Design Idea DI-60
LinkSwitch® Low Cost 2.5 W
Regulated Charger/Adapter
Input Voltage
Topology
Device
Power Output
Output Voltage
Application
Flyback
Charger/Adapter
2.5 W
85-265 VAC
LNK501
5.5 V
Design Highlights
• Replacesalineartransformerbasedpowersupplyatthesame
or lower cost, with better output regulation
• No-load input power consumption < 300 mW, at 230 VAC
input: meets worldwide energy conservation guidelines
• Extremely simple circuit – requires only 29 components!
• Output voltage (CV) tolerance: ±10% at peak power point
• Output current (CC) tolerance: ±25% when L ≤ ±10%
• Features short-circuit, open loop and thermal protection
• Typically about 70% efficient!
The bias/feedback winding (T1, pins 3-4) provides U1 with
CONTROLpincurrent. InCVmode,thatcurrentiscontrolled
bytheU2-phototransistor. Atstart-upandinCCmode,when
U2 is OFF, R5 provides the current path. Diode D6 is in the
return leg of the winding, making it shield the core from the
primary winding. The bias winding and the primary-to-
secondary shield winding (T1, pin 1) both reduce EMI.
Resistor R3 and C4 reduce EMI.
CONTROL pin capacitor C6 stores energy and supplies it
back to U1 at start-up, determines the “restart attempt rate”
in the auto-restart mode, shunts high frequency switching
noise around U1 and provides U1 with the instantaneous
MOSFET gate-drive current it requires. The combined
voltages of VR1, R7 and the U2-LED determine the output
voltage. Resistor R8 provides bias current to VR1. The
output voltage can be fine-tuned by adjusting the values of
R7 and R8.
• Meets EN550022 B EMI without a Y-1 Safety capacitor
• Ultra-low leakage current: < 5 µA at 265 VAC input
Operation
Fusible resistor RF1 gives short-circuit fault protection and
limitsstart-upinrushcurrent. InductorsL1andL2andcapacitors
C1 and C2 form a low-cost pi (π) filter that attenuates conducted
EMI. Transformer (T1) winding phasing and D7 orientation let
no secondary winding current flow when the U1-MOSFET is
ON, so the primary winding current stores its energy in the core
of T1. When the U1-MOSFET turns OFF, the energy stored in
T1 drives current out of the secondary winding, forward biasing
D7, charging C7 and developing/maintaining the output voltage
across C7. The RCD network of C3, D5, R1 and R2 clamps the
flybackvoltageacrosstheprimarywinding,astheU1-MOSFET
LinkSwitch solutions must only operate in discontinuous
2
~
conduction mode. PO 0.5 L I f, where P = Output Power,
~
O
L = transformer primary inductance, I = LinkSwitch peak
current, f = Switching frequency and η = Efficiency. I2f is
accurately controlled for LinkSwitch; therefore, PO is
proportional to L.
turns OFF.
C3
T1 EE13
Lp = 2.36 mH
8
D7
11DQ06
L1
1.0 mH
D1-D4
1N4005 x 4
1 nF
5.5 V, 0.45 A
RTN
500 V
12T
26 AWG TIW
1
1
5
4
114T
34 AWG
2
3
L
D5
1N4007G
C4
100 pF
500 V
85 - 265
VAC
PC817A
PC817A
N
U2
D
S
U2
C
LinkSwitch
L2
Ferrite
Bead
U1
LNK501
VR1
5.1 V
2%
C6
220 nF
D6
BAV20
Figure 1. 2.5 W LinkSwitch Based Charger/Adapter.
PI-3695-103103
December 2004
DI-60
www.powerint.com
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