Design Idea DI-85
®
LinkSwitch-LP 2 W Charger:
Replaces Unregulated Linear Solutions
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
Charger
LNK564P
2 W
90-265 VAC
6 V
Flyback
Design Highlights
Past the peak power point, cycle skipping ceases and U1
limits the supplyʼs output current by reducing its oscillator
(MOSFET switching) frequency, as the voltage on the FB pin
drops. If the load demand causes the FB pin voltage to drop
below an auto-restart threshold voltageVFB(AR) of 0.8V(1Vto
1.5 V on the supply output) for more than 100 ms, the IC goes
into auto-restart mode. In auto-restart, MOSFET switching is
enabled for about 100 ms, approximately every 800 ms, until
the FB pin voltage increases above 0.8 V.
• Low-cost, low parts-count solution: 14-17 components
• Proprietary IC design and winding techniques enable a
Clampless™ design with simple Filterfuse™ input stage
• ±5% over-temperature threshold – with hysteretic
recovery – keeps PCB temperatures below safety limits
• Auto-restart: output short circuit and open loop
protection
• IC creepage distance >3.2 mm: no arcing in high
humidity
Due to the frequency jittering of the U1 internal oscillator and
the E-Shield™ winding techniques used in constructing the
transformer (T1), conducted EMI is adequately attenuated by
the LC filter formed by L1 and C1. Inductor L1 serves as both
adifferentialmodechokeandafuse. ThisFilterfuseissleeved
with heat-shrink tubing, and its winding wire diameter was
selected so that it will open like a fuse if any single component
fails shorted. Thanks to the tight current limit tolerance of
the LinkSwitch-LP family and construction techniques used
on T1, the primary winding can be left Clampless, since the
peakdrainvoltagedoesnotapproachthe700Vdrain-to-source
breakdown voltage (BVDSS) of U1.
• Easily meets all EPS energy efficiency standards
• Meets CISPR-22 Class B EMI with good margin
Operation
This LinkSwitch-LP based flyback converter (Figure 1)
provides an output VI curve (Figure 2) similar to that of an
unregulatedline frequencytransformer basedsupply, but with
output current that is limited past the maximum rated output
power (peak power point).
From no-load to the 2 W peak power point, the LNK564P
(U1) regulates the output voltage by skipping switching
cycles, based on the current delivered into the FEEDBACK
(FB) pin. At the peak power point, the supply delivers
>300 mA of load current at >5.7 VDC.
With no optocoupler and such a low parts count, this supply
is cost competitive with the unregulated line frequency
transformer based solution it was designed to replace.
C5
220 µF
25 V
VR1*
D1
L1
D4
UF4002
RF1*
R3 1N5240B
T1
EE16
6 V,
1N4937
3300 µH
8.2 Ω
2 kΩ
10 V
L
0.33 A
2
1
7
6
J-1
J3-2
J3-1
RTN
2.5 W
C1
10 µF
400 V
90-265
VAC
J-2
4
N
D2
1N4005
5
R1
37.4 kΩ
D3
1N4005
C4*
D
S
100 pF
C3
330 nF
50 V
FB
BP
250 VAC
LinkSwitch-LP
U1
R2
3 kΩ
LNK564P
C2
0.1 µF
*Optional components
50 V
PI-4106-101105
Figure 1. LNK564 Based 6 V, 330 mA, 2 W, Low-Cost, Flyback Charger Power Supply.
February 2006
DI-85
www.powerint.com