Design Idea DI-78
®
LinkSwitch-HF Low Cost 1.6 W
CV/CC Charger/Adapter
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
Charger/Adapter
LNK353
1.6 W
85-265 VAC
5.2 V
Flyback
Design Highlights
through the use of transformer shield windings, a snubber (R4
and C9) across the output diode, and by the internal switching
frequency jitter of LinkSwitch-HF.
•
•
Low cost, low component count solution
No-loadpowerconsumption<300mW–meetsworldwide
energy efficiency requirements
LinkSwitch-HF uses ON/OFF control to regulate the output.
Current >49 µA fed from the BYPASS pin capacitor C5 into
the FEEDBACK pin disables the MOSFET for that switching
cycle.
•
•
•
Low leakage current design, <10 µA
Universal AC input range – single design worldwide
Secondary-side feedback gives accurate voltage and
constant current output
•
•
Small size and weight
Meets EN55022 B EMI limits
To maximize efficiency, a Schottky diode (D7) is used to rectify
the secondary. Filtering is provided by a low ESR capacitor
C6 and for most battery chargers, no additional filtering is
required.
Operation
The AC input is rectified and filtered and applied to one
side of the primary winding. The other end of the primary is
connected to the DRAIN pin of U1. During the on-time of
U1, energy is stored in the transformer and delivered to the
load when U1 switches off. A simple RCD clamp (D5, R2,
C3 and R3) limits the peak drain voltage.
During CV operation the output voltage is determined by the
sum of the Q1 VBE voltage and VR1. As the voltage increases
above this level, Q1 is biased on and current is fed through the
optocoupler LED, thus inhibiting switching cycles to maintain
regulation. The transition to CC operation occurs when the
voltage developed across R9 and R8 exceeds the LED forward
voltage of U2 (~1V). Resistors R7 and R8 limit the optocoupler
current during transients. Resistor R5 limits the base current of
Q1 and R6 provides a bias current through VR1 (~0.5 mA).
A simple pi (π) filter formed by C1, L1 and C2 together with
RF1 are sufficient to attenuate differential conducted EMI.
Common mode and radiated EMI generation is minimized
C9
680 pF
100 V
C10
100 pF
Y1
R4
62 Ω
C3
L2
Bead
R5
5.1 Ω
R6
1.2 kΩ
R3
2.2 nF
5.2 V,
320 mA
200 kΩ 400 V
1
4
7
6
J3
BR1
MD5S
0.5 A,
600 V
D7
10BQ060
Q1
C1
C7
100 µF
10 V
R1
2N3906
2.2 µF
400 V
200 kΩ
RF1
8.2 Ω
R2
VR1
J1
NC NC
200 Ω
C6
BZX79-
B5V1
5.1 V, 2%
470 µF
10 V
T1
EP10
R7
390 Ω
D5
85-265
VAC
Lp = 900 µH
1N4007G
C2
2.2 µF
400 V
R8
100 Ω
1%
J2
D
S
FB
BP
U2B
PC817D
U2A
PC817D
LinkSwitch-HF
U1
R9
3 Ω
C5
L1
Bead
LNK353
0.1 µF
1/2 W
J4
50 V
RTN
PI-3899-072204
Figure 1. Schematic of a 1.6 W, 5.2 V Constant Voltage, 320 mA Constant Current Charger Using LNK353P.
November 2004
DI-78
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