System and method for providing RF power to a load

• US 6,271,508 B1
• Filed: 03/17/2000
• Issued: 08/07/2001
• 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:
  US09/528,182
• Time in Patent Office: 508 Days
• Field of Search: 219/663, 219/665, 219/666, 219/667, 219/661, 219/670, 219/650, 219/779, 323/234, 323/355, 323/364
• US Class Current: 219/663
• 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 tank circuit coupled to an output of the RF power supply, comprising:

a direct current (DC) voltage source;

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

an admittance sensor, coupled to the frequency controller and to the tank circuit, that produces a signal representative of the admittance of the tank circuit, wherein

said admittance signal is provided to said frequency controller; and

said frequency controller uses said admittance signal to modify said frequency of said alternating voltage so that said frequency of said alternating voltage tracks a resonant frequency of the tank circuit.

2. The RF power supply of claim 1, wherein said admittance sensor comprises

a current sensor that provides a current signal that represents the amount of the current provided to the tank circuit;

a voltage sensor that provides a voltage signal that represents the amount of voltage applied to the tank circuit; and

a divider that receives said voltage signal and said current signal and produces said admittance signal that represents the admittance of the tank circuit.

3. A method for inductively heating a workpiece, comprising the steps of:

generating an alternating voltage having a first frequency;

providing said alternating voltage to a tank circuit;

generating a first admittance signal that represents the admittance of the tank circuit at said first frequency;

setting said frequency of said alternating voltage to a second frequency;

generating a second admittance signal that represents the admittance of the tank circuit at said second frequency;

comparing the magnitude of said first admittance signal to said second admittance signal; and

modifying said frequency of said alternating voltage as a function of a result of said comparison so that said frequency of said alternating voltage tracks a resonant frequency of the tank circuit.

4. A method for inductively heating a workpiece placed in proximity to an inductor coil of a tank circuit, wherein the tank circuit has a resonant frequency, comprising the steps of:

applying an alternating voltage to the tank circuit, said alternating voltage having a frequency, and said alternating voltage having a first voltage level;

determining a coarse estimate of the resonant frequency of the tank circuit;

based on said coarse estimate, determining a fine estimate of the resonant frequency of the tank circuit;

setting said frequency of said alternating voltage to said fine estimate of said resonant frequency;

increasing said voltage level of said alternating voltage from said first voltage level to a second voltage level after setting said frequency of said alternating voltage to said fine estimate of said resonant frequency; and

while said voltage level is being increased and until a heat off indication is generated, modifying said frequency of said alternating voltage so that said frequency of said alternating voltage tracks the resonant frequency of the tank circuit.

5. The method of claim 4, wherein said step of determining a coarse estimate of the resonant frequency comprises the steps of:

setting said frequency of said alternating voltage to a first frequency;

storing a value representing said first frequency;

determining an admittance value of the tank circuit at said first frequency;

changing said frequency of said alternating voltage by an offset amount;

determining an admittance value of the tank circuit at the new frequency;

comparing said admittance value of the tank circuit at the new frequency to said admittance value of the tank circuit at said first frequency;

if the tank circuit is a series resonant tank circuit and said admittance value of the tank circuit at the new frequency is greater than said admittance value of the tank circuit at said first frequency, storing a value representing said new frequency of said alternating voltage;

if the tank circuit is a parallel resonant tank circuit and said admittance value of the tank circuit at the new frequency is less than said admittance value of the tank circuit at said first frequency, storing a value representing said new frequency of said alternating voltage.

6. The method of claim 4, wherein said step of modifying said frequency of said alternating voltage so that said frequency of said alternating voltage tracks the resonant frequency of the tank circuit 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 (2), said admittance determined in step (4), and said admittance determined in step (6).

7. The method of claim 4, wherein said step of modifying said frequency of said alternating voltage so that said frequency of said alternating voltage tracks the resonant frequency of the tank circuit 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).

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

(1) applying an alternating voltage to the tank circuit, said alternating voltage having a frequency;

(2) determining an admittance of the tank circuit;

(3) increasing said frequency of said alternating voltage;

(4) determining said admittance of the tank circuit;

(5) comparing said admittance determined in step (2) with said admittance determined in step (4);

(6) if said admittance determined in step (4) is less than said admittance determined in step (2), then increasing said frequency of said alternating voltage, otherwise decreasing said frequency of said alternating voltage.

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

(1) applying an alternating voltage to the tank circuit, said alternating voltage having a frequency;

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

(3) setting said frequency of said alternating voltage to F+, where F+ equals said 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 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 (2), said admittance determined in step (4), and said admittance determined in step (6).

10. A method for inductively heating a workpiece, comprising the steps of:

applying an alternating voltage to a tank circuit, the alternating voltage having a frequency and a first voltage level, the tank circuit having an initial resonant frequency;

determining a coarse estimate of the initial resonant frequency of the tank circuit;

based on the coarse estimate, determining a fine estimate of the initial resonant frequency of the tank circuit;

setting the frequency of the alternating voltage to the fine estimate of the resonant frequency;

increasing the voltage level of the alternating voltage from the first voltage level to a second voltage level after determining the fine estimate of the resonant frequency; and

while the voltage level is being increased and until a heat off indication is received, modifying the frequency of the alternating voltage so that the frequency of the alternating voltage tracks the resonant frequency of the tank circuit.

11. The method of claim 10, further comprising the step of ceasing applying the alternating voltage to the tank circuit after receiving the heat off indication.