Start circuit for generation of pulse width modulated switching pulses for switch mode power supplies

In a DC/DC converter power supply including a DC voltage source, a power transformer including a primary winding and a secondary winding, rectifier and filter means connected to said secondary winding for generating an output DC voltage from the current output by secondary winding, pulse generator means for generating a train of pulses of varying pulse width as a function of the present value of said output voltage, power switch means coupled to said primary winding such that when said power switch means closes, said DC voltage source is coupled across said primary winding, means for coupling said train of pulses to said power switch means, said pulses causing said power switch means to close for the duration of each said pulse, and means for coupling ongoing power to said pulse generator means once said pulse generator means begins operation, the improvement comprising start means responsive to the initiation of said DC voltage source for starting the operation of said pulse generator means, said start means comprising:

• relaxation oscillator means connected to said DC voltage source for periodically generating a start pulse at a predetermined frequency, said relaxation oscillator means including a capacitor and means for discharging said capacitor when the voltage across said capacitor reaches a predetermined breakdown voltage level;
• means for coupling said start pulse to said power switch means, said start pulse causing said power switch means to close for a predetermined duration;
• means connected to said secondary winding for powering said pulse generator means from the energy coupled to said secondary winding by said switch means closures;
• and disabling means for selectively disabling said relaxation oscillator means when said pulse generator means begins to be powered by said ongoing power, said disabling means including a diode having its anode connected to the electrical junction of said relaxation oscillator means and said power switch means, a disabling switch means for disabling said start means coupled across said capacitor and a resistor-capacitor circuit means connected between the cathode of said diode and said disabling switch means such that the time constant of said resistor-capacitor circuit means couples a voltage to said disabling switch means sufficient to turn on said switch and thereby disable said relaxation oscillator means during the time said power switch means is switched on by said train of pulses.

In a DC/DC converter power supply including a DC voltage source generated from a rectified AC voltage source, a power transformer including a primary winding and a secondary winding, rectifier and filter means connected to said secondary winding for generating an output DC voltage from the current output by said secondary winding, pulse generator means for generating a train of pulses of varying pulse width as a function of the present value of said output voltage, power switch means coupled to said primary winding such that when said power switch means closes, said DC voltage source is coupled across said primary winding, means for coupling said train of pulses to said power switch means, said pulses causing said power switch means to close for the duration of each said pulse, and means for coupling ongoing power to said pulse generator means once said pulse generator means begins operation, the improvement comprising start means responsive to the initiation of said DC voltage source for starting the operation of said pulse generator means, said start means comprising:

• relaxation oscillator means connected to said DC voltage source for periodically generating a start pulse at a predetermined frequency, means for coupling said start pulse to said power switch means, said start pulse causing said power switch means to close for a predetermined duration, means connected to said secondary winding for powering said pulse generator means from the energy coupled to said secondary winding by said switch means closures, disabling means for selectively disabling said relaxation oscillator means when said pulse generator means begins to be powered by said ongoing power;
• and delay means coupled to said disabling means for delaying electrical removal of said disabling means from said start means for a preselected period after shutdown of said power supply, said delay means includes charge pump means connected to said AC voltage source and to said disabling means, said charge pump including a charge pump resistor connected to said AC voltage source and to a first charge pump capacitor connected to the anode of a first charge pump diode and to the cathode of a second charge pump diode, the cathode of said first charge pump diode being connected to a first side of a second charge pump capacitor, the anode of said second charge pump diode being connected to a second side of said second charge pump capacitor, such that when the voltage on the end of said charge pump resistor connected to said AC voltage source is increasing, said second charge pump diode is back biased and said first charge pump diode is forward biased such that current flows through said first charge pump diode to charge said second charge pump capacitor and when the end of said charge pump resistor connected to said AC voltage source is going negative, said first charge pump diode is back biased and said second charge pump diode is forward biased such that current flows from said second charge pump capacitor through said second charge pump diode to charge said first charge pump capacitor.

In a DC/DC converter power supply including a DC voltage source, a power transformer including a primary winding and a secondary winding, rectifier and filter means connected to said secondary winding for generating an output DC voltage from the current output by said secondary winding, pulse generator means for generating a train of pulses of varying pulse width as a function of the present value of said output voltage, power switch means coupled to said primary winding such that when said power switch means closes, said DC voltage source is coupled across said primary winding, means for coupling said train of pulses to said power switch means, said pulses causing said power switch means to close for the duration of each said pulse, and means for coupling ongoing power to said pulse generator means once said pulse generator means begins operation, the improvement comprising start means responsive to the initiation of said DC voltage source for starting the operation of said pulse generator means, said start means comprising:

• relaxation oscillator means connected to said DC voltage source for periodically generating a start pulse at a predetermined frequency, means for coupling said start pulse to said power switch means, said start pulse causing said power switch means to close for a predetermined duration, means connected to said secondary winding for powering said pulse generator means from the energy coupled to said secondary winding by said switch means closures, disabling means for selectively disabling said relaxation oscillator means when said pulse generator means begins to be powered by said ongoing power;
• and- rapid start means for reducing the time period between the initiation of start attempts by said start means after shutdown of said power supply, said rapid start means comprising:
• a start diode connected to a secondary winding of said transformer;
• a start capacitor connected to the cathode of said start diode;
• a start resistor in series with said start capacitor;
• and
• a supply capacitor in series with said start resistor, such that said train of pulses from said pulse generating means causes high current pulses to flow from said secondary winding through said start diode, said start capacitor and said start resistor charging said supply capacitor rapidly during operation of said start means, and such that said rapid start capacitor limits current to said supply capacitor when said pulse generator means begins operation.