Rev. D 12/11

17

PFS704-729EG

www.powerint.com

PCB Design Guidelines and Design Example

The line-sense network and the feedback circuit use large

resistance values in order to minimize power dissipation in the

feedback network and the line-sense network. Care should be

taken to place the feedback circuit and the line-sense network

components away from the high voltage and high current nodes

to minimize any interference. Any noise injected in the feedback

network or the line-sense network will typically manifest as

degradation of power factor. Excessive noise injection can lead

to waveform instability or dissymmetry.

The EMI filter components should be clustered together to

improve filter effectiveness. The placement of the EMI filter

components on the circuit board should be such that the input

circuit is located away from the drain node of the HiperPFS, the

output diode of the PFC or the PFC inductor.

A filter or decoupling capacitor should be placed at the output

of the bridge rectifier. This capacitor together with the

X capacitance in the EMI filter and the differential inductance of

the EMI filter section and the source impedance, works as a

filter to reduce the switching frequency current ripple in the

input current. This capacitor also helps to minimize the loop

Figure 19. PCB Layout Example for System Power Supply consisting of a PFC and a Second Stage Converter.

area of the switching frequency current loop thereby reducing

EMI.

The connection between the HiperPFS drain node, output

diode drain terminal and the PFC inductor should be kept as

small as possible.

A low loss ceramic dielectric capacitor should be connected

between the cathode of the PFC output diode and the source

terminal of the HiperPFS. This ensures that the loop area of the

loop carrying high frequency currents at the transition of

switch-off of the MOSFET small and helps to reduce radiated

EMI due to high frequency pulsating nature of the diode current

traversing through the loop.

During placement of components on the board, it is best to

place the VOLTAGE MONITOR pin, FEEDBACK pin and VCC

pin decoupling capacitors close to the HiperPFS before the

other components are placed and routed. Power supply return

trace from the GROUND pin should be separate from the trace

connecting the feedback circuit components to the GROUND pin.

PFC

Output

EMI Filter

Thermistor

Shorting Relay

Bridge

Rectifier

PFC

Inductor

HiperPFS

Auxiliary Supply for PFC –

from Standby Power Supply

Second Stage

Converter

Input

Capacitor

PFC Output

Capacitor

PI-6238-012611

AC Input