Mitsubishi V19-V21, Tehnical Manual

Page: 57 / 71

8-5
HV Regulation
The HV Regulation circuitry is shown in Figure 8-4 .
Controlling the duty cycle of the drive applied to the
HV Output transistor, Q5A51, regulates the HV. The
amount of energy stored in the Flyback transformer
is determined by how long Q5A51 conducts. In-
creasing conduction time increases HV, and decreas-
ing conduction time reduces HV.
A sample of the HV is derived from the HV Block
ground return circuitry. DC feedback (HV FB DC)
is directed through an Op-amp in IC5A01 to the HV-
IN input at pin 6 of IC5A00. AC feedback (HV FB
AC) is applied directly to pin 6 to speed up the op-
eration.
in excessive HV. HV FB DC can safely be measured
after buffering by the Op-amp in IC5A01.
The sample of the HV is compared to the HV-ADJ
voltage from the Convergence circuitry, input at pin
11 of IC5A00. If the HV feedback voltage is higher
than the HV-ADJ voltage, conduction time is de-
creased. Conversely, if the feedback voltage is lower,
conduction time is increased.
X-Ray Protect
Figure 8-5 illustrates the X-Ray Protect circuitry. It
is the same as in the V17 and V18 chassis. The X-
Ray Protect circuitry shuts the TV down if High
Voltage, Beam Current or arcing is excessive.
D5A57 provides a sample of the HV by rectifying
the signal from pin 5 of the Flyback. Beam current is
detected by monitoring the voltage at the ground
return for the HV winding
in the Flyback, pin 8 of
T5A51.
The voltage from D5A57 is
applied to the (-) input of
an Op-amp in IC5A02.
Reference IC5A03 hold the
(+) input at 10 volts. If the
voltage for D5A57 exceeds
the 10 volt reference, the
output of the Op-amp goes
Low, pulling the X-Ray line
Low. The TV µPC re-
sponds by turning the TV
Off.
A similar Op-amp configu-
ration monitors beam cur-
rent. Pin 8 of the Flyback
is connected through
D5A59 to the (+) input (pin
5) of another OP-amp in
IC5A02. The (-) input (pin
6) is a reference voltage
derived from R5A66,
R5A67 and the 12V supply.
Caution: Do not measure HV FB DC at the DK
connector. This will load down the circuit resulting