Panasonic BT-M1950Y Service Manual page 113

2. Operation
esurge protection
A varistor (VA001) is used for surge protection. Compared to a discharge tube or spark gap, the varistor has the
following advantages with respect to surge voltage occurring from lightning or other cause.
• Large nonohmic response and wide voltage range. Voltage can be stabilized and surge absorbed in a wide range.
• Excellent surge absorption and fast response.
A rush resistor (R060) is provided for rush current protection.
ePWM
switching regulator
Power on to switching start
AC voltage through the line filter is rectified by diode bridge D001. The rectified DC voltage via starter transistor
0005 forms the initial power supply of the switching control lC001.
The DC voltage is also applied from the switching transformer to the drain of FET 0004. In this state, the FET begins
switching operation when the switching pulse is applied to the FET gate.
The 1coo1 internal oscillator begins operation when power is supplied to pin 16. Although the oscillator is
synchronized to the external sync input at pin 13, in the initial state, the oscillation is free running. The free running
frequency is determined by C019 and R023 at pin 11.
A squarewave output from the internal comparator is sent via pin 9 and buffer 0001 to drive transistors 0002 and ·
0003. A totem pole connection is used for 0002 and 0003. When the base potential is High, 0002 switches on;
and when Low, 0003 switches on. The resulting output allows adequate current to flow in the FET drain.
When switching operation reaches normal, the IC001 power supply is rectified by D012 and DC voltage can be
used.
TOI
w1-1Ylli:I.JUI
0001
SW. TRANSFORMER
r---,
._
_______________________
-.-
__
~--___.LINE
FILTER
JUUL
ICOOI
COMPARATOR
FREE
RUN
FREQ.
Q005
STARTER
vcc
vcc
G
Q004
SW.OUT
TO ICOOI
@PIN 0012
--------1.13------
EXT. SYNC IN
Fig. 2-1 PWM switching regulator
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