OFF TIME CONTROL METHOD FOR SWITCHING REGULATOR

• US 20100201335A1
• Filed: 01/13/2010
• Published: 08/12/2010
• Est. Priority Date: 01/13/2009
• Status: Active Grant

• Current Assignee: Monolithic Power Systems Incorporated
• Original Assignee: Monolithic Power Systems Incorporated
• Inventors: Zhang, Junming, Shi, Yang, Ren, Yuancheng, Lu, Huanyu, Li, En
• Granted Patent: US 8,971,061 B2
• Time in Patent Office: Days
• US Class Current: 323/284
• CPC Class Codes: H02M 3/33507 with automatic control of the output voltage or current, e.g. flyback converters H02M3/33561, H02M3/33569 take precedence

Abstract

The present invention provides an off time control method and switching regulator using it. The current flowing through a switch is compared with a current threshold, and the switch is turned off if the current flowing through the switch is larger than the current threshold. The off time of the switch is determined by the load. The current threshold is variable at light load to prevent generating the audible noise and improve the whole efficiency.

Claims

1. A switching regulator using off time control, comprising:

a switch; and

a control circuit, controlling the on and off of said switch;

wherein said control circuit compares the current flowing through said switch with a current threshold and turns off said switch when the current flowing through said switch is larger than said current threshold, said control circuit further determines the off time of said switch based on the magnitude of a load; and

said current threshold is variable at light load.

2. The switching regulator of claim 1, wherein at light load said current threshold is increased and decreased along with the load.

3. The switching regulator of claim 2, further comprising:

a voltage feedback circuit, sensing the output voltage of said switching regulator and generating a feedback signal accordingly; and

a current sensing circuit, electrically coupled to said switch, sensing the current flowing through said switch and generating a current sensing signal representative of it;

wherein said control circuit comprises a threshold adjusting circuit which is electrically coupled to said voltage feedback circuit to receive said feedback signal and generate a threshold accordingly; and

said control circuit compares said current sensing signal with said threshold and turns off said switch based on the comparison result.

4. The switching regulator of claim 3, wherein said current threshold is maintained constant at non-light load.

5. The switching regulator of claim 4, wherein said threshold adjusting circuit comprises:

a variable threshold generating circuit, electrically coupled to said voltage feedback circuit, receiving said feedback signal and generating a variable threshold accordingly, wherein said variable threshold is increased and decreased along with the load; and

a threshold comparison circuit, electrically coupled to said variable threshold generating circuit, comparing said variable threshold with a constant threshold and outputting the minimum one as said threshold compared with said current sensing signal.

6. The switching regulator of claim 5, wherein said threshold comparison circuit comprises two diodes the anodes of which are electrically connected together to output said threshold compared with said current sensing signal, the cathode of one diode is electrically connected to said variable threshold generator to receive said variable threshold, the cathode of the other diode is electrically connected to receive said constant threshold.

7. The switching regulator of claim 5, wherein said feedback signal is increased and decreased reversely with the load, and said variable threshold is the difference between a reference and the multiple of said feedback signal.

8. The switching regulator of claim 3, wherein said control circuit further comprises:

a charging current source, one terminal of said charging current source receives a supply voltage;

a charging capacitor, electrically connected between said charging current source and the ground; and

a discharging switch, electrically connected to said charging capacitor in parallel;

wherein said control circuit compares the voltage across said charging capacitor with said feedback signal and turns on said switch based on the comparison result.

9. The switching regulator of claim 8, wherein said control circuit further comprises:

a first comparator, the non-inverting input terminal of said first comparator is electrically connected to said charging capacitor to receive the voltage across it, the inverting input terminal of said first comparator is electrically coupled to said voltage feedback circuit to receive said feedback signal;

a second comparator, the non-inverting input terminal of said second comparator is electrically connected to said current sensing circuit to receive said current sensing signal, the inverting input terminal of said second comparator is electrically connected to said threshold adjusting circuit to receive said threshold;

a flip-flop, the set and reset terminals of said flip-flop are electrically connected to the output terminals of said first and second comparators separately, the output terminal of said flip-flop is electrically coupled to said switch and discharging switch to control the on and off of them.

10. The switching regulator of claim 9, wherein said control circuit further comprises two diodes the cathodes of which are electrically connected together and connected to the inverting input terminal of said first comparator, the anode of one diode is electrically connected to a voltage source, the anode of the other diode is electrically connected to said voltage feedback circuit to receive said feedback signal.

11. The switching regulator of claim 1, wherein said switch regulator comprises a flyback converter.

12. An off time control method, comprising:

comparing the current flowing through a switch with a current threshold, and turning off said switch if the current flowing through said switch is larger than said current threshold;

determining the off time of said switch based on the load; and

varying said current threshold at light load.

13. The method of claim 12, wherein at light load said current threshold is increased and decreased along with the load.

14. The method of claim 13, further comprises:

sensing the output voltage of said switching regulator and generating a feedback signal accordingly;

sensing the current flowing through said switch and generating a current sensing signal representative of it;

generating a threshold according to said feedback signal; and

comparing said current sensing signal with said threshold and turning off said switch based on the comparison result.

15. The method of claim 14, further comprising maintaining said current threshold constant at non-light load.

16. The method of claim 15, further comprising:

generating a variable threshold according to said feedback signal, wherein said variable threshold is increased and decreased along with the load;

comparing said variable threshold with a constant threshold and using the smaller one as said threshold compared with said current sensing signal.

17. The method of claim 16, wherein said feedback signal is increased and decreased inversely with the load, and said variable threshold is the difference between a reference and the multiple of said feedback signal.