Design Idea DI-59
®
LinkSwitch
Low Cost 2.5 W
CV/CC Charger or Adapter
Application
Charger/Adapter
Device
LNK500
Power Output
2.5 W
Input Voltage
85-265 VAC
Output Voltage
5.5 V
Topology
Flyback
Design Highlights
• Replaces a linear transformer based power supply at the
same or lower cost, with better output regulation
• No-load input power consumption is less than 300 mW, at
230 VAC input: meets worldwide conservation guidelines
• Extremely simple circuit – requires only 23 components!
• No alternate path of secondary-side feedback is required:
output feedback is obtained through the main transformer!
• Output Voltage (CV) tolerance
±10%
at peak power point
• Output Current (CC) tolerance
±25%,
when L
≤
±10%
• Features the following auto-recovering protection functions:
output short-circuit, open feedback loop, thermal shutdown
• Typically about 68% efficient!
• Meets EN550022 B EMI limits, without a Y-1 type Safety
capacitor between the primary and the secondary
• Ultra-low leakage current: < 5
µA
at 265 VAC input
T1 drives current out of the secondary winding, forward
biasing D6, charging C6 and developing/maintaining the
output voltage across C6.
The RCD network of C3, D5 and R1 has two functions:
1) It clamps the reflection of the output voltage (V
OR
) on the
primary winding, as the MOSFET turns OFF.
2) It holds V
OR
, for use as output feedback, eliminating the
cost and complexity of a separate feedback path.
Resistor R1 attenuates the switching noise from the V
OR
.
Resistor R2 determines the amount of feedback current that
flows into the CONTROL pin of U1. The output voltage can
be fine-tuned by varying the value of R2. CONTROL pin
capacitor C4 stores power and supplies it back to U1 during
start-up, determines the restart “attempt” frequency in the
auto-restart mode, shunts high frequency noise around U1
and provide U1 with the instantaneous MOSFET gate-drive
current it requires. Resistor R3, capacitor C5, the 22-turn
core cancellation winding and the 5-turn shield winding all
reduce EMI.
LinkSwitch
based solutions are designed to operate only in
the discontinuous conduction mode.
P
O
~ 0.5 L I
2
f,
where
P
O
= Output Power,
L
= transformer primary inductance,
I
=
LinkSwitch
peak current,
f
= Switching frequency and
η
= Efficiency.
I
2
f
is accurately controlled for the
LinkSwitch;
therefore,
P
O
is proportional to
L.
T1 EE13
L
P
= 2.3 mH
5
1
Operation
Fusible resistor RF1 gives short-circuit fault protection and
limits start-up inrush current. Diodes D1–D4 provide full
bridge rectification that charges capacitors C1 and C2. Inductors
L1 and L2 and capacitors C1 and C2 form a low-cost pi (π) filter
that attenuates conducted EMI. Transformer (T1) winding
phasing and D6 orientation let no secondary winding current
flow when the MOSFET in U1 is ON, so the current that flows
through the primary winding stores its energy in the core of T1.
When the MOSFET in U1 is turned OFF, the energy stored in
L1
1.5 mH
LinkSwitch
D
C
S
5.5 V, 0.45 A
U1
LNK500
L
85-265
VAC
N
RF1
8.2
Ω
1 W Fusible
D1-D2
1N4007
C2
4.7
µF
400 V
C4
0.22
µF
25 V
C3
100 nF
63 V
FILM
90T, #36
2
22T, #34
3
R3
100
Ω
C5
100 pF
500 V
15T, #30, TIW
6
2
5T,
#30 x 3
C6
330
µF
16 V
D6
SB1100
R2
17.4 kΩ
1%
R5
22 kΩ
C1
4.7
µF
400 V
L2
Ferrite
Bead
D5
1N4937
R1
100
Ω
RTN
C7
R4
33
Ω
150 pF
100 V
D3-D4
1N4007
Figure 1. 2.5 W LinkSwitch Based Charger/Adapter
DI-59
PI-3692-091903
www.powerint.com
September 2003
POWERINT [ POWER INTEGRATIONS, INC. ]
