Design Idea DI-60
®
LinkSwitch
Low Cost 2.5 W
Regulated Charger/Adapter
Application
Charger/Adapter
Device
LNK501
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
<
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!
• Meets EN550022 B EMI without a Y-1 Safety capacitor
• Ultra-low leakage current: < 5
µA
at 265 VAC input
The bias/feedback winding (T1, pins 3-4) provides U1 with
CONTROL pin current. In CV mode, that current is controlled
by the U2-phototransistor. At start-up and in CC mode, 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.
LinkSwitch
solutions must only operate in 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
LinkSwitch;
therefore,
P
O
is
proportional to
L.
Operation
Fusible resistor RF1 gives short-circuit fault protection and
limits start-up inrush current. 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 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
flyback voltage across the primary winding, as the U1-MOSFET
turns OFF.
D1-D4
1N4005 x 4
L1
1.0 mH
C3
1 nF
500 V
T1 EE13
Lp = 2.36 mH
8
D7
11DQ06
5.5 V, 0.45 A
12T
26 AWG TIW
1
1
114T
34 AWG
2
D5
1N4007G
PC817A
N
D
C
5
4
3
RTN
L
85 - 265
VAC
C4
100 pF
500 V
PC817A
U2
U2
LinkSwitch
U1
LNK501
L2
Ferrite
Bead
S
C6
220 nF
D6
BAV20
VR1
5.1 V
2%
PI-3695-103103
Figure 1. 2.5 W LinkSwitch Based Charger/Adapter.
DI-60
www.powerint.com
December 2004
POWERINT [ POWER INTEGRATIONS, INC. ]
