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File name ctc185powersupply.pdf

Power Supply 5 Introduction The power supply in the CTC185 is a non-isolated switching power supply that uses a CTC185 Main MOSFET (Metal Oxide Semiconductor Field Effect Transistor) as the switching Regulator device. The supply uses a winding on the IHVT to provide a voltage boost and a series PWM controlled MOSFET to regulate the output voltage to 130 volt DC. The supply has no alignments because it uses a precision 1% voltage reference IC (U4103). The control circuit is synchronized to the horizontal oscillator by using a flyback pulse as a timing reference for a sawtooth ramp generator. When the chassis is in standby mode, Q4114 is kept on by a drain to gate resistor which forces the output voltage to be equal to the RAW B+ voltage. T4401 IHVT RAW B+ 3 C4007 1 CR4103 CR4105 V2 L4102 Q4114 I1 REG B+ 130VDC C4153 V1 GATE DRIVE V3 FLYBACK PULSE PIN 8 OF IHVT HORIZ. SYNC CIRCUIT AND SAWTOOTH GEN. V4 COMPARATOR V5 ERROR AMP AND REFERENCE Figure 1, Main Regulator Block Diagram The winding on the IHVT at pins 1 and 3 sums an inverted retrace pulse on the RAW B+ (V1 in figure 1). This provides the pre-boost so the supply will regulate even when RAW B+ falls below 130 volts (figure 2). The number of turns on the winding determines the lowest voltage at which the supply will regulate. The 25 and 27 inch chassis will regulate the +130V with as little as 95 volts AC RMS and the 19 and 20 inch chassis with as little as 90 volts AC RMS. After the pre-boost, the supply acts as a buck converter in that the output voltage is equal to the input voltage times the duty cycle. When Q4114 is turned on, current flows through CR4105, L4102 and Q4114 charge up C4153 . When retrace begins, the voltage on the anode of CR4105 begins to drop but the current I 1 (figure 3) continues to flow through it because of inductor L4102 . When the voltage V2 (figure 3) decreases to -0.7 volts, CR4105 turns off and CR4103 turns on and conducts I 1 until it drops to zero. During this time Q4114 remains on to conduct the current I1 until it drops to zero. The timing of the turn off of Q4114 can be seen in figure 3. The falling edge of voltage V3 is the signal that turns off Q4114. The turn off time of Q4114 is fixed and the turn on time varies in response to the PWM (Pulse Width Modulator) control circuit. 6 Power Supply Figure 2 Figure 3 Power Supply 7 T4401 IHVT RAW B+ 3 C4007 1 CR4105 V2 L4102 CR4103 Q4114 I1 REG B+ 130VDC C4153 V1 GATE DRIVE V3 FLYBACK PULSE PIN 8 OF IHVT HORIZ. SYNC CIRCUIT AND SAWTOOTH GEN. V4 COMPARATOR V5 ERROR AMP AND REFERENCE Figure 4, Main Regulator Block Diagram(repeated) The PWM is made up of the horizontal sync circuit and sawtooth generator block, the comparator block, and the error amp and reference block (figure 4). The horizontal sync and sawtooth block generates a sawtooth voltage V4 (figure 5). This signal and the error voltage V5 from the error amp block are fed into the comparator block which generates the gate drive vo