Panasonic EURO 4 Chassis Technical Manual page 136

Chapter 4
2.5. Protection Circuitry
2.5.1. Thermal Shut-down
This circuit triggers the latch circuit when the body of
•
the IC exceeds 140 C
2.5.2. Overvoltage Protection
The Overvoltage protection (OVP) is used to trigger
the latch circuit (mentioned later) when the Vcc
voltage at pin 4 exceeds approximately 22V.
Although the OVP circuit basically functions to protect
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.
2.5.3. Overcurrent
The Overcurrent protection (OCP) is performed
pulse-by-pulse by detecting the peak drain current of
the FET in every pulse and which is used to reverse
the internal oscillator output of IC4801.
The FET drain current is detected by inputting the
voltage drop developed across R4805, R4807 into pin
1 of IC4801 via the noise filter circuit R4806, C4807.
When this input voltage exceeds the internally
generated reference signal (0.73V) the drive output is
pulled LOW, resulting in the internal FET transistor of
IC4801 switching OFF and the power supply
stopping.
2.5.4. Latch
The latch circuit is used to keep the output from the
oscillator low stopping the power supply operating
when the over voltage (OVP) and thermal shut-down
(TSD) circuits are in operation.
Vin
16V
(TYP)
10V
(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
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.
53
3DQDVRQLF
EURO 4D Supplement
figure 1