Design Idea DI-77

<200 mW No-Load Consumption

(a similarly rated linear transformer adapter will typically

consume approximately 1 W under no-load). This low no-load

consumption is the result of the inherent cycle skipping feature

of the TinySwitch-II and careful transformer design.

Key Design Points

• Minimize secondary-side power consumption. Zener diode

VR1 receives its bias current from the opto-LED in U2.

Using a low current device minimizes consumption while

providing an output voltage tolerance of ±5%. A TL431

can be used for tighter output voltage accuracy.

• Minimize clamp losses: design the transformer to have a low

reflected voltage (V

• Minimize leakage inductance: select wire gauge sizes that

completely fill each winding layer of the transformer.

• Minimize intra-winding capacitance and no-load

consumption: put a layer of tape between each primary

winding layer.

• Use >200 kΩ resistor in the RCD clamp to further reduce

power losses if the following two conditions are met: 1) the

EMIperformanceisnotcompromised,and2)thereisenough

drain voltage (BV

www.powerint.com

Figure 1. TinySwitch-II 3.0 W Adapter.

DI-77

®

May 2004

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

Adapter

TNY263P

3 W

85-265 VAC

9 V / 330 mA

Flyback

PI-3877-060404

D

S

BP

EN

85 - 265

VAC

RTN

N

L

9 V, 330 mA

RF1

8.2

Ω

1.0 W

D1

1N4007

D2

1N4007

D3

1N4007

D4

1N4007

C1

4.7

µF

400 V

L2

Bead

L3

Bead

L1

1.0 mH

C3

1 nF

1 kV

C7

1

µF

50 V

U1

TNY263P

C4

0.1

µF

50 V

U2

PC817A

D6

11DQ06

NC NC

T1

EE16

Lp = 1.6 mH

C5

330

µF

16 V

C6

100

µF

25 V

R2

16

Ω

R1

200 k

Ω

1/2 W

R3

200

Ω

D5

1N4007G

VR1

BZX79-B8V2

C2

4.7

µF

400 V

TinySwitch-II

1

3

10

8

Design Highlights

• Replaces a linear transformer based power supply at the

same or lower cost, but with much higher performance

• <200 mW no-load consumption (at 115/230 VAC)

• Meets CISPR 22-B with >15 dB margin, w/o Y capacitor

• Low cost, low component count solution: only 24 parts!

• TinySwitch-II On/Off regulation scheme produces ±5%

output regulation over temperature with a 2% Zener

• Does not require a transformer bias winding

Operation

ThisTinySwitch-IIbasedflybackconverterproducesanisolated

3W, 9VDC output from an 85VAC to 265VAC input. Fusible

resistorRF1givesshortcircuitfaultprotectionandlimitsstartup

inrush current. Inductors L1 and L2 and capacitors C1 and C2

form a low-cost pi (π) filter that attenuates conducted EMI. In

addition, the internal frequency jitter of the TinySwitch-II, the

use of shield windings, an output RC snubber, and a primary-

side RCD clamp makes the circuit meet CISPR-22 Class B

conducted EMI limits without aYcapacitor, resulting in a very

low value of AC leakage current (<300 µA).

The key performance characteristic of this circuit is the

extremely low no-load power consumption of <200 mW