Philips EM7U LCOS, Service Manual

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MAIN POWER SUPPLY (Figure 24)
Power is applied to the Main Power supply when the set is turned On. The RAW-DC is
switched to the Main Power supply located on the Main Power supply board. Voltage is
applied to the Primary winding of 5310 via Pins 5 and 3. Voltage is then applied to Pin 7 of
7302. In the Startup mode, voltage from the internal current limiting resistor Rs is routed to
Pin 1 where it charges Capacitor 2344. When the charge on Capacitor 2344 reaches 5.8
volts, Pin 1 is switched to the Internal Supply connection. The Oscillator and PWM drive are
switched On to drive 5310. IC 7302 will continue to drive the Transformer, 5310, until the
charge on 2344 drops below 4.8 volts. The Startup cycles will continue until the secondary
reference voltage, the 9 volts supply, reaches the correct level. When the 9-volt supply
reaches the correct level, 2.5 volts will be applied to the input of Shunt Regulator 7306.
Shunt Regulator 7306 will then turn On causing current to flow through the LED in
Optoisolator 6307. This will turn the transistor inside 6307 to turn On. Voltage from Pins 1
and 2 of the Hot secondary is rectified by 6305 and filtered by 2341. Optoisolator 6307
switches this voltage to Pin 1 of 7302 to provide the operating voltage for the IC.
The secondary pr36, +12, +9, and +22 volt supplies. Shunt Regulator 7304 and
FET 7303 regulate the +12 volt supply.
The PWR_FAIL Main Power Fail detection circuit signals the Microprocessor when power is
about to be lost to the set while it is On. Voltage from Pin 9 of 5310 is rectified by 6309 to
produce a negative voltage, which is filtered by Capacitor 2365. This voltage along with the
+9 volt supply is applied to the input of Shunt regulator 7309 via resistor network of 3354,
3355, and 3353. As long as the negative voltage is present on 2365, the voltage on the input
of 7309 will be less than the 2.5 volts necessary to turn it On. Transistor 7310 will be turned
Off, keeping the PWR_FAIL line Low. When the switched mode supply ceases operation,
Capacitor 2365 will discharge quickly, due to its low value, allowing the voltage on the input of
7309 to increase to 2.5 volts. Shunt Regulator 7309 will turn On, turning transistor 7310 On,
causing the PWR_FAIL line to go High. This will signal the Microprocessor to shut everything
down before the main supply voltage drops.
Regulation of the secondary is accomplished by monitoring the reference voltage, the +9 volt
supply, to control the On time of the internal FET of 7302. If the +9 volt supply increases,
Shunt Regulator 7306 will conduct harder increasing the amount of current through 6307.
This will cause the LED to increase in brightness, reducing the internal resistance of the tran-
sistor inside the IC. The increase in current will cause the voltage across the internal sensing
resistor Re inside 7302 to increase. The PWM will then reduce the On time of the internal
FET, decreasing the secondary voltage. If the +9 volt supply should decrease, 7306 will con-
duct less, the transistor inside 6307 will conduct less, causing less current to flow through the
sensing resistor Re. The PWM will sense the lower voltage across Re, increasing the On
time of the internal FET to increase the secondary voltage.
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