Design Idea DI-84
®
TinySwitch-II 3 W Charger:
<30 mW No-load Consumption
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
Charger
TNY264P
3 W
85-265 VAC
5 V, 600 mA
Flyback
The bias winding should provide enough current to fully
disable 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. This allows a simple
flyback winding to be used. Figure 2 shows that the bias
winding and choice of R2 should provide approximately
600 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
• Meets CEC active mode efficiency with good margin
The circuit meets CISPR-22 Class B conducted EMI limits
without aYcapacitor, and therefore has very lowAC leakage
current. Superior EMI performance is achieved via the
TinySwitch-II frequency jitter, an output RC snubber, use of
the bias winding as a shield, and careful selection of clamp
Zener voltage.
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
600 µA at no-load. Figure 2 shows the details.
• Minimizesecondarycircuitbiascurrents. Uselowcurrent
feedback Zeners for best tolerance. The very low Zener
bias current in this design will provide better than ±
10%
output voltage tolerance.
• Designtransformerwithlowreflectedvoltagetominimize
clamplosses.Alargerdevice(TNY266)mayenablefurther
reduction in VOR
This 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. The bias winding disables the
internal high voltage current source, which normally powers
the IC from the DRAIN pin, thereby further reducing power
consumption.
• Wind transformer for lowest leakage inductance. Choose
wire gauges to completely fill winding layers.
C3
470 pF
100V
R6
33 Ω
T1
L3
Ferrite
Bead
T1 EE13
LP = 1.9 mH
L1
1.0 mH
D7
11DQ06
5 V, 0.6 A
1
8
C6
470 µF
10 V
VR1
BZY97-
C130
8T
24 AWG T.I.
7
3
102T
D1
D2
R3
1.5 kΩ
32 AWG
1N4005 1N4005
RF1
8.2 Ω
1.0 W
D5
C5
47 µF
16 V
15T
1N4007G
32 AWG
L
2
R1
4
R2
9.2 kΩ
Q1
2N3906
200 Ω
1/2 W
D6
1N4148
Fusible
C1
4.7 µF
400V
C2
4.7 µF
400V
85 - 265
VAC
C7
100 µF
10V
D
U1
EN/UV
BP
TNY264P
TinySwitch-II
N
VR2
BZX79-
B5V1
S
R4
C4
U2
PC817A
820 Ω
0.1 µF
D3
1N4005
D4
1N4005
50 V
L2
Ferrite
Bead
R5
2.4 Ω
2 W
RTN
Figure 1. TinySwitch-II 3.0 W Cell Phone Charger.
PI-3659-060205
June 2005
DI-84
www.powerint.com