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Design Idea DI-28

<30 mW No-load Consumption

September 2002

DI-28

®

Application

Charger

TNY264P

5 V, 600 mA

Power Output

3 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Flyback

Figure 1. TinySwitch-II 3.0 W Cell Phone Charger.

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

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.

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.

The no-load performance is achieved by using a transformer

bias winding as a low voltage source for TinySwitch-II operating

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.

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, 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.

The circuit meets CISPR-22 Class B conducted EMI limits

without a Y capacitor and therefore has very low AC leakage

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.

Key Design Points

• Design bias winding circuit to provide approximately

550

µA at no-load. Figure 2 shows the details.

•Minimize secondary circuit bias currents. Use low current

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.

• Design transformer with low reflected voltage to minimize

clamp losses. A bigger device (TNY266) may help to

further reduce V

•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.

PI-3079-091402

D

S

BP

EN

85 - 265

VAC

RTN

N

L

+5 VDC

RF1

8.2

Ω

1.0 W

D1

1N4005

D2

1N4005

D3

1N4005

D4

1N4005

C1

4.7

µF

400 V

L1

1.0 mH

VR1

BZY97C200

C3

2.2 nF

1 kV

D5

1N4937

U1

TNY264P

L2

Bead

2.2

µH

C4

0.1

µF

50 V

U2

PC817A

D7 11DQ06

8T

24 AWG

105T

33 AWG

18

7

3

4

2

13T x2

31 AWG

D6 1N4148

C5

47

µF

16 V

R2

5.1 k

Ω

C6

470

µF

10 V

U2

R4

820

Ω

R5

2.7

Ω

2 W

R3

1.5 k

Ω

L3

Ferrite

Bead

VR3

BZX79B5V1

C7

100

µF

10 V

C2

4.7

µF

400 V

Q1

2N3906

TinySwitch-II

®