LNK362-364
CY1
100 pF
250 VAC
C4
L1
6.2 V,
330 µF
16 V
T1
1 mH
322 mA
EE16
4
5
9
8
J3
J4
D5
1N4934
3
NC NC
VR1
BZX79-
B5V1
RF1
D1
D2
R1
3.9 k
1/8 W
8.2 Ω
1N4005 1N4005
2.5 W
J1
5.1 V, 2%
R2
390 Ω
1/8 W
C1
3.3 µF
400 V
C2
3.3 µF
400 V
85-265
VRMS
R3
1 k
1/8 W
U2
PC817A
J2
D
S
FB
BP
LinkSwitch-XT
U1
LNK362P
D3
D4
C3
100 nF
50 V
1N4005 1N4005
L2
1 mH
PI-4162-110205
Figure 5. 2 W Universal Input CV Adapter Using LNK362.
The rectified and filtered input voltage is applied to the primary
winding of T1. The other side of the primary is driven by the
integrated MOSFETin U1. No primary clamp is required as the
low value and tight tolerance of the LNK362 internal current
limit allows the transformer primary winding capacitance to
provide adequate clamping of the leakage inductance drain
voltage spike.
Applications Example
A 2 W CV Adapter
The schematic shown in Figure 5 is a typical implementation of
a universal input, 6.2 V ±7%, 322 mA adapter using LNK362.
ThiscircuitmakesuseoftheClamplesstechniquetoeliminatethe
primary clamp components and reduce the cost and complexity
of the circuit.
The secondary of the flyback transformer T1 is rectified by D5,
a low cost, fast recovery diode, and filtered by C4, a low ESR
capacitor. The combined voltage drop across VR1, R2 and the
LED of U2 determines the output voltage. When the output
voltage exceeds this level, current will flow through the LED
of U2. As the LED current increases, the current fed into the
FEEDBACK pin of U1 increases until the turnoff threshold
current (~49 µA) is reached, disabling further switching cycles
of U1.At full load, almost all switching cycles will be enabled,
and at very light loads, almost all the switching cycles will be
disabled, giving a low effective frequency and providing high
light load efficiency and low no-load consumption.
The Ecosmart features built into the LinkSwitch-XT family
allow this design to easily meet all current and proposed
energyefficiencystandards,includingthemandatoryCalifornia
Energy Commission (CEC) requirement for average operating
efficiency.
The AC input is rectified by D1 to D4 and 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, L1
and L2, differential mode noise attenuator. Resistor R1 damps
ringing caused by L1 and L2.
Resistor R2 provides 1 mA through VR1 to bias the Zener
closer to its test current. Resistor R1 allows the output voltage
to be adjusted to compensate for designs where the value of the
Zener may not be ideal, as they are only available in discrete
voltage ratings. For higher output accuracy, the Zener may be
replaced with a reference IC such as the TL431.
This simple input stage, together with the frequency jittering of
LinkSwitch-XT, a low value Y1 capacitor and PIʼs E-Shield™
windings within T1, allow the design to meet both conducted
and radiated EMI limits with >10 dBµV margin. The low value
of CY1 is important to meet the requirement for a very low
touch current (the line frequency current that flows through
CY1) often specified for adapters, in this case <10 µA.
B
11/05
4
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