Design Idea DI-76
®
LinkSwitch
3.1 W Low Parts Count Power Supply
Application
Device
Power Output
Input Voltage
Output Voltage Topology
Appliance/Industrial Control
LNK520P
3.1 W
100-375 VDC
12 V Flyback
Design Highlights
restart(forhardshortcircuits)andthermalshutdowneffectively
produce a very rugged design.
•
Replaces a linear transformer based power supply at the
same or lower cost, but with much better performance
High Efficiency: >75% at >100 mA of load current
Primary side regulated: requires no optocoupler
<300 mW no-load power consumption at 375 VDC
Very low parts count: only 9 components!
LinkSwitch derives all feedback information from the primary
side, thus no optocoupler is required. During output diode
(D4) conduction, the output voltage is reflected through the
transformer turns ratio, providing a primary feedback voltage
(VOR), which is rectified by diode D3 and held by capacitor C4.
Resistor R3 limits the effects of transformer leakage on the
feedback voltage. Resistor R2 feeds the VOR signal as a current
to the CONTROL pin of the LinkSwitch, controlling duty cycle
(and primary current limit during overload conditions). When
there is little or no feedback signal (such as a short circuit) the
LinkSwitch goes into auto-restart, limiting output current to
approximately 40 mA.
•
•
•
•
•
12 V output: ideal for driving motors, relays, or SCRs
Operation
The very low parts count LinkSwitch flyback circuit shown
below can be used as a general-purpose power supply or
linear adapter replacement.
The design provides regulated output voltage during normal
loads as well as a soft output-current limit during overload
conditions(suchasastalledmotorordefectiverelay).Adding
to this, the other LinkSwitch self-protection features of auto-
Capacitor C3 provides device decoupling. The extra winding
on pin 3 of T1 is a shield that reduces EMI.
LinkSwitch
12 V, 260 mA
T1
D
S
1
+
C3
+
U1
C
69T
1 µF
10 V
LNK520
5
34 AWG
C5
30T
C4
4
220 µF
30 AWG
T.I.W.
0.1 µF
100 V
FILM
R2
17.8 kΩ
25 V
43T
35 AWG
6
*C2
100 nF
400 V
DC IN
100-375
VDC
3
D3
1N4937
RTN
D4
UF4003
1 A
55T
38 AWG
200 V
R3
150 Ω
EE13
Lp = 2.484 mH
*Optional
Component
PI-3883-070604
Figure 1. DC-Input, Isolated LinkSwitch 3.12 W Flyback Converter.
July 2004
DI-76
www.powerint.com