®
Design Idea DI-28
TinySwitch®-II 3 W Charger:
<30 mW No-load Consumption
Topology
Flyback
Device
Power Output
Input Voltage
Output Voltage
Application
Charger
3 W
85-265 VAC
TNY264P
5 V, 600 mA
the internal current source at no-load. Other load conditions
are not important as the device will be powered from the
DRAIN pin if bias is lost, allowing a simple flyback winding
to be used. Figure 2 shows that the bias winding and choice
of R2 should provide approximately 550 µA at no-load to
minimize consumption.
Design Highlights
• Less than 30 mW no-load power consumption (for
115/230 VAC input)
• Meets CISPR-22 Class B without Y capacitor
• Low cost, low component count solution
The circuit meets CISPR-22 Class B conducted EMI limits
without aYcapacitorandthereforehasverylowACleakage
current. TinySwitch-II frequency jitter, use of the bias
winding as a shield and capacitor C3 to soften leakage
inductance spikes combine to provide the EMI performance.
Operation
The TinySwitch-II flyback converter in Figure 1 generates a
constant voltage, constant current (CV/CC) 5 V,
600 mA output. Typical applications include wall-mounted
chargers for cell phones, PDAs and other battery powered
portable equipment.
Key Design Points
The key performance characteristic of the circuit shown is the
extremely low no-load consumption of <30 mW. A linear
transformer charger of similar rating will typically consume
1 W to 4 W at no-load. At $0.12/kWh, the TinySwitch-II can
therefore reduce energy costs by $1 to $4 per year.
• Design bias winding circuit to provide approximately
550 µA at no-load. Figure 2 shows the details.
• Minimizesecondarycircuitbiascurrents. Uselowcurrent
feedback Zeners for best tolerance. The very low bias
current in this design will provide approximately ±10%
output voltage tolerance. A precision reference (e.g.
TL431) can be used to reduce this if required.
• Designtransformerwithlowreflectedvoltagetominimize
clamp losses. A bigger device (TNY266) may help to
further reduce VOR.
• Wind transformer for lowest leakage inductance. Choose
wire gauges to completely fill winding layers.
• Winding transformer with tape between primary layers
further reduces intra-winding capacitance and no-load
consumption.
The no-load performance is achieved by using a transformer
biaswindingasalowvoltagesourceforTinySwitch-IIoperating
current. Even without this winding, a TinySwitch-II circuit will
consume <300 mW at no-load. However, by providing external
bias, the internal high voltage current source, which normally
powers the IC from the DRAIN pin, is disabled and a further
reduction in consumption is therefore achieved.
Thebiaswindingshouldprovideenoughcurrenttofullydisable
D7 11DQ06
1
8
+5 VDC
L3
C6
L1
1.0 mH
8T
24 AWG
Ferrite
Bead
C3
470 µF
10 V
2.2 nF
R3
1.5 kΩ
VR1
BZY97C200
1 kV
7
3
D6 1N4148
D1
D2
105T
33 AWG
RF1
8.2 Ω
1.0 W
1N4005 1N4005
C7
D5
1N4937
13T x2
31 AWG
4
R2
5.1 kΩ
C5
100 µF
2
47 µF
Q1
2N3906
10 V
16 V
L
C2
4.7 µF
400 V
C1
4.7 µF
400 V
85 - 265
VAC
TinySwitch-II
VR3
BZX79B5V1
U1
R4
820 Ω
TNY264P
N
D
S
EN
BP
U2
D3
1N4005
D4
1N4005
L2
Bead
2.2 µH
C4
0.1 µF
50 V
R5
2.7 Ω
2 W
U2
PC817A
RTN
PI-3079-091402
Figure 1. TinySwitch-II 3.0 W Cell Phone Charger.
September 2002
DI-28
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