Panasonic KX-F910BX Service Manual page 156

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 Q101contines 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
(Q101)
Ei
V
L
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 increased voltage detected in the error detecting circuit, then drives the main
transistor. In this power supply the duty ratio is defined by changing the ON period of main transistor.
This is shown as follows.
When the output voltage of 24V circuit becomes higher, the current of photo coupler PC101 increases, the pulse width of
output control IC becomes narrow and the ON period of Q101 becomes shorter.
[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, L201, D203.
D (D201)
+
C
L
-
V
L
T101
Ei
Eo
Ts dTs
dTs
(1-d)Ts
T
T
Eo
In the equivalent circuit:
When SW is ON, current flows
SW ¤ L
When SW is OFF, Current flows
R
L ¤ D ¤ R
L
The value of inductance rectifiers increasing
current during ON period.
IL = Ei / L*d*Ts
The value of inductance rectifiers decreasing
T
current during OFF period.
IL = Eo/L(1-d) * Ts
From equations (1) and (2),
Eo = d/(I-d) * Ei
| 165 |
: ON TIME OF Q101
on
: PERIOD OF OSCILLATION
s
L
(1)
(2)
KX-F910BX