NEC FP1355 Service Manual

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Contents

NEC FP1355 Service Manual Manual pdf 31 page image

1 - 251.4 High voltage blockThe high voltage circuit is composed of the high-voltage regulator IC701, MOS-FET Q701flyback transformer (FBT) T701, operation amplifier IC702 and their peripheral circuits.1.4.1 High voltage control circuitThe IC701 is an IC for high voltage control, with the block diagram given in Fig.13. The OFF trigger PWMcontrol system is adopted to carry out high voltage control. The built-in VCO circuit gets synchronizedwhen the horizontal synchronous signal (hereafter SYNC signal) is input from Pin 3 of IC701 (horizontalsynchronous input terminal). Thus, the MOS-FET Q701 is controlled to turn OFF with the timing of theSYNC signal changing over to HI, and is called OFF trigger system.Further, the DC voltage, output after the feedback signal (IC701 Pin 11) from FBT T701 is compared with thehigh-voltage set voltage (IC701 Pin 12) from IC101 in the internal error amplifier, is compared with the saw-tooth-waveform of VCO to control the pulse output DUTY so as to keep the high voltage constant (PWM control).The timing chart for OFF trigger PWM control system is given in Fig.14. Set the high voltage by selecting[HVADJ] in OSD and manipulating +/- button (Standard value: 27.0kV).1.4.2 Protective function circuit(1) Start and stop of high-voltage regulator IC701The IC701 starts operation when Vcc voltage (power voltage applied to Pin 2 and Pin 10) attains thelevel of 8.4 Vtyp, and the operation stops when the Vcc voltage is less than 7.4 Vtyp.(2) IC701 overcurrent protection (OCP) functionIt detects the peak value of the drain current in MOS-FET Q701 per pulse and stops DRIVE when thevoltage in Pin 6 of IC701 detecting the end-to-end voltage of the source resistors (R706 and R707),exceeds 1.0 Vtyp until the next SYNC signal is input.(3) IC701 overload protection (OLP) functionThis function brings the system to the latch stop when OCP gets continuously activated due tocontinuous overload.It forms time constant using C716. With OCP activated and C716 charged and the voltage in Pin 8 ofIC701 exceeding 2.5 Vtyp, IC701 gets set to Latch mode, bringing the control operation to stop. Thisstatus does not get released (reset) until the Vcc voltage (power voltage applied to Pin 2 and Pin 10)in IC701 is less than 7.4 Vtyp.(4) Over-voltage protection function for anode voltage (X-ray protector)A voltage proportional to the high voltage is generated in Pin 6 of T701 due to the winding ratiobetween secondary and tertiary winding inside FBT T701. This voltage is then rectified by D707 andC708 and is further divided by R708 and R709 before being input in Pin 17 of microcomputer IC101for comparison with the X-Pro set value. In case the voltage exceeds the set value, the output in Pin33 of microcomputer IC101 gets fixed to LOW (P-OFF mode). With the mode set to P-OFF, theapplication of voltage Vcc to IC701 stops, causing the IC701 operation to stop. The status continuesuntil the power SW is turned OFF. The overvoltage protection function is set to operate when thehigh voltage level reaches 30kV (with the beam current is approx. 1mA).(5) Overcurrent protection function for beam current (beam protector)The beam current is supplied from +12V power source through R722. Since the end-to-end voltage ofR722 varies according to the beam current, the voltage drop due to R722 becomes large if the beamcurrent increases. The voltage in Pin 9 of FBT T701 (the voltage obtained by subtracting the voltagedrop due to beam current from the +12V power voltage) undergoes resistance division by R723 andR724, and is then input into Pin 6 of operation amplifier IC702 for comparison with Pin 5 of IC702(reference voltage). The voltage is then output (Pin 7 of IC702) and is input into Pin 16 of microcom-puter IC101. The voltage in Pin 7 of IC702 (output terminal) is output linearly due to the fluctuation inbeam current. However, if the terminal voltage in Pin 16 of IC101 exceeds Beam-Pro setting value (ABLdata +70 : Max. 254), the output of Pin 33 of IC101 gets fixed to LOW (P-OFF mode). With the modeset to P-OFF, the application of Vcc voltage to IC701 stops, causing the IC701 operation to stop. Thisstatus continues until the power switch is turned OFF. The overcurrent protection function is set tooperate when the beam current reaches the level of approximately 1300μA.