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6-17. HV. B+ Control Circuit
The B+ control circuit generates the supply voltage for the horizontal scanning circuit. This supply voltage
is approximately proportion to the H frequency in order to keep the scanning amplitude constant when the
horizontal frequency changes. The circuit is a step-up converter and it uses the current mode regulation
principle.
The feedback signal is derived from pin 6 of T440 via R453, C453 and R452 to the error amplifier on pin
15 of I401. The value of R325 is chosen to get a suitable gain for the error amplifier. A saw-tooth
waveform related to the output current of Q440 as a I-sense is fed to pin16 of I401 via R442, R440 and
C326.
Both of the feedback and saw-tooth I-sense waveform are fed to a comparator in I401 to generate a PWM
(Pulse Width Modulation) control signal output at pin28 of I401.
A square waveform control output on pin28 of I401 is then fed to Q440 via buffer Q438, Q439. Q440 is
designed as a step-up voltage converter and the DC voltage (56V) is fed to the Drain of Q440 via L440.
The energy is stored in L440 when Q440 is on; the energy is released when Q440 is off. D442, C444
Rectifier and filter gives a DC voltage from 69…165V depend on Horiz. Freq. 30…70KHz. The step-up
converter supplies DC voltage (69V … 165V) to pin9 of T440. The snub circuit C441, R441, R444 and
C442 is for the noise transaction.
Fail-Safe Protection
The fail-safe shut down protection is activated when anode voltage exceeds certain radiation safety limit.
The fail-safe control voltage is derived from pin9 of T440 via R449/ R450 and D449 and pin 6 of T440 via
D453, R447, D446, then applied to the pin 25 of I401, the input at pin25 of I401 is a DC voltage with
typical shut down voltage of about 8V. I401 will block the H-drive output and H&V blanking output when
pin25 is over 8V. The protection can be reset and return to normal operation either by power off and on or
by I
2
C.
Rotation
The control signal on pin 3 of I501 is to adjust the display tilt. When you press " + "key, the voltage on
base of Q203 rises and causes current Ic of Q202 increased. By virtue of current flow, rotation coil makes
the display a bit tilt. The display will tilt anti-clockwise when " - " press.
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