Panasonic UF-890 Service Manual page 251

The value of output voltage is
Eo=d/(1-d)*Ei
d=Ton/Ts
Equivalent circuit model for the RCC.
SW
Ei (Q101)
VL
VL
In the actual circuit, the fixed output voltages are obtained by changing the winding ratio of transformer
T101. 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, -12 VDC, +5 VP and +5 VDC. The +24 VDC output is protected by the
Error Detection Circuit, and the +5 VP and -12 VDC outputs are protected by the circuitry inside of the
voltage regulator IC, +5 VDC is protected by ZD251.
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 Q101. 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
photocoupler PC101 increases, the output pulse width of the control circuit decreases and the ON time
period of Q101 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.
D(D201)
Eo
L C
T101
Ei
Eo
Ts dTs
IL
(1-d)Ts dTs
Ton : On time of Q101
Ts : Period of oscillation
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
RL
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),
Eo=d/(1-d)*Ei
T
251