Read Sharp 66DW-18H (serv.man3) Service Manual online
Sharp Electronics (UK) Ltd
Technical Support
February 99
Page 27
VERTICAL OUTPUT STAGE
The Vertical output stage in the CW100 chassis are operated in a ‘Class D’ configuration; similar to
the CS chassis. The output transistors are acting as switches, and not amplifiers. The advantages of
this type of biasing system are minimal power consumption with minimal distortion, however a
disadvantage is the noise that is left on the signal.
the CS chassis. The output transistors are acting as switches, and not amplifiers. The advantages of
this type of biasing system are minimal power consumption with minimal distortion, however a
disadvantage is the noise that is left on the signal.
Figure 1 Block Diagram of a 50Hz Class D vertical output Stage
It can be seen in Figure 1 that our vertical drive is fed into a switching amplifier along with line pulses.
The resultant of mixing these two signals is that at the switching amplifier output we have a PWM
signal with a fix frequency of 15.625kHz but the width of the pulses will vary depending on the
amplitude of the vertical drive signal.
The resultant of mixing these two signals is that at the switching amplifier output we have a PWM
signal with a fix frequency of 15.625kHz but the width of the pulses will vary depending on the
amplitude of the vertical drive signal.
From the output of the switching amplifier the PWM signal passes through a class 'D' amplifier circuit
and it is converted back to an analogue signal by means of a low pass filter before being applied to
the vertical scanning coils
and it is converted back to an analogue signal by means of a low pass filter before being applied to
the vertical scanning coils
Line frequency is chosen for the PWM frequency because each field is made up of 312.5 lines and
therefore, our PWM signal will have a different size pulse per line and the scan will move down the
screen one line by line. The CW100 chassis has 100Hz field scan therefore, the line frequency will
31.25kHz, but the principle is exactly the same.
therefore, our PWM signal will have a different size pulse per line and the scan will move down the
screen one line by line. The CW100 chassis has 100Hz field scan therefore, the line frequency will
31.25kHz, but the principle is exactly the same.
C
IRCUIT
D
ESCRIPTION
From the output of the switching amplifier shown in Figure 2 (IC501) the PWM signal is fed to the
base of Q504 & Q505.Starting with a positive going PWM signal; Q504 will turn on & Q505 turns off.
Allowing current to flow from the +13V rail, through Q504 collector/emitter junction, D502 & D522
turning on Q508, allowing current to flow from ground to the –13V rail via R523, vertical scanning
coils, L501 & C514 (LPF), & Q508 collector/emitter junction.
base of Q504 & Q505.Starting with a positive going PWM signal; Q504 will turn on & Q505 turns off.
Allowing current to flow from the +13V rail, through Q504 collector/emitter junction, D502 & D522
turning on Q508, allowing current to flow from ground to the –13V rail via R523, vertical scanning
coils, L501 & C514 (LPF), & Q508 collector/emitter junction.
Next when the PWM signal goes negative Q505 will turn on & Q504 turns off. Allowing current to flow
from the -13V rail, through Q505 collector/emitter junction, D502 & D522 turning on Q508, allowing
current to flow from +13V to ground via Q507 collector/emitter junction t vertical scanning coils, L501
& C514 (LPF), &. R523
from the -13V rail, through Q505 collector/emitter junction, D502 & D522 turning on Q508, allowing
current to flow from +13V to ground via Q507 collector/emitter junction t vertical scanning coils, L501
& C514 (LPF), &. R523
To prevent Q507 & Q508 being switched on simultaneously damaging Q507 & Q508 due to
excessive current flow. Crossover distortion is introduced into the drive signal to Q507 & Q508 by the
action of C507, D502, D522, C508, D501 & D521.
excessive current flow. Crossover distortion is introduced into the drive signal to Q507 & Q508 by the
action of C507, D502, D522, C508, D501 & D521.
L.P.F
C l a s s D
O u t p u t
+
-
-
Switching
Amplifier
Amplifier
Line Pulses
Frame Drive
To Frame
Scan Coils
Scan Coils
PWM Signal
15.625kHz
15.625kHz
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