Resonant frequency tracking system and method for use in a radio frequency (RF) power supply

• US 20030071034A1
• Filed: 11/25/2002
• Published: 04/17/2003
• Est. Priority Date: 07/10/1998
• Status: Active Grant

• Current Assignee: Ambrell Corporation (inTEST Corporation)
• Original Assignee: Daniel J. Lincoln, Gary A. Schwenck, Leslie L. Thompson
• Inventors: Schwenck, Gary A., Thompson, Leslie L., Lincoln, Daniel J.
• Granted Patent: US 6,730,894 B2
• Time in Patent Office: Days
• US Class Current: 219/666
• CPC Class Codes: H02M 7/48 using discharge tubes with control electrode or semiconductor devices with control electrode
• CPC Class Codes: H05B 6/04 Sources of current
• CPC Class Codes: H05B 6/06 Control, e.g. of temperature, of power

Abstract

An RF power supply that is capable of tracking rapid changes in the resonant frequency of a load and capable of quickly responding to varying load conditions so as to deliver the desired amount of power. The present invention also provides an RF power supply capable of delivering a wide range of power over a broad frequency range to a load that is remotely located from the power supply. According to one embodiment, the RF power supply includes a direct current (DC) voltage source that provides a DC voltage within a predetermined voltage range; an amplifier, coupled to the DC voltage source, that provides an alternating voltage to a tank circuit connected to an output of the RF power supply; a frequency controller, coupled to the amplifier, to set the frequency of the alternating voltage produced by the amplifier; and a sensor, coupled to the load, to provide a signal to the frequency controller, where the frequency controller sets the frequency of the alternating voltage based on the signal received from the sensor.

Claims

1. A radio frequency (RF) power supply for providing RF power to a load placed within an inductor coil of a tank circuit coupled to an output of the RP power supply, comprising:

a direct current (DC) voltage source that provides a DC voltage within a first predetermined range;

an amplifier, coupled to said DC voltage source, to provide an alternating voltage to the tank circuit;

a frequency controller, coupled to said amplifier, to control a frequency of said alternating voltage provided by said amplifier; and

a sensor, coupled to the tank circuit and to said frequency controller, comprising a current sensor that measures current flowing into the tank circuit and a voltage sensor that measures a voltage across the tank circuit.

2. The RF power supply of claim 1, wherein said sensor further comprises a divider that receives from said current sensor a first signal corresponding to said current flowing into the tank circuit, receives from said voltage sensor a second signal corresponding to said voltage across the tank circuit, and outputs a third signal to an analog to digital (A/D) converter that outputs a digitized signal corresponding to said third signal to said frequency controller.

3. The RF power supply of claim 2, wherein said third signal represents an admittance of the tank circuit.

4. The RF power supply of claim 2, wherein said third signal represents an impedance of the tank circuit.

5. The radio frequency power supply of claim 1, wherein said current sensor includes:

a first full wave bridge connected to an input of said tank circuit;

a first filter connected to said full wave bridge; and

a first gain stage connected to said filter.

6. The radio frequency power supply of claim 1, wherein said voltage sensor includes:

a second full wave bridge connected to said input of said tank circuit;

a second filter connected to said second full wave bridge; and

a second gain stage connected to said second filter.

7. The RF power supply of claim 1, wherein the tank circuit includes a capacitance connected in parallel with the inductor coil and wherein said frequency controller controls said frequency of the alternating voltage provided by said amplifier so as to continuously minimize an admittance of the tank circuit.

8. The RF power supply of claim 1, wherein the tank circuit includes a capacitance connected in series with the inductor coil and wherein said frequency controller controls said frequency of said alternating voltage provided by said amplifier so as to continuously maximize the admittance of the tank circuit.

9. The RF power supply of claim 1, further comprising RF cables for remotely locating the tank circuit from the RF power supply.

10. The RF power supply of claim 9, wherein a length of said RF cables is on the order of 200 feet.

11. A radio frequency (RF) power supply for providing RF power to a load, wherein the load is placed within an inductor coil of a tank circuit coupled to an output of the RF power supply, comprising:

a direct current (DC) voltage source that provides a DC voltage within a first predetermined range;

an amplifier, coupled to said DC voltage source, to provide an alternating voltage to the tank circuit;

a frequency controller, coupled to said amplifier, to control a frequency of the alternating voltage provided by said amplifier, wherein said frequency controller includes a processor coupled to a frequency synthesizer, wherein an output of said frequency synthesizer controls said frequency of said alternating voltage; and

a sensor, coupled to the tank circuit, to provide a signal to said processor, wherein said processor controls said output of said frequency synthesizer based on said signal received from said sensor, thereby controlling said frequency of said alternating voltage.

12. The RF power supply of claim 11, wherein said frequency synthesizer is a direct digital synthesizer.

13. The RF power supply of claim 11, wherein said frequency synthesizer is coupled to said amplifier through a delay circuit and a driver circuit.

14. The RF power supply of claim 11, further comprising RF cables for remotely locating the tank circuit from the RF power supply.

15. The RF power supply of claim 14, wherein a length of said RF cables is on the order of 200 feet.

16. A radio frequency (RF) power supply for providing RF power to a load, wherein the load is placed within a tank circuit coupled to an output of the RF power supply, comprising:

a direct current (DC) voltage source that produces a DC output voltage within a first predetermined range, said DC voltage source comprising a pulse width modulator (PWM) with hysteretic current mode control.

an amplifier, coupled to said DC voltage source, to provide an alternating voltage to the tank circuit;

a frequency controller, coupled to said amplifier, to control a frequency of said alternating voltage provided by said amplifier; and

a sensor, coupled to the load, to provide a signal to said frequency controller, wherein said frequency controller sets the frequency of said alternating voltage based on said signal received from said sensor.

17. The RF power supply of claim 16, wherein said PWM comprises a switch for hard switching a DC input voltage to produce pulses of energy, wherein said pulses of energy are filtered through an inductor and a capacitor to produce said DC output voltage, and wherein a diode is connected to said inductor for enabling current to flow through said inductor when said switch is in an off state.

18. The RF power supply of claim 17, wherein said PWM further comprises:

delaying means coupled between said switch and said diode for delaying said diode reverse recovery current; and

voltage spike prevention means coupled to said switch for preventing voltage spikes on said output of said switch.

19. The RE power supply of claim 17, wherein said pulse width modulator further comprises means for preventing excessive power loss from occurring in said switch during an on time transition.

20. The RF power supply of claim 17, wherein said pulse width modulator further comprises means for providing zero current switching at turn-on of said switch, thereby minimizing turn-on transition power dissipation.

21. The RF power supply of claim 17, wherein said pulse width modulator further comprises an inductor coupled between an output of said switch and a cathode of said diode, and a snubber to prevent voltage spikes on said output of said switch.

22. The RF power supply of claim 21, wherein said snubber is a resonant LC snubber.

23. The RF power supply of claim 22, wherein said snubber comprises:

a capacitor coupled to said output of said switch; and

an inductor coupled to said capacitor and an input of said switch.

24. The RF power supply of claim 16, further comprising RF cables for remotely locating the tank circuit from the RF power supply.

25. The RF power supply of claim 24, wherein a length of said RF cables is on the order of 200 feet.

26. A method for inductively heating a workpiece placed in an inductor coil of a tank circuit, comprising the steps of:

(1) generating an alternating voltage having a frequency;

(2) providing said alternating voltage to the tank circuit;

(3) sensing an admittance of the tank circuit; and

(4) altering said frequency of said alternating voltage provided to the tank circuit so as to deliver a desired level of power to the workpiece.

27. The method according to claim 26, wherein if the tank circuit is a parallel resonant tank circuit, said frequency is altered in step (4) so as to minimize said admittance of the tank circuit.

28. The method according to claim 26, wherein if the tank circuit is a series resonant tank circuit, said frequency is altered in step (4) so as to maximize said admittance of the tank circuit.

29. A method for inductively heating a workpiece placed in an inductor coil of a tank circuit, comprising the steps of:

generating an alternating voltage having a frequency;

providing said alternating voltage to the tank circuit;

sensing an impedance of the tank circuit; and

altering said frequency of said alternating voltage provided to the tank circuit so as to deliver a desired level of power to the workpiece.

30. A method for inductively heating a workpiece placed in an inductor coil of a tank circuit, comprising the steps of:

generating an alternating voltage having a frequency;

providing said alternating voltage to the tank circuit;

sensing power reflected from the tank circuit; and

altering said frequency of said alternating voltage provided to the tank circuit so as to deliver a desired level of power to the workpiece.

31. A method for inductively heating a workpiece placed in an inductor coil of a tank circuit, comprising the steps of:

generating an alternating voltage having a frequency;

providing said alternating voltage to the tank circuit;

sensing forward power; and

altering said frequency of said alternating voltage provided to the tank circuit so as to deliver a desired level of power to the workpiece.

32. A method for inductively heating a workpiece placed in an inductor coil of a tank circuit, wherein the tank circuit has a resonant frequency within a known frequency range, comprising the steps of:

(1) applying an alternating voltage to the tank circuit, said alternating voltage having a frequency within the known frequency range, and said alternating voltage having a first voltage level;

(2) determining a course estimate of the resonant frequency;

(3) based on said course estimate, determining a fine estimate of the resonant frequency;

(4) rapidly increasing said voltage level of said alternating voltage from said first voltage level to a second voltage level; and

(5) while said voltage level is being increased and until a heat off indication is generated, continuously tracking the resonant frequency.

33. The method of claim 32, wherein the step of determining a course estimate of the resonant frequency comprises the steps of:

(1) determining a first admittance value of the tank circuit;

(2) storing said first admittance value in a first memory location and storing a first frequency value representing said frequency of said alternating voltage in a second memory location;

(3) changing said frequency of said alternating voltage by an offset amount;

(4) determining a second admittance value of the tank circuit;

(5) comparing said second admittance value to said admittance value stored in said first memory location;

(6) if the tank circuit is a series resonant tank circuit, storing said second admittance value in said first memory location if said second admittance value is greater than said admittance value stored in said first memory location, and storing a second frequency value representing said frequency of said alternating voltage in said second memory location; and

(7) if the tank circuit is a parallel resonant tank circuit storing said second admittance value in said first memory location if said second admittance value is less than said admittance value stored in said first memory location, and storing a third frequency value representing the frequency of said alternating voltage in said second memory location.

34. The method according to claim 32, wherein said offset amount is a function of said frequency of said alternating voltage and a quality factor of the tank circuit.

35. The method of claim 32, wherein the step of tracking the resonant frequency comprises the steps of:

(1) setting said frequency of said alternating voltage to a predetermined frequency;

(2) determining an admittance of the tank circuit at said predetermined frequency;

(3) setting said frequency of said alternating voltage to F+, where F+ equals said predetermined frequency plus a first offset amount;

(4) determining said admittance of the tank circuit at said F+ frequency;

(5) setting said frequency of said alternating voltage to F−

, where F−

equals said predetermined frequency minus a second offset amount;

(6) determining said admittance of the tank circuit; and

(7) changing said frequency of said alternating voltage based on said admittance determined in step (1), said admittance determined in step (3), and said admittance determined in step (5).

36. The method according to claim 35, wherein said first offset amount is a function of said predetermined frequency and a quality factor of the tank circuit.

37. The method of claim 32, wherein the step of tracking the resonant frequency comprises the steps of:

(1) determining an admittance of the tank circuit and storing said admittance in a first memory location;

(2) decreasing said frequency of said alternating voltage;

(3) determining said admittance of the tank circuit;

(4) comparing said admittance stored in said first memory location with said admittance determined in step (3);

(5) if said admittance determined in step (3) is less than or equal to said admittance stored in said first memory location, storing said admittance determined in step (3) in said first memory location, and returning to step (2), otherwise continuing to step (6);

(6) increasing said frequency of said alternating voltage;

(7) determining said admittance of the tank circuit and storing said admittance in said first memory location;

(8) increasing said frequency of said alternating voltage;

(9) determining said admittance of the tank circuit;

(10) if said admittance determined in step (9) is less than or equal to said admittance stored in said first memory location, storing said admittance determined in step (9) in said first memory location, and returning to step (8), otherwise continuing to step (11); and

(11) decreasing said frequency of said alternating voltage and returning to step (1).

38. The method of claim 32, wherein said first voltage level is approximately the minimum voltage level that provides a sufficient signal to measure said admittance of the tank circuit over a range of frequencies.

39. The method of claim 38, wherein said second voltage level is approximately a full scale voltage.

40. The method of claim 39, wherein said step of increasing said voltage level of said alternating voltage from said first voltage level to a second voltage level occurs in approximately 30 milliseconds.

41. A radio frequency (RF) power supply for providing RF power to a load placed within an inductor coil of a tank circuit, comprising:

a direct current (DC) voltage source that provides a DC voltage within a first predetermined range;

an amplifier, coupled to said DC voltage source, to provide an alternating voltage to the tank circuit;

a frequency controller, coupled to said amplifier, to control a frequency of said alternating voltage provided by said amplifier; and

a power sensor being coupled to the tank circuit and to said frequency controller.

42. The system of claim 41, wherein said power sensor includes means for sensing forward power.

43. The system of claim 41, wherein said power sensor includes means for sensing reflected power.

44. The system of claim 41, wherein said power sensor includes means for sensing both forward and reflected power.

45. A direct current (DC) voltage source that produces a DC output voltage, comprising:

a pulse width modulator (PWM), wherein said PWM comprises a switch for hard switching a DC input voltage at a controlled pulse width and duty cycle to produce pulses of energy;

a filter that filters said pulses of energy to produce the DC output voltage; and

a diode coupled to said filter, wherein

said PWM further comprises an inductor coupled between an output of said switch and a cathode of said diode, and a snubber to prevent voltage spikes on an output of said switch.

46. A direct current (DC) voltage source that produces a DC output voltage, comprising:

a pulse width modulator (PWM), wherein said PWM comprises a switch for hard switching a DC input voltage at a controlled pulse width and duty cycle to produce pulses of energy;

a filter that filters said pulses of energy to produce the DC output voltage; and

a diode coupled to said filter, wherein

said PWM further comprises means for providing zero current switching at turn-on of said switch, thereby minimizing turn-on transition power dissipation.