Higher Harmonic Circuit - NEC FP1355 Service Manual

1.1.4

Higher harmonic circuit

(1) The pulsating waveform rectified in the full wave mode by D901 is switched throughout the full cycle
by the frequency of several tens kHz or more. Through this, the input current waveform becomes
an average of the switching currents of the partial cycles, thus becoming the sine waveform in the
macro. (See Fig.1)
(2) For the AC input voltage, the AC input current of the sine wave type in the same phase flows to
achieve 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 uses
MC33262P of Motorola. (See Fig. 2)
(4) After the sub power circuit operates, P-SUS signal becomes HI when +5V voltage is supplied to the
MPU. Then, Q902 is turned ON, the voltage of approx. +18V is supplied to pin8 (VCC terminal) of
IC901 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 is
divided with R913, R914, R915, R916 and R917 (2.5VDC), and is input to pin1 of IC901 (error
amplifier input).
(6) The output of the error amplifier and the divided waveform of the pulsating voltage input to pin3 of
IC901 sets the threshold voltage of the current sense comparator to control the Q901 flowing
current 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. When
the 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 load
through 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 of
L903. The zero current detector monitors that the auxiliary coil voltage drops beyond the thresh
old 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 input
current of the sine wave in the same phase with the AC input voltage is gained on the input side.
Peak
0
ON
MOSFET
Q901
OFF
Figure 1. L903 coil current
Average
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