Method and apparatus for providing harmonic inductive power

• US 20060076338A1
• Filed: 11/01/2005
• Published: 04/13/2006
• Est. Priority Date: 07/02/2003
• Status: Active Grant

• Current Assignee: ITherm Technologies, L.P., Optitherm Technologies, Co.
• Original Assignee: ITherm Technologies, L.P., Optitherm Technologies, Co.
• Inventors: Kagan, Valery
• Granted Patent: US 7,279,665 B2
• Time in Patent Office: Days
• US Class Current: 219/201
• CPC Class Codes: H05B 2206/024 the resistive heat generated in the induction coil is conducted to the load
• CPC Class Codes: H05B 6/04 Sources of current
• CPC Class Codes: H05B 6/06 Control, e.g. of temperature, of power

Abstract

Method and apparatus for providing harmonic inductive power, and more particularly for delivering current pulses providing a desired amount of pulse energy in high frequency harmonics to a load circuit for inductive heating of an article. By controlling the shape and/or frequency of such current pulses, the apparatus and method can be used to enhance the rate, intensity and/or power of inductive heating delivered by the heater coil and/or to enhance the lifetime or reduce the cost and complexity of an inductive heating power supply. Of particular significance, the apparatus and method may be used to significantly increase the power inductively delivered to a ferromagnetic or other inductively heated load, without requiring an increase of current in the heater coil. This enables new heating applications, and in some known applications, decreases the energy consumption or cooling requirements and/or increase the lifetime of the heater coil.

Claims

14. A method comprising:

providing a power supply circuit for delivering current pulses with high frequency harmonics in a load circuit for inductive heating of an article;

prior to the delivery of the current pulses, determining an impedance parameter of the load circuit and determining an energy content of the current pulses based upon the impedance parameter.

15. The method of claim 14, comprising:

monitoring a response of the load circuit for changes to the impedance parameter.

16. The method of claim 14, comprising:

determining the energy content of the current pulses based upon one or more limitations of the power supply circuit, wherein the limitations include limitations in voltage, current spike, RMS current, switching frequency and temperature.

17. The method of claim 14, wherein the impedance parameter is used to detect a presence, absence or a change in:

an input to the power supply;

a connection of the load circuit to the power supply;

a failure of a heater coil in the load circuit;

a loss or change of magnetic coupling during heating of the article; and

contact between one or more turns of a heater coil of the load circuit.

18. A method for inductive heating of a load circuit having variable impedance parameters comprising:

providing a signal to determine one or more impedance parameters of the load circuit; and

supplying to the load circuit current pulses providing high frequency harmonics in the load circuit based on the determined one or more impedance parameters.

19. The method of claim 18, wherein the load circuit includes a heater coil generating a magnetic flux for inductive heating of an article, and wherein the variable impedance parameters of the load circuit are based on one or more of:

variations in the heater coil; and

variations in magnetic coupling between the heater coil and the article.

20. A method of dynamic heating control comprising:

supplying current pulses providing a desired amount of pulse energy in high frequency harmonics in a load circuit for inductive heating of an article;

supplying a signal for determining one or more impedance parameters of the load circuit during heating; and

modifying the energy content of the current pulses based upon the determined one or more impedance parameters.

21. A method comprising:

supplying current pulses with high frequency harmonics in a load circuit for inductive heating of an article;

determining one or more impedance parameters of the load circuit;

determining an energy content of the current pulses based on the determined one or more impedance parameters and a desired power delivery to the load circuit.

22. A method of delivering inductive power from a power supply circuit to a load circuit coupled to the power supply circuit, comprising:

supplying current pulses with high frequency harmonics in the load circuit for inductive heating of an article;

determining one or more limitations of the power supply circuit;

determining one or more impedance parameters of the load circuit; and

determining, based on the one or more determined impedance parameters and limitations, an energy content of the current pulses for delivery of a desired power to the load circuit within the limitations of the power supply circuit.

23. The method of claim 22, wherein the power supply circuit includes a charging circuit coupled to the load circuit, the method comprising:

determining an impedance parameter of the charging circuit based on a frequency response of the charging circuit.

24. The method of claim 22, comprising:

determining an impedance parameter of the load circuit based on a frequency of oscillation of the load circuit.

25. The method of claim 24, wherein the frequency of oscillation is determined by monitoring consecutive zero crossings of a voltage or current supplied to the load circuit.

26. The method of claim 22, wherein the desired power is determined by determining a damping coefficient of the load circuit.

27. The method of claim 26, wherein the damping coefficient is determined by monitoring the amplitude of consecutive peaks of a voltage or current supplied to the load circuit.

28. A power supply control apparatus comprising:

a charging circuit;

a load circuit coupled to the charging circuit;

a switching device for controlling the charging circuit to deliver current pulses in the load circuit during an on-time of the switching device; and

a monitoring and control circuit for controlling the on-time and off-time of the switching device during a heating cycle to provide a desired amount of pulse energy in high frequency harmonics in the load circuit.

1. A method of delivering inductive power from a power supply circuit to a load circuit for inductive heating of an article, wherein the power supply circuit includes a charging circuit coupled to the load circuit, the method comprising:

determining an impedance parameter of the load circuit;

determining an impedance parameter of the charging circuit; and

supplying to the load circuit, based on the determined impedance parameters of the load circuit and charging circuit, current pulses providing a desired amount of pulse energy in high frequency harmonics in the load circuit for inductive heating of the article.

2. The method of claim 1, wherein the power supply circuit includes a switching device for controlling the charging circuit and the method includes determining an on-time (t_on) of the switching device for providing the desired current pulses.

3. The method of claim 2, wherein the method includes determining an off-time (t_off) of the switching device for providing the desired current pulses.

4. The method of claim 3, wherein t_on and t_off are determined to enable delivery in the current pulses of at least 50% of the energy stored in the charging circuit.

5. The method of claim 3, wherein t_on and t_off are determined to enable delivery in the current pulses of at least 90% of the energy stored in the charging circuit.

6. The method of claim 1, wherein at least 50% of the pulse energy is in high frequency harmonics.

7. The method of claim 1, wherein at least 90% of the pulse energy is in high frequency harmonics.

8. The method of claim 1, wherein the load circuit has a damping ratio in the range of 0.01 to 0.2.

9. The method of claim 1, wherein the method is intermittently employed during a cycle of heating the article to detect changes in at least one of the determined impedance parameters.

10. The method of claim 1, including modifying the impedance parameter of the charging circuit based on a desired power delivery to the load circuit.

29. The apparatus of claim 28, wherein the monitoring and control circuit controls an opening time of the switching device by monitoring current in the charging and load circuits.

30. The apparatus of claim 28, wherein the desired amount is at least 50%.

31. The apparatus of claim 28, wherein the switching device couples the charging circuit and load circuit so that at least 50% of the energy stored in the charging circuit is delivered to the load circuit.

32. The apparatus of claim 31, wherein the switching device couples the charging circuit and load circuit so that at least 90% of the energy stored in the charging circuit is delivered to the load circuit.

34. The apparatus of claim 28, wherein the switching device couples the charging circuit to the load circuit such that current oscillates through the switching device during the delivery of the current pulses to the load circuit.

35. The apparatus of claim 28, wherein the switching device couples the charging circuit to the load circuit such that energy is left stored in the charging circuit to achieve a non-zero current condition in the load circuit on subsequent charging cycles.

37. The apparatus of claim 28, wherein the monitoring circuit includes means for monitoring consecutive zero crossings of the current or voltage in the current pulse and determining a desired shape and frequency of the current pulse based on such monitoring.

38. The apparatus of claim 28, wherein the load circuit has a damping ratio in a range of 0.01 to 0.2.

39. The apparatus of claim 38, wherein the load circuit has a damping ratio in a range of 0.05 to 0.1.

40. The apparatus of claim 28, including a signal generator to provide a signal for determining at least one impedance parameter of the load circuit.

41. The apparatus of claim 28, including means for monitoring the response of the load circuit for changes to at least one impedance parameter of the load circuit.

42. The apparatus of claim 28, wherein the load circuit includes a heater coil magnetically coupled to a ferromagnetic or conductive article.

43. The apparatus of claim 28, including means for monitoring zero crossings of voltage or current supplied to the load circuit.

44. The apparatus of claim 28, including a means for monitoring the amplitude of consecutive peaks of a voltage or current supplied to the load circuit.

45. The apparatus of claim 28, wherein the switching device comprises a plurality of switches disposed in parallel.

46. The apparatus of claim 28, including means for preventing current in the load circuit from flowing back through the charging circuit.

32-1. The apparatus of claim 28, wherein the switching device couples the charging circuit to the load circuit so that for an input voltage U_D to the charging circuit, a voltage of at least 2U_D is delivered to the load circuit.

36. The apparatus of claim 33, wherein the on-time and/or off-time is controlled to achieve a substantially zero current condition through the switch, while neither the load circuit nor the charging circuit have a zero current condition.

47. A method for generating current pulses providing a desired amount of pulse energy in high frequency harmonics in a load circuit for inductive heating of an article, the method comprising:

generating current pulses with high frequency harmonics, each pulse comprising at least one steeply varying portion for delivering at least 50% of the pulse energy in the load circuit in high frequency harmonics;

controlling the on/off timing of the current pulses to generate a plurality of such pulses as a desired current signal for inductive heating.

48. The method of claim 47, wherein the on/off timing is controlled to produce two or three oscillations in each current pulse.

49. The method of claim 47, wherein the on/off timing is controlled so that each current pulse ends after its amplitude falls by at least 50% from an amplitude of a maximum peak in the current pulse.

50. The method of claim 49, wherein the on/off timing is controlled so that each current pulse ends after its amplitude falls by at least 75% from an amplitude of a maximum peak in the current pulse.

51. The method of claim 50, wherein the on/off timing is controlled so that each current pulse ends after its amplitude falls by at least 90% from an amplitude of a maximum peak in the current pulse.

53. The method of claim 47, wherein the on/off timing is controlled such that each current pulse includes at least one steeply varying portion having a maximum rate of change at least 5 times greater than a maximum rate of change of a sinusoidal signal of the same fundamental frequency and RMS current amplitude.

54. The method of claim 53, wherein maximum rate of change is at least 10 times greater.

55. The method of claim 54, wherein the maximum rate of change is at least 20 times greater.

56. The method of claim 53, wherein an upper limit of the maximum rate of change is determined based on a voltage limit of the load circuit.

57. The method of claim 47, wherein the on/off timing is controlled such that each current pulse contains at least two complete oscillation cycles before damping to a level below 10% of an amplitude of a maximum peak in the current pulse.

52. The method of 51, wherein the on/off timing is controlled so that each current pulse ends after its amplitude falls by at least 95% from an amplitude of a maximum peak in the current pulse.