Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 11
When the magnetic field within T701 collapses, the back emf generated across the primary winding
causes large voltage spikes to appear across pins 12 and 17. If these spikes are not suppressed, then
damage will occur to the primary side of the power supply.
To prevent this from happening a snubber circuit, consisting of D711, R715 and C710 is used. Thus
when pin 12 becomes positive with respect to pin 17, D711 will conduct and current will flow through
R715. The spike will be damped by the action of C710 and R715. FB701 removes any HF noise.
Power on control
Primary processor power supply
As the chopper supply stage is shut down during the standby mode, it is necessary to provide power to
the control circuitry from another source. This is achieved by supplying the mains voltage to a shunt
regulator circuit as shown in the diagram below.
Figure 9 : Primary Processor Power Supply
This simple circuit provides a positive five volt supply to the reset IC, IC701 and microprocessor, IC702.
Power supply turn on
At mains application, the primary microprocessors reset IC, IC701, will provide a positive going reset
pulse to initialise IC702. At the same time the ceramic filter, CF701 will start to oscillate. This sequence
of events will initialise IC702, the key scan and remote control function will become operational.
When pin 15 ( PA1 ) of IC702 goes low, either by pressing one of the four buttons on the front of the
television or by use of the remote control, the base bias to Q702 will be removed and it will turn off, its
collector voltage will therefore rise. The power supply will now start up in line with the description given
previously.
If pin 15 ( PA1 ) of IC702 goes high, it will turn on Q702 via R792 and D730. This action will remove the
voltage on the gate of Q701 and turns it off. As this condition is permanent ( for the period of time that
pin 15 is at five volts ), the power supply will turn off until a new power on command is issued by IC701..
Communication is maintained with the system microprocessor, IC101, via the two opto couplers. IC704
is used to send data from IC702 to IC101 for control purposes - button pressing and remote control.
IC703 is used to control IC702 from IC101, i.e. to shut down the set if a protection fault occurs. Note that
the reset pin of IC701 is also connected to this opto coupler via D706, so that if the transistor conducts
heavily, pin 6 of IC702 will go low. IC702 will now turn the power supply off which will remove the supply
to the main processor and IC703 will stop conducting therefore pin 6 will be allowed to rise, resetting
IC702 and in turn powering up the supply again. Note that the supply to IC702 is always there.