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STANDBY SUPPLY (Figure 23)
The Standby supply is located on the Main Power board. It provides the +5VSTBY supply to
the set whenever power is applied.
When power is first applied, the RAWDC_STBY is applied to the primary winding of 5210 and
to Pin 7 of 7202. Current through resistor Rs charges Capacitor 2244 connected to Pin 1 of
7202. When the charge on 2244 reaches 5.8 volts, the Internal Supply switch is switched to
internal. The Oscillator and PWM turn On to drive the internal FET. The PWM will continue
to drive the FET until the charge on Capacitor 2244 drops below 4.8 volts. Capacitor 2244
will again charge and the process will repeat. With each startup cycle, energy is transferred
to the secondary windings of 5210. When the +5VSTBY supply reaches 5 volts, the input of
7206 will go to 2.5 volts. This will turn Shunt Regulator 7206 On, causing current to flow
through optoisolator 6207. Voltage from Pins 1 and 2 of 5210 is rectified by 6205 to produce
the operating voltage for 7202. When Optoisolator 6207 turns On, the operating voltage is
applied to Pin 1 of 7202. The supply is in a normal operating mode.
Regulation of the secondary is accomplished by monitoring the reference voltage via Shunt
Regulator 7206 and optoisolator 6207. The +5VSTBY is the reference voltage for the
Standby supply. If the 5-volt standby supply should increase, Shunt Regulator 7206 will con-
duct harder, causing the LED in Optoisolator 6207 to get brighter. The internal resistance of
the transistor inside 6207 will be reduced. The internal regulator inside 7202 connected to
Pin 1 works to keep Pin 1 at 5.8 volts. If the current through 6207 increases, the current flow
through the 5.8-volt regulator will increase. This increase in current will cause the voltage
across the sensing resistor Re to increase. The PWM will then reduce the On time of the
internal FET, reducing the secondary voltage. If the 5-volt standby voltage should decrease,
the current through the sensing resistor Re will decrease, reducing the voltage across the
resistor. This will cause the PWM to turn the internal FET On for a longer period of time,
increasing the secondary voltage.
The +5.2 volt supply is switched by FET 7203 when the +12 volt supply from the Main Power
supply is switched On. If the Standby Supply should fail or Power is removed from the set,
the Standby Power Fail circuit will signal the Microprocessor in time to allow it to shut down.
Voltage from Pin 8 of 5210 is rectified by 6209 to produce a negative voltage. This voltage
and the +5 volt standby voltage is applied to the input of Shut Regulator 7209. The combined
voltage keeps the input to 7209 below the turn On voltage of 2.5 volts. A small Capacitor,
2265, filters the negative voltage that is rectified by 6209. If power is removed from the set,
2265 will discharge quickly because of its small value. The input to 7209 will then go above
2.5 volts, turning it On. This will turn transistor 7210 On, switching the Standby Power Fail
line High. Transistor 7210 will also turn On, turning the +5.2 volt supply Off.
Page 12
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