Induction heating apparatus

The present invention relates to an induction heating apparatus. In order to cope with various types of containers without increasing an operating frequency of an induction heating apparatus, the present invention compares a resistance value of a container with a predetermined reference resistance value, and determines an operating mode of a switching device according to a result of the comparison. According to the present invention, it is possible to use various types of containers without increasing an operating frequency of an induction heating apparatus, by adjusting a resonance frequency of a working coil according to a resistance value of a container used in the induction heating apparatus.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a U.S. National Stage Application under 35 U.S.C. § 371 of PCT Application No. PCT/KR2018/011797, filed Oct. 5, 2018, which claims priority to Korean Patent Application No. 10-2017-0131690, filed Oct. 11, 2017, whose entire disclosures are hereby incorporated by reference.

1. TECHNICAL FIELD

The present disclosure relates to an induction heating apparatus.

2. BACKGROUND ART

Various types of cooking apparatuses are used to heat food at homes and restaurants. Conventionally, gas stoves that are fueled by gas have been widely used. However, in recent years, apparatuses for heating an object subject to heating such as a cooking container, e.g., a pot, which use electricity instead of gas, have been used.

Processes for heating an object using electricity are largely classified into resistance heating and induction heating. Electric resistance heating is a process in which an object subject to heating is heated by delivering heat, generated when electric current flows in a metallic resistance wire or a non-metallic heating element such as silicon carbide, to the object through radiation or conduction. Induction heating is a process in which an object itself subject to heating is heated by generating eddy currents in the object which consists of a metallic ingredient, using a magnetic field that is generated around a working coil when a predetermined magnitude of high-frequency electric power is supplied to the working coil.

The theory of induction heating is specifically described as follows. When an electric power source is supplied to an induction heating apparatus, a predetermined magnitude of high-frequency voltage is supplied to a coil. Accordingly, an induction magnetic field is generated around a working coil that is placed in the induction heating apparatus. When a magnetic line of force of the induction magnetic field that is generated as described above passes through the bottom of an object subject to heating, which includes a metallic ingredient and is placed on the induction heating apparatus, eddy currents are generated inside the bottom of the object. Then the generated eddy currents flow through the bottom of the object, and the object is heated.

When the induction heating apparatus is used, the upper plate of the induction heating apparatus is not heated while the object subject to heating is heated. Accordingly, when the object is lifted from the upper plate of the induction heating apparatus, the induction magnetic field around the coil vanishes, and the object stops generating heat immediately. Additionally, the working coil of the induction heating apparatus does not generate heat. Accordingly, temperature of the upper place of the induction heating apparatus remains low while food is cooking, thereby making it possible to ensure safety.

Further, the induction heating apparatus heats the object itself that is subject to heating through induction heating, thereby making it possible to ensure high energy efficiency, compared to a gas stove or a resistance heating apparatus. Furthermore, the induction heating apparatus can heat an object that is subject to heating for a shorter period of time than other types of heating apparatuses. When power output of the induction heating apparatus is higher, the induction heating apparatus can heat an object subject to heating more quickly.

In general, a conventional induction heating apparatus has an operating frequency of 70 kHz or less to reduce switching losses that are caused by electromagnetic interference (EMI) when the induction heating apparatus is driven and that are caused by switching operations of a switching device in the induction heating apparatus.

When the induction heating apparatus is used to heat a container that consists of a material such as aluminum with a relatively low resistance value, a relatively low voltage and a relatively large current are supplied to the working coil of the induction heating apparatus to generate electric power for heating. When a low voltage and a large current are supplied to the working coil of the induction heating apparatus, conduction loss occurs to the switching device, and an amount of heat generated by the working coil increases.

To solve the problem, when a container that consists of a material such as aluminum is used, an operating frequency may be increased to increase a resistance value of the container. However, the operating frequency of the induction heating apparatus is limitedly increased due to the switching losses of the switching devices.

DISCLOSURE

Technical Problem

The present disclosure provides an induction heating apparatus for which various types of containers may be used without increasing an operating frequency of the induction heating apparatus by adjusting a resonance frequency of a working coil based on a resistance value of the container which is used on the induction heating apparatus.

Objectives of the present disclosure are not limited to what has been described. Additionally, other objectives and advantages that have not been mentioned may be clearly understood from the following description and may be more clearly understood from embodiments. Further, it will be understood that the objectives and advantages of the present disclosure may be realized via means and a combination thereof that are described in the appended claims.

Technical Solution

As described above, the induction heating apparatus compares a resistance value of a container with a predetermined reference resistance value, and determines an operating mode of a switching device based on a result of the comparison, to be applied to various type of containers without increasing an operating frequency of the induction heating apparatus.

In the present disclosure, a switching device may operate in four modes and the like such as the triple frequency mode, the double frequency mode, the half-bridge mode, and the full-bridge mode.

In the triple frequency mode, a resonance frequency of a working coil of the induction heating apparatus is set to be three times as much as an operating frequency of the switching device. Additionally, in the double frequency mode, a resonance frequency of the working coil of the induction heating apparatus is set to be two times as much as an operating frequency of the switching device. A resonance frequency of the working coil is adjusted by adjusting capacitance of a variable capacitor unit that is included in an inverter unit.

Further, in the half-bridge mode and the full-bridge mode, a resonance frequency of the working coil is set to be the same as an operating frequency of the switching device. Magnitude of voltage that is delivered to the working coil, i.e., bridge voltage, when the induction heating apparatus operates in the half-bridge mode, is half the magnitude of bridge voltage when the induction heating apparatus operates in the full-bridge mode.

That is, according to the present disclosure, a resonance frequency of the working coil may be adjusted by adjusting capacitance of a variable capacitor unit that is included in an inverter unit, based on a resistance value of a container. Accordingly, the induction heating apparatus may be used for various types of containers with different resistance values without adjusting an operating frequency of the induction heating apparatus.

The induction heating apparatus according to an embodiment includes a rectifying unit that rectifies an AC voltage supplied by a power source and that outputs the rectified AC voltage, a smoothing unit that smooths the rectified AC voltage and that outputs a DC voltage, an inverter unit108that includes a first switching device, a second switching device, a third switching device, a fourth switching device, and a variable capacitor unit, that converts the DC voltage through switching operations and that outputs resonance current, a working coil that heats a container using the resonance current supplied by the inverter unit, and a control unit that compares a resistance value of the container with a predetermined reference resistance value, that determines an operating mode of the switching device based on results of the comparison, and that adjusts capacitance of the variable capacitor unit on the basis of the operating mode.

In an embodiment, the control unit may set the operating mode to a triple frequency mode when a resistance value of the container, which is measured in a state in which an operating frequency of a switching device included in the inverter unit is set to a predetermined first operating frequency, is less than a predetermined first reference resistance value. The control unit may adjust capacitance of the variable capacitor unit such that a resonance frequency of the working coil is three times as much as an operating frequency of the switching device.

Additionally, in an embodiment, the control unit may set the operating mode to a double frequency mode when a resistance value of the container, which is measured in a state in which an operating frequency of a switching device included in the inverter unit is set to a predetermined second operating frequency, is less than a predetermined second reference resistance value. The control unit may adjust capacitance of the variable capacitor unit such that a resonance frequency of the working coil is two times as much as an operating frequency of the switching device.

Further, in an embodiment, the control unit may set the operating mode to a half-bridge mode when a resistance value of the container, which is measured in a state in which an operating frequency of a switching device included in the inverter unit is set to a predetermined third operating frequency, is less than a predetermined third reference resistance value.

Further, in an embodiment, the control unit may set the operating mode to a full-bridge mode when a resistance value of the container, which is measured in a state in which an operating frequency of a switching device included in the inverter unit is set to a predetermined third operating frequency, is greater than or equal to a predetermined third reference resistance value.

Further, in an embodiment, the control unit may set the operating frequency to a predetermined restricted frequency when a resistance value of the container, which is measured in a state in which an operating frequency of a switching device included in the inverter unit is set to a predetermined third operating frequency, is greater than or equal to a predetermined fourth reference resistance value.

Furthermore, in an embodiment, the control unit may adjust capacitance of the variable capacitor unit such that a resonance frequency of the working coil is the same as an operating frequency of the switching device.

Advantageous Effects

An induction heating apparatus according to the present disclosure may be used for various types of containers without increasing an operating frequency of the induction heating apparatus by adjusting a resonance frequency of a working coil based on a resistance value of the containers used on the induction heating apparatus.

BEST MODE

The above-described objectives, features and advantages are specifically described hereunder with reference to the attached drawings. Accordingly, one having ordinary skill in the art may readily implement the technical spirit of the present disclosure. Further, in describing the present disclosure, publicly-known technologies in relation to the disclosure are not specifically described if they are deemed to make the gist of the disclosure unnecessarily vague. Below, embodiments are specifically described with reference to the attached drawings. In the drawings, identical reference numerals denote identical or similar elements.

FIG.1is a block diagram illustrating an induction heating apparatus according to an embodiment of the present disclosure.

Referring toFIG.1, an induction heating apparatus according to an embodiment of the present disclosure includes a rectifying unit104, a smoothing unit106, an inverter unit108, and a working coil (WC). Additionally, the induction heating apparatus may further include a detecting unit14.

The rectifying unit104rectifies AC input power that is supplied from a power source102, and outputs a rectified power source voltage. The rectifying unit104may include a plurality of diodes. For instance, the rectifying unit104may consist of a first diode (D1) and a second diode (D2) that are connected in series with each other, and a third diode (D3) and a fourth diode (D4) that are connected in series with each other.

The smoothing unit106smooths the power source voltage that is rectified by the rectifying unit104and outputs a DC voltage. The smoothing unit106may consist of an inductor (L) and a capacitor (C) that are connected in series with each other.

The inverter unit108includes a plurality of switching devices. In an embodiment of the present disclosure, the inverter unit108includes four switching devices, i.e., a first switching device (T1), a second switching device (T2), a third switching device (T3), and a fourth switching device (T4).

The first switching device (T1) and the second switching device (T2) are connected in series with each other, and are complementarily turned on and turned off by switching signals (S1, and S2) that are supplied by a below-described driving unit12. Likewise, the third switching device (T3), and the fourth switching device (T4) are connected in series with each other, and are complementarily turned on and turned off by switching signals (S3, and S4) that are supplied by the driving unit12.

The complementary turn-on and turn-off of the switching devices are referred to as switching operations. The inverter unit108converts the DC voltage that is supplied by the smoothing unit106to output an AC voltage, through switching operations of the switching devices (T1, T2, T3, and T4).

Additionally, the inverter unit108includes an inductor (Lr) and a variable capacitor unit (C1, C2, and C3) for converting the AC voltage that is output by switching operations of the switching devices into resonance current. The inductor (Lr) and variable capacitor unit (C1, C2, and C3) are connected in series with the working coil (WC), and resonance current is supplied to the working coil (WC) through resonance by the AC voltage that is supplied by switching operations of the switching devices.

A relay unit110that includes relays for optionally connecting each capacitor with an output terminal (a) between the first switching device (T1) and the second switching device (T2) is connected to the variable capacitor unit (C1, C2, and C3). As described below, the relays that are included in the relay unit110may be optionally opened or closed by a control unit10. Capacitance of the variable capacitor unit (C1, C2, and C3) may be determined on the basis of the number of the relays that are opened or closed by the control unit10.

In the present disclosure, capacitance of the variable capacitor unit (C1, C2, and C3) may be adjusted by adjusting the opening and closing of the relays included in the relay unit110by control of the control unit10. As described above, resonance frequencies of resonance current that flows in the working coil (WC) may be adjusted by adjusting capacitance of the variable capacitor unit (C1, C2, and C3).

FIG.1illustrates a variable capacitor unit that includes three capacitors (C1, C2, and C3) for convenience of description. However, the number of the capacitors, and the capacitance of each of the capacitors that constitute the variable capacitor unit may vary depending on embodiments. Additionally, the number of the relays that constitute the relay unit110may vary based on the number of the capacitors that constitute the variable capacitor unit.

The working coil (WC) heats a container that is placed around the working coil (WC), using resonance current that is supplied from the inverter unit108. When resonance current is supplied to the working coil (WC), eddy currents occur between the working coil (WC) and the container, a body of the container is heated, and contents in the container are heated.

The detecting unit14measures at least one of input current and input voltage that are supplied to the working coil (WC) and provides a result of the measurement to the control unit10. The control unit10may calculate a resistance value of the container that is currently placed around the working coil (WC), using the result of the measurement that is delivered by the detecting unit14.

The control unit10compares the resistance value of the container, which is calculated as described above, with a predetermined reference resistance value. The control unit10determines an operating mode of the switching devices (T1, T2, T3, and T4) that are included in the inverter unit10, on the basis of results of comparison between the resistance value of the container and the reference resistance value. In the present disclosure, the switching devices (T1, T2, T3, and T4) may operate in any one of the triple frequency mode, the double frequency mode, the half-bridge mode, and the full-bridge mode.

Additionally, the control unit10may determine capacitance of the variable capacitor unit (C1, C2, and C3) on the basis of the determined operating mode. The control unit10may control opening and closing of the relay unit110on the basis of the determined capacitance and may select a capacitor that will be connected between the output terminal (a) between the first switching device (T1) and the second switching device (T2), and the working coil (WC), among the capacitors (C1, C2, and C3) that constitute the variable capacitor unit.

Further, the control unit10delivers a control signal to the driving unit12on the basis of the determined operating mode. The driving unit12generates switching signals (S1, S2, S3, and S4) that are supplied to the switching devices (T1, T2, T3, and T4), based on the control signal that is delivered by the control unit10. Operating frequencies of the switching devices (T1, T2, T3, and T4) are determined according to waveforms of the switching signals (S1, S2, S3, and S4) that are generated by the driving unit12.

Below, a process during which the control unit10determines operating modes of the switching devices (T1, T2, T3, and T4), and operation of the induction heating apparatus in each operating mode are described with reference toFIGS.1to5.

When performance of a heating operation is asked by a user in a state in which a container is placed near the working coil (WC), the control unit10sets an operating frequency of the switching devices (T1, T2, T3, and T4) to a predetermined first operating frequency. In an embodiment, the first operating frequency may be set to be three times (3×fmin) as much as a minimum frequency (fmin). The minimum frequency (fmin) denotes a minimum value among operating frequencies that may drive the switching devices (T1, T2, T3, and T4) included in the induction heating apparatus.

The control unit10supplies switching signals to the switching devices (T1, T2, T3, and T4) through the driving unit12in a state in which the control unit10sets the operating frequency of the switching devices (T1, T2, T3, and T4) to the first operating frequency. When resonance current is supplied to the working coil (WC) through switching operations of the switching devices (T1, T2, T3, and T4), the control unit10calculates a resistance value of the container that is placed near the working coil (WC), using input voltage and input current that are detected by the detecting unit14.

The control unit10compares the calculated resistance value of the container with a predetermined first reference resistance value. In an embodiment, the first reference resistance value (Rpot,t,max) may be set as in the following formula.

In [Formula 1], Gmaxdenotes a ratio of input voltage to output voltage of the inverter unit108, i.e., a maximum voltage gain that is a maximum value among voltage gains, and Vindenotes a voltage value that is supplied by a power source102. Additionally, Prateddenotes maximum rated power of the induction heating apparatus.

When the resistance value of the container is less than the first reference resistance value, as a result of comparison between the calculated resistance value of the container and the first reference resistance value, the control unit10sets an operating mode to a triple frequency mode.

FIG.2illustrates a waveform of resonance current of the working coil, a waveform of bridge voltage (Vab), and a waveform of the switching signal (S1, S2, S3, and S4) when a drive mode of the switching devices of the induction heating apparatus according to an embodiment is a triple frequency mode. The bridge voltage (Vab) denotes a voltage value between the output terminal (a) and the output terminal (b), which is output by the switching operation of the switching devices (T1, T2, T3, and T4).

As described above, when the operating mode of the induction heating apparatus is determined as the triple frequency mode, the control unit10supplies a control signal to the driving unit12to output a switching signal (S1, S2, S3, and S4) of the waveform illustrated inFIG.2.

Further, the control unit10sets capacitance (Cr.t) of a variable capacitor unit as in the following formula such that resonance current is output three times during one period (P1) of the switching signal (S1, S2, S3, and S4), in other words, such that a resonance frequency of the working coil (WC) in which resonance current flows is three times as much as an operating frequency of the switching devices (T1, T2, T3, and T4), as illustrated inFIG.2.

In [Formula 2], fr.tdenotes a frequency that is three times as much as an operating frequency of the switching devices (T1, T2, T3, and T4), and Lrdenotes an inductance value of an inductor (Lr) illustrated inFIG.1.

The control unit10controls the relay unit110such that capacitance of the variable capacitor unit (C1, C2, and C3) is identical to the set capacitance (Cr.t) and selects a capacitor that will be connected. After setting the capacitance of the variable capacitor unit (C1, C2, and C3), the control unit10generates the switching signals (S1, S2, S3, and S4) as inFIG.2through the driving unit12and drives the switching devices (T1, T2, T3, and T4), and accordingly, a heating operation is performed.

When the resistance value of the container is greater than or the same as the first reference resistance value, as a result of comparison between the calculated resistance value of the container and the first reference resistance value, the control unit10sets an operating frequency of the switching devices (T1, T2, T3, and T4) to a predetermined second operating frequency. In an embodiment, the second operating frequency may be set to be two times (2×fmin) as much as the minimum frequency (fmin).

The control unit10supplies switching signals to the switching devices (T1, T2, T3, and T4) through the driving unit12in a state in which the control unit10sets the operating frequency of the switching devices (T1, T2, T3, and T4) to the second operating frequency. When resonance current is supplied to the working coil (WC) through switching operations of the switching devices (T1, T2, T3, and T4), the control unit10calculates a resistance value of the container that is placed near the working coil (WC), using input voltage and input current that are detected by the detecting unit14.

The control unit10compares the calculated resistance value of the container with a predetermined second reference resistance value. In an embodiment, the second reference resistance value (Rpot,d,max) may be set as in the following formula.

When the resistance value of the container is less than the second reference resistance value, as a result of comparison between the calculated resistance value of the container and the second reference resistance value, the control unit10sets an operating mode to a double frequency mode.

FIG.3illustrates a waveform of resonance current of the working coil, a waveform of bridge voltage (Vab), and a waveform of the switching signal (S1, S2, S3, and S4) when a drive mode of the switching devices of the induction heating apparatus according to an embodiment is a double frequency mode.

As described above, when the operating mode of the induction heating apparatus is determined as the double frequency mode, the control unit10supplies a control signal to the driving unit12to output a switching signal (S1, S2, S3, and S4) of the waveform illustrated inFIG.3.

Further, the control unit10sets capacitance (Cr.d) of a variable capacitor unit as in the following formula such that resonance current is output two times during one period (P2) of the switching signal (S1, S2, S3, and S4), in other words, such that a resonance frequency of the working coil (WC) in which resonance current flows is two times as much as an operating frequency of the switching devices (T1, T2, T3, and T4), as illustrated inFIG.3.

In [Formula 4], fr.ddenotes a frequency that is two times as much as an operating frequency of the switching devices (T1, T2, T3, and T4),

The control unit10controls the relay unit110such that capacitance of the variable capacitor unit (C1, C2, and C3) is identical to the set capacitance (Cr.d) and selects a capacitor that will be connected. After setting the capacitance of the variable capacitor unit (C1, C2, and C3), the control unit10generates the switching signals (S1, S2, S3, and S4) as inFIG.3through the driving unit12and drives the switching devices (T1, T2, T3, and T4), and accordingly, a heating operation is performed.

When the resistance value of the container is greater than or the same as the second reference resistance value, as a result of comparison between the calculated resistance value of the container and the second reference resistance value, the control unit10sets an operating frequency of the switching devices (T1, T2, T3, and T4) to a predetermined third operating frequency. In an embodiment, the third operating frequency may be set to the minimum frequency (fmin).

The control unit10supplies switching signals to the switching devices (T1, T2, T3, and T4) through the driving unit12in a state in which the control unit10sets the operating frequency of the switching devices (T1, T2, T3, and T4) to the third operating frequency. When resonance current is supplied to the working coil (WC) through switching operations of the switching devices (T1, T2, T3, and T4), the control unit10calculates a resistance value of the container that is placed near the working coil (WC), using input voltage and input current that are detected by the detecting unit14.

The control unit10compares the calculated resistance value of the container with a predetermined third reference resistance value. In an embodiment, the third reference resistance value (Rpot,h,max) may be set as in the following formula.

When the resistance value of the container is less than the third reference resistance value, as a result of comparison between the calculated resistance value of the container and the third reference resistance value, the control unit10sets an operating mode to a half-bridge mode.

FIG.4illustrates a waveform of resonance current of the working coil, a waveform of bridge voltage (Vab), and a waveform of the switching signal (S1, S2, S3, and S4) when a drive mode of the switching devices of the induction heating apparatus according to an embodiment is a half-bridge mode.

As described above, when the operating mode of the induction heating apparatus is determined as the half-bridge mode, the control unit10supplies a control signal to the driving unit12to output a switching signal (S1, S2, S3, and S4) of the waveform illustrated inFIG.4.

Further, the control unit10sets capacitance (Cr.h) of a variable capacitor unit as in the following formula such that a resonance frequency of the working coil (WC) in which resonance current flows is the same as an operating frequency of the switching devices (T1, T2, T3, and T4), as illustrated inFIG.4.

In [Formula 6], fr.hdenotes a frequency that is the same as an operating frequency of the switching devices (T1, T2, T3, and T4).

The control unit10controls the relay unit110such that capacitance of the variable capacitor unit (C1, C2, and C3) is identical to the set capacitance (Cr.h) and selects a capacitor that will be connected. After setting the capacitance of the variable capacitor unit (C1, C2, and C3), the control unit10generates the switching signals (S1, S2, S3, and S4) as inFIG.4through the driving unit12and drives the switching devices (T1, T2, T3, and T4), and accordingly, a heating operation is performed.

When the resistance value of the container is greater than or the same as the third reference resistance value, as a result of comparison between the calculated resistance value of the container and the third reference resistance value, the control unit10sets an operating mode to a full-bridge mode.

Then the control unit10compares the calculated resistance value of the container with a preset fourth reference resistance value. In an embodiment, the fourth reference resistance value (Rpot,f,max) may be set as in the following formula.

When the resistance value of the container is less than the fourth reference resistance value, as a result of comparison between the calculated resistance value of the container and the fourth reference resistance value, the control unit10drives the induction heating apparatus in the full-bridge mode.

FIG.5illustrates a waveform of resonance current of the working coil, a waveform of bridge voltage (Vab), and a waveform of the switching signal (S1, S2, S3, and S4) when a drive mode of the switching devices of the induction heating apparatus according to an embodiment is a full-bridge mode.

As described above, when the operating mode of the induction heating apparatus is determined as the full-bridge mode, the control unit10supplies a control signal to the driving unit12to output a switching signal (S1, S2, S3, and S4) of the waveform illustrated inFIG.5.

Further, the control unit10sets capacitance (Cr.f) of a variable capacitor unit as in the following formula such that a resonance frequency of the working coil (WC) in which resonance current flows is identical to an operating frequency of the switching devices (T1, T2, T3, and T4), as illustrated inFIG.5.

In [Formula 8], fr.fdenotes a frequency that is the same as an operating frequency of the switching devices (T1, T2, T3, and T4).

The control unit10controls the relay unit110such that capacitance of the variable capacitor unit (C1, C2, and C3) is identical to the set capacitance (Cr.f), and selects a capacitor that will be connected. After setting the capacitance of the variable capacitor unit (C1, C2, and C3), the control unit10generates the switching signals (S1, S2, S3, and S4) as inFIG.5through the driving unit12and drives the switching devices (T1, T2, T3, and T4), and accordingly, a heating operation is performed.

When the resistance value of the container is greater than or the same as the fourth reference resistance value, as a result of comparison between the calculated resistance value of the container and the fourth reference resistance value, the control unit10sets an operating frequency of the switching devices (T1, T2, T3, and T4) to a predetermined restricted frequency while setting an operating mode to the full-bridge mode. When the resistance value of the container is greater than or the same as the fourth reference resistance value, the induction heating apparatus may not operate at maximum power in the full-bridge mode. Accordingly, the control unit10limits an operating frequency of the switching devices (T1, T2, T3, and T4) to a restricted frequency, e.g., a resonance frequency of the working coil (WC).

FIG.6is a flow chart illustrating a control method of an induction heating apparatus according to an embodiment of the present disclosure.

When performance of a heating operation is asked by a user in a state in which a container is placed near a working coil (WC), a control unit10sets an operating frequency of switching devices (T1, T2, T3, and T4) to a predetermined first operating frequency (602).

The control unit10supplies switching signals to the switching devices (T1, T2, T3, and T4) through a driving unit12in a state in which the control unit10sets the operating frequency of the switching devices (T1, T2, T3, and T4) to the first operating frequency. When resonance current is supplied to the working coil (WC) through switching operations of the switching devices (T1, T2, T3, and T4), the control unit10detects a resistance value of the container that is placed near the working coil (WC), using input voltage and input current that are detected by the detecting unit14(604).

Next, the control unit10compares the detected resistance value of the container with a predetermined first reference resistance value (K1) (606). When the resistance value of the container is less than the first reference resistance value (K1) as a result of the comparison (606), the control unit10sets an operating mode of the switching devices (T1, T2, T3, and T4) to a triple frequency mode (608).

Accordingly, the control unit10delivers control signals to the driving unit12to generate switching signals corresponding to the triple frequency mode, controls a relay unit110such that a resonance frequency of the working coil (WC) is three times as much as the operating frequency of the switching devices (T1, T2, T3, and T4), and adjusts capacitance of a variable capacitor unit (C1, C2, and C3). After the triple frequency mode is set as described above, the control unit10supplies switching signals to the switching devices (T1, T2, T3, and T4) through the driving unit12, and performs a heating operation (632).

When the resistance value of the container is greater than or the same as the first reference resistance value (K1) as a result of the comparison (606), the control unit10sets the operating frequency of the switching devices (T1, T2, T3, and T4) to a second operating frequency (610), and supplies switching signals to the switching devices (T1, T2, T3, and T4) through the driving unit12. When resonance current is supplied to the working coil (WC) through switching operations of the switching devices (T1, T2, T3, and T4), the control unit10detects a resistance value of the container that is placed near the working coil (WC), using input voltage and input current that are detected by the detecting unit14(612).

Next, the control unit10compares the detected resistance value of the container with a predetermined second reference resistance value (K2) (614). When the resistance value of the container is less than the second reference resistance value (K2) as a result of the comparison (614), the control unit10sets the operating mode of the switching devices (T1, T2, T3, and T4) to a double frequency mode (616).

Accordingly, the control unit10delivers control signals to the driving unit12to generate switching signals corresponding to the double frequency mode, controls the relay unit110such that a resonance frequency of the working coil (WC) is two times as much as the operating frequency of the switching devices (T1, T2, T3, and T4), and adjusts capacitance of the variable capacitor unit (C1, C2, and C3). After the double frequency mode is set as described above, the control unit10supplies switching signals to the switching devices (T1, T2, T3, and T4) through the driving unit12, and performs a heating operation (632).

When the resistance value of the container is greater than or the same as the second reference resistance value (K2) as a result of the comparison (614), the control unit10sets the operating frequency of the switching devices (T1, T2, T3, and T4) to a third operating frequency (618), and supplies switching signals to the switching devices (T1, T2, T3, and T4) through the driving unit12. When resonance current is supplied to the working coil (WC) through switching operations of the switching devices (T1, T2, T3, and T4), the control unit10detects a resistance value of the container that is placed near the working coil (WC), using input voltage and input current that are detected by the detecting unit14(620).

Next, the control unit10compares the detected resistance value of the container with a predetermined third reference resistance value (K3) (622). When the resistance value of the container is less than the third reference resistance value (K3) as a result of the comparison (622), the control unit10sets the operating mode of the switching devices (T1, T2, T3, and T4) to a half-bridge mode (624).

Accordingly, the control unit10delivers control signals to the driving unit12to generate switching signals corresponding to the half-bridge mode, controls the relay unit110such that a resonance frequency of the working coil (WC) is the same as the operating frequency of the switching devices (T1, T2, T3, and T4), and adjusts capacitance of the variable capacitor unit (Ci, C2, and C3). After the half-bridge mode is set as described above, the control unit10supplies switching signals to the switching devices (T1, T2, T3, and T4) through the driving unit12, and performs a heating operation (632).

When the resistance value of the container is greater than or the same as the third reference resistance value (K3) as a result of the comparison (622), the control unit10sets the operating mode of the switching devices (T1, T2, T3, and T4) to a full-bridge mode (626). Then the control unit10compares the detected resistance value of the container with a predetermined fourth reference resistance value (K4) (628).

When the resistance value of the container is greater than or the same as the fourth reference resistance value (K4) as a result of the comparison (628), the control unit10sets the operating frequency of the switching devices (T1, T2, T3, and T4) to a predetermined restricted frequency, limits the operating frequency (630), and performs a heating operation (632).

When the resistance value of the container is less than the fourth reference resistance value (K4) as a result of the comparison (628), the control unit10supplies switching signals to the switching devices (T1, T2, T3, and T4) through the driving unit12, and performs a heating operation (632).

The present disclosure that is described above may be replaced, changed and modified in different ways by one having ordinary skill in the art to which the disclosure pertains without departing from the technical spirit of the disclosure. Thus, the disclosure should not be construed as being limited to the embodiments and the attached drawings set forth herein.