Design Idea DI-60

Regulated Charger/Adapter

DI-60

®

Application

Charger/Adapter

LNK501

5.5 V

Power Output

2.5 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Flyback

Figure 1. 2.5 W LinkSwitch Based Charger/Adapter.

Design Highlights

• Replaces a linear transformer based power supply at the same

or lower cost, with better output regulation

• No-load input power consumption < 300 mW, at 230 VAC

input: meets worldwide energy conservation guidelines

• Extremely simple circuit – requires only 29 components!

• Output voltage (CV) tolerance:

±10% at peak power point

• Output current (CC) tolerance:

±25% when L ≤ ±10%

• Features short-circuit, open loop and thermal protection

• Typically about 70% efficient!

• Meets EN550022 B EMI without a Y-1 Safety capacitor

• Ultra-low leakage current: < 5

µA at 265 VAC input

Operation

Fusible resistor RF1 gives short-circuit fault protection and

limits start-up inrush current. Inductors L1 and L2 and capacitors

C1 and C2 form a low-cost pi (

π) filter that attenuates conducted

EMI. Transformer (T1) winding phasing and D7 orientation let

no secondary winding current flow when the U1-MOSFET is

ON, so the primary winding current stores its energy in the core

of T1. When the U1-MOSFET turns OFF, the energy stored in

T1 drives current out of the secondary winding, forward biasing

D7, charging C7 and developing/maintaining the output voltage

across C7. The RCD network of C3, D5, R1 and R2 clamps the

flyback voltage across the primary winding, as the U1-MOSFET

turns OFF.

The bias/feedback winding (T1, pins 3-4) provides U1 with

CONTROL pin current. In CV mode, that current is controlled

by the U2-phototransistor. At start-up and in CC mode, when

U2 is OFF, R5 provides the current path. Diode D6 is in the

return leg of the winding, making it shield the core from the

primary winding. The bias winding and the primary-to-

secondary shield winding (T1, pin 1) both reduce EMI.

Resistor R3 and C4 reduce EMI.

CONTROL pin capacitor C6 stores energy and supplies it

back to U1 at start-up, determines the “restart attempt rate”

in the auto-restart mode, shunts high frequency switching

noise around U1 and provides U1 with the instantaneous

MOSFET gate-drive current it requires. The combined

voltages of VR1, R7 and the U2-LED determine the output

voltage. Resistor R8 provides bias current to VR1. The

output voltage can be fine-tuned by adjusting the values of

R7 and R8.

LinkSwitch solutions must only operate in discontinuous

conduction mode. P

O

L = transformer primary inductance, I = LinkSwitch peak

current, f = Switching frequency and

η = Efficiency. I

accurately controlled for LinkSwitch; therefore, P

O

is

proportional to L.

~

~

PI-3695-103103

85 - 265

VAC

L

N

RTN

5.5 V, 0.45 A

D1-D4

1N4005 x 4

L1

1.0 mH

C3

1 nF

500 V

D5

1N4007G

U1

LNK501

U2

U2

L2

Ferrite

Bead

C6

220 nF

PC817A

PC817A

12T

26 AWG TIW

D7

11DQ06

T1 EE13

Lp = 2.36 mH

D6

BAV20

1

1

8

4

5

3

2

114T

34 AWG

C4

100 pF

500 V

LinkSwitch

VR1

5.1 V

2%

D

S

C

www.powerint.com

December 2004