METHOD AND APPARATUS FOR SENSING A CURRENT IN A CIRCUIT

• US 20080068193A1
• Filed: 11/13/2007
• Published: 03/20/2008
• Est. Priority Date: 05/04/2005
• Status: Active Grant

• Current Assignee: Power Integrations Incorporated
• Original Assignee: Power Integrations Incorporated
• Inventors: Balakrishnan, Balu, Odell, Arthur
• Granted Patent: US 7,656,304 B2
• Time in Patent Office: Days
• US Class Current: 340/664
• CPC Class Codes: G01R 19/16552 in I.C. power supplies
• CPC Class Codes: H02J 5/00 Circuit arrangements for transfer of electric power between ac networks and dc networks H02J3/36 takes precedence
• CPC Class Codes: H02J 7/02 for charging batteries from ac mains by converters

Abstract

Techniques are disclosed to sense a current in a circuit. For instance, current sense circuit according to the teachings of the present invention includes a current sense resistor coupled to an input of the current sense circuit. The current sense resistor is coupled to receive a current to be sensed from the input of the current sense circuit. The current to be sensed is converted to a current sense voltage. A first PN junction diode is coupled to the current sense resistor. A light emitting diode (LED) is coupled to the first PN junction diode to provide a current sense threshold substantially proportional to a difference between a forward voltage drop of the LED and a forward voltage drop of the first PN junction diode. The first PN junction diode is coupled to be biased from the forward voltage drop of the LED. The LED is coupled to the current sense resistor to generate an output when the current sense voltage from the current sense resistor reaches the current sense threshold.

Claims

1. A current sense circuit, comprising:

a current sense resistor coupled to an input of the current sense circuit, the current sense resistor to receive a current to be sensed from the input of the current sense circuit to convert the current to be sensed to the current sense voltage;

a first PN junction diode of a first transistor coupled to the current sense resistor;

a second resistor coupled between a collector of the first transistor and a base of the first transistor; and

a light emitting diode (LED) coupled to the first PN junction diode to provide a current sense threshold substantially a function of a difference between a forward voltage drop of the LED and a forward voltage drop of the first PN junction diode, the current sense threshold substantially equal to a voltage drop across the second resistor, the first PN junction diode coupled to be biased from the forward voltage drop of the LED, the LED coupled to the current sense resistor to generate an output when the current sense voltage from the current sense resistor reaches the current sense threshold.

2. The current sense circuit of claim 1 further comprising a third resistor coupled between the LED and the second resistor.

3. The current sense circuit of claim 2 further comprising a second PN junction diode of a second transistor, wherein an emitter of the second transistor is coupled to the current sense resistor and wherein a base of the second transistor is coupled to the collector of the first transistor.

4. The current sense circuit of claim 1 further comprising a third transistor coupled to the first transistor and coupled to the LED, wherein the collector of the first transistor is coupled to a collector of the third transistor, the collector of the third transistor coupled to drive the LED such that an output of the current sense circuit comprises light output generated by the LED.

5. The current sense circuit of claim 1 further comprising an optocoupler coupled to the current sense resistor, wherein the optocoupler includes the LED.

6. The current sense circuit of claim 1 wherein the current sense circuit is included in a secondary side circuit of an isolated power supply.

7. A current sense circuit, comprising:

a current sense resistor coupled to a direct current (DC) output of a power supply, wherein the current sense circuit is to be powered entirely from the DC output, wherein the current sense resistor is to convert a current at the DC output into a current sense voltage;

a first PN junction diode of a first transistor coupled to the current sense resistor; and

a second resistor coupled between a collector of the first transistor and a base of the first transistor, wherein a current sense threshold is substantially equal to a voltage across the second resistor, wherein the current sense circuit is to generate an output when the current sense voltage reaches the current sense threshold to sense the current at the DC output.

8. The current sense circuit of claim 7 wherein the output of the current sense circuit comprises a light output of a light emitting diode (LED) coupled to the first PN junction diode.

9. The current sense circuit of claim 8 wherein the voltage across the second resistor is substantially a function of a difference between a forward voltage drop of the LED and a forward voltage drop of the first PN junction diode.

10. The current sense circuit of claim 9 wherein the LED is off when an output voltage at the DC output of the power supply is too low.

11. The current sense circuit of claim 7 wherein the output of the current sense threshold is substantially temperature compensated.

12. The current sense circuit of claim 7 wherein the output of the current sense circuit is to be used as a feedback signal to regulate a current at the DC output.

13. The current sense circuit of claim 12 wherein the current at the DC output is maintained substantially at a constant current level over a voltage range at the DC output.

14. The current sense circuit of claim 13 wherein the current at the DC output is reduced below the constant current level outside of the voltage range at the DC output.