FAN7621 データシートの表示（PDF） - Fairchild Semiconductor

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FAN7621

PFM Controller for Half-Bridge Resonant Converters

Fairchild Semiconductor 

5.3 Overload Protection (OLP): Overload is defined as

the load current exceeding its normal level due to an

unexpected abnormal event. In this situation, the

protection circuit should trigger to protect the power

supply. However, even when the power supply is in the

normal condition, the overload situation can occur during

the load transition. To avoid premature triggering of

protection, the overload protection circuit should be

designed to trigger only after a specified time to

determine whether it is a transient situation or a true

overload situation. Figure 27 shows a typical overload

protection circuit. By sensing the resonant capacitor

voltage on the control pin, the overload protection can be

implemented. Using RC time constant, shutdown delay

can be also introduced. The voltage obtained on the

control pin is given as:

VCON

=

2(CB

CB

+ Csense

)

VCr

p

−

p

(7)

where VCrp-p is the amplitude of the resonant capacitor

voltage.

5.4 Over-Voltage Protection: (OVP): When the LVCC

reaches 23V, OVP is triggered. This protection is used

when auxiliary winding of the transformer to supply VCC

to the controller is utilized.

5.5 Thermal Shutdown (TSD): If the temperature of the

junction exceeds approximately 130°C, the thermal

shutdown triggers.

6. PCB Layout Guideline: Duty imbalance problems

may occur due to the radiated noise from main

transformer, the inequality of the secondary-side leakage

inductances of main transformer, and so on. Among

them, 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. Figure 28 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 turns on, respectively. The primary

current does not enclose any component of controller.

In addition, it is helpful to reduce the duty imbalance to

make the loop configured between CON pin and opto-

coupler as small as possible, as shown in the red line in

Figure 28.

Figure 28. Example for Duty Balancing

© 2009 Fairchild Semiconductor Corporation

FAN7621 • Rev. 1.0.1

12

www.fairchildsemi.com

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