www.powerint.com

September 2003

DI-59

Application

Charger/Adapter

LNK500

5.5 V

Power Output

2.5 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Flyback

Design Highlights

• Replaces a linear transformer based power supply at the

same or lower cost, with better output regulation

• No-load input power consumption is less than 300 mW, at

230 VAC input: meets worldwide conservation guidelines

• Extremely simple circuit – requires only 23 components!

• No alternate path of secondary-side feedback is required:

output feedback is obtained through the main transformer!

• Output Voltage (CV) tolerance

±10% at peak power point

• Output Current (CC) tolerance

±25%, when L ≤ ±10%

• Features the following auto-recovering protection functions:

output short-circuit, open feedback loop, thermal shutdown

• Typically about 68% efficient!

•Meets EN550022 B EMI limits, without a Y-1 type Safety

capacitor between the primary and the secondary

• 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. Diodes D1–D4 provide full

bridge rectification that charges capacitors C1 and C2. 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 D6 orientation let no secondary winding current

flow when the MOSFET in U1 is ON, so the current that flows

through the primary winding stores its energy in the core of T1.

When the MOSFET in U1 is turned OFF, the energy stored in

Figure 1. 2.5 W LinkSwitch Based Charger/Adapter

Design Idea DI-59

LinkSwitch

®

Low Cost 2.5 W

CV/CC Charger or Adapter

T1 drives current out of the secondary winding, forward

biasing D6, charging C6 and developing/maintaining the

output voltage across C6.

The RCD network of C3, D5 and R1 has two functions:

1) It clamps the reflection of the output voltage (V

primary winding, as the MOSFET turns OFF.

2) It holds V

cost and complexity of a separate feedback path.

Resistor R1 attenuates the switching noise from the V

Resistor R2 determines the amount of feedback current that

flows into the CONTROL pin of U1. The output voltage can

be fine-tuned by varying the value of R2. CONTROL pin

capacitor C4 stores power and supplies it back to U1 during

start-up, determines the restart “attempt” frequency in the

auto-restart mode, shunts high frequency noise around U1

and provide U1 with the instantaneous MOSFET gate-drive

current it requires. Resistor R3, capacitor C5, the 22-turn

core cancellation winding and the 5-turn shield winding all

reduce EMI.

LinkSwitch based solutions are designed to operate only in

the discontinuous conduction mode. P

O

P

I = LinkSwitch peak current, f = Switching frequency and

η = Efficiency. I

therefore, P

C1

4.7 µF

400 V

C2

4.7 µF

400 V

C4

0.22 µF

25 V

C5

100 pF

500 V

C7

150 pF

100 V

C6

330 µF

16 V

R2

17.4 kΩ

1%

R1

100 Ω

R3

100 Ω

R5

22 kΩ

R4

33 Ω

D5

1N4937

D6

SB1100

C3

100 nF

63 V

FILM

5.5 V, 0.45 A

RTN

RF1

8.2 Ω

1 W Fusible

D1-D2

1N4007

D3-D4

1N4007

LinkSwitch

D

S

C

U1

LNK500

90T, #36

15T, #30, TIW

22T, #34

1

T1 EE13

L

5

6

2

2

L1

1.5 mH

PI-3692-091903

85-265

VAC

L

N

5T,

#30 x 3

L2

Ferrite

Bead

3

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