Control method and circuit with indirect input voltage detection by switching current slope detection

1. A circuit for detecting an input voltage of a power converter, comprising:

a current sense circuit generating a current signal in response to a switching current of an inductance device;

a detection circuit coupled to sense a slope of the current signal; and

a signal generation circuit generating an input-voltage signal in accordance with the slope of the current signal;

wherein the level of the input-voltage signal is corrected to the input voltage of the power converter.

2. The circuit as claimed in claim 1 , wherein the slope of the current signal is sensed when a power transistor of the power converter is turned on.

3. The circuit as claimed in claim 1 , wherein a first signal is generated by the detection circuit sampling the current signal during a first period when a power transistor of the power converter is turned on, a second signal is generated by the detection circuit sampling the current signal during a second period when the power transistor is turned on, the slope of the current signal is determined in accordance with the first signal and the second signal.

4. The circuit as claimed in claim 1 , wherein the detection circuit comprises:

a first capacitor coupled to sample-and-hold the current signal through a first switch during a first period after a power transistor of the power converter being turned on;

a second capacitor coupled to sample-and-hold the current signal through a second switch during a second period after the power transistor being turned on; and

a third capacitor coupled to sample-and-hold the differential voltage of the first capacitor and the second capacitor for generating a slope signal;

wherein the slope signal is correlated to the slope of the current signal.

5. The circuit as claimed in claim 4 , wherein the first switch is controlled by a first sample signal, the second switch is controlled by a second sample signal, the first sample signal and the second sample signal are generated by an oscillation circuit of the power converter.

6. The circuit as claimed in claim 5 , wherein the oscillation circuit further generates an oscillation signal coupled to enable the power transistor, the first sample signal and the second sample signal are synchronized with the oscillation signal.

7. The circuit as claimed in claim 1 , wherein the signal generation circuit comprises:

an operational amplifier coupled to amplify the slope of the current signal for generating the input-voltage signal.

8. The circuit as claimed in claim 1 , wherein the inductive device is a transformer.

9. An input voltage detection method for detecting an input voltage of a power converter, comprising:

generating a current signal in response to a switching current of an inductance device;

detecting the waveform of the current signal to generate a slope signal; and

generating an input-voltage signal in accordance with the slope signal, the level of the input-voltage signal corrected to the input voltage of the power converter.

10. The input voltage detection method as claimed in claim 9 , wherein the slope signal is correlated to a slope of the switching current.

11. The input voltage detection method as claimed in claim 9 , further comprising:

sampling the current signal during a first period to generate a first signal when a power transistor of the power converter being turned on;

sampling the current signal during a second period to generate a second signal when the power transistor being turned on; and

generating the slope signal in accordance with the first signal and the second signal.

12. The input voltage detection method as claimed in claim 9 , wherein the inductive device is a transformer.

13. A circuit for detecting an input voltage of a power converter, comprising:

a detection circuit coupled to sense a switching current of an inductance device for generating a slope signal; and

a signal generation circuit generating an input-voltage signal in accordance with the slope signal;

wherein the level of the input-voltage signal is corrected to the input voltage of the power converter.

14. The circuit as claimed in claim 13 , wherein the slope signal is correlated to a slope of the switching current.

15. The circuit as claimed in claim 13 , wherein a first signal is generated by the detection circuit sampling the switching current during a first period when a power transistor of the power converter is turned on, a second signal is generated by the detection circuit sampling the switching current signal during a second period when the power transistor is turned on, the slope signal is determined in accordance with the first signal and the second signal.

16. The circuit as claimed in claim 13 , wherein the detection circuit, comprises:

a first capacitor coupled to sample-and-hold the switching current through a first switch during a first period after a power transistor of the power converter being turned on;

a second capacitor coupled to sample-and-hold the switching current through a second switch during a second period after the power transistor being turned on; and

a third capacitor coupled to sample-and-hold the differential voltage of the first capacitor and the second capacitor for generating the slope signal;

wherein the slope signal is correlated to a slope of the switching current.

17. The circuit as claimed in claim 16 , wherein the first switch is controlled by a first sample signal, the second switch is controlled by a second sample signal, the first sample signal and the second sample signal are generated by an oscillation circuit of the power converter.

18. The circuit as claimed in claim 17 , wherein the oscillation circuit further generates an oscillation signal coupled to enable the power transistor, the first sample signal and the second sample signal are synchronized with the oscillation signal.

19. The circuit as claimed in claim 13 , wherein the signal generation circuit comprises:

an operational amplifier coupled to amplify the slope signal for generating the input-voltage signal.