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Design Idea DI-27

<10 mW No-load Consumption

September 2002

DI-27

®

Application

Adapter

TNY254

12 V

Power Output

3 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Flyback

Figure 1. 3 W AC Adapter with <10 mW No-load Consumption.

Design Highlights

• Extremely low no-load power consumption (<10 mW at

115/230 VAC input)

• Simple RC snubber/clamp reduces EMI

• Low cost, low component count solution

Operation

The TinySwitch flyback converter in Figure 1 generates an

isolated 12 VDC, 3 W output from 85 VAC to 265 VAC input.

Typical applications include wall mounted AC adapters for

consumer products.

The key performance characteristic of the circuit shown is

extremely low no-load consumption of <10 mW. A linear

transformer adapter of similar rating will typically consume

1 W to 4 W at no-load. At $0.12/kWh, the TinySwitch can

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

The no-load performance is achieved using a transformer bias

winding as a low voltage source for TinySwitch operating

current. Even without this winding, a TinySwitch circuit will

consume <100 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. Figure 2 shows that the

bias winding and choice of C5 and R3 should provide

250

µA to 300 µA at no-load to minimize consumption.

Zener VR3 is required to clamp the BYPASS pin voltage.

The output Zener VR2 is not biased, minimizing the

secondary circuit consumption. A low current Zener there-

fore provides the best output voltage tolerance.

Components R2 and C3 provide a low cost clamp and also

help reduce EMI by softening switching edges. This type of

snubber increases switching losses and would therefore

normally be avoided in low consumption circuits. However,

with TinySwitch, these losses are negligible at no-load as

switching frequency is so low. This is illustrated in Figure 3

where replacing R2 and C3 with a Zener clamp across the

primary winding reduces consumption by <0.25 mW.

®

PI-3011-091802

D

S

EN/UV

BP

R2

1.5 kΩ

1/2 W

R3

4.7 kΩ

R1

4.7 kΩ

RF1

8.2 Ω

C3

68 pF

1 kV

U1

TNY254P

C4

0.1 µF

50 V

C5

100 µF

10 V

C6

180 µF

16 V

C7

100 µF

16 V

C1 & C2

3.3 µF

400 V

D6 UF4003

D5 1N4148

D1 & D2

1N4007

D3 & D4

1N4007

VR3

BZX79-

B6V2

VR2

BZX79B11

110

9

3

4

2

L2

Ferrite

Bead

RTN

+12 VDC

85 - 265

VAC

TinySwitch

L1

1.0 mH

L

N

U2

PC817A

U2

223T

36 AWG

49T

28 AWG

30T

x2

37 AWG

C8

2.2 nF Y1