Oki B 4100 Maintenance Manual page 432

(1) Noise filter circuit
The noise filter consists of L and C to reduce the normal mode noise and common mode noise
incoming from the AC line.
(2) Rectifying and smoothing circuit
The input AC voltage is rectified by diodes D1 to D4 and smoothed by capacitor C6 for DC
voltage supply to the inverter. The rash current at power ON is prevented by TH1.
(3) Switching circuit (RCC type)
This flyback converter (RCC) type switching circuit converts the DC input current to a high
frequency pulse train by switching Q1 ON and OFF. Energy is accumulated in the primary
winding of transformer T1 during the Q1 OFF period, and the accumulated energy is released to
the secondary side in the OFF period for power supply. On/OFF control is performed by gate
voltage control by Q3 via photocoupler PC1 from 5V output.
The overcurrent protection circuit is actuated upon rise in the Q3 base voltage through ZD2 when
the voltage of the bias winding as positive feedback rises due to the rise in the load current. When
a voltage exceeding the zenor voltage of ZD4 and ZD5 is input, ZD4 and ZD5 become conductive
to actuate the overvoltage protection circuit.
If the input voltage is excessive, ZD4 and ZD5 are broken down in the short mode to operate the
overcurrent protection circuit to reduce the output voltage.
The overcurrent protection circuit is automatically reset after the overcurrent state is eliminated.
If ZD4 and ZD5 are broken down by shorting, they should be replaced for recovery from the
overvoltage state.
40055101TH Rev.4
Explanation on Operation
Fig. 1
Fig. 2
Fig. 3
Q1 drain-source
voltage waveform
0
Q1 drain current
waveform
0
Q1 gate voltage
waveform
0
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