Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 5
Figure 3 : Mains Rectifier Circuit
D701, D702, D703 and D704 full wave rectify the incoming mains and this rectified voltage is smoothed
by C705. The resultant 320VDC is now fed to the switched mode power supply.
D705 is used to limit possible damage caused to the bridge rectifier from internally generated noise that
could come close to the PIV of the diodes. Note that in some versions of the CA10 chassis, this diode
is not fitted.
Standby operation
Because of proposed legislation, which will limit the stand by power consumption, changes have been
made to the operation of the set during the stand by mode. This is achieved by muting the operation
of the power supply in the standby mode so that no oscillation takes place. The operation of the power
supply described below, therefore, relates to when the set is running. How the set comes out of standby
will be covered in the Power on control section of these notes.
Start up operation
Rectified and smoothed mains voltage ( +320VDC ) is used to provide the start up voltage to the gate
of Q701. Current flows via R713 and R714 causing C714 to charge up, the earth return for the capacitor
is via R718 and winding 15/14 of T701. This allows a voltage to be built up on the gate of Q701 ( this
will turn on Q701 when it gets to about four and a half volts ). D712 ensures that the steady DC on the
gate of Q701 does not exceed twelve volts as any voltage higher than this may damage the device.
C712 removes any high frequency pulses that may damage Q701.
As Q701 turns on, current will flow via the bridge rectifier, pins 17 and 12 of T701 and the source/drain
of Q701 finally returning to the bridge rectifier by the earth return resistor, R716. This will induce an emf
into the primary winding of the chopper transformer, which will be reflected by the secondaries via the
magnetic induction within the transformer itself.
The voltage generated by the winding connected between pins 14 and 15 will also increase as the
conduction of Q701 increases. This increase in voltage will cause D716 to conduct when its zener
voltage is reached ( 5.6 volts ). Therefore C713 will start to charge and when the base/emitter voltage
of Q703 reaches 0.65 volts it will conduct and effectively remove the gate supply. Q701 is now turned
off and current ceases to flow in the primary winding of T701 and the emf will start to decay.
As the secondary voltages decay, pin 15 of T701 will go negative and this negative charge is transferred
via C714 to the gate of Q701, holding the FET fully off for a time period decided by the charging rate of
C714 ( via R713/4 ) and the discharge rate of C713 ( via the base/emitter junction of Q703 ). Once the
magnetic field has collapsed totally, the cycle is repeated.