Design Idea DI-89

CV Power Adapter

Design Highlights

• Low-cost, low parts-count CV solution: 20 components

• Proprietary IC design and winding techniques enable a

Clampless

™ drain-node

• ± 5% over-temperature threshold – with hysteretic

recovery – keeps PCB temperatures below safety limits

• Auto-restart: output short circuit and open loop protection

• IC creepage > 3.2 mm: no arcing in humid environments

• Easily meets all EPS energy efficiency standards

• Meets CISPR-22 Class B EMI with sufficient margin

Operation

ThisLinkSwitch-XTbasedflybackconverter(Figure1)provides

2 W of tightly regulated constant voltage (CV) output power,

while meeting the active-mode efficiency and no-load power

consumption requirements of all harmonized energy efficiency

(EPA, CEC) standards (see Figure 2 and Figure 3).

Diodes D1– D4 rectify theAC input. The resulting DC is filtered

by bulk storage capacitors C1 and C2. Components L1, L2, C1

and C2 form a conducted EMI noise filter. Resistor R1 dampens

the ringing of the filter. Switching frequency jitter and

PIʼs E-Shield transformer construction technology enable

this design to meet EN55022 Class-B conducted EMI with

good margin (see Figure 4). Y capacitor C4 (optional) can

improve the unit-to-unit repeatability of EMI scans.

ThissupplyalsotakesadvantageofPIʼsClamplesstransformer

techniques, which uses T1ʼs primary winding capacitance to

clamp the voltage spike that its leakage inductance causes,

each time the MOSFET in U1 turns off. Therefore, this

converter has no primary clamp components connected to

the drain-node.

Fromno-loaduntilmaximumoutputpower(2W)isdelivered,

the LNK362P (U1) regulates the output voltage by skipping

switching cycles, based on the current delivered into the

feedback(FB)pin. Iftheoutputisoverloadedandnofeedback

(<49µA)isreceivedwithina40msperiod,U1goesintoauto-

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

for about 40 ms approximately every 800 ms if no feedback

is received within the 40 ms window of enabled switching.

www.powerint.com

Figure 1. LNK362 Based 6.2 V, 322 mA, 2 W, Low-cost, Flyback CV Output Power Adapter.

DI-89

®

November 2005

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

Adapter

LNK362P

2 W

85-265 VAC

6.2 V

Flyback

PI-4205-110805

D

S

FB

BP

D1

1N4005

D2

1N4005

D5

1N4934

D3

1N4005

D4

1N4005

RF1

8.2 k

2.5 W

R1

3.9 k

1/8 W

R3

390

Ω

1/8 W

R2

1 k

1/8 W

6.2 V,

322 mA

85-265

VRMS

J3-1

J3-2

J2

J1

L1

1 mH

L2

1 mH

C1

3.3

µF

400 V

C2

3.3

µF

400 V

C4*

100 pF

250 VAC

C5

330

µF

16 V

VR1

BZX79-

B5V1

5.1 V, 2%

*Optional component

T1

EE16

4

5

3

9

8

NC NC

C3

100 nF

50 V

U2

PC817A

U1

LNK362P

LinkSwitch-XT