LNK353/354
CY1
100 pF
D6
SS14
5.7 V,
400 mA
T1
EE16
5
3
4
5
9
8
J3-2
C3
R5
C6
2.2 nF
400 V
D1
D2
68 Ω
R1
100 kΩ
R6
6.8 Ω
R7
220 Ω
330 µF
16 V
1N4005 1N4005
RTN
J3-1
C5
2.2 nF
R3
200 Ω
RF1
8.2 Ω
2.5 W
Q1
NC NC
D5
1N4007GP
MMST
3906
J1
C1
C2
4.7 µF
400 V
VR1
BZX79B5V1
5.1 V, 2%
85-265
VAC
4.7 µF
400 V
R8
390 Ω
R4
5.1 kΩ
J2
D
S
R9
200 Ω
FB
BP
LinkSwitch-HF
U1
LNK354P
D3
D4
1N4005 1N4005
U2B
U2A
R10
2.4 Ω
1 W
PC817D PC817D
C4
100 nF
L1
1 mH
PI-3891-070204
Figure 5. Universal Input, 5.7 V, 400 mA, Constant Voltage, Constant Current Battery Charger Using LinkSwitch-HF.
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.
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.
In constant voltage (CV) mode, the output voltage is set by the
ZenerdiodeVR1andtheemitter-basevoltageofPNPtransistor
Q1. The VBE of Q1 divided by the value of R7 sets the bias
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.
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.
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.
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.
F
2/05
4
POWERINT [ Power Integrations ]
