Toshiba TLP-X2000B, Service Manual

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6–12
Chapter 6
3. DC/DC converter
The main parts of this switching converter are transformer T201, switching MOSFET Q103, control IC
IC102 and output diodes D201, D202 and D203. This converter is Fly Back type.
This means that energy is transferred from the primary to secondary when MOSFET Q103 is off.
The main output is 6V line and auxiliary output is 14V and 17V line.
a. Start up
When AC input is supplied to this power supply unit, power source of IC102 is supplied through R125.
And then IC102 begins switching operation and the converter starts up.
Once the converter starts up, power source of IC102 is supplied from auxiliary winding of T201.
b. Output Voltage Control
Output voltage is controlled by Pulse Width Modulation (PWM).
The voltage divided by R208, R209, R210 and R223 in the 6V line is detected, and this voltage is
compared with the reference voltage of shunt regulator IC201. Photo coupler PC101 feeds back the
comparison from secondary to primary. And output voltage is adjusted by the level of current drawn
from the FB pin of IC102.
When 6V output voltage is above the control level, IC102 to shorten the on-time (duty cycle) of
MOSFET Q103. This cause the average output to decrease. When the output is below the control
level, on-time (duty cycle) is increase, thereby increasing the average output voltage.
c. Overcurrent Protection
If the power supply output becomes overload, the current of MOSFET Q103 is limited by the maximum
threshold voltage of the IS pin in IC102 and output voltage drops. If the state continues as it is, an
overload protection function of IC102 operates to stop the IC in the latch mode.
d. Overvoltage Protection
Overvoltage state is detected when output voltage exceeds the zener voltage of zener diode (ZD201,
ZD202, ZD203). When zener diode is in conduction state, through the photo coupler PC102, the CS
pin voltage of IC102 is increased
forcibly to the threshold level for the latch mode. And power supply is
shut down.
e. Overheat Protection
The abnormality of heat sink temperature of Q101, Q102 and Q103 is detected by PTC thermistor
PR101. Through the diode D113, the CS pin voltage of IC102 is increased
forcibly to the threshold level
for the latch mode
and power supply is shut down. It operates at the time of abnormal state such as
overload or FAN-Locked.