www.powerint.com

Design Idea DI-18

CV/CC Charger or Adapter

September 2002

DI-18



Application

Charger/Adapter

LNK501

5.5 V

Power Output

2.75 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Flyback

Design Highlights

• Replaces a linear transformer based supply at the same or

lower cost but with much higher performance

• <0.3 W consumption at zero load meets worldwide guidelines

(EC’s 0.3 W, USA’s 1 W for example)

• Extremely simple circuit – only 17 components (14 with

integrated bridge) for production-worthy design

• Primary based CV/CC output – no secondary sense

components required

•

±10% output voltage and ±20% output current tolerances at

peak power point

• Fully protected for thermal, short circuit and open loop faults

• >70% efficiency

•Meets CISPR22B/EN55022B and FCC B EMI limits

•Meets 2.5 kV EN61000-4-5 differential surge

• Ultra-low leakage current design <5

µA

• EE13 core for low cost and small size

Operation

The AC input is rectified and filtered by D1-D4, C1 and C2.

Conducted EMI filtering is provided both by a

π filter (C1, L1

and C2) and a differential filter (RF1 and C1). Together with

a shield in the transformer (formed from part of the primary) the

design meets conducted EMI limits with no Y-capacitor between

primary and secondary. Resistor RF1 also functions as a fuse.

Figure 1. LinkSwitch 2.75 W Charger Power Supply: 85 VAC to 265 VAC Input, 5.5 V, 0.5 A Output

LinkSwitch derives all feedback information from the primary.

During output diode conduction, the output voltage

transformed through the turns ratio is sampled and held by

C4. The feedback voltage across C4 (V

feedback current by R1 and fed into the CONTROL pin. This

feedback current regulates the output by PWM control during

CV operation, and by reducing the internal current limit

during CC operation. Below an output voltage of ~2 V

LinkSwitch enters auto-restart, limiting average output current

to <50 mA. The nominal transition from CV to CC occurs at

5.5 V, 0.5 A. The output envelope characteristic and

specification limits are shown in Figure 2.

Together with D5, C4 and R1 are also part of the primary

clamp, limiting the peak drain to source voltage due to

leakage inductance. Resistor R2 filters the leading edge

leakage inductance spike, reducing the error in the feedback

voltage. The CONTROL pin capacitor C3 provides energy

storage for supply startup and sets auto-restart timing during

fault conditions.

Key Design Points

• Select transformer turns ratio to give a V

Lower values reduce power capability, higher values

increase no-load consumption.

• R1 provides 2.3 mA into the CONTROL pin at the

peak power point at 85 VAC. The value can be

adjusted to center the output voltage.

®

C1

4.7 µF

400 V

C2

4.7 µF

400 V

C3

0.22 µF

50 V

L1

1 mH

R1

20.5 kΩ

1%

R2

100 Ω

D5

1N4937

C4

0.1 µF

100 V

D6

11DQ06

1A, 60 V

C5

470 µF

10 V

U1

LNK501

LinkSwitch

5.5 V, 0.5 A

RTN

RF1

10 Ω 1 W

Fusible

D1, D2, D3, D4

1N4005 X 4

104 T

34 AWG

12 T

2 x 30 AWG

EE13

15 T

30 AWG

T.I.W.

T1

3

4

1

5

6

PI-3364-091602

D

S

C

85 - 265 VAC