Panasonic KX-F900 Service Manual page 165

I KX-F900
The circuit in the previous page, when the transistor Q101 is ON, secondary rectifier diode D201 is OFF and the energy is
charged in the transformer T101. Q101 continues being ON while the voltage is generated by control winding (B). Q101 is
tuned OFF by control circuit, then each windings of T101 changes the polarity and rectifier diode D201 truns ON.
The charged energy of T101 supplies power through D201 to output load. And the voltage of control winding is decreased
and QlOlcontines being OFF state. When all energy is discharged through D201, Q101 is turned ON again and it makes the
polarity of each windings of T101 in reverse and goes to self oscillation. When input voltage Ei is high, the ON period of Q101
becomes shorter, and when load current is high, the ON period of Q101 becomes longer.
The value of output voltage is
Eo = d/(1-d)*Ei
d = Ton/Ts
SW D (D201) Eo
_o ' 1_ on
Ei (Q101)
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To,: ON TIME OF Q101
T, : PERIOD OF OSCILLATION
In the equivalent circuit:
When SW is ON, current flows
SW --* L
When SW is OFF, Current flows
L-* D--" RL
The value of inductance rectifiers increasing
current during ON period.
IL = Ei/L*d*Ts (1)
The value of inductance rectifiers decreasing
current during OFF period.
IL = Eo/L(1-d) * Ts (2)
From equations (1) and (2),
Eo = d/(I-d) * Ei
In the actual circuit, the fixed output voltage can be obtained by changing the winding ration of transformer T101. In this
converter circuit, the duty ratio of ON period and OFF period of the transistor produces output. In this power supply, the bias
winding is also built-in in the transformer and the output value is one. 24V output voltage is stabilized and changes the duty
ratio.
[Control Circuit And Error Detecting Circuit]
The control circuit amplifies the output with increasedvoltage detected in the error detecting circuit, then drives the main
transistor. I n this powersupply the duty ratiois definedby changing the ON periodof main transistor.
This isshownas follows.
When the outputvoltageof24V circuitbecomes higher, the currentof photocouplerPC101 increases,the pulse widthof
outputcontrolIC becomesnarrowand the ON period ofQ101 becomesshorter.
[Over Current Limiter (O.C.L.)]
IC101 rectifiers the highest voltage with resistors R105 and R106 detecting the current in the primary side. When the current
is supplied higher than the highest voltage, it switches to ratch mode which stops oscillating.
[DC-DC converter]
Output 5V, is made by DC-DC Converter. 5V output is rectified by IC202, Q201, 1_201, D 203.
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