Abstract: An electromagnetic cooking device is provided having a controller and a plurality of RF feeds configured to introduce electromagnetic radiation into an enclosed cavity to heat up a food load. The controller is configured to: select a heating target; generate a heating strategy to determine a sequence of desired heating patterns; cause the RF feeds to output an RF signal to thereby excite the enclosed cavity; monitor the created heating patterns to measure resonances in the enclosed cavity and store a map of efficiency in frequency and phase domains from which the controller identifies resonant modes and Q-factors associated therewith; continue to monitor the created heating patterns and store maps of efficiency in the frequency and phase domains until a specified change is detected in at least one Q-factor; and when the specified change in the at least one Q-factor is identified, stop cooking the food load.

Abstract: This disclosure relates to determining a material transition point such as a melt-point, and to determining an annealing parameter based on the determined material transition point. Changes in a parameter associated with an electromagnetic circuit coupled to an object subject to heating are monitored. A material transition point is determined upon detecting a predetermined change in the parameter. The annealing parameter is derived from the determined material transition point.

Abstract: Systems and methods for real-time control of temperature within a plasma chamber are described. One of the methods includes sensing a voltage in real time of a rail that is coupled to a voltage source. The voltage source supplies a voltage to multiple heater elements of the plasma chamber. The voltage that is sensed is used to adjust one or more duty cycles of corresponding one or more of the heater elements. The adjusted one or more duty cycles facilitate achieving and maintaining a temperature value within the plasma chamber over time.

Abstract: A Transistor Resonant Characteristic Sensor (TReCS) includes a sensing element positioned along electronic equipment so that the sensing element is electromagnetically coupled to the electronic equipment. The sensing element includes a coil. The sensing element is configured to detect magnetic oscillations associated with a characteristic signal generated by the electronic equipment. The TReCS sensor further includes an evaluation circuit connected to the sensing element for monitoring health state of the electronic equipment. The evaluation circuit includes one or more processing elements configured to diagnose health state of the electronic equipment based on extracted baseband information associated with the characteristic signal.

Abstract: An induction cooking system. The induction cooking system includes a base, one or more side walls, an induction coil, and an infrared temperature sensor. The base includes a base surface associated therewith. The base surface includes a window. The window is disposed within the base surface. The one or more side walls define a well above the base surface. The well is configured to receive a vessel disposed above the base surface. The induction coil is disposed within the base. The induction coil defines a first surface that is disposed below the base surface. The induction coil also defines a second surface that is disposed opposite from the first surface. The induction coil further defines an aperture disposed adjacent to the window and extending from a first surface toward a second surface of the induction coil. The infrared temperature sensor is disposed adjacent to the window and within the aperture.

Abstract: A power converter circuit and an associated method of converting an AC power supply. The power converter circuit comprises: a supply rectifier circuit (2) for rectifying an AC supply power to generate a rectified supply power; an inverter circuit (3) for receiving the rectified supply power to generate an inverted supply power; a load rectifier circuit (4) for rectifying the inverted supply power to generate a rectified load power for supplying a load current to a load (5); and a boost circuit (6) driven by the load current to provide a boosted voltage to the rectified supply power.

Abstract: A system for magnetic characterization of an induction heating wire including a conductor having a first end and a second end longitudinally opposed from the first end, wherein the induction heating wire extends along a portion of the conductor and is electrically isolated from the conductor, an alternating current power source electrically coupled with the conductor to pass an electric current between the first end and the second end, a current sensor positioned to sense the electric current, a sensing wire including a first lead and an opposed second lead, wherein the sensing wire defines a first loop having a first polarity and a second loop having a second, opposite polarity, the second loop being connected to the first loop at a crossover, and wherein the induction heating wire extends through the first loop, and a voltage sensor positioned to sense a voltage across the first lead and the second lead.

Abstract: Induction heating apparatus and methods are presented for pipe end heating using a solenoid coil to heat a controlled length of a pipe workpiece by adjusting the relative positions of the solenoid coil with a flux diverter positioned relative to the pipe end to divert flux from the coil to control inductive heating of the pipe workpiece end.

Abstract: An induction heating device and method are disclosed. The device may include a support plate and a coil assembly and a plurality of ferrite bars or the like interposed between the coil assembly and the support plate. Additionally, a central polymeric element may be directly in contact with and interposed between the coil assembly and the support plate. The central element may have a plurality of radial seats configured for engagement with at least one end of the ferrite bars, the seats are configured to locate the bars in a predetermined position.

Abstract: A control circuit (24) includes a calculation means (28) which determines ON time and OFF time of a main switching element (14) and a drive pulse generating means (30) which generates drive pulses that turn the main switching element (14) ON and OFF. A control function formula which prescribes a relationship between an output voltage (Vo) and an output differential value (Vd) by a negative linear function and the like is defined in the calculation means (28). The calculation means (28) samples an input voltage signal (Vi) and an output voltage signal (Vo) at a timing synchronized with a switching cycle of the main switching element (14), and calculates the ON time and OFF time of the main switching element (14) thereafter so as to satisfy the control function formula. The drive pulse generating means (30) generates drive pulses (V14) which turn the main switching element (14) ON and OFF on the basis of the ON and OFF time determined by the calculation means (28).

Abstract: A high-voltage gate driver circuit configured to drive a high-side power switch and a low-side power switch includes an active dv/dt triggered ESD protection circuit coupled between a protected node and a power rail node. The active dv/dt triggered ESD protection circuit includes a dv/dt circuit controlling an ESD protection transistor connected between the protected node and the power rail node. The ESD protection transistor is turned on when an ESD event occurs at the protected node to conduct ESD current from the protected node to the power rail node. The dv/dt circuit is charged up after a time constant to disable the ESD protection transistor.

Abstract: The image forming device includes a reception unit, an image forming unit, a switching unit, a detection unit, and a charging unit. The switching unit switches the image forming device between a first mode in which the reception unit is not supplied a voltage from a power source, is supplied power from a secondary battery, and receives image forming requests, and a second mode in which the image forming unit is supplied the voltage and performs image forming. The detection unit detects a value of an amount of energy of the secondary battery. In the second mode, the charging unit charges the secondary battery by supplying the voltage to the secondary battery when the value is equal to or less than a threshold value, and the charging unit boosts the voltage and supplies the boosted voltage to the secondary battery when the value is greater than the threshold value.

Abstract: In some aspects of the invention, a system for operating a plurality of plasma and/or induction heating processing systems includes an operating unit that has a display device on which a graphic user interface can be displayed, at least two power generators that supply power to a plasma process or an induction heating process, and a network that connects the operating unit to the power generators to transmit signals between the operating unit and the power generators. The graphic user interface includes a static region and a dynamic region, and a selection device for selecting information to be displayed in the dynamic region.

Abstract: An image forming apparatus uses a switching element to switch current flowing to an excitation coil, so that a heating layer in a fixing member produces heat that fixes an image to a transported sheet. Based on a change in temperature of the switching element over time, a prediction unit determines, at a predetermined first time point, a predicted temperature of the switching element at a second time point at which a tip of the sheet is scheduled to arrive at the fixing member. When the predicted temperature is at least a predetermined value, a control unit controls power supplied to the excitation coil by restricting switching of the switching element and lifting the restriction, so that by the second time point the detected temperature of the fixing member reaches a temperature necessary for fixing.

Abstract: The present invention relates to a contactlessly chargeable heater. In one example, the contactlessly chargeable heater includes: an induction patch emitting magnetic field by means of current applied to an inner coil; and a heating body, receiving the magnetic field from the charging patch to perform a contactless charging operation, and including a conductive metal yarn for emitting heat via the power generated through the contactless charging operation. The conductive metal yarn is woven together using threads within the heating body.

Abstract: Apparatus and process for heating and melting a material in a susceptor vessel are provided wherein phase synchronized ac voltage is supplied from a separate power source to each one of at least two induction coils in separate zones around the vessel. Power magnitude from each source to an induction coil is controlled by pulse width control of the source's output voltage. Output frequency from each source is either fixed or variable based upon the electrically conductive state of the material. Optional electromagnetic stirring is achieved by establishing a phase shift between the voltage outputs of the power supplies after the material in the susceptor vessel has melted.

Abstract: The present disclosure provides systems and methods for protecting switching elements in an induction heating system. A switching power loss associated with a switching element of the induction heating system can be calculated and an operating frequency of the induction heating system can be adjusted based upon the switching power loss. According to one aspect, the switching power loss can be classified into one of a plurality of threat zones based upon the magnitude of the switching power loss and the operating frequency can be adjusted based upon the threat zone into which the switching power loss is classified. According to another aspect, the switching power loss can be calculated based at least in part on a duty cycle of an output signal. The duty cycle of the output signal can provide an indication of the proximity of the operating frequency of the induction heating system to resonance.

Abstract: A fixing apparatus includes an induction heating coil configured to heat a heat generating member including a conductive heating element, a boosting circuit configured to boost a DC voltage obtained by rectifying AC power, a switching element configured to input a DC voltage boosted by the boosting circuit and to supply a high-frequency current to the induction heating coil, a driving circuit configured to drive the switching element, a temperature detection unit configured to detect a temperature of the heat generating member, and a control unit configured to control power supplied to the induction heating coil by controlling a boosting ratio of the boosting circuit and a driving frequency of the switching element by the driving circuit so that the temperature detected by the temperature detection unit reaches a target temperature.

Abstract: A rethermalizing station includes a well defined by an exterior wall, a food pan configured to be inserted into the well and to hold a food item, a first induction coil surrounding the exterior wall of the well, the first induction coil configured to warm the food item via inductive heating of the food pan, a first temperature sensor configured to detect a temperature of the food pan, and a control unit coupled to the first induction coil and the first temperature sensor, the control unit configured to control the first induction coil in response to the temperature of the food pan detected by the first temperature sensor such that temperature of the food pan is maintained at a targeted temperature.

Abstract: An induction heating apparatus is capable of stop heating without excessively boosting output voltages of a booster circuit and a power factor correction circuit. The induction heating apparatus includes a boosting function unit, an inverter circuit, and a booster circuit controller. The boosting function unit includes a power factor correction circuit and a booster circuit, and boosts an input direct-current power to a direct-current voltage having a peak value larger than the peak value of the input direct-current power by turning on/off a switching element. The inverter circuit includes a heating coil, and inputs the direct-current voltage output by the boosting function unit to generate a high frequency current in the heating coil by turning on or off a different switching element. The booster circuit controller stops a boosting operation of the boosting function unit without a prescribed delay from the stop of an operation of the inverter circuit.

Abstract: An induction hob (100) is provided. The induction hob comprises a solid plate (105), a plurality of electrically activatable coil members (1101-1108) arranged underneath the plate, said coil members defining corresponding cooking zones (1151-1158) of the induction hob, and a control unit (135) configured to select power levels for the cooking zones. In the solution according to one or more embodiments of the present invention, upon selection of first (P1) and second (P6) power levels for non-adjacent first (1151) and second (1156) cooking zones, respectively, the control unit is configured to automatically select for each intermediate cooking zone (1152-1155) between said first and second cooking zones a corresponding power level (P2-P5) obtained by interpolation of the first and second power levels with a predefined interpolating function.

Abstract: The purpose of the present invention is to minimize switching losses of an inverter. An induction heating device includes: a plurality of induction heating coils (20) which are disposed adjacent with each other; a plurality of inverters (30), each of which has a capacitor (40) serially connected to each of the induction heating coils (20), and converts a DC voltage into a square wave voltage; and a control circuit (15) which controls so as to align the phase of coil currents flowing though the plurality of the induction heating coils (20), wherein the control circuit (15) controls the timing at which the square wave voltage transitions such that an instantaneous value of the square wave voltage is preserved in either the DC voltage or a turnover voltage, when the coil current zero crosses.

Abstract: An induction heat treatment method with which temperature control is enabled, condition setting is easy, and the quality of a treatment object can be stabilized includes: a data acquiring step of heating and quench-hardening a sample of the treatment object to thereby acquire process data; a storing step of storing the process data; a checking step of checking the power supply output transition data and the quenching timing data as to validity based on the temperature transition data stored in the storing step; and a mass production step of performing heat treatment of the treatment object in accordance with the power supply output transition data and the quenching timing data stored in the storing step and checked as to validity in the checking step.

Abstract: A cooking appliance. The cooking appliance includes a cook top for a kitchen. One side of the cook top includes a cooking area with cooking areas. The cooking areas may be operated via an induction cooker. The bottom side of the cooking appliance has a display. The display can provide useful information thereon such as the time. The cook top can be arranged in a first horizontal position for use of the cooking areas, and folded to a second vertical position during non use to display information on the display. The cooking appliance also includes a data reader, such as an RFID reader or bar code scanner to obtain data from a food product package. The data may include suggested recipes, cooking temperatures, and/or cooking times. The obtained data may be used to assist in controlling the operation of the cooking areas.

Abstract: A heating apparatus, in particular a hob heating apparatus, with at least one heating connection for at least one heating element and at least one frequency unit. In order to provide a heating apparatus of the generic type with relatively high operational reliability, it is proposed that the heating apparatus comprises a protective unit, which is provided for determining the existence of a conducting path between the frequency unit and the at least one heating connection.

Abstract: An induction hardening monitoring apparatus (20) comprising: a current sensor (21) for detecting output current from a high-frequency inverter (11); a voltage sensor for detecting a voltage generated in a heating coil (14) connected between output terminals of the high-frequency inverters (11) together with a capacitor (12) in an equivalent circuit manner; and a controller (23) for monitoring a hardening processing based on a detection signal from the current sensor (21) and a detection signal from a voltage sensor (22), wherein the controller (23) monitors a hardening processing by calculating an effective value of output current from the high-frequency inverter (11) based on a detection signal from the current sensor (21) and calculating an effective value of voltage generated in a heating coil (14) based on a detection signal from the voltage sensor (22); or further calculating load impedance based on each effective value.

Abstract: An induction heating cooker includes a switch element and an inductive coil, which is coupled between a power voltage and a first terminal of the switch element. A second terminal of the switch element is coupled to a common voltage. A control circuit for controlling the inducting heating cooker includes first and second comparators and a pulse generator. The first comparator receives voltages of two terminals of the inductive coil and thus outputs a trigger signal. The second comparator receives a reference voltage and a voltage of the first terminal of the switch element, and enables a fading signal when the voltage of the first terminal is higher than the reference voltage. When the trigger signal is enabled, the pulse generator outputs a pulse to control the switch element. When the fading signal is enabled, the pulse generator reduces a pulse width of the pulse.

Abstract: An apparatus and process are provided for induction heating of a workpiece. The workpiece is moved through an inductor to inductively heat treat the workpiece with electric power of varying frequency and duty cycle or amplitude control to control the magnitude of electric power as the frequency changes. Alternatively the workpiece may be stationary and the inductor can be moved along the workpiece, or combined and coordinated movement of both the workpiece and inductor can be used.

Abstract: A method for detecting and compensating the effects of disturbances in controlling induction heating systems by monitoring the temperature of the cooking vessel or its content on the basis of at least one electrical measurement correlated to said temperature. An analysis in time and/or frequency domain of said electrical measurement is carried out in order to detect and compensate the effect of cooking vessel movements and/or mains line voltage fluctuation or similar electrical noises.

Abstract: A fixing apparatus includes an induction heating coil configured to heat a heat generating member including a conductive heating element, a boosting circuit configured to boost a DC voltage obtained by rectifying AC power, a switching element configured to input a DC voltage boosted by the boosting circuit and to supply a high-frequency current to the induction heating coil, a driving circuit configured to drive the switching element, a temperature detection unit configured to detect a temperature of the heat generating member, and a control unit configured to control power supplied to the induction heating coil by controlling a boosting ratio of the boosting circuit and a driving frequency of the switching element by the driving circuit so that the temperature detected by the temperature detection unit reaches a target temperature.

Abstract: An induction heating cooker and a control circuit therefor are provided. The cooker includes a switch element and an inductive coil, which is coupled between a power voltage and a first terminal of the switch element. A second terminal of the switch element is coupled to a common voltage. The control circuit includes first and second comparators and a pulse generator. The first comparator receives voltages of two terminals of the inductive coil and thus outputs a trigger signal. The second comparator receives a reference voltage and a voltage of the first terminal of the switch element, and enables a fading signal when the voltage of the first terminal is higher than the reference voltage. When the trigger signal is enabled, the pulse generator outputs a pulse to control the switch element. When the fading signal is enabled, the pulse generator reduces a pulse width of the pulse.

Abstract: An article of composite material includes a plurality of plies of material consolidated through the application of pressure and heating, in which each material ply is made by a resin matrix reinforced with fiber material. The article includes heating electrical resistance and temperature sensing devices embedded in the composite material, which are respectively placed in at least one interface zone between the material plies and are suitable to allow a temperature control of the article in service.

Abstract: An induction heating fuser unit including a series resonance circuit that includes an induction coil and condenser includes a phase comparing unit that compares a phase of a pulse for driving the series resonance circuit with a phase of a current flowing through the induction coil to obtain a phase difference; a resonant frequency following oscillation unit that changes an oscillation frequency such that a driving frequency of the series resonance circuit follows a resonant frequency of the series resonance circuit by using the phase difference obtained by the phase comparing unit; and a PWM signal generating unit that generates a pulse for driving the series resonance circuit based on the oscillation frequency. The phase comparing unit, the resonant frequency following oscillation unit, and the PWM signal generating unit are controlled digitally. Accordingly, PWM control is accomplished without considering a temperature change or a change of numerical value of component.

Abstract: In some aspects of the invention, a system for operating a plurality of plasma and/or induction heating processing systems includes an operating unit that has a display device on which a graphic user interface can be displayed, at least two power generators that supply power to a plasma process or an induction heating process, and a network that connects the operating unit to the power generators to transmit signals between the operating unit and the power generators. The graphic user interface includes a static region and a dynamic region, and a selection device for selecting information to be displayed in the dynamic region.

Abstract: In one exemplary embodiment, the induction heating system includes an induction heating power source. The induction heating power source is operable to identify an induction heating device coupled to the induction heating power source. The exemplary induction heating power source is operable to automatically limit power based on the identity of the induction heating device.

Abstract: The invention relates to a method for heating accumulator cells of an electrical energy accumulator, at least one first accumulator cell and at least one second accumulator cell being arranged in series. According to the invention, the first accumulator cell and the second accumulator cell alternately feed at least one inductive component which is electrically connected to an intermediate tap between the first and second accumulator cells. The invention also relates to a corresponding device.

Abstract: A supplying power adjusting apparatus has excellent temperature response and excellent stabilities to power supply change and load change. The apparatus is provided with a semiconductor inverter for high-speed switching power control, which converts a direct current rectified by a rectifying circuit (10) into alternating current power in response to a control signal and supplies a heater (7) with the power; a temperature change detecting circuit (24) for detecting the temperature change of the heater (7); a power supply change detecting circuit (22) for detecting the power change of the rectifying circuit (10); a load change detecting circuit (23) for detecting the change of the alternating current power supplied to the heater (7); and a power control signal generating circuit (15) which calculates a power quantity to be supplied to the heater (7) and controls the frequency or duty ratio of a control signal to be added to the power controlling semiconductor inverter in response to the calculated results.

Abstract: To provide a heat treatment providing a product with quality as desired, high frequency induction heating treatment equipment capable of temperature control to provide such heat treatment includes temperature controlling means for adjusting in temperature an object to be treated, and quenching means for determining when the object heated should be cooled. The temperature controlling means includes heating means, temperature measuring means involved in temperature control, and temperature adjusting means. The quenching means includes temperature measuring means involved in quenching, and heat treatment adjusting means.

Abstract: An induction heating cooking device has an inverter including a resonant circuit, and a heating output control part. The resonant circuit has a resonant capacitor and a heating coil that is magnetically coupled to a load. The inverter has first and second switching elements. The heating output control part performs control by inverting the rate values of the driving periods of the first and second switching elements. The driving of the inverter is thus controlled so that substantially the same heating output is obtained to average the losses of the first and second switching elements.

Abstract: A method and apparatus is featured for providing linear control of a regulated electrical property (e.g., current or voltage) in a switch mode power supply of a thermal processing system that includes an inductive element and at least one switching element. The method and apparatus feature structure, or steps, for generating a nonlinear model that predict values for a regulated electrical property of the inductive element based on a given duty cycle of the at least one switching element and structure, or steps, for generating a nonlinear model that determines a duty cycle for the at least one switching element based on the nonlinear predictive model for the regulated electrical property, the nonlinear model determining the duty cycle such that a linear relationship results between the regulated electrical property and a selected value.

Abstract: A method for detecting and compensating the effects of disturbances in controlling induction heating systems by monitoring the temperature of the cooking vessel or its content on the basis of at least one electrical measurement correlated to said temperature. An analysis in time and/or frequency domain of said electrical measurement is carried out in order to detect and compensate the effect of cooking vessel movements and/or mains line voltage fluctuation or similar electrical noises.

Abstract: A PWM power regulator device has a processor, an input interface circuit, a zero phase detecting circuit and a thyristor. The input interface circuit is connected to a PWM output terminal of an external digital control circuit and the processor. The processor calculates a period and duty cycle of a PWM signal from the PWM output terminal and then outputs a triggering signal to the thyristor. The thyristor is triggered at different conduct angle according to the triggering signal and the triggering signal is determined by the processor according to the PWM signal. Therefore, the power regulator device is directly connected to the PWM output terminal of the digital control circuit without digital to analog converter.

Abstract: A heating device for a field-device display module includes a heating element. The heating element is adapted to the shape of the field-device display module and is equipped such that it converts an electrical current to thermal energy. The heating element can be coupled to the field-device display module such that by the heating element the field-device display module can be heated.

Abstract: An improved system and methods useful for the cooking of one or more food items by the application of a non-ambient temperature developed within a temperature retentive element. The temperature retentive element can be sized and shaped to form a cooking box in which cooking at a temperature relative to the non-ambient temperature may proceed and from which the food item, when fully cooked, may be served.

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.

Abstract: The invention relates to a method for controlling an induction cooking appliance with at least one coil, wherein the power of the coil is adjusted as a function of a position of a cooking utensil on the coil. The invention also relates to an induction cooking appliance for heating a cooking utensil, which has at least one coil and a drive unit for the coil, the induction cooking appliance being designed to implement the mentioned method.

Abstract: An induction heat treatment method with which temperature control is enabled, condition setting is easy, and the quality of a treatment object can be stabilized includes: a data acquiring step of heating and quench-hardening a sample of the treatment object to thereby acquire process data; a storing step of storing the process data; a checking step of checking the power supply output transition data and the quenching timing data as to validity based on the temperature transition data stored in the storing step; and a mass production step of performing heat treatment of the treatment object in accordance with the power supply output transition data and the quenching timing data stored in the storing step and checked as to validity in the checking step.

Abstract: The invention relates to an induction heating cooker and a control circuit therefor. The cooker includes a switch element and an inductive coil, which is coupled between a power voltage and a first terminal of the switch element. A second terminal of the switch element is coupled to a common voltage. The control circuit includes first and second comparators and a pulse generator. The first comparator receives voltages of two terminals of the inductive coil and thus outputs a trigger signal. The second comparator receives a reference voltage and a voltage of the first terminal of the switch element, and enables a fading signal when the voltage of the first terminal is higher than the reference voltage. When the trigger signal is enabled, the pulse generator outputs a pulse to control the switch element. When the fading signal is enabled, the pulse generator reduces a pulse width of the pulse.

Abstract: An induction heating cooker and a control method thereof. The induction heating cooker includes a plurality of heating coil groups, each of the heating coil groups including a plurality of heating coils connected in series, a plurality of inverters to individually supply high-frequency voltages to the heating coil groups, respectively, and a controller to control operations of the inverters such that the high-frequency voltages are supplied to the heating coil groups, respectively, based on the numbers of heating coils upon which at least one cooking container is placed within the respective heating coil groups. Therefore, it is possible to effectively heat the cooking container even though the container is put on any position of a cooking plate irrespective of a specific position of the cooking plate. Also, it is possible to effectively heat the container regardless of the size of the container.

Abstract: An induction heating apparatus capable of stably operating at least one switching element contained therein is provided with a control circuit 5 which comprises a resonance waveform detector 6 for detecting a high frequency AC waveform supplied from an inverter circuit 3 to a heating coil 4 to produce a detection signal DS1 corresponding to high frequency AC power waveform; a phase comparator 8 for producing an adjusting signal PH corresponding to a phase difference between detection signal DS1 from resonance waveform detector 6 and drive signal D1 from drive circuit 7; and an addition circuit 13 for superimposing the drive signal D1 from drive circuit 7 on detection signal DS1 from resonance waveform detector 6 to supply to phase comparator 8the superimposed signal on a level same as or over operation threshold value VTH for the phase comparator 8.