Panasonic EURO 4 Chassis Technical Manual page 134

Chapter 4
2.2. Start Up Operation
The start-up circuit, which consists of resistors R4802
and R4803, is used to start and stop operations of the
power supply IC IC4801. This is achieved by
detecting the voltage that appears at the Vcc terminal,
pin 4.
At start-up, capacitor C4810 is charged via the
start-up circuit resistors R4802, R4803 which applies
approximately 16V to pin 4 of IC4801.
The voltage applied to pin 4 of IC4801 is then fed to the
internal start circuit, over-voltage monitor and latch
circuit which are used to control the oscillator and
drive stage. When pin 4 reaches approximately 16V,
IC4801 begins to operate and drive the internal FET
transistor into conduction resulting in current flow via
the primary winding of T4801, pin 3 (drain terminal)
and pin 2 (source terminal) of IC4801.
Once IC4801 begins to operate the supply voltage at
pin 4 is supplied via the rectifying diode D4808 and
smoothing capacitor C4810 which is fed from the drive
winding of the switching transformer T4801.
This supply voltage which is fed from the drive winding
V1 of T4801 is initially unable to provide the supply
voltage demanded and so the voltage at pin 4
decreases.
The charge held by C4810 however, slows this
decreasing voltage at pin 4 long enough to prevent the
shut down voltage of 10V being reached, thus
allowing the drive winding voltage at V1 to become
established.
Also located at pin 4 is a zener diode D4801 which is
used as an over-voltage device to protect the input pin
4 of IC4801. The zener diode D4801 which operates
as a CROW BAR device will conduct If an
over-voltage situation occurs and the zener diodes
break-over voltage is exceeded, this will create a short
circuit resulting in the over-current protection circuit
discussed in chapter 4 section 2.5.3. becoming
active, the end result being the shut down of the power
supply.
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3DQDVRQLF
EURO 4D Supplement