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Design Idea DI-84

<30 mW No-load Consumption

June 2005

DI-84

®

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

Operation

The TinySwitch-II flyback converter in Figure 1 generates

TinySwitch-II

TinySwitch-II

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

TinySwitch-II

TinySwitch-II

therefore reduce energy costs by $1 to $4 per year.

This no-load performance is achieved by using a transformer

biaswindingasalowvoltagesourceforTinySwitch-IIoperating

TinySwitch-II

TinySwitch-II

current. Even without this winding, a TinySwitch-II circuit will

TinySwitch-II

TinySwitch-II

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.

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.

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

TinySwitch-II

TinySwitch-II

the bias winding as a shield, and careful selection of clamp

Zener voltage.

Key Design Points

• 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 V

• Wind transformer for lowest leakage inductance. Choose

wire gauges to completely fill winding layers.

Fusible

PI-3659-060205

L3

Ferrite

Bead

R4

820

Ω

5 V, 0.6 A

RTN

U2

PC817A

D

S

EN/UV

BP

TinySwitch-II

85 - 265

VAC

L

N

U1

TNY264P

RF1

8.2

Ω

1.0 W

D1

1N4005

D2

1N4005

D3

1N4005

D4

1N4005

D5

1N4007G

L1

1.0 mH

L2

Ferrite

Bead

R1

200

Ω

1/2 W

C2

4.7

µF

400V

C1

4.7

µF

400V

VR1

BZY97-

C130

1

2

R6

33

Ω

C3

470 pF

100V

R5

2.4

Ω

2 W

C7

100

µF

10V

D7

11DQ06

R3

1.5 k

Ω

C6

470

µF

10 V

VR2

BZX79-

B5V1

Q1

2N3906

R2

9.2 k

Ω

D6

1N4148

C5

47

µF

16 V

T1

C4

0.1

µF

50 V

102T

32 AWG

15T

32 AWG

8T

24 AWG T.I.

T1 EE13

4

3

7

8

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

Charger

TNY264P

3 W

85-265 VAC

5 V, 600 mA

Flyback

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