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Design Idea DI-58

CV/CC Charger or Adapter

September 2003

DI-58



Application

Charger/Adapter

LNK500

5.5 V

Power Output

1.5 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 15 components for

production-worthy design

• Primary based CV/CC output – no secondary sense

components required

•

±10% output voltage and ±25% output current tolerances at

peak power point

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

• >62% efficiency (>70% with R1 replaced by an inductor)

•Meets CISPR22B/EN55022B EMI limits with low cost

resistive input filter

• 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, D2, C1 and C2.

Conducted EMI filter is provided both by a

π filter (C1, R1 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 1.5 W Charger Power Supply: 85 VAC to 265 VAC Input, 5.5 V, 0.27 A Output

LinkSwitch derives all feedback information from the primary.

During output diode conduction, the output voltage reflected

through the turns ratio is sampled and held by C4. The

feedback voltage across C4 (V

current by R2 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 less

than 50 mA. The nominal transition from CV to CC occurs

at 5.5 V, 0.27 A. The output envelope characteristic and

specification limits are shown in Figure 2.

Together with D3, C4 and R2 are also part of the primary clamp,

limiting the peak drain to source voltage due to leakage

inductance. Resistor R3 filters the leading edge leakage

inductance spike, reducing the error in the feedback voltage.

The CONTROL pin capacitor C3 provides energy storage for

supply start-up and sets auto-restart timing during fault

conditions.

Key Design Points

• Select transformer turns ratio to give a V

to 60 V. Lower values reduce power capability; higher

values increase no-load consumption.

• R2 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, C2

2.2 µF

400 V

C3

0.22 µF

R2

21 kΩ

R3

100 Ω

D3

1N4937

C4

0.1 µF

100 V

FILM

D4

UF4002

1A 100V

5.5 V, 0.27 A

L

N

RTN

RF1

10 Ω

Fusible

T1

85-265

VAC

D1

1N4007

15 T

30 AWG

T.I.W.

C5

220 µF

16V

LinkSwitch

D

S

C

U1

LNK500

104T

34 AWG

12T

2 x 30 AWG

1

5

6

4

3

D2

1N4007

R1

68 Ω

Flameproof

PI-3687-091903

EE13

L