Q0465R View Datasheet(PDF) - Fairchild Semiconductor

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Q0465R

Green-Mode Fairchild Power Switch (FPS™) for Quasi-Resonant Operation - Low EMI and High Efficiency

Fairchild Semiconductor 

PCB Layout Guide

Due to the combined scheme, FPS shows better noise

immunity than conventional PWM controller and

MOSFET discrete solutions. Furthermore, internal drain

current sense eliminates noise generation caused by a

sensing resistor. There are some recommendations for

PCB layout to enhance noise immunity and suppress the

noise inevitable in power-handling components.

There are typically two grounds in the conventional

SMPS: power ground and signal ground. The power

ground is the ground for primary input voltage and

power, while the signal ground is ground for PWM

controller. In FPS, those two grounds share the same

pin, GND. Normally the separate grounds do not share

the same trace and meet only at one point, the GND pin.

More, wider patterns for both grounds are good for large

currents by decreasing resistance.

Capacitors at the VCC and FB pins should be as close as

possible to the corresponding pins to avoid noise from

the switching device. Sometimes Mylar® or ceramic

capacitors with electrolytic for VCC is better for smooth

operation. The ground of these capacitors needs to

connect to the signal ground (not power ground).

The cathode of the snubber diode should be close to the

Drain pin to minimize stray inductance. The Y-capacitor

between primary and secondary should be directly

connected to the power ground of DC link to maximize

surge immunity.

Because the voltage range of feedback and sync line is

small, it is affected by the noise of the drain pin. Those

traces should not draw across or close to the drain line.

When the heat sink is connected to the ground, it should

be connected to the power ground. If possible, avoid

using jumper wires for power ground and drain.

Figure 36. Recommended PCB Layout

Mylar® is a registered trademark of DuPont Teijin Films.

© 2008 Fairchild Semiconductor Corporation

FSQ0465RS/RB Rev. 1.0.1

17

www.fairchildsemi.com

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