Patent ID: 7652898

Claim:
A soft start circuit, comprising: said soft start circuit connected to a pulse width modulation (PWM) controller for generating a carrier signal, having a function for modulating amplitude to a pulse width, and including a first terminal for providing a reference power source, a second terminal receiving a voltage for determining an amplitude of the frequency signal, a third terminal receiving a first value for determining a frequency of the frequency signal, and a fourth terminal receiving a second value for determining the frequency of the frequency signal; a duty ratio establishing unit for using the reference power source to generate a first voltage, and outputting the first voltage to the second terminal; a variable switching unit including a first capacitor charged with a current provided from the first terminal and a first switch turned off by using a charged voltage of the first capacitor; and a frequency controlling unit including first and second resistors coupled in parallel to the third terminal and a second capacitor for providing a fixed voltage to the fourth terminal, allowing the current to flow to the second resistor when the first switch is turned on, and allowing the current to flow to the second resistors when the first switch is turned off.