Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support GroupPage 8
Figure 6 : Power Supply Regulation Circuit - Secondary Side
Voltage regulation
Voltage regulation is provided by the feed back through the opto coupler, IC705. The 150 volt line is
used as the controlling source and a reference is provided from the +13V supply. Protection is also
incorporated via the +7V supply for faults that may occur on either of the five volt supplies ( STB5V and
+5V ).
Current flowing through the opto coupler LED will effect its brightness and therefore the conduction of
the light sensitive transistor. This will control the time taken to charge C713 from the supply generated
by D715 and R722 and therefore will control the turn on time of Q703, note that the earth return for C713
is via R716.
As the collector of Q703 is connected to C714 this action will in turn affect its charging time and the
on/off time of Q701. As the charge/discharge of C714 is controlled by the switching of Q701 its base
capacitor, C713, is the timing control influence. As the discharge of C713 is constant, the off time will
remain at approximately the same period during each cycle, because its discharge path remains the
same through R716. Whereas the charging of C714 will control the on time, i.e. when its charge is
above the threshold turn on voltage on Q701 or +4.5V. Therefore it can be noted that an increase in load
will slow down the frequency of operation.
Note that D715, R722 and C716, on the primary side of the opto coupler, provide a voltage supply circuit
to the collector of the transistor contained within IC705. Also note that on the secondary side of the
regulation circuit, the supply for the reference zener diode, D717 is taken from the +13V supply.
Supply increase
If the 150 volt supply line were to increase, the voltage on the base of Q705 would also increase. As
the emitter of Q705 is held steady by the zener diode D717, at 6.2V, the collector will start to drop. This