Panasonic UF-890 Service Manual page 250

ETXDN218A7D / DU2L1B146 (115V)
Input Filter Circuit
AC line voltage travels to the rectifying circuit through the line filter. The line filter eliminates RFI noise
which may otherwise pass to the AC line from the power supply unit. It also protects the power supply
unit from transient noise which may pass into the unit from the AC line.
Rectifying and Smoothing Circuit
As soon as power is applied to the Power Supply Unit, AC line voltage is rectified by C105 and is
smoothed by capacitor C107. The protection circuit at the time of start-up is controlled by an IC (IC101)
and resistors R103 and R110.
Inrush Current Protection Circuit
When the capacitor C105 is not charged by the AC input, an inrush current, or current surge, appears at
the input side. Power thermistor TH101 limits the inrush current.
Converter Circuit
A hybrid IC (IC101), in combination with transformer T1, form a switching power supply circuit using the
RCC (Ringing Choke Converter) system.
Ei
Q101
Main Switching Circuit
In the above circuit, when the main switching transistor, Q101, is turned On, input voltage, Ei, is supplied
to the primary winding of transformer T101. However, no current will flow through diode D201 of the
secondary side, due to reverse polarity of the secondary winding causing no current flow within T1. But
the transformer charges with energy. When Q101 is turned Off, the supply voltage to the primary winding
shuts off and the windings of T101 change polarity, allowing D201 to conduct, releasing the energy
accumulated in T101 to the circuit. When the energy is discharged through D201, Q101 turns on, once
again reversing the polarity on T101 windings, creating a self-oscillation circuit.
T101
P
B
Control Circuit
P --- Primary Winding
S --- Secondary Winding
B --- Control Winding
D201
Eo
+
S
250