Control Circuit - Sharp Sf-2052 Serviceanleitung

In the case of the 200V series, the smoothing capacitors C707 ∼
C709 are charged by the electric field effect transistors Q714 and
Q715 in synchronization with the IC Z701 control signal by the reactor
L706 to extend the conduction angle of the input current (then the
general capacitor input system), improving the harmonic waves.
A rush current is also prevented by always limiting charging of the
smoothing capacitor.
The DC output (unstable state) obtained in the above procedures has
continuous waveforms as shown in Fig. 2, and its pulsing is rather
greater than in the general capacitor input system.
(4) Main switching circuit
The unstable Dc output is turned on/off by the electric field effect
transistors Q702 and Q703 using the transformer T701 as a load to
transmit energy through the transformer T701 to the secondary side
while he electric field transistors are on.
The on/off control is performed by the rectangular waveform which is
PWM-controlled at a certain frequency (about 110KHz) by the control
IC Z701. The capacitors C718 and C719 and resistors R714 and
R715 form the snubber of the electric field effect transistors Q702 and
Q703. In the composition of the diode D706, the capacitor C721 and
the transistor Q707, "R: resistance" section in the reset circuit of the
transformer T701 called as diode CR is substituted with the power
zenor function obtained by the transistor Q707 and the avalanche
diode D707 (D707, D708 in the 200V series).

(5) Control circuit

This circuit generates controlled PWM waveforms to stabilize the sec-
ondary output of the transformer T701. This function is performed by
the control IC Z701. After detection and amplification of the master
output (+24V in this power unit), therefore, the output is passed to the
primary side and the PWM rectangular waveforms are generated by
the control IC Z701.
The IC (Z701) has the overcurrent protecting function described later
and the shut down function against an overvoltage signal.
(5)-1. Master output voltage detection, error amplifying
circuit and transmitting circuit to the primary side
The master output voltage (+24V) is detected by the transistor Q709
to amplify the error.
The error-amplified output is passed to the primary side by the photo
coupler PC703.
Resistors R757 and R758 provide the reference voltage for error
amplification, resistors R763, R762, and R746 and variable resistor
RV702 perform detection of error amplification, and resistors R761,
RX705, and RX 706 and capacitors CX703 and CX704 function as
the phase correction of error amplification.
(5)-2. Generation of PWM rectangular waveform by the
control IC
The control IC (Z701) generates PWM rectangular waveforms based
on the error amplification output transmitted to the primary side by the
photo coupler PC703.
The frequency of PWB rectangular waveforms (the operating fre-
quency of this power unit) is fixed to about 110KHz regardless of
variations in loads.
The power supply method to the control IC (Z701) differs in the
starting state and in the steady state of this power unit. In the starting
state, the operation is started by the voltage of the smoothing capaci-
tor C723 charged by diodes D701, D702, D741, and D742, and resis-
tors R706 and R707, and diode D717. In the steady state, the output
of transformer T701 (between windings 7 ∼ A) is passed through the
rectifying diode D714 and smoothed by the smoothing capacitor
C722 and stabilized by transistor Q706 and zenor diode D715. Then
the smoothing capacitor C723 is charged to supply power to the
control IC (Z701).
(6) Secondary rectifying/smoothing output circuit
The output obtained from the transformer T701 is outputted through
the smoothing choke coil L707 and the smoothing capacitor. The
+38V output is composed of rectifying diode RC706 and the smooth-
ing capacitors C739 ∼ C741. The outputs of 10V, +5V, and –20V are
obtained from the D/D convertor of +24V described later.
Regulation after the secondary rectifying and smoothing is as de-
scribed below.
(6)-1. +24V
The +24V is stabilized as the master circuit as described above. To
perform ripple compression,the LC filter is composed of the reactor
L709 and the capacitor C742.
(6)-2. +38V
In the +38V, the LC filter is composed of the reactor L708 and the
capacitor C734 for ripple compression.
(7) Main switching circuit (+10V, +5V, –20V)
As stated before, to obtain outputs of +10V, +5V, and –20V, he
transformer T702 is turned on/off by the electric field effect transistor
Q701, and energy is transmitted to the secondary side through the
transformer T702 during OFF period of the electric field effect transis-
tor. (The RCC system)
ON/OFF operation is performed at about 20KHz ∼ 200KHz depending
on variations in load caused by self oscillation.
The capacitor C745 and the resistor R765 form the snubber of the
electric field effect transistor Q710. The reset circuit of the trans-
former T702, generally called diode CR, is composed of the diode
D740 and capacitor C744 and resistors R751 and R776.
(8) Control circuit
This circuit generates controlled rectangular waveforms to stabilize
the secondary output from the transformer T702. This function is
performed by transistors Q711 and Q712. The transistor Q712 has
the overcurrent protection function described later.
(8)-1. Master output voltage detection, error
amplification circuit and transmitting circuit
The master output voltage is detected and the error is amplified by
the error amplifying IC (Z702). The error-amplified output is transmit-
ted to the transistor Q711.
Resistors R782, R781, R780, and RX712 are used for detection of
error amplification. Resistors R783, RX711, and RX710, and capaci-
tors CX709 and CX708 are used for phose correction of error amplifi-
cation.
(8)-2.
Generation of rectangular waveform by self oscillation
Oscillation is started by the starting resistors R748, R772, and
RX707. Oscillation is stabilized through the transformer T702 (be-
tween windings 8 ∼ 9) output, the resistors R773 and R768, and the
capacitor CX705. The electric field effect transistor Q710 is driven
(turned off) by the transistor Q712 based on the error amplification
output transmitted by the transistor Q711. (The self oscillating type)
(9) Secondary rectifying/smoothing output circuit
The output of the transformer T702 is passed through the rectifying
diode and the smoothing capacitor.
The +10V is composed of the rectifying diode C707, the smoothing
capacitor C746. The +5V is composed of the rectifying diode RC708
and the smoothing capacitors C748 and C749. The –20V output is
composed of the rectifying diode D737 and the smoothing capacitor
C751.
(9)-1. +5V
As stated before, the +5V is stabilized as the master circuit. For ripple
compression, the LC filter is composed of the reactor L712 and the
capacitor C750.
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