB540
6
RTN
15 V, 30 mA
3
D2
BAV21
4
1
C1*
0.01
µF
1 kV
C6
1
µF
50 V
U2
LTV817
R6
300
Ω
R7
510
Ω
R9
16.2 kΩ
1%
R10
12.1 kΩ
1%
(primary
referenced)
T1
D
L
TOPSwitch-GX
C
U1
TOP242Y
S
CONTROL
X
F
C2
0.1
µF
R5
6.8
Ω
C5
47
µF
C8
4.7
µF
10 V
U3
TL431
C7
0.33
µF
R11
10 kΩ
1%
PI-2963-092402
* Optional
Figure 1. TOPSwitch-GX 17 W PC Standby.
DI-17
www.powerint.com
November 2002
DI-17
higher efficiency than an RCD clamp, maximizing available
output power for the Blue Angel 5 W input power limit.
A secondary shunt regulator (U3) together with R6, R7, R9,
R10 and R11 are used to sense both the 3.3 V and 5 V outputs.
Control loop compensation is accomplished with capacitor
C7, R5 and C5. Resistor R6 sets the DC gain while R7
provides bias to the TL431 (U3). The values shown provided
satisfactory phase margin and bandwidth.
TRANSFORMER PARAMETERS
Core Material
Bobbin
EEL19 (Nippon Ceramic NC-2H)
2
A
LG
= 720 nH/T
EEL19 8 pin
(TDK BE-9-118CPH or equivalent)
Primary: 147T x 34 AWG
Bias: 17T x 34 AWG
3.3 V: 4T x 3 x 27 AWG T.I.W.
5.0 V: 2T x 3 x 27 AWG T.I.W.
(T.I.W. = Triple Insulated Wire)
Primary (1-2), tape,
3.3 V (7, 8-6), 5 V (5-7, 8), tape
Bias (3-4), tape
Primary: 2.3 mH
±10%,
Leakage: 75
µH
(maximum)
650 kHz (minimum)
Winding Details
Key Design Points
• Decoupling capacitor (C1) should be used if the standby
supply is far from the main input bulk capacitor.
• The value of R1 is set according to the equation:
R1 = (V
UV
- 2.5)/50
µA.
• Minimize secondary trace leakage inductance to improve
output cross regulation.
• Y1 capacitor (CY1) should be connected between secondary
return and primary positive DC rail to minimize potential
coupling into the
TOPSwitch-GX
SOURCE pin during
common mode line surge events.
• Connect bias winding return past SOURCE pin to route
common mode surge currents away from
TOPSwitch-GX.
• A soft-finish capacitor (C8) eliminates output start-up
overshoot.
• Capacitor C2 should be located close to U1.
Winding Order
(Pin Numbers)
Inductance
Primary Resonant Frequency
Table 1. Transformer Construction Information.
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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 (including circuits external to the
products and transformer construction) 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
,
LinkSwitch
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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