Design Idea DI-116
®
TinySwitch-III
28 W Multiple Output Supply with <50 mW
No-Load Consumption
Application
Device
Power Output Input Voltage
90-265 VAC
Output Voltage
Topology
DVD / Set-top Box
TNY280P 25 W (28 W pk)
5 V / 12 V / -12 V / -24 V
Flyback
The internal MOSFET in U1 conducts current through the
primary winding of T1 during each enabled switching cycle.
When the primary current reaches the MOSFET current
limit, it turns off and the energy in T1 is transferred to the
secondary.
Design Highlights
•
•
•
Simple, low cost, low parts count solution
Low input power at no-load: <50 mW at 265 VAC
Low standby power consumption: <90 mW input with
5 V, 5 mA load
•
•
High efficiency: >74% at 90 VAC
The primary clamp (D5, VR3, R1, R2 and C3) limits the
maximum peak drain voltage below the 700 V breakdown
voltage of the internal MOSFET.
>10 dBµV EMI margin to EN55022B conducted EMI
limits, even with output connected to safety earth ground
Simple output overvoltage latching shutdown circuit
protects load under fault conditions
•
•
Outputovervoltageprotection(OVP)isprovidedbyU2,R12,
R13,VR1andVR2. If the feedbackloopwere toopencircuit
due to a failure of U3, VR1 orVR2 would turn on U2 causing
the current into the BP/M pins of U1 to exceed 6.5 mA, and
U1 to latch off. The values of VR1 and VR2 determine the
output voltage at which this occurs. Once triggered, cycling
the AC power resets the OVP latch.
On-time extension provides 28 ms hold-up time
Operation
TheDVDorsettopboxsupplyshowninFigure1wasdesigned
as a Flyback converter using a TNY280P device (U1). In this
circuit, the 10 µF value of C4 selects the increased current
limit level of U1, which allows the supply to deliver up to
28 W of peak power during startup or load transients (opening
the DVD tray).
To reduce no-load input power and increase light-load
efficiency, resistor R3 feeds the supply current for U1 from
the auxiliary bias winding on the transformer.
C6
C18
1 nF
250 VAC
330 pF R3
100 V 16 Ω
VR4
VR3
BZY97C200 1 nF
200 V
1 kV
C3
BZX79-
VR1
C8
C7
680 µF
16 V
L2
B8V2 1N5244B
8.2 V
220 µF
16 V
D7 SB360
3.3 µH
T1
14 V
+12 V,
1.15 A
8
6
7
2
3
C9
R4
C11
220 µF
10 V
L3
3.3 µH
1 nF
50 V
10 Ω
D1
1N4937
D2
1N4937
+5 V,
2.1 A
R2
NC
VR2
C12
C10
R1
C13
100 nF
50 V
D8
SB530
200 Ω
BZX79-B4V7
4.7 V 2%
2.2 µF
2200 µF
560 Ω
1/2 W
50 V
10 V
RTN
R11
10 Ω
D9
BAV20
10
R6
10 Ω
D5
FR107
F1
2A
-12 V,
20 mA
L1
10 mH
C15
100 nF
50 V
C14
R12
R8
D10
2.2 µF
100 Ω
56 Ω
BAV20
50 V
-24 V,
15 mA
9
4
RV1
275 VAC
C1
47 µF
400V
C2
90-265
VAC
47 µF
R7
400V
10 Ω
C5
1 µF
100 V
D6
BAV21
R13
1 kΩ
R9
1 kΩ
U3-A
U2-A
5
TinySwitch-III
U1
TNY280P
EE25
R15
115 kΩ
1%
R14
15 kΩ
1%
C16
220 nF
50 V
C18
D
S
U2-B
PC817D
100 nF
50 V
EN/UV
BP/M
C17
1 µF
50 V
D3
D4
1N4007GP 1N4007GP
RTN
R3
100 kΩ
C4
10 µF
50 V
U3-B
PC817D
U4
TL431
2%
R10
3 kΩ
R16
10 kΩ
1%
PI-4416-090606
Figure 1. 25 W, 28 W pk Multiple Output Supply Using TNY280P.
September 2006
DI-116
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