A causal sampling circuit is developed to measure a reflected voltage and demagnetizing time of the transformer. It includes a signal-generation circuit to generate a sample signal for sampling the reflected voltage of the transformer. A ramp signal of the sampling circuit is generated in response to the demagnetizing of the transformer. A first reference signal is generated in accordance with the magnitude of the ramp signal after the transformer is fully demagnetized. A second reference signal is generated in response to the ramp signal and a bias signal. The sample signal is enabled in response to the demagnetizing of the transformer. The sample signal is disabled once the second reference signal is higher than the first reference signal. A sample-and-hold circuit is coupled to the transformer to sample the reflected voltage of the transformer in response to the sample signal.

A system and method for delivering regulated power and current to an output load has a flyback transformer having a primary winding and a secondary winding. The secondary winding delivers stored energy to the output load. An oscillator circuit is provided for generating a periodical signal. A switching circuit is coupled to the flyback transformer and the oscillator circuit for energizing the primary winding to a reference current level each cycle of the oscillator circuit. The oscillator circuit has an integrator for deriving a time integral of a voltage at the primary winding. The oscillator circuit has a peak detector coupled to the integrator for holding a peak value of the time integral. The oscillator circuit further has a ramp generator for producing a ramp signal. A comparator is provided for comparing the peak value with the ramp signal and generating the periodical signal whenever the ramp signal exceeds the peak value.

A detection circuit for detecting the demagnetizing time of a magnetic device is provided. An input circuit is coupled to the magnetic device for detecting a magnetizing voltage and a demagnetizing voltage of the magnetic device. A control circuit is coupled to the input circuit for generating a demagnetizing-time signal in response to the magnetizing voltage, the demagnetizing voltage, and a magnetizing time. The magnetizing time is correlated to the enable period of the magnetizing voltage. The demagnetizing time of the magnetic device is represented by the demagnetizing-time signal.

A novel switching power supply device no more in need of a stand-by signal for turning on/off a switching element is provided. The switching power supply device is configured as having a main switching element Q1, wherein the switching power supply circuit is connected with a pulse oscillation circuit IC1 for outputting pulse signal; the pulse oscillation circuit and an output terminal of the main switching element are connected with an input section of an internal detection circuit 12; the internal detection circuit 12 is configured so as to control timing of switching of the main switching element Q1 upon detection of the pulse signal supplied from the pulse oscillation circuit IC1.

A linear-predict sampling circuit is developed to generate a feedback signal by detecting a demagnetized voltage of the transformer. A switching signal is generated in response to the feedback signal for regulating the output of the power converter. A signal-generation circuit is used to generate a sample signal in response to a first signal, a second signal, and the switching signal. The first signal is correlated to a magnetized voltage of the transformer. The second signal is correlated to the demagnetized voltage of the transformer. A sample-and-hold circuit is coupled to the transformer to generate the feedback signal by sampling the demagnetized voltage of the transformer in response to the sample signal. The feedback signal is correlated to the output voltage of the power converter.