Panasonic EURO 4 Chassis, Technical Manual

Page: 15 / 163

3DQDVRQLF
15
8.6. Protection Circuitry
8.6.1.Thermal Shut-down
This circuit triggers the latch circuit when the body of
the IC exceeds 140
•
C
8.6.2.Overvoltage Protection
The Overvoltage protection (OVP) is used to trigger
the latch circuit (mentioned later) when the Vcc
voltage at pin 4 exceeds 22V.
Although the OVP circuit basically functions as
protection of the Vcc terminal, it also prevents over
voltage at the secondary output, since the Vcc
terminal is supplied from the drive winding of
transformer T801, whose voltage is proportional to
its output voltage from the secondary windings.
8.6.3.Overcurrent
The Overcurrent protection (OCP) is performed
pulse-by-pulse by detecting the peak of the drain
current of the MOS-FET in every pulse and which is
used to reverse the internal oscillator output of
IC801 .
The MOS-FET drain current is detected by inputting
the voltage drop developed across R810, R811 into
pin 1 of
IC801 via the noise filter circuit R809, C815.
When this input voltage exceeds the internally
generated reference signal of 0.73V the drive output
is pulled LOW, resulting in the internal MOS-FET of
IC801 switching OFF and the power supply
stopping.
8.6.4.Latch
The latch circuit is used to keep the output from the
oscillator low stopping the power supply operating
when the overvoltage (OVP) and thermal shut-down
(TSD) circuits are in operation.
16V
(TYP)
10V
Vin
(TYP)
In this condition the Vcc input (pin 4) voltage
decreases until the the Vcc input reaches the
shut-down voltage of 10V. At this point pin 4 begins
to rise again but when it reaches the start up level
(16V) the latch circuit continues to stop the drive.
When the latch is ON, Vcc pin 4 voltage increases
and decreases within the 10V to 16V range, as
shown in the above fig. and is prevented from rising
normally.
Cancellation of the latch is achieved by switching
OFF the TV and disconnecting the AC input to the
circuit.
8.6.5.150V Line Protection
In-conjunction with the above mentioned protection
circuits, the 150V line is separately monitored for
over voltage protection (OVP) and over current
protection (OCP).
The OVP is monitored by zener diode D875, which
when its breakover voltage is exceeded causes
transistor Q855 to conduct. This results in the base
of Q853 being pulled LOW, causing Q853 to be
switched OFF.
When transistor Q853 is no longer conducting then
no current flow takes place via the standby relay
winding and the collector/emitter junction of
transistor Q853 . The result of no current flow via the
standby relay winding causes the normally open
relay contact of the standby relay RL801 to open,
stopping the supply of the mains voltage to the
power supply circuit causing the TV to switch into
standby.
Likewise where an over current situation occurs then
an increased voltage drop across current limiting
resistor R877 develops. This increased voltage drop
biases on Q857 which feeds a HIGH level via the
emitter/collector junction of Q857 to the base of
Q855 . When transistor Q855 conducts, Q853
becomes non-conducting resulting in the TV being
switched into standby mode.