Patent Publication Number: US-2022232673-A1

Title: Induction heating apparatus and method for controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2021-0008193, filed in Korea on Jan. 20, 2021, and Korean Patent Application No. 10-2022-0006854, filed in Korea on Jan. 17, 2022, the disclosure of which are incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Disclosed herein are an induction heating apparatus and a method for controlling the same. 
     2. Background 
     Induction heating apparatuses are devices that generate eddy current in a metallic container and heat the container, using a magnetic field generated around a working coil. When an induction heating apparatus is driven, alternating current is supplied to the working coil. An induced magnetic field is generated around the working coil disposed in the induction heating apparatus. When magnetic line of force of the induced magnetic field generated passes through a bottom of the metallic container over the working coil, eddy current is generated inside the bottom of the container. Accordingly, the eddy current generated flows in the container, and the container itself is heated. 
       FIG. 1  is a view showing a waveform of alternating current supplied to a working coil when a container is heated by an induction heating apparatus of the related art. In  FIG. 1 , time point P 1  is a time point at which a power level of a heating zone of the induction heating apparatus is set, an instruction to initiate heating is input, and current starts to flow in the working coil corresponding to the heating zone. 
     When a user sets a power level and inputs an instruction to initiate heating in a state in which a container is placed in (or provided on) the heating zone, an inverter circuit generates alternating current as a switching signal is supplied to the inverter circuit electrically connected to the working coil, at time point P 1 . Accordingly, the alternating current generated by the inverter circuit is supplied to the working coil. 
     After the AC current starts to be supplied to the working coil at the time point P 1 , the peak values of the AC current supplied to the working coil until the time point P 2  are maintained at IA 1  and IA 2 , respectively. Accordingly, the amount of change in the alternating current supplied to the working coil (i.e., a peak-to-peak value) is maintained at IA 1 -IA 2  in the period of P 1 -P 2 . 
     After the alternating current starts to be supplied to the working coil at time point P 1 , a controller of the induction heating apparatus controls the inverter circuit to match an output power value of the working coil and a required power value corresponding to the power level set by the user. In  FIG. 1 , time point P 3  is a time point at which the working coil outputs power having the same value as the required power value. 
     By the way, while the output power value of the working coil is adjusted, a range of the alternating current supplied to the working coil can increase temporarily. For example, in the period of P 2 -P 3  of  FIG. 1 , the peak value of the alternating current supplied to the working coil increases to IA 3  greater than IA 1  and decreases to IA 4  less than IA 2 . Accordingly, a change in the amount of the alternating current supplied to the working coil in the period of P 2 -P 3  is IA 3 -IA 4 . The change of IA 3 -IA 4  is greater than the change of IA 1 -IA 2  of the alternating current supplied to the working coil in the period of P 1 -P 2 . 
     When the output power value of the working coil matches the required power value at time point P 3 , the peak value of the alternating current supplied to the working coil is maintained at IA 5  and IA 6  respectively. 
     As magnitude of the current supplied to the working coil increases, noise generated by the working coil increases. In the related art, during an initial process in which the induction heating apparatus starts a heating operation, the peak-to-peak value of the current supplied to the working coil can increase excessively and instantly in the period (e.g., period P 2 -P 3 ) that is earlier than the time point at which the output power value of the working coil matches the required power value. 
     As described above, when the instruction to initiate heating is input and the container starts to be heated, an excessively loud noise can be generated due to an instant increase in the peak-to-peak value of the current supplied to the working coil, as described above. The initial driving noise can cause inconvenience to the user each time the user uses the induction heating apparatus, and in some cases, the user can wrongly think that the induction heating apparatus fails. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Arrangements and embodiments may be described in detail with reference to the following drawings in which like reference numerals refer to like elements and wherein: 
         FIG. 1  is a view showing a waveform of alternating current supplied to a working coil when a container is heated by an induction heating apparatus of the related art; 
         FIG. 2  is an exploded perspective view showing an induction heating apparatus in one embodiment; 
         FIG. 3  is a circuit diagram of the induction heating apparatus in one embodiment; 
         FIG. 4  is a graph showing a relationship between a driving frequency of an inverter circuit and an output power value of a working coil in one embodiment; 
         FIG. 5  is a view showing a waveform of each of the switching signals and a waveform of current supplied to the working coil when duty cycles of the switching signals input to switching elements belonging to a first group are set to 0.05, in one embodiment; 
         FIG. 6  is a view showing a waveform of each of the switching signals and a waveform of current supplied to the working coil when the duty cycles of the switching signals input to the switching elements belonging to the first group are set to 0.25, in one embodiment; 
         FIG. 7  is a view showing a waveform of each of the switching signals and a waveform of current supplied to the working coil when the duty cycles of the switching signals input to the switching elements belonging to the first group are set to 0.5, in one embodiment; 
         FIG. 8  is a view showing a waveform of each switching signal and a waveform of current supplied to the working coil when the duty cycle of the switching signal input to each switching element is set to 0.05, in one embodiment; 
         FIG. 9  is a view showing a waveform of each switching signal and a waveform of current supplied to the working coil when the duty cycle of the switching signal input to each switching element is set to 0.25, in one embodiment; 
         FIG. 10  is a view showing a waveform of each switching signal and a waveform of current supplied to the working coil when the duty cycle of the switching signal input to each switching element is set to 0.5, in one embodiment; 
         FIG. 11  is a flow chart showing a method for controlling an induction heating apparatus in one embodiment; 
         FIG. 12  is a flow chart showing a method for controlling an induction heating apparatus in another embodiment; 
         FIG. 13  is a graph showing changes in a driving frequency of an inverter circuit and a duty cycle of a switching signal when the working coil is driven at a high power level in one embodiment; and 
         FIG. 14  is a graph showing changes in a driving frequency of an inverter circuit and a duty cycle of a switching signal when the working coil is driven at a low power level in one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The above-described aspects, features and advantages are specifically described hereunder with reference to the accompanying drawings such that one having ordinary skill in the art to which the present disclosure pertains can easily implement the technical idea of the disclosure. In the disclosure, detailed descriptions of known technologies in relation to the disclosure are omitted if they are deemed to make the gist of the disclosure unnecessarily vague. Below, preferred embodiments according to the disclosure are specifically described with reference to the accompanying drawings. In the drawings, identical reference numerals can denote identical or similar components. 
       FIG. 2  is an exploded perspective view showing an induction heating apparatus in one embodiment. An induction heating apparatus  10  in one embodiment may include a case  102  constituting a main body, and a cover plate  104  coupled to the case  102  and sealing the case  102 . 
     The cover plate  104  may be coupled to an upper surface of the case  102  and seal a space, formed inside the case  102 , from the outside. The cover plate  104  may include an upper plate  106  on which a container for cooking a food item is placed. In one embodiment, the upper plate  106  may be made of tempered glass such as ceramic glass. However, a material for the upper plate  106  may vary depending on embodiments. 
     Heating zones  12 ,  14  respectively corresponding to working coil assemblies  122 ,  124  may be formed on the upper plate  106 . For a user to recognize positions of the heating zones  12 ,  14  easily, lines or figures corresponding to the heating zones  12 ,  14  may be printed or displayed on the upper plate  106 . 
     The case  102  may be formed into a cuboid, an upper portion of which is open. The working coil assemblies  122 ,  124  for heating a container may be disposed in the space formed inside the case  102 . Additionally, an interface  114  may be disposed inside the case  102 , and the interface  114  may allow the user to supply power or be used to adjust a power level of each of the heating zones  12 ,  14 , and display information on the induction heating apparatus  10 . The interface  114  may be implemented as a touch panel that is capable of inputting and displaying information at the touch of the touch panel. However, an interface  114  having a different structure may be used depending on embodiments. 
     Additionally, on the upper plate  106 , a manipulation zone  118  may be disposed in a position corresponding to a position of the interface  114 . For the user&#39;s manipulation, characters or images and the like may be printed in the manipulation zone  118 , in advance. The user may perform desired manipulation by touching a specific point of the manipulation zone  118  with reference to the characters or images that are printed in the manipulation zone  118  in advance. Additionally, information output by the interface  114  may be displayed through the manipulation zone  118 . 
     The user may set a power level of each of the heating zones  12 ,  14  through the interface  114 . The power level may be displayed in the manipulation zone  118  as numbers (e.g., 1, 2, 3, . . . , 9). When a power level of each of the heating zones  12 ,  14  is set, a required power value and a heating frequency of a working coil corresponding to each of the heating zones  12 ,  14  may be determined. A controller may drive each working coil, based on the determined heating frequency, such that an actual output power value of each working coil matches a required power value set by the user. 
     A power supply  112  (or power supply device) for supplying power to the working coil assemblies  122 ,  124  or the interface  114  may be disposed in the space formed inside the case  102 . 
     In the embodiment of  FIG. 2 , two working coil assemblies (i.e., a first working coil assembly  122  and a second working coil assembly  124 ) are disposed inside the case  102 , for example. However, three or more working coil assemblies may be disposed inside the case  102 , depending on embodiments. 
     The working coil assemblies  122 ,  124  may include a working coil that forms an induced magnetic field using a high-frequency alternating current supplied by the power supply unit  112 , and an insulating sheet for protecting a coil from heat generated by a working coil forming an induced magnetic field. In  FIG. 2 , the first working coil assembly  122  may include a first working coil  132  for heating a container placed in a first heating zone  12 , and a first insulating sheet  130 , for example. Additionally, though not illustrated, the second working coil assembly  124  may include a second working coil and a second insulating sheet. Depending on embodiments, the insulating sheet may not be provided. 
     Each of the working coils may be provided with a temperature sensor, in a central portion thereof. In  FIG. 4 , a temperature sensor  134  may be disposed in a central portion of the first working coil  132 , for example. The temperature sensor may measure a temperature of a container placed in each of the heating zones. In one embodiment, the temperature sensor may be a thermistor temperature sensor having a variable resistance whose resistance value changes according to the temperature of the container, but the type of the temperature sensor is not limited thereto. 
     In one embodiment, the temperature sensor may output a sensing voltage corresponding to a temperature of a container, and the sensing voltage output from the temperature sensor may be delivered to the controller. The controller may ascertain the temperature of the container, based on magnitude of the sensing voltage output from the temperature sensor, and when the temperature of the container is a predetermined reference value or greater, may perform an overheat preventing operation by decreasing an actual power value of a working coil or stopping driving of a working coil. 
     Furthermore, though not illustrated in  FIG. 2 , a substrate may be disposed in the space formed inside the case  102 , and a plurality of circuits or elements including the controller may be mounted onto the substrate. The controller may drive each of the working coils according to the user&#39;s instruction to initiate heating, input through the interface  114 , to perform a heating operation. When the user inputs an instruction to end heating through the interface  114 , the controller may stop the driving of the working coils to end the heating operation. 
       FIG. 3  is a circuit diagram of the induction heating apparatus in one embodiment. The induction heating apparatus  10  in one embodiment may include a rectifying circuit  202 , smoothing circuits L 1 , C 1 , an inverter circuit  204 , a working coil  132 , a controller  2 , and a driving circuit  22 . 
     The rectifying circuit  202  may include a plurality of diode elements D 1 , D 2 , D 3 , D 4 . The rectifying circuit  202  may be a bridge diode circuit, as illustrated in  FIG. 3 , and/or may be another circuit depending on embodiments. The rectifying circuit  202  may rectify AC voltage supplied by a power supply device  20  (or by a power supply) and output voltage having a pulse waveform. 
     The smoothing circuits L 1 , C 1  may smooth the voltage rectified by the rectifying circuit  202  and output DC link voltage. The smoothing circuits L 1 , C 1  may include a first inductor L 1  and a DC link capacitor C 1 . 
     The inverter circuit  204  may include a first switching element SW 1 , a second switching element SW 2 , a third switching element SW 3 , and a fourth switching element SW 4 . Other numbers of switching elements may also be include as part of the inverter circuit  204 . 
     As illustrated in  FIG. 3 , the inverter circuit  204  of the induction heating apparatus  10  in one embodiment may be implemented as a full-bridge circuit including four switching elements SW 1 , SW 2 , SW 3 , SW 4 . In another embodiment, the inverter circuit  204  may be implemented as a half-bridge circuit including two switching elements (i.e., a first switching element SW 1  and a second switching element SW 2  in  FIG. 3 ). 
     The first switching element SW 1 , the second switching element SW 2 , the third switching element SW 3  and the fourth switching element SW 4  may be respectively turned on and turned off by a first switching signal S 1 , a second switching signal S 2 , a third switching signal S 3  and a fourth switching signal S 4 . Each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  may be turned on when each of the switching signals S 1 , S 2 , S 3 , S 4  is at a high level, and turned off when each of the switching signals S 1 , S 2 , S 3 , S 4  is at a low level. 
     In  FIG. 3 , each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  is an IGBT element, for example. However, each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  may be another type of switching element (e.g., a BJT or an FET and the like), depending on embodiments. 
     Any of the switching elements SW 1 , SW 2 , SW 3 , SW 4  may be turned on and turned off complementarily. For example, in any operation mode, the second switching element SW 2  may be turned off (turned on) while the first switching element SW 1  is turned on (turned off). The switching elements that are turned on and turned off complementarily are referred to as ‘complementary’ switching elements. 
     Any of the switching elements SW 1 , SW 2 , SW 3 , SW 4  may be turned on and turned off identically. For example, in any operation mode, the first switching element SW 1  and the third switching element SW 3  may be turned on and turned off with the same timing. The switching elements that are turned on and turned off with the same timing are referred to as switching elements ‘belonging to the same group’. 
     The first switching element SW 1  and the third switching element SW 3  may be referred to as switching elements belonging to a first group (i.e., a high side), and the second switching element SW 2  and the fourth switching element SW 4  may be referred to as switching elements belonging to a second group (i.e., a low side). 
     If the inverter circuit  204  is implemented as a half-bridge circuit (i.e., a circuit only including the first switching element SW 1  and the second switching element SW 2 ) in another embodiment, the first switching element SW 1  may be a switching element belonging to the first group, and the second switching element SW 2  may be a switching element belonging to the second group. 
     DC link voltage input to the inverter circuit  204  may be converted into alternating current as a result of the turn-on and turn-off operations (i.e., a switching operation) of the switching elements SW 1 , SW 2 , SW 3 , SW 4  included in the inverter circuit  204 . The alternating current converted by the inverter circuit  204  may be supplied to the working coil  132 . As a resonance phenomenon occurs in the working coil  132 , eddy current may flow in a container, and the container may be heated. 
     Each of the first switching element S 1 , the second switching element S 2 , the third switching element S 3  and the fourth switching element S 4  may be a pulse width modulation (PWM) signal having a predetermined duty cycle. 
     When the alternating current output from the inverter circuit  204  is supplied to the working coil  132 , the working coil  132  may be driven. As a result of the driving of the working coil  132 , a container placed over (or provided on) the working coil  132  may be heated while eddy current flows in the container. During the driving of the working coil  132 , magnitude of thermal energy supplied to the container may vary depending on magnitude of power actually generated as a result of the driving of the working coil (i.e., an actual output power value of the working coil). 
     When the induction heating apparatus  10  is powered on as a result of manipulation of the interface of the induction heating apparatus  10  by the user, the induction heating apparatus may be put on standby for driving as power is supplied to the induction heating apparatus from the power supply device  20  (or from the power supply). The user may then place a container over a working coil (or on the working coil) of the induction heating apparatus and set a power level for the container, to give the working coil an instruction to initiate heating. When the instruction to initiate heating is given by the user, a power value required of the working coil  132  (i.e., a required power value of the working coil  132 ) may be determined based on the power level set by the user. 
     Having received the user&#39;s instruction to initiate heating, the controller  2  may determine a frequency (i.e., a heating frequency) corresponding to the required power value of the working coil  132 , and supply a control signal corresponding to the determined heating frequency to the driving circuit  22 . Accordingly, the switching signals S 1 , S 2 , S 3 , S 4  may be output from the driving circuit  22 , and as the switching signals S 1 , S 2 , S 3 , S 4  are respectively input to the switching elements SW 1 , SW 2 , SW 3 , SW 4 , the working coil  132  may be driven. As a result of the driving of the working coil  132 , the container may be heated while eddy current flows in the container. 
     In one embodiment, the controller  2  may determine a heating frequency that is a frequency corresponding to a power level of a heating zone set by the user. For example, when the user sets a power level of a heating zone, the controller  2  may gradually decrease a driving frequency of the inverter circuit  204 , in a state in which the driving frequency of the inverter circuit  204  is set to a predetermined reference frequency, until an output power value of the working coil  132  matches a required power value corresponding to the power level set by the user. The controller  2  may determine a frequency at a time when the output power value of the working coil  132  matches the required power value as a heating frequency. 
     The controller  2  may supply a control signal corresponding to the determined heating frequency to the driving circuit  22 . The driving circuit  22  may output switching signals S 1 , S 2 , S 3 , S 4  that have a duty ratio corresponding to the heating frequency determined by the controller  2 , based on the control signal output from the controller  2 . As a result of the input of the switching signals S 1 , S 2 , S 3 , S 4 , alternating current may be supplied to the working coil  132  while the switching elements SW 1 , SW 2 , SW 3 , SW 4  are turned on and turned off complementarily. 
     In one embodiment, the controller  2  may perform a noise suppressing operation for suppressing an initial driving noise before the heating frequency is determined. Embodiments may be described of a process (or operation) in which the controller  2  of the induction heating apparatus  10  performs a noise suppressing operation and a process (or operation) in which a heating frequency of the inverter circuit  204  is determined. 
       FIG. 4  is a graph showing a relationship between a driving frequency of an inverter circuit and an output power value of a working coil in one embodiment. In  FIG. 4 , FR denotes a resonance frequency of the inverter circuit  204 . In one embodiment, the driving frequency of the inverter circuit  204  may be determined as a value greater than the resonance frequency FR. Additionally, in  FIG. 4 , PR denotes a required power value corresponding to a power level set for a heating zone, and FH denotes a driving frequency (i.e., a heating frequency) of the inverter circuit  204  when an output power value of the working coil  132  matches the required power value PR. 
     Before the heating frequency FH is determined as described above, the controller  2  may perform a noise suppressing operation. To perform the noise suppressing operation, the controller  2  may set a driving frequency of the inverter circuit  204  to a predetermined first driving frequency F 1 . For example, the first driving frequency F 1  may be set to 120 kHz. However, the first driving frequency F 1  may be set to different values depending on embodiments. 
     When the driving frequency of the inverter circuit  204  is set to the first driving frequency F 1  as illustrated in  FIG. 4 , an output power value P 1  of the working coil  132  may be much lower than the required power value PR. Accordingly, a container may hardly be heated. This means that a peak-to-peak value of current output from the inverter circuit  204  is very low when the driving frequency of the inverter circuit  204  is set to the first driving frequency F 1 . When the driving frequency of the inverter circuit  204  is set to the first driving frequency F 1 , noise, caused by the driving of the working coil  132 , may be minimized. 
     In a state in which the driving frequency of the inverter circuit  204  is set to the first driving frequency F 1 , the controller  2  may supply switching signals S 1 , S 2 , S 3 , S 4  to the inverter circuit  204 . The controller  2  may then gradually decrease the driving frequency of the inverter circuit  204  by a predetermined unit size (e.g., 1 kHz) until the driving frequency of the inverter circuit  204  reaches to a second driving frequency F 2 . For example, the second driving frequency F 2  may be set to 60 kHz. However, the second driving frequency F 2  may be set to different values depending on embodiments. 
     When the driving frequency of the inverter circuit  204  is set to the second driving frequency F 2  as illustrated in  FIG. 4 , an output power value P 2  of the working coil  132  may be much lower than the required power value PR. Accordingly, the container may hardly be heated. This means that a peak-to-peak value of current output from the inverter circuit  204  is very low when the driving frequency of the inverter circuit  204  is set to the second driving frequency F 2 . Accordingly, although the driving frequency of the inverter circuit  204  decreases to the second driving frequency F 2 , noise, caused by the driving of the working coil  132 , may still be minimized. 
     As described above, the noise suppressing operation for suppressing an initial driving noise of the working coil  132  may be performed by decreasing the driving frequency of the inverter circuit  204  from the first driving frequency F 1  to the second driving frequency F 2 . 
     When the driving frequency of the inverter circuit  204  is decreased to the second driving frequency F 2 , the controller  2  may determine a heating frequency corresponding to the power level that is input by the user to heat the container. For example, the controller  2  may gradually decrease the driving frequency of the inverter circuit  204  by a predetermined unit size (e.g., 1 kHz) while comparing the output power value of the working coil  132  with the required power value PR when the driving frequency of the inverter circuit  204  is set to the second driving frequency F 2 . The controller  2  may determine a driving frequency at a time when the output power value of the working coil  132  matches the required power value PR as a heating frequency FH. 
     When the heating frequency FH is determined, the controller  2  may supply a control signal corresponding to the heating frequency FH to the driving circuit  22 . Accordingly, the driving circuit  22  may supply switching signals S 1 , S 2 , S 3 , S 4  corresponding to the heating frequency FH to the inverter circuit  204 , and the working coil  132  may output power having a value the same as the required power value PR to heat the container using current supplied by the inverter circuit  204 . 
     The noise suppressing operation performed by the controller  2  may include the below-described control of a duty cycle of a switching signal as well as the above-described control of a driving frequency of the inverter circuit  204 . 
       FIG. 5  is a view showing a waveform of each of the switching signals and a waveform of current supplied to the working coil when duty cycles of the switching signals input to switching elements belonging to a first group are set to 0.05, in one embodiment.  FIG. 6  is a view showing a waveform of each of the switching signals and a waveform of current supplied to the working coil when the duty cycles of the switching signals input to the switching elements belonging to the first group are set to 0.25, in one embodiment.  FIG. 7  is a view showing a waveform of each of the switching signals and a waveform of current supplied to the working coil when the duty cycles of the switching signals input to the switching elements belonging to the first group are set to 0.5, in one embodiment. 
       FIGS. 5 to 7  show a waveform of each of the switching signals S 1 , S 2 , S 3 , S 4  output by the driving circuit  22 . Additionally,  FIGS. 5 to 7  show a waveform of current supplied to the working coil  132  when each of the switching signals S 1 , S 2 , S 3 , S 4  is supplied to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  of the inverter circuit  204 . 
     Before the heating frequency FH is determined as described above, the controller  2  may perform a noise suppressing operation. To perform the noise suppressing operation, the controller  2  may adjust duty cycles of the switching signals S 1 , S 3  input to some of the switching elements SW 1 , SW 2 , SW 3 , SW 4  included in the inverter circuit  204  (e.g., the switching elements SW 1  and SW 3  belonging to the first group). An embodiment may be described in which the duty cycles of the switching signals S 1 , S 3  input to the switching elements SW 1  and SW 3  belonging to the first group are adjusted. However, in another embodiment, duty cycles of the switching signals S 2 , S 4  input to the switching elements SW 2 , SW 4  belonging to the second group may be adjusted. 
     As illustrated in  FIG. 5 , a duty cycle of a switching signal is defined as a ratio (T 1 /T 12 ) of on-time T 1  to a total of on-time T 1  and off-time T 2  of the switching signal (i.e., a single cycle time of the switching signal). 
     When the noise suppressing operation is started, the controller  2  may set duty cycles of the switching signals S 1 , S 3 , input to the switching elements SW 1  and SW 3  belonging to the first group, to a predetermined first reference duty cycle, e.g., 0.05 (see  FIG. 5 ). The controller  2  may then increase the duty cycles of the switching signals S 1 , S 3  input to the switching elements SW 1  and SW 3  belonging to the first group by a predetermined unit size (e.g., 0.01) until the duty cycles of the switching signals S 1 , S 3  input to the switching elements SW 1  and SW 3  belonging to the first group increase to a second reference duty cycle (e.g., 0.5). The first reference duty cycle may be set to different values depending on embodiments. 
       FIG. 6  shows waveforms of the switching signals when the duty cycles of the switching signals S 1 , S 3  input to the switching elements SW 1  and SW 3  belonging to the first group increase to 0.25.  FIG. 7  shows waveforms of the switching signals when the duty cycles of the switching signals S 1 , S 3  input to the switching elements SW 1  and SW 3  belonging to the first group increase to the second reference duty cycle (e.g. 0.5). The second reference duty cycle may be set to different values depending on embodiments. 
     In  FIGS. 5 to 7 , the duty cycles of the switching signals S 2 , S 4  input to the switching elements SW 2  and SW 4  belonging to the second group may be fixed to the second reference duty cycle (e.g., 0.5). The duty cycles of the switching signals S 2 , S 4  input to the switching elements SW 2  and SW 4  belonging to the second group may be set to different values depending on embodiments. 
     As illustrated in  FIGS. 5 to 7 , a range (a peak-to-peak value) of alternating current supplied to the working coil  132  may gradually increase, as the duty cycles of the switching signals S 1 , S 3  input to the switching elements SW 1  and SW 3  belonging to the first group gradually increase from the first reference duty cycle (e.g., 0.05) to the second reference duty cycle (e.g., 0.5). For example, in  FIGS. 5 to 7 , the peak-to-peak value of the alternating current supplied to the working coil  132  may gradually increase to IA 1 -IA 2 , IA 3 -IA 4 , and IA 5 -IA 6  respectively. 
     Since the duty cycles of the switching signals S 1 , S 3  input to the switching elements SW 1  and SW 3  belonging to the first group are adjusted as described above, alternating current having a peak-to-peak value may not be instantaneously supplied to the working coil  132  during an initial driving process of the working coil  132 , and the peak-to-peak value of the current supplied to the working coil  132  may gradually increase, unlike that of the related art. Accordingly, during the initial driving process of the working coil  132 , the peak-to-peak value of the current supplied to the working coil  132  may be prevented from increasing excessively and instantly, and an initial driving noise may not be made. 
       FIG. 8  is a view showing a waveform of each switching signal and a waveform of current supplied to the working coil when the duty cycle of the switching signal input to each switching element is set to 0.05, in one embodiment.  FIG. 9  is a view showing a waveform of each switching signal and a waveform of current supplied to the working coil when the duty cycle of the switching signal input to each switching element is set to 0.25, in one embodiment.  FIG. 10  is a view showing a waveform of each switching signal and a waveform of current supplied to the working coil when the duty cycle of the switching signal input to each switching element is set to 0.5, in one embodiment. 
       FIGS. 8 to 10  show a waveform of each of the switching signals S 1 , S 2 , S 3 , S 4  output by the driving circuit  22 . Additionally,  FIGS. 8 to 10  show a waveform of current supplied to the working coil  132  when each of the switching signals S 1 , S 2 , S 3 , S 4  is supplied to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  of the inverter circuit  204 . 
     In another embodiment, to perform the noise suppressing operation, the controller  2  may adjust the duty cycle of the switching signal S 1 , S 2 , S 3 , S 4  input to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  included in the inverter circuit  204 , as illustrated in  FIGS. 8 to 10 . 
     When the noise suppressing operation is started, the controller  2  may set the duty cycle of the switching signal S 1 , S 2 , S 3 , S 4 , input to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4 , to a predetermined first reference duty cycle, e.g., 0.05 (see  FIG. 8 ). The controller  2  may then increase the duty cycle of the switching signal S 1 , S 2 , S 3 , S 4  input to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  by a predetermined unit size (e.g., 0.01) until the duty cycle of the switching signal S 1 , S 2 , S 3 , S 4  input to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  increases to a second reference duty cycle (e.g., 0.5). The first reference duty cycle may be set to different values depending on embodiments. 
       FIG. 9  shows waveforms of the switching signals when the duty cycle of the switching signal S 1 , S 2 , S 3 , S 4  input to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  increases to 0.25.  FIG. 10  shows waveforms of the switching signals when the duty cycle of the switching signal S 1 , S 2 , S 3 , S 4  input to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  increases to the second reference duty cycle (e.g., 0.5). 
     As illustrated in  FIGS. 8 to 10 , a range (a peak-to-peak value) of alternating current supplied to the working coil  132  may gradually increase as the duty cycle of the switching signal S 1 , S 2 , S 3 , S 4  input to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  gradually increases from the first reference duty cycle (e.g., 0.05) to the second reference duty cycle (e.g., 0.5). For example, in  FIGS. 8 to 10 , the peak-to-peak value of the alternating current supplied to the working coil  132  may gradually increase to IA 7 -IA 8 , IA 9 -IA 10  and IA 11 -IA 12  respectively. 
     Since the duty cycle of the switching signal S 1 , S 2 , S 3 , S 4  input to each of the switching elements SW 1 , SW 2 , SW 3 , SW 4  is adjusted as described above, alternating current having a peak-to-peak value may not be instantaneously supplied to the working coil  132  during an initial driving process of the working coil  132 , and the peak-to-peak value of the current supplied to the working coil  132  may gradually increase, unlike that of the related art. Accordingly, during the initial driving process of the working coil  132 , the peak-to-peak value of the current supplied to the working coil  132  may be prevented from increasing excessively and instantly, and an initial driving noise may not be made. 
       FIG. 11  is a flow chart showing a method for controlling an induction heating apparatus in one embodiment. The controller  2  of the induction heating apparatus  10  in one embodiment may receive an input power level of a heating zone formed on an upper plate of the induction heating apparatus  10  ( 1102 ). When the power level of the heating zone is input, a required power value of a working coil corresponding to the heating zone may be determined. 
     The controller  2  may then adjust a duty cycle of a switching signal input to a switching element included in the inverter circuit  204  and a driving frequency of the inverter circuit  204 , and perform a noise suppressing operation ( 1104 ). 
     In one embodiment, the noise suppressing operation performed by the controller  2  may include setting duty cycles of switching signals, input to switching elements belonging to a first group, to a predetermined first reference duty cycle, setting a driving frequency of the inverter circuit  204  to a predetermined first reference frequency, supplying a switching signal to each of the switching elements included in the inverter circuit  204 , based on the first reference duty cycle and the first reference frequency, increasing the duty cycles of the switching signals, supplied to the switching elements belonging to the first group, to a predetermined second reference duty cycle, and decreasing the driving frequency of the inverter circuit  204  to a predetermined second reference frequency. 
     In one embodiment, during the noise suppressing operation, duty cycles of switching signals input to switching elements belonging to a second group may be set to a predetermined third reference duty cycle. 
     In one embodiment, the third reference duty cycle may be set to a duty cycle the same as the second reference duty cycle. 
     In another embodiment, the noise suppressing operation performed by the controller  2  may include setting a duty cycle of a switching signal, input to each of the switching elements included in the inverter circuit  204 , to a predetermined first reference duty cycle, setting a driving frequency of the inverter circuit  204  to a predetermined first reference frequency, supplying a switching signal to each of the switching elements included in the inverter circuit  204 , based on the first reference duty cycle and the first reference frequency, increasing a duty cycle of a switching signal supplied to each of the switching elements included in the inverter circuit  204  to a predetermined second reference duty cycle, and decreasing the driving frequency of the inverter circuit  204  to a predetermined second reference frequency. 
     When completing the noise suppressing operation, the controller  2  may determine a heating frequency corresponding to the power level that was previously input ( 1106 ). The determining of a heating frequency corresponding to the power level ( 1106 ) may include comparing an output power value of the working coil  132  with the required power value while decreasing the driving frequency of the inverter circuit  204 , and determining a driving frequency at a time when the output power value matches the required power value as the heating frequency. 
     When the heating frequency is determined, the controller  2  may drive the working coil  132  by supplying a switching signal to each switching element included in the inverter circuit  204 , based on the determined heating frequency ( 1108 ). 
       FIG. 12  is a flow chart showing a method for controlling an induction heating apparatus in another embodiment. To heat a container placed in (or provided on) a heating zone, the user may input a power level of the heating zone, in which the container to be heated is placed, through a manipulation zone  118  of the induction heating apparatus  10 . Accordingly, the controller  2  may receive the input power level of the heating zone ( 1202 ). When the power level of the heating zone is input, the controller  2  may calculate an output power value corresponding to the input power level, and determine the calculated output power value as a required power value of a working coil  132  corresponding to the heating zone. 
     After the power level is input, the controller  2  may perform a noise suppressing operation including steps  1204  to  1214 , to suppress initial driving noise that is generated before the working coil  132  starts to heat the container. 
     When the noise suppressing operation is started, the controller  12  may set duty cycles of switching signals input to switching elements belonging to a first group to a first reference duty cycle ( 1204 ). For example, the controller  2  may set the duty cycles of the switching signals S 1 , S 3 , input to the switching elements SW 1 , SW 3  belonging to the first group, to a predetermined first reference duty cycle (e.g., 0.05), in the circuit illustrated in  FIG. 3 . 
     Additionally, the controller  2  may set a driving frequency of the inverter circuit  204  to a predetermined first reference frequency ( 1206 ). For example, the controller  2  may set the driving frequency of the inverter circuit  204  to the predetermined first reference frequency (e.g., 120 kHz), in the circuit illustrated in  FIG. 3 . 
     The controller  2  may supply a switching signal to each of the switching elements included in the inverter circuit, based on the first reference duty cycle set in operation  1204  and the first reference frequency set in operation  1206  ( 1208 ). Accordingly, alternating current generated from the inverter circuit may be supplied to the working coil. 
     The controller  2  may then determine whether the duty cycles of the switching signals input to the switching elements belonging to the first group are the same as a predetermined second reference duty cycle (e.g., 0.5) ( 1210 ). 
     When the duty cycles of the switching signals input to the switching elements belonging to the first group are not the same as the second reference duty cycle in operation  1210 , the controller  2  may increase the duty cycles of the switching signals input to the switching elements belonging to the first group by a predetermined unit size (e.g., 0.01) ( 1212 ). As a result of repetition of operation  1210  and operation  1212 , the duty cycles of the switching signals input to the switching elements belonging to the first group may gradually increase from the first reference duty cycle to the second reference duty cycle. 
       FIG. 12  shows an embodiment in which the duty cycles of the switching signals input to the switching elements belonging to the first group gradually increase from the first reference duty cycle to the second reference duty cycle. In another embodiment, duty cycles of switching signals input to all the switching elements included in the inverter circuit  204  may gradually increase from the first reference duty cycle to the second reference duty cycle. 
     When the duty cycles of the switching signals input to the switching elements belonging to the first group are the same as the second reference duty cycle in operation  1210 , the controller  2  may determine whether the driving frequency of the inverter circuit is the same as a predetermined second reference frequency (e.g., 60 kHz) ( 1214 ). 
     When the duty cycles of the switching signals input to the switching elements belonging to the first group are not the same as the second reference duty cycle in operation  1214 , the controller  2  may decrease the driving frequency of the inverter circuit by a predetermined unit size (e.g., 1 kHz) ( 1216 ). As a result of repetition of operation  1214  and operation  1216 , the driving frequency of the inverter circuit may gradually decrease from the first reference frequency to the second reference frequency. 
     When the duty cycles of the switching signals input to the switching elements belonging to the first group are the same as the second reference duty cycle in operation  1214 , the noise suppressing operation may end. 
     When the noise suppressing operation is completed, the controller  2  may determine a heating frequency corresponding to the power level which was input in operation  1202  ( 1218 ). For example, the controller  2  may compare the output power value of the working coil  132  with the required power value while decreasing the driving frequency of the inverter circuit  204  by a predetermined unit size (e.g., 1 kHz) from the second reference frequency (e.g., 60 kHz). The controller  2  may determine a driving frequency at a time when the output power value of the working coil  132  matches the required power value as a frequency for heating the container (i.e., a heating frequency). 
     When the heating frequency is determined, the controller  2  may supply a switching signal to each of the switching elements included in the inverter circuit, based on the heating frequency ( 1220 ). In operation  1220 , a duty cycle of a switching signal S 1 , S 2 , S 3 , S 4  supplied to each of the switching elements may be set to the same duty cycle. Accordingly, while alternating current output from the inverter circuit is supplied to the working coil  132 , the working coil  132  may output power having the same value as the required power value, and the container may be heated. 
     In the embodiment of  FIG. 12 , when the noise suppressing operation is performed, the driving frequency of the inverter circuit may be adjusted after the duty cycles of the switching signals input to the inverter circuit is adjusted. However, in another embodiment, the duty cycles of the switching signals may be adjusted after the driving frequency of the inverter circuit is adjusted. In yet another embodiment, the duty cycles of the switching signals and the driving frequency of the inverter circuit may be adjusted at the same time. 
     It is an object of the present disclosure to provide an induction heating apparatus and a method for controlling the same that reduces initial driving noise generated at a time when a power level of a heating zone is set and a container starts to be heated. 
     The object may be solved by features of the independent claims. Aspects according to the present disclosure are not limited to the above ones, and other aspects and advantages that are not mentioned above can be clearly understood from the following description and can be more clearly understood from the embodiments set forth herein. Additionally, the aspects and advantages in the present disclosure can be realized via means and combinations thereof that are described in the appended claims. 
     In one embodiment, when a power level of a heating zone is input, a noise suppressing operation may be performed to suppress initial driving noise that can be generated before a frequency for heating a container (i.e., a heating frequency) is determined. 
     A controller of an induction heating apparatus in one embodiment may perform the noise suppressing operation by adjusting duty cycles of switching signals input to switching elements included in an inverter circuit and a driving frequency of the inverter circuit respectively. 
     In one embodiment, the controller may gradually increase duty cycles of switching signals input to switching elements belonging to a first group (e.g., a high side), among the switching elements included in the inverter circuit, from a predetermined first reference duty cycle to a predetermined second reference duty cycle. In another embodiment, the controller may gradually increase a duty cycle of a switching signal input to each of the switching elements included in the inverter circuit. 
     Additionally, in one embodiment, the controller may gradually decrease a driving frequency of the inverter circuit from a predetermined first reference frequency to a predetermined second reference frequency. 
     As a result of the control over the duty cycles and the driving frequency, magnitude of alternating current supplied to a working coil may gradually increase until an output power value of the working coil becomes identical with a required power value. Accordingly, a temporary increase in the magnitude of the alternating current supplied to the working coil, which would otherwise occur in the related art, may be prevented. Thus, the initial driving noise generated by a working coil in the related art may be suppressed. 
     According to one aspect, the induction heating apparatus may include a working coil disposed in a position corresponding to a position of a heating zone, an inverter circuit including a plurality of switching elements and configured to supply current to the working coil, a driving circuit configured to supply a switching signal to each of the switching elements included in the inverter circuit, and a controller configured to determine a driving frequency of the inverter circuit, supply a control signal based on the driving frequency to the driving circuit, and drive the working coil. The controller may be configured to perform a control method according to any one of the herein described embodiments. 
     The controller may be configured to receive an input power level of the heating zone, to adjust duty cycles of switching signals input to the switching elements included in the inverter circuit and a driving frequency of the inverter circuit and to perform a noise suppressing operation, to determine a heating frequency corresponding to the power level, and based on the heating frequency, to supply a switching signal to each of the switching elements included in the inverter circuit and drive the working coil. The controller may be additionally or alternatively configured for setting duty cycles of switching signals, input to switching elements belonging to a first group, to a predetermined first reference duty cycle, setting a driving frequency of the inverter circuit to a predetermined first reference frequency, supplying a switching signal to each of the switching elements included in the inverter circuit, based on the first reference duty cycle and the first reference frequency, increasing the duty cycles of the switching signals, supplied to the switching elements belonging to the first group, to a predetermined second reference duty cycle, and decreasing the driving frequency of the inverter circuit to a predetermined second reference frequency. 
     The noise suppressing operation may include setting duty cycles of switching signals, input to switching elements belonging to a first group, to a predetermined first reference duty cycle, setting a driving frequency of the inverter circuit to a predetermined first reference frequency, supplying a switching signal to each of the switching elements included in the inverter circuit, based on the first reference duty cycle and the first reference frequency, increasing the duty cycles of the switching signals, supplied to the switching elements belonging to the first group, to a predetermined second reference duty cycle, and decreasing the driving frequency of the inverter circuit to a predetermined second reference frequency. 
     In one embodiment, during the noise suppressing operation, duty cycles of switching signals input to switching elements belonging to a second group may be set to a predetermined third reference duty cycle. 
     In one embodiment, the third reference duty cycle may be set to a duty cycle the same as the second reference duty cycle. 
     In one embodiment, the noise suppressing operation may include setting a duty cycle of a switching signal, input to each of the switching elements included in the inverter circuit, to a predetermined first reference duty cycle, setting a driving frequency of the inverter circuit to a predetermined first reference frequency, supplying a switching signal to each of the switching elements included in the inverter circuit, based on the first reference duty cycle and the first reference frequency, increasing a duty cycle of a switching signal supplied to each of the switching elements included in the inverter circuit to a predetermined second reference duty cycle, and decreasing the driving frequency of the inverter circuit to a predetermined second reference frequency. 
     In one embodiment, the controller may compare an output power value of the working coil with a required power value while decreasing the driving frequency of the inverter circuit, and determine a driving frequency at a time when the output power value matches the required power value as the heating frequency. The required power value may correspond to the input power level. 
     According to a further aspect, a method for controlling an induction heating apparatus, in particular for an induction heating apparatus according to any one of the herein described embodiments, may include receiving an input power level of a heating zone, performing a noise suppressing operation by adjusting duty cycles of switching signals input to switching elements included in an inverter circuit and a driving frequency of the inverter circuit, determining a heating frequency corresponding to the power level, and driving a working coil by supplying a switching signal to each switching element included in the inverter circuit, based on the heating frequency. 
     Performing the noise suppressing operation may include setting duty cycles of switching signals, input to switching elements belonging to a first group, to a predetermined first reference duty cycle, setting a driving frequency of the inverter circuit to a predetermined first reference frequency, supplying a switching signal to each of the switching elements included in the inverter circuit, based on the first reference duty cycle and the first reference frequency, increasing the duty cycles of the switching signals, supplied to the switching elements belonging to the first group, to a predetermined second reference duty cycle, and decreasing the driving frequency of the inverter circuit to a predetermined second reference frequency. 
     During the noise suppressing operation, duty cycles of switching signals input to switching elements belonging to a second group may be set to a predetermined third reference duty cycle. 
     The third reference duty cycle may be set to a duty cycle the same as the second reference duty cycle. 
     Performing the noise suppressing operation may include setting a duty cycle of a switching signal, input to each of the switching elements included in the inverter circuit, to a predetermined first reference duty cycle, setting a driving frequency of the inverter circuit to a predetermined first reference frequency, supplying a switching signal to each of the switching elements included in the inverter circuit, based on the first reference duty cycle and the first reference frequency, increasing a duty cycle of a switching signal supplied to each of the switching elements included in the inverter circuit to a predetermined second reference duty cycle, and decreasing the driving frequency of the inverter circuit to a predetermined second reference frequency. 
     Determining a heating frequency corresponding to the power level may include comparing an output power value of the working coil with a required power value while decreasing the driving frequency of the inverter circuit, and determining a driving frequency at a time when the output power value matches the required power value as the heating frequency. The required power value may correspond to the input power level. 
     In the embodiments of the present disclosure, when a power level of a heating zone is set and a container starts to be heated, initial driving noise may be reduced. Thus, a user may experience no inconvenience caused by noise while using an induction heating apparatus and user satisfaction may improve. Additionally, the possibility that the user misunderstood that the induction heating device is malfunctioning due to the initial driving noise is lowered. 
       FIG. 13  is a graph showing changes in a driving frequency of an inverter circuit and a duty cycle of a switching signal when the working coil is driven at a high power level in one embodiment. 
     In  FIG. 13 , data  1301  indicates a driving frequency of the inverter circuit, and data  1302  indicates a duty cycle of a switching signal input to the switching elements of the inverter circuit. 
     In one embodiment, the working coil may be driven at a high power level (e.g., a power level of 4 or more among power levels of 1 to 9). When the user sets the power level of the working coil to a high power level, the working coil is continuously driven until heating is finished. 
     In  FIG. 13 , a first section A 1  is a section in which the driving of the working coil is started according to an instruction to initiate heating input by the user. In the first section A 1 , the driving frequency of the inverter circuit is set to a predetermined first reference frequency FR 1  (e.g., 120 kHz). In addition, the duty cycle of the switching signal input to the inverter circuit in the first section A 1  increases from a predetermined first reference duty cycle DR 1  (e.g., 10%) to a second reference duty cycle DR 2  (e.g., 50%). That is, the noise suppressing operation is performed in the first section A 1  to suppress noise generated during the initial driving of the working coil. 
     After the noise suppressing operation is completed, in the second section A 2 , the driving frequency of the inverter circuit decreases from the first reference frequency FR 1  to a predetermined second reference frequency FR 2  (e.g., 80 kHz). 
     In the third section A 3 , the controller  2  determines the heating frequency FH corresponding to the power level input by the user. Also, in the third section A 3 , the controller  2  may perform a container detection operation for determining whether a heatable container exists above the working coil based on the resonance current value of the working coil. 
     In the fourth section A 4 , the driving frequency of the inverter circuit decreases from the second reference frequency FR 2  to the heating frequency FH (e.g., 40 kHz). Accordingly, the output power value of the working coil gradually increases. 
     In the fifth section A 5 , the inverter circuit is driven at the heating frequency FH. Accordingly, the container is heated to correspond to the power level set by the user. 
     Meanwhile, the duty cycle of the switching signal input to the inverter circuit in the second section A 2  to the fifth section A 5  is maintained as the second reference duty cycle DR 2 . 
     According to the embodiment shown in  FIG. 13 , noise generated during the initial driving of the working coil is reduced compared to the related art by the noise suppressing operation performed in the first section A 1 . In addition, since the driving frequency of the inverter circuit is gradually adjusted to the heating frequency FH from the first reference frequency FR 1  and the second reference frequency FR 2 , noise due to a sudden change in the driving frequency does not occur. 
       FIG. 14  is a graph showing changes in a driving frequency of an inverter circuit and a duty cycle of a switching signal when the working coil is driven at a low power level in one embodiment. 
     In  FIG. 14 , data  1401  and  1403  indicate driving frequencies of the inverter circuit, and data  1402  and  1404  indicate duty cycles of switching signals input to the switching elements of the inverter circuit. 
     In one embodiment, the working coil may be driven at a low power level (e.g., a power level of 3 or less among power levels of 1 to 9). When the user sets the power level of the working coil to a low power level, the working coil is discontinuously driven until heating is finished, that is, in an on/off manner. 
     For example, as shown in  FIG. 14 , when the working coil is driven at a low power level, the driving of the working coil is controlled so that the on sections (ON 1 , ON 2 ) in which the working coil is driven and the off sections (OFF 1 , OFF 2 ) in which the working coil is not driven appear alternately. The controller  2  may control the driving of the working coil so that the average value of the output power values of the working coils accumulated in each of the on sections ON 1  and ON 2  respectively correspond to the required power value. 
     It is noted that the power level values belonging to the high power level and the power level values belonging to the low power level may vary according to embodiments. 
     In  FIG. 14 , the first on section ON 1  includes the first section A 1  to the fourth section A 4 . 
     The first section A 1  is a section in which the driving of the working coil is started according to an instruction to initiate heating input by the user. In the first section A 1 , the driving frequency of the inverter circuit is set to a predetermined first reference frequency FR 1  (e.g., 120 kHz). 
     In addition, the duty cycle of the switching signal input to the inverter circuit in the first section A 1  is increases from a predetermined first reference duty cycle DR 1  (e.g., 10%) to a second reference duty cycle DR 2  (e.g., 50%). That is, the noise suppressing operation is performed in the first section A 1  to suppress noise generated during the initial driving of the working coil. 
     After the noise suppressing operation is completed, in the second section A 2 , the driving frequency of the inverter circuit decreases from the first reference frequency FR 1  to the second predetermined reference frequency FR 2 . 
     In the third section A 3 , the controller  2  determines the heating frequency FH corresponding to the power level input by the user. In the embodiment of  FIG. 14 , the heating frequency FH (e.g., 50 kHz) is identical to the second reference frequency FR 2 . Also, in the third section A 3 , the controller  2  may perform a container detection operation for determining whether a heatable container exists above the working coil based on the resonance current value of the working coil. 
     In the fourth section A 4 , the driving frequency of the inverter circuit is maintained at the heating frequency FH, that is, the second reference frequency FR 2 . 
     Meanwhile, the duty cycle of the switching signal input to the inverter circuit in the second section A 2  to the fourth section A 4  is maintained as the second reference duty cycle DR 2 . 
     When the fourth section A 4  ends, the driving of the working coil is stopped during the first off section OFF 1 . 
     When the first off section OFF 1  ends, the second on section ON 2  starts. The second on section ON 2  includes a first section A 1  to a fourth section A 4 . The driving process of the working coil in the first section A 1  to the fourth section A 4  included in the second on section ON 2  is as same as the driving process of the working coil in the first section A 1  to the fourth section included in the first on section ON 1 . 
     When the second on section ON 2  ends, the driving of the working coil is stopped during the second off section OFF 2 . 
     Although not shown, the working coil is driven so that the on section and the off section appear alternately even after the second off section OFF 2 . 
     According to the embodiment shown in  FIG. 14 , the noise suppressing operation is performed in the first section A 1  whenever each of the on sections ON 1  and ON 2  starts. Therefore, whenever the on section starts, noise generated due to the initial driving of the working coil is reduced compared to the related art. In addition, since the driving frequency of the inverter circuit is gradually adjusted from the first reference frequency FR 1  to the heating frequency FH, noise due to a sudden change in the driving frequency does not occur. 
     The embodiments are described above with reference to a number of illustrative embodiments thereof. However, the present disclosure is not intended to limit the embodiments and drawings set forth herein, and numerous other modifications and embodiments can be devised by one skilled in the art. Further, the effects and predictable effects based on the configurations in the disclosure are to be included within the range of the disclosure though not explicitly described in the description of the embodiments. 
     It will be understood that when an element or layer is referred to as being “on” another element or layer, the element or layer can be directly on another element or layer or intervening elements or layers. In contrast, when an element is referred to as being “directly on” another element or layer, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     Spatially relative terms, such as “lower”, “upper” and the like, may be used herein for ease of description to describe the relationship of one element or feature to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “lower” relative to other elements or features would then be oriented “upper” relative to the other elements or features. Thus, the exemplary term “lower” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Embodiments are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures). As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.