Panasonic UF-E1 Service Manual page 142

UF-E1
6.2.6. Description of Power Supply Board Functions
The power supply unit, as shown in the block diagram, has an AC input of 230 V, 50/60 Hz and supplies
outputs of 24 V, 5 V, and 3.3 V. (Refer to Figure 6.2.6.1)
Filter,
AC Input
Rectifying and
230V
50/60HZ
Smoothing Circuit
1. Filter, Rectifying-Smoothing Circuit
The filter section removes noise that come from power supply unit so that the noise will not be dis-
charged to outside. At the same time, it also prevents outside noise from coming into the unit.
The rectifying-smoothing section rectifies alternate current, using Diodes D10 through D 13, smoothes
the current at Condenser C5, and provides direct current voltage to the switching section. Also, ther-
mister NTC1 prevents rush current at the time when power is turned on.
2. Switching Section
The switching circuit uses automatic ringing choke converter method. By repeated on and off of MOS
FET Q1, the direct current voltage supplied by the rectifier smoothing circuit is converted to high-fre-
quency pulses. During the time when Q1 is on, the energy is accumulated in the primary coil of the
transformer T1 and the energy is discharged to the secondary side when Q1 is off. The electric power
is supplied in this manner.
The frequency changes in the output load. As the load becomes heavier, on-period becomes longer.
Control over the constant voltage is achieved by providing feedback on the control circuit from the 24V
output via photo-coupler PC1.
The excess current protection circuit detects the expansion of the on-period caused by heavier load of
the output and extends the off-period of Q1, using the control circuit, which reduces the energy accu-
mulating in the primary coil of the transformer T1.
Protection from excess voltage is achieved by the function of the excess current protection circuit in the
control circuit. The rising 24V voltage on the secondary output is sensed by the Zener Diode D104,
which is between the 24V output and the ground, when there is conductance on the Zener Diode
D104, putting it into excess current condition.
3. 24V Circuit
By smoothing and rectifying on the transformer T1 output by diode D101 and electrolysis condenser
C101, 24V output is supplied. The voltage is adjusted by the volume VR101.
4. 5V Circuit
After rectifying and smoothing transformer T1 output by diodes D201 and D202 and electrolytic con-
denser C20, 5V output is obtained, which is voltage stabilized by voltage reduction regulator circuit.
The control of constant voltage is achieved by first detecting the 5V output by shunt regulator IC201.
Then, through Q202, Q201 based current is controlled for constant voltage control.
5. 3.3V Circuit
After rectifying and smoothing transformer T1 output by diodes D201 and D202 and electrolytic con-
denser C20, 3.3V output is obtained, which is voltage stabilized by regulator IC, IC301.
Switching Section
Control
Circuit
Figure 6.2.6.1
138
24V 24V
Rectifying and
Circuit
Smoothing Circuit
5V
Rectifying and
Smoothing Circuit
24V
5V
5V
Circuit
3.3V
3.3V
Circuit
GND