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Soft start scheme for resonant converters having variable frequency control

  • US 6,154,375 A
  • Filed: 10/08/1999
  • Issued: 11/28/2000
  • Est. Priority Date: 10/08/1999
  • Status: Expired due to Fees
First Claim
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1. A resonant mode power supply comprising:

  • means for generating a d.c. supply voltage having a first terminal and a second terminal;

    a first switching element having a first terminal coupled to said first terminal of said generating means, and a second terminal connected to a supply node;

    a second switching element having a first terminal coupled to said supply node and a second terminal coupled to said second terminal of said generating means;

    a transforming comprising a primary winding having a first end, and a second end connected to the second terminal of said second switching element, and a secondary winding having a first end, a second end and a central tap connected to ground;

    a capacitor for coupling the first end of the primary winding to the supply node;

    a sensing resistor for connecting the second end of the primary winding to said second terminal of said generating means;

    a first diode for connecting the first end of said secondary winding to an output node;

    a second diode for connecting the second end of said secondary winding to said output node; and

    a controller connected to control inputs of said first and second switching elements for alternatively turning on and off said first and second switching elements for inducing an oscillation in an oscillating circuit including at least said capacitor and said primary winding of said transformer, wherein a frequency of said oscillation is inversely proportional to an amount of power being delivered by said resonant mode power supply, characterized in that said resonant mode power supply further comprises a soft-start circuit having means for sweeping the frequency of oscillation from a maximum value to a minimum value, said sweeping means using the function
    
    
    space="preserve" listing-type="equation">f(x)=1-e.sup.-twhereby the rate of decrease of the frequency reduces as the amount of power delivered to the output increases and as the output voltage approaches a regulation value.

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