Design Idea DI-58
LinkSwitch
Low Cost 1.5 W
CV/CC Charger or Adapter
Application
Charger/Adapter
Device
LNK500
Power Output
1.5 W
Input Voltage
85-265 VAC
Output Voltage
5.5 V
Topology
Flyback
Design Highlights
• Replaces a linear transformer based supply at the same or
lower cost but with much higher performance
• <0.3 W consumption at zero load meets worldwide guidelines
(EC’s 0.3 W, USA’s 1 W for example)
• Extremely simple circuit – only 15 components for
production-worthy design
• Primary based CV/CC output – no secondary sense
components required
•
±10%
output voltage and
±25%
output current tolerances at
peak power point
• Fully protected for thermal, short circuit and open loop faults
• >62% efficiency (>70% with R1 replaced by an inductor)
• Meets CISPR22B/EN55022B EMI limits with low cost
resistive input filter
• Ultra-low leakage current design <5
µA
• EE13 core for low cost and small size
LinkSwitch
derives all feedback information from the primary.
During output diode conduction, the output voltage reflected
through the turns ratio is sampled and held by C4. The
feedback voltage across C4 (V
OR
) is converted into feedback
current by R2 and fed into the CONTROL pin. This feedback
current regulates the output by PWM control during CV
operation, and by reducing the internal current limit during
CC operation. Below an output voltage of ~2 V,
LinkSwitch
enters auto-restart, limiting average output current to less
than 50 mA. The nominal transition from CV to CC occurs
at 5.5 V, 0.27 A. The output envelope characteristic and
specification limits are shown in Figure 2.
Together with D3, C4 and R2 are also part of the primary clamp,
limiting the peak drain to source voltage due to leakage
inductance. Resistor R3 filters the leading edge leakage
inductance spike, reducing the error in the feedback voltage.
The CONTROL pin capacitor C3 provides energy storage for
supply start-up and sets auto-restart timing during fault
conditions.
Operation
The AC input is rectified and filtered by D1, D2, C1 and C2.
Conducted EMI filter is provided both by a
π
filter (C1, R1 and
C2) and a differential filter (RF1 and C1). Together with a
shield in the transformer (formed from part of the primary), the
design meets conducted EMI limits with no Y-capacitor between
primary and secondary. Resistor RF1 also functions as a fuse.
Key Design Points
• Select transformer turns ratio to give a V
OR
of 40 V
to 60 V. Lower values reduce power capability; higher
values increase no-load consumption.
• R2 provides 2.3 mA into the CONTROL pin at the peak
power point at 85 VAC. The value can be adjusted to
center the output voltage.
T1
1
C3
0.22
µF
C4
0.1
µF
100 V
FILM
104T
34 AWG
12T
2 x 30 AWG
3
EE13
D4
L
P
= 1.36 mH UF4002
1A 100V
4
5
15 T
30 AWG
T.I.W.
C5
220
µF
16V
RTN
5.5 V, 0.27 A
R1
68
Ω
Flameproof
LinkSwitch
D
C
S
U1
LNK500
L
85-265
VAC
N
RF1
10
Ω
Fusible
D1
1N4007
R2
21 kΩ
C1, C2
2.2
µF
400 V
6
D3
1N4937
R3
100
Ω
D2
1N4007
Figure 1. LinkSwitch 1.5 W Charger Power Supply: 85 VAC to 265 VAC Input, 5.5 V, 0.27 A Output
DI-58
PI-3687-091903
www.powerint.com
September 2003
POWERINT [ POWER INTEGRATIONS, INC. ]
