D

S

FB

BP

J3-1

RTN

6 V,

0.33 A

J3-2

L

J-1

J-2

N

D1

1N4937

RF1*

8.2

Ω

2.5 W

C1

10

µF

400 V

C2

0.1

µF

50 V

L1

3300

µH

D4

UF4002

D3

1N4005

C5

220

µF

25 V

C3

330 nF

50 V

C4*

100 pF

250 VAC

*Optional components

R3

2 k

Ω

VR1*

1N5240B

10 V

R2

3 k

Ω

R1

37.4 k

Ω

T1

EE16

2

1

7

6

4

5

D2

1N4005

90-265

VAC

LinkSwitch-LP

PI-4106-101105

U1

LNK564P

www.powerint.com

Design Idea DI-85

Replaces Unregulated Linear Solutions

February 2006

DI-85

®

Design Highlights

• Low-cost, low parts-count solution: 14-17 components

• Proprietary IC design and winding techniques enable a

Clampless

™ design with simple Filterfuse™ input stage

• ±5% over-temperature threshold – with hysteretic

recovery – keeps PCB temperatures below safety limits

• Auto-restart: output short circuit and open loop

protection

• IC creepage distance >3.2 mm: no arcing in high

humidity

• Easily meets all EPS energy efficiency standards

• Meets CISPR-22 Class B EMI with good margin

Operation

This LinkSwitch-LP based flyback converter (Figure 1)

provides an output VI curve (Figure 2) similar to that of an

unregulated line frequency transformer based supply, but with

output current that is limited past the maximum rated output

power (peak power point).

From no-load to the 2 W peak power point, the LNK564P

(U1) regulates the output voltage by skipping switching

cycles, based on the current delivered into the FEEDBACK

(FB) pin. At the peak power point, the supply delivers

>300 mA of load current at >5.7 VDC.

Past the peak power point, cycle skipping ceases and U1

limits the supplyʼs output current by reducing its oscillator

(MOSFET switching) frequency, as the voltage on the FB pin

drops. If the load demand causes the FB pin voltage to drop

below an auto-restart threshold voltage V

1.5 V on the supply output) for more than 100 ms, the IC goes

into auto-restart mode. In auto-restart, MOSFET switching is

enabled for about 100 ms, approximately every 800 ms, until

the FB pin voltage increases above 0.8 V.

Due to the frequency jittering of the U1 internal oscillator and

the E-Shield™ winding techniques used in constructing the

transformer (T1), conducted EMI is adequately attenuated by

the LC filter formed by L1 and C1. Inductor L1 serves as both

a differential mode choke and a fuse. This Filterfuseis sleeved

with heat-shrink tubing, and its winding wire diameter was

selected so that it will open like a fuse if any single component

fails shorted. Thanks to the tight current limit tolerance of

the LinkSwitch-LP family and construction techniques used

on T1, the primary winding can be left Clampless, since the

peakdrainvoltagedoesnotapproachthe700Vdrain-to-source

breakdown voltage (BV

With no optocoupler and such a low parts count, this supply

is cost competitive with the unregulated line frequency

transformer based solution it was designed to replace.

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

Charger

LNK564P

2 W

90-265 VAC

6 V

Flyback

Figure 1. LNK564 Based 6 V, 330 mA, 2 W, Low-Cost, Flyback Charger Power Supply.