Panasonic EURO 4 Chassis Technical Manual page 130

Chapter 4
1.6. Protection Circuitry
1.6.1. Thermal Shut-down
This circuit triggers the latch circuit when the body of
•
the IC exceeds 140 C
1.6.2. 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.
1.6.3. 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.
1.6.4. 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.
Vin
16V
(TYP)
10V
(TYP)
figure 1
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
figure 1 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.
1.6.5. 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. When Q855 conducts its
collector is pulled LOW, this LOW level is then fed via
connector E15 pin 2 to the W-Board via connector
W7. On the W-Board this resultant LOW level is then
fed to the base of Q853 which is pulled LOW, resulting
in Q853 being switched OFF.
When transistor Q853 is no longer conducting then no
current flow takes place via the standby relay (RL801)
winding and the collector/emitter junction of transistor
Q853. The result of no current flow via the standby
relay (RL801) winding causes the normally open relay
contact of the standby relay RL801 to open, stopping
the supply of the mains voltage to the main power
supply circuit located on E-Board 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 again Q853
becomes non-conducting resulting in the TV being
switched into standby mode.
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3DQDVRQLF
EURO 4D Supplement