Circuit Description
GB 84
A10PTV2.0 NTSC
9.
Figure 9-2
Figure 9-3
9.2
Power Factor Correction Circuit
The input to most switching power supplies consists of a
bridge rectifier and a large electrolytic capacitor when AC
power is applied, the Capacitor will charge to approximately
1.4 times the RMS value of the applied AC voltage. This type
of supply does not draw current from the AC power source
through out the entire AC cycle. When the capacitor charge
falls to point “A”, the instantaneous value of the AC voltage
exceeds the charge of the capacitor. The bridge diodes are
then forward biased, causing current to flow from the AC
source. Current will continue to flow from the AC line until the
AC sinusoidal voltage reaches its peak at Point “B”. At this
point, the charge on the capacitor will exceed the AC line
voltage reversing the bridge diodes. This results in an AC
current waveform that is narrow and distorted compared to
the AC voltage waveform. Non sinusoidal waveforms have a
high harmonic content, with excessive peaks, which results
in a low power factor of 0.5 to 0.6. Power Factor is a ratio of
real power divided by apparent power. Excessive harmonics
and peak currents reduce the efficiency of the power
distribution system. The A10 Projection TV without Power
Factor correction has a high harmonic content of 85 percent,
creating a Power Factor of 0.5. High current spikes will also
be created in the AC supply. With the Power Factor
correction circuit, the peak current is limited, with the
harmonic content being reduced. The Power Factor is
increased to 0.99. The ideal Power Factor is one, which
occurs when the current is sinusoidal and in phase with the
voltage. Many countries limit the current harmonic content
of equipment supplied by the AC Mains. The A10 Power
Factor correction circuit uses a Boost regulator to smooth out
the current draw from the ac line improving the Power Factor
to 0.99, which is accomplished by drawing current from the
AC source throughout the entire ac cycle making the current
waveform sinusoidal. Input to the module is connected to the
ac Mains. The output supplies are approximately 376 volts
dc to the Main and Standby Switched mode Power Supply
circuit. The Boost Regulator circuit produces a higher output
voltage than the input voltage. The regulator drive circuit
compares 6103’s output voltage, the input voltage from the
bridge, and the voltage across the current sensing resistors
to control the On time of 7104. This will maintain the output
voltage at 376 volts DC and limit the input current to
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