Rev. E 09/09

4

LNK623-626

www.powerint.com

Applications Example

Circuit Description

This circuit is configured as a three output, primary-side

regulated flyback power supply utilizing the LNK626PG. It can

deliver 7 W continuously and 10 W peak (thermally limited) from

an universal input voltage range (85 – 265 VAC). Efficiency is

>67% at 115 VAC/230 VAC and no-load input power is

<140 mW at 230 VAC.

Input Filter

AC input power is rectified by diodes D1 through D4. The

rectified DC is filtered by the bulk storage capacitors C1 and

C2. Inductor L1, L2, C1 and C2 form a pi (π) filter, which

attenuates conducted differential-mode EMI noise. This

configuration along with Power Integrations transformer

EN55022 class B with good margin without requiring a

Y capacitor. Fuse F1 provides protection against catastrophic

failure. Negative temperature coefficient thermistor RT1 limits

the inrush current when AC is first applied to below the

maximum rating of diodes D1 through D4. Metal oxide varistor

RV1 clamps the AC input during differential line transients,

protecting the input components and maintaining the peak

drain voltage of U1 below its 700 V BV

For differential

surge levels at or below 2 kV this component may be omitted.

LNK626 Primary

The LNK626PG device (U1) incorporates the power switching

device, oscillator, CV control engine, startup, and protection

functions. The integrated 700 V MOSFET provides a large drain

voltage margin in universal input AC applications, increasing

reliability and also reducing the output diode voltage stress by

allowing a greater transformer turns ratio. The device can be

completely self-powered from the BYPASS pin and decoupling

capacitor C4. In this design a bias circuit (D6, C6 and R4) was

added to reduce no load input power below 140 mW.

The rectified and filtered input voltage is applied to one side of

the primary winding of T1. The other side of the transformer’s

primary winding is driven by the integrated MOSFET in U1. The

leakage inductance drain voltage spike is limited by the clamp

circuit D5, R1, R2, C3 and VR1. The zener bleed clamp

arrangement was selected for lowest no-load input power but in

applications where higher no-load input power is acceptable

VR1 may be omitted and the value of R1 increased to form a

standard RCD clamp.

Output Rectification

The secondaries of the transformer are rectified by D7, D8 and

D9. A Schottky barrier type was used for the main 5 V output

for higher efficiency. The +12 V and -22 V outputs use an

ultrafast rectifier diode. The main output is post filtered by L3

and C10 to remove switching frequency ripple. Resistors R7,

R8 and R9 provide a preload to maintain the output voltages

within their respective limits when unloaded. To reduce high

frequency ringing and associated radiated EMI an RC snubber

formed by R10 and C13 was added across D7.

Figure 4.

7 W (10 W peak) Multiple Output Flyback Converter for DVD Applications with Primary Sensed Feedback.

PI-5205-102208

D

S

FB

BP

R3

6.34 k

1%

C13

270 pF

R6

4.02 k

1%

R1

5.1 k

1/8 W

R2

390

R4

6.2 k

C4

1 F

50 V

C5

680 pF

50 V

C8

1000 F

10 V

C10

470 F

10 V

C11

47 F

50 V

C9

47 F

25 V

C6

10 F

50 V

R5

47 k

1/8 W

U1

LNK626PG

LinkSwitch-CV

D8

UF4003

D7 SB540

D6

1N4148

D9

UF4003

R9

39 k

1/8 W

R8

24 k

1/8 W

R7

510

1/8 W

T1

EEL19

16

7

11

8,9,10

12

5

4

2

3

C1

22 F

400 V

C2

22 F

400 V

C3

820 pF

1 kV

F1

3.15 A

RT1

10

85 - 265

VAC

L

N

D1

FR106

D2

FR106

VR1

1N5272B

D5

1N4007

D3

1N4007

D4

1N4007

L1

3.5 × 7.6 mm

Ferrite Bead

L2

680 uH

L3

10 H

12 V, 0.1 A

5 V, 1.7 A

RTN

-22 V, 15 mA

RV1

275 V

R10

47