FAN7621SSJX データシートの表示（PDF） - Fairchild Semiconductor

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FAN7621SSJX

PFM Controller for Half-Bridge Resonant Converters

Fairchild Semiconductor 

6. Current Sensing Using Resistor: FAN7621S senses

drain current as a negative voltage, as shown in Figure

23 and Figure 24. Half-wave sensing allows low power

dissipation in the sensing resistor, while full-wave

sensing has less switching noise in the sensing signal.

7. PCB Layout Guidelines: Duty imbalance problems

may occur due to the radiated noise from the main

transformer, the inequality of the secondary-side leakage

inductances of main transformer, and so on. It is one of

the dominant reasons that the control components in the

vicinity of RT pin are enclosed by the primary current flow

pattern on PCB layout. The direction of the magnetic field

on the components caused by the primary current flow is

changed when the high- and low-side MOSFET turns on

by turns. The magnetic fields with opposite direction from

each other induce a current through, into, or out of the RT

pin, which makes the turn-on duration of each MOSFET

different. It is strongly recommended to separate the

control components in the vicinity of RT pin from the

primary current flow pattern on PCB layout. Error!

Reference source not found. shows an example for the

duty-balanced case. The yellow and blue lines show the

primary current flows when the lower-side and higher-

side MOSFETs turn on, respectively. The primary current

does not enclose any component of controller.

Figure 23. Half-Wave Sensing

Figure 24. Full-Wave Sensing

Figure 25. Example for Duty Balancing

© 2009 Fairchild Semiconductor Corporation

FAN7621S • Rev. 1.0.1

11

www.fairchildsemi.com

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