Figure 5.11, Early Switching Regulator Circuit. On the left is a power transformer. The secondary does not have a center tap. The secondary is connected to a bridge rectifier as in earlier circuits. The positive output from the bridge goes to the positive of a capacitor, C1. The negative of C1 goes to the negative output of the bridge and they both go to the negative output of the power supply. The positive of C1 goes to the drain of an f e t. The source goes through a resistor, R1, to the cathode of a zener diode. The cathode goes to the gate of the fet. The zener's anode goes to the negative output. The positive of C1 goes to the positive power pin of an op amp. The negative power pin goes to the negative output of the supply. The positive of C1 goes to the emitter of a P N P transistor, Q1. The collector of the transistor goes to the cathode of a diode, D6. The anode of D6 goes to negative. The collector also goes to the positive output through an inductor. There is a capacitor, C2 connected across the output terminals from positive to negative. The positive output goes to one end of R2. The other end of R2 goes to the top of a pot, R3. The bottom of R3 goes to negative through R4. The wiper of R3 goes to the inverting input of the op amp. The cathode of the zener diode goes to the noninverting input. The output of the op amp goes to the inverting input of a comparator. A comparator behaves much like an op amp that does not have any feedback applied. If the noninverting input is more positive than the inverting input the output is a logic high level. If the noninverting input is more negative than the inverting input the output is a logic low level. The noninverting input is fed by a triangular wave generator. The output goes through a resistor, R5, to the base of Q1. When the triangular wave is higher than the op amp's output the transistor is turned off. When the op amp's output is higher than the wave, the transistor is on. End verbal description.
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