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

• US 6,781,357 B2
• Filed: 09/23/2002
• Issued: 08/24/2004
• 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:
  US10/253,307
• Publication Number:
  US 20030076078A1
• Time in Patent Office: 701 Days
• Field of Search: 323/282, 323/283, 323/284, 323/285, 323/235, 323/286, 323/287, 323/288, 363/79, 363/80, 363/97, 363/131, 363/235
• US Class Current: 323/282
• 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 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;

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

2. The power supply regulation circuit of claim 1 wherein the variable current limit threshold is further varied between the first level and the second level during a time when the switch is off.

3. The power supply regulation circuit of claim 1 further comprising an oscillator adapted to provide a sawtooth waveform, the variable current limit threshold is adapted to be varied in response to the sawtooth waveform provided by the oscillator.

4. The power supply regulator circuit of claim 3 further comprising a comparator coupled to the current sense circuitry to compare the current in the switch with the variable current limit threshold.

5. The power supply regulator circuit of claim 4 further comprising a latch adapted to provide the control signal, the latch coupled to be reset in response to the comparator.

6. The power supply regulator of claim 1 wherein the switch comprises a metal oxide field effect transistor (MOSFET).

7. A power supply, comprising:

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

current sense circuitry coupled to the switching device to sense a current in the switching device, the current sense circuitry having a variable current limit threshold that is varied between first and second levels during a time when the switching device is on such that the power supply output is regulated to provide an output characteristic having an approximately constant output current below an output voltage threshold.

8. The power supply of claim 7 wherein the power supply is adapted to transition from providing an approximately constant output voltage to providing the approximately constant output current when the output current threshold is reached.

9. The power supply of claim 7 wherein the variable current limit threshold is further varied between the first level and the second level during a time when the switch is off.

10. The power supply of claim 7 wherein the regulation circuit further comprises an oscillator adapted to provide a sawtooth waveform, the variable current limit threshold is adapted to be varied in response to the sawtooth waveform provided by the oscillator.

11. The power supply of claim 10 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.

12. The power supply of claim 11 wherein the regulation circuit further comprises a latch adapted to provide a control signal coupled to be received by the switching device, the latch coupled to be reset in response to the comparator.

13. The power supply of claim 7 wherein the switching device comprises a metal oxide field effect transistor (MOSFET).

14. A method for regulating 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;

disabling the switching device in response to one of the current through the switching device reaching a variable current limit threshold or a signal derived from an oscillator; and

varying the variable current limit threshold between first and second levels during a time when the switching device is on such that the power supply output is regulated to provide an output characteristic having an approximately constant output current below an output voltage threshold.

15. The method of claim 14 wherein disabling the switching device in response to the current through the switching device reaching the variable current limit threshold comprises comparing the current through the switching device with the variable current limit threshold.

16. The method of claim 14 further comprising varying the variable current limit threshold between the first and second levels during a time when the switching device is off.

17. The method of claim 14 wherein varying die variable current limit threshold between first and second levels comprises:

generating a sawtooth waveform; and

deriving the variable current limit threshold from the sawtooth waveform.

18. The method of claim 14 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.

19. The method of claim 18 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.

20. The method of claim 14 wherein the signal derived from the oscillator includes one of a maximum duty cycle signal or an output of a PWM circuit coupled to the oscillator.