Inductor current zero-crossing detection method and circuit and switching power supply thereof

1. A method of detecting an inductor current zero-crossing in a switching power supply, the method comprising:

a) determining a present output voltage of said switching power supply;

b) generating a zero-crossing detection threshold voltage with a predetermined fixed value when said output voltage is less than or equal to a predetermined minimum output voltage; and

c) generating a zero-crossing detection signal according to said zero-crossing detection threshold voltage, wherein said zero-crossing detection signal is used to turn off a synchronous rectifier switch in said switching power supply, wherein said zero-crossing detection threshold voltage is generated to vary with said output voltage when said output voltage is greater than said predetermined minimum output voltage.

2. The method of claim 1 , wherein said generating said zero-crossing detection threshold voltage to vary with said output voltage comprises generating said zero-crossing detection threshold voltage in a linear relationship with said output voltage.

3. The method of claim 1 , wherein said generating said zero-crossing detection threshold voltage to vary with said output voltage comprises generating said zero-crossing detection threshold voltage to increase with said output voltage.

4. The method of claim 3 , wherein said generating said zero-crossing detection signal according to said zero-crossing detection threshold voltage comprises comparing said zero-crossing detection threshold voltage against a zero voltage.

5. The method of claim 1 , wherein said generating said zero-crossing detection threshold voltage to vary with said output voltage comprises generating said zero-crossing detection threshold voltage to decrease when said output voltage is increasing.

6. The method of claim 5 , wherein said generating said zero-crossing detection signal according to said zero-crossing detection threshold voltage comprises comparing said zero-crossing detection threshold voltage against said output voltage.

7. A method of detecting an inductor current zero-crossing in a switching power supply, the method comprising:

a) determining a present output voltage of said switching power supply according to a present input voltage of said switching power supply and a duty cycle of a pulse-width modulation (PWM) signal, wherein said output voltage is equal to a product of said input voltage and said duty cycle;

b) generating a zero-crossing detection threshold voltage according to said output voltage; and

c) generating a zero-crossing detection signal according to said zero-crossing detection threshold voltage, wherein said zero-crossing detection signal is used to turn off a synchronous rectifier switch in said switching power supply.

8. An inductor current zero-crossing detection circuit configured in a switching power supply, the circuit comprising:

a) an output voltage sampling circuit configured to determine a present output voltage of said switching power supply;

b) a zero-crossing detection threshold voltage generating circuit configured to generate a zero-crossing detection threshold voltage with a predetermined fixed value when said output voltage is less than or equal to a predetermined minimum output voltage; and

c) a zero-crossing detection circuit configured to generate a zero-crossing detection signal according to said zero-crossing detection threshold voltage, wherein said zero-crossing detection signal is used to turn off a synchronous rectifier switch in said switching power supply, and wherein said zero-crossing detection threshold voltage is generated to vary with said output voltage when said output voltage is greater than said predetermined minimum output voltage.

9. The circuit of claim 8 , wherein said zero-crossing detection threshold voltage varies with said output voltage by a linear relationship with said output voltage.

10. The circuit of claim 8 , wherein said zero-crossing detection threshold voltage varies with said output voltage by increasing with said output voltage.

11. The circuit of claim 10 , further comprising:

a) a zero-crossing comparator having a first input terminal coupled to an output terminal of said zero-crossing detection threshold voltage generating circuit, and a second input terminal coupled to ground;

b) a first voltage controlled current source configured to provide a variable current that increases along with said output voltage when said output voltage is greater than said minimum output voltage;

c) a first constant current source and said first voltage controlled current source being coupled to said first input terminal; and

d) a first switch coupled between a first node and said first input terminal, wherein said first switch is controllable by a pulse-width modulation (PWM) signal, and wherein a main power switch and an inductor of said switching power supply are coupled at said first node.

12. The circuit of claim 8 , wherein said zero-crossing detection threshold voltage varies with said output voltage by decreasing when said output voltage increases.

13. The circuit of claim 12 , further comprising:

a) a zero-crossing comparator having a second input terminal coupled to an output terminal of said zero-crossing detection threshold voltage generating circuit, and a first input terminal coupled to an output terminal of said switching power supply;

b) a second voltage controlled current source configured to provide a variable current that increases along with said output voltage when said output voltage is greater than said minimum output voltage;

c) a second constant current source and said second voltage controlled current source being coupled to said second input terminal; and

d) a second switch coupled between a first node and a second input terminal of said zero-crossing comparator, wherein said second switch is controllable by said PWM signal, and wherein a main power switch and an inductor of said switching power supply are coupled at said first node.

14. An inductor current zero-crossing detection circuit configured in a switching power supply, the circuit comprising:

a) an output voltage sampling circuit configured to determine a present output voltage of said switching power supply;

b) a zero-crossing detection threshold voltage generating circuit configured to generate a zero-crossing detection threshold voltage according to said output voltage;

c) a zero-crossing detection circuit configured to generate a zero-crossing detection signal according to said zero-crossing detection threshold voltage, wherein said zero-crossing detection signal is used to turn off a synchronous rectifier switch in said switching power supply;

d) an input current generating circuit having an input terminal coupled to a voltage input terminal of said switching power supply, and an output terminal coupled to a first terminal of a third switch; and

e) a sampling resistor having a first terminal coupled to a second terminal of said third switch, and a second terminal of said sampling resistor being coupled to ground, wherein said third switch is controllable by said PWM signal.

15. A switching power supply, comprising:

a) said inductor current zero-crossing detection circuit of claim 8 ; and

b) a power converter.