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Standby Power Supply
Figure 3
Standby B+ is applied to the Standby Switching regulator 7218 via Pins 5 and 3 of 5202. An internal
switch in 7218 charges capacitor 2283 connected to Pin 1 of the IC. When the charge on 2283
reaches 5.8 volts, the internal switch switches to internal. The IC is now being powered by the charge
on capacitor 2283. The internal FET drives the transformer 5202 until the charge on capacitor 2283
reaches 4.8 volts. The IC repeats the cycle until the 5- volt Standby voltage reaches the correct level.
Shunt regulator 7212 then turns On, turning the optoisolator 7213 On. The operating voltage for 7218
is then supplied by the rectified Hot secondary voltage from Pin 1 of 5202. The internal regulator
keeps Pin 1 at 5.8 volts. Regulation is accomplished by monitoring the 5-volt standby voltage. If the 5-
volt supply increases, the shunt regulator 7212 will conduct more, causing the resistance of the
transistor inside 7218 to decrease. The sensing resistor, Re, inside 7218 will sense the increase in
current and reduce the On time of the internal FET, which will lower the 5-volt supply to the correct
level.
In Standby, the 5-volt Standby voltage is the only voltage being applied to the set. The +15, +9, and
+5.2-volt sources are turned Off. When the set is turned On, the Standby Line goes Low, turning
transistor 7214 Off, turning 7205 On, which turns 7215 On. This switches the +15-volt supply to the
set. The +15-volt supply then switches transistors 7216 and 7217 On to switch the +9-volt and +5.2-
volt supplies to the set. When the +15UV (Under Voltage detection) line goes High, transistor 7220
will turn On, which will turn 7205 Off. This will cause 7215 to turn Off, switching the +15, the +9, and
the +5.2-volt supplies Off. This will cause the set to shut down.
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