Method and apparatus for maintaining a constant load current with line voltage in a switch mode power supply

• US 7,110,270 B2
• Filed: 07/15/2004
• Issued: 09/19/2006
• Est. Priority Date: 09/27/2001
• Status: Expired due to Term

• Current Assignee: Power Integrations Incorporated
• Original Assignee: Power Integrations Incorporated
• Inventors: Wong, Kent, Matthews, David Michael Hugh, Balakrishnan, Balu, Djenguerian, Alex B.
• Primary Examiner(s): Patel, Rajnikant B.
• Application Number:
  US10/892,300
• Publication Number:
  US 20040251886A1
• Time in Patent Office: 796 Days
• Field of Search: 363/21.1, 363/21.12, 363/97, 363/21.4, 363/56, 363/49, 323/288, 323/222, 323/284, 323/282
• US Class Current: 363/21.07
• CPC Class Codes: H02M 1/0009 Devices or circuits for detecting current in a converter
• 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
• CPC Class Codes: H02M 3/33523 with galvanic isolation between input and output of both the power stage and the feedback loop
• CPC Class Codes: H03K 17/0822 in field-effect transistor switches

Abstract

A power supply including a regulation circuit that maintains an approximately constant load current with line voltage. In one embodiment, a regulation circuit includes a semiconductor switch and current sense circuitry to sense the current in the semiconductor switch. The current sense circuitry has a current limit threshold. The regulation circuit current limit threshold is varied from a first level to a second level during the time when the semiconductor switch is on. One embodiment of the regulation circuit is used in a power supply having an output characteristic having an approximately constant output voltage below an output current threshold and an approximately constant output current below an output voltage threshold.

Claims

1. A power supply regulator, comprising:

a comparator having first and second inputs and an output, the first input of the comparator adapted to sense a current adapted to flow between first and second terminals of a switch, the second input of the comparator coupled to receive a variable current limit threshold signal; and

a control circuit adapted to generate a control signal in response the output of the comparator, the control signal adapted to be coupled to a control terminal of the switch to control switching of the switch to provide a power supply to have an output characteristic having an approximately constant output current below an output threshold voltage, the variable current limit threshold signal adapted to vary between a first level and a second level during a time when the switch is adapted to be on in response to the control signal.

2. The power supply regulator of claim 1 wherein the current limit threshold signal adapted to vary between the first level and the second level during a time when the switch is adapted to be off in response to the control signal.

3. The power supply regulator of claim 1 further comprising an oscillator adapted to generate a sawtooth waveform, wherein variable current limit threshold signal is generated in response to the sawtooth waveform.

4. The power supply regulator of claim 3 wherein the control circuit includes a latch adapted to provide the control signal, wherein the latch includes a reset input coupled to the output of the comparator.

5. The power supply regulator of claim 4 wherein the latch further includes a set input coupled to an output of the oscillator.

6. A power supply regulator, comprising:

a comparator having first and second inputs and an output, the first input of the comparator to sense a voltage developed by a switch, during an on time of the switch, the second input of the comparator coupled to receive a current limit threshold signal to increase during the on time of the switch; and

a control circuit to generate a control signal in response to the output of the comparator, the control signal to be coupled to a control terminal of the switch to control switching of the switch to provide a power supply to have an output characteristic having an approximately constant output current below an output threshold voltage, the variable current limit threshold signal to vary between a first level and a second level during a time when the switch is to be on in response to the control signal.

7. The power supply regulator of claim 6 further comprising an oscillator to generate a sawtooth waveform, wherein variable current limit threshold signal is generated in response to the sawtooth waveform.

8. The power supply regulator of claim 7 wherein the control circuit includes a latch to provide the control signal, wherein the latch includes a reset input coupled to the output of the comparator.

9. The power supply regulator of claim 8 wherein the latch further includes a set input coupled to an output of the oscillator.

10. A power supply regulation circuit, the circuit comprising:

a switch including a first terminal, a second terminal and a control terminal, said switch being operable to couple or decouple the first terminal and the second terminal in response to a control signal received at the control terminal;

voltage sense circuitry to sense a voltage developed across the switch during an on time of the switch, representative of a current in the switch, the voltage sense circuitry having a variable current limit threshold to increase between a first level and a second level during a time when the switch is on, the control signal responsive to the variable current limit threshold to provide a power supply with an output characteristic having an approximately constant output voltage below an output current threshold and an approximately constant output current below an output voltage threshold.

11. The power supply regulation circuit of claim 10 further comprising an oscillator to provide a sawtooth waveform, the variable current limit threshold is to be varied in response to the sawtooth waveform provided by the oscillator.

12. The power supply regulator circuit of claim 11 further comprising a comparator coupled to the voltage sense circuitry to compare the current in the switch with the variable current limit threshold.

13. The power supply regulator circuit of claim 12 further comprising a latch to provide the control signal, the latch coupled to be reset in response to the comparator.

14. The power supply regulator of claim 10 wherein the switch comprises a metal oxide field effect transistor (MOSFET).

15. A power supply coupled to receive an input voltage, comprising:

an energy transfer element coupled between a power supply input and a power supply output; and

a regulation circuit coupled between the power supply input and the energy transfer element, the regulation circuit including;

a switch coupled between the power supply input and the energy transfer element to control delivery of energy to the power supply output;

current sense circuitry to sense a voltage developed across the switch during an on time of the switch, representative of a current in the switch, the current sense circuitry having a variable current limit threshold to increase between first and second levels during a time when the switching device is on.

16. The power supply of claim 15 wherein the power supply is to transition from providing an approximately constant output voltage to providing an approximately constant output current when an output current threshold is reached.

17. The power supply of claim 15 wherein the regulation circuit further comprises an oscillator to provide a sawtooth waveform, the variable current limit threshold is to be varied in response to the sawtooth waveform provided by the oscillator.

18. The power supply of claim 17 wherein the regulation circuit further comprises a comparator coupled to the current sense circuitry to compare the current in the switch with the variable current limit threshold.

19. The power supply of claim 18 wherein the regulation circuit further comprises a latch to provide a control signal coupled to be received by the switching device, the latch coupled to be reset in response to the comparator.

20. The power supply of claim 15 wherein the switching device comprises a metal oxide field effect transistor (MOSFET).

21. A method for regulating a power supply, comprising:

enabling or disabling a flow of energy from a power supply input to a power supply output with a switching device in response to a control signal;

sensing a current through the switching device by sensing a voltage developed across the switching device during an on time of the switching device;

disabling the switching device in response to the current through the switching device reaching a variable current limit threshold; and

increasing the current limit threshold between first and second levels during a time when the switching device is on.

22. The method of claim 21 varying the variable current limit threshold such that the power supply output is regulated to provide an approximately constant output voltage below an output current threshold and an approximately constant output current below an output voltage threshold.

23. The method of claim 21 wherein increasing the current limit threshold between first and second levels comprises:

generating a sawtooth waveform; and

deriving the variable current limit threshold from the sawtooth waveform.

24. The method of claim 21 wherein disabling the switching device comprises resetting a latch from which the control signal is provided in response to the current through the switching device reaching the variable current limit threshold.

25. The method of claim 24 further comprising receiving a feedback signal responsive to an output level of the power supply output and resetting the latch in response to the feedback signal.