Toshiba TDP-SW20 Service Manual page 96

2) Power Factor Correction Control Circuit
This circuit has 4 functions as below.
A) Generate stable voltage
B) Reduce input harmonic currents
C) Over current protection
D) Over voltage protection
A) Generate stable voltage
This circuit operate as step up to 370Vdc(typ.) and voltage control. Actually, Q102 and Q103 are
switched by MC101.
Initial voltage setting of 370V output (between pin 1 and pin 3 of CN102) has set at 370V (typ.)
by VR101. (Input voltage : 85Vac , max. load)
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 pin18 of MC101), then compare the sine waveform
and control Q102 and Q103 switching as correct sine waveform.
C) Over current protection
The peak current through Q102 and Q103 is detected at pin 13 of MC101 as the voltage across R109.
When the drain current of Q102 and Q103 goes over a certain limit, 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 abnormal, pin 5 of MC101 to be pulled down,
Q106 to be turn on, pin 8 of IC101 to be pulled up, and power supply to be shut down.
3) Switching Converter
The main parts of switching converter are transformer T101, switching MOS FET Q107 and output
diode D201.This converter is Fly Back type.
This means that energy is transferred from the primary to secondary when Q107 is off.
The main output is 6.5V output and auxiliary output is 15V output.
4) Feedback Control
A) Start up
When AC input is ON, power source of IC101 is supplied from R128 to R131 and start switching and
then the converter is start. 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 R208 and R209, it is compared against the reference
voltage of IC201.Optoisolator PC101 feeds back the comparison from secondary to primary by
adjust the level of current drawn from pin 2 of IC101.
When 6.5V output voltage is above the control level, IC101 to shorten the on-time (duty cycle) of
Q107. 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 Control
The peak current through Q107 is detected at pin 3 and pin 4 of IC101 as the voltage across R136.
When the drain current of Q107 goes over a certain limit, IC101 turns off Q107.
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