Patent Publication Number: US-2023141743-A1

Title: Inductive heating arrangement

Description:
PRIORITY CLAIM 
     The present application is a Continuation of U.S. application Ser. No. 16/343,255, filed Apr. 18, 2019, which is a National Phase entry of PCT Application No. PCT/EP2017/076771, filed Oct. 19, 2017, which claims priority from U.S. Provisional Application No. 62/410,056, filed Oct. 19, 2016, each of which is hereby fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an inductive heating arrangement. 
     BACKGROUND 
     Articles such as cigarettes, cigars and the like burn tobacco during use to create tobacco smoke. Attempts have been made to provide alternatives to these articles, which articles burn tobacco, by creating products that release compounds without burning. Examples of such products are so-called heat-not-burn products, also known as tobacco heating products or tobacco heating devices, which release compounds by heating, but not burning, the material. The material may be for example tobacco or other non-tobacco products or a combination, such as a blended mix, which may or may not contain nicotine. 
     SUMMARY 
     According to a first aspect of the present disclosure there is provided an inductive heating arrangement for use with a device for heating smokable material to volatilize at least one component of said smokable material, the inductive heating arrangement comprising; a susceptor arrangement that is heatable by penetration with a varying magnetic field to heat the smokable material; at least a first inductor coil and a second inductor coil, the first inductor coil for generating a first varying magnetic field for heating a first section of the susceptor arrangement and the second inductor coil for generating a second varying magnetic field for heating a second section of the susceptor arrangement; a control circuit for controlling the first inductor coil and the second inductor coil, wherein, the control circuit is configured so that when one of the first and second coils is actively being driven to generate a varying magnetic field the other of the first and second inductor coils is inactive and wherein the control circuit is configured so that the inactive one of the first and second inductor coils is prevented from carrying a current induced by the active one of the first and second inductor coils sufficient to cause significant heating of the susceptor arrangement. 
     According to a second aspect of the present disclosure there is provided an aerosol provision device for providing an inhalable aerosol, the device comprising: the heating arrangement according to the first aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG.  1    illustrates schematically an apparatus arranged to heat smokable material. 
         FIG.  2    illustrates a cross sectional view of a heating arrangement for the apparatus of  FIG.  1   . 
         FIG.  3    is a diagram of a first circuit for controlling the heating arrangement of  FIG.  2   . 
         FIG.  4    is a trace of voltage as a function of time across a component of the first circuit of  FIG.  3   . 
         FIG.  5    is a diagram of a second alternative circuit for controlling the heating arrangement of  FIG.  2   . 
         FIG.  6    is a diagram of the first circuit shown in  FIG.  3    schematically shown connected to additional circuitry. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the term “smokable material” includes materials that provide volatilized components upon heating, typically in the form of an aerosol. “Smokable material” includes any tobacco-containing material and may, for example, include one or more of tobacco, tobacco derivatives, expanded tobacco, reconstituted tobacco or tobacco substitutes. “Smokable material” also may include other, non-tobacco, products, which, depending on the product, may or may not contain nicotine. “Smokable material” may for example be in the form of a solid, a liquid, a gel or a wax or the like. “Smokable material” may for example also be a combination or a blend of materials. 
     Apparatus is known that heats smokable material to volatilize at least one component of the smokable material, typically to form an aerosol which can be inhaled, without burning or combusting the smokable material. Such apparatus is sometimes described as a “heat-not-burn” apparatus or a “tobacco heating product” or “tobacco heating device” or similar. Similarly, there are also so-called e-cigarette devices, which typically vaporize a smokable material in the form of a liquid, which may or may not contain nicotine. The smokable material may be in the form of or be provided as part of a rod, cartridge or cassette or the like which can be inserted into the apparatus. A heater for heating and volatilizing the smokable material may be provided as a “permanent” part of the apparatus or may be provided as part of the smoking article or consumable which is discarded and replaced after use. A “smoking article” in this context is a device or article or other component that includes or contains in use the smokable material, which is heated to volatilize the smokable material, and optionally other components in use. 
     Referring to  FIG.  1   , an apparatus  100  arranged to heat smokable material is shown. The apparatus  100  can be used to heat smokable material (not shown in  FIG.  1   ) to volatilize at least one component of the smokable material. In this example, the apparatus  100  comprises an elongate outer housing  101 . The apparatus  100  may comprise any suitable material or materials, for example, the outer housing  101  may comprise plastic or metal. The apparatus  100  has a mouthpiece  101   a  through which a user can draw a material that has been volatilized in the apparatus  100 . 
     The apparatus  100  has a heating chamber  102  that contains a heating arrangement  103  for heating the smokable material (not shown). The heating chamber  102  is in fluid flow communication with the mouthpiece  101   a.    
     The apparatus  100  further comprises a controller  106  and a DC power source  108 . The controller  106  may comprise control circuitry and a micro-processor arrangement configured and arranged to control the heating arrangement  103  as discussed further below. 
     The power source  108  may be a battery, which may be a rechargeable battery or a non-rechargeable battery. Examples include nickel cadmium batteries, although any suitable batteries may be used. The power source  108  is electrically coupled to the heating arrangement  103  to supply electrical power when required and under control of the controller  106  to heat the smokable material (as discussed, to volatize the aerosol generating material without causing it to combust or undergo pyrolysis). 
     The apparatus  100  may further comprise an actuator  110 , for example, a user operable push button  110  on the exterior of the housing  101  and coupled to the controller  106 . 
     The apparatus  100  may further comprise one or more air inlets  112  formed through the housing  101  and into the heating chamber  102 . 
     In use, heat produced by the heating arrangement  103  heats the smokable material in the heating chamber  102  to generate aerosol and/or a gas or vapor. As a user inhales on the mouthpiece  101   a  air is drawn into the heating chamber  102  through the one or more air inlets  112  and the combination of the drawn air and aerosol and/or gas vapor passes into the mouthpiece  101   a  for inhalation by a user. 
     Referring now to  FIG.  2   , there will be described an example of the heating arrangement  103  in which example the heating arrangement  103  is an inductive heating arrangement that provides heat by inductive heating. 
     The heating arrangement  103  comprises a susceptor  202 . The susceptor  202  comprises a first susceptor region  202   a  and a second susceptor region  202   b.  In this example, the susceptor  202  is a single tubular member made from a material that can be inductively heated, (i.e. the susceptor  202  generates heat when in the vicinity of a varying magnetic field). In some examples, the susceptor  202  may have a cross-sectional shape other than circular, for example, square, rectangular, triangular or any other suitable shape. In some examples, the susceptor  202  may not be tubular and could be a blade susceptor. In some examples, the susceptor  202  may comprise a magnetically permeable material. A varying magnetic field generates eddy currents in the susceptor  202 , which eddy currents flow against the electrical resistance of the susceptor  202  to generate heat. In some examples, the susceptor  202  may be made from iron, steel, aluminum or the like. 
     In examples in which the susceptor  202  is composed of a magnetic material, a varying magnetic field additionally causes heating due to the well-known hysteresis effect. 
     The heating arrangement  103  also comprises first and second inductor coils  204 , 206 , respectively. The first and second inductor coils  204 , 206  are made from an electrically conducting material. In one example, the first and second inductor coils  204 , 206  are made from copper. In another example, the first and second inductor coils  204 , 206  are made from copper Litz wire, specifically. In this example, the first and second inductor coils  204 , 206  are wound in a helical fashion around a central longitudinal axis of the susceptor  202 . The first and second inductor coils  204 , 206 , in this example, are wound co-axially around the susceptor  202 , that is, the central longitudinal axes of the wound first and second inductor coils  204 , 206  and the central longitudinal axis of the susceptor  202  coincide. In this example, the first and second inductor coils  204 , 206  wound around the tubular susceptor  202  also have a tubular shape. In other examples, where the susceptor  202  is of a different cross-sectional shape, the first and second inductor coils  204 , 206  wound around the susceptor  202  may have the same cross-sectional shape as the susceptor  202 . The first inductor coil  204  comprises a first end  204   a  and a second end  204   b  and the second inductor coil  206  comprises a first end  206   a  and a second end  206   b.  The first end  204   a  of the first inductor coil  204  is closer to an end of the susceptor  202  corresponding to the first susceptor region  202   a  than it is to the centre of the susceptor  202 , and the second end  204   b  of the first inductor coil  204  is closer to the centre of the susceptor  202  than it is to the end of the susceptor  202  corresponding to the first susceptor region  202   a.  On the other hand, the first end  206   a  of the second inductor coil  206  is closer to an end of the susceptor  202  corresponding to the second susceptor region  202   b  than it is to the centre of the susceptor  202 , and the second end  206   b  of the second inductor coil  206  is closer to the centre of the susceptor  202  than it is to the end of the susceptor  202  corresponding to the second susceptor region  202   b.    
     In the example of  FIG.  2   , the first and second inductor coils  204 , 206  generate a varying magnetic field when a varying electrical current flows through them. In this example, when a varying current flows through the first inductor coil  204 , it generates a corresponding varying magnetic field, which field causes only the part of the susceptor  202  substantially closest to the first inductor coil  204  to generate heat. In other words, the varying magnetic field generated by the first inductor coil  204  causes substantially localized heating in the first susceptor region  202   a  of the susceptor  202 . Similarly, when a varying current flows through the second inductor coil  206 , it generates a corresponding varying magnetic field, which field causes only the part of the susceptor  202  substantially closest to the second inductor coil  206  to generate heat. In other words, the varying magnetic field generated by the second inductor coil  206  causes substantially localized heating in the second susceptor region  202   b  of the susceptor  202 . Thus, the first and second inductor coils  204 , 206  can be operated to heat substantially the entire length of the susceptor  202 . More specifically, the first inductor coil  204  can be operated to heat the first susceptor region  202   a  and the second inductor coil  206  can be operated to heat the second susceptor region  202   b.    
     In one example, one of the inductor coils may be operated for an extended period of time in order to substantially locally heat its respective susceptor region. In some examples, the inductor coils  204 , 206  may be operated alternatively, each inductor coil being operated for a respective given interval of time while the other inductor coil is not operated. The given intervals of time for each inductor coil may be such that substantially the entire length of the susceptor  202  is evenly heated, or they may be such that the susceptor  202  is un-evenly heated. In examples of the apparatus  100 , smokable material can be placed in the volume within the tubular susceptor  202 . In some examples, the smokable material may be contained in a smokable material wrapper or container (not shown), which container can be inserted into the volume within the tubular susceptor  202 . The smokable material container may be composed of a material that allows a desired amount of heat from the susceptor  202  to reach the smokable material in order to heat it. In another example, the smokable material may be formed into a long string or a rope like element, which can be inserted into the volume within the tubular susceptor  202 . In another example, the smokable material may be in the form of pellets or tablets of smokable material that can be inserted into the volume within the tubular susceptor  202 . In examples of the apparatus  100 , suitable means for directing air through the tubular susceptor  202  may be included. 
     In examples of the heating arrangement  103 , the heat generated by the susceptor  202  heats the smokable material to volatilize at least one component of the smokable material. Since the heating of the susceptor  202  can be localized, the smokable material can be heated in a localized fashion. For example, if the first susceptor region  202   a  is heated, only the smokable material within the volume of the first susceptor region  202   a  can be expected to be heated. Similarly, if the second susceptor region  202   b  is heated, only the smokable material within the volume of the second susceptor region  202   b  can be expected to be heated. 
     In this example, the heating arrangement  103  further comprises first and second temperature sensing elements  208 , 210 , respectively. The first temperature sensing element  208  is placed in contact with the susceptor  202  at a position substantially near the middle of the first inductor coil  204 , that is, in the middle of the first susceptor region  202   a  as shown in  FIG.  2   . Similarly, the second temperature sensing element  210  is placed in contact with the susceptor  202  at a position substantially near the middle of the second inductor coil  206 , that is, in the middle of the second susceptor region  202   b  as shown in  FIG.  2   . Accordingly, the temperature sensing element  208  detects the temperature of the susceptor  202  in the middle of the first susceptor region  202   a,  and the temperature sensing element  210  detects the temperature of the susceptor  202  in the middle of the second susceptor region  202   b.  In other examples, a number of temperature sensing elements other than two may be used. In other examples, temperature sensing elements may be positioned differently. 
     In the example of  FIG.  2   , the heating arrangement  103  also comprises a magnetic conductor  212  surrounding the first and second inductor coils  204 , 206 . In this example, the magnetic conductor  212  is a tubular member arranged co-axially with respect to the first and second inductor coils  204 , 206 . The magnetic conductor  212  is made from a high permeability and low-loss material, and acts to substantially confine the magnetic field generated by the first and second inductor coils  204 , 206  within the volume enclosed by the magnetic conductor  212 . In some examples, a magnetic conductor may surround only one of the first and second inductor coils  204 , 206 , for example the coil nearest the mouth end of the apparatus  100 . In some examples, a first magnetic conductor may surround the first inductor coil  204  and a second magnetic conductor may surround the second inductor coil  206 , with the first and second magnetic conductors having a gap between them. In other examples, the heating arrangement  103  may not comprise any such magnetic conductor. 
     In this example, the controller  106  is configured to control the heating arrangement  103 . The controller  106  comprises circuitry that controls the operation of the first and second inductor coils  204 , 206  in order to control the heating arrangement  103 . 
     Referring now to  FIG.  3   , there is illustrated an example of a circuit comprised in the controller  106 . In this example, the circuit  300  is configured to control current flow through the first and second inductor coils  204 , 206  which are connected in the circuit  300  as shown in  FIG.  3   . The circuit  300  is configured to control both the first and second inductor coils  204 , 206  such that only one of the first and second inductor coils  204 , 206  operates to significantly heat its respective susceptor region  202   a,    202   b  at a given time. In other words, the topology of circuit  300  allows the same circuitry to be used to operate two separate inductor coils at different times to heat the susceptor  202 . 
     It will be understood that when the first inductor coil  204  is being controlled to generate a varying magnetic field, a voltage will be induced in the second inductor coil  206  and vice versa. However in this example, the topology of the circuit  300  is such that when one of the inductor coils is controlled to generate a varying magnetic field, that is, to heat the susceptor  202 , an induced current sufficient to cause significant heating of the susceptor  202  is prevented from flowing in the other inductor coil. More specifically, when the first inductor coil  204  is being operated, current sufficient to cause significant heating of the susceptor  202  is prevented from flowing in the second inductor coil  206 , and when the second inductor coil  206  is being operated, current sufficient to cause significant heating of the susceptor  202  is prevented from flowing in the first inductor coil  204 . In general, in examples of the apparatus  100 , the controller  106  comprises circuitry arranged such that when one of the inductor coils  204 ,  206  is being operated to heat the susceptor  202 , a current sufficient to cause significant heating of the susceptor  202  is prevented from flowing in the other inductor coil  204 ,  206 . Thus, when one of the inductor coils is being operated to heat the susceptor  202 , the other inductor coil is prevented from significantly heating the susceptor  202 . As a result, since the first and second inductor coils  204  and  206  operate to substantially locally heat susceptor regions  202   a  and  202   b  respectively, localized heating of the susceptor  202  can be achieved. 
     In the example of  FIG.  3   , the circuit  300  includes first and second resonator sections  302  and  304 . The first inductor coil  204  is arranged to form part of the first resonator section  302  of the circuit  300 , and the second inductor coil  206  is arranged to form part of the second resonator section  304  of the circuit  300 . The first resonator section  302  also comprises a first capacitor  306  comprising a first terminal  306   a  and a second terminal  306   b,  and a first switch  308 . Similarly, the second resonator section  304  further comprises a second capacitor  310  comprising a first terminal  310   a  and a second terminal  310   b,  and a second switch  312 . The first switch  308  is arranged to turn the first resonator section  302  on and off, and the second switch  312  is arranged to turn the second resonator section  304  on and off. In some examples, the components of circuit  300  may be arranged differently (in a different topography) to as shown in  FIG.  3   . In some examples, additional or alternative components may be included. 
     In this example, the first and second switches  308  and  312  of the circuit  300  are field effect transistors (FETs). More specifically, in this particular example, the first and second FETs  308  and  312  are N-channel FETs. As will be appreciated by those skilled in the art, the first FET  308  comprises a drain terminal  308   a,  a source terminal  308   b  and a gate terminal  308   c  and the second FET  312  comprises a drain terminal  312   a,  a source terminal  312   b  and a gate terminal  312   c.    
     The first and second resonator sections  302  and  304 , in this particular example, are LC (inductor/capacitor) resonator sections. In other words, each resonator section  302 ,  304  is equivalent to an LC resonator circuit. 
     A power supply connection  314  connects the second end  204   b  of the first coil  204 , the second terminal  306   b  of the first capacitor  306 , the second end  206   b  of the second coil  206  and the second terminal  310   b  of the second capacitor  310  to the positive terminal of the (DC) power source  108  (not illustrated in  FIG.  3   ). The first end  204   a  of the first inductor coil  204  and the first terminal  306   a  of the first capacitor  306   a  are connected to the drain terminal  308   a  of the first FET  308  and, likewise, the first end  206   a  of the second inductor coil  206  and the first terminal  310   a  of the second capacitor  310  are connected to the drain terminal  312   a  of the second FET  312 . A negative terminal connection  316  connects the source terminal  308   b  of the first FET  308  and the source terminal  312   b  of the second FET  312  to the negative terminal of the power source  108 . 
     As will be well understood by those skilled in the art, an N-channel FET is in an ‘ON’ state when an appropriate control voltage is applied to its gate so that a conductive path exists between its drain and source. However, as will also be well understood by those skilled in the art, when an N-channel FET is in an ‘OFF’ state (i.e. when the appropriate control voltage is not applied to its gate) it effectively acts as a diode. In  FIG.  3   , the diode functionality that the first FET  308  exhibits when in its OFF state is represented by a first diode  308   d  and the diode functionality that the second FET  312  exhibits when in its OFF state is represented by a second diode  312   d.  The first diode  308   d  has its cathode connected to the first end  204   a  of the first inductor coil  204  and the first terminal  306   a  of the first capacitor  306 , and its anode connected to the negative terminal connection  316 , and the second diode  312   d  has its cathode connected to the first end  206   a  of the second inductor coil  206  and the first terminal  310   a  of the first capacitor  310 , and its anode connected to the negative terminal connection  316 . 
     In this example, having regard to the first and second FETs  308  and  312 , and the topology of the circuit  300 , the phasing of the first and second inductor coils  204  and  206  with respect to each other is chosen such that when the first inductor coil  204  is being operated, current sufficient to cause significant heating of the susceptor  202  is prevented from flowing in the second inductor coil  206 , and when the second inductor coil  206  is being operated, current sufficient to cause significant heating of the susceptor  202  is prevented from flowing in the first inductor coil  204 . 
     In this example, one of the first  204  and second  206  inductor coils is controlled to heat the susceptor by having its corresponding first  308  or second  312  switch (as the case may be) being repeatedly turned on and off at a fast switching rate while the other one of the first  204  and second  206  inductor coils remains inactive and its corresponding first  308  or second  312  switch (as the case may be) remains off. More specifically, the first inductor coil  204  is controlled to heat the susceptor  202  when the first switch  308  is turned on and off at a first switching rate while the second switch  312  remains off, and the second inductor coil  206  is controlled to heat the susceptor  202  when the second switch  312  is turned on and off at a second switching rate while the first switch  308  remains off. A controller  318  is provided in the circuit  300  to control the switching on and off of whichever of the first  308  and second  312  FETs is being operated. The first and second switching rates may be different or the same. 
     The operation of the first  204  and second  206  inductor coils during this fast switching is explained in more detail below. 
     The functioning of the example circuit  300  will now be described in more detail in the context of the first inductor coil  204  being operated to heat the susceptor  202  when the first FET  308  is being rapidly turned on and off by the controller  318 . 
     When the first FET  308  is on, a DC current flows between the power supply connection  314  and the negative terminal connection  316 , and through the first inductor coil  204 . This DC current is driven by the power supply  108 . It will be understood that when a current flows through the first inductor coil  204 , the first inductor coil  204  generates a magnetic field as a result of the current. The first inductor coil  204  stores energy in the magnetic field it generates. When the first FET  308  is on, the voltage across the first FET  308  is zero. The first capacitor  306  is short circuited by the first FET  308  being on. In other words, when the first FET  308  is on, current between the power supply connection  314  and the negative terminal connection  316  flows through the first FET  308 . 
     After the first inductor  204  has been allowed to generate a magnetic field due to the first FET  308  being on for a given amount of time and the first FET  308  is subsequently turned off, the current driven by the power supply  108  through the first inductor coil  204  begins to drop off. The first inductor coil  204  resists this change in current and generates an induced voltage using the energy that was stored in the magnetic field generated by the first inductor coil  204  when the first FET  308  was on and a direct current was flowing through the first inductor coil  204 . Accordingly, when the first FET  308  is turned off after being on, the first capacitor  306  and the first inductor coil  204  resonate with each other. 
       FIG.  4    shows the voltage across the first FET  308 , as indicated by a voltage trace  400 , when the first FET  308  is turned off and on twice during a time period that the first inductor  204  is being operated to heat the susceptor  202 . 
     The voltage trace  400  comprises a first section  400   a  when the first FET  308  is on, a second section  400   b  to  400   d  when the first FET  308  is switched off, a third section  400   f  when the first FET  308  is switched on again, a fourth section  400   g  when the first FET  308  is switched off again and a fifth section  400   h  when the first FET  308  is subsequently switched on again. 
     The voltage across the first FET  308  is zero when the first FET  308  is on in sections  400   a,    400   f  and  400   h.    
     When the first FET  308  is turned off as indicated by section  400   b  to  400   d  and also by section  400   g,  the first inductor coil  204  uses the energy stored in its magnetic field (which magnetic field was the result of a current flow when the first FET  308  was on) to induce a voltage that resists a drop in the current flowing through the first inductor coil  204  as a result of the first FET  308  being off. The voltage induced in the first inductor coil  204  causes a corresponding variation in voltage across the first FET  308 . During this variation in voltage, the first inductor coil  204  and the first capacitor  306  resonate with each other. The voltage  400  initially increases (see for example  400   b ) as the induced voltage in the first inductor coil  204  increases to oppose a drop in current due to the first FET  308  being off, reaches a peak (see for example  400   c ) and then, as the energy stored in the magnetic field of the first inductor coil  204  diminishes, decreases back to zero (see for example  400   d ). 
     The varying voltage  400   b  to  400   d  and  400   g  produces a corresponding varying current and, since during the off time of the first FET  308 , the first capacitor  306  and the first inductor  204  act as a resonant LC circuit, the total impedance of the first resonant section  302  is at a minimum during this time. It will therefore be understood that the maximum magnitude of the varying current flowing through the first resonant section  302  will be relatively large. 
     This relatively large varying current accordingly causes a relatively large varying magnetic field in the first inductor  204  which causes the susceptor  202  to generate heat. The time period over which the voltage across the first FET  308  varies as indicated by section  400   b  to d and by section  400   g  in this example depends on the resonant frequency of the first resonant section  302 . As will be appreciated by those skilled in the art, the resonant frequency of the first resonant section  302  depends on the inductance of the first inductor coil  204  and the capacitance of the first capacitor  306 . 
     Referring again to  FIG.  4   , when the first FET  308  is off and the voltage across the first FET  308  subsequently decreases back towards 0V (e.g. the voltage is substantially at level  402  at point  400   e ), the controller  318  turns the first FET  308  back on so that a dc current driven by the power supply  108  flows again through the first inductor coil  204 , and the first inductor coil  204  can store energy in the form of a magnetic field for the next time the first FET  308  is switched off to turn the resonant section  302  on as indicated by section  400   g.    
     The time for which the controller  318  keeps the first FET  308  on (e.g. part  400   f ) may be selected in accordance with the amount of energy that is desired to be stored in the first inductor coil  204 , a part of which energy will be used to heat the susceptor  202  during the next off time of the first FET  308  as indicated by  400   g  (on time of the resonant part  202 ). The amount of energy that may be stored in the first inductor coil  204  for a given on time  400   f  of the first FET  308  will depend on such factors as the voltage provided by the power supply  108  and the number of turns on the first inductor coil  204 , for example. It will be understood that in this example, when the voltage across the first FET  308  reaches 0V, the voltage at the drain  308   a  of the first FET  308  also reaches 0V. 
     As the controller  318  repeatedly switches first FET  308  on and off in this way at a switching rate, the above described process is continuously repeated to heat the susceptor  202 . 
     Although the above description of the functioning of the circuit  300  is presented in the context of the first inductor coil  204  being operated to heat the susceptor  202 , it will be understood that the second inductor coil  206  forming part of the second resonator section  304  will be operated in substantially the same way, with the second FET  312  performing functions equivalent to the first FET  308  and the second capacitor  310  performing functions equivalent to the first capacitor  306 . 
     As previously mentioned above, the circuit  300  allows the first and second inductor coils  204 , 206  to be operated such that only one of the first and second coils  204  and  206  operates to significantly heat the susceptor  202  at a given time. This is achieved firstly by switching, at a given switching rate, one of the first or second FETs  308  or  312  while the other FET remains off. 
     Secondly, when one of the first  204  and second  206  inductor coils is being operated to heat the susceptor  202  (as is described above with respect to the first coil  204 ), the circuit  300  is specifically configured so that a voltage that is induced in the other non-operating coil by the varying magnetic field of the operating coil does not cause a significant current to flow through the non-operating coil which itself would cause a varying magnetic field which could heat the susceptor. More specifically, when the first inductor coil  204  is being operated to heat the susceptor  202 , a current sufficient to cause significant heating of the susceptor  202  is prevented from flowing through the second inductor coil  206 , and when the second inductor coil  206  is being operated to heat the susceptor  202 , a current sufficient to cause significant heating of the susceptor  202  is prevented from flowing through the first inductor coil  204 . 
     This is necessary because, as mentioned above, the first  308  and second  312  FETs effectively act as diodes when switched off and so may conduct a current if they are forward biased (i.e. the FETs are not perfect switches). Accordingly, it is important that the circuit  300  is configured so that when one of the first  204  and  206  inductor coils is being operated to heat the susceptor  202 , the voltage induced across the other non-operative inductor coil does not forward bias the intrinsic diode of the FET associated with that non-operative inductor coil but instead reverse biases it. 
     It will be understood by those skilled in the art that when considering two inductors which are magnetically coupled, their winding relative to each other determines in which direction the varying magnetic field generated by one inductor drives a current/induces a voltage in the other inductor. 
     The direction in which current is driven/voltage is induced in a coil can be determined by applying the well-known “right hand rule” relating direction of current to the direction of a magnetic field. The relative winding of the first and second inductor coils  204  and  206  may be referred to as the phasing of the first and second inductor coils  204  and  206 . 
     In the topology of the circuit  300  in  FIG.  3    the first inductor coil  204  and the second inductor coil  206  are wound in opposite senses as indicated by the dots which appears at the first end  204   a  of the first inductor coil  204  and at the first end  206   a  of the second coil  206 . An example of the heating arrangement  103  is also shown connected to the circuit  300  to illustrate that the first and second inductor coils  204 , 206  are physically wound in opposite directions in this example. 
     Accordingly, when, for example, the controller  318  is repeatedly switching on and off the first FET  308  (as described above) in order to cause the first inductor coil  204  to heat the susceptor  202 , each time the first FET  308  is switched off, a positive voltage is generated at the first end  204   a  of the first inductor coil  204  and a corresponding smaller positive voltage is induced at the first end  206   a  of the second inductor coil  206  due to magnetic coupling. This results in the intrinsic diode of the second FET  312  being reverse biased and hence current is substantially prevented from flowing through the second FET  312 . Although some current may flow between the second inductor coil  206  and second capacitor  310 , a current sufficient to generate a varying magnetic field strong enough to significantly heat the susceptor  202  is prevented from flowing through the second inductor coil  206  due to the second FET  312  being reverse biased. Also, as a result of the second FET  312  being reverse biased and a current being prevented from flowing through it when the first FET  308  is repeatedly being switched on and off and the second FET  312  remains off, a significant amount of energy is prevented from building up in the second inductor coil  206  so that a significant amount of energy is not drawn away from the first inductor coil  204  being operated to heat the susceptor  202 . 
     Similarly, when, for example, the controller  318  is repeatedly switching on and off the second FET  312  in order to cause the second inductor coil  206  to heat the susceptor  202 , each time the second FET  312  is switched off, a positive voltage is generated at the first end  206   a  of the second inductor coil  206  and a corresponding smaller positive voltage is induced at the first end  204   a  of the first inductor coil  204  due to magnetic coupling. This results in the intrinsic diode of the first FET  308  being reverse biased and hence current is substantially prevented from flowing in the first FET  308 . Although some current may flow between the first inductor coil  204  and first capacitor  306 , a current sufficient to generate a varying magnetic field strong enough to significantly heat the susceptor  202  is prevented from flowing through the first inductor coil  204  due to the first FET  308  being reverse biased. Also, as a result of the first FET  308  being reverse biased and a current being prevented from flowing through it when the second FET  312  is repeatedly being switched on and off and the first FET  308  remains off, a significant amount of energy is prevented from building up in the first inductor coil  204  so that a significant amount of energy is not drawn away from the second inductor coil  206  being operated to heat the susceptor  202 . 
     Thus, preventing a current sufficient to significantly heat the susceptor from flowing in the non-operating inductor coil in this way provides the additional advantage of preventing the non-operating inductor coil from taking a significant amount of energy away from the operating coil in order to generate its own magnetic field, which energy is used to generate a varying current and therefore magnetic field by the operating coil in order to heat the susceptor  202 . 
     Referring now to  FIG.  5   , there is illustrated a second circuit  500  configured to control the first and second inductor coils  204  and  206  comprised in the controller  106 . 
     Many of the components of the circuit  500  are identical to corresponding components in the circuit  300  and function in an identical way. These components have been given the same reference numerals as they have in  FIG.  3    and in the interests of brevity will not be described in detail again. In some examples, the components of circuit  500  may be arranged differently (in a different topography) to as shown in  FIG.  5   . In some examples, additional or alternative components may be included. 
     In this example, the circuit  500  is used to control a heating arrangement in which both the first and second inductor coils  204 , 206  are wound in the same direction relative to each other (have the same phasing) as indicated by the dots which appears at the first end  204   a  of the first inductor coil  204  and at the second end  206   b  of the second coil  206 . An example of the heating arrangement  103  is shown connected to the circuit  500  to illustrate that the first and second inductor coils  204 , 206  are physically wound in the same direction in this example. One difference in the circuit topology of the circuit  500  compared with the circuit  300  is the configuration of the wiring connecting the second inductor coil  206  to the rest of the circuit. As mentioned above, in circuit  300 , the second end  206   b  of the second coil  206  connects to the positive terminal of the power source  108  via the power supply connection  314 , and the first end  206   a  of the second coil  206  connects to the drain terminal  312   a  of the second FET  312 . In contrast, in circuit  500 , the first end  206   a  of the second coil  206  connects to the positive terminal of the power source  108  via the power supply connection  314 , and the second end  206   b  of the second coil  206  connects to the drain terminal  312   a  of the second FET  312 . 
     Although, in this example, the inductor coils  204 , 206  are wound in the same direction, the topology of circuit  500  is such that when the first inductor coil  204  is being operated, each time the first FET  308  is switched off, the voltage induced across the second inductor coil  206  reverse biases the intrinsic diode  312   d  of the second FET  312 , and that when the second inductor coil  206  is being operated, each time the second FET  312  is switched off, the voltage induced across the first inductor coil  204  reverse biases the intrinsic diode  308   d  of the first FET  308 . As in the case of the circuit  300 , in the example of the controller  318  repeatedly switching the first FET  308  on and off to cause the first inductor coil  204  to heat the susceptor  202 , each time the first FET  308  is switched off, a positive voltage is generated at the first end  204   a  of the first inductor coil  204 . However, in contrast to circuit  300 , in circuit  500  a corresponding smaller positive voltage is induced at the second end  206   b  (instead of the first end  206   a  as is the case for circuit  300 ) of the second inductor coil  206  due to magnetic coupling. Since in circuit  500 , the second end  206   b  of the second inductor coil  206  connects to the drain terminal  312  of the second FET  312 , each time the first FET  308  is switched off, the second FET  312  is reverse biased. Hence current is substantially prevented from flowing through the second FET  312 . Conversely, when the controller  318  repeatedly switches the second FET  312  on and off to cause the second inductor coil  206  to heat the susceptor  202 , each time the second FET  312  is switched off, the first FET  308  becomes reverse biased in a similar manner, and hence current is substantially prevented from flowing through the first FET  308 . Thus, although in circuit  500  the inductor coils  204 , 206  are physically wound in the same sense, the circuit  500  provides the advantages mentioned above with respect to the circuit  300  by having the inductor coils  204 , 206  connected to the circuit in a way such that a substantial current is prevented from flowing in the inactive inductor coil. 
     However, the topology of the circuit  500  may require a more difficult printed circuit board layout with high current traces. In some examples, the simpler topology of the circuit  300  may be preferred. 
     Referring now to  FIG.  6   , an example controller  318   a,    318   b  for controlling the circuit  300  of  FIG.  3    will be described. In  FIG.  6   , the circuit  300  of  FIG.  3    is reproduced except that the controller  318  is represented as two different sections, namely, a zero voltage detector section  318   a  and a switching section  318   b,  and that an example heater arrangement  103  is not shown connected to the circuit. 
     When one of the first  308  and second  312  FETs is being repeatedly switched on and off, as described above with respect to  FIG.  3   , to operate a respective one of the first  204  and second  206  inductor coils, the zero voltage detector section  318   a  detects when, after the respective FET has been switched off, the voltage across that FET has returned to zero (e.g. point  400   e  in  FIG.  4   ) or, is close to zero, and in response to the zero voltage detector section  318   a  making this detection, the switching section  318   b  switches the respective FET on again. 
     The switch controller section  318   a  is a zero voltage detection circuit comprising first  600  and second  602  small signal diodes, a pull up resistor  604 , and a logic power source  606 . Taking the example of the second inductor coil  206  being operated to heat the susceptor  202 , the functioning of the switch controller section  318   a,  will now be described. 
     When the second inductor coil  206  is being operated to heat the susceptor  202 , the first FET  308  remains off. When the first FET  308  remains off, the first small signal diode  600  has either no bias or is reverse biased depending on the voltages at the logic power source  606  and the power supply connection  314 , that is, the voltage at the cathode end of the first small signal diode  600  is either substantially the same as or higher than the voltage at the anode end of the first small signal diode  600 . 
     During the switching at the switching rate of the second FET  312 , when the second FET  312  is off and the voltage across it varies as indicated by  4   b - d  of  FIG.  4   , the second small signal diode  602  is reversed biased. At the end of this variation in voltage, when the voltage reaches 0V as indicated by  400   e,  or is close to 0V, the second small signal diode  602  becomes forward biased. The second switch controller section  318   b,  in this example, includes one or more flip-flop means (not shown) that can be set to switch the first and second FETs  308  and  318  on or off. Accordingly, when the second small signal diode  602  is forward biased at  400   e,  the signal from the second small signal diode  602  is provided to a flip-flop means (not shown) included in the second switch controller section  318   b  in order to set it to switch the second FET  312  on. When the second FET  312  is on, the second small signal diode  602  is reverse biased. 
     As already described above with reference to the  FIG.  3    circuit, the second FET  312  remains on until a required amount of energy is stored in the associated inductor coil  206 . In this example, the magnitude of the current flowing in the second inductor coil  206  may be measured by suitable current measuring means (not shown) that may be included in the second switch controller means  318   b.  Once the magnitude of the current in the second inductor coil  206  is at a level corresponding to the desired amount of energy being stored in the second inductor  206 , the flip-flop is reset in order to switch the second FET  312  off and initiate another variation of the voltage  400   g  across the first capacitor  310 . 
     In one particular example, the logic power source  606  provides a voltage of  2 . 5 V, and the signal from the second small signal diode  602  is provided to the flip-flop means mentioned above included in the second switch controller section  318   b.  The flip-flop means switches at half of the voltage of the logic power source  606 , that is, at 1.25V in this example. This means that the forward bias voltage of the second small signal diode  602  and the voltage at the second FET  312  drain must sum to 1.25V in order that the digital logic circuit turns the second FET  312  on. In this example therefore, the second FET  312  is switched on when its drain  312   a  is at 0.55V rather than at 0V as referenced above. It should be noted that ideally, switching should occur at 0V across the FET  312  for maximum efficiency. This zero voltage switching advantageously prevents the second FET  312  from discharging the second capacitor and thereby wasting energy stored in the second capacitor  310 . 
     However, the loss in efficiency due to the use of this digital logic circuit as opposed to, for example, an analogue comparator circuit can be thought to be offset by the advantageous saving in circuit parts and cost. In this example 0.55V is an acceptable voltage across the second FET  312  at which to switch the second FET  312  back on. 
     It should be noted that in this example, the zero voltage switching as described above occurs in steady state conditions, that is, when the repeated switching of the second FET  312  is ongoing. In order to commence operation of the second inductor coil  206  by commencing the repeated switching of the second FET  312 , an additional signal may be provided to the second FET  312 . 
     Although in the above description the functioning of the zero voltage detection circuit  318   a  is described in relation to controlling switching of the second FET  312 , it will be understood that the zero voltage detection circuit  318   a  functions in the same way, using the first small signal diode  600  instead of the second small signal diode  602 , to control the first FET  308 . 
     It will be appreciated that the first and second inductor coils  204 , 206 , in some examples, may have at least one characteristic different from each other. For example, the first inductor  204  may have at least one characteristic different from the second inductor coil  206 . More specifically, in one example, the first inductor coil  204  may have a different value of inductance than the second inductor coil  206 . In another example, the first and second inductor coils be different lengths such that the first inductor coil  204  is wound over a larger section of the susceptor  202  or vice versa. In another example, the first inductor coil  204  may comprise a different number of turns than the second inductor coil  206 . In yet another example, the first inductor coil  204  may be composed of a different material to the second inductor coil  206 . It is envisaged that the first inductor coil  204  may have one or more different characteristics to the second inductor coil  206  based on, for example, how the smokable material within the volume of the susceptor is desired to be heated. In some examples, the first and second inductor coils  204  and  206  may be substantially identical. 
     The above examples have been described with the circuits  300  and  500  comprising N-channel FETs. However, in some examples, circuits comprising P-channel FETs may be used instead. For example, P-channel FETs may be used in circuit  300  if the connection  314  shown in  FIG.  3    is instead connected to the negative terminal of the power source  108 , and the connection  316  is instead connected to the positive terminal of the power source  108 . 
     The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilized and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future.