Main Supply Actual Operation - Toshiba 1997 G Series Troubleshooting Manual

6. Main Supply Actual operation

Refer to Figure 7 diagram and waveforms.
1. Start-up
When power is applied to the set, a start-up pulse of
16V is applied to pin 9 of IC Q801. At the same time,
the charging of C869 (pin 8) induces a delay to the in-
ternal latch circuit to prevent the Over Voltage Protect
(OVP) from engaging, and C866 (pin 7) sets the switch-
ing frequency high to reduce the surge current. After
the initial start-up, the circuit operates at its nominal
frequency (70-80 kHz), and the Drive Circuit (see page
9) supplys 17V to pin 9.
2. Output switching element
Two power MOSFETs in push-pull configuration, op-
erate the switching. The on-off timing of each MOS-
FET is controlled by the logic inside Q801. To avoid
shorting the MOSFETs, they are never turned on at the
same time. Between the time one transistor turns off
and the other turns on, both MOSFETs are off. This off
time is called dead time, and is determined by
R867(pin3).
3. Basic Oscillation
The frequency of the internal oscillator is determined
by the charge and discharge of capacitor C862 (pin 4),
and is controlled by the feedback into pin 5 through
the Oscillator control block. The oscillator generates a
ramp waveform at Pin 4. The ramp waveform charges
up to 4 V (typical) and discharges to about 2.5 V. The
charging time is the output-on period for one of the
MOSFETs, and the discharging time is the off period
for both MOSFETs (see OSC OUT SIGNAL waveform
of Figure 7). The lowest oscillation frequency is deter-
mined by capacitor C862 and resistor R867.
4. Frequency Control
Current flowing out of the CONT terminal (Pin 5) var-
ies the charging time of oscillator capacitor C862, which
in turn, controls the frequency of the Output (Pin 14)
signal. The control current is determined by the
photocoupler. The photocoupler phototransister side cur-
rent is determined by the feedback current of the photo-
diode side. The photodiode current is determined by
the error amp inside of Z801, which is monitoring the
+125V source. Thus, the terminal current (CONT) cor-
responds to the feedback from the +125V output.
5.CD terminal (Pin 8) - Latch Delay
The Latch circuit shuts the power supply off (shut-down)
when a fault is detected. Shut-down occurs by detecting
errors from the following:
• Over voltage protection (OVP) circuit
• Thermal shock detection (TSD) circuit
• Over current protection (OCP) circuit
• Loss of and no recovery of Main B+
The charging time of capacitor C869 connected to the
CD terminal (Pin 8) is used to delay the operation of the
latch circuit when power is initially applied. If the unit
goes into shut-down, temporarily remove AC power to
reset the latch circuit.
7.OC terminal (Pin 11) - Over Current Detect
This is to detect over-current in the LC series resonant
circuit.
8.Over voltage protection (OVP) circuit
If the Vcc terminal (Pin 9) exceeds 22V (typical), the
latch circuit is engaged (shutdown) .
9.Thermal shock detection (TSD) circuit
This is to make the Latch circuit operate when the IC's
internal temperature exceeds 150°C.
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