AN-6755

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.2 • 1/14/11

8

Printed Circuit Board (PCB) Layout

High-frequency switching current / voltage makes PCB

layout a very important design consideration. Good PCB

layout minimizes excessive EMI and helps the power

supply survive during surge / ESD tests.

Guidelines:

To improve EMI performance and reduce line frequency

ripples, the output of the bridge rectifier should be

connected to capacitor C1 first, then to the switching

circuits.

The high-frequency current loop shown in Figure 18 is C1 –

current loop should be as small as possible. Keep the traces

related to the drain of the MOSFET and RCD snubber

should be kept way from control circuits to prevent

unnecessary interference. If a heatsink is used for the

MOSFET, connect this heatsink to ground.

be connected together, then to the current-loop ground 2 at a

single point close to the ground connection of capacitor C3.

auxiliary winding, D1, C2, D2, and C3 should also be

kept small. Place C3 close to the FAN6755 for good

decoupling. For high-level surge, this auxiliary ground must

be connected to the bulk capacitor directly. This method can

improve the surge capability of the system.

Two suggestions with different pro and cons for ground

connections are offered:

interference for sense signals.

the earth ground is not available on the power supply.

Regarding the EMI discharge path, the charges go from

secondary through the transformer stray capacitance to

back to the mains. Control circuits should not be placed on

the discharge path. Point discharge for common choke can

decrease high-frequency impedance and increase EMI

immunity.

Should a Y-cap between primary and secondary be

required, connect this Y-cap to the positive terminal of C1.

If this Y-cap is connected to the primary GND, it should be

Point discharge of this Y-cap also helps for EMI; however,

the creepage between these two pointed ends should be

large enough to satisfy the requirements of applicable

standards.

Figure 18.Layout Considerations