Switching power converter with current sensing transformer auxiliary power supply

• US 8,198,874 B2
• Filed: 06/30/2009
• Issued: 06/12/2012
• Est. Priority Date: 06/30/2009
• Status: Active Grant

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Cirrus Logic Incorporated
• Inventors: Melanson, John L.
• Primary Examiner(s): STERRETT, JEFFREY L
• Application Number:
  US12/495,648
• Publication Number:
  US 20100327838A1
• Time in Patent Office: 1,078 Days
• Field of Search: 323/222, 323/282, 323/285, 323/286, 323/287
• US Class Current: 323/222
• CPC Class Codes: H02M 1/0006 Arrangements for supplying an adequate voltage to the control circuit of converters
• CPC Class Codes: H02M 1/0009 Devices or circuits for detecting current in a converter
• CPC Class Codes: H02M 3/156 with automatic control of output voltage or current, e.g. switching regulators

Abstract

A switching power converter having a current sensing transformer providing input to an auxiliary power supply provides efficient current sensing, while reducing the cost of the magnetic coupling element. The auxiliary power supply and current sense circuit both receive input from a secondary winding of a current sensing transformer having a primary winding coupled in series with the converter'"'"'s main magnetic coupling element. To provide accurate sensing, the magnetization the current sensing transformer is accounted for. The magnetization is compensated for in the current sensing result, current sensing is performed during a part of the cycle in which charging of the auxiliary power supply is disabled, or the core of the current sensing transformer is made large, raising its mutual inductance. In another alternative technique, a circuit node can be pre-charged to a value that cancels the offset due to the magnetization current.

Claims

1. A switched-power circuit, comprising:

a magnetic coupling element for coupling an input of the switched-power circuit to an output of the switched-power circuit and having at least one winding;

a switching circuit for controlling energizing of a primary winding of the magnetic coupling element from an input voltage source connected to the input of the switched-power circuit;

a current sensing transformer having a primary winding coupled in series with the at least one winding of the magnetic coupling element and the switching circuit, and having a secondary winding;

a control circuit for controlling switching of the switching circuit;

a current sensing circuit coupled to the secondary winding of the current sensing transformer for providing an indication of a magnitude of current conducted through the at least one winding of the magnetic coupling element; and

an auxiliary power supply having an input coupled to the secondary winding of the current sensing transformer and an output coupled to the control circuit for providing a supply voltage to the control circuit.

2. The switched-power circuit of claim 1, wherein the current sensing circuit corrects the indication of the magnitude of the current to compensate for a mutual inductance of the current sensing transformer.

3. The switched-power circuit of claim 2, wherein the current sensing circuit approximates the mutual inductance of the secondary winding of the current sensing transformer from an estimation determined from a measurement of a voltage across the secondary winding of the current sensing transformer.

4. The switched-power circuit of claim 2, wherein the current sensing circuit approximates the mutual inductance of the current sensing transformer from a predetermined value of the mutual inductance of the current sensing transformer.

5. The switched-power circuit of claim 1, wherein the current sensing circuit further comprises a circuit for selectively reducing a voltage across the secondary winding of the current sensing transformer during a measurement interval in which the indication of the magnitude of the current is determined, whereby an effect of a magnetization current of the current sensing transformer on the indication of the magnitude of the current is substantially reduced.

6. The switched-power circuit of claim 5, wherein the auxiliary power supply further comprises a circuit for isolating the output of the auxiliary power supply from the secondary winding of the current sensing transformer when the voltage across the secondary winding is selectively reduced.

7. The switched-power circuit of claim 5, wherein voltage across the secondary winding of the current sensing transformer is only reduced during the measurement interval if the switched-power circuit is operating in continuous conduction mode.

8. The switched-power circuit of claim 1, wherein the auxiliary power supply circuit further comprises a pre-charging circuit for magnetizing the secondary winding of the current sensing transformer prior to a measurement cycle to a value opposite the magnetization current caused by the voltage across the secondary winding, whereby a magnetization current of the current sensing transformer is compensated for by the magnetizing of the secondary winding.

9. A method of operating a switched-power circuit, comprising:

switching an input voltage source across at least one winding of a magnetic coupling element to transfer power to an output of the switched-power circuit;

sensing a current flowing through the at least one winding of the magnetic coupling element using a sensing transformer having a primary winding coupled in series with the at least one winding of the magnetic coupling element; and

generating an auxiliary power supply from the secondary winding of the current sensing transformer for providing a supply voltage to a control circuit of the switched-power circuit.

10. The method of claim 9, further comprising correcting the indication of the magnitude of the current to compensate for a mutual inductance of the current sensing transformer.

11. The method of claim 10, wherein the correcting comprises approximating the mutual inductance of the secondary winding of the current sensing transformer from an estimation determined by measuring a voltage across the secondary winding of the current sensing transformer.

12. The method of claim 10, wherein the correcting comprises approximating the mutual inductance of the current sensing transformer from a predetermined value of the mutual inductance of the current sensing transformer.

13. The method of claim 9, further comprising selectively reducing a voltage across the secondary winding of the current sensing transformer during a measurement interval in which the indication of the magnitude of the current is determined, whereby an effect of a magnetization current of the current sensing transformer on the indication of the magnitude of the current is substantially reduced.

14. The method of claim 13, further comprising isolating the output of the auxiliary power supply during the selectively reducing.

15. The method of claim 13, wherein the selectively reducing only reduces the voltage across the secondary winding of the current sensing transformer during the measurement interval if the switched-power circuit is operating in continuous conduction mode.

16. The method of claim 9, further comprising magnetizing the secondary winding of the current sensing transformer prior to a measurement cycle to a value opposite the magnetization current caused by the voltage across the secondary winding, whereby a magnetization current of the current sensing transformer is compensated for by the magnetizing of the secondary winding.

17. An integrated circuit, comprising:

a switching control circuit for controlling a switch for charging an external magnetic coupling element through at least one primary winding;

at least one input terminal for coupling to a secondary winding of a current sensing transformer;

a current sensing circuit having an input coupled to the at least one input terminal for providing an indication of a magnitude of current conducted through the at least one winding of the external magnetic coupling element; and

an auxiliary power supply having an input coupled to the at least one input terminal and an output coupled to the switching control circuit for providing a supply voltage to the control circuit.

18. The integrated circuit of claim 17, wherein the current sensing circuit corrects the indication of the magnitude of the current to compensate for a mutual inductance of the current sensing transformer.

19. The integrated circuit of claim 17, wherein the current sensing circuit further comprises a circuit for selectively reducing a voltage across the secondary winding of the current sensing transformer during a measurement interval in which the indication of the magnitude of the current is determined, whereby an effect of a magnetization current of the current sensing transformer on the indication of the magnitude of the current is substantially reduced.

20. The integrated circuit of claim 19, wherein the auxiliary power supply further comprises a circuit for isolating the output of the auxiliary power supply from the secondary winding of the current sensing transformer when the voltage across the secondary winding is selectively reduced.

21. The integrated circuit of claim 19, wherein the voltage across the secondary winding of the current sensing transformer is only reduced during the measurement interval if the switched-power circuit is operating in continuous conduction mode.

22. The integrated circuit of claim 17, wherein the auxiliary power supply circuit further comprises a pre-charging circuit for magnetizing the secondary winding of the current sensing transformer prior to a measurement cycle to a value opposite the magnetization current caused by the voltage across the secondary winding, whereby a magnetization current of the current sensing transformer is compensated for by the magnetizing of the secondary winding.