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Fig. 7 Oscillating operation at constant voltage control
For the control of the output, the feedback current (FB current) flowing to a photocoupler flows across
R4 and voltage drop VR4 thus caused is added to VR5.
The VR5 voltage (peak value of ID) which is necessary for the inversion of the Comp.1 is controlled
by VR4 (FB current) to change the height of ID peak value and VR4 controls the secondary side output
(current mode control).
R4 has a function of stabilizing the operation at light load.
Normally bias by VR4 increases at light load.
Without R4, R5 increases abnormally due to noise caused by surge current at MOS FET off, resulting
in wrong operation of Comp.1.
In order to prevent this trouble, the OCP/FB terminal (pin 1) is connected by R4 to the ground at low
impedance(560 ), while the MOS FET is off.
Then, the impedance of the OCP/FB terminal is reduced around by 1/2 (when R4=680 ) and the
bias of R4 is reduced by around 1/2 at the moment of turning on.
Thus, C5 absorbs noise caused at turn-on of the MOS FET.
In the conventional M6540 series, the photocoupler varies the slope of charging time until reaching the
threshold voltage Vth(1)=0.73V, but in F6500 series, DC bias control is used.
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