Patent Publication Number: US-2021185774-A1

Title: System and method for tuning an induction circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application from U.S. application Ser. No. 15/790,414 entitled SYSTEM AND METHOD FOR TUNING AN INDUCTION CIRCUIT, filed on Oct. 23, 2017, by Salvatore Baldo et al., now U.S. Pat. No. ______, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to an induction cooktop and, more particularly, to a circuit configuration and method of operation for an induction cooktop. 
     BACKGROUND 
     Induction cooktops are devices which exploit the phenomenon of induction heating for food cooking purposes. The disclosure provides for a power circuit for an induction cooktop configured to provide improved performance while maintaining an economical design. The improved performance may be provided by an increased range of operating power for induction cooktops. Accordingly, the disclosure provides for systems and methods of controlling the operating power of induction cooktops. 
     SUMMARY 
     According to one aspect of the present invention, a method for controlling a heating operation of an induction cooktop. A direct current (DC) power is generated from an alternating current (AC) power source and supplied to a first resonant inverter and a second resonant inverter via a power supply bus. A switching frequency of each of the first resonant inverter and the second resonant inverter is controlled and, in response to the switching frequency, supplied to a plurality of induction coils of the resonant inverters, such that an electromagnetic field is generated. A selective tuning operation of the first resonant inverter or the second resonant inverter includes controlling a connection of a capacitor to either the first resonant inverter or the second resonant inverter. 
     According to another aspect of the present invention, an induction cooking system includes a power supply bus configured to generate direct current (DC) power, a first resonant inverter, and a second resonant inverter in connection with the power supply bus. A plurality of induction coils are configured to generate an electromagnetic field in connection with the plurality of resonant inverters. At least one switch is configured to control a connection of a tuning capacitor with either the first resonant inverter or the second resonant inverter. The system further includes at least one controller configured to control a switching frequency of each of the first resonant inverter and the second resonant inverter supplied to the plurality of induction coils of the resonant inverters. The switching frequency controls the electromagnetic field. The controller is further configured to control the connection of the tuning capacitor with either the first resonant inverter or the second resonant inverter via the at least one switch. 
     According to yet another aspect of the present invention, a method for controlling an induction heating system is disclosed. The method includes generating a direct current (DC) power from an alternating current (AC) power source and supplying the DC power to a first resonant inverter and a second resonant inverter via a power supply bus. A switching frequency of each of the first resonant inverter and the second resonant inverter is controlled generating an electromagnetic field in response to the switching frequency supplied to a plurality of induction coils of the resonant inverters. A selective tuning operation of either the first resonant inverter or the second resonant inverter is applied by controlling a connection of a tuning capacitor to either the first resonant inverter or the second resonant inverter. The selective tuning operation includes connecting the tuning capacitor in parallel with a first dedicated capacitor of the first resonant inverter in a first configuration, and alternatively connecting the tuning capacitor in parallel with a second dedicated capacitor of the second resonant inverter in a second configuration. 
     These and other objects of the present disclosure may be achieved by means of a cooktop incorporating the features set out in the appended claims, which are an integral part of the present description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further objects and advantages of the present disclosure may become more apparent from the following detailed description and from the annexed drawings, which are provided by way of a non-limiting example, wherein: 
         FIG. 1  is a top view of a cooktop according to the present disclosure; 
         FIG. 2  is a schematic representation of an exemplary embodiment of a driving circuit for an induction cooking system; 
         FIG. 3  is a schematic representation of an exemplary embodiment of a driving circuit for an induction cooking system; 
         FIG. 4  is a schematic representation of an exemplary embodiment of a driving circuit for an induction cooking system; 
         FIG. 5  is a plot of a system response of an exemplary embodiment of an inverter; 
         FIG. 6  is a plot of power generated by two different resonant capacitors over a range of switching frequencies demonstrating a shift in an operating frequency; and 
         FIG. 7  is a schematic representation of an exemplary embodiment of a driving circuit for an induction cooking system in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the device as oriented in  FIG. 1 . However, it is to be understood that the device may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
     Conventional induction cooktops may comprise a top surface made of glass-ceramic material upon which cooking units are positioned (hereinafter “pans”). Induction cooktops operate by generating an electromagnetic field in a cooking region on the top surface. The electromagnetic field is generated by inductors comprising coils of copper wire, which are driven by an oscillating current. The electromagnetic field has the main effect of inducing a parasitic current inside a pan positioned in the cooking region. In order to efficiently heat in response to the electromagnetic field, the pan may be made of an electrically conductive ferromagnetic material. The parasitic current circulating in the pan produces heat by dissipation; such heat is generated only within the pan and acts without directly heating the cooktop. 
     Induction cooktops have a better efficiency than electric cooktops (i.e. a greater fraction of the absorbed electric power is converted into heat that heats the pan). The presence of the pan on the cooktop causes the magnetic flux close to the pan itself causing the power to be transferred towards the pan. The disclosure provides for a device and method for increasing the performance of a Quasi Resonant inverter that may be used in economical induction cooktops. In particular, the methods and devices proposed increase the regulation range of AC-AC Quasi Resonant (QR) inverters arranged in couples to supply two independent induction pancake coils. 
     QR inverters or resonant inverters are widely used as AC current generators for induction cooktops. Such inverters, also called Single Ended inverters, are particularly attractive because they only require one solid state switch and only one resonant capacitor to generate a variable frequency/variable amplitude current to feed the induction coil. When properly designed and matched with their load, QR inverters are known to operate in a so called “soft-switching” mode of operation. The soft switching mode operates by a switching device commutating when either the voltage across it and/or the current flowing into it are null. In this sense, QR inverters may provide a reasonable compromise between cost and energy conversion efficiency. 
     One drawback of QR inverters is that the output power may be limited to a narrow range in the soft-switching mode of operation. In particular, when the output power being regulated falls below a given limit, the inverter fails in operating in a soft switching mode, leading to a dramatic and unmanageable increase in thermal losses and electro-magnetic interference (EMI). On the other hand, when the power being regulated exceeds a given limit, the resonating voltage across the solid state switch exceeds its maximum rating, leading to instantaneous and irreversible damage of the switching device itself. These two limitations may lead to a relatively low regulation range of the output power. The regulation range is defined as the ratio between a maximum power achievable and the minimum power achievable. The maximum power achievable is limited by a maximum voltage across the switch. The minimum power achievable is limited by a deep loss of a zero voltage switching at turn on. 
     The aforementioned limitations become exacerbated when multiple inverters are required to operate simultaneously and in synchronized manner. The limitations are compiled when operating two inverters because the frequency interval of allowed operation is reduced to the interval common frequency between the inverters. The common frequency interval is necessarily narrower than the individual frequency interval allowed by each of the individual QR inverters. More often than not, when the impedance of the induction coils are very different than one another, it is impossible to operate the coils simultaneously and at the same frequency without incurring severe inverter overstress. The systems and methods described herein substantially increase both the individual and the joint frequency operating regulation range of a dual QR inverter system without reducing efficiency and while preserving the soft switching operation. For clarity, the QR inverters discussed herein may be referred to as resonant inverters or inverters. 
     Referring to  FIG. 1 , a top view of a cooktop  10  is shown. The cooktop  10  may comprise a plurality of cooking hobs  12  oriented on a ceramic plate  14 . Beneath the ceramic plate  14  and corresponding to each of the hobs  12 , a plurality of induction coils  16  may be disposed in a housing  18 . The induction coils  16  may be in communication with a controller  20  configured to selectively activate the induction coils  16  in response to an input to a user interface  22 . The controller  20  may correspond to a control system configured to activate one or more of the induction coils  16  in response to an input or user selection. The induction coils  16  may each comprise a driving circuit controlled by the controller  20  that utilizes a switching device (e.g. a solid state switch) to generate a variable frequency/variable amplitude current to feed the induction coils  16 . In this configuration, the induction coils  16  are driven such that an electromagnetic field is generated to heat a pan  24 . Further discussion of the driving circuits of the induction coils  16  is provided in reference to  FIGS. 2-4 . 
     The user interface  22  may correspond to a touch interface configured to perform heat control and selection of the plurality of hobs  12  as represented on a cooking surface  28  of the cooktop  10 . The user interface  22  may comprise a plurality of sensors  30  configured to detect a presence of an object, for example a finger of an operator, proximate thereto. The sensors  30  may correspond to any form of sensors. In an exemplary embodiment, the sensors  30  may correspond to capacitive, resistive, and/or optical sensors. In an exemplary embodiment, the sensors  30  correspond to capacitive proximity sensors. 
     The user interface  22  may further comprise a display  32  configured to communicate at least one function of the cooktop  10 . The display  32  may correspond to various forms of displays, for example, a light emitting diode (LED) display, a liquid crystal display (LCD), etc. In some embodiments, the display  32  may correspond to a segmented display configured to depict one or more alpha-numeric characters to communicate a cooking function of the cooktop  10 . The display  32  may further be operable to communicate one or more error messages or status messages of the cooktop  10 . 
     Referring now to  FIGS. 2-4 , a schematic view of a driving circuit  42  is shown. In order to identify specific exemplary aspects of the driving circuits  42 , the various embodiments of the driving circuits  42  are referred to as a first driving circuit  42   a  demonstrated in  FIG. 2 , a second driving circuit  42   b  demonstrated in  FIG. 3 , and a third driving circuit  42   c  demonstrated in  FIG. 4 . For common elements, each of the specific exemplary embodiments may be referred to as the driving circuit  42 . Though specific features are discussed in reference to each of the first, second, and third driving circuits, each of the embodiments may be modified based on the combined teachings of the disclosure without departing from the spirit of the disclosure. 
     The driving circuit  42  comprises a plurality of inverters  44  configured to supply driving current to a first induction coil  16   a  and a second induction coil  16   b . The inverters  44  may correspond to resonant or QR inverters and each may comprise a switching device  46  (e.g. a first switching device  46   a  and a second switching device  46   b ). The switching devices  46  may correspond to solid state power switching devices, which may be implemented as an insulated-gate bipolar transistor (IGBT). The switching devices  46  may be supplied power via a direct current (DC) power supply  48  and may be controlled via a control signal supplied by the controller  20 . In this configuration, the controller  20  may selectively activate the induction coils  16  by controlling a switching frequency supplied to the switching devices  46  to generate the electromagnetic field utilized to heat the pan  24 . As discussed in the following detailed description, each of the driving circuits  42  may provide for an increased range in a switching frequency (f SW ) of the plurality of inverters  44  to drive the induction coils  16 . The induction coils  16  may correspond to independent induction coils or independent pancake coils. 
     The DC power supply  48  may comprise a bridge rectifier  50  and an input filter  51  configured to supply DC voltage to a DC-bus  52  from an alternating current (AC) power supply  54 . In this configuration, the current DC-bus  52  may be conducted to the inverters  44  across a DC-bus capacitor  56  separating the DC-bus  52  from a ground  58  or ground reference node. In this configuration, the DC power supply  48  may be configured to rectify periodic fluctuations in the AC power to supply DC current to the inverters  44 . The DC power supply  48  may be commonly implemented in each of the exemplary driving circuits  42  demonstrated in  FIG. 2  and is omitted from  FIGS. 3 and 4  to more clearly demonstrate the elements of the driving circuits  42 . 
     Still referring to  FIGS. 2-4 , the first inverter  44   a  and the second inverter  44   b  are in conductive connection with the DC-Bus  52  of the DC power supply  48 . The first inverter  44   a  may comprise a first dedicated resonant capacitor  60   a  and the first induction coil  16   a . The first dedicated resonant capacitor  60   a  may be connected in parallel with the first induction coil  16   a  from the DC-bus  52  to the first switching device  46   a . The second inverter  44   b  comprises a second dedicated resonant capacitor  60   b  and the second induction coil  16   b . The second dedicated resonant capacitor  60   b  may be connected in parallel with the second induction coil  16   b  from the DC-bus  52  to the second switching device  46   b . In an exemplary embodiment, the dedicated resonant capacitors  60  are dimensioned to establish the resonance in a desired frequency range in conjunction with a third resonant capacitor that may be selectively connected in parallel with either the first dedicated resonant capacitor  60   a  or the second dedicated resonant capacitor  60   b . The third resonant capacitor may be referred to herein as a tuning capacitor  62 . Examples of frequency ranges for operation of the inverters  44  are discussed further in reference to  FIGS. 5 and 6 . 
     The tuning capacitor  62  may be selectively connectable in parallel with either the first dedicated resonant capacitor  60   a  or the second dedicated resonant capacitor  60   b  via a two-way switch  64 . For example, the controller  20  of the cooktop  10  may be configured to control the switch  64  to a first switch configuration conductively connecting the tuning capacitor  62  in parallel with the first dedicated resonant capacitor  60   a  and the first induction coil  16   a . The first switch configuration as discussed herein is demonstrated in  FIG. 2 . The controller  20  may further be configured to control the switch  64  to a second switch configuration conductively connecting the tuning capacitor  62  in parallel with the second dedicated resonant capacitor  60   b  and the second induction coil  16   b . In this way, the driving circuit  42   a  may be operable to selectively shift the operating frequency range supplied to a load of the first induction coil  16   a  or the second induction coil  16   b.    
     Referring now to  FIG. 3 , in some embodiments, the driving circuit  42   b  may comprise a second switch or a relay switch  72 . The relay switch  72  may be configured to selectively disconnect the tuning capacitor  62  from the inverters  44 . In this configuration, the controller  20  may be configured to control the two-way switch  64  and the relay switch  72 . Accordingly, the controller  20  may be configured to control the two-way switch  64  to a first switch configuration conductively connecting the tuning capacitor  62  in parallel with the first dedicated resonant capacitor  60   a  and the first induction coil  16   a . The controller  20  may further be operable to control the two-way switch  64  to a second switch configuration conductively connecting the tuning capacitor  62  in parallel with the second dedicated resonant capacitor  60   b  and the second induction coil  16   b . Finally, the controller  20  may control the relay switch  72  to selectively disconnect the tuning capacitor  62  from both of the first inverter  44   a  and the second inverter  44   b.    
     Referring now to  FIG. 4 , in yet another embodiment, the driving circuit  42   c  may comprise a first two-way switch  64   a  and a second two-way switch  64   b . The controller  20  may control the first two-way switch  64   a  to selectively shift the operating frequency of the first inverter  44   a  and the second inverter  44   b  as discussed in reference to  FIGS. 2 and 3 . Additionally, the second two-way switch  64   b  may be connected between the tuning capacitor  62  and the first two-way switch  64   a . The second two-way switch  64   b  may be configured to selectively connect the tuning capacitor  62  to the first two-way switch  64   a  in a first switching configuration. Additionally, the second two-way switch  64   b  may be configured to selectively connect the tuning capacitor  62  to the ground  58  in parallel with the DC-bus capacitor  56  in a second switching configuration. 
     In operation, the controller  20  may control the second two-way switch  64   b  to selectively connect the tuning capacitor  62  to the first two-way switch  64   a  in the first switch configuration. Additionally, the controller  20  may control the second two-way switch  64   b  to selectively connect the tuning capacitor  62  to the ground  58 . By connecting the tuning capacitor  62  to the ground  58  in parallel with the DC-bus capacitor  56 , the controller  20  may limit electro-magnetic interference (EMI). Accordingly, the various configurations of the driving circuits  42  may provide for improved operation of the induction cooktop  10 . 
     Referring now to  FIG. 5 , a plot of power generated by an exemplary embodiment of the inverter  44  is shown. The plot demonstrates the performance of the inverter  44  with two different values of the dedicated resonant capacitor  60  and similar loads (e.g. the pan  24 ). The plot demonstrates the power generated by two different exemplary inverter configurations to a range of switching frequencies (f SW ). For example, the power output range of the inverter  44  is shown over a first operating range  82  for the dedicated resonant capacitor  60  having a capacitance of 270 nF. For comparison, the power output range of the inverter  44  is shown over a second operating range  84  for the dedicated resonant capacitor  60  having a capacitance of 330 nF. 
     As demonstrated in  FIG. 5 , the first operating range  82  corresponds to a comparatively lower capacitance and varies from a power output of 674 W at a switching frequency (f SW ) of 40 kHz to 1831 W at f SW =32 kHz. The second operating range  84  corresponds to a comparatively higher capacitance and varies from a power output of 758 W at f SW =36 kHz to 1964 W at f SW =29 kHz. Accordingly, increasing the capacitance of the dedicated resonant capacitor  60  of the inverter  44  may provide for a shift lower than the operating range of the switch frequency (f SW ) while increasing the power output. These principles may similarly be applied to adjust the operating range and power output of the exemplary inverters  44  of the driving circuits  42  by adjusting the effective capacitance with the tuning capacitor  62  to suit a desired mode of operation. 
     Referring now to  FIG. 6 , a system response of the driving circuit  42  resulting from a frequency shift caused by adding the tuning capacitor  62  is shown. As previously discussed, the controller  20  may selectively connect the tuning capacitor  62  in parallel to either the first inverter  44   a  or the second inverter  44   b . As previously discussed, the tuning capacitor  62  may be added in parallel to either the first dedicated resonant capacitor  60   a  or the second dedicated resonant capacitor  60   b  by the controller  20 . Depending on the particular embodiment or the driving circuit  42 , the controller  20  may add the tuning capacitor  62  in parallel by controlling the first two-way switch  64   a  in combination with either the second two-way switch  64   b  or the relay switch  72 . Accordingly, the controller  20  may be configured to selectively adjust an operating frequency range of either the first inverter  44   a  or the second inverter  44   b.    
     In operation, the operating frequency of each of the inverters may not only differ based on the design of the inverters  44  but also in response to load changes or differences in the diameter, magnetic permeability and conductivity of the conductive ferromagnetic material of the pans or cooking accessories on the cooktop  10 . In the exemplary embodiment shown in  FIG. 6 , each of the first inverter  44   a  and the second inverter  44   b  comprises a dedicated resonant capacitor  60  of 270 nF. However, due to differences in load on each of the induction coils  16  and other variables, the operating ranges differ significantly. For example, in the exemplary embodiment, the first inverter  44   a  has a first operating range  92  that varies from 710 W at f SW =30.8 kHz to 1800 W at f SW =25 kHz. The second inverter  44   b  has a second operating range  94  that varies from 670 W at f SW =40 kHz to 1825 W at f SW =32.3 kHz. Note that neither the first operating range  92  nor the second operating range  94  provide for soft-switching operation between 30.8 kHz and 32.3 kHz and do not overlap in the operating range of the switching frequency (f SW ). 
     During operation it may be advantageous to limit intermodulation acoustic noise. However, as demonstrated, the first operating range  92  and the second operating range  94  do not have an overlapping range of operation in the soft-switching region. However, by adjusting the effective capacitance of the second dedicated resonant capacitor  60   b  by adding the tuning capacitor  62  in parallel, the second operating range  94  is shifted to an adjusted operating range  96 . Though discussed in reference to shifting the second operating range  94  of the second inverter  44   b , the controller  20  may be configured to similarly shift the first operating range  92  of the first inverter  44   a . In general, the controller  20  may identify the higher operating range of the switch frequency (f SW ) of the first inverter  44   a  and the second inverter  44   b  and control at least one of the switches (e.g.  64   a ,  64   b , and  72 ) to apply the tuning capacitor  62  in parallel with the corresponding dedicated resonant capacitor (e.g.  60   a  or  60   b ). In this way, the controller  20  may shift the operating range of the first inverter to at least partially overlap with the operating range of the second inverter. 
     Still referring to  FIG. 6 , the adjusted operating range  96  varies from approximately 750 W at 36 kHz to 1960 W at 29 kHz. Accordingly, the first operating range  92  of the first inverter  44   a  and the adjusted operating range  96  of the second inverter  44   b  may provide for a common frequency range  98 . In this configuration, the controller  20  may control each of the inverters  44  with the same switching frequency within the common frequency range  98  to achieve simultaneous operation while limiting acoustic noise. The effects of applying the tuning capacitor  62  to the inverters  44  are summarized in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Performance changes resulting from applying tuning capacitor 62 
               
            
           
           
               
               
               
               
            
               
                 Switch Configuration 
                 Frequency Range 
                 Pmax 
                 Pmin 
               
               
                   
               
               
                 Dedicated Resonant 
                 Shift Upward 
                 Decrease 
                 Decrease 
               
               
                 Capacitor 
                 (increase) 
               
               
                 Dedicated Resonant 
                 Shift Downward 
                 Increase 
                 Increase 
               
               
                 Capacitor with Tuning 
                 (decrease) 
               
               
                 Capacitor 
               
               
                   
               
            
           
         
       
     
     From Table 1, the performance changes of the inverter  44  with and without the tuning capacitor  62  are summarized. In response to the tuning capacitor  62  being added in parallel with the dedicated resonant capacitor  60 , the range of the switching frequency (f SW ) is shifted downward or decreased. Additionally, the maximum power (P max ) output from the inverter  44  increases and the minimum power (P min ) increases. In this way, the controller  20  may control at least one of the switches (e.g.  64   a ,  64   b , and  72 ) to adjust the operating range of one of the inverters  44 . In some cases, the shifting of the operating range may provide for the common frequency range  98  of the inverters  44  to achieve simultaneous operation while limiting acoustic noise. 
     Accordingly, based on the first operating range  92 , the second operating range  94 , and the adjusted operating range  96 , the controller  20  may be configured to control the inverters  44  to operate within their respective operating ranges. For example, in the case that only one of the two inverters  44  is active, the controller  20  may be configured to connect the tuning capacitor  62  to the corresponding induction coil  16  (e.g.  16   a  or  16   b ). The controller  20  may connect the tuning capacitor  62  via the first two-way switch  64   a  if a set-point power of an operating range (e.g.  92  or  94 ) exceeds the maximum power deliverable by that inverter ( 44   a  or  44   b ) with only the dedicated resonant capacitor ( 60   a  or  60   b ). Otherwise, when the set-point power of the inverters  44  are within the operating ranges ( 92  or  94 ), the controller  20  may disconnect the tuning capacitor  62  by controlling the second two-way switch  64   b  or the relay switch  72 . 
     In the case where both inverters  44  are required to deliver power simultaneously, the controller  20  may connect the tuning capacitor  62  to one of the induction coils  16  such that the first inverter  44   a  and the second inverter  44   b  have the common operating frequency range  98 . For example, the controller  20  may connect the tuning capacitor  62  in parallel with the second inverter  44   b . Accordingly, the first operating range  92  of the first inverter  44   a  and the adjusted operating range  96  of the second inverter  44   b  may provide for the common frequency range  98 . In this configuration, the controller  20  may control each of the inverters  44  with the same switching frequency within the common frequency range  98  to achieve simultaneous operation while limiting acoustic noise. Finally, in the case where both inverters  44  are required to deliver power simultaneously and the operating frequency ranges  92  and  94  already include an overlapping frequency range, the controller  20  may disconnect the tuning capacitor  62  by controlling the second two-way switch  64   b  or the relay switch  72 . 
     Referring now to  FIG. 7 , a diagram of yet another embodiment of a driving circuit  42 ,  42   d  for a cooktop  10  is shown. The driving circuit  42   d  may comprise a plurality of half-bridge, series resonant inverters  100 . For example, the driving circuit  42   d  may comprise a first series resonant inverter  100   a  and a second series resonant inverter  100   b . The first series resonant inverter  100   a  may comprise the first induction coil  16   a  and a plurality of dedicated resonant capacitors  102   a  and  102   b . Additionally, the first series resonant inverter  100   a  may comprise a plurality of switching devices  104  (e.g. a first switching device  104   a  and a second switching device  104   b ). The first switching device  104   a  may be connected from the DC-bus  52  to a first side of the first induction coil  16   a . The second switching device  104   b  may be connected from the ground  58  to the first side of the first induction coil  16   a . A first dedicated capacitor  102   a  may be connected from the DC-bus  52  to a second side of the first induction coil  16   a . Additionally, a second dedicated capacitor  102   b  may be connected from the ground  58  to the second side of the first induction coil  16   a.    
     The second series resonant inverter  100   b  may comprise the second induction coil  16   b  and a plurality of dedicated resonant capacitors  102   c  and  102   d . The second series resonant inverter  100   b  may further comprise a plurality of switching devices  104  (e.g. a third switching device  104   c  and a fourth switching device  104   d ). The third switching device  104   c  may be connected from the DC-bus  52  to a first side of the second induction coil  16   b . The fourth switching device  104   d  may be connected from the ground  58  to the first side of the second induction coil  16   b . A third dedicated capacitor  102   c  may be connected from the DC-bus  52  to a second side of the second induction coil  16   b . Additionally, a fourth dedicated capacitor  102   d  may be connected from the ground  58  to the second side of the second induction coil  16   b.    
     The switching devices  104  may correspond to solid state power switching devices, similar to the switching devices  104 , which may be implemented as an insulated-gate bipolar transistor (IGBT). The switching devices  104  may be supplied power via DC-bus  52  of the DC power supply  48  and may be controlled via a control signal supplied by the controller  20 . In this configuration, the controller  20  may selectively activate the induction coils  16  by controlling a switching frequency supplied to the switching devices  104  to generate the electromagnetic field utilized to heat the pan  24 . 
     The tuning capacitor  62  may be selectively connected to the second side of the first induction coil  16   a  or connected to the second side of the second induction coil  16   b  by the two-way switch  64 . For example, in a first configuration, the switch  64  may connect the tuning capacitor  62  in parallel with the second dedicated capacitor  102   b . In a second configuration, the switch  64  may connect the tuning capacitor  62  in parallel with the fourth dedicated capacitor  102   d . Accordingly, the driving circuit  42   d  may be operable to selectively shift the operating frequency range supplied to a load of the first induction coil  16   a  or the second induction coil  16   b  by controlling the switch  64 . 
     It will be understood by one having ordinary skill in the art that construction of the described device and other components is not limited to any specific material. Other exemplary embodiments of the device disclosed herein may be formed from a wide variety of materials, unless described otherwise herein. 
     For purposes of this disclosure, the term “coupled” (in all of its forms, couple, coupling, coupled, etc.) generally means the joining of two components (electrical or mechanical) directly or indirectly to one another. Such joining may be stationary in nature or movable in nature. Such joining may be achieved with the two components (electrical or mechanical) and any additional intermediate members being integrally formed as a single unitary body with one another or with the two components. Such joining may be permanent in nature or may be removable or releasable in nature unless otherwise stated. 
     It is also important to note that the construction and arrangement of the elements of the device as shown in the exemplary embodiments is illustrative only. Although only a few embodiments of the present innovations have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited. For example, elements shown as integrally formed may be constructed of multiple parts or elements shown as multiple parts may be integrally formed, the operation of the interfaces may be reversed or otherwise varied, the length or width of the structures and/or members or connector or other elements of the system may be varied, the nature or number of adjustment positions provided between the elements may be varied. It should be noted that the elements and/or assemblies of the system may be constructed from any of a wide variety of materials that provide sufficient strength or durability, in any of a wide variety of colors, textures, and combinations. Accordingly, all such modifications are intended to be included within the scope of the present innovations. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the desired and other exemplary embodiments without departing from the spirit of the present innovations. 
     It will be understood that any described processes or steps within described processes may be combined with other disclosed processes or steps to form structures within the scope of the present device. The exemplary structures and processes disclosed herein are for illustrative purposes and are not to be construed as limiting. 
     It is also to be understood that variations and modifications can be made on the aforementioned structures and methods without departing from the concepts of the present device, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise. 
     The above description is considered that of the illustrated embodiments only. Modifications of the device will occur to those skilled in the art and to those who make or use the device. Therefore, it is understood that the embodiments shown in the drawings and described above is merely for illustrative purposes and not intended to limit the scope of the device, which is defined by the following claims as interpreted according to the principles of patent law, including the Doctrine of Equivalents.