Design Idea DI-18
LinkSwitch
Low Cost 2.75 W
CV/CC Charger or Adapter
Application
Charger/Adapter
Device
LNK501
Power Output
2.75 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 17 components (14 with
integrated bridge) for production-worthy design
• Primary based CV/CC output – no secondary sense
components required
•
±10%
output voltage and
±20%
output current tolerances at
peak power point
• Fully protected for thermal, short circuit and open loop faults
• >70% efficiency
• Meets CISPR22B/EN55022B and FCC B EMI limits
• Meets 2.5 kV EN61000-4-5 differential surge
• 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
transformed through the turns ratio is sampled and held by
C4. The feedback voltage across C4 (V
OR
) is converted into
feedback current by R1 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 <50 mA. The nominal transition from CV to CC occurs at
5.5 V, 0.5 A. The output envelope characteristic and
specification limits are shown in Figure 2.
Together with D5, C4 and R1 are also part of the primary
clamp, limiting the peak drain to source voltage due to
leakage inductance. Resistor R2 filters the leading edge
leakage inductance spike, reducing the error in the feedback
voltage. The CONTROL pin capacitor C3 provides energy
storage for supply startup and sets auto-restart timing during
fault conditions.
Operation
The AC input is rectified and filtered by D1-D4, C1 and C2.
Conducted EMI filtering is provided both by a
π
filter (C1, L1
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.
D1, D2, D3, D4
1N4005 X 4
L1
1 mH
Key Design Points
• Select transformer turns ratio to give a V
OR
of 40-60 V.
Lower values reduce power capability, higher values
increase no-load consumption.
• R1 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
C3
0.22
µF
50 V
C4
0.1
µF
100 V
1
104 T
34 AWG
4
12 T
2 x 30 AWG
3
EE13
L
P
=2.55 mH
5
5.5 V, 0.5 A
C5
470
µF
10 V
RTN
15 T
30 AWG
T.I.W.
6
D6
11DQ06
1A, 60 V
LinkSwitch
D
C
S
U1
LNK501
R1
20.5 kΩ
1%
RF1
10
Ω
1 W
Fusible
85 - 265 VAC
C1
4.7
µF
400 V
C2
4.7
µF
400 V
D5
1N4937
R2
100
Ω
PI-3364-091602
Figure 1. LinkSwitch 2.75 W Charger Power Supply: 85 VAC to 265 VAC Input, 5.5 V, 0.5 A Output
DI-18
www.powerint.com
September 2002
POWERINT [ POWER INTEGRATIONS, INC. ]
