Power Off Operation Of The Dc-To-Dc Converter; Protection Of The Dc-To-Dc Converter - Panasonic TC-26LX20 Technical Manual

enters pin 6 of IC7001 to begin the oscillation. The pulses that ar e output at pin 1 of the
oscillator enter the transformer T7004, causing it to build a magnetic field. The output of
the transformer is rectified into 121Vdc and rovided to the LCD panel via the connector p
P4. When the rectified output of the transformer reaches 90Vdc, the diodes D7043,
D7044, D7045, and D7046 go into conduction, turning on transistor Q7010 and thereby,
turning off IC7005. This eliminates the start up voltage of IC7001. The oscillator
continues to operate using the run supply created by a secondary of the transformer
T7004 and the diode D7014. See Figure 19.

Power Off Operation of the DC-to-DC converter

When the unit is turned off, the 120V_Stop command is provided at pin 2 of the
connector P1/AP10. This causes the transistors Q7008 and Q7009 to turn on and stop
the conduction of IC7005 and IC7006. See Figure 20.
The 120V_Stop line is designed to stop the operation of the backlight DC-to-DC
converter if there is a drop or an increase in the 24 volts supply to the AP board. Pin 2
of the connector P1/AP10 inputs a High to the P board causing the transistors Q7008
and Q7009 to turn on. This causes IC7006 to immediately turn on and short pins 6 and
7 of IC7001. This action stops the oscillation of IC7001. To keep the oscillation of the IC
from starting again, the DC supply is grounded through Q7008.

Protection of the DC-to-DC converter

Over-current protection of the circuit is provided at pin 7 of the IC7001 from the emitter
output of Q7018.
Over-voltage protection of the primary is provided via the diode D7054. If the rectified
voltage of D7018 exceeds the operating voltage of IC7001, the diode D7054 conducts
and applies DC to the base of Q7019. The transistor Q7019 turns on and causes Q7018
to turn off, effectively removing the feedback voltage to pin 7 of IC7001. This action
stops the operation of the oscillator.
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