SAMSUNG HCP4752XAA
Service Manual

Alignment and Adjustment

2-6 High Voltage Part
2-6-1 PWM REG Circuit
For the existing high voltage REG circuit (input voltage variation type), a dynamic REG response is not provided. So it is difficult for both beam linearity and uniformity in screen size to be maintained on the screen with rapidly changing beams. A PWM (Pulse Width Modulation) type of high voltage, however, provides the maintenance of beam linearity and uniformity in screen size via a quick response to beam change by performing sync lock every 1H line, and detecting beam fluctuation at 1H line, and then controlling the IC current of high voltage output circuit. 1. High Voltage Fluctuation Detect (DC Detect) FBT pin 11 detects DC high voltage fluctuation. The detected DC high voltage value is input to PWM IC471 pin1 through R473, VR471, R471, and then it is input to a differential AMP circuit that differentiates the gap after comparing with the reference voltage input to pin2. 2. High Voltage Fluctuation Detect (AC Detect) To check AC high voltage fluctuation, the output from FBT is detected by using a capacitor inside the high voltage distributor. The detection of AC high voltage fluctuation, a detection of dynamic beam current change is required in order to keep beam linearity and uniformity in size. Regarding the capacitor, a capacity of less than 3000P should be applied to a PWM type. (The existing type needs a capacity of about 6000P.) AC detect circuit eliminates unnecessary high frequency by using C476, D472. Also, AC gain is limited to + / - 0.7V (D472). This AC gain is combined with the detection value of DC high voltage fluctuation by using C478. 3. PWM IC OSC Sync Lock A PWM type IC needs sync lock for PWM pulse and horizontal scan line. The standard time constant of OSC circuit is determined by C487, R475 (PWM IC pins 5 and 6). And the standard OSC frequency is about 27 kHz . The horizontal frequency of scan line is 31.5kHz(NT), 31.25kHz(PAL), so sync lock for this horizontal frequency should be performed using 4. Dead Time (HV Protect) Dead Time (PWM IN pin4) consists of C481, delays high voltage for a certain time to soft start in power on, a x-ray protection circuit. The voltage of Dead Time is detected by FBT pin7 and through DC Feedback. The normal voltage of Dead Time is +27V. When high voltage increases, however, detected voltage is in proportion to high voltage. Then, the detected voltage is applied to ICR01S(TL431). If the voltage is over 2.5V (normal:about 2.25V), TL431 turns ON, the base port of QR401S becomes low, and then an emitter current flows. At this time, a high voltage protection point is set. When QR401S turns ON, high voltage is applied to PWM IC pin4 and then muted. sync lock circuit. The sync lock circuit consists of Q481(Tr KSC815-Y), D479, D478, and C492. The input AFC signal is connected to PWM IC pin 5 through D479 so that it can be negative Trig.

5. Output Circuit The voltages, which are detected form an error detection circuit of PWM IC (Differential AMP) and Dead Time, each is applied to PWM conparator . Due to these detection coltages, Q1, Q2 (Output TR) parallel operate. Q482 (External TR), however, functions as a buffer; natches inpedance between the output port of PWM IC and the final output TR(IRFS640). The PWM pulse (applied to the final output FET (IRFS640 GATE) varies the IC current of high voltage TR(Q473) by adjusting the load impedance of starage Trans (T431). Due to this variation of current, the gain for Q473 emitter pulse changes T444(FBT)makes this emitter pulse became high voltage. Such

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