Design Idea DI-76

LinkSwitch

3.1 W Low Parts Count Power Supply

Design Highlights

• 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!

• 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-

restart(forhardshortcircuits)andthermalshutdowneffectively

produce a very rugged design.

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

(V

Resistor R3 limits the effects of transformer leakage on the

feedback voltage. Resistor R2 feeds the V

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.

Capacitor C3 provides device decoupling. The extra winding

on pin 3 of T1 is a shield that reduces EMI.

www.powerint.com

Figure 1. DC-Input, Isolated LinkSwitch 3.12 W Flyback Converter.

DI-76

®

July 2004

Application

Device

Power Output

Input Voltage

Output Voltage Topology

Appliance/Industrial Control

LNK520P

3.1 W

100-375 VDC

12 V

Flyback

LinkSwitch

T1

1

5

6

3

+

+

D

S

C

PI-3883-070604

*C2

100 nF

400 V

R2

17.8 k

Ω

U1

LNK520

D3

1N4937

R3

150

Ω

C3

1

µF

10 V

C4

0.1

µF

100 V

FILM

30T

30 AWG

T.I.W.

69T

34 AWG

43T

35 AWG

55T

38 AWG

C5

220

µF

25 V

D4

UF4003

1 A

200 V

EE13

Lp = 2.484 mH

*Optional

Component

RTN

12 V, 260 mA

DC IN

100-375

VDC

4