Sony KDP-57XBR2 Training Manual page 53

TO HV
R8119
PROTECT 2
IC8008/3
7.2V
R8174
HV
3
BLOCK
CN8006
D8041
RD13M
+12V
Q8025
D BOARD
R8177
+135V
R8178
R8167
+12V
C8090
PWM Stage: Figure 11-4 illustrates the second half of the HV regulator. There are two must-haves for a PWM
circuit to function: The first is a sample of the voltage to be regulated, and a sawtooth or triangular shaped
waveform. The horizontal pulse is utilized to provide the stable timing of the sawtooth signal applied at pin 6 of
IC8002. The pulse will be amplified and shaped by peaking and integrators located in the upper half of the
diagram. Pin 5 of IC8002 will receive a sample representing high voltage level from the circuit previously described
in Figure 11-3. Since the sawtooth signal is applied to the inverting input of IC8002/pin6, any rise of the DC
voltage at the non-inverting input at pin 5 will cause the comparator to output a high only when the level of the
sawtooth is greater than the DC voltage input. Since the sawtooth enters the inverting input, the positive duration
of the pulse is increased and the HV regulator driver, Q8008, will be turned on for a longer period.
This pulse can be used to control the on time of an amplifier (such as in this case), or filtered to provide a DC
correction voltage. This is the essence of how all PWM regulators function. An incorrect DC monitor voltage or
incorrect level, shape and frequency of the sawtooth reference will generate erroneous PWM timing pulses.
Since the reference signal is generated from the H pulse, frequency deviations are unlikely in this circuit.
The PWM will be output to Q8016 and provide control of the conduction of Q8008 at the emitter of HV output
Q8007 (Figure 11-2). Since Q8008 is a FET, its gate has high impedance at its input. Q8015 will assist in pulling
the signal down to maintain a uniform pulse shape. The signal will also be sent to HV drive stage to control the
voltage at Q8003 (Figure 9-1).
IC8007 1/4
6
7
5
C8068
Q8024
N
P
D8039
R8166
RD10ES
R8173
D8042
C8094
HZT33
Q8026
P
R8172
FIGURE 11-3 - HV REGULATION CONTROL
11. RA-4/4A High Voltage Circuit Troubleshooting
R8128 R8139
R8136 C8082
C8070
C8076
9
R8126
10
6.9V
IC8007 1/4
R8127
C8052
14.2V
R8195
R8196
R8168
50
8
R8138
2
1.5V
3
C8074
TO IC8002/5
HV REGULATION
PWM
R8093
1
IC8007 1/4
R8119
2
R8201
1
11.3TVP14 7/11/02