ViewSonic E95 - Page 50

4.5.3 Horizontal Scanning and High Voltage Power Supply Circuit
The horizontal scanning is accomplished by Q415, D410, C428, and DY (P401). Except the basic
horizontal scan output, Linearity coil T403 corrects the asymmetrical distortion.
(C427/C432/C434/C435/C436/C437/C438) are S-Correction capacitors, Q420/Q421/Q422/Q423/Q424
switched on/off according to different H-frequency.
The conducting period of damper diode D410 completes the first half part of horizontal scan. The
conducting period of horizontal output transistor Q415 completes the second half of scan.
The retrace capacitor is C428, the charge and discharge actions of retrace capacitor via yoke after
Q415 turned off, which is designed for the same resonant frequency with the main scan circuit.
In the high voltage drive circuit, the output of Q401 is applied to the primary winding of flyback
transformer (FBT) T401 and drives the flyback transformer to supply CRT anode voltage of about 25.8KV
during retrace period. T402 pin6 voltage (B+) steps down by IC402, Q402. Feedback components are
C416, R425 and VR402.
Several driven power sources are developed in the FBT secondary, including
40Vp-p: XPR pulse (pin10), -130Vp-p: G1 negative pulse (pin4), 400Vp-p: V Focus VCC (pin3).
4.5.4 Buck Converter and width control
In order to maintain same scan width (raster) within 30KHz-95KHz or 30KHz-86KHz, the scan
supply B+ tracked with continuos H-frequency is necessary, and the design is implemented by a
convention method-of buck converter. The buck converter mainly composes of a P-channel MOSFET
(Q408), choke T402 and rectifier diode D409.
The PWM control circuit is integrated in IC401 pin5 is inverting, pin3 is output of error amplifier,
both for frequency compensation and gain setting. PWM pulse width output at pin6 is modulated
according to internal error amplifier output. Pin4 current sense is controlled by Q403, R431, ZD401 and
R432 for horizontal size width limitation.
4.5.5 The Pincushion/Trapezoid/Top Hook/Bottom Hook Correction
The voltage across R418 can be varied in accordance with a vertical parabolic or sawtooth waveform,
which is generated by IC401 pin11. The peak yoke current is decreased in proportion to the feedback of
R418, as the voltage across R418 is changed for compensating the pincushion, trapezoid and Hook effect.
The waveform is adjusted by IC301 SCL SDA control IC401 pin18 pin 19.
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