A circuit for detecting a knee of a voltage oscillating waveform, applicable to an isolated load circuit, which includes an isolated transformer with a primary side winding, a second side winding, and an auxiliary winding, a primary side switch, and at least one operating element, wherein the primary side switch is electrically connected to the primary side winding of the isolated transformer, the operating element is electrically connected to the secondary side winding of the isolated transformer, the auxiliary winding is disposed at the same side of the primary winding, utilized for detecting the knee of the voltage oscillating waveform of the auxiliary winding and a total oscillating time so as to control an average output current flowing through the operating element by using the knee and the total oscillating time as the primary side switch is turned off, and the circuit for detecting a knee of a voltage oscillating waveform comprising:
- a filter, electrically connected to the auxiliary winding for receiving and filtering the voltage oscillating waveform to generate a filtered voltage;
- a first comparator, having a first comparing input, a second comparing input, and a first comparing output, the first comparing input being electrically connected to the filter through a first switch for receiving at least a first reference voltage which is a difference between the filtered voltage and a first preset voltage and is positive, the second comparing input being electrically connected to the auxiliary winding for receiving the voltage oscillating waveform, and the first comparing output being utilized for transmitting a first signal as the voltage oscillating waveform reaches the first reference voltage;
- a second comparator, having a third comparing input, a fourth comparing input, and a second comparing output, the third comparing input is electrically connected to the filter through a second switch for receiving at least one second reference voltage which is a difference between a reference stabilized voltage and a second preset voltage and is greater than the first reference voltage, the second preset voltage is positive, the fourth comparing input being electrically connected to the auxiliary winding for receiving the voltage oscillating waveform, and the second comparing output being utilized for transmitting a second signal as the voltage oscillating waveform reaches the second reference voltage;
- a time recorder, electrically connected to the first comparing output of the first comparator, the second comparing output of the second comparator, the first switch, and the second switch for recording the total oscillating time;
- and
- a knee detecting module, electrically connected to the first comparator, the second comparator, and the time recorder, for receiving at least one of the first signal and the second signal to control the time recorder accordingly;
- wherein, as the first switch is conducted, the time recorder counts a preset first oscillating time, the knee detecting module triggers the time recorder to recount the first oscillating time when the knee detecting module receives the first signal within the first oscillating time, and the knee detecting module triggers the time recorder to record the first oscillating time when the knee detecting module does not receive the first signal within the first oscillating time and then the knee detecting module further conducts the second switch to count a preset second oscillating time, the knee detecting module triggers the time recorder to recount the second oscillating time when the knee detecting module receives the second signal within the second oscillating time, and the knee detecting module triggers the time recorder to record the second oscillating time when the knee detecting module does not receive the second signal within the second oscillating time, and the knee detecting module detects the knee and triggers to the time recorder to sum up the first oscillating time and the second oscillating time to record the total oscillating time when the knee detecting module does not receive the second signal within the second oscillating time but receives the first signal;
- wherein the knee detecting module comprises:
- a first detecting switch, electrically connected to the first comparator and the time recorder;
- a second detecting switch, electrically connected to the first comparator and the time recorder;
- a third detecting switch, electrically connected to the second comparator and the time recorder;
- a fourth detecting switch, electrically connected to the second comparator and the time recorder;
- and
- a processing unit, electrically connected to the first detecting switch and the third detecting switch;
- wherein, within the first oscillating time, the first detecting switch and the fourth detecting switch are conducted by the time recorder, and the second detecting switch and the third detecting switch are turned off by the time recorder, and within the second oscillating time, the second detecting switch and the third detecting switch are conducted by the time recorder, and the first detecting switch and the fourth detecting switch are turned off by the time recorder.