Patent ID: 7764521

Claim:
An apparatus for providing multiple functions or protections for a power converter including a transformer having a primary winding connected between a power input and a power switch, and a secondary winding connected to a power output, and operative to deliver power from the power input to the power output by switching the power switch by a control signal, the apparatus comprising: an auxiliary winding for generating an induced voltage by inductively coupling the primary or secondary winding in such a manner that when the power switch is at a first switch state, the induced voltage reflects an input information of the power converter, and when the power switch is at a second switch state, the induced voltage reflects an output information of the power converter; a detection circuit for obtaining the input and output information from the induced voltage so as to implement the multiple functions or protections, the detection circuit including a first resistor connected between the auxiliary winding and a detection terminal for generating a first detection signal including the input information from the induced voltage when the power switch is at the first switch state, and a second resistor serially connected to the first resistor via the detection terminal for generating a second detection signal including the output information by dividing the induced voltage when the power switch is at the second switch state, whereby the first and second detection signals are used to implement the multiple functions or protections; and a constant voltage-second control circuit for monitoring the first detection signal to prevent the power converter from having a duty cycle too short and keep a constant voltage even though the input voltage at the power input is varying when the power converter is at a light loading.