LNK353/354
CY1
100 pF
T1
EE16
D6
SS14
9
5.7 V,
400 mA
J3-2
C6
330
µF
16 V
Q1
MMST
3906
VR1
BZX79B5V1
5.1 V, 2%
R9
200
Ω
R6
6.8
Ω
R7
220
Ω
RTN
J3-1
D1
1N4005
RF1
8.2
Ω
2.5 W
J1
85-265
VAC
J2
D2
1N4005
R1
100 kΩ
C3
2.2 nF
400 V
R3
200
Ω
5 4 5
R5
68
Ω
C5
2.2 nF
3
NC NC
8
D5
1N4007GP
C1
4.7
µF
400 V
C2
4.7
µF
400 V
LinkSwitch-HF
D
FB
BP
R4
5.1 kΩ
R8
390
Ω
D3
1N4005
D4
1N4005
U1
LNK354P
S
L1
1 mH
C4
100 nF
U2B
U2A
PC817D PC817D
R10
2.4
Ω
1W
PI-3891-070204
Figure 5. Universal Input, 5.7 V, 400 mA, Constant Voltage, Constant Current Battery Charger Using LinkSwitch-HF.
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 diode VR1 and the emitter-base voltage of PNP transistor
Q1. The V
BE
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.
During load transients, R6 and R8 ensure that the ratings of Q1 are
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
sense resistor exceeds the optocoupler diode forward conduction
voltage of approximately 1 V. The current through the LED
is therefore determined by the output current and CC control
dominates over the CV feedback loop. CC control is maintained
even under output short circuit conditions.
4
F
2/05
POWERINT [ POWER INTEGRATIONS, INC. ]
