Patent Publication Number: US-2021186109-A1

Title: A resonant circuit for an aerosol generating system

Description:
PRIORITY CLAIM 
     The present application is a National Phase entry of PCT Application No. PCT/US2019/049076, filed Aug. 30, 2019, which claims priority from GB Application No. 1814202.6 filed Aug. 31, 2019, each of which is hereby fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a resonant circuit for an aerosol generating system, more specifically a resonant circuit for inductively heating a susceptor arrangement to generate an aerosol. 
     BACKGROUND 
     Smoking 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 by creating products that release compounds without combusting. Examples of such products are so-called “heat not burn” products or tobacco heating devices or products, which release compounds by heating, but not burning, material. The material may be, for example, tobacco or other non-tobacco products, which may or may not contain nicotine. 
     SUMMARY 
     According to a first aspect of the present disclosure, there is provided a resonant circuit for an aerosol generating system, the resonant circuit comprising: an inductive element for inductively heating a susceptor arrangement to heat an aerosol generating material to thereby generate an aerosol; and a switching arrangement that, in use, alternates between a first state and a second state to enable a varying current to be generated from a DC voltage supply and flow through the inductive element to cause inductive heating of the susceptor arrangement; wherein the switching arrangement is configured to alternate between the first state and the second state in response to voltage oscillations within the resonant circuit which operate at a resonant frequency of the resonant circuit, whereby the varying current is maintained at the resonant frequency of the resonant circuit. 
     The resonant circuit may be an LC circuit comprising the inductive element and a capacitive element. 
     The inductive element and the capacitive element may be arranged in parallel and the voltage oscillations may be voltage oscillations across the inductive element and the capacitive element. 
     The switching arrangement may comprise a first transistor and a second transistor, arranged such that, when the switching arrangement is in the first state the first transistor is OFF and the second transistor is ON and when the switching arrangement is in the second state the first transistor is ON and the second transistor is OFF. 
     The first transistor and the second transistor may each comprise a first terminal for turning that transistor ON and OFF, a second terminal and a third terminal, and the switching arrangement may be configured such that first transistor is adapted to switch from ON to OFF when the voltage at the second terminal of the second transistor is equal to or below a switching threshold voltage of the first transistor. 
     The first transistor and the second transistor may each comprise a first terminal for turning that transistor ON and OFF, a second terminal and a third terminal, and the switching arrangement may be configured such that second transistor is adapted to switch from ON to OFF when the voltage at the second terminal of the first transistor is equal to or below a switching threshold voltage of the second transistor. 
     The resonant circuit may further comprise a first diode and a second diode and the first terminal of the first transistor may be connected to the second terminal of the second transistor via the first diode, and the first terminal of the second transistor may be connected to the second terminal of the first transistor via the second diode, whereby the first terminal of the first transistor is clamped at low voltage when the second transistor is ON and the first terminal of the second transistor is clamped at low voltage when the first transistor is ON. 
     The first diode and/or the second diode may be Schottky diodes. 
     The switching arrangement may be configured such that first transistor is adapted to switch from ON to OFF when the voltage at the second terminal of the second transistor is equal to or below a switching threshold voltage of the first transistor plus a bias voltage of the first diode. 
     The switching arrangement may be configured such that second transistor is adapted to switch from ON to OFF when the voltage at the second terminal of the first transistor is equal to or below a switching threshold voltage of the second transistor plus a bias voltage of the second diode. 
     The first transistor and the second transistor may each comprise a first terminal for turning that transistor ON and OFF, a second terminal and a third terminal, and the circuit may further comprise a third transistor and a fourth transistor. The first terminal of the first transistor may be connected to the second terminal of the second transistor via the third transistor and the first terminal of the second transistor may be connected to the second terminal of the first transistor via the fourth transistor. The third and fourth transistors may be field effect transistors. 
     Each of the third transistor and the fourth transistor may have a first terminal for turning that transistor ON and OFF, and each of the third transistor and the fourth transistor may be configured to be switched ON when a voltage greater than or equal to a threshold voltage is applied to its respective first terminal. 
     The resonant circuit may be configured to be activated by the application of a voltage greater than or equal to the threshold voltage to the first terminals of both the third transistor and the fourth transistor to thereby turn the third and fourth transistor ON. 
     In some examples, the resonant circuit does not comprise a controller configured to actuate the switching arrangement. 
     The resonant frequency of the resonant circuit may change in response to energy being transferred from the inductive element to the susceptor arrangement. 
     The resonant circuit may comprise a transistor control voltage for supplying a control voltage to the first terminals of the first transistor and the second transistor. 
     The resonant circuit may comprise a first pull-up resistor connected in series between the first terminal of the first transistor and the transistor control voltage and a second pull-up resistor connected in series between the first terminal of the second transistor and the transistor control voltage. 
     The third transistor may be connected between the control voltage and the first terminal of the first transistor and the fourth transistor may be connected between the control voltage and the second transistor. 
     The first transistor and/or the second transistor may be field effect transistors. 
     A first terminal of the DC voltage supply may be connected to first and second points in the resonant circuit wherein the first point and the second point are electrically located to either side of the inductive element. 
     A first terminal of the DC voltage supply may be connected to a first point in the resonant circuit wherein the first point is electrically connected to a central point of the inductive element such that current flowing from the first point can flow in a first direction through a first portion of the inductive element and in a second direction through a second portion of the inductive element. 
     The resonant circuit may comprise at least one choke inductor positioned between the DC voltage supply and the inductive element. 
     The resonant circuit may comprise a first choke inductor and a second choke inductor wherein the first choke inductor is connected in series between the first point and the inductive element and the second choke is connected in series between the second point and the inductive element. 
     The resonant circuit may comprise a first choke inductor, wherein the first choke inductor is connected in series between the first point in the resonant circuit and the central point of the inductive element. 
     According to a second aspect of the present disclosure there is provided an aerosol generating device comprising the resonant circuit according to the first aspect. 
     The aerosol generating device may be configured to receive a first consumable component having a first susceptor arrangement and the aerosol generating device may be configured to receive a second consumable component having a second susceptor arrangement, wherein the varying current is maintained at a first resonant frequency of the resonant circuit when the first consumable component is coupled to the device and at a second resonant frequency of the resonant circuit when the second consumable component is coupled to the device. 
     The aerosol generating device may comprise a receiving portion, the receiving portion configured to receive either one of the first consumable component or the second consumable component such that the first or second susceptor arrangement is provided in proximity to the inductive element. 
     The inductive element may be an electrically conductive coil, wherein the device is configured to receive at least a part of the first or second susceptor arrangement within the coil. 
     According to a third aspect of the present disclosure there is provided a system comprising an aerosol generating device according to the second aspect and a susceptor arrangement. 
     The susceptor arrangement may be formed of aluminum. 
     The susceptor arrangement may be arranged in a consumable comprising the susceptor arrangement and aerosol generating material. 
     According to a fourth aspect of the present disclosure there is provided a kit of parts comprising a first consumable component comprising a first aerosol generating material and a first susceptor arrangement, and a second consumable component comprising a second aerosol generating material and a second susceptor, the first and second consumable components configured for use with the aerosol generating device according to the second aspect. 
     The first consumable component may have a different shape compared to the second consumable component. 
     The first susceptor arrangement may have a different shape or be formed from a different material compared to the second consumable component. 
     The first and second consumable components may be selected from the group comprising: a stick, a pod, a cartomizer, and a flat sheet. 
     The first susceptor arrangement or the second susceptor arrangement may be formed of aluminum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically an aerosol generating device according to an example. 
         FIG. 2  illustrates schematically a resonant circuit according to an example. 
         FIG. 3  illustrates schematically a resonant circuit according to a second example. 
         FIG. 4  illustrates schematically a resonant circuit according to a third example. 
         FIG. 5  illustrates schematically a resonant circuit according to a fourth example. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Induction heating is a process of heating an electrically conducting object (or susceptor) by electromagnetic induction. An induction heater may comprise an inductive element, for example, an inductive coil and a device for passing a varying electric current, such as an alternating electric current, through the inductive element. The varying electric current in the inductive element produces a varying magnetic field. The varying magnetic field penetrates a susceptor suitably positioned with respect to the inductive element, generating eddy currents inside the susceptor. The susceptor has electrical resistance to the eddy currents, and hence the flow of the eddy currents against this resistance causes the susceptor to be heated by Joule heating. In cases where the susceptor comprises ferromagnetic material such as iron, nickel or cobalt, heat may also be generated by magnetic hysteresis losses in the susceptor, i.e. by the varying orientation of magnetic dipoles in the magnetic material as a result of their alignment with the varying magnetic field. 
     In inductive heating, as compared to heating by conduction for example, heat is generated inside the susceptor, allowing for rapid heating. Further, there need not be any physical contact between the inductive heater and the susceptor, allowing for enhanced freedom in construction and application. 
     An induction heater may comprise an LC circuit, having an inductance L provided by an induction element, for example the electromagnet which may be arranged to inductively heat a susceptor, and a capacitance C provided by a capacitor. The circuit may in some cases be represented as an RLC circuit, comprising a resistance R provided by a resistor. In some cases, resistance is provided by the ohmic resistance of parts of the circuit connecting the inductor and the capacitor, and hence the circuit need not necessarily include a resistor as such. Such a circuit may be referred to, for example as an LC circuit. Such circuits may exhibit electrical resonance, which occurs at a particular resonant frequency when the imaginary parts of impedances or admittances of circuit elements cancel each other. 
     One example of a circuit exhibiting electrical resonance is an LC circuit, comprising an inductor, a capacitor, and optionally a resistor. One example of an LC circuit is a series circuit where the inductor and capacitor are connected in series. Another example of an LC circuit is a parallel LC circuit where the inductor and capacitor are connected in parallel. Resonance occurs in an LC circuit because the collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor, while the discharging capacitor provides an electric current that builds the magnetic field in the inductor. The present disclosure focuses on parallel LC circuits. When a parallel LC circuit is driven at the resonant frequency, the dynamic impedance of the circuit is at maximum (as the reactance of the inductor equals the reactance of the capacitor), and circuit current is at a minimum. However, for a parallel LC circuit, the parallel inductor and capacitor loop acts as a current multiplier (effectively multiplying the current within the loop and thus the current passing through the inductor). Driving the RLC or LC circuit at or near the resonant frequency may therefore provide for effective and/or efficient inductive heating by providing for the greatest value of the magnetic field penetrating the susceptor. 
     A transistor is a semiconductor device for switching electronic signals. A transistor typically comprises at least three terminals for connection to an electronic circuit. In some prior art examples, an alternating current may be supplied to a circuit using a transistor by supplying a drive signal which causes the transistor to switch at a predetermined frequency, for example at the resonant frequency of the circuit. 
     A field effect transistor (FET) is a transistor in which the effect of an applied electric field may be used to vary the effective conductance of the transistor. The field effect transistor may comprise a body B, a source terminal S, a drain terminal D, and a gate terminal G. The field effect transistor comprises an active channel comprising a semiconductor through which charge carriers, electrons or holes, may flow between the source S and the drain D. The conductivity of the channel, i.e. the conductivity between the drain D and the source S terminals, is a function of the potential difference between the gate G and source S terminals, for example generated by a potential applied to the gate terminal G. In enhancement mode FETs, the FET may be OFF (i.e. substantially prevent current from passing therethrough) when there is substantially zero gate G to source S voltage, and may be turned ON (i.e. substantially allow current to pass therethrough) when there is a substantially non-zero gate G-source S voltage. 
     An n-channel (or n-type) field effect transistor (n-FET) is a field effect transistor whose channel comprises an n-type semiconductor, where electrons are the majority carriers and holes are the minority carriers. For example, n-type semiconductors may comprise an intrinsic semiconductor (such as silicon for example) doped with donor impurities (such as phosphorus for example). In n-channel FETs, the drain terminal D is placed at a higher potential than the source terminal S (i.e. there is a positive drain-source voltage, or in other words a negative source-drain voltage). In order to turn an n-channel FET “on” (i.e. to allow current to pass therethrough), a switching potential is applied to the gate terminal G that is higher than the potential at the source terminal S. 
     A p-channel (or p-type) field effect transistor (p-FET) is a field effect transistor whose channel comprises a p-type semiconductor, where holes are the majority carriers and electrons are the minority carriers. For example, p-type semiconductors may comprise an intrinsic semiconductor (such as silicon for example) doped with acceptor impurities (such as boron for example). In p-channel FETs, the source terminal S is placed at a higher potential than the drain terminal D (i.e. there is a negative drain-source voltage, or in other words a positive source-drain voltage). In order to turn a p-channel FET “on” (i.e. to allow current to pass therethrough), a switching potential is applied to the gate terminal G that is lower than the potential at the source terminal S (and which may for example be higher than the potential at the drain terminal D). 
     A metal-oxide-semiconductor field effect transistor (MOSFET) is a field effect transistor whose gate terminal G is electrically insulated from the semiconductor channel by an insulating layer. In some examples, the gate terminal G may be metal, and the insulating layer may be an oxide (such as silicon dioxide for example), hence “metal-oxide-semiconductor”. However, in other examples, the gate may be made from other materials than metal, such as polysilicon, and/or the insulating layer may be made from other materials than oxide, such as other dielectric materials. Such devices are nonetheless typically referred to as metal-oxide-semiconductor field effect transistors (MOSFETs), and it is to be understood that as used herein the term metal-oxide-semiconductor field effect transistors or MOSFETs is to be interpreted as including such devices. 
     A MOSFET may be an n-channel (or n-type) MOSFET where the semiconductor is n-type. The n-channel MOSFET (n-MOSFET) may be operated in the same way as described above for the n-channel FET. As another example, a MOSFET may be a p-channel (or p-type) MOSFET, where the semiconductor is p-type. The p-channel MOSFET (p-MOSFET) may be operated in the same way as described above for the p-channel FET. An n-MOSFET typically has a lower source-drain resistance than that of a p-MOSFET. Hence in an “on” state (i.e. where current is passing therethrough), n-MOSFETs generate less heat as compared to p-MOSFETs, and hence may waste less energy in operation than p-MOSFETs. Further, n-MOSFETs typically have shorter switching times (i.e. a characteristic response time from changing the switching potential provided to the gate terminal G to the MOSFET changing whether or not current passes therethrough) as compared to p-MOSFETs. This can allow for higher switching rates and improved switching control. 
       FIG. 1  illustrates schematically an aerosol generating device  100 , according to an example. The aerosol generating device  100  comprises a DC power source  104 , in this example a battery  104 , a circuit  150  comprising an inductive element  158 , a susceptor arrangement  110 , and aerosol generating material  116 . 
     In the example of  FIG. 1 , the susceptor arrangement  110  is located within a consumable  120  along with the aerosol generating material  116 . The DC power source  104  is electrically connected to the circuit  150  and is arranged to provide DC electrical power to the circuit  150 . The device  100  also comprises control circuitry  106 , in this example the circuit  150  is connected to the battery  104  via the control circuitry  106 . 
     The control circuitry  106  may comprise means for switching the device  100  on and off, for example in response to a user input. The control circuitry  106  may for example comprise a puff detector (not shown), as is known per se, and/or may take user input via at least one button or touch control (not shown). The control circuitry  106  may comprise means for monitoring the temperature of components of the device  100  or components of a consumable  120  inserted in the device. In addition to the inductive element  158 , the circuit  150  comprises other components which are described below. 
     The inductive element  158  may be, for example a coil, which may for example be planar. The inductive element  158  may, for example, be formed from copper (which has a relatively low resistivity). The circuitry  150  is arranged to convert an input DC current from the DC power source  104  into a varying, for example alternating, current through the inductive element  158 . The circuitry  150  is arranged to drive the varying current through the inductive element  158 . 
     The susceptor arrangement  110  is arranged relative to the inductive element  158  for inductive energy transfer from the inductive element  158  to the susceptor arrangement  110 . The susceptor arrangement  110  may be formed from any suitable material that can be inductively heated, for example a metal or metal alloy, e.g., steel. In some implementations, the susceptor arrangement  110  may comprise or be entirely formed from a ferromagnetic material, which may comprise one or a combination of example metals such as iron, nickel and cobalt. In some implementations, the susceptor arrangement  110  may comprise or be formed entirely from a non-ferromagnetic material, for example aluminum. The inductive element  158 , having varying current driven therethrough, causes the susceptor arrangement  110  to heat up by Joule heating and/or by magnetic hysteresis heating, as described above. The susceptor arrangement  110  is arranged to heat the aerosol generating material  116 , for example by conduction, convection, and/or radiation heating, to generate an aerosol in use. In some examples, the susceptor arrangement  110  and the aerosol generating material  116  form an integral unit that may be inserted and/or removed from the aerosol generating device  100 , and may be disposable. In some examples, the inductive element  158  may be removable from the device  100 , for example for replacement. The aerosol generating device  100  may be hand-held. The aerosol generating device  100  may be arranged to heat the aerosol generating material  116  to generate aerosol for inhalation by a user. 
     It is noted that, as used herein, the term “aerosol generating material” includes materials that provide volatilized components upon heating, typically in the form of vapor or an aerosol. Aerosol generating material may be a non-tobacco-containing material or a tobacco-containing material. For example, the aerosol generating material may be or comprise tobacco. Aerosol generating material may, for example, include one or more of tobacco per se, tobacco derivatives, expanded tobacco, reconstituted tobacco, tobacco extract, homogenzied tobacco or tobacco substitutes. The aerosol generating material can be in the form of ground tobacco, cut rag tobacco, extruded tobacco, reconstituted tobacco, reconstituted material, liquid, gel, gelled sheet, powder, or agglomerates, or the like. Aerosol generating material also may include other, non-tobacco, products, which, depending on the product, may or may not contain nicotine. Aerosol generating material may comprise one or more humectants, such as glycerol or propylene glycol. 
     Returning to  FIG. 1 , the aerosol generating device  100  comprises an outer body  112  housing the DC power supply  104 , the control circuitry  106  and the circuit  150  comprising the inductive element  158 . The consumable  120  comprising the susceptor arrangement  110  and the aerosol generating material  116  in this example is also inserted into the body  112  to configure the device  100  for use. The outer body  112  comprises a mouthpiece  114  to allow aerosol generated in use to exit the device  100 . 
     In use, a user may activate, for example via a button (not shown) or a puff detector (not shown), the circuitry  106  to cause a varying, e.g. alternating, current to be driven through the inductive element  108 , thereby inductively heating the susceptor arrangement  110 , which in turn heats the aerosol generating material  116 , and causes the aerosol generating material  116  thereby to generate an aerosol. The aerosol is generated into air drawn into the device  100  from an air inlet (not shown), and is thereby carried to the mouthpiece  104 , where the aerosol exits the device  100  for inhalation by a user. 
     The circuit  150  comprising the inductive element  158 , and the susceptor arrangement  110  and/or the device  100  as a whole may be arranged to heat the aerosol generating material  116  to a range of temperatures to volatilize at least one component of the aerosol generating material  116  without combusting the aerosol generating material. For example, the temperature range may be about 50° C. to about 350° C., such as between about 50° C. and about 300° C., between about 100° C. and about 300° C., between about 150° C. and about 300° C., between about 100° C. and about 200° C., between about 200° C. and about 300° C., or between about 150° C. and about 250° C. In some examples, the temperature range is between about 170° C. and about 250° C. In some examples, the temperature range may be other than this range, and the upper limit of the temperature range may be greater than 300° C. 
     It will be appreciated that there may be a difference between the temperature of the susceptor arrangement  110  and the temperature of the aerosol generating material  116 , for example during heating up of the susceptor arrangement  110 , for example where the rate of heating is large. It will therefore be appreciated that in some examples the temperature at which the susceptor arrangement  110  is heated to may, for example, be higher than the temperature to which it is desired that the aerosol generating material  116  is heated. 
     Referring now to  FIG. 2 , there is illustrated an example circuit  150 , which is a resonant circuit, for inductive heating of the susceptor arrangement  110 . The resonant circuit  150  comprises the inductive element  158  and a capacitor  156 , connected in parallel. 
     The resonant circuit  150  comprises a switching arrangement M 1 , M 2  which, in this example, comprises a first transistor M 1  and a second transistor M 2 . The first transistor M 1  and the second transistor M 2  each comprise a respective first terminal G 1 , G 2 , second terminal D 1 , D 2  and third terminal S 1 , S 2 . The second terminals D 1 , D 2  of the first transistor M 1  and the second transistor M 2  are connected to either side of the parallel inductive element  158  and the capacitor  156  combination, as will be explained in more detail below. The third terminals S 1 , S 2  of the first transistor M 1  and the second transistor M 2  are each connected to earth  151 . In the example illustrated in  FIG. 2  the first transistor M 1  and the second transistor M 2  are both MOSFETS and the first terminals G 1 , G 2  are gate terminals, the second terminals D 1 , D 2  are drain terminals and the third terminals S 1 , S 2  are source terminals. 
     It will be appreciated that in alternative examples other types of transistors may be used in place of the MOSFETs described above. 
     The resonance circuit  150  has an inductance L and a capacitance C. The inductance L of the resonant circuit  150  is provided by the inductive element  158 , and may also be affected by an inductance of the susceptor arrangement  110  which is arranged for inductive heating by the inductive element  158 . The inductive heating of the susceptor arrangement  110  is via a varying magnetic field generated by the inductive element  158 , which, in the manner described above, induces Joule heating and/or magnetic hysteresis losses in the susceptor arrangement  110 . A portion of the inductance L of the resonant circuit  150  may be due to the magnetic permeability of the susceptor arrangement  110 . The varying magnetic field generated by the inductive element  158  is generated by a varying, for example alternating, current flowing through the inductive element  158 . 
     The inductive element  158  may, for example, be in the form of a coiled conductive element. For example, inductive element  158  may be a copper coil. The inductive element  158  may comprise, for example, a multi-stranded wire, such as Litz wire, for example a wire comprising a number of individually insulated wires twisted together. The AC resistance of a multi-stranded wire is a function of frequency and the multi-stranded wire can be configured in such a way that the power absorption of the inductive element is reduced at a driving frequency. As another example, the inductive element  158  may be a coiled track on a printed circuit board, for example. Using a coiled track on a printed circuit board may be useful as it provides for a rigid and self-supporting track, with a cross section which obviates any requirement for multi-strand wire (which may be expensive), which can be mass produced with a high reproducibility for low cost. Although one inductive element  158  is shown, it will be readily appreciated that there may be more than one inductive element  158  arranged for inductive heating of one or more susceptor arrangements  110 . 
     The capacitance C of the resonant circuit  150  is provided by the capacitor  156 . The capacitor  156  may be, for example, a Class 1 ceramic capacitor, for example a COG type capacitor. The total capacitance C may also comprise the stray capacitance of the resonant circuit  150 ; however, this is or can be made negligible compared with the capacitance provided by the capacitor  156 . 
     The resistance of the resonant circuit  150  is not shown in  FIG. 2  but it should be appreciated that a resistance of the circuit may be provided by the resistance of the track or wire connecting the components of the resonance circuit  150 , the resistance of the inductor  158 , and/or the resistance to current flowing through the resonance circuit  150  provided by the susceptor arrangement  110  arranged for energy transfer with the inductor  158 . In some examples, one or more dedicated resistors (not shown) may be included in the resonant circuit  150 . 
     The resonant circuit  150  is supplied with a DC supply voltage V 1  provided from the DC power source  104  (see  FIG. 1 ), e.g. from a battery. A positive terminal of the DC voltage supply V 1  is connected to the resonant circuit  150  at a first point  159  and at a second point  160 . A negative terminal (not shown) of the DC voltage supply V 1  is connected to earth  151  and hence, in this example, to the source terminals S of both the MOSFETs M 1  and M 2 . In examples, the DC supply voltage V 1  may be supplied to the resonant circuit directly from a battery or via an intermediary element. 
     The resonant circuit  150  may therefore be considered to be connected as an electrical bridge with the inductive element  158  and the capacitor  156  in parallel connected between the two arms of the bridge. The resonant circuit  150  acts to produce a switching effect, described below, which results in a varying, e.g. alternating, current being drawn through the inductive element  158 , thus creating the alternating magnetic field and heating the susceptor arrangement  110 . 
     The first point  159  is connected to a first node A located at a first side of the parallel combination of the inductive element  158  and the capacitor  156 . The second point  160  is connected to a second node B, to a second side of the parallel combination of the inductive element  158  and the capacitor  156 . A first choke inductor  161  is connected in series between the first point  159  and the first node A, and a second choke inductor  162  is connected in series between the second point  160  and the second node B. The first and second chokes  161  and  162  act to filter out AC frequencies from entering the circuit from the first point  159  and the second point  160  respectively but allow DC current to be drawn into and through the inductor  158 . The chokes  161  and  162  allow the voltage at A and B to oscillate with little or no visible effects at the first point  159  or the second point  160 . 
     In this particular example, the first MOSFET M 1  and the second MOSFET M 2  are n-channel enhancement mode MOSFETs. The drain terminal of the first MOSFET M 1  is connected to the first node A via a conducting wire or the like, while the drain terminal of the second MOSFET M 2  is connected to the second node B, via a conducting wire or the like. The source terminal of each MOSFET M 1 , M 2  is connected to earth  151 . 
     The resonant circuit  150  comprises a second voltage source V 2 , gate voltage supply (or sometimes referred to herein as a control voltage), with its positive terminal connected at a third point  165  which is used for supplying a voltage to the gate terminals G 1 , G 2  of the first and second MOSFETs M 1  and M 2 . The control voltage V 2  supplied at the third point  165  in this example is independent of voltage V 1  supplied at the first and second points  159 ,  160 , which enables variation of voltage V 1  without impacting the control voltage V 2 . A first pull-up resistor  163  is connected between the third point  165  and the gate terminal G 1  of the first MOSFET M 1 . A second pull-up resistor  164  is connected between the third point  165  and the gate terminal G 2  of the second MOSFET M 2 . 
     In other examples, a different type of transistor may be used, such as a different type of FET. It will be appreciated that the switching effect described below can be equally achieved for a different type of transistor which is capable of switching from an “on” state to an “off” state. The values and polarities of the supply voltages V 1  and V 2  may be chosen in conjunction with the properties of the transistor used, and the other components in the circuit. For example, the supply voltages may be chosen in dependence on whether an n-channel or p-channel transistor is used, or in dependence on the configuration in which the transistor is connected, or the difference in the potential difference applied across terminals of the transistor which results in the transistor being in either on or off. 
     The resonant circuit  150  further comprises a first diode d 1  and a second diode d 2 , which in this example are Schottky diodes, but in other examples any other suitable type of diode may be used. The gate terminal G 1  of the first MOSFET M 1  is connected to the drain terminal D 2  of the second MOSFET M 2  via the first diode d 1 , with the forward direction of the first diode d 1  being towards the drain D 2  of the second MOSFET M 2 . 
     The gate terminal G 2  of the second MOSFET M 2  is connected to the drain D 1  of the first second MOSFET M 1  via the second diode d 2 , with the forward direction of the second diode d 2  being towards the drain D 1  of the first MOSFET M 1 . The first and second Schottky diodes d 1  and d 2  may have a diode threshold voltage of around 0.3V. In other examples, silicon diodes may be used having a diode threshold voltage of around 0.7V. In examples, the type of diode used is selected in conjunction with the gate threshold voltage, to allow desired switching of the MOSFETs M 1  and M 2 . It will be appreciated that the type of diode and gate supply voltage V 2  may also be chosen in conjunction with the values of pull-up resistors  163  and  164 , as well as the other components of the resonant circuit  150 . 
     The resonant circuit  150  supports a current through the inductive element  158  which is a varying current due to switching of the first and second MOSFETs M 1  and M 2 . Since, in this example the MOSFETs M 1  and M 2  are enhancement mode MOSFETS, when a voltage applied at the gate terminal G 1 , G 2  of one of the first and second MOSFETs is such that a gate-source voltage is higher than a predetermined threshold for that MOSFET, the MOSFET is turned to the ON state. Current may then flow from the drain terminal D 1 , D 2  to the source terminal S 1 , S 2  which is connected to ground  151 . The series resistance of the MOSFET in this ON state is negligible for the purposes of the operation of the circuit, and the drain terminal D can be considered to be at ground potential when the MOSFET is in the ON state. The gate-source threshold for the MOSFET may be any suitable value for the resonant circuit  150  and it will be appreciated that the magnitude of the voltage V 2  and resistances of resistors  164  and  163  are chosen dependent on the gate-source threshold voltage of the MOSFETs M 1  and M 2 , essentially so that voltage V 2  is greater than the gate threshold voltage(s). 
     The switching procedure of the resonant circuit  150  which results in varying current flowing through the inductive element  158  will now be described starting from a condition where the voltage at first node A is high and the voltage at the second node B is low. 
     When the voltage at node A is high, the voltage at the drain terminal D 1  of the first MOSFET M 1  is also high because the drain terminal D 1  of M 1  is connected, directly in this example, to the node A via a conducting wire. At the same time the voltage at the node B is held low and the voltage at the drain terminal D 2  of the second MOSFET M 2  is correspondingly low (the drain terminal of M 2  being, in this example, directly connected to the node B via a conducting wire). 
     Accordingly, at this time, the value of the drain voltage of M 1  is high and is greater than the gate voltage of M 2 . The second diode d 2  is therefore reverse-biased at this time. The gate voltage of M 2  at this time is greater than the source terminal voltage of M 2 , and the voltage V 2  is such that the gate-source voltage at M 2  is greater than the ON threshold for the MOSFET M 2 . M 2  is therefore ON at this time. 
     At the same time, the drain voltage of M 2  is low, and the first diode d 1  is forward biased due to the gate voltage supply V 2  to the gate terminal of M 1 . The gate terminal of M 1  is therefore connected via the forward biased first diode d 1  to the low voltage drain terminal of the second MOSFET M 2 , and the gate voltage of M 1  is therefore also low. In other words, because M 2  is on, it is acting as a ground clamp, which results in the first diode d 1  being forward biased, and the gate voltage of M 1  being low. As such, the gate-source voltage of M 1  is below the ON threshold and the first MOSFET M 1  is OFF. 
     In summary, at this point the circuit  150  is in a first state, wherein: 
     voltage at node A is high; 
     voltage at node B is low; 
     first diode d 1  is forward biased; 
     second MOSFET M 2  is ON; 
     second diode d 2  is reverse biased; and 
     first MOSFET M 1  is OFF. 
     From this point, with the second MOSFET M 2  being in the ON state, and the first MOSFET M 1  being in the OFF state, current is drawn from the supply V 1  through the first choke  161  and through the inductive element  158 . Due to the presence of inducting choke  161 , the voltage at node A is free to oscillate. Since the inductive element  158  is in parallel with the capacitor  156 , the observed voltage at node A follows that of a half sinusoidal voltage profile. The frequency of the observed voltage at node A is equal to the resonant frequency f 0  of the circuit  150 . 
     The voltage at node A reduces sinusoidally in time from its maximum value towards 0 as a result of an energy decay at node A. The voltage at node B is held low (because MOSFET M 2  is on) and the inductor L is charged from the DC supply V 1 . The MOSFET M 2  is switched off at a point in time when the voltage at node A is equal to or below the gate threshold voltage of M 2  plus the forward bias voltage of d 2 . When the voltage at node A has finally reached zero, the MOSFET M 2  will be fully off. 
     At the same time, or shortly after, the voltage at node B is taken high. This happens due to the resonant transfer of energy between the inductive element  158  and the capacitor  156 . When the voltage at node B becomes high due to this resonant transfer of energy, the situation described above with respect to the nodes A and B and the MOSFETs M 1  and M 2  is reversed. That is, as the voltage at A reduces towards zero, the drain voltage of M 1  is reduced. The drain voltage of M 1  reduces to a point where the second diode d 2  is no longer reverse biased and becomes forward biased. Similarly, the voltage at node B rises to its maximum and the first diode d 1  switches from being forward biased to being reverse biased. As this happens, the gate voltage of M 1  is no longer coupled to the drain voltage of M 2  and the gate voltage of M 1  therefore becomes high, under the application of gate supply voltage V 2 . The first MOSFET M 1  is therefore switched to the ON state, since its gate-source voltage is now above the threshold for switch-on. As the gate terminal of M 2  is now connected via the forward biased second diode d 2  to the low voltage drain terminal of M 1 , the gate voltage of M 2  is low. M 2  is therefore switched to the OFF state. 
     In summary, at this point the circuit  150  is in a second state, wherein: 
     voltage at node A is low; 
     voltage at node B is high; 
     first diode d 1  is reverse biased; 
     second MOSFET M 2  is OFF; 
     second diode d 2  is forward biased; and 
     first MOSFET M 1  is ON. 
     At this point, current is drawn through the inductive element  158  from the supply voltage V 1  through the second choke  162 . The direction of the current has therefore reversed due to the switching operation of the resonant circuit  150 . The resonant circuit  150  will continue to switch between the above-described first state in which the first MOSFET M 1  is OFF and the second MOSFET M 2  is ON, and the above-described second state in which the first MOSFET M 1  is ON and the second MOSFET M 2  is OFF. 
     In the steady state of operation, energy is transferred between the electrostatic domain (i.e., in the capacitor  156 ) and the magnetic domain (i.e., the inductor  158 ), and vice versa. 
     The net switching effect is in response to the voltage oscillations in the resonant circuit  150  where we have an energy transfer between the electrostatic domain (i.e., in the capacitor  156 ) and the magnetic domain (i.e., the inductor  158 ), thus creating a time varying current in the parallel LC circuitry, which varies at the resonant frequency of the circuit. This is advantageous for energy transfer between the inductive element  158  and the susceptor arrangement  110  since the circuitry  150  operates at its optimal efficiency level and therefore achieves more efficient heating of the aerosol generating material  116  compared to circuitry operating off resonance. The described switching arrangement is advantageous as it allows the circuit  150  to drive itself at the resonant frequency under varying load conditions, for example when a different susceptor is coupled to the inductive element. What this means is that in the event that the properties of the circuitry  150  change (for example if the susceptor  110  is present or not, or if the temperature of the susceptor changes, or even physical movement of the susceptor element  110 ), the dynamic nature of the circuitry  150  continuously adapts its resonant point to transfer energy in an optimal fashion, thus meaning that the circuitry  150  is always driven at resonance. Moreover, the configuration of the circuit  150  is such that no external controller or the like is required to apply the control voltage signals to the gates of the MOSFETS to effect the switching. 
     In examples described above, with reference to  FIG. 2 , the gate terminals G 1 , G 2  are supplied with a gate voltage via a second power supply which is different to the power supply for the source voltage V 1 . However, in some examples, the gate terminals may be supplied with the same voltage supply as the source voltage V 1 . In such examples, the first point  159 , second point  160 , and third point  165  in the circuit  150  may, for example, be connected to the same power rail. In such examples, it will be appreciated that the properties of the components of the circuit must be chosen to allow the described switching action to take place. For example, the gate supply voltage and diode threshold voltages should be chosen such that the oscillations of the circuit trigger switching of the MOSFETs at the appropriate level. The provision of separate voltage values for the gate supply voltage V 2  and the source voltage V 1  allows for the source voltage V 1  to be varied independently of the gate supply voltage V 2  without affecting the operation of the switching mechanism of the circuit. 
     The resonant frequency f 0  of the circuit  150  may be in the MHz range, for example in the range 0.5 MHz to 4 MHz, for example in the range 2 MHz to 3 MHz. It will be appreciated that the resonant frequency f 0  of the resonant circuit  150  is dependent on the inductance L and capacitance C of the circuit  150 , as set out above, which in turn is dependent on the inductive element  158 , capacitor  156  and additionally the susceptor arrangement  110 . That is, it can be considered that the resonant frequency changes in response to energy being transferred from the inductive element to the susceptor arrangement. As such, the resonant frequency f 0  of the circuit  150  can vary from implementation to implementation. For example, the frequency may be in the range 0.1 MHz to 4 MHz, or in the range of 0.5 MHz to 2 MHz, or in the range 0.3 MHz to 1.2 MHz. In other examples, the resonant frequency may be in a range different from those described above. Generally, the resonant frequency will depend on the characteristics of the circuitry, such as the electrical and/or physical properties of the components used, including the susceptor arrangement  110 . 
     It will also be appreciated that the properties of the resonant circuit  150  may be selected based on other factors for a given susceptor arrangement  110 . For example, in order to improve the transfer of energy from the inductive element  158  to the susceptor arrangement  110 , it may be useful to select the skin depth (i.e. the depth from the surface of the susceptor arrangement  110  within which current density falls by a factor of 1/e, which is at least a function of frequency) based on the material properties of the susceptor arrangement  110 . The skin depth differs for different materials of susceptor arrangements  110 , and reduces with increasing drive frequency. On the other hand, for example, in order to reduce the proportion of power supplied to the resonant circuit  150  and/or driving element  102  that is lost as heat within the electronics, it may be beneficial to have a circuit which drives itself at relatively lower frequencies. Since the drive frequency is equal to the resonant frequency in this example, the considerations here with respect to drive frequency are made with respect to obtaining the appropriate resonant frequency, for example by designing a susceptor arrangement  110  and/or using a capacitor  156  with a certain capacitance and an inductive element  158  with a certain inductance. In some examples, a compromise between these factors may therefore be chosen as appropriate and/or desired. 
     The resonant circuit  150  of  FIG. 2  has a resonant frequency f 0  at which the current I is minimized and the dynamic resistance is maximized. The resonant circuit  150  drives itself at this resonant frequency and therefore the oscillating magnetic field generated by the inductor  158  is maximum, and the inductive heating of the susceptor arrangement  110  by the inductive element  158  is maximized. 
     In some examples, inductive heating of the susceptor arrangement  110  by the resonant circuit  150  may be controlled by controlling the supply voltage provided to the resonant circuit  150 , which in turn may control the current flowing in the resonant circuit  150 , and hence may control the energy transferred to the susceptor arrangement  110  by the resonant circuit  150 , and hence the degree to which the susceptor arrangement  110  is heated. In other examples, it will be appreciated that the temperature of the susceptor arrangement  110  may be monitored and controlled by, for example, changing the voltage supply (e.g., by changing the magnitude of the voltage supplied or by changing the duty cycle of a pulse width modulated voltage signal) to the inductive element  158  depending on whether the susceptor arrangement  110  is to be heated to a greater or lesser degree. 
     As mentioned above, the inductance L of the resonant circuit  150  is provided by the inductive element  158  arranged for inductive heating of the susceptor arrangement  110 . At least a portion of the inductance L of resonant circuit  150  is due to the magnetic permeability of the susceptor arrangement  110 . The inductance L, and hence resonant frequency f 0  of the resonant circuit  150  may therefore depend on the specific susceptor(s) used and its positioning relative to the inductive element(s)  158 , which may change from time to time. Further, the magnetic permeability of the susceptor arrangement  110  may vary with varying temperatures of the susceptor  110 . 
       FIG. 3  shows a second example of a resonant circuit  250 . The second resonant circuit  250  comprises many of the same components as the resonant circuit  150  and like components in each of the resonant circuits  150   250  are provided with the same reference numerals and will not be described in detail again. 
     The second circuit  250  differs from the first circuit  150  in that the second circuit  250  does not comprise the diodes d 1 , d 2 , via which the gate terminals G 1 , G 2  of each of the transistors M 1 , M 2  are respectively connected to the drain terminals D 1 , D 2  of the other of the transistors M 1 , M 2 . Instead of the diodes d 1 , d 2  which are included in the first circuit  150 , the second circuit  250  comprises a third MOSFET M 3  and a fourth MOSFET M 4 . 
     In the second circuit  250 , the gate G 1  of the first MOSFET M 1  is connected to the drain D 2  of the second MOSFET M 2  via the third MOSFET M 3 . The gate G 2  of the second MOSFET M 2  is similarly connected to the drain D 1  of the first MOSFET M 1  via a fourth MOSFET M 4 . The control voltage V 2  is supplied from the point  165  to gate terminals G 3 , G 4  of both the third MOSFET M 3  and the fourth MOSFET M 4 . In an example, such as the example represented by  FIG. 3 , the gate terminals G 3 , G 4  of the third MOSFET M 3  and the fourth MOSFET M 4  are connected to one another via an electrical conductor, for example an electrical track, and the voltage V 2  supplied to a point on the electrical conductor. It will be appreciated that each of the third MOSFET M 3  and the fourth MOSFET M 4  has a gate threshold voltage such that when a voltage greater than the threshold voltage is applied to its gate terminal G 3 , G 4 , the respective MOSFET M 3 , M 4  is turned “on” such that current may flow from its drain terminal to its source terminal. In examples, the voltage V 2  is greater than the threshold voltages of the third and fourth MOSFETs M 3 , M 4  such that applying the control voltage V 2  turns the third and fourth MOSFETs M 3 , M 4  to the ON state. In an example, the threshold voltage of the third MOSFET M 3  is equal to the threshold voltage of the fourth MOSFET M 4 . In some examples, the second circuit  250  may comprise one of more pull-down resistors (not shown in  FIG. 3 ) connected between the gates G 1 , G 2  of the first and second MOSFETs M 1 , M 2  and ground. 
     The second circuit  250  operates as a self-oscillating circuit which causes a varying current to flow through the inductive element  158  in the manner described with reference to the first example circuit  150  with reference to  FIG. 2 . Differences in the behavior of the second circuit  250  from that of the first example circuit  150  due to the use of MOSFETs M 3 , M 4  rather than diodes d 1 , d 2 , will become apparent from the following description. 
     The switching procedure of the second circuit  250  which results in a varying current flowing through the inductive element  158  will now be described. 
     When the voltage V 2  is applied to the gates G 3 , G 4  of the third and fourth MOSFETs M 3 , M 4 , the third and fourth MOSFETs are turned “on”. Providing that a voltage V 1 , at this point, each of the first, second, third and fourth MOSFETs M 1 -M 4  is in the ON state. At this point, the voltages at nodes A and B start to fall. Certain imbalances may exist in the circuit  250 , for example differences in resistance between the MOSFETs M 1 -M 4 , or the properties of the values of inductors present in the circuit. These imbalances act such that the voltage at one of the nodes A or B begins to fall faster than the voltage at the other of these nodes A, B. The MOSFET M 1 , M 2  corresponding to the node A, B at which the voltage falls fastest will remain in the ON state. The other of the MOSFETS M 1 , M 2 , corresponding with the other of nodes A, B is switched to the OFF state. The following describes the situation wherein the voltage at node A begins oscillating and the voltage at the node B remains at zero. However, equally, it may be the case that it is the voltage at the node B which begins oscillating while the voltage at node A remains at zero volts. 
     When the voltage at node A rises, the voltage at the drain terminal D 1  of the first MOSFET M 1  also rises because the drain terminal D 1  of first MOSFET M 1  is connected to the node A via a conducting wire. At the same time, the voltage at the node B is held low and the voltage at the drain terminal D 2  of the second MOSFET M 2  is correspondingly low (the drain terminal D 2  of the second MOSFET M 2  being, in this example, directly connected to the node B via a conducting wire). 
     As the voltage at the node A and the drain D 1  of the first MOSFET M 1  rises, the voltage at the gate G 2  of the second MOSFET M 2  rises. This is due to the drain D 1  being connected via the fourth MOSFET M 4  to the gate G 2  of the second MOSFET M 2  and the fourth MOSFET M 4  being “on” due to the voltage V 2  being applied to its gate terminal G 4 . 
     As the voltage at the drain D 1  of the first MOSFET M 1  rises, the voltage at the gate G 2  of the second MOSFET M 2  continues to rise until it reaches a maximum voltage value V max . The maximum voltage value V max  reached at the gate G 2  of the second MOSFET M 2  is dependent on the control voltage V 2  and the gate-source voltage of the fourth MOSFET M 4  (V gsM4 ). The maximum value V max  may be expressed as V max =V 2 −V gsM4 . 
     After a half cycle of oscillation at the resonant frequency of the circuit  250 , the voltage at the drain D 1  of the first MOSFET M 1  begins decreasing. The voltage at the drain D 1  of the first MOSFET M 1  decreases until it reaches 0V. At this point, the first MOSFET M 1  turns from “off” to “on” and the second MOSFET M 2  turns from “on” to “off”. 
     The circuit then continues to oscillate in a similar manner as described above, except with the node A remaining at zero volts while the node B is free to oscillate. That is, the voltage at the drain D 2  of the second MOSFET M 2  and at the node B then begins rising, while the voltage at the drain D 1  of the first MOSFET M 1  and the node A remains at zero. 
     As the voltage at the node B and the drain D 2  of the second MOSFET M 2  rises, the voltage at the gate G 1  of the first MOSFET M 1  rises since the drain D 2  is connected via the third MOSFET M 3  to the gate G 1  of the first MOSFET M 1  and the third MOSFET M 3  is “on” due to the voltage V 2  being applied to its gate terminal G 3 . 
     As the voltage at the drain D 2  of the second MOSFET M 2  rises, the voltage at the gate G 1  of the first MOSFET M 1  continues to rise until it reaches a maximum voltage value V max . The maximum voltage value V max  reached at the gate G 1  is dependent on the control voltage V 2  and the gate-source voltage of the third MOSFET M 3  (V gsM3 ). The maximum value V max  may be expressed as V max =V 2 −V gsM3 . In this example, the gate-source voltages of the third and fourth MOSFETs M 3 , M 4  are equal to one another, i.e. V gsM3 =V gsM4 . 
     After a half cycle of oscillation at the resonant frequency of the second circuit  250 , the voltage at the drain D 2  of the second MOSFET M 2  begins decreasing. The voltage at the drain D 2  of the second MOSFET M 2  decreases until it reaches 0V. At this point, the second MOSFET M 2  turns from “off” to “on” and the first MOSFET M 1  turns from “on” to “off”. 
     In the manner described with reference to the first example circuit  150 , when the second MOSFET M 2  is in the ON state, and the first MOSFET M 1  is in the OFF state, current is drawn from the supply V 1  through the first choke  161  and through the inductive element  158 . When the first MOSFET M 1  is in the ON state, and the second MOSFET M 2  is in the OFF state, current is drawn from the supply V 1  through the second choke  162  and through the inductive element  158 . The second example circuit  250  therefore oscillates in the same manner as described for the first example circuit  150  of  FIG. 2 , with the direction of the current reversing with each switching operation of the circuit  250 . 
     The use of third and fourth MOSFETs M 3 , M 4 , in some examples, may be advantageous because it may allow for lower energy losses. That is, the first example circuit  150  may result in resistive losses due to some current draw through the pull-up resistors  163 ,  164  to ground  151 . For example, when the first MOSFET M 1  is in the ON state, the second diode d 2  is forward biased and thus a small current may be drawn through the second pull-up resistor  164 , resulting in resistive losses. Similarly, when the second MOSFET M 2  is in the ON state, there may be resistive losses due to current drawn through the first pull-up resistor  163 . The second example circuit in examples may omit the resistors  163 ,  164 . The second example circuit  250  may reduce such losses by substituting the pull-up resistors  163 ,  164  and the diodes d 1 , d 2  for third and fourth MOSFETs M 3 , M 4 . For example, in the second example circuit  250 , when the first MOSFET M 1  is in the OFF state the current drawn through the third MOSFET M 3  may be essentially zero. Similarly, in the second example circuit  250 , when the second MOSFET M 2  is in the OFF state the current drawn through the fourth MOSFET M 4  may be essentially zero. Thus, resistive losses may be reduced by use of the arrangement shown in the second circuit  250 . Further, energy may be required to charge and discharge the gates G 1 , G 2  of first MOSFET M 1  and second MOSFET M 2 . The second circuit  250  may provide for this energy to be effectively provided from the nodes A and B. 
     Example circuits above have been described comprising two choke inductors  161 ,  162 . In another example, an example inductive heating circuit may comprise only one choke inductor. In such an example circuit, the inductor coil  158  may be “center-tapped”. 
       FIG. 4  shows a third example circuit  350  which is a variation on the first example circuit  150  and in which the coil  158  is a center-tapped coil and a single choke inductor  461  replaces the first and second choke inductors  161 ,  162 . The susceptor  110  is omitted from  FIG. 4  for clarity purposes. Again, components that are the same as those in the circuit  150  illustrated in  FIG. 2  are given the same reference numerals in  FIG. 4  as they are in  FIG. 1 . 
     In the third circuit  350 , voltage V 1  is applied via the choke inductor  461  to a center of the inductor coil  158 , at a single point  459  as opposed to at first and second points  159 ,  160  in the first example circuit  150 . Rather than, as in the first and second example circuits  150 ,  250 , current being drawn alternately through the first choke  161  and the second choke  162  as the current in the circuit changes direction due to the resonant oscillations of the circuit, current is drawn through the single choke inductor  461  and alternately drawn through a first part  158   a  of the inductor  158  and through a second part  158   b  of the inductor  158  as the current oscillations in the circuit  350  change direction due to the switching operation of the MOSFETs M 1 , M 2 . The third circuit  350  operates in an equivalent manner to the first circuit  150  in other respects. 
     A fourth example circuit is shown in  FIG. 5 . Again, components that are the same as those in the circuit  150  illustrated in  FIG. 2  are given the same reference numerals in  FIG. 4  as they are in  FIG. 1 . The fourth circuit  450  differs from the third circuit  350  in that, rather than comprising the single capacitor  156  of the third circuit  350 , the fourth circuit  450  is provided with a first capacitor  156   a  and a second capacitor  156   b.  The fourth circuit  450 , similarly to the third circuit  350  comprises a center-tapped arrangement with the inductor comprising a first part  158   a  and a second part  158   b.  The voltage V 1  is applied via the choke inductor  461  to a center of the inductor coil  158  (as in the arrangement of  FIG. 4 ) and, further, the center of the inductor coil  158  is electrically connected to a point between the first capacitor  156   a  and the second capacitor  156   b.  Two adjacent circuit loops are therefore provided, one comprising the first inductor part  158   a  and the first capacitor  156   a  and the other comprising the second inductor part  158   b  and the second capacitor  156   b.  The fourth circuit  450  operates in an equivalent manner to the third circuit  350  in other respects. 
     The center-tapped arrangement described with reference to  FIG. 4  and  FIG. 5  can equally be applied in an arrangement which uses third and fourth MOSFETs instead of diodes, in the manner described with reference to  FIG. 3 . The use of a center-tapped arrangement may be advantageous since the number of parts required to assemble the circuit may be reduced. For example, the number of choke inductors may be reduced from two to one. 
     In examples described herein the susceptor arrangement  110  is contained within a consumable and is therefore replaceable. For example, the susceptor arrangement  110  may be disposable and for example integrated with the aerosol generating material  116  that it is arranged to heat. The resonant circuit  150  allows for the circuit to be driven at the resonance frequency, automatically accounting for differences in construction and/or material type between different susceptor arrangements  110 , and/or differences in the placement of the susceptor arrangements  110  relative to the inductive element  158 , as and when the susceptor arrangement  110  is replaced. Furthermore, the resonant circuit is configured to drive itself at resonance regardless of the specific inductive element  158 , or indeed any other component of the resonant circuit  150  used. This is particularly useful to accommodate for variations in manufacturing both in terms of the susceptor arrangement  110  but also with regards to the other components of the circuit  150 . For example, the resonant circuit  150  allows the circuit to remain driving itself at the resonant frequency regardless of the use of different inductive elements  158  with different values of inductance, and/or differences in the placement of the inductive element  158  relative to the susceptor arrangement  110 . The circuit  150  is also able to drive itself at resonance even if the components are replaced over the lifetime of the device. 
     In some examples, the aerosol generating device  100  is configured to be usable with a plurality of different types of consumables each of which consumables comprises a different type of susceptor arrangement to the other consumables. 
     The different susceptor arrangements may be formed, for example, of different materials or be of different shapes or different sizes or different combinations of different materials or shapes or sizes. 
     In use, the resonant frequency of the circuit  150  is dependent upon the particular susceptor arrangement of whichever type of consumable is coupled to, for example inserted into, the device  100 . However, the alternating frequency through the inductive element  158  of the resonant circuit, due to the self-oscillating arrangement of the circuit  150 , is configured to self-adjust to match changes in the resonant frequency caused by the coupling of a different susceptor/consumable to the inductive element. Accordingly, the circuit is configured to heat a given susceptor arrangement at the resonant frequency of the circuit  150  when that consumable is coupled to the device  100 , regardless of the properties of the susceptor arrangement or consumable. 
     In some examples, the aerosol generating device  100  is configured to receive a first consumable having a first susceptor arrangement and the device is also configured to receive a second consumable having a second susceptor arrangement that is different to the first susceptor arrangement. 
     For example, the device  100  may be configured to receive a first consumable comprising an aluminum susceptor of a particular size and also be configured to receive a second consumable comprising a steel susceptor, which may be of a different shape and/or size to the aluminum susceptor. 
     The varying current in the circuit  150  is maintained at a first resonant frequency of the resonant circuit  150  when the first consumable is coupled to the device and is maintained at a second resonant frequency of the resonant circuit when the second consumable is coupled to the device  100 . 
     The aerosol generating device  100  in examples comprises a receiving portion for receiving a consumable. The receiving portion may be configured to receive a plurality of types of consumables, such as the first consumable or the second consumable.  FIG. 1  shows the aerosol generating device  100  in receipt of a consumable  120 , which is schematically shown to be received in a receiving portion  130  of the aerosol generating device  100 . The receiving portion  130  may be a cavity or chamber in the body  112  of the device. When the consumable  120  is in the receiving portion  130 , the susceptor arrangement  110  of the consumable  120  is arranged in proximity for inductive coupling and heating by the inductive element  158 . 
     The device  100  may be configured to receive a plurality of different consumables of different shapes. 
     In examples, as mentioned above, the inductive element  158  is an electrically conductive coil. In such examples, at least a part of the susceptor arrangement of a consumable may be configured to be received within the coil. This may provide efficient inductive coupling between the susceptor arrangement and the inductive element and as such provide for efficient heating of the susceptor arrangement. 
     Operation of the aerosol generating device  100  comprising resonant circuit  150 , will now be described, according to an example. Before the device  100  is turned on, the device  100  may be in an ‘off’ state, i.e. no current flows in the resonant circuit  150 . The device  150  is switched to an ‘on’ state, for example by a user turning the device  100  on. Upon switching on of the device  100  the resonant circuit  150  begins drawing current from the voltage supply  104 , with the current through the inductive element  158  varying at the resonant frequency f 0 . The device  100  may remain in the on state until a further input is received by the controller  106 , for example until the user no longer pushes the button (not shown), or the puff detector (not shown) is no longer activated, or until a maximum heating duration has elapsed. The resonant circuit  150  being driven at the resonant frequency f 0  causes an alternating current I to flow in the resonant circuit  150  and the inductive element  158 , and hence for the susceptor arrangement  110  to be inductively heated. As the susceptor arrangement  110  is inductively heated, its temperature (and hence the temperature of the aerosol generating material  116 ) increases. In this example, the susceptor arrangement  110  (and aerosol generating material  116 ) is heated such that it reaches a steady temperature T MAX . The temperature T MAX  may be a temperature which is substantially at or above a temperature at which a substantial amount of aerosol is generated by the aerosol generating material  116 . The temperature T MAX  may be between around 200 and around 300° C. for example (although of course may be a different temperature depending on the material  116 , susceptor arrangement  110 , the arrangement of the overall device  100 , and/or other requirements and/or conditions). The device  100  is therefore in a ‘heating’ state or mode, wherein the aerosol generating material  116  reaches a temperature at which aerosol is substantially being produced, or a substantial amount of aerosol is being produced. It should be appreciated that in most, if not all cases, as the temperature of the susceptor arrangement  110  changes, so too does the resonant frequency f 0  of the resonant circuit  150 . This is because magnetic permeability of the susceptor arrangement  110  is a function of temperature and, as described above, the magnetic permeability of the susceptor arrangement  110  influences the coupling between the inductive element  158  and the susceptor arrangement  110 , and hence the resonant frequency f 0  of the resonant circuit  150 . 
     The present disclosure predominantly describes an LC parallel circuit arrangement. As mentioned above, for an LC parallel circuit at resonance, the impedance is maximum and the current is minimum. Note that the current being minimum generally refers to the current observed outside of the parallel LC loop, e.g., to the left of choke  161  or to the right of choke  162 . Conversely, in a series LC circuit, current is at maximum and, generally speaking, a resistor is required to be inserted to limit the current to a safe value which can otherwise damage certain electrical components within the circuit. This generally reduces the efficiency of the circuit because energy is lost through the resistor. A parallel circuit operating at resonance does not require such restrictions. 
     In some examples, the susceptor arrangement  110  comprises or consists of aluminum. Aluminum is an example of a non-ferrous material and as such has a relative magnetic permeability close to one. What this means is that aluminum has a generally low degree of magnetization in response to an applied magnetic field. Hence, it has generally been considered difficult to inductively heat aluminum, particularly at low voltages such as those used in aerosol provision systems. It has also generally been found that driving circuitry at resonance frequency is advantageous as this provides optimum coupling between the inductive element  158  and susceptor arrangement  110 . For aluminum, it is observed that a slight deviation from the resonant frequency causes a noticeable reduction in the inductive coupling between the susceptor arrangement  110  and the inductive element  158 , and thus a noticeable reduction in the heating efficiency (in some cases to the extent where heating is no longer observed). As mentioned above, as the temperature of the susceptor arrangement  110  changes, so too does the resonant frequency of the circuit  150 . Therefore, in the case where the susceptor arrangement  110  comprises or consists of a non-ferrous susceptor, such as aluminum, the resonant circuit  150  of the present disclosure is advantageous in that the circuitry is always driven at the resonant frequency (independent of any external control mechanism). This means that maximum inductive coupling and thus maximum heating efficiency is achieved at all times enabling aluminum to be efficiently heated. It has been found that a consumable including an aluminum susceptor can be heated efficiently when the consumable includes an aluminum wrap forming a closed electrical circuit and/or having a thickness of less than 50 microns. 
     In examples where the susceptor arrangement  110  forms part of a consumable, the consumable may take the form of that described in PCT/EP2016/070178, the entirety of which is incorporated herein by reference. 
     The above examples are to be understood as illustrative examples of the disclosure. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the examples, or any combination of any other of the other examples. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.