Patent Publication Number: US-2023141390-A1

Title: Induction heating device and control method thereof

Description:
TECHNICAL FIELD 
     Disclosed herein are an induction heating device and a control method thereof. 
     BACKGROUND ART 
     Various types of cooking devices are used at homes or restaurants to heat food items. Gas ranges that use gas as a fuel have been widely used as one of the cooking devices. Currently, devices that heat a container by using electric energy are used. 
     Methods in which a container is heated using electric energy include a resistance heating method and an induction heating method. In terms of the resistance heating method, heat energy is generated when electric energy is supplied to a metallic resistance wire or a non-metallic heat generation element such as silicon carbide, and the generated heat energy is used to heat a container. In terms of the induction heating method, a magnetic field is generated around a working coil when electric energy is supplied to the working coil, eddy current is produced in a metallic container by the magnetic field, and the container itself is heated. 
     The principle of the induction heating method is described in more detail as follows. As power is supplied to an induction heating device, a high-frequency voltage having predetermined magnitude is supplied to a working coil. Accordingly, an induction field is produced around the working coil in the induction heating device. When a magnetic line of force of the produced induction field passes through the bottom of a metallic vessel placed on the working coil, eddy currents are generated at the inside of the bottom of the vessel. As the generated eddy currents flow through the vessel, the vessel itself is heated. 
     The induction heating devices includes two or more heating zones and two or more working coils corresponding to the two or more heating zones. For example, in the case of an induction heating device having two heating zones, when a user provides containers respectively on the two heating zones to perform cooking at the same time, power for driving is supplied to each of the two working coils. Each of the working coils operates at a driving frequency corresponding to a required output value set by the user. 
     When a difference between the driving frequencies of two working coils is included in an audio frequency band (e.g., 2 kHz to 20 kHz) at a time of a simultaneous operation of the two working coils, interference noise caused by the driving of the working coils is generated. The generated interference noise causes inconvenience to the user who uses the induction heating device, and leads the user to think that the induction heating device is not working correctly. 
     One of the ways of removing interference noise that is generated when two or more working coils operate is to adjust the driving frequency of an operating working coil such that the difference between the driving frequencies of the working coils is outside the audio frequency band. However, as the driving frequency of the operating working coil is adjusted, the actual output value of the working coil does not match a required output value set by the user. 
     For example, as the driving frequency of the operating working coil decreases to remove interference noise, the actual output value of the working coil becomes greater than the required output value, raising the possibility that a container overheats. Additionally, as the driving frequency of the operating working coil increases to remove interference noise, the actual output value of the working coil becomes less than the required output value, raising the possibility that a container is not sufficiently heated. 
     DESCRIPTION OF INVENTION 
     Technical Problems 
     The objective of the present disclosure is to provide an induction heating device and a control method thereof that maintains the output value of each working coil at the same value as a required output value set by a user while preventing the generation of interference noise, when two or more working coils operate at the same time. 
     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. 
     Technical Solutions 
     In one embodiment of the present disclosure, control for removing interference noise is performed based on a difference between a first driving frequency and a second driving frequency, when a first working coil operating at the first driving frequency and a second working coil operating at the second driving frequency operate at the same time. 
     That is, an induction heating device of one embodiment sets the difference between the first driving frequency and the second driving frequency to be equal to a predetermined noise avoidance value (e.g., 22 kHZ) when the difference between the first driving frequency and the second driving frequency is included in a first predetermined reference range (e.g., 8 kHz or greater and less than 20 kHZ). 
     Additionally, the induction heating device of one embodiment sets the first driving frequency and the second driving frequency to the same value when the difference between the first driving frequency and the second driving frequency is included in a second predetermined reference range (e.g., 2 kHz or greater and less than 8 kHZ). Based on the control, the difference between the first driving frequency and the second driving frequency is outside an audio frequency band or becomes 0, and interference noise is removed. 
     Further, the induction heating device according to the present disclosure adjusts the magnitude of the first driving frequency such that the difference between the first driving frequency and the second driving frequency may be set to the same value as the predetermined noise avoidance value when the difference between the first driving frequency and the second driving frequency is included in the first predetermined reference range. Since the magnitude of the first driving frequency is adjusted as described above, the actual output value of the first working coil may differ from a required output value of the first working coil set by the user. 
     To solve the problem, the induction heating device of one embodiment adjusts the duty ratio of a switching signal provided to a first inverter circuit that supplies electric current to the first working coil. As the duty ratio of the switching signal provided to the first inverter circuit is adjusted, the output value of the first working coil corresponding to the driving frequency of the first working coil changes. To solve the problem, the induction heating device of one embodiment may adjust the actual output value of the first working coil to be equal to the required output value of the first working coil by adjusting the duty ratio of the switching signal provided to the first inverter circuit, after adjusting the first driving frequency. 
     The induction heating device of one embodiment comprises a first working coil, a first inverter circuit configured to operate at a first driving frequency that corresponds to a first required output value of the first working coil and supplying electric current to the first working coil, a second working coil, a second inverter circuit configured to operate at a second driving frequency that corresponds to a second required output value of the second working coil and is greater than the first driving frequency and supplying electric current to the second working coil, and a controller configured to set a difference between the first driving frequency and the second driving frequency to be equal to a predetermined noise avoidance value by adjusting a duty ratio of a switching signal input to the first inverter circuit and magnitude of the first driving frequency, when the difference between the first driving frequency and the second driving frequency is included in a first predetermined reference range. 
     In one embodiment of the present disclosure, when the difference between the first driving frequency and the second driving frequency is included in the first predetermined reference range, the controller changes the duty ratio of the switching signal input to the first inverter circuit to a predetermined reference duty ratio and changes magnitude of the first driving frequency to a predetermined noise avoidance frequency. 
     In one embodiment of the present disclosure, the noise avoidance frequency is a value obtained by subtracting the noise avoidance value from the second driving frequency. 
     In one embodiment of the present disclosure, the controller increases the duty ratio of the switching signal input to the first inverter circuit when an actual output value of the first working coil is less than the first required output value after the magnitude of the first driving frequency changes. 
     In one embodiment of the present disclosure, the controller decreases the duty ratio of the switching signal input to the first inverter circuit when an actual output value of the first working coil is greater than the first required output value after the magnitude of the first driving frequency changes. 
     In one embodiment of the present disclosure, an actual output value of the first working coil is maintained to be the same as the first required output value after the duty ratio of the switching signal input to the first inverter circuit is adjusted. 
     In one embodiment of the present disclosure, the controller sets the first driving frequency and the second driving frequency to the same value when the difference between the first driving frequency and the second driving frequency is included in a second predetermined reference range. 
     In one embodiment of the present disclosure, a control method of an induction heating device, which comprises a first working coil, a first inverter circuit configured to operate at a first driving frequency that corresponds to a first required output value of the first working coil and supplying electric current to the first working coil, a second working coil, and a second inverter circuit configured to operate at a second driving frequency that corresponds to a second required output value of the second working coil and is greater than the first driving frequency and supplying electric current to the second working coil, comprises calculating a difference between the first driving frequency and the second diving frequency, comparing the difference between the first driving frequency and the second driving frequency with a predetermined reference range, and setting the difference between the first driving frequency and the second driving frequency to be equal to a predetermined noise avoidance value by adjusting a duty ratio of a switching signal input to the first inverter circuit and magnitude of the first driving frequency, when the difference between the first driving frequency and the second driving frequency is included in the predetermined reference range. 
     In one embodiment of the present disclosure, setting the difference between the first driving frequency and the second driving frequency to be equal to a predetermined noise avoidance value comprises changing the duty ratio of the switching signal input to the first inverter circuit to a predetermined reference duty ratio when the difference between the first driving frequency and the second driving frequency is included in the first predetermined reference range, and changing the magnitude of the first driving frequency to a predetermined noise avoidance frequency. 
     In one embodiment of the present disclosure, the noise avoidance frequency is a value obtained by subtracting the noise avoidance value from the second driving frequency. 
     In one embodiment of the present disclosure, setting the difference between the first driving frequency and the second driving frequency to be equal to a predetermined noise avoidance value further comprises increasing the duty ratio of the switching signal input to the first inverter circuit when an actual output value of the first working coil is less than the first required output value after the magnitude of the first driving frequency changes. 
     In one embodiment of the present disclosure, setting, setting the difference between the first driving frequency and the second driving frequency to be equal to a predetermined noise avoidance value further comprises decreasing the duty ratio of the switching signal input to the first inverter circuit when an actual output value of the first working coil is greater than the first required output value after the magnitude of the first driving frequency changes. 
     In one embodiment of the present disclosure, an actual output value of the first working coil is maintained to be the same as the first required output value after the duty ratio of the switching signal input to the first inverter circuit is adjusted. 
     The control method of one embodiment further comprises setting the first driving frequency and the second driving frequency to the same value when the difference between the first driving frequency and the second driving frequency is included in a second predetermined reference range. 
     Advantageous Effects 
     In the embodiments of the present disclosure, when two or more working coils operate at the same time, the output value of each working coil is maintained at the same value as a required output value set by a user, and the generation of interference noise caused by the working coils is prevented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is an exploded perspective view showing an induction heating device of one embodiment. 
         FIG.  2    is a circuit diagram of the induction heating device of one embodiment. 
         FIG.  3    is a graph showing output values corresponding to driving frequencies of each working coil when two working coils operate. 
         FIG.  4    is a waveform diagram showing the duty ratio of a switching signal input to an inverter circuit of the induction heating device of one embodiment. 
         FIG.  5    is a graph showing a change in the output values of a working coil, based on the adjustment of duty ratios of switching signals input to the inverter circuit of the induction heating device of one embodiment and the adjustment of driving frequencies of the working coil. 
         FIG.  6    is a graph showing a change in the driving frequency and output value of a working coil, under a divide mode noise avoidance control of the induction heating device of one embodiment. 
         FIG.  7    is a graph showing a change in the driving frequency and output value of a working coil, under a divide mode noise avoidance control of an induction heating device of another embodiment. 
         FIG.  8    is a graph showing a change in the driving frequency and output value of a working coil, under a divide mode noise avoidance control of an induction heating device of yet another embodiment. 
         FIG.  9    is a flowchart showing a control method of an induction heating device of one embodiment. 
         FIG.  10    is a flowchart showing a control method of an induction heating device of another embodiment. 
         FIG.  11    is a flowchart showing a process of performing divide mode noise avoidance control in the control method of an induction heating device of another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The above-described aspects, features and advantages are specifically described hereafter with reference to the accompanying drawings such that one having ordinary skill in the art to which the present disclosure pertains can embody the disclosure easily. In the disclosure, detailed description of known technologies in relation to the disclosure is omitted if it is deemed to make the gist of the disclosure unnecessarily vague. Hereafter, 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.  1    is an exploded perspective view showing an induction heating device of one embodiment. 
     Referring to  FIG.  1   , the induction heating device  10  of one embodiment comprises a case  102  constituting a main body of the induction heating device  10 , and a cover plate  104  being coupled to the case  102  and sealing the case  102 . 
     The cover plate  104  is coupled to the upper surface of the case  102 , and seals a space formed in the case  102  from the outside. The cover plate  104  comprises an upper plate part  106  on which a container for cooking a food item is placed. In one embodiment of the present disclosure, the upper plate part  106  may be made of a tempered glass material such as ceramic glass, but the material for the upper plate part  106  may vary depending on embodiments. 
     A heating zone  12 ,  14  respectively corresponding to a working coil assembly  122 ,  124  is formed on the upper plate part  106 . To allow a user to recognize the position of the heating zone  12 ,  14  clearly, lines or figures corresponding to the heating zone  12 ,  14  may be printed or expressed on the upper plate part  106 . 
     The case  102  may be formed into a cuboid the upper portion of which is open. The working coil assembly  122 ,  124  for heating a container is disposed in the space of the case  102 . Additionally, an interface part  114  is provided in the case  102 , and performs the function of allowing the user to supply power or to adjust a power level of each heating zone  12 ,  14  and the function of displaying information on the induction heating device  10 . The interface part  114  may be embodied as a touch panel that enables a touch-based input of information input and a display of information, but depending on embodiments, an interface  114  having a different structure may be used. 
     A manipulation zone  118  is formed on the upper plate part  106  and disposed in a position corresponding to the position of the interface part  114 . For the user&#39;s manipulation, characters or images and the like may be printed in the manipulation zone  118 . The user may touch a specific point of the manipulation zone  118  with reference to the characters or images printed in advance in the manipulation zone  118 , to perform a desired manipulation. Further, information output by the interface part  114  may be displayed through the manipulation zone  118 . 
     A power supply part  112  for supplying power to the working coil assembly  122 ,  124  or the interface part  114  is disposed in the space of the case  102 . 
     In the embodiment of  FIG.  1   , two working coil assemblies, i.e., a first working coil assembly  122  and a second working coil assembly  124 , disposed in the case  102  are illustrated exemplarily, but depending on embodiments, three or more working coil assemblies may be disposed in the case  102 . 
     The working coil assemblies  122 ,  124  comprises a heat insulation sheet for protecting a coil from heat that is generated by a working coil forming an induction field, with high-frequency AC currents supplied by the power supply part  112 , and by a container. For example, in  FIG.  1   , the first working coil assembly  122  comprises a first working coil  132  for heating a container placed in a first heating zone  12 , and a first heat insulation sheet  130 . Though not illustrated, the second working coil assembly  124  comprises a second working coil and a second heat insulation sheet. Depending on embodiments, the heat insulation sheet may be excluded. 
     Additionally, a temperature sensor is disposed in the central portion of each working coil. For example, in  FIG.  1   , a temperature sensor  134  is disposed in the central portion of the first working coil  134 . The temperature sensor measures the temperature of a container placed in each heating zone. In one embodiment of the present disclosure, the temperature sensor may be a thermistor sensor that has variable resistance where a resistance value varies depending on the temperature of a container, but not be limited. 
     The temperature sensor in one embodiment of the present disclosure outputs a sensing voltage corresponding to the temperature of a container, and the sensing voltage output from the temperature sensor is transferred to a controller. The controller confirms the temperature of the container, based on the magnitude of the sensing voltage output from the temperature sensor, and when the temperature of the container is a predetermined reference value or greater, performs a predetermined overheat protection operation. 
     Though not illustrated in  FIG.  1   , a substrate on which a plurality of circuits or elements comprising the controller is mounted may be disposed in the space of the case  102 . The controller drives each working coil according to the user&#39;s instruction to initiate heating, which is input through the interface part  114 , and performs a heating operation. As the user inputs an instruction to end heating through the interface part  114 , the controller stops the driving of the working coil and ends the heating operation. 
       FIG.  2    is a circuit diagram of the induction heating device of one embodiment. 
     Referring to  FIG.  2   , the induction heating device  10  of one embodiment comprises a first rectifier circuit  202 , a first smoothing circuit L 1 , C 1 , a first inverter circuit  204 , a first working coil  132 , a second rectifier circuit  212 , a second smoothing circuit L 2 , C 2 , a second inverter circuit  214 , and a second working coil  142 , a first driving circuit  22 , a second driving circuit  24 , and a controller  2 . 
     The first rectifier circuit  202  and the second rectifier circuit  212  rectify an AC input voltage supplied from an input power source  20  and respectively output a voltage having a pulse waveform. The first rectifier circuit  202  and the second rectifier circuit  212  may be respectively a circuit (e.g., a bridge rectifier circuit) comprising a plurality of diode elements. 
     The first smoothing circuit L 1 , C 1  and the second smoothing circuit L 2 , C 2  smooth the voltage rectified by the first rectifier circuit  202  and the second rectifier circuit  212  and respectively output a DC link voltage. The first smoothing circuit L 1 , C 1  comprises a first inductor L 1  and a first DC link capacitor C 1 , and the second smoothing circuit L 2 , C 2  comprises a second inductor L 2  and a second DC link capacitor C 2 . 
     The first inverter circuit  204  and the second inverter circuit  214  convert the DC link voltage output from the first smoothing circuit L 1 , C 1  and the second smoothing circuit L 2 , C 2  respectively to an AC voltage for driving the first working coil  132  and the second working coil  142 . 
     In one embodiment of the present disclosure, the first inverter circuit  204  and the second inverter circuit  214  may be comprised of a different topology. For example, as illustrated in  FIG.  2   , the first inverter circuit  204  may be embodied as a full bridge inverter circuit comprising four switching elements, i.e., 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 , and the second inverter circuit  214  may be embodied as a half bridge inverter circuit comprising two switching elements, i.e., a fifth switching element SW 5 , a sixth switching element SW 6  and two capacitors C 3 , C 4 . However, the topology of the first inverter circuit  204  and the second inverter circuit  214  may vary depending on embodiments. 
     The topologies of the first inverter circuit  204  and the second inverter circuit  214  differ as described above, such that a first driving frequency of the first working coil  132  that is driven by the first inverter circuit  204 , and a second driving frequency of the second working coil  142  that is driven by the second inverter circuit  204  may be set differently. When two inverter circuits are respectively embodied as a full bridge inverter circuit and a half bridge inverter circuit as illustrated in  FIG.  2   , a difference between the driving frequencies of the working coils increases. Accordingly, as will be described later, when the driving frequency is adjusted to remove interference noise, the amount of adjustment of the driving frequency decreases. 
     The first switching element SW 1  and the second switching element SW 2 , included in the first inverter circuit  204 , are alternately turned on/turned off by a first switching signal S 1  and a second switching signal S 2  output from a first driving circuit  22 . Additionally, the third switching element SW 3  and the fourth switching element SW 4 , included in the first inverter circuit  204 , are alternately turned on/turned off by a third switching signal S 3  and a fourth switching signal S 4  output from the first driving circuit  22 . 
     Further, the fifth switching element SW 5  and the sixth switching element SW 6 , included in the second inverter circuit  214 , are alternately turned on/turned off by a fifth switching signal S 5  and a sixth switching signal S 6  output from a second driving circuit  24 . 
     In the present disclosure, two switching elements&#39; being turned on/turned off alternately denotes that one switching element is turned on while the other switching element is turned off. For example, in  FIG.  2   , the first switching element SW 1  and the third switching element SW 3  are turned on while the second switching element SW 2  and the fourth switching element SW 4  are turned off. When the first switching element SW 1  and the third switching element SW 3  are turned off while the second switching element SW 2  and the fourth switching element SW 4  are turned on. Since the switching elements&#39; alternate turn-on/turn-off is repeated, a DC link voltage input to each inverter circuit  204 ,  214  is converted into an AC voltage. Thus, AC current for driving each working coil  132 ,  142  is supplied to each working coil  132 ,  142 . 
     In the present disclosure, the first switching signal S 1 , the second switching signal S 2 , the third switching signal S 3 , the fourth switching signal S 4 , the fifth switching signal S 5  and the sixth switching signal S 6  are respectively a pulse width modulation (PWM) signal that has a predetermined duty ratio. 
     As AC current output from the first inverter circuit  204  is supplied to the first working coil  132 , the first working coil  132  operates. As the first working coil  132  operates, while eddy current flows in a container placed on the first working coil  132 , the container is heated. Likewise, as AC current output from the second inverter circuit  214  is supplied to the second working coil  142 , the second working coil  214  operates. The magnitude of heat energy supplied to a container varies depending on the magnitude of power that is generated as each working coil operates, i.e., the output value of each working coil. 
     The controller  2  determines a driving frequency of each working coil  132 ,  142  such that the driving frequency corresponds to each heating zone&#39;s power level set by the user. In one embodiment of the present disclosure, the controller  2  may determine a driving frequency of each working coil  132 ,  142  with reference to a table data on which driving frequencies corresponding to power levels are recorded or a relational expression (or a function) between each power level and each driving frequency. 
     The controller  2  supplies a control signal corresponding to the determined driving frequency to each driving circuit  22 ,  24 . Each of the driving circuits  22 ,  25  outputs a switching signal S 1 , S 2 , S 3 , S 4 , S 5 , S 6  having a duty ratio corresponding to the diving frequency determined by the controller  220 , based on the control signal output from the controller  2 . 
     As the user turns on (powers on) the induction heating device  10  by manipulating a control interface of the induction heating device  10 , power is supplied from the input power source  20  to the induction heating device, and the induction heating device is put on standby for driving. Then the user places a container on a working coil of the induction heating device, and gives an instruction to initiate heating to the working coil by setting a power level for the container. As the user gives the instruction to initiate heating, an output value required of the working coil  132 , i.e., a required output value, is determined based on the power level set by the user. 
     Having received the user&#39;s instruction to initiate heating, the controller  220  sets a driving frequency corresponding to the required output value and supplies a control signal corresponding to the set driving frequency to the driving circuit  22 ,  24 . Accordingly, as switching signals S 1 , S 2 , S 3 , S 4 , S 5 , S 6  are output from the driving circuit  22 ,  24 , the working coil  132 ,  142  operates. As the working coil  132 ,  142  operates, the container is heated while eddy current flows in the container. 
     Additionally, as the first working coil  132  and the second working coil  142  operate respectively after the user sets a power level of the first heating zone  12  and a power level of the second heating zone  14 , the above-described interference noise may be generated. The controller  2  in one embodiment calculates a difference between a first driving frequency of the first working coil  132  and a second driving frequency of the second working coil  142 , compares the calculated difference with a predetermined reference range and performs noise avoidance control. In the present disclosure, the difference between the first driving frequency and the second driving frequency denotes a value obtained by subtracting the lower of the two driving frequencies from the higher of the two driving frequencies. 
       FIG.  3    is a graph showing output values corresponding to driving frequencies of each working coil when two working coils operate. 
       FIG.  3    is a graph  31  showing output values based on driving frequencies of the first working coil  132 , and a graph  32  showing output values based on driving frequencies of the second working coil  142 , respectively. 
     As illustrated in  FIG.  3   , the magnitude of a resonance frequency fr 1  of the first working coil  132  is less than that of a resonance frequency fr 2  of the second working coil  142 . Accordingly, the magnitude of the first driving frequency fd 1  of the first working coil  132  is also less than that of the second driving frequency fd 2  of the second working coil  142 . 
     In one embodiment of the present disclosure, the driving frequency of each working coil is greater than a limiting frequency. The limiting frequency is the same as or greater than the resonance frequency of each working coil. Accordingly, as the magnitude of the driving frequency of each working coil increases as illustrated in  FIG.  3   , the output value of each working coil decreases. 
     The controller  2  calculates a difference between the first driving frequency fd 1  of the first working coil  132  and the second driving frequency fd 2  of the second working coil  142 , and compares the calculated difference with a first predetermined reference range and a second predetermined reference range and performs noise avoidance control. 
     In one embodiment of the present disclosure, the first reference range and the second reference range may vary respectively depending on embodiments. For example, the first reference range may be set to 8 kHz or greater and less than 20 kHZ, and the second reference range may be set to 2 kHz or greater and less than 8 kHZ. However, the boundary values (e.g., 2, 8 and 20) of the first reference range and the second reference range may vary depending on embodiments. 
     If the difference between the first driving frequency and the second driving frequency is included in the first reference range, the controller  2  performs divide mode noise avoidance control. In the present disclosure, the divide mode noise avoidance control denotes changing the difference between the first driving frequency and the second driving frequency to a value greater than a maximum boundary value (e.g., 20 kHz) of an audio frequency band (e.g., 2 kHz to 20 kHz) and removing interference noise. 
     As the divide mode noise avoidance control starts, the controller  2  adjusts a duty ratio of a switching signal input to the first inverter circuit  204  having the first driving frequency less than the second driving frequency and adjusts the magnitude of the first driving frequency, and sets the difference between the first driving frequency and the second driving frequency to be equal to a predetermined noise avoidance value (e.g., 22 kHz). Under the control, the difference between the first driving frequency and the second driving frequency are outside the audio frequency band, and interference noise is removed. The noise avoidance value varies depending on embodiments. 
     If the difference between the first driving frequency and the second driving frequency is included in the second reference range, the controller  2  performs coupling mode noise avoidance control. In the present disclosure, the coupling mode noise avoidance control denotes changing the difference between the first driving frequency and the second driving frequency to 0 that is a value not included in the audio frequency band (e.g., 2 kHz to 20 kHz) and removing interference noise. 
     As the coupling mode noise avoidance control starts, the controller  2  sets the first driving frequency and the second driving frequency to the same value. In an example, the controller  2  increases the magnitude of the first driving frequency to be equal to the second frequency value or decreases the magnitude of the second driving frequency to be equal to the first driving frequency. In another example, the controller  2  may change the first driving frequency and the second driving frequency respectively to a predetermined frequency. Under the control, the difference between the first driving frequency and the second driving frequency becomes 0, causing no interference noise. 
     If the difference between the first driving frequency and the second driving frequency is not included in the first reference range and the second reference range, the controller  2  does not change the first driving frequency and the second driving frequency. The difference between the first driving frequency and the second driving frequency outside the first reference range and the second reference range denotes that the difference between the first driving frequency and the second driving frequency is outside the audio frequency band and that interference noise is not generated. Accordingly, the controller  2  does not change the first driving frequency and the second driving frequency and maintains the first driving frequency and the second driving frequency. 
       FIG.  4    is a waveform diagram for describing duty ratios of switching signals input to an inverter circuit of the induction heating device of one embodiment. 
     When the difference between the first driving frequency and the second driving frequency is included in the first reference range as described above, the controller  2  adjusts the duty ratio of the switching signal input to the first inverter circuit and the magnitude of the first driving frequency, and sets the difference between the first driving frequency and the second driving frequency to be equal to the noise avoidance value. 
       FIG.  4    shows the wave form of a switching signal input to the first inverter circuit  204  or the second inverter circuit  214  of the induction heating device  10  according to the present disclosure. As illustrated, the voltage of the switching signal changes to a high level or a low level alternately depending on a predetermined cycle T. When the voltage of the switching signal is at a high level, the switching element is turned on, and when the voltage of the switching signal is at a low level, the switching element is turned off. 
     The section where the voltage of the switching signal is at a high level within one cycle T of the switching signal is referred to as an on-duty section, and the section where the voltage of the switching signal is at a low level is referred to as an off-duty section. In the present disclosure, the duty ratio of a switching signal is defined as a ratio of the on-duty section t 1  in one cycle T of the switching signal. In an example, under the assumption that one cycle of a switching signal is 1 second and the time of the on-duty section is 0.5 second, the duty ratio of the switching signal is 0.5 or 50%. In another example, under the assumption that one cycle of a switching signal is one second and that the time of the on-duty section is 0.2 second, the duty ratio of the switching signal is 0.2 or 20%. 
     Additionally, the switching signals provided to the inverter circuit are alternately at a high level voltage and a low level voltage. In an example, when the voltages of the first switching element SW 1  and the third switching element SW 3  input to the first inverter circuit  204  are at a high level, the voltages of the second switching signal SW 2  and the fourth switching signal SW 4  are at a low level. In another example, when the voltage of the fifth switching signal SW 5  input to the second inverter circuit  214  is at a low level, the voltage of the sixth switching signal SW 6  is at a high level. 
     Hereafter, a control method according to the present disclosure is described based on the controller  2 &#39;s adjustment of the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  among the switching signals input to the first inverter circuit  204 . However, in another embodiment, the controller  2  may also adjust the duty ratios of the second switching signal SW 2  and the fourth switching signal SW 4 . 
       FIG.  5    is a graph showing a change in the output values of a working coil, based on the adjustment of duty ratios of switching signals input to the inverter circuit of the induction heating device of one embodiment and the adjustment of driving frequencies of the working coil. 
       FIG.  5    respectively shows a graph  51  that shows the driving frequency and the output value of the first working coil  132  when the duty ratio of the first switching signal SW 1  and the third switching signal SW 3  input to the first inverter circuit  204  is 50%, and a graph  52  that shows the driving frequency and the output value of the first working coil  132  when the duty ratio of the first swishing signal SW 1  and the third switching signal SW 3  input to the first inverter circuit  204  is 20%. 
     In one embodiment of the present disclosure, as the controller  2  adjusts the duty ratio of the switching signal input to the inverter circuit as illustrated in  FIG.  5   , the output value corresponding to the driving frequency of the working coil changes. 
     For example, when the first working coil  132  operates at a driving frequency of X 1  at the first switching signal SW 1  and the third switching signal SW 3 &#39;s duty ratio of 50%, an actual output value of the first working coil  132  becomes P 1 . 
     If the driving frequency of the first working coil  132  changes to X 2  in the state where the duty ratio of the first switching signal SW 1  and the third switching signal SW 3  is maintained at 50%, The actual output value of the first working coil  132  becomes much greater than P 1 . Accordingly, the actual output value of the first working coil  132  becomes greater than a first required output value of the first working coil  132 , corresponding to a power level set by the user. 
     However, when the duty ratio of the first switching signal SW 1  and the third switching signal SW 3  decreases to 20% in the state where the driving frequency of the first working coil  132  is maintained at X 1 , the actual output value of the first working coil  132  decreases to P 2 . 
     Additionally, when the driving frequency of the first working coil  132  decreases to X 2  in the state where the duty ratio of the first switching signal SW 1  and the third switching signal SW 3  is maintained at 20%, the actual output value of the first working coil  132  becomes P 1  again. 
     Under the control, when the controller  2  changes the driving frequency of the first working coil  132  from X 1  to X 2 , the actual output value of the first working coil  132  may not increase and be maintained at P 1 . 
     Hereafter, embodiments in relation to the controller  2 &#39;s performance of the divide mode noise avoidance control in the induction heating device of one embodiment are described with reference to  FIGS.  6  to  8   . In the embodiments of  FIGS.  6  to  8   , the magnitude of the first driving frequency fd 1  is less than the magnitude of the second driving frequency fd 2 . 
       FIG.  6    is a graph showing a change in the driving frequency and output value of a working coil, under the divide mode noise avoidance control of the induction heating device of one embodiment. 
       FIG.  6    respectively shows a graph  61  that shows the driving frequency and the output value of the first working coil  132  when the duty ratio of the first switching signal SW 1  and the third switching signal SW 3  input to the first inverter circuit  204  is 50%, and a graph  62  that shows the driving frequency and the output value of the second working coil  142  when the duty ratio of the fifth swishing signal SW 5  input to the second inverter circuit  214  is 20%. 
     As the user sets power levels of the first heating zone  12  and the second heating zone  14  respectively and inputs an instruction to initiate heating, the controller  2  provides a control signal respectively to the first driving circuit  22  and the second driving circuit  24 . Accordingly, the first working coil  132  operates at a first driving frequency fd 1  corresponding to a first required output value of P 1  corresponding to the power level set by the user. Further, the second working coil  142  operates at a second driving frequency fd 2  corresponding to a second required output value of P 2  corresponding to the power level set by the user. 
     As the first working coil  132  and the second working coil  142  operate respectively, the controller  2  calculates a difference between the first driving frequency fd 1  of the first working coil  132  and the second driving frequency fd 2  of the second working coil  142 . 
     The controller  2  determines whether the calculated difference between the first driving frequency fd 1  and the second driving frequency fd 2  are included in the first reference range (8 kHz or greater and less than 20 kHZ). When the difference between the first driving frequency fd 1  and the second driving frequency fd 2  is included in the first reference range (8 kHz or greater and less than 20 kHZ) as a result of the determination, the controller  2  performs the divide mode noise avoidance control to remove interference noise. 
     As the divide mode noise avoidance control starts, the controller  2  changes the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  input to the first inverter circuit  204  to a predetermined reference duty ratio (e.g., 20%). Accordingly, the output value of the first working coil  132  in graph  63  becomes entirely less than the output value of the first working coil  132  in graph  61  where the duty ratio is 50%, at the same driving frequency. 
     The reference duty ratio may vary depending on embodiments. In one embodiment of the present disclosure, the reference duty ratio may be set to the lowest (a minimum duty ratio) of the duty ratios of the first switching signal SW 1  and the third switching signal SW 3 , which can be set by the controller  2 , but not limited. 
     After changing the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  to the predetermined reference duty ratio (e.g., 20%), the controller  2  changes the driving frequency of the first working coil  132  to a noise avoidance frequency fd 1 ′. The noise avoidance frequency fd 1 ′ is set to a value fd 2 −22 kHz obtained by subtracting the noise avoidance value (22 kHz) from the second driving frequency fd 2 . 
     After the driving frequency of the first working coil  132  changes to the noise avoidance frequency fd 1 ′, the controller  2  determines whether the actual output value of the first working coil  132  is the same as the first required output value set by the user. Since the actual output value of the first working coil  132  is maintained at the same value as the first required output value of P 1  as illustrated in  FIG.  6   , the controller  2  ends the divide mode noise avoidance control. 
     If the driving frequency of the first working coil  132  changes to the noise avoidance frequency fd 1 ′ in the state where the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  are 50%, the actual output value of the first working coil  132  becomes greater than the first working coil  132 &#39;s first required output value of P 1  set by the user. 
     However, in the control method of the present disclosure, as the driving frequency of the first working coil  132  changes to the noise avoidance frequency fd 1 ′ after the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  decrease to the reference duty ratio, the actual output value of the first working coil  132  may be maintained at the same value as the first working coil  132 &#39;s first required output value of P 1  set by the user. 
     Further, as the driving frequency of the first working coil  132  changes to the noise avoidance frequency fd 1 ′, interference noise is removed since a difference 22 kHz between the noise avoidance frequency fd 1 ′, which is the driving frequency of the first working coil  132 , and the second driving frequency fd 2  is not included in the audio frequency band (2 kHz to 20 kHz). 
       FIG.  7    is a graph showing a change in the driving frequency and output value of a working coil, under a divide mode noise avoidance control of an induction heating device of another embodiment. 
       FIG.  7    respectively shows a graph  71  that shows the driving frequency and the output value of a first working coil  132  when the duty ratio of a first switching signal SW 1  and a third switching signal SW 3  input to a first inverter circuit  204  is 50%, and a graph  72  that shows the driving frequency and the output value of a second working coil  142  when the duty ratio of a fifth swishing signal SW 5  input to a second inverter circuit  214  is 50%. 
     As the user sets power levels of the first heating zone  12  and the second heating zone  14  respectively and inputs an instruction to initiate heating, a controller  2  provides a control signal respectively to the first driving circuit  22  and the second driving circuit  24 . Accordingly, the first working coil  132  operates at a first driving frequency fd 1  corresponding to a first required output value of P 1  corresponding to the power level set by the user. Further, the second working coil  142  operates at a second driving frequency fd 2  corresponding to a second required output value of P 2  corresponding to the power level set by the user. 
     As the first working coil  132  and the second working coil  142  operate respectively, the controller  2  calculates a difference between the first driving frequency fd 1  of the first working coil  132  and the second riving frequency fd 2  of the second working coil  142 . 
     The controller  2  determines whether the calculated difference between the first driving frequency fd 1  and the second riving frequency fd 2  is included in the first reference range (8 kHz or greater and less than 20 kHZ). When the difference between the first driving frequency fd 1  and the second riving frequency fd 2  is included in the first reference range (8 kHz or greater and less than 20 kHZ) as a result of the determination, the controller  2  performs the divide mode noise avoidance control to remove interference noise. 
     As the divide mode noise avoidance control starts, the controller  2  changes the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  input to the first inverter circuit  204  to a predetermined reference duty ratio (e.g., 20%). Accordingly, the output value of the first working coil  132  in graph  73  becomes entirely less than the output value of the first working coil  132  in graph  71  where the duty ratio is 50%, at the same driving frequency. 
     The reference duty ratio may vary depending on embodiments. In one embodiment of the present disclosure, the reference duty ratio may be set to the lowest (a minimum duty ratio) of the duty ratios of the first switching signal SW 1  and the third switching signal SW 3 , which can be set by the controller  2 , but not limited. 
     After changing the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  to the predetermined reference duty ratio (e.g., 20%), the controller  2  changes the driving frequency of the first working coil  132  to a noise avoidance frequency fd 1 ′. The noise avoidance frequency fd 1 ′ is set to a value fd 2 −22 kHz obtained by subtracting the noise avoidance value (22 kHz) from the second driving frequency fd 2 . 
     After the driving frequency of the first working coil  132  changes to the noise avoidance frequency fd 1 ′, the controller  2  determines whether the actual output value of the first working coil  132  is the same as the first required output value set by the user. When the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  are the reference duty ratio (20%), and the driving frequency of the first working coil  132  is the noise avoidance frequency fd 1 ′ as illustrated in  FIG.  7   , the actual output value of the first working coil  132  becomes P 3  less than the first required output value of P 1 . 
     Since the actual output value of the first working coil  132  is less than the first required output value of P 1  set by the user as a result of the determination, the controller  2  increases the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  to a value (e.g., 30%) greater than the reference duty ratio. Accordingly, the output value of the first working coil  132  in graph  74  becomes entirely greater than the output value of the first working coil  132  in graph  73  where the duty ratio is 20%, at the same driving frequency. 
     After the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  change to 30%, the controller  2  determines whether the actual output value of the first working coil  132  is the same as the first required output value set by the user. Since the actual output value of the first working coil  132  is the same as the first required output value of P 1  as a result of the determination, the controller  2  ends the divide mode noise avoidance control. 
       FIG.  8    is a graph showing a change in the driving frequency and output value of a working coil, under a divide mode noise avoidance control of an induction heating device of yet another embodiment. 
       FIG.  8    respectively shows a graph  81  that shows the driving frequency and the output value of a first working coil  132  when the duty ratio of a first switching signal SW 1  and a third switching signal SW 3  input to a first inverter circuit  204  is 50%, and a graph  82  that shows the driving frequency and the output value of a second working coil  142  when the duty ratio of a fifth swishing signal SW 5  input to a second inverter circuit  214  is 50%. 
     As the user sets power levels of the first heating zone  12  and the second heating zone  14  respectively and inputs an instruction to initiate heating, a controller  2  provides a control signal respectively to the first driving circuit  22  and the second driving circuit  24 . Accordingly, the first working coil  132  operates at a first driving frequency fd 1  corresponding to a first required output value of P 1  corresponding to the power level set by the user. Further, the second working coil  142  operates at a second driving frequency fd 2  corresponding to a second required output value of P 2  corresponding to the power level set by the user. 
     As the first working coil  132  and the second working coil  142  operate respectively, the controller  2  calculates a difference between the first driving frequency fd 1  of the first working coil  132  and the second riving frequency fd 2  of the second working coil  142 . 
     The controller  2  determines whether the calculated difference between the first driving frequency fd 1  and the second riving frequency fd 2  is included in the first reference range (8 kHz or greater and less than 20 kHZ). When the difference between the first driving frequency fd 1  and the second riving frequency fd 2  is included in the first reference range (8 kHz or greater and less than 20 kHZ) as a result of the determination, the controller  2  performs the divide mode noise avoidance control to remove interference noise. 
     As the divide mode noise avoidance control starts, the controller  2  changes the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  input to the first inverter circuit  204  to a predetermined reference duty ratio (e.g., 30%). Accordingly, the output value of the first working coil  132  in graph  83  becomes entirely less than the output value of the first working coil  132  in graph  81  where the duty ratio is 50%, at the same driving frequency. 
     The reference duty ratio may vary depending on embodiments. In one embodiment of the present disclosure, the reference duty ratio may be set to the lowest (a minimum duty ratio) of the duty ratios of the first switching signal SW 1  and the third switching signal SW 3 , which can be set by the controller  2 , but not limited. 
     After changing the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  to the predetermined reference duty ratio (e.g., 30%), the controller  2  changes the driving frequency of the first working coil  132  to a noise avoidance frequency fd 1 ′. The noise avoidance frequency fd 1 ′ is set to a value fd 2 −22 kHz obtained by subtracting the noise avoidance value (22 kHz) from the second driving frequency fd 2 . 
     After the driving frequency of the first working coil  132  changes to the noise avoidance frequency fd 1 ′, the controller  2  determines whether the actual output value of the first working coil  132  is the same as the first required output value set by the user. When the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  are the reference duty ratio (30%), and the driving frequency of the first working coil  132  is the noise avoidance frequency fd 1 ′ as illustrated in  FIG.  8   , the actual output value of the first working coil  132  becomes P 3  greater than the first required output value of P 1 . 
     Since the actual output value of the first working coil  132  is greater than the first required output value of P 1  set by the user as a result of the determination, the controller  2  decreases the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  to a value (e.g., 20%) less than the reference duty ratio. Accordingly, the output value of the first working coil  132  in graph  84  becomes entirely less than the output value of the first working coil  132  in graph  83  where the duty ratio is 30%, at the same driving frequency. 
     After the duty ratios of the first switching signal SW 1  and the third switching signal SW 3  change to 20%, the controller  2  determines whether the actual output value of the first working coil  132  is the same as the first required output value set by the user. Since the actual output value of the first working coil  132  is the same as the first required output value of P 1  as a result of the determination, the controller  2  ends the divide mode noise avoidance control. 
     As shown in the embodiments of  FIGS.  6  to  8   , in the divide mode noise avoidance control performed by the controller  2  of the induction heating device  10  according to the present disclosure, the duty ratio of a switching signal input to the inverter circuit of a driving frequency (e.g., a first driving frequency) having a relatively small magnitude changes to the reference duty ratio. Then since the driving frequency (e.g., the first driving frequency) having a relatively small magnitude changes to the noise avoidance frequency, interference noise caused by a simultaneous operation of the first working coil  132  and the second working coil  142  is removed. 
     When the actual output value of the working coil (e.g., the first working coil  132 ) the driving frequency of which changes is different from the first required output value set by the user after the interference noise is removed based on the change in the frequency, the controller  2  additionally adjusts the duty ratio of the switching signal input to the inverter circuit of the driving frequency (e.g., the first driving frequency) having a relatively small magnitude. Based on the control, even after the interference noise is removed, the actual output value of each working coil may be maintained to be the same as the first required output value set by the user. 
       FIG.  9    is a flowchart showing a control method of an induction heating device of one embodiment. 
     As a first working coil  132  operates at a first driving frequency while a second working coil  142  operates at a second driving frequency greater than the first driving frequency, a controller  2  calculates a difference between the first driving frequency and the second driving frequency ( 902 ). 
     The controller  2  compares the calculated difference between the first driving frequency and the second driving frequency with a predetermined reference range ( 904 ). In one embodiment of the present disclosure, the reference range may comprise a first reference range for divide mode noise avoidance control, and a second reference range for coupling mode noise avoidance control. 
     When the difference between the first driving frequency and the second driving frequency is included in the first reference range as a result of the comparison, the controller  2  adjusts the duty ratio of a switching signal S 1 , S 3  input to a first inverter circuit  204  that supplies electric current to the first working coil  132 , and the magnitude of the first driving frequency, and sets the difference between the first driving frequency and the second driving frequency to be equal to a predetermined noise avoidance value K ( 906 ). 
     In one embodiment of the present disclosure, step  906  comprises changing the duty ratio of the switching signal input to the first inverter circuit  204  to a predetermined reference duty ratio and changing the magnitude of the first driving frequency to a noise avoidance frequency when the difference between the first driving frequency and the second driving frequency is included in the first predetermined reference range. The noise avoidance frequency is defined as a value obtained by subtracting the noise avoidance value k from the second driving frequency. 
     In one embodiment of the present disclosure, step  906  comprises increasing the duty ratio of the switching signal input to the first inverter circuit  204  when the actual output value of the first working coil  132  is less than a first required output value of the first working coil  132  after the magnitude of the first driving frequency changes. 
     In one embodiment of the present disclosure, step  906  comprises decreasing the duty ratio of the switching signal input to the first inverter circuit  204  when the actual output value of the first working coil  132  is greater than the first required output value after the magnitude of the first driving frequency changes. 
     Based on the above-described control, after the duty ratio of the switching signal input to the first inverter circuit  204  is adjusted, the actual output value of the first working coil  132  is maintained to be the same as the first required output value. 
       FIG.  10    is a flowchart showing a control method of an induction heating device of another embodiment. 
     As a first working coil  132  operates at a first driving frequency while a second working coil  142  operates at a second driving frequency greater than the first driving frequency, a controller  2  calculates a difference between the first driving frequency and the second driving frequency ( 1002 ). 
     The controller  2  determines whether the calculated difference between the first driving frequency and the second driving frequency is included in a first reference range (e.g., 8 kHz or grater and less than 20 kHZ) ( 1004 ). When the difference between the first driving frequency and the second driving frequency is included in the first reference range as a result of the determination, the controller  2  performs divide mode noise avoidance control ( 1006 ) and removes interference noise caused by a simultaneous operation of the first working coil  132  and the second working coil  142 . 
       FIG.  11    is a flowchart showing a process of performing divide mode noise avoidance control in the control method of an induction heating device of another embodiment. 
     As the divide mode noise avoidance control ( 1006 ) starts, the controller  2  changes the duty ratio of the switching signal (e.g., S 1  and S 3 ) input to the first inverter circuit  204  changes to a predetermined reference duty ratio (e.g., 20%) ( 1102 ). The reference duty ratio may vary depending on embodiments. 
     Then the controller  2  decreases the magnitude of the first driving frequency ( 1104 ). In one embodiment of the present disclosure, the controller  2  changes the first driving frequency to a predetermined noise avoidance frequency to decrease the magnitude of the first driving frequency. The noise avoidance frequency is defined as a value obtained by subtracting a noise avoidance value k from the second driving frequency. 
     After the magnitude of the first driving frequency decreases, the controller  2  determines whether the actual output value of the first working coil  132  is the same as a first required output value of the first working coil  132 , set by the user ( 1106 ). 
     When the actual output value of the first working coil  132  is not the same as the first required output value of the first working coil  132 , set by the user, as a result of the determination, the controller  2  adjusts the duty ratio of the switching signal (e.g., S 1  and S 3 ) input to the first inverter circuit  204  ( 1108 ). When the actual output value of the first working coil  132  is less than the first required output value of the first working coil  132 , set by the user, in step  1108 , the controller  2  increases the duty ratio of the switching signal input to the first inverter circuit  204 . On the contrary, when the actual output value of the first working coil  132  is greater than the first required output value of the first working coil  132 , set by the user, in step  1108 , the controller  2  decreases the duty ratio of the switching signal input to the first inverter circuit  204 . After ending the adjustment of the duty ratio, the controller  2  performs step  1106  again. 
     When determining that the actual output value of the first working coil  132  is the same as the first required output value of the first working coil  132 , set by the user, in step  1106 , the controller  2  ends the divide mode noise avoidance control ( 1006 ). 
     Referring back to  FIG.  10   , when determining that the difference between the first driving frequency and the second driving frequency is not included in the first reference range in step  1104 , the controller  2  determines whether the difference between the first driving frequency and the second driving frequency is included in the second reference range (e.g., 2 kHz or greater and less than 8 kHZ) ( 1008 ). 
     When the difference between the first driving frequency and the second driving frequency is included in the second reference range as a result of the determination, the controller  2  performs coupling mode noise avoidance control ( 1010 ). In one embodiment of the present disclosure, the step ( 101 ) of performing the coupling mode noise avoidance control comprises setting the first driving frequency and the second driving frequency to the same value. 
     When the difference between the first driving frequency and the second driving frequency is not included in the second reference range as a result of the determination in step  1008 , the controller  2  maintains the first driving frequency and the second driving frequency since interference noise is not caused by a simultaneous operation of the first working coil  132  and the second working coil  142 . 
     In the above-described control method according to the present disclosure, interference noise caused by a simultaneous operation of two working coils is removed. Additionally, in the process of adjusting the driving frequency of a working coil to remove interference noise, the actual output value of the working coil may be maintained to be the same as the required output value. 
     The embodiments are described above with reference to a number of illustrative embodiments thereof. However, embodiments are not limited to the embodiments and drawings set forth herein, and numerous other modifications and embodiments can be made by one skilled in the art within the technical scope of the disclosure. Further, predictable effects as well as the effects based on the configurations in the disclosure are to be included within the scope of the disclosure though not explicitly described in the description of the embodiments.