NEC FP1355 Service Manual

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NEC FP1355 Service Manual Manual pdf 8 page image

1 - 21.1.4 Higher harmonic circuit(1) The pulsating waveform rectified in the full wave mode by D901 is switched throughout the full cycleby the frequency of several tens kHz or more. Through this, the input current waveform becomesan average of the switching currents of the partial cycles, thus becoming the sine waveform in themacro. (See Fig.1)(2) For the AC input voltage, the AC input current of the sine wave type in the same phase flows toachieve the power circuit of improved power factor and reduced higher harmonic wave component.(3) L903 is the choke coil, Q901 is MOS FET, D902 is the rectifying diode, C911 is the block capacitor,and IC901 is the power factor improved controller. The power factor improved controller usesMC33262P of Motorola. (See Fig. 2)(4) After the sub power circuit operates, P-SUS signal becomes HI when +5V voltage is supplied to theMPU. Then, Q902 is turned ON, the voltage of approx. +18V is supplied to pin8 (VCC terminal) ofIC901 through D929 from pin2 of T902, and the following operation is started.(5) The pulsating voltage waveform rectified in the full wave mode by D901 is divided with R904, R905,R906, R907 and R908 (100VAC : 1.1Vp-p and 240VAC: 2.9Vp-p), and is input to pin3 of IC901(Multiplier input). Moreover, the output (+side of C911: 400VDC) of the higher harmonic circuit isdivided with R913, R914, R915, R916 and R917 (2.5VDC), and is input to pin1 of IC901 (erroramplifier input).(6) The output of the error amplifier and the divided waveform of the pulsating voltage input to pin3 ofIC901 sets the threshold voltage of the current sense comparator to control the Q901 flowingcurrent from zero to the peak line of the AC input voltage in the sine wave pattern.(7) When Q901 is turned ON, the drain current of Q901 flows to R910 and R937 to drop the voltage,and the voltage generated by the voltage drop is input to pin4 (current sense input) of IC901. Whenthe voltage reaches the threshold voltage of the current sense comparator, Q901 is turned OFF.(8) When Q901 is turned OFF, the accumulated energy of L903 starts to be supplied to the loadthrough D902.(9) As the accumulated energy of L903 drops, the auxiliary coil voltage (pin8 of L903) also drops.When it reaches the threshold voltage of *zero current detector, Q901 will be turned ON again.* Pin 5 of IC901 is the zero current detection terminal to input the auxiliary coil voltage of pin10 ofL903. The zero current detector monitors that the auxiliary coil voltage drops beyond the threshold voltage. Thus, the accumulated energy of L903 is indirectly detected.(10) The above operation is repeated to continue the oscillating operation. Thus, the DC voltage (L903,Q901, D902 and C911 compose the voltage rise circuit.) is gained on the output, and the AC inputcurrent of the sine wave in the same phase with the AC input voltage is gained on the input side.0PeakIQ901AverageID902ONMOSFETQ901OFFFigure 1. L903 coil current