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DI-92

B

9/05

Power Integrations

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Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does

not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND

SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A

PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (transformer construction and

circuits external to the products) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to

Power Integrations. A complete list of Power Integrations' patents may be found at

www.powerint.com. Power Integrations grants its customers a license

under certain patent rights as set forth at http://www.powerint.com/ip.htm.

The PI logo,

TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, EcoSmart, PI Expert and PI FACTS are trademarks of

Power Integrations, Inc. Other trademarks are property of their respective companies.

Copyright 2005, Power Integrations, Inc.

For the latest updates, visit

www.powerint.com

Figure 2. Conducted EMI Plot of the Schematic Shown in

Figure 1 – Background Scans Taken at 110 VAC:

Foreground Scans Taken at 230 VAC.

Figure 3. Output Current (Top Trace) and Input Current

(Bottom Trace), 50 mA/div, 5

µs/div.

Key Design Points

• The circuit shown in Figure 1 has a total output current

tolerance of ±12% (including ∆T of 50 °C).

• To prevent noise coupling to the input, place the input filter

components physically away from the source node of the

LinkSwitch-TN

andL1inductor. TheDCinputfiltercapacitors

C1 and C2 can be placed as a barrier between the AC input

and these two components.

• The circuit shown in Figure 1 uses a low cost resistive pi (π)

filter for differential mode filtering. For output power greater

than 0.5 W, an inductive pi filter is recommended.

• For better EMI performance, operate the circuit strictly in

DCM (see Figure 3 – output current decays to zero in every

switching cycle).

• A second rectifier diode may be placed in the return leg of

theAC input (not used in Figure 1). This may give improved

EMI performance and better surge withstand capability.

• The maximum number of LED strings determines the

total output current and is limited by the current limit of

the LNK302 device and the inductance of L2.

1.0

0.15

10.0

100.0

-20

-10

0

10

20

30

40

50

60

80

70

MHz

QP

AV

0

20

25

30

15

10

5

35

40

45

50

Time (

µs)

0 mA

0 mA

Populated Circuit Board