®
Design Idea DI-18
Lin k S w it c h Low Cost 2.75 W
CV/CC Charger or Adapter
Topology
Flyback
Device
Power Output
Input Voltage
Output Voltage
Application
Charger/Adapter
2.75 W
85-265 VAC
LNK501
5.5 V
LinkSwitchderivesallfeedbackinformationfromtheprimary.
During output diode conduction, the output voltage
transformed through the turns ratio is sampled and held by
C4. The feedback voltage across C4 (VOR) is converted into
feedbackcurrentbyR1andfedintotheCONTROLpin. This
feedbackcurrentregulatestheoutputbyPWMcontrolduring
CV operation, and by reducing the internal current limit
during CC operation. Below an output voltage of ~2 V
LinkSwitchentersauto-restart,limitingaverageoutputcurrent
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.
Design Highlights
• Replaces a linear transformer based supply at the same or
lower cost but with much higher performance
• <0.3Wconsumptionatzeroloadmeetsworldwideguidelines
(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
• Fullyprotectedforthermal,shortcircuitandopenloopfaults
• >70% efficiency
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.
• 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
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
ashieldinthetransformer(formedfrompartoftheprimary)the
designmeetsconductedEMIlimitswithnoY-capacitorbetween
primary and secondary. Resistor RF1 also functions as a fuse.
Key Design Points
• Select transformer turns ratio to give a VOR 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.
L1
1 mH
D1, D2, D3, D4
1N4005 X 4
Lin k S w it c h
T1
D
S
5.5 V, 0.5 A
5
1
4
C3
0.22 µF
50 V
U1
15 T
C
C5
470 µF
10 V
104 T
34 AWG
LNK501
30 AWG
T.I.W.
6
RF1
10 Ω 1 W
Fusible
RTN
12 T
C4
R1
20.5 kΩ
1%
D6
11DQ06
1A, 60 V
2 x 30 AWG
3
0.1 µF
100 V
C1
C2
EE13
LP=2.55 mH
4.7 µF
400 V
4.7 µF
400 V
85 - 265 VAC
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
September 2002
DI-18
www.powerint.com
POWERINT [ Power Integrations ]
