Toshiba TDP-TW95E Service Manual page 51

B) Reduce input harmonic current
Normally switching power supply circuit is capacitor input type. Input current of this power supply
has many harmonics, because of the conduction angle of input current is narrow.
Therefore, an input current is distorted and this is a cause of low power factor.
Main purpose of Power Factor Correction Control Circuit is reducing input harmonic current. MC101
senses input voltage(through R102 to pin13 of MC101),then compare the sine waveform and
control Q1,Q2(MC102) switching as correct sine waveform.
C) Over current protection
The peak current through Q1,Q2 (MC102) is detected at pin 9 of MC101 as the voltage across
R110, R162.
When the drain current of Q1,Q2 (MC102) goes over a certain limit, control ON-duty of them, and
MC101 turns off.
D) Over voltage protection
Lamp output voltage is adjusted at 1 pin of MC101 voltage by VR101.Even if this Connection to be
open,it must be protected. So we prepared other circuit for over voltage protection. Lamp voltage is
detected at pin 2 of MC101,when lamp voltage to be abnornal,pin 4 of MC101 to be pulled down,
Q107 to be turn on, pin 8 of IC101 to be pulled up, and power supply to be shut down.
E) (Inrush current suppression circuit)
Not related by PFC,but include inrush current suppression circuit.
This circuit is to reduce (big) charge current through CNl102 lamp drive(Ballast)power supply and
Charge current C116,at AC input timing.
This circuit composed of C109, CR101, TH101, CR101 and C, R.
3) Switching Converter
The main parts of switching converter are transformerT101, switching MOS FET Q3 (MC102) and
output diode D201, D203.This converter is Fly Back type.
This means that energy is transferred from the primary to secondary when Q3 (MC102) is off.
The main output is 6.5V output and auxiliary output is 15V output.
4) Control Circuit
a) Start up
When AC input is ON, power source of IC101 is supplied from R135~R138 and start switching and
then the converter is Starts. Once the converter begins switching, power source of IC101 is supplied
from B2-B1 winding of T101.
b) Output Voltage Control
Output voltage is controlled by pulse Width Modulation (PWM).
6.5V output voltage is sensed at between R213 and R214; it is compared against the reference
voltage of lC20l.Optoisolator PC101 feeds back the comparison from secondary to primary by adjust
The level of current drawn from pin 2 of IC101.
When6.5V output voltage is above the control level,IC101 to shorten the on-time (duty cycle) of
Q3 (MC102).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) Over Current Limit
The peak current through Q3(MClO2) is detected at pin 3 and pin 4 of IC101 as the voltage across
R131 and R132.
When the drain current of Q3 (MC102) goes over a certain limit, ICl101 turns off Q3 (MClO2).
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