Power Supply Board Section - Panasonic KX-FLM652CX Service Manual

KX-FLM652CX

7.16. POWER SUPPLY BOARD SECTION

This power supply board uses the switching regulator method.
F1
F2
Filter
Input
Circuit
Block
[Filter circuit]
The filter circuit eliminates the noise from power supply unit not to leak it outside, and also the circuit prevents an incoming
noise. Varistor Z1 deals with the excessive surge voltage from thunder, etc.
[Rectifier and Smoothing Circuit]
AC voltage is rectified by D10, D11, D12 and D13, and smoothed by C5. Then DC voltage is supplied to the switching circuit.
[Switching Circuit]
This circuit has self-excited Ringing Choke Converter.
This system supplies DC voltage through the following process: The DC voltage provided from the Rectifier and Smoothing
Circuit is converted into RF pulse by switching ON/OFF MOS FET Q1 repeatedly. While Q1 is switched ON, the energy is
charged in the primary of T1, and it will be discharged to the secondary of T1 while Q1 is OFF.
[Switching Control Circuit]
The constant voltage is controlled by the feedback from the 24V output via the photo coupler PC1
The overcurrent protection circuit detects the extension of the ON time caused by heavier output load, and makes the control
circuit prolong the OFF time of Q1. Thus the circuit eliminates the charged energy in the primary of T1. When the output voltage
rises above 24V, the zener diode D104 between 24V output and GND breaks down. The over voltage protection works when
the overload causes the overcurrent protection circuit of the control circuit to be activated.
[24V Circuit]
The output from the transformer T1 is rectified by D101 and smoothed by C101, and then 24V is output.
[5V circuit]
The output from the transformer T1 is rectified by D501 and smoothed by C501, and then the stabilized 5V is output through
the automatic voltage regulator IC501.
[3.3V Circuit]
The 3.3V output from above-mentioned 24V circuit is regulated at the step-down chopper circuit. In this method, the energy is
supplied to the load via L271while MOS FET Q271 is turned ON, and the charged energy in L271 will be supplied to the load
via D271 during the OFF time.
The shunt regulator IC201 detects the 3.3V output and feeds back, and then the constant voltage is controlled.
When the output voltage goes down and Q274 is turned OFF, the short-circuit protection circuit keeps the power OFF until the
power is turned ON again.
[Heater Circuit]
When HTRCTL signal turns "high", the voltage is supplied to the gate 1 of the triac SCR1 via the photo coupler PC2, which
causes the triac SCR1 to be turned ON. Consequently the heater output is turned ON.
Dummy load method (to quickly check the power supply output)
Refer to POWER SUPPLY BOARD SECTION (P.152).
AC OUT
AC Output
Circuit Block
D10-D13,C5
Q1
Rectification
and smoothing
Circuit Block
Control
Circuit Block
PC2
Optical Isolator
Transformer
24V Circuit
T1
Switching
Circuit
Block
PC1
Photo-Coupler
204
Block
IC501
5V Circuit
Block
3.3V Circuit
Block
HTRCTL
24V
PWRDWN
5V
3.3V
GND