NEC MultiSync FP1355 FP1355 FP1355, Руководство по обслуживанию

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1.1.5 Sub power circuit
(1) The sub power uses PRC control regulator STR-G6352 (IC903) produced by Sanken Electric. (See Fig.3)
(2) When the power switch is turned ON, the rectified and smoothened DC voltage (AC voltage
x ) is supplied to pin4 of IC903, through R950, R951 and R952. When pin4 reaches
approx. 17V, the built-in output FET is put into operation. (Since Q902 is OFF, IC902 and
IC 903 do not operate.)
(3) This also induces the voltage at pin2 of T902 and on the secondary side. These outputs
are respectively rectified, and are used as the power for control on the primary side and
the power for the MPU.
(4) IC903 mo5V and -15V output on the secondary side by IC922 (Shunt regulator),
and suppresses the voltage regulation by feeding back to pin 5 of IC903 via IC912
(Photocoupler).
(5) When the voltage on the secondary side starts, the MPU will be put into operation and the
P-SUS signal line will become HIGH.
(6) This information is transmitted to the primary side via IC913 to turn ON Q902.
When Q902 is turned ON, the power for control on the primary side will be supplied to
IC901 and IC902 to operate the higher harmonic circuit. Thus, the main power circuit will
be put into operation.
1.1.6 Main power circuit
(1) The main power circuit adopts the flyback type switching power of pseudo-reosonance
operation. This is composed of a Sanken brand hybrid IC STR-F6676 (IC902) that inte-
grates the power MOS-FET and control IC.
The circuit operation is described as follows. (See Fig. 4.)
(2) The timing at that the power MOS-FET is turned ON is consistent with the bottom point of
the voltage resonant waveform after the transformer (T901) discharges the energy to the
secondary side, that is, a half cycle of the resonant frequency determined by LP value
(primary coil inductor value) of T901, and C914 (resonant capacitor). This is called
pseudo-reosonance operation. The advantage of such an effect is that the switching loss
is reduced by turning it ON when the voltage between the drain sources of the power MOS-
FET becomes the lowest.
(3) Like the higher harmonic circuit, voltage of approx. +18V is supplied to the Vcc terminal
(Pin 4) of IC902 (STR-F6676) via D929 from pin2 of T902 when Q902 is turned ON by the
P-SUS signal from the MPU.
When the voltage of Pin 4 of IC902 reaches 16V, the control circuit will be put into opera-
tion to turn ON the integrated MOS-FET.
(4) When MOS-FET is turned ON, the capacitor C1 in IC will be charged to approx. 6.5V. On
the other hand, the drain current flows to R928, and the voltage generated by the voltage
drop is applied to pin1 (OCP/FB terminal) of IC902.
When the voltage of Pin 1 reaches approx. 0.73V, the comparator (Comp. 1) in IC will be
activated to turn OFF MOS-FET.
(5) The voltage between both ends of C1 drops to approx. 3.7V. the oscillator output will be
reversed again to turn ON MOS-FET.
The above is repeated to continue the oscillation operation.
(6) Here, IC902 mo215V of the output on the secondary side with IC921 (error ampli-
fier) and feeds back it to pin1 of IC902 via IC911 (photocoupler), thus suppressing the
voltage fluctuation of the primary side.
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