LNK353/354

F

2/05

4

Applications Example

A 2.4 W CC/CV Charger Adapter

The circuit shown in Figure 5 is a typical implementation of

a 5.7 V, 400 mA, constant voltage, constant current (CV/CC)

battery charger.

The input bridge formed by diodes D1-D4, rectifies the AC

input voltage. The rectified AC is then filtered by the bulk

storage capacitors C1 and C2. Resistor RF1 is a flameproof,

fusible, wire wound type and functions as a fuse, inrush current

limiter and, together with the π filter formed by C1, C2 and L1,

differential mode noise attenuator.

This simple EMI filtering, together with the frequency jittering

of LinkSwitch-HF (U1), a small value Y1 capacitor (CY1),

and shield windings within T1, and a secondary-side RC

snubber (R5, C5), allows the design to meet both conducted

and radiated EMI limits. The low value of CY1 is important

to meet the requirement of low line frequency leakage current,

in this case <10 µA.

The rectified and filtered input voltage is applied to the primary

winding of T1. The other side of the transformer primary is

driven by the integrated MOSFET in U1. Diode D5, C3, R1

and R3 form the primary clamp network. This limits the peak

drain voltage due to leakage inductance. Resistor R3 allows the

use of a slow, low cost rectifier diode by limiting the reverse

current through D5 when U1 turns on. The selection of a slow

diode improves efficiency and conducted EMI.

Output rectification is provided by Schottky diode D6. The low

forward voltage provides high efficiency across the operating

range and the low ESR capacitor C6 minimizes output voltage

ripple.

In constant voltage (CV) mode, the output voltage is set by the

Zener diodeVR1 and the emitter-base voltage of PNPtransistor

Q1. The V

current through VR1 (~2.7 mA). When the output voltage

exceeds the threshold voltage determined by Q1 and VR1, Q1

is turned on and current flows through the LED of U2. As the

LED current increases, the current fed into the FEEDBACK

pin increases, disabling further switching cycles of U1. At

very light loads, almost all switching cycles will be disabled,

giving a low effective switching frequency and providing low

no-load consumption.

Duringloadtransients,R6andR8ensurethattheratingsofQ1are

not exceeded while R4 prevents C4 from being discharged.

Resistors R9 and R10 form the constant current (CC) sense

circuit. Above approximately 400 mA, the voltage across the

senseresistorexceedstheoptocouplerdiodeforwardconduction

voltage of approximately 1 V. The current through the LED

is therefore determined by the output current and CC control

dominatesovertheCVfeedbackloop.CCcontrolismaintained

even under output short circuit conditions.

D

S

FB

BP

RF1

8.2

Ω

2.5 W

85-265

VAC

5.7 V,

400 mA

J3-2

RTN

J3-1

J1

J2

R3

200

Ω

R5

68

Ω

R1

100 k

Ω

R4

5.1 k

Ω

R6

6.8

Ω

R9

200

Ω

R10

2.4

Ω

1 W

R8

390

Ω

U2A

PC817D

VR1

BZX79B5V1

5.1 V, 2%

R7

220

Ω

U2B

PC817D

D1

1N4005

D2

1N4005

D5

1N4007GP

D3

1N4005

D4

1N4005

D6

SS14

C6

330

µF

16 V

Q1

MMST

3906

L1

1 mH

CY1

100 pF

C1

4.7

µF

400 V

C3

2.2 nF

400 V

5

3

4 5

9

8

T1

EE16

NC NC

U1

LNK354P

C2

4.7

µF

400 V

C4

100 nF

C5

2.2 nF

LinkSwitch-HF

PI-3891-070204

Figure 5. Universal Input, 5.7 V, 400 mA, Constant Voltage, Constant Current Battery Charger Using LinkSwitch-HF.