RF power supply

• US 6,521,874 B2
• Filed: 08/07/2001
• Issued: 02/18/2003
• Est. Priority Date: 07/10/1998
• Status: Expired due to Term

• Current Assignee: Ambrell Corporation (inTEST Corporation)
• Original Assignee: Ameritherm, Inc. (inTEST Corporation)
• Inventors: Thompson, Leslie L., Lincoln, Daniel J., Schwenck, Gary A.
• Primary Examiner(s): Leung, Philip H.
• Application Number:
  US09/922,655
• Publication Number:
  US 20010045427A1
• Time in Patent Office: 560 Days
• Field of Search: 219/663, 219/665, 219/666, 219/667, 219/661, 219/670, 219/650, 219/779, 363/95, 363/97, 363/17, 323/234, 323/265, 323/285, 323/293
• 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. In an induction heating system for heating a material, wherein the induction heating system comprises a circuit having a radiator for heating the material by inducing a current in the material and having a resonant frequency that changes while the material is being heated, a method for efficiently heating the material, comprising the steps of:

generating a signal having a frequency;

amplifying the signal;

providing the amplified signal to the circuit; and

during an entire heating cycle;

continuously measuring the power applied to the circuit and the power reflected from the circuit, and determining a ratio of the applied power and the reflected power; and

continuously altering the frequency of the signal provided to the circuit so that the frequency follows the resonant frequency of the circuit, wherein the frequency is altered based on determined ratios of the applied power and the reflected power.

2. A method of using an RF power supply to inductively heat a material, wherein the RF power supply applies an RF signal at a frequency in an RF range to a circuit, wherein the material is placed in close proximity to a portion of the circuit so that when the RF signal is applied to the circuit a current is induced in the material, comprising the steps of:

(1) determining an estimate of the resonant frequency of the circuit and setting the frequency of the RF signal to the estimated resonant frequency of the circuit;

(2) after performing step (1) then performing the steps of;

(2a) sensing applied RF power furnished from said power supply to the circuit and sensing reflected RF power that is reflected back to said power supply from said circuit; and

(2b) determining the ratio of said applied power to said reflected power;

(3) after performing step (2a) decreasing the frequency of the RF signal applied to the circuit;

(4) after performing step (3) then performing the steps of;

sensing applied RF power furnished from said power supply to the circuit;

sensing reflected RF power that is reflected back to said power supply from said circuit; and

determining the ratio of said applied power to said reflected power;

(5) determining whether the ratio determined in step (4) is greater than, equal to, or less than the ratio determined in step (2);

(6) increasing the frequency of the RF signal if it is determined that the ratio determined in step (4) is greater than or equal to the ratio determined in step (2); and

(7) decreasing the frequency of the RF signal if it is determined that the ratio determined in step (4) is less than the ratio determined in step (2).

3. In an induction heating system for heating a material, wherein the induction heating system comprises a circuit having a radiator for heating the material by inducing a current in the material and having a resonant frequency that changes while the material is being heated, a method for efficiently heating the material, comprising the steps of:

placing the material close enough to the radiator so that an electromagnetic field originating at the radiator will induce in the material a current that is capable of heating the material;

generating a signal having a frequency;

amplifying the signal;

providing the amplified signal to the circuit;

measuring the power applied to the circuit;

measuring the power reflected from the circuit; and

tracking the changes in the resonant frequency of the circuit while the material is being heated by either increasing or decreasing the frequency of the signal so that the frequency of the signal follows the resonant frequency of the circuit, wherein the decision as to whether to increase or decrease the frequency of the signal is based, at least in part, on measurements of both the applied and reflected power.

4. An induction heating system for heating a material, comprising:

a circuit, wherein the circuit has a resonant frequency that changes while the material is being heated;

a signal generator that generates a signal having a frequency;

an amplifier coupled between the circuit and the signal generator that amplifies the signal, wherein the amplified signal is provided to the circuit;

a forward power sensor that senses the applied power furnished to the circuit;

a reflected power sensor that senses the power that is reflected from the circuit; and

a processor coupled to the forward power sensor, the reflected power sensor, and the signal generator, wherein the processor controls the signal generator so that the frequency of the signal generated by the signal generator continuously tracks the resonant frequency of the circuit while the material is being heated, and wherein the processor determines a ratio of the applied and reflected power and uses the determined ratio in controlling the signal generator.

5. The induction heating system of claim 4, wherein the circuit includes a radiator for inducing a current in the material when the amplified signal is provided to the circuit.

6. The induction heating system of claim 5, wherein the radiator is an induction heating coil.

7. The induction heating system of claim 4, wherein the signal generator comprises a frequency synthesizer.

8. The induction heating system of claim 4, wherein the processor determines the ratio of the applied power to the reflected power and uses the determined ratio in controlling the signal generator.

9. The induction heating system of claim 4, wherein the processor determines the ratio of the reflected power to the applied power and uses the determined ratio in controlling the signal generator.

10. An induction heating system for heating a material, comprising:

a circuit, wherein the circuit has a resonant frequency that changes while the material is being heated;

a signal generating means for generating a signal having a frequency;

a signal amplification means, coupled between the circuit and the signal generator, for amplifying the signal, wherein the amplified signal is provided to the circuit;

a forward power sensing means for measuring the applied power furnished to the circuit;

a reflected power sensing means for measuring the power that is reflected from the circuit; and

a processing means, coupled to the forward power sensor, the reflected power sensor, and the signal generator, for controlling the signal generator so that the frequency of the signal generated by the signal generator continuously tracks the resonant frequency of the circuit while the material is being heated, wherein the processor means determines a ratio of the applied and reflected power and uses the determined ratio in controlling the signal generator.

11. The induction heating system of claim 10, wherein the circuit includes a radiator for inducing a current in the material when the amplified signal is provided to the circuit.

12. The induction heating system of claim 11, wherein the radiator is an induction heating coil.

13. The induction heating system of claim 10, wherein the signal generating means comprises a frequency synthesizer.

14. The induction heating system of claim 10, wherein the processing means determines the ratio of the applied power to the reflected power and uses the determined ratio in controlling the signal generator.

15. The induction heating system of claim 10, wherein the processing means determines the ratio of the reflected power to the applied power and uses the determined ratio in controlling the signal generator.

16. A method for tracking the resonant frequency of a circuit comprising the steps of:

(1) generating a signal having a frequency (F);

(2) providing the signal to the circuit;

(3) setting a direction flag to either a first value or a second value;

(4) measuring the power applied to the circuit and the power reflected from the circuit;

(5) if the direction flag is set to the first value, then decrease the frequency of the signal provided to the circuit to a new frequency (Fnew), where Fnew equals F minus an offset amount;

(6) if the direction flag is set to the second value, then increase the frequency of the signal provided to the circuit to a new frequency (Fnew), where Fnew equals F plus an offset amount;

(7) measuring the power applied to the circuit and the power reflected from the circuit;

(8) determining which of F and Fnew appears closer to the resonant frequency of the circuit, wherein the determination is based at least in part on the applied and reflected power as measured in steps (4) and (7);

(9) if F appears closer to the resonant frequency of the circuit and the direction flag is set to the first value, then set the direction flag to the second value, increase the frequency of the signal provided to the circuit to so that the frequency is equal to F, and return to step (4); and

(10) if F appears closer to the resonant frequency of the circuit and the direction flag is set to the second value, then set the direction flag to the first value, decrease the frequency of the signal provided to the circuit to so that the frequency is equal to F, and return to step (4).

17. The method of claim 16, wherein the step of determining which of F and Fnew appears closer to the resonant frequency of the circuit comprises the steps of:

calculating a ratio of the applied and reflected power measured in step (4);

calculating a ratio of the applied and reflected power measured in step (7);

and comparing the two calculated ratios.