Toshiba TDP-ST20E Service Manual page 53

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 R106, R107, R156 to pin1 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 2 ,pin 4 of MC101 as the voltage across R114.
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
PFC output voltage is adjusted at 12 pin of MC101 voltage by VR101. Even if this Connection to
be open, it must be protected.
Over voltage protection stops oscillation in Q1 and Q2 by activating the over voltage protection of PFC
control IC in MC101 when PFC output voltage becomes over voltage.And the over voltage protection starts
oscillation again when voltage goes down to a certain level, and it repeats oscillation and stop.
E) (Inrush current suppression circuit)
Not related by PFC, but include inrush current suppression circuit . (TH101)
This circuit is to reduce (big) charge current through P2 lamp drive (Ballast) power supply and
Charge current C114, at AC input timing.
3) Switching Converter
The main parts of switching converter are transformer T101, switching MOS FET Q3 (MC102) and
output diode D201, D202. 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 12V 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 R129 R132 and start switching
and then the converter is start. Once the converter begins switching, power source of IC101
is supplied from B1 winding of T101.
b) Output Voltage Control
The stabilization of output voltage is controlled by Pulse Width Modulation (PWM).
It is stabilized at 12V by resistance ratio of R206 and R207 based on the standard voltage at
IC201.This stabilization is done by feedback from the secondary circuit to the primary circuit
through Optoisolator PC102 and adjusting the level of current drawn from pin 2 of IC101.
When 12V 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 (MC102) is detected at pin 3 of IC101 as the voltage
at both ends of R147.
When the drain current of Q3 (MC102) goes over a certain limit, IC101 turns off Q3 (MC102).
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Chapter 6