Design Idea DI-75

Regulated Charger/Adapter

Design Highlights

• Replaces a linear transformer based power supply at the

same or lower cost, with better output regulation

• No-loadinputpowerconsumption<300mWat230VAC

input: meets worldwide energy efficiency guidelines

• Extremely simple circuit – requires only 26 components!

• Output voltage (CV) tolerance: ±5% at peak power

point

• Output current (CC) tolerance: ±25% when L ≤ ±10%

• Features short-circuit, open loop and thermal protection

• Greater than 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

capacitorsC1andC2formalow-costpi(π)filterthatattenuates

conducted EMI. Transformer (T1) winding phasing and D7

orientation let no secondary winding current flow when the

U1-MOSFETis 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 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 feedback path. Diode D6 is in the return leg of

thewinding,makingitshieldthecorefromtheprimarywinding.

The bias winding and the primary-to-secondary shield winding

both reduce EMI.

CONTROL pin capacitor C6 stores energy and supplies it 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

controlled for LinkSwitch; therefore P

www.powerint.com

DI-75

May 2004

PI-3869-051204

85 - 265

VAC

L

N

RTN

5.0 V, 0.5 A

RF1

8.2

Ω

1 W

Fusible

D1-D4

1N4005 x 4

C1

4.7

µF

400 V

L1

1.0 mH

C3

470 pF

500 V

D5

1N4007G

U1

LNK520

U2

U2

L2

Ferrite

Bead

C6

220 nF

PC817A

PC817A

C5

1

µF

50 V

12T

26 AWG TIW

D7

11DQ06

T1 EE13

Lp = 2.40 mH

D6

1N4005G

3

3

7

1

8

2

4

C7

330

µF

16 V

R4

15

Ω

R2

150 k

Ω

R1

100

Ω

114T

35 AWG

C2

4.7

µF

400 V

LinkSwitch

R8

910

Ω

R7

120

Ω

VR1

4.7 V

2%

R5

7.5 k

Ω

D

S

C

Figure 1. 2.5 W LinkSwitch Based Charger/Adapter.

~~

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

Charger/Adaper

LNK520

2.5 W

85-265 VAC

5.0 V

Flyback