OKIFAX 610, Service Handbook

Page: 145 / 147

8.6.2 Power Circuit Diagram
(1) Circuit Description
(a) EMI filter
C1, C2, C3, C4, and L3(NF1) constitute EMI filter circuit. As the operating freqt of this power
supply circuit is 60KHx, radio frequency interference wave will incur under the st electricate of
the highe frequency in order to prevent it from inte other electric products via its input terminal.
(b) The limiting of inrush current
The action of R1 (PT1) thermal resistor is to limit the transient enormous cu occurred due to rapid
charge of C5 on switching on, the feature of the thermal resistor is to become allmost short due to
the passing of current to cause the dropping of resistance on switching on, it will not cause too
much voltage dropping after the circuit operates normally.
The thermal resistor retains considerable temperature for sometime after switchin the resistance
is still bery low by this time. If restart immediately after bring the a of limiting inrush current into
fully play, the circuit and switch are easy to incur fault resulting from the flow of enormous current
over the primary side; accordingly it requires to wait for 20 seconds at least before restarting after
switched off.
(c) AC-DC rectifying and filtering
DB1 is bridge rectifier of the part of input and C5 is filter condenser. The input AC voltage is
rectfied and filtered by DB1 and C5 and changed to be DC voltage; the value of the DC voltage
after rectifying is approx. 1.4 times of AC input RMS value.
(d) Switch parts and action principle
The switch adopts MOS-FET, the witching feature is excellent in both high speed high frequency.
The waveform of Q1 is triggerred by the switch of pin 6 of IC2. While Q1 becomes conductive,
initial current will folw through the primary side winding of the transformer gradually and store
electric potential in it; on accout of the polarities of the input and output windings of the current
limiting coil of the transformer are opposite, all output diodes bear inverse bias, no energy is
shifted to load at the time.
While FET is not conductive, the drain of Q1 should bear over two times input voltage, owing to
the disappearing of magnetic field resulting in opposite polarities of the windings, the diode on the
secondary side will become conductive to charge the capacity of the output side and cause the
presenting of voltage on the output side to output current. D1, R4 and DZ1 (D5, C6, R7) from a
limiting circuit to lilmit the voltage wave peak to be produced by the leaking conductance of the
transformer while Q1 is turned off to store energy after the turning off of Q1, the energy will be
shifted to C5 condenser. IC1 is primary and secondary sides insulating light coupling IC.
Use of each pin of IC2 is as follows:
Pin 1: Feedback from +5V of secondary is used at +5V regulator.
Pin2: NC.
Pin 3: Current sense from R8 (R9) of over-current detection.
Pin 4: Over-load delay.
Pin 5: Connected to ground.
Pin 6: Wave width control output to drive Q1.
Pin 7: Connected to Vcc.
Pin 8: Restart delay