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Switching power supply VCO

  • US 4,797,803 A
  • Filed: 12/23/1987
  • Issued: 01/10/1989
  • Est. Priority Date: 12/23/1987
  • Status: Expired due to Term
First Claim
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1. A voltage controlled oscillator (VCO) of the type used in a symmetrical switching power supply wherein the frequency of a squarewave control signal produced at an output terminal of said VCO, said control signal controlling a switching means to drive current symmetrically positive and negative through a magnetic device, controls a DC output voltage of said power supply, and wherein said frequency of said control signal is controlled by a VCO input signal, said VCO comprising:

  • a voltage comparator means having a plus(+) terminal and a minus(-) terminal, wherein if a voltage at said plus(+) terminal exceeds a voltage at said minus(-) terminal, said output terminal of said voltage comparator means will be at a high voltage, and if said voltage at said minus(-) terminal exceeds that voltage at said plus(+) terminal, said output terminal of said voltage comparator means will be at a low voltage;

    a first resistor with one end coupled to said output terminal of said voltage comparator means;

    a capacitor coupled between said resistor and a ground wherein said capacitor is coupled to said minus(-) terminal of said voltage comparator means, wherein said capacitor charges up when said output terminal of said voltage comparator means is at a high voltage and discharges when said output terminal of said voltage comparator means is at a low voltage;

    a voltage divider formed by a plurality of resistors for providing a high bias voltage and a low bias voltage corresponding to voltages at each end of one of said plurality of resistors, wherein each end of said one of said plurality of resistors is coupled to sai plus(+) terminal of said voltage comparator means via a diode, and wherein said plus(+) terminal of said voltage comparator means is also coupled to said output terminal of said voltage comparator means via a second resistor so that when said output terminal of said voltage comparator means is at a high voltage, said high bias voltage will be coupled to said plus(+) terminal, and when said output terminal of said voltage comparator means is at a low voltage, said low bias voltage will be coupled to said plus(+) terminal, resulting in said capacitor charging to approximately said high bias voltage when said output terminal of said voltage comparator means is at a high voltage and discharging to said low bias voltage when said output terminal of said voltage comparator means is at a low voltage; and

    ,a transistor means coupled to said voltage divider for shunting said one of said plurality of resistors forming said voltage divider and controlling the voltage difference between said high bias voltage and said low bias voltage, in response to said VCO input signal applied to a control terminal of said transistor, in order to control the frequency of said voltage at said output terminal of said voltage comparator means, said voltage at said output terminal being a squarewave.

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