Panasonic Facsimile UF-590, Service Manual

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In the actual circuit, the fixed output voltages are obtained by changing the winding ratio of transformer
T100. In this converter circuit, the output voltages are stabilized by controlling the duty cycle of the ON
and OFF timing of the transistor. In this power supply, the bias winding is built into the transformer. The
power supply has four outputs, +24 VDC, -5 VDC, +5 VP and +5 VDC. The +24 VDC output is protected
by the Error Detection Circuit, and the +5VDC, +5 VP and -5 VDC outputs are protected by the circuitry
inside of the voltage regulator IC.
Control Circuit and Error Detection Circuit
The control circuit amplifies the output of the duty cycle according to the error voltage detected by the
Error Detection Circuit, and drives the main transistor Q100. The method used to change the duty cycle
is to change the ON time period. When the output voltage of the +24 VDC circuit rises, the current of
photo coupler PC103 increases, the output pulse width of the control circuit decreases and the ON time
period of Q100 decreases. This control circuit decides the minimum OFF time period by itself. When the
oscillation frequency becomes higher and the OFF time period becomes minimum, the OFF time period
remains unchanged and only the ON time period decreases. This way, there is a upper limit of the
oscillation frequency and the duty cycle is expanded.
The value of output voltage is
Eo=d/(1-d)*Ei
d=Ton/Ts
Equivalent circuit model for the RCC.
Ton : ON time of Q100
In the equivalent circuit ; When SW is ON, current
flows
SW L
When SW is OFF, current flows
L D RL
The value of inductance increase current between
ON period. (d*Ts)
IL=Ei/L*d*Ts . . . . . . . . . . . . . . . . . .(1)
The value of inductance decrease current between
OFF period. ((1-d)*Ts) . . . . . . . . . . . . . . . .(2)
From equation (1) and (2),
E0=d/(1-d)*Ei
Ts : Period of oscillation
VL
VL
L C
T100
D(D102)
Eo
Eo
Ts dTs
RL
Ei
SW
(Q100)
T
IL
Ei
(1-d)Ts dTs