INDUCTIVE HEATING ARRANGEMENT FOR HEATED AEROSOL-FORMING SUBSTRATES

Abstract

The present invention relates to an inductive heating arrangement for heating an aerosol-forming substrate. The heating arrangement comprises a DC power supply and an electronic power supply circuit comprising a DC / AC inverter connected to the DC power supply. The DC / AC inverter comprises a resonant switching power amplifier with at least one transistor switch, at least one transistor switch driver circuit associated with the transistor switch, and an LC load network. The LC load network comprises at least one capacitor and at least one inductor, wherein the inductor is configured to generate an alternating magnetic field during operation of the heating arrangement to inductively heat the aerosol-forming substrate.The transistor switch controller circuit comprises an adjustable oscillator configured to output a switching signal to the transistor switch having an adjustable switching frequency. The heating arrangement further comprises a current sensor for determining the DC supply current drawn from the DC power source during operation of the heating arrangement, and a controller configured to receive a current signal from the current sensor indicative of the DC supply current and to tune the switching frequency of the switching signal in response to the received current signal to maintain the DC supply current drawn from the DC power source within a predetermined range. The invention further relates to an aerosol generating device and an aerosol generating system comprising such an inductive heating arrangement.Furthermore, the invention relates to a method and apparatus for calibrating such a heating arrangement.

Description

INDUCTIVE HEATING ARRANGEMENT FOR HEATED AEROSOL-FORMING SUBSTRATES The present invention relates to an inductive heating arrangement for heating an aerosol-forming substrate. The invention further relates to an aerosol-generating device and an aerosol-generating system comprising such an inductive heating arrangement. In addition, the invention relates to a method and apparatus for calibrating such a heating arrangement. Inductive heating arrangements used to generate inhalable aerosols by inductively heating aerosol-forming substrates capable of forming inhalable aerosols are generally known from the prior art. In general, such heating arrangements may comprise an induction source for inductively heating a susceptor arrangement that is in thermal proximity or in direct physical contact with the substrate to be heated. The induction source is configured to generate an alternating magnetic field that induces at least one of the heat-generating eddy currents or hysteresis losses in the susceptor arrangement.While the induction source is typically part of an aerosol generating device, the susceptor arrangement may be part of the device or part of an aerosol generating article comprising the substrate to be heated and configured to be received in an aerosol generating device that includes the induction source. To generate the alternating magnetic field, the heating arrangement may comprise a DC / AC inverter including a resonant switching power amplifier. The amplifier comprises a switching device and a resonant LC charging network comprising a capacitor and an inductor, wherein the inductor generates the alternating magnetic field during operation. While such resonant switching inverters have proven advantageous with respect to the rapid generation of the heat required in the susceptor, the output power may be subject to variations from a nominal target output power. In principle, the variation in output power can be limited by using electronic components with tight tolerances. However, such components are expensive or place high demands on their manufacture. Therefore, it would be desirable to have an inductive heating arrangement for heating an aerosol-forming substrate, as well as an aerosol-generating device and system comprising such a heating arrangement, that combines the advantages of prior art solutions¹ while mitigating their disadvantages. In particular, it would be desirable to have an inductive heating arrangement for heating an aerosol-forming substrate, as well as an aerosol-generating device and system comprising such a heating arrangement, that allows for less stringent requirements to be placed on component tolerances, while keeping performance variations within acceptable limits. According to the present invention, an inductive heating arrangement is provided for heating an aerosol-forming substrate. The heating arrangement comprises a DC power supply and an electronic power supply circuit comprising a DC / AC inverter connected to the DC power supply. The DC / AC inverter comprises a resonant switching power amplifier with at least one transistor switch, at least one transistor switch driver circuit associated with the transistor switch, and an LC charging network. The LC charging network comprises at least one capacitor and at least one inductor, wherein the inductor is configured to generate an alternating magnetic field during operation of the heating arrangement to inductively heat the aerosol-forming substrate by means of a susceptor arrangement.The transistor switch driver circuit comprises an adjustable oscillator configured to output a switching signal to the transistor switch that has an adjustable switching frequency. According to the invention, it has been found that the variations in output power observed with prior art devices are due to a deviation of the actual resonant curve from the nominal resonant curve of the LC load network. Consequently, the switching frequency of the switching device, which until now has been selected with respect to a desired operating point on the nominal resonant curve, is effectively at a different operating point on the actual resonant curve, which, in turn, is associated with an output power different from the desired target value. In particular, it has been found that a tolerance of more than ±1 percent of the nominal inductance of the inductor and the nominal capacitance of the capacitor of the LC load network can result in a variation of more than ±17 percent of the nominal target output power of the heating arrangement.Such variations in output power across different devices are generally unacceptable. Instead of using capacitors and inductors with tighter tolerances, the present invention proposes using a transistor switch driver circuit comprising an adjustable oscillator configured to output a switching signal to the transistor switch, which has an adjustable switching frequency. This allows the switching frequency of the switching signal to be adjusted and, therefore, the effective operating point on the actual resonant curve of the LC load network to be shifted to a position associated with a desired target output power. For example, the switching frequency of the switching signal can be adjusted to a desired range around the actual resonant frequency of the LC load network.Similarly, the switching frequency of the switching signal can be adjusted to a desired offset from the actual resonant frequency of the LC load network. In other words, by adjusting the switching frequency, the resonant switching power amplifier can be tuned to the appropriate operating point on the actual resonant curve, which is associated with an acceptably small range around the desired target output power. As a result, the variation in output power can be significantly limited, while at the same time, the tolerance requirements for the capacitors and inductors can be reduced.For example, instead of using capacitors and inductors that have a tolerance of ±1 percent or less of the nominal capacitance and inductance, respectively, the adjustable oscillator allows the use of a capacitor with a capacitance tolerance of ±2 percent or more and an inductor with an inductance tolerance of even ±5 percent or more. Specifically, the tolerance of the capacitance of the capacitor in an LC charging network can range from ±2 percent to ±4 percent of the capacitor's nominal capacitance value. Similarly, the tolerance of the inductance of the inductor in an LC charging network can range from ±3 percent to ±7 percent, particularly from ±4 percent to ±6 percent, and preferably ±5 percent of the inductor's nominal inductance value. As used herein, the term tolerance refers to the permissible limit or limits of variation in a physical property (deviation from a nominal value); here, the capacitance of the capacitor and the inductance of the inductor, respectively. Using capacitors and inductors with tolerances within these ranges results in significant cost reductions, as the manufacturing of these components is less demanding. In general, the inductive heating arrangement can be configured to generate a high-frequency alternating magnetic field. As mentioned in this description, the high-frequency alternating magnetic field can be in the range between 500 kHz (kilohertz) and 30 MHz (megahertz), particularly between 5 MHz (megahertz) and 15 MHz (megahertz), preferably between 5 MHz (megahertz) and 10 MHz (megahertz). Preferably, the switching frequency is adjustable within a range of ±20 percent, particularly ±10 percent, and preferably ±5 percent around a center frequency. In general, the center frequency can be any frequency within the frequency ranges of the alternating magnetic field defined above. Preferably, the center frequency is between 6.5 MHz and 7.1 MHz, particularly between 6.7 MHz and 6.9 MHz, for example, at 6.78 MHz. Similarly, the switching frequency can be set within a range of 5.4 MHz (Mega-Hertz) to 8 MHz (Mega-Hertz), in particular between 6.0 MHz (Mega-Hertz) and 7.5 MHz (Mega-Hertz), preferably between 6.4 MHz (Mega-Hertz) and 7.2 MHz (Mega-Hertz).On the one hand, these tuning ranges can be large enough to adjust the switching power amplifier to its desired operating point and thus limit the variation in output power. On the other hand, these tuning ranges are small enough to keep the costs of the adjustable oscillator low, while still allowing the switching frequency to be precisely tuned and held stable once set at a specific value. Preferably, the adjustable oscillator is a MEMS-based (microelectromechanical system) adjustable oscillator, a voltage-controlled oscillator (VCO), or a Colpitts oscillator. As used in this description, the term switching power amplifier refers to an electronic amplifier comprising one or more amplifier transistors operated as electronic switches, and not as linear gain devices as in other amplifiers. The switching power amplifier may be a Class C power amplifier, a Class D power amplifier, or a Class E power amplifier. A class D amplifier is an electronic amplifier comprising two transistor switches and two transistor switch driver circuits to turn the two transistor switches on and off. The transistor switches are controlled at high frequency to ensure that one of the two transistor switches is off at the moment the other one is turned on. In contrast, a class E power amplifier includes only one transistor switch. Furthermore, class E power amplifiers are known for their minimal power dissipation in the switching transistor during switching transitions. The circuit achieves high efficiency by using only the transistor switch at zero current (on-to-off switching) or zero voltage (off-to-on switching) points, thus minimizing power losses in the switch. Class E power amplifiers are widely known and described in detail, for example, in the article "Class-E RF Power Amplifiers" by Nathan O. Sokal, published in the bimonthly magazine QEX, January / February 2001 issue, page 920, by the American Radio Relay League (ARRL), Newington, CT, USA.In general, a class E power amplifier may include a transistor switch and an LC load network configured to operate at a low ohmic load, wherein the LC load network comprises a series connection of a capacitor and an inductor. Specifically, the LC load network may comprise a series connection of the capacitor and the inductor. Furthermore, the LC load network includes a shunt capacitor. Preferably, the class E power amplifier is a single-ended, first-order class E power amplifier having a single transistor switch. In at least one transistor switch of the power amplifier, the switching transistor can be any type of transistor and can be implemented as a bipolar junction transistor (BJT). More preferably, however, the transistor switch is represented as a field-effect transistor (FET) such as a metal-oxide-semiconductor field-effect transistor (MOSFET) or a metal-semiconductor field-effect transistor (MESFET). Due to the small number of components, the volume of the power supply electronic circuit can be kept extremely small. This extremely small volume of the power supply electronic circuits is possible because the LC charging network inductor is used directly as the inductor for inductive coupling to the susceptor arrangement. The small volume also allows the overall dimensions of the entire inductive heating arrangement to remain small. The DC power source may generally comprise any suitable DC power source configured to provide a DC supply voltage and DC supply current to the power supply electronic circuitry. Preferably, the DC power source comprises one or more batteries, such as a lithium iron phosphate battery. The DC power source may be rechargeable. In particular, the DC power source may comprise one or more rechargeable batteries. The DC power source may have a capacity that allows for the storage of sufficient energy for one or more user experiences. For example, the DC power source may have sufficient capacity to allow for the continuous generation of aerosol for a period of approximately six minutes or a period that is a multiple of six minutes.In another example, the DC power supply may have sufficient capacity to allow a predetermined number of discrete puffs or activations of the inductive heating arrangement. The DC supply voltage of the power supply may be in the range of approximately 2.5 volts to approximately 4.5 volts, and the DC supply current may be in the range of approximately 2.5 amps to approximately 5 amps (corresponding to a DC power supply in the range of approximately 6.25 watts to approximately 22.5 watts). The DC power supply may include a DC power choke. As used in this description, the term DC power choke refers to an inductor used to block higher frequencies while passing direct current (DC) and lower frequencies of alternating current (AC) in an electrical circuit. In general, the output power of an inductive heating device is a function of both the DC supply current and the DC supply voltage drawn from the DC power source. Assuming the DC supply voltage remains essentially constant for a certain period, the DC supply current drawn from the DC power source during operation of the heating device is essentially proportional to, and therefore indicative of, the output power of the heating device. Consequently, to determine a signal indicative of the output power of the heating device, the inductive heating device may include a current sensor to measure the DC supply current drawn from the DC power source during operation.Preferably, the current sensor comprises a sensing resistor and a current shunt amplifier. The current sensor, in particular the current shunt amplifier, can be configured to output a current signal indicative of the DC supply current drawn from the DC power source during operation of the heating arrangement. To account for a possible decrease in the DC supply voltage over time, the inductive heating arrangement may also include a voltage sensor to determine the DC supply voltage drawn from the DC power source. For this purpose, the voltage sensor may include a voltage divider. In particular, the voltage sensor may be configured to output a voltage signal indicative of the DC supply voltage drawn from the DC power source during operation of the heating arrangement. Therefore, the output power of the inductive heating arrangement can be determined as a function of both the DC supply current and the DC supply voltage drawn from the DC power source. Alternatively, as will be described in more detail below, the current sensor and the optional voltage sensor, or at least parts of these, such as the current shunt amplifier, the sensing resistor, or the voltage divider, may be part of a calibration apparatus for adjusting the output power of the heating arrangement to the adjustable oscillator (microelectromechanical system), a voltage-controlled oscillator (VCO), or a Colpitts oscillator operating point. Regardless of whether the current sensor and the optional voltage sensor are part of the heating arrangement or a calibration apparatus, the configuration described above allows the output power of the heating arrangement to be adjusted to a desired working point by measuring at least the DC supply current drawn from the DC power source, and possibly in addition to the DC supply voltage, while adjusting the switching frequency of the switching signal provided by the adjustable oscillator to the transistor switch until the measured DC supply current is within a predetermined reference range.The predetermined reference range of the measured DC supply current or of the current signal indicative of this can be determined in advance (e.g., by calibration) in order to indicate a desired heating performance of the heating arrangement, in particular a desired output energy of the heating arrangement. To adjust the output power of the heating arrangement in this manner, the inductive heating arrangement may comprise a controller operatively coupled at least to the current sensor and the transistor switch driver circuit. In particular, the inductive heating arrangement may comprise a controller operatively coupled at least to the current sensor and the transistor switch driver circuit in a feedback loop configuration. The controller can be configured to receive a current signal from the current sensor indicating the DC supply current and to adjust the switching frequency of the switching signal in response to, in particular based on, the received current signal to adjust the DC supply current drawn from the DC power source to be within a predetermined reference range.As used in the present description, the term "adjust the switching frequency of the switching signal in response to the received current signal" may mean that the controller is configured to set the adjustable oscillator to emit a switching signal that has a determined operating switching frequency in response to, in particular as a function of, the received current signal, for which (i.e., for which the operating switching frequency) the current signal received from the current sensor is within a predetermined reference range of the current signal.In other words, the controller can be configured to set the switching frequency of the switching signal to correspond to a determined operating switching frequency in response to, in particular as a function of, the received current signal, for which (i.e., for which operating switching frequency) the current signal received from the current sensor is within a predetermined reference range of the current signal. That is, the present invention may relate to an inductive heating arrangement for heating an aerosol-forming substrate, wherein the heating arrangement comprises a DC power source and an electronic power supply circuit comprising a DC / AC inverter connected to the DC power source, wherein the DC / AC inverter comprises a resonant switching power amplifier with at least one transistor switch, at least one transistor switch driver circuit associated with the transistor switch, and an LC charging network comprising at least one capacitor and at least one inductor, wherein the inductor is configured to generate an alternating magnetic field during operation of the heating arrangement for inductively heating the aerosol-forming substrate.wherein the transistor switch driver circuit comprises an adjustable oscillator configured to output a switching signal to the transistor switch having an adjustable switching frequency, wherein the heating arrangement further comprises a current sensor for determining the DC supply current drawn from the DC power source during operation of the heating arrangement, and a controller operatively coupled to the current sensor and the transistor switch driver circuit, wherein the controller is configured to receive a current signal from the current sensor indicating the DC supply current and to configure the adjustable oscillator to output a switching signal having an operating switching frequency determined in response to, in particular as a function of, the received current signal, for which (i.e.,for which the operating switching frequency) the current signal received from the current sensor is within a predetermined reference range of the current signal. In particular, the controller can be configured to compare the current signal received from the current sensor with a predetermined reference range of the current signal. The operating switching frequency of the switching signal at which the DC supply current drawn from the DC power source during operation is within the predetermined reference range, and in particular at which the current signal received from the current sensor is within the predetermined reference range, can be determined by the heating system controller or by a calibration device for adjusting the heating system's output power, which is not part of the heating system itself. For example, such a calibration device can be used to adjust the heating system's output power during its manufacturing process. In the latter case, the heating system controller can be operationally coupled to a calibration device. The calibration device can be configured to determine, in response to, and in particular based on, the current signal, an operating switching frequency for which the DC supply current drawn from the DC power source is within a predetermined range. For this purpose, the calibration device can include a calibration controller. The heating system controller can be configured to transmit the current signal to the calibration device, in particular to its calibration controller, and to receive a signal from the calibration device, in particular from its calibration controller, indicating the determined operating switching frequency.The heating arrangement controller can use this signal, which is determined in response to, in particular as a function of, the current signal, to adjust the switching frequency of the switching signal to the determined operating switching frequency to adjust the DC supply current drawn from the DC power source to be within a predetermined reference range.Although in this configuration the operating switching frequency of the switching signal for which the DC supply current drawn from the DC power source is within the predetermined range is determined by the calibration apparatus, this configuration remains within the scope of the design. The controller is configured to receive a current signal from the current sensor indicating the DC supply current and to adjust the switching frequency of the switching signal in response to, and in particular based on, the received current signal to adjust the DC supply current drawn from the DC power source to a predetermined reference range. Further details of the calibration apparatus are described below. Furthermore, the heating arrangement controller can be configured (by yourself) to determine (in response to, in particular based on, the current signal) an operating switching frequency of the switching signal for which the DC supply current drawn from the DC power source during operation is in the predetermined reference range, in particular for which the current signal received from the current sensor is in the predetermined reference range of the current signal. The controller may comprise a microprocessor, for example, a programmable microprocessor, a microcontroller, or an application-specific integrated circuit (ASIC), or other electronic circuit capable of providing control. Preferably, the adjustment of the heating element's output energy is performed during the heating element's manufacture, i.e., as a type of calibration process. Preferably, this is done using a reference susceptor arrangement, particularly in cases where the heating element does not comprise a susceptor arrangement itself, but where the susceptor arrangement is part of an aerosol-generating article. Once the operating switching frequency is determined during the calibration process, this frequency can be programmed into the adjustable oscillator so that it outputs a fixed switching signal at that frequency. In other words, once calibrated, the adjustable oscillator's operating switching frequency will remain constant for the duration of the calibration. The controller can configure the adjustable oscillator, as mentioned previously, to output a switching signal at the specified operating switching frequency.In other words, the controller can be configured to set the switching frequency of the switching signal to correspond to the predetermined operating switching frequency for which the DC supply current drawn from the DC power source is within the predetermined reference range. The inductor or heating device used to generate the alternating magnetic field may comprise at least one induction coil, i.e., a single induction coil or a plurality of induction coils. The number of induction coils may depend on the number of elements, the size, and the shape of the susceptor arrangement. The induction coil or coils may be shaped to fit the shape of a housing for an aerosol-generating device of which the heating arrangement may be a part. For example, at least one induction coil can be a helical coil or a flat coil, specifically a flat coil or a curved flat coil. Using a flat spiral coil allows for a compact design that is robust and inexpensive to manufacture. Using a helical induction coil advantageously allows for the generation of a homogeneous alternating magnetic field. As used in this description, a flat spiral coil means a coil that is generally flat, where the winding axis of the coil is normal to the surface on which the coil lies. The flat spiral induction coil can have any convenient shape within the plane of the coil. For example, the flat spiral coil can be circular, or it can have a generally oblong or rectangular shape.However, the phrase "flat spiral coil," as used herein, covers both flat coils and flat spiral coils formed to conform to a curved surface. For example, the induction coil may be a curved flat coil arranged on the circumference of a preferably cylindrical coil support, such as a ferrite core. Furthermore, the flat spiral coil may comprise, for example, two layers of a four-turn flat spiral coil or a single layer of a four-turn flat spiral coil. One or more induction coils may be housed within a heating arrangement, or a main body or housing of an aerosol generating device comprising the heating arrangement. The one or more induction coils may be wound around a preferably cylindrical coil support, such as a ferrite core. As described above, the susceptor arrangement to be exposed to the alternating magnetic field generated by the inductor of the heating arrangement may be part of an aerosol generating device, and the heating arrangement may also be part of that device. Alternatively, the susceptor may be part of an aerosol generating article for use with a device comprising the aerosol-forming substrate to be heated. It is also possible that a portion of the susceptor arrangement may be part of an aerosol generating article, while another portion of the susceptor arrangement may be part of an aerosol generating device. In particular, in the first case, the heating arrangement according to the present invention may comprise a susceptor arrangement.As part of the heating arrangement, the susceptor arrangement can be arranged within the alternating magnetic field generated by the inductor during operation of the heating arrangement. As used in this description, the term susceptor arrangement refers to a component comprising at least one susceptor material capable of converting electromagnetic energy into heat when subjected to an alternating magnetic field. This conversion may result from at least one of the hysteresis losses or eddy currents induced in the susceptor material, depending on the material's electrical and magnetic properties. Hysteresis losses occur in ferromagnetic or ferrimagnetic susceptor materials because the magnetic domains within the material change under the influence of the alternating magnetic field. Eddy currents are induced in electrically conductive susceptor materials. In the case of an electrically conductive ferromagnetic or ferrimagnetic susceptor material, heat is generated due to both eddy currents and hysteresis losses. In general, the susceptor assembly may comprise or be formed from any material that can be induction-heated to a temperature sufficient to generate an aerosol from an aerosol-forming substrate. The susceptor assembly may comprise a metal or carbon. The susceptor assembly may comprise a ferromagnetic material, for example, ferritic iron, ferromagnetic steel, or stainless steel. A preferred susceptor may be formed from 400 series stainless steels, for example, grade 410, grade 420, or grade 430 stainless steel. Another suitable susceptor may comprise aluminum. The susceptor arrangement may comprise a variety of geometric configurations. The at least one susceptor may comprise or be one of a susceptor rod, susceptor bar, susceptor sheet, susceptor strip, or susceptor plate. When the susceptor arrangement is part of the aerosol-generating device, the susceptor rod, susceptor bar, susceptor sheet, susceptor strip, or susceptor plate may project into a receiving cavity of the device, preferably into an insertion opening of the cavity for inserting an aerosol-generating article into the cavity. The susceptor assembly may comprise, or be, a filament susceptor, a mesh susceptor, or a wick susceptor. Likewise, the susceptor assembly may comprise, or be, a susceptor sleeve, a susceptor cup, a cylindrical susceptor, or a tubular susceptor. Preferably, the internal void of the susceptor sleeve, susceptor cup, cylindrical susceptor, or tubular susceptor is configured to detachably receive at least a portion of the aerosol-forming substrate to be heated. The aforementioned susceptor assembly may have any cross-sectional shape, for example, circular, oval, square, rectangular, triangular, or any other suitable shape. The susceptor arrangement may comprise a single susceptor material. Alternatively, the susceptor arrangement may comprise at least a first susceptor material and a second susceptor material. While the first susceptor material may be optimized with respect to heat loss and thus heating efficiency, the second susceptor material may be used as a temperature marker. For this purpose, the second susceptor material preferably comprises a ferrimagnetic or ferromagnetic material. In particular, the second susceptor material may be chosen to have a Curie temperature corresponding to a predefined heating temperature. At its Curie temperature, the magnetic properties of the second susceptor material change from ferromagnetic or ferrimagnetic to paramagnetic, which is accompanied by a temporary change in its electrical resistance.Therefore, by monitoring a corresponding change in the electric current drawn from the DC power source, it is possible to detect when the second susceptor material has reached its Curie temperature and thus when the predefined heating temperature has been reached. The present invention further relates to an aerosol generating device used to generate an aerosol by inductively heating an aerosol-forming substrate. The aerosol generating device comprises an inductive heating arrangement according to the present invention and as described herein. As used herein, the term aerosol generating device refers to an electrically operated device capable of interacting with at least one aerosol-forming substrate, particularly an aerosol-forming substrate provided within an aerosol-generating article, such as to generate an aerosol by heating the substrate via induction heating. Preferably, the aerosol generating device is a mouth-capture device for generating an aerosol that is directly inhalable by a user through the user's mouth. In particular, the aerosol generating device is a portable aerosol generating device. As used herein, the term “aerosol-forming substrate” refers to a substrate capable of releasing volatile compounds that can form an aerosol upon heating. Preferably, the aerosol-forming substrate is intended to be heated rather than burned to release volatile compounds that can form an aerosol. The aerosol-forming substrate may be part of the aerosol-generating article. The aerosol-forming substrate may be a solid aerosol-forming substrate, a gel-like aerosol-forming substrate, a liquid aerosol-forming substrate, or any combination thereof. The aerosol-forming substrate may comprise at least one of the solid and liquid components. The aerosol-forming substrate may comprise a tobacco-containing material containing volatile tobacco flavor compounds, which are released from the substrate upon heating.In particular, the aerosol-forming substrate may be a tobacco-containing aerosol-forming substrate. Alternatively or additionally, the aerosol-forming substrate may comprise a non-tobacco material. The aerosol-forming substrate may further comprise an aerosol former. Examples of suitable aerosol formers are glycerin and propylene glycol. The aerosol-forming substrate may further comprise other additives and ingredients, such as nicotine or flavorings. The aerosol-forming substrate may also be a paste-like material, a pouch of porous material comprising an aerosol-forming substrate, or, for example, loose tobacco mixed with a gelling or adhesive agent, which could include a common aerosol former such as glycerin, and which is compressed or molded into a cap. The aerosol generating device may comprise a receiving cavity for detachably receiving the aerosol-forming substrate to be heated, in particular for detachably receiving at least a portion of an aerosol-generating article comprising the aerosol-forming substrate to be heated. The receiving cavity may be embedded in a housing of the aerosol generating device. The aerosol generating device may include a general controller for controlling the operation of the device. The general controller may include, or be the controller of, the heating arrangement mentioned above. The controller may include a microprocessor, for example, a programmable microprocessor, a microcontroller, an application-specific integrated circuit (ASIC), or other electronic circuitry capable of providing control. The aerosol generating device may also include a power source, in particular a DC power source, such as a battery. Specifically, the aerosol generating device may include a single power source. Preferably, the device's DC power source may include, or be, the DC power source of the heating arrangement mentioned above. The aerosol generating device may comprise a main body that preferably includes the DC power source and at least parts of the electronic power supply circuitry of the heating arrangement, in particular the at least one transistor switch, the at least one transistor switch driver circuit, and the LC charging network capacitor, and if present, the shunt capacitor and the DC power choke. The LC charging network inductor may also be part of the main body. In addition to the main body, the aerosol generating device may further comprise a nozzle, particularly if the aerosol generating article used with the device does not include a nozzle. The nozzle can be mounted to the main body of the device. The nozzle can be configured to close the receiving cavity when mounted on the main body. To attach the nozzle to the main body, a proximal end portion of the main body may include a magnetic or mechanical mounting, such as a bayonet or snap-fit ​​mounting, which engages with a corresponding counterpart on a distal end portion of the nozzle. If the device does not include a nozzle, an aerosol generating article for use with the aerosol generating device may include a nozzle, such as a filter plug. The aerosol-generating device may comprise at least one air inlet, in particular an air inlet that allows air to enter the receiving cavity. For example, the air inlet may be formed by an insertion opening in the receiving cavity used to insert the aerosol-forming substrate or the aerosol-generating article into the cavity. The aerosol-generating device may also comprise at least one air outlet, for example, an air outlet at the nozzle (if present). Preferably, the aerosol-generating device comprises an air path extending from the at least one air inlet through the receiving cavity, and possibly beyond an air outlet at the nozzle, if present. Preferably, the aerosol-generating device comprises at least one air inlet in continuous communication with the receiving cavity.For example, the aerosol generating system 15 may comprise an air path extending from an insertion opening of the receiving cavity along an internal surface of the receiving cavity and possibly beyond through the aerosol forming substrate within the article and a nozzle to the user's mouth. As described above, the aerosol generating device may further comprise a susceptor arrangement for exposure to the alternating magnetic field generated by the inductor of the heating arrangement. As such, the susceptor arrangement may be, in particular, a part of the heating arrangement. Details of the susceptor have been described above. Other features and advantages of the aerosol generating device according to the present invention have been described above with respect to the heating arrangement according to the present invention and therefore apply equally. In accordance with the invention, an aerosol generating system is also provided. The system comprises an aerosol generating device according to the present invention and as described herein, as well as an aerosol generating article for use with the device. As used herein, the term aerosol-generating article refers to an article comprising at least one aerosol-forming substrate capable of releasing volatile compounds upon heating that can form an aerosol. That is, the article comprises at least one aerosol-forming substrate that is heated by the device. Preferably, the aerosol-generating article comprises at least one aerosol-forming substrate that is intended to be heated, rather than burned, to release volatile compounds that can form an aerosol. The aerosol-generating article may be a consumable, particularly a consumable that is discarded after a single use. The aerosol-generating article may be a tobacco product. For example, the article may be a cartridge containing a liquid or solid aerosol-forming substrate to be heated.Alternatively, the article may be a bar-shaped article, particularly a tobacco article, resembling conventional cigarettes and including a solid aerosol-forming substrate. Preferably, the aerosol-generating article has a circular or elliptical cross-section, or an oval, square, rectangular, triangular, or polygonal one. As described above, the susceptor arrangement to be exposed to the alternating magnetic field generated by the heating arrangement's inductor may be part of the aerosol generating device or part of the aerosol generating article of the aerosol generating article. Accordingly, the aerosol generating system article may comprise a susceptor arrangement in thermal contact or thermal proximity to the aerosol-forming substrate, wherein the susceptor arrangement is disposed within the alternating magnetic field generated by the inductor during the operation of the heating arrangement, when the article is received into the receiving cavity of the device. The article may comprise one or more of the following elements: a first support element, a substrate element, a second support element, a cooling element, and a nozzle element. Preferably, the aerosol-generating article comprises at least a first support element, a second support element, and a substrate element located between the first and second support elements. All the aforementioned elements may be arranged sequentially along a longitudinal axis of the article in the order described above, wherein the first support element is preferably located at a distal end of the article and the filter element is preferably located at a proximal end of the article. Each of the aforementioned elements may be essentially cylindrical. In particular, all elements may have the same external cross-sectional shape.Furthermore, the elements may be enclosed by an outer wrapping that holds them together and maintains the convenient cross-sectional shape of the bar-shaped article. Preferably, the wrapping is made of paper. The substrate element may comprise at least one aerosol-forming substrate and, if present, the susceptor arrangement. Preferably, the susceptor arrangement is in thermal contact with or near the aerosol-forming substrate. As used herein, the term nozzle element refers to a portion of the article through which the aerosol exits the aerosol-generating article. In particular, the nozzle element can be placed in a user's mouth to directly inhale an aerosol from the article. Preferably, the nozzle element includes a filter. At least one of the first support element and the second support element may comprise a central air passage. Preferably, at least one of the first support element and the second support element may comprise a hollow cellulose acetate tube. Alternatively, the first support element may be used to cover and protect the distal front end of the substrate element. The aerosol cooling element is an element with a large surface area and low suction resistance, for example, 15 mmWG to 20 mmWG. During use, an aerosol formed by volatile compounds released from the substrate element is drawn through the aerosol cooling element before being transported to the proximal end of the aerosol-generating article. Additional features and advantages of the aerosol generating system according to the present invention have been described with respect to the aerosol generating device and heating arrangement according to the present invention and therefore apply equally. The present invention further relates to a method for calibrating an inductive heating arrangement in accordance with the present invention and as described herein. The method comprises: operationally couple the heating arrangement to a reference susceptor arrangement; - operate the heating arrangement to heat the reference susceptor arrangement; - determine the actual DC supply current drawn from the DC power source during operation of the heating arrangement; - determine an operating switching frequency of the switching signal for which the actual DC supply current drawn from the DC power source is within the predetermined range; - Configure the adjustable oscillator to emit a switching signal that has the specified operating switching frequency. In particular, the present invention may further relate to a method for calibrating an inductive heating arrangement according to the present invention and as described herein, wherein the method comprises: - operationally couple the heating arrangement to a reference susceptor arrangement; - operate the heating arrangement to heat the reference susceptor arrangement; - determine the actual DC supply current drawn from the DC power source during operation of the heating arrangement; - Tune the switching frequency of the switching signal while determining the actual DC supply current drawn from the DC power source until the actual DC supply current is within a predetermined range; - determine an operating switching frequency of the switching signal for which the actual DC supply current drawn from the DC power source is within the predetermined range; - Configure the adjustable oscillator to emit a switching signal that has the specified operating switching frequency. As described above with respect to the heating arrangement according to the present invention, it has been found that the output power of the heating arrangement can be calibrated to a desired operating point by measuring at least the DC supply current drawn from the DC power source, by determining in response to, in particular based on the DC supply current or a signal indicative thereof, an operating switching frequency of the switching signal for which the DC supply current drawn from the DC power source during operation is within a predetermined reference range, in particular for which a current signal received from the current sensor is within a predetermined reference range of the current signal,and subsequently by configuring the adjustable oscillator to output a switching signal having a switching frequency at which the actual DC supply current drawn from the DC power source is within a predetermined range. In particular, the operating switching frequency of the switching signal can be determined by calculating a frequency shift using a previously known function between the DC current and the switching frequency (current shift as a function of frequency shift) that will be applied to the switching frequency at which the DC supply current associated with the received current signal was measured. This can be achieved either by the heating arrangement controller or by a calibration device operatively coupled to the heating arrangement controller. Then,The DC current can be measured again at the switched switching frequency to verify that, in operation, the DC supply current drawn from the DC power source is within the predetermined range for the switched switching frequency. Similarly, the output power of the heating arrangement can be calibrated to a desired operating point by measuring at least the DC supply current drawn from the DC power source 19, while adjusting the switching frequency of the switching signal until the measured DC supply current falls within a predetermined reference range, and subsequently by setting the adjustable oscillator to output a switching signal with a switching frequency at which the actual DC supply current drawn from the DC power source falls within a predetermined range. Advantageously, this calibration allows for less stringent component tolerance requirements while keeping output power performance variations within acceptable limits. The actual DC supply current can be continuously determined while the switching frequency is being adjusted. Alternatively, the switching frequency can be adjusted in stages, particularly through a plurality of frequency values, where the respective actual DC supply current is determined for each adjustment stage, specifically for each frequency value. In particular, the stage of adjusting the switching frequency to a new frequency value and the stage of determining the respective actual DC supply current can be alternated. Similarly, the actual DC supply current can be measured at a single, pre-specified switching frequency, for example, at a predefined switching frequency as specified by the adjustable oscillator supplier. This measurement can be repeated for verification or to determine an average value. The new switching frequency at which the output energy is within the desired range can then be determined by calculating a frequency shift using a previously known function between the DC current and the switching frequency (current shift as a function of frequency shift). This shift can then be applied to the previously specified switching frequency at which the single DC supply current was measured, associated with the received current signal.This process does not involve a feedback loop or a comparison of the DC current signal received from the current sensor with a predetermined reference range of the current signal. Preferably, this type of calibration is performed during the manufacturing of the heating arrangement. Additional features and advantages of the calibration method according to the present invention have been described with respect to the heating arrangement according to the present invention and therefore apply equally. The present invention further relates to a calibration apparatus for calibrating the output energy of an inductive heating arrangement according to the present invention and as described herein, particularly for use in a method for calibrating an inductive heating arrangement according to the present invention and as described herein. The calibration apparatus may comprise a reference susceptor arrangement and a current sensor for determining the DC supply current drawn from the DC power source during operation of the heating arrangement. The reference susceptor arrangement is preferably of the same type as the susceptor arrangement that will be used later in the operation of the heating arrangement to heat the aerosol-forming substrate. Furthermore, the reference susceptor arrangement may be positioned in the same location relative to the heating arrangement's inductor as the actual susceptor arrangement that will be used later to heat the aerosol-forming substrate. Preferably, the current sensor comprises a sensing resistor and a current shunt amplifier. The current sensor, in particular the current shunt amplifier, can be configured to output a current signal indicative of the DC supply current drawn from the DC power source during operation of the heating arrangement. To account for a possible decrease in the DC supply voltage over time, the calibration apparatus may further include a voltage sensor to determine the DC supply voltage drawn from the DC power source. For this purpose, the voltage sensor may include a voltage divider. The voltage sensor is preferably configured to output a voltage signal indicative of the DC supply voltage drawn from the DC power source during operation of the heating arrangement. Therefore, the output power of the inductive heating arrangement can be determined as a function of both the DC supply current and the DC supply voltage drawn from the DC power source. The current sensor and, if present, the voltage sensor can be operatively coupled to the heating arrangement controller to provide the controller with respective signals indicating the DC supply current and DC supply voltage. This, in turn, allows the heating arrangement controller to determine whether the measured DC supply current is within a predetermined reference range while adjusting the switching frequency of the switching signal. Alternatively, the calibration apparatus may comprise a calibration controller itself that is operatively coupled to the calibration apparatus's current sensor and operatively coupled to the heating arrangement's transistor switch driver circuit in a feedback-loop configuration. Like the heating arrangement's controller, the calibration controller may be configured to receive a current signal from the current sensor indicating the DC supply current and to adjust the switching frequency of the switching signal in response to the received current signal to adjust the DC supply current drawn from the DC power source to a predetermined interval. In addition, the calibration controller can be configured to determine an operating switching frequency for the switching signal at which the DC supply current drawn from the DC power source is within a predetermined range. The calibration controller can also be configured to set the adjustable oscillator to output a switching signal with the predetermined operating switching frequency. The present invention further relates to a calibration apparatus for calibrating the output energy of an inductive heating arrangement according to the present invention and as described herein, particularly for use in a method for calibrating an inductive heating arrangement according to the present invention and as described herein, wherein the calibration apparatus comprises a reference susceptor arrangement inductively coupled to the inductor of the heating arrangement and a calibration controller operatively coupled to the controller of the heating arrangement.The calibration controller can be configured to receive the current signal communicated from the heating arrangement controller, to determine in response to, in particular as a function of, the current signal, the operating switching frequency of the switching signal for which the DC supply current drawn from the DC power source is in the predetermined range, and to communicate a signal indicating the determined operating switching frequency to the heating arrangement controller. As mentioned earlier regarding the calibration method, the actual DC supply current communicated to the calibration controller can be a single (average) value measured at a single (fixed) prespecified switching frequency, e.g., at a predefined switching frequency as specified by the adjustable oscillator supplier, which is then used to calculate a frequency shift that is applied to the prespecified switching frequency at which the (singular) DC supply current was measured associated with the received current signal to determine the new switching frequency for which the output power is in the desired range.Consequently, the calibration controller can be configured to determine the operating switching frequency of the switching signal by calculating a frequency shift using a predetermined function between the DC current and the switching frequency (current shift as a function of frequency shift). This shift is applied to the switching frequency at which the DC supply current associated with the received current signal was measured. The DC current can then be measured again at the switched switching frequency to verify that, during operation, the DC supply current drawn from the DC power source is within the predetermined range for the switched switching frequency. Additional features and advantages of the calibration apparatus according to the present invention have been described with respect to the heating arrangement and the calibration method according to the present invention and therefore apply equally. The invention is defined in the claims. However, a non-exhaustive list of non-limiting examples is provided below. Any one or more of the features of these examples may be combined with any one or more features of another example, embodiment, or aspect described herein. Example Ex 1: An inductive heating arrangement for heating an aerosol-forming substrate, wherein the heating arrangement comprises a DC power source and an electronic power supply circuit comprising a DC / AC inverter connected to the DC power source, wherein the DC / AC inverter comprises a resonant switching power amplifier with at least one transistor switch, at least one transistor switch driver circuit associated with the transistor switch, and an LC charging network comprising at least one capacitor and at least one inductor, wherein the inductor is configured to generate an alternating magnetic field during operation of the heating arrangement for inductively heating the aerosol-forming substrate.wherein circuit 23 of the transistor switch driver comprises an adjustable oscillator configured to output a switching signal to the transistor switch having an adjustable switching frequency. Example Ej2: The inductive heating arrangement according to example Ejl, wherein the switching frequency is adjustable within a range of ±20 percent, in particular ±10 percent, preferably ±5 percent around a center frequency. Example Ej3: The inductive heating arrangement in accordance with example Ej2, wherein the center frequency is in a range between 6.5 MHz and 7.1 MHz, in particular between 6.7 MHz and 6.9 MHz. Example Ej4: The inductive heating arrangement in accordance with any of the above examples, wherein the switching frequency is adjustable within a range between 5.4 MHz and 8 MHz, in particular between 6.0 MHz and 7.5 MHz, preferably between 6.4 MHz and 7.2 MHz. Example Ej5: The inductive heating arrangement in accordance with any of the above examples, wherein the adjustable oscillator is an adjustable oscillator (microelectromechanical system), a voltage-controlled oscillator (VCO), or a Colpitts oscillator. Example Ej6: The inductive heating arrangement in accordance with any of the above examples, wherein a tolerance of a capacitor capacitance is in a range between ±2 percent and ±4 percent of a nominal capacitor capacitance value. Example Ej7: The inductive heating arrangement in accordance with any of the above examples, wherein a tolerance of an inductor inductance is in a range between ± 3 percent and ± 7 percent, in particular between ± 4 percent and ± 6 percent, preferably ± 5 percent of a nominal inductance value of the inductor. Example Ej8: The inductive heating arrangement in accordance with any of the above examples, wherein the switching power amplifier is one of a class C power amplifier, a class D power amplifier, and a class E power amplifier. Example Ej9: The inductive heating arrangement in accordance with any of the above examples, wherein the LC charging network comprises a series connection of the capacitor and the inductor. Example EjlO: The inductive heating arrangement in accordance with any of the above examples, wherein the LC charging network comprises a shunt capacitor. Example Ejl 1: The inductive heating arrangement in accordance with any of the above examples, further comprising a current sensor for determining the DC supply current drawn from the DC power source during operation of the heating arrangement. Example Ejl2: The inductive heating arrangement in accordance with any of the above examples, wherein the current sensor comprises a sensing resistor and a current shunt amplifier. Example Ejl3: The inductive heating arrangement according to either Examples Ejll or Ejl2, further comprising a controller operatively coupled to the current sensor and the transistor switch controller circuit in a feedback loop configuration, wherein the controller is configured to receive a current signal from the current sensor indicative of the DC supply current and to tune the switching frequency of the switching signal in response to the received current signal to tune the DC supply current drawn from the DC power source to be within a predetermined range. Example Ej 14: The inductive heating arrangement in accordance with example Ejl3, wherein the controller is configured to determine an operating switching frequency of the switching signal for which, in operation, the DC supply current drawn from the DC power source is in the predetermined range. Example Ejl5: The inductive heating arrangement in accordance with example Ejl4, wherein the controller is configured to set the adjustable oscillator to emit a switching signal having the determined operating switching frequency. Example Ejl6: The inductive heating arrangement in accordance with any of the above examples, further comprising a susceptor arrangement, wherein the susceptor arrangement is disposed within the alternating magnetic field generated by the inductor during operation of the heating arrangement. Example Ex 17: The inductive heating arrangement in accordance with any of the above examples, further comprising a voltage sensor for determining the DC supply voltage drawn from the DC power source. Example Ejl8: The inductive heating arrangement according to example Ejl7, wherein the voltage sensor comprises a voltage divider. Example Ex 19: An aerosol generating device used to generate an aerosol by inductively heating an aerosol-forming substrate, wherein the device comprises an inductive heating arrangement in accordance with any of the above examples. Example Ej20: The aerosol generating device according to example Ejl9, wherein the device comprises a receiving cavity for detachably receiving the aerosol forming substrate to be heated, in particular for detachably receiving an aerosol generating article comprising the aerosol forming substrate to be heated. Example Ej21: An aerosol generating system comprising an aerosol generating device in accordance with either Example Ej 19 or Example Ej20, and an aerosol generating article for use with the device, wherein the article comprises an aerosol forming substrate to be heated by the device. Example Ej22: The aerosol generating system according to example Ej21, wherein the article comprises a susceptor arrangement in thermal contact or thermal proximity to the aerosol forming substrate, wherein the susceptor arrangement is disposed within the alternating magnetic field generated by the inductor during the operation of the heating arrangement, when the article is received in the receiving cavity of the device. Example Ej23: A method for calibrating an inductive heating arrangement in accordance with any one of Examples Ej 1 to Ej 18, the method comprising: - operationally couple the heating arrangement to a reference susceptor arrangement; - operate the heating arrangement to heat the reference susceptor arrangement; - determine the actual DC supply current drawn from the DC power source during operation of the heating arrangement; - Adjust the switching frequency of the switching signal while determining the actual DC supply current drawn from the DC power source until the actual DC supply current is within a predetermined range; - determine an operating switching frequency of the switching signal for which the actual DC supply current drawn from the DC power source is within the predetermined range; - Configure the adjustable oscillator to emit a switching signal that has the specified operating frequency switching frequency. Example Ej24: A calibration apparatus for calibrating an inductive heating arrangement in accordance with any of Examples Ej1 to Ej18, in particular for use in a method in accordance with Example 23, wherein the calibration apparatus comprises a reference susceptor arrangement inductively coupled to the inductor of the heating arrangement and a current sensor for determining the DC supply current drawn from the DC power source during operation of the heating arrangement. Example Ej25: The calibration apparatus in accordance with example Ej24, wherein the current sensor comprises a sensing resistor and a current shunt amplifier. Example Ej26: The calibration apparatus in accordance with either of Examples Ej24 or Ej25, wherein the current sensor is configured to emit a current signal indicative of the DC supply current drawn from the DC power source during operation of the heating arrangement. Example Ej27: The calibration apparatus in accordance with any of Examples Ej24 to Ej26, further comprising a voltage sensor for determining the DC supply voltage drawn from the DC power source. Example Ej28: The calibration apparatus in accordance with example Ej27, wherein the voltage sensor comprises a voltage divider. Example Ej29: The calibration apparatus in accordance with either of Examples Ej27 or Ej28, wherein the voltage sensor is configured to emit a voltage signal indicative of the DC supply voltage drawn from the DC power source during operation of the heating arrangement. Example Ej30: The calibration apparatus in accordance with any of Examples Ej24 to Ej29, wherein the current sensor and, if present, the voltage sensor is operatively coupled to the heating arrangement controller. Example Ej31: The calibration apparatus in accordance with any of Examples Ej24 to Ej29, further comprising a calibration controller operatively coupled to the current sensor of the calibration apparatus and operatively coupled to the transistor switch driver circuit of the heating arrangement in a feedback loop configuration, wherein the calibration controller is configured to receive a current signal from the current sensor indicating the DC supply current and to adjust the switching frequency of the switching signal in response to the received current signal to adjust the DC supply current drawn from the DC power source to be within a predetermined range. Example Ej32: The inductive heating arrangement in accordance with Example Ej31, wherein the calibration controller is configured to determine an operating switching frequency of the switching signal for which, in operation, the DC supply current drawn from the DC power source is in the predetermined range. Example Ej33: The inductive heating arrangement in accordance with Example Ej32, wherein the calibration controller is configured to set the adjustable oscillator to emit a switching signal having the determined operating switching frequency. Now, examples will also be described with reference to the figures in which: Figure 1 schematically illustrates an illustrative embodiment of an aerosol generating system 1 according to the present invention; Figure 2 shows a first illustrative embodiment of an inductive heating arrangement according to the present invention, which can be used in the aerosol generating device according to Figure 1; Figure 3 schematically illustrates the nominal resonance curve and the actual resonance curve of the inductive heating arrangement according to Figure 2; Figure 4 illustrates an embodiment of a method for calibrating the output energy of the heating arrangement according to Figure 2; Figure 5 shows an example of the current sensor 40 used in the heating arrangement according to;and Figure 6 shows a second illustrative embodiment of an inductive heating arrangement according to the present invention, which can be used alternatively in the aerosol generating device according to Figure 1.; Figure 1 schematically illustrates an illustrative embodiment of an aerosol generating system 1 according to the present invention. The system 1 comprises an aerosol generating device 10 and an aerosol generating article 100 for use with the device, comprising an aerosol-forming substrate that is heated to form an inhalable aerosol. The aerosol-generating article 100 is a rod-shaped consumable comprising four elements arranged sequentially in coaxial alignment: an aerosol-forming rod segment 110, a support element 140 having a central air passage, an aerosol cooling element 150, and a nozzle element 160 comprising a filter. The aerosol-forming rod segment 110 is disposed at a distal end of article 100 and comprises a strip-shaped susceptor arrangement 120 and the aerosol-forming substrate 130 to be heated. Conversely, the nozzle element 160 is disposed at a proximal end of article 100, enabling a user to take a puff from it. The support element 140 and the aerosol cooling element 150 are disposed between them. Each of the four elements is essentially cylindrical, all having essentially the same diameter.The four elements are enclosed by an outer sheath 170 to hold them together and maintain the desired circular cross-sectional shape of the bar-shaped article 100. The sheath 170 is preferably made of paper. Further details of the article, particularly the four elements, are described in WO 2015 / 176898 A1. The aerosol-generating device 10 comprises a cylindrical receiving cavity 20 defined within a proximal portion 12 of the device 10 to receive at least a distal portion of the article 100. The device 10 further comprises an inductive heating arrangement 30 including an inductor 31 for generating a high-frequency alternating magnetic field. In the present embodiment, the inductor 31 is a helical coil circumferentially surrounding the cylindrical receiving cavity 20.The coil 31 is arranged so that the susceptor arrangement 120 of the aerosol-generating article 100 is exposed to the alternating magnetic field when article 100 is coupled to device 10. Therefore, when the inductive heating arrangement 30 is activated, the susceptor arrangement 120 heats up due to eddy currents and / or hysteresis losses induced by the alternating magnetic field within the susceptor arrangement 120, depending on the magnetic and electrical properties of the material. The susceptor arrangement 120 heats up to an operating temperature sufficient to vaporize the aerosol-forming substrate 130 surrounding the susceptor arrangement 120 within article 100.Within a distal portion 13, the aerosol generating device 10 further comprises a DC power source 50 and a controller 60 (illustrated in Figure 1 schematically only) for powering and controlling the heating process. Although it is part of the aerosol generator article 100, the susceptor arrangement 120 can be considered as part of the inductive heating arrangement 30. The same applies to the DC power source 50 and the controller 60. During use of system 1, when a user takes a puff at the nozzle element 160 of article 100, air is drawn into cavity 20 at the edge of an article insertion opening 25 within cavity 20. The airflow further extends to the distal end of cavity 20 through a passage formed between the inner surface of the cylindrical cavity 20 and the outer surface of article 100. At the distal end of cavity 20, the airflow enters the aerosol-generating article 100 through the substrate element 110 and then passes through the support element 140, the aerosol cooling element 150, and the nozzle element 160, where it finally exits article 100. In the substrate element 110, vaporized material from the aerosol-forming substrate 130 is entrained into the airflow.Subsequently, as it passes through the support element 140, the cooling element 150 and the nozzle element 160, the airflow containing the vaporized material is cooled to form an inhalable aerosol that escapes from article 10 through the nozzle element 160. Figure 2 shows an illustrative embodiment of the inductive heating arrangement 30 according to the present invention, which can be implemented in the aerosol generating device 10 according to Figure 1. According to the invention, the inductive heating arrangement 30 comprises a DC / AC inverter that is connected to the DC power source 50.In the present embodiment, the DC / AC inverter includes a class E power amplifier, i.e., a resonant switching power amplifier, comprising the following components: a transistor switch 36 comprising a field-effect transistor (FET), for example a metal-oxide-semiconductor field-effect transistor (MOSFET), a transistor switch supply circuit 37 including an oscillator for supplying a switching signal (gate source voltage) to the transistor switch 36, and an LC load network 33 comprising a shunt capacitor C1 and a series connection of a capacitor C2 and an inductor L2. The inductor L2 corresponds to the inductor 31 shown in Figure 1, which is configured to generate an alternating magnetic field within the cavity 20 during the operation of system 1.Furthermore, a DC power choke L1 is provided to supply the DC supply voltage +V_DC from the DC power source 50. Also shown in Figure 2 is the ohmic resistance R, representing the total equivalent resistance or total resistive load 38, which in operation 30 corresponds to the sum of the ohmic resistance of the inductor 31, marked as L2, and the ohmic resistance of the susceptor arrangement 120 shown in Figure 1. If no item is inserted into the cavity 20, the equivalent resistance or resistive load 38 corresponds only to the ohmic resistance of the inductor 31. Further details of the inductive heating arrangement 30 according to the present modality, in particular with regard to its operating principle, are described, for example, in WO 2015 / 177046 AL. Due to manufacturing tolerances of the electronic components, particularly capacitor C2 and inductor L2, the actual resonance curve of the LC 33 load network may deviate from the nominal resonance curve. This is shown in Figure 3, where the solid curved line represents the nominal resonance curve 510 of the LC 33 load network as defined by the nominal values ​​of capacitor C2 and inductor L2, and the dashed curved line represents the actual resonance curve 520 of the LC 33 load network as defined by the actual values ​​of capacitor C2 and inductor L2.Up to this point, the switching frequency f_l of the switching signal to be supplied by the transistor switch 37 to the transistor switch 36 has been determined with respect to the nominal resonant curve 510, that is, with respect to a specific operating point 511 on the nominal resonant curve 510 that is associated with a desired target output energy P_l. However, as further shown in Figure 3, a deviation from the nominal resonant curve 510 can cause the effective operating point of the power amplifier for the given switching frequency f_l to be at a point 522 on the actual resonant curve 520 that is associated with an output energy P_2 different from the desired target output P_1. Therefore, this deviation can result in a deviation of the output energy of the heating arrangement 30 from its nominal value.For example, a tolerance of more than ±1 percent of the nominal inductance of inductor L2 and the nominal capacitance of capacitor C2 of the LC charging network 33 can result in a variation of more than ±17 percent in the output power of the heating arrangement 30. Typically, this is unacceptable. Instead of using capacitors and inductors with tighter tolerances, the heating arrangement 30 according to the present invention comprises a transistor switch 37 drive circuit comprising an adjustable oscillator 39 configured to output a switching signal to the transistor switch 36 having an adjustable switching frequency. As also shown in Figure 3, this allows the switching frequency of the switching signal to be adjusted to change the effective operating point on the actual resonant curve of the LC charging network to a point 31. 521 or a sufficiently small interval around a point 521 that is associated with the desired target output energy P_l. Therefore, by adjusting the switching frequency of the switching signal from the value f_l to the value f_2 as shown in Figure 3, the resonant switching power amplifier can be effectively tuned to a desired operating point. As a result, the variation in output energy can be significantly limited, while at the same time, the requirements for the tolerances of the capacitors and inductors can be adjusted to a lower value. In the present example shown in Figure 3, the switching frequency is adjusted to a desired interval offset to the actual resonant frequency of the LC load network. The adjustment of the heating array's output energy is preferably performed during the array's manufacture, i.e., as a type of calibration process. This is preferably done using a reference susceptor array (not shown in Figure 2), particularly in cases where the actual susceptor array used to heat the aerosol-forming substrate is part of an aerosol-generating article, as shown in Figure 1. Figure 4 illustrates one embodiment of a method 300 for calibrating the output energy of the heating arrangement 30 in accordance with Figure 2. In a first step 301, a reference susceptor arrangement (not shown) is operatively coupled to the heating arrangement 30. With reference to the aerosol-generating device 10 in Figure 1, the reference susceptor arrangement may, for example, be inserted into the receiving cavity 20 of the device 10. Preferably, the reference susceptor arrangement is of the same type and is placed in the same position in the receiving cavity 20 as the susceptor arrangement 120 within the aerosol-generating article 100 that is to be used with the device later to heat the substrate. In a further step 302, the heating arrangement 10 is operated to heat the reference susceptor arrangement.During operation of the heating arrangement, the actual DC supply current drawn from the DC power source 50 is determined. For this purpose, the heating arrangement 30 may include a current sensor 40, as shown in Figure 2. The details of the current sensor will be described later with reference to Figure 5. The determined actual DC supply current is compared to a predetermined range. When the determined actual DC supply current is not within a predetermined range, the switching frequency of the switching signal provided by the supply circuit from transistor switch 32 to transistor switch 36 is changed to bring the effective operating point on the actual resonant curve of the LC load network to a position or range sufficiently small to achieve a desired target output power. This is shown in step 303 in Figure 4.The switching frequency is adjusted until the actual DC supply current is within a predetermined range associated with a desired target output power (see Figure 3). The actual DC supply current can be continuously determined while adjusting the switching frequency. Alternatively, the switching frequency can be tuned in stages, particularly through multiple frequency values, where the respective actual DC supply current is determined for each tuning stage, specifically for each frequency value. The steps of adjusting the switching frequency to a new frequency value and determining the respective actual DC supply current can be alternated.In a subsequent stage 304, when the actual DC supply current is within a predetermined range, an operating switching frequency for the switching signal is determined. This frequency corresponds to the actual DC supply current drawn from the DC power source during operation and is within the predetermined range. Finally, in stage 305, the adjustable oscillator is configured to output a switching signal with the predetermined operating switching frequency. To determine the DC supply current drawn from the DC power source 50 during operation of the heating arrangement, the inductive heating arrangement 30 may comprise a current sensor 40 as shown in Figure 2. The current sensor 40 is configured to output a current signal indicating the DC supply current. As shown in Figure 5, the current sensor 40 may comprise a sensing resistor 41 and a current shunt amplifier 42. To account for a possible decrease in the DC supply voltage over time, the inductive heating arrangement 30 may further comprise a voltage sensor 45 to determine the DC supply voltage drawn from the DC power source 50. As shown in Figure 2, the voltage sensor 45 may comprise a voltage divider.The voltage sensor 45 can be configured to output a voltage signal indicative of the DC supply voltage drawn from the DC power source during operation of the heating arrangement. Therefore, the output power of the inductive heating arrangement can be determined as a function of both the DC supply current and the DC supply voltage drawn from the DC power source 50. To adjust the output power of the heating arrangement, in particular to adjust the switching frequency of the adjustable oscillator 39 to a desired operating point on the actual resonance curve of the LC load network, for example, within a desired range around the resonance frequency of the LC load network 33, the current sensor 40, the adjustable oscillator 39, and, if present, the voltage sensor 45 are operatively coupled to the controller 60 in a feedback loop configuration, as shown in Figure 2. The controller 60 can be the same as that shown in Figure 1 and configured to receive the current signal from the current sensor 40 indicating the DC supply current.Furthermore, controller 60 is configured to tune the switching frequency of the switching signal output by the adjustable oscillator in response to the received current signal to tune the DC supply current drawn from the DC power source to the predetermined range, as described earlier with respect to the calibration method shown in Figure 4. Controller 60 can also be configured to determine an operating switching frequency for the switching signal at which the DC supply current drawn from the DC power source 50 is within the predetermined range. Additionally, controller 60 can be configured to set the adjustable oscillator 39 to output a switching signal with the predetermined operating switching frequency.That is, the determined operating switching frequency can be written into the adjustable oscillator 39 so that the oscillator 39 emits a fixed switching signal that has the determined operating switching frequency. Instead of using an internal current sensor 40, the calibration method can also be performed using a separate calibration apparatus. In addition to the reference susceptor arrangement mentioned above, the calibration apparatus may include a current sensor to determine the DC supply current drawn from the DC power source during operation of the heating arrangement. The current sensor can be configured in the same manner as described above with respect to the current sensor 40 shown in Figure 2 and Figure 5. The calibration apparatus may also include a voltage sensor as described above to determine the DC supply voltage drawn from the DC power source 50 to account for a possible decrease in the DC supply voltage over time. The current sensor and, if present, the voltage sensor of the calibration apparatus can be operatively coupled to the controller 60 of the heating arrangement 30 or to the aerosol generating device, respectively. Alternatively, instead of using the internal controller 60, the calibration apparatus can comprise a calibration controller itself. The calibration controller can be operatively coupled to the current sensor of the calibration apparatus and to the transistor switch driver circuit of the heating arrangement in a feedback loop configuration. The calibration controller can be configured in the same manner as described above with respect to the internal controller 60 shown in Figure 1 and Figure 2. Figure 6 shows a second illustrative embodiment of an inductive heating arrangement 430 according to the present invention, which may be used alternatively in the aerosol generating device according to Figure 1. With respect to the inductive heating arrangement 430, it comprises the same or similar features to the inductive heating arrangement 30 according to Figure 2; the same reference symbols are used in Figure 5, but are increased by 400. Instead of a class E amplifier, the inductive heating arrangement 430 according to Figure 6 comprises a class D amplifier comprising two transistor switches 436.1 and 436.2 to provide a high-frequency oscillating current to an inductor 431, which in turn is used to generate the alternating magnetic field for heating the susceptor arrangement.As in Figure 2, the inductor 431 is again denoted by L2, and the combined ohmic resistance 438 of the inductor 431 and the susceptor arrangement is denoted by R. The inductive heating arrangement 430 further comprises a DC power supply 450 connected to the two transistor switches 436.1 and 436.2. Two transistor driver circuits 437.1 and 437.2 are provided for switching two of the transistors 436.1 and 436.2 on and off. The switches are controlled at high frequency so as to ensure that one of the two transistor switches 436.1 and 436.2 is off at the time the other of the two transistors is turned on. To allow adjustment of the output energy of the inductive heating arrangement 430, each of the two transistor switch driver circuits 437.1 and 437.2 comprises an adjustable oscillator 439.1 and 439.2 configured to emit a switching signal to the respective transistor switch 436.1 and 436.2 which has an adjustable switching frequency. By means of this, the heating arrangement 430 in accordance with Figure 6 can be calibrated in the same way as described above with respect to Figure 4. The values ​​of C1 and C2 can be chosen to maximize the efficient dissipation of energy in the susceptor arrangement. For the purposes of this description and the appended claims, unless otherwise stated, all numbers expressing quantities, percentages, etc., are to be understood as modified in all cases by the term "approximately." Furthermore, all intervals include the maximum and minimum points described and include any intermediate intervals therewith, which may or may not be specifically enumerated in this description. In this context, therefore, a number A is understood to be A ± 5 percent of A. Within this context, a number A may be considered to include numerical values ​​that are within the general standard error for the measurement of the property that modifies the number A.The number A, in some cases as used in the appended claims, may deviate by the percentages listed above, provided that the amount by which A deviates does not materially affect the basic and novel feature(s) of the claimed invention. Furthermore, all intervals include the maximum and minimum points described and include any intermediate intervals therein, which may or may not be specifically listed herein.

Claims

1. An inductive heating arrangement for heating an aerosol-forming substrate, wherein the heating arrangement comprises a DC power source and an electronic power supply circuit comprising a DC / AC inverter connected to the DC power source, wherein the DC / AC inverter comprises a resonant switching power amplifier with at least one transistor switch, at least one transistor switch driver circuit associated with the transistor switch, and an LC charging network comprising at least one capacitor and at least one inductor, wherein the inductor is configured to generate an alternating magnetic field during operation of the heating arrangement for inductively heating the aerosol-forming substrate.wherein the transistor switch driver circuit comprises an adjustable oscillator configured to output a switching signal to the transistor switch having an adjustable switching frequency, wherein the heating arrangement further comprises a current sensor for determining the DC supply current drawn from the DC power source during operation of the heating arrangement, and a controller configured to receive a current signal from the current sensor indicating the DC supply current and to adjust the switching frequency of the switching signal in response to the received current signal to adjust the DC supply current drawn from the DC power source to a predetermined interval.

2. The inductive heating arrangement according to claim 1, wherein the switching frequency is adjustable within a range of ±20 percent, in particular ±10 percent, preferably ±5 percent around a center frequency.

3. The inductive heating arrangement according to any of the preceding claims, wherein the switching frequency is adjustable in a range between 5.4 MHz and 8 MHz, in particular between 6.0 MHz and 7.5 MHz, preferably between 6.4 MHz and 7.2 MHz.

4. The inductive heating arrangement according to any of the preceding claims, wherein a tolerance of a capacitor capacitance is in the range of ±2 percent and ±4 percent of a nominal capacitor capacitance value.

5. The inductive heating arrangement according to any of the preceding claims, wherein a tolerance of an inductor inductance is in the range of ±3 percent and ±7 percent, in particular between ±4 percent and ±6 percent, preferably ±5 percent of a nominal inductance value of the inductor.

6. The inductive heating arrangement according to any preceding claim, wherein the switching power amplifier is one of a class C power amplifier, a class D power amplifier, and a class E power amplifier.

7. The inductive heating arrangement according to any preceding claim, wherein the controller is configured to set the adjustable oscillator to emit a switching signal having a determined operating switching frequency in response to the received current signal, for which the operating switching frequency of the DC supply current drawn from the DC power source is in the predetermined range.

8. The inductive heating arrangement according to any preceding claim, wherein the controller is operatively coupled to a calibration apparatus configured to determine in response to the current signal the operating switching frequency of the switching signal for which, in operation, the DC supply current drawn from the DC power source is in the predetermined range, wherein the controller is configured to communicate the current signal to the calibration apparatus and to receive a signal communicated from the calibration apparatus indicative of the determined operating switching frequency.

9. The inductive heating arrangement according to any one of claims 1 to 7, wherein the controller is operatively coupled to the current sensor and the transistor switch controller circuit in a feedback loop configuration.

10. The inductive heating arrangement according to claim 9, wherein the controller is configured to determine, in particular in response to the current signal, the operating switching frequency of the switching signal for which, in operation, the DC supply current drawn from the DC power source is in the predetermined range.

11. The inductive heating arrangement according to any preceding claim, further comprising a susceptor arrangement, wherein the susceptor arrangement is disposed within the alternating magnetic field generated by the inductor during operation of the heating arrangement.

12. A method for calibrating an inductive heating arrangement according to any one of claims 1 to 11, the method comprising: operatively coupling the heating arrangement to a reference susceptor arrangement; operating the heating arrangement to heat the reference susceptor arrangement; determining the actual DC supply current drawn from the DC power source during operation of the heating arrangement; determining an operating switching frequency of the switching signal for which the actual DC supply current drawn from the DC power source is within the predetermined range; setting the adjustable oscillator to emit a switching signal having the determined operating switching frequency.

13. The method according to claim 12, wherein determining the operating switching frequency comprises adjusting the switching frequency of the switching signal while determining the actual DC supply current drawn from the DC power source until the actual DC supply current is within a predetermined range.

14. A calibration apparatus for calibrating an inductive heating arrangement according to any of claims 1 to 11, particularly for use in a method according to claim 12, wherein the calibration apparatus comprises a reference susceptor arrangement inductively coupled to the inductor of the heating arrangement and a calibration controller operatively coupled to the controller of the heating arrangement, wherein the calibration controller is configured to receive the current signal communicated from the controller of the heating arrangement, to determine, in response to the current signal, the operating switching frequency of the switching signal for which the DC supply current drawn from the DC power source is within the predetermined range.and to communicate a signal indicating the predetermined operating switching frequency to the heating arrangement controller.

15. The calibration apparatus according to claim 14, wherein the calibration controller is configured to determine the operating switching frequency of the switching signal by calculating a frequency shift using a previously known function between the DC current and the switching frequency to be applied to the switching frequency at which the DC supply current associated with the received current signal was measured.

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