Sony VPL-PX20 Service Manual page 70

3. Basic Operations
In conventional rectification circuits which do not use the active filter, peak recharge current is generated in a part of the
AC voltage waveform due to the capacity components of the smoothing capacitor. This causes such problems of the
effective power such as later phase power consumption, high frequency, etc., and may cause subsequent problems in other
equipment. To prevent this, this power supply circuit is equipped with a step-up DC-DC converter in the smoothing
circuit and adopts an active filter which compares the input current waveform with the input voltage waveform by
appropriate control.
Fig. 2 shows the principle of the active filter circuit. When MOS-FET Q2100 turns ON, energy is accumulated in the
choke coil L2100. When it goes OFF, energy is discharged, and current flows to the load side.
Fig. 3 shows the relation between the voltage and current. The average input current for each switching period of the
MOS-FET Q2100 is half of the peak current flowing to the choke coil L2100. Since these peak values can be placed on the
curve proportionate to half the AC voltage input, they appear like resistance load on the AC voltage from the input side of
the active filter.
Maximum Switching current
Switching Current
Inductor Current
VPL-PX20/PX30
D2100
L2100
Control Circuit
Fig. 2 Princple of Active Filter
Pulse Voltage
Fig. 3 Puls Voltage and current
D2108
+V
+
Q2100
C2112&C2113
-V
D2100 : Bridge rectifier
L2100 : Smoothing choke coil
C2112,C2113 : Smoothing capacitor
IC2100
Q2100 : MOS-FET
D2108 : Smoothing diode
IC2100 : Control IC
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