INDUCTIVE HEATING DEVICE FOR HEATING AN AEROSOL-FORMING SUBSTRATE

Abstract

An inductive heating device (1) comprises a device housing (10) a DC power source (11), a power supply electronics (13) comprising a DC/AC inverter (132) including a Class-E power amplifier with a transistor switch (1320), a transistor switch driver circuit (1322), and an LC load network (1323) configured to operate at low ohmic load (1324), the LC load network (1323) comprising a shunt capacitor (C1) and a series connection of a capacitor (C2) and an inductor (L2), and a cavity (14) arranged in the device housing (10), the cavity (14) having an internal surface shaped to accommodate at least a portion of the aerosol-forming substrate (20), wherein the cavity (14) is arranged such that the inductor (L2) is inductively coupled to the susceptor (21) of the aerosol-forming substrate (20) during operation.

Claims

1. Inductive heating device for heating an aerosol-forming substrate, the inductive heating device comprising: a device housing a DC power source having a DC supply voltage, a power supply electronics configured to operate at high frequency, the power supply electronics comprising a DC/AC inverter connected to the DC power source, the DC/AC inverter including a Class-E power amplifier including a transistor switch, a transistor switch driver circuit, and an LC load network configured to operate at low ohmic load wherein the LC load network comprises a shunt capacitor and a series connection of a capacitor and an inductor having an ohmic resistance, and a cavity arranged in the device housing, the cavity having an internal surface shaped to accommodate at least a portion of the aerosol-forming substrate wherein, when, the portion of the aerosol-forming substrate is accommodated in the cavity, the inductor of the LC load network is configured to inductively couple to a susceptor positioned within the aerosol-forming substrate to heat the aerosol-forming substrate during operation.

2. Inductive heating device according to claim 1, wherein device is configured for heating an aerosol-forming substrate of a smoking article.

3. Inductive heating device according to claim 2, wherein the device is configured for heating a tobacco-laden solid aerosol-forming substrate of a smoking article.

4. Inductive heating device according to claim 1, wherein the DC supply voltage of the DC power source is in the range of about 2.5 Volts to about 4.5 Volts, and wherein the DC supply amperage of the DC power source is in the range of about 2.5 Amperes to about 5 Amperes.

5. Inductive heating device according to claim 1, wherein the total volume of the power supply electronics is equal to or smaller than 2 cm.sup.3.

6. Inductive heating device according to claim 1, wherein the inductor of the LC load network comprises a helically wound cylindrical inductor coil having an oblong shape and defining an inner volume in the range of about 0.15 cm.sup.3 to about 1.10 cm.sup.3, and wherein the helically wound cylindrical inductor coil is positioned on or adjacent the internal surface of the cavity.

7. Inductive heating device according to claim 1, wherein the device housing has a substantially cylindrical shape with the cavity being arranged at the proximal end of the device housing and with the DC power source being arranged at the distal end of the device housing, and wherein the power supply electronics is arranged between the DC power source and the cavity.

8. Inductive heating device according to claim 1, wherein the DC power source comprises a rechargeable DC battery.

9. Inductive heating device according to claim 1, wherein the power supply electronics further comprises a microcontroller which is programmed to interrupt generation of AC power by the DC/AC inverter as the temperature of the susceptor positioned within the aerosol-forming substrate has exceeded a Curie temperature of the susceptor during operation, and which is programmed to resume generation of AC power as the temperature of the susceptor has cooled down below this Curie temperature again.

10. Inductive heating device according to claim 1, wherein the class E power amplifier has an output impedance and wherein the power supply electronics further comprises a matching network for matching the output impedance of the class E power amplifier to the low ohmic load.

11. Inductive heating system comprising an inductive heating device according to claim 1 and an aerosol-forming substrate, at least a portion of the aerosol-forming substrate being accommodated in the cavity of the inductive heating device with a susceptor positioned within the aerosol-forming substrate, such that the inductor of the LC load network of the DC/AC inverter of the inductive heating device is inductively coupled to the susceptor positioned within the aerosol-forming substrate to heat the aerosol-forming substrate during operation.

12. Inductive heating system according to claim 11, wherein the aerosol-forming substrate is an aerosol-forming substrate of a smoking article.

13. Inductive heating system according to claim 12, wherein the aerosol-forming substrate of the smoking article is a tobacco-laden solid aerosol-forming substrate.

14. Kit comprising an inductive heating device according to claim 1 and an aerosol-forming substrate, the inductive heating device and the aerosol-forming substrate being configured such that in operation at least a portion of the aerosol-forming substrate is accommodated in the cavity of the inductive heating device with a susceptor positioned within the aerosol-forming substrate, such that the inductor of the LC load network of the DC/AC inverter of the inductive heating device is inductively coupled to the susceptor positioned within the aerosol-forming substrate.

15. Kit according to claim 14, wherein the aerosol-forming substrate is an aerosol-forming substrate of a smoking article.

16. Kit according to claim 15, wherein the aerosol-forming substrate of the smoking article is a tobacco-laden solid aerosol-forming substrate.

17. Method of operating an inductive heating system, the method comprising the steps of: providing a DC power source having a DC supply voltage, providing a power supply electronics configured to operate at high frequency, the power supply electronics comprising a DC/AC inverter connected to the DC power source, the DC/AC inverter including a Class-E power amplifier including a transistor switch, a transistor switch driver circuit, and an LC load network configured to operate at low ohmic load, wherein the LC load network comprises a shunt capacitor and a series connection of a capacitor and an inductor having an ohmic resistance, providing a cavity capable of accommodating at least a portion of an aerosol-forming substrate, the cavity being arranged such that upon accommodation of the portion of the aerosol-forming substrate in the cavity the inductor of the LC load network is inductively coupled to a susceptor positioned within the aerosol-forming substrate, and providing an aerosol-forming substrate and inserting at least a portion of the aerosol-forming substrate into the cavity such that the inductor of the LC load network is inductively coupled to the susceptor positioned within the aerosol-forming substrate.

18. Method according to claim 17, wherein the DC power source is a rechargeable battery, and wherein the method further comprises the step of charging the rechargeable battery prior to inserting the portion of the aerosol-forming substrate into the cavity.

19. Method according to claim 17, wherein the aerosol-forming substrate comprises the susceptor, and wherein the inductor of the LC load network is configured to inductively couple to the susceptor of the aerosol-forming substrate during operation.

20. Inductive heating device according to claim 1, wherein the aerosol-forming substrate comprises the susceptor, and wherein the inductor of the LC load network is configured to inductively couple to the susceptor of the aerosol-forming substrate during operation.

Description

[0060] Further advantageous aspects of the invention will become apparent from the following description of embodiments with the aid of the drawings in which:

[0061] FIG. 1 shows the general heating principle underlying the invention,

[0062] FIG. 2 shows a block diagram of an embodiment of the inductive heating device and system according to the invention,

[0063] FIG. 3 shows an embodiment of the inductive heating device with the essential components arranged in a device housing,

[0064] FIG. 4 shows an embodiment of essential components of the power electronics of the inductive heating device according to the invention (without matching network),

[0065] FIG. 5 shows an embodiment of the inductor of the LC load network in form of a helically wound cylindrical inductor coil having an oblong shape,

[0066] FIG. 6 shows a detail of the LC load network including the inductivity and ohmic resistance of the coil, and in addition shows the ohmic resistance of the load.

[0067] In FIG. 1 the general heating principle underlying the instant invention is schematically illustrated. Schematically shown in FIG. 1 are a helically wound cylindrical inductor coil L2 having an oblong shape and defining an inner volume in which there is arranged a portion or all of an aerosol-forming substrate 20 of a smoking article 2, the aerosol-forming substrate comprising a susceptor 21. The smoking article 2 comprising the aerosol-forming substrate 20 with the susceptor 21 is schematically represented in the enlarged cross-sectional detail shown separately on the right hand side of FIG. 1. As mentioned already, the aerosol-forming substrate 20 of the smoking article 2 may be a tobacco-laden solid substrate, however, without being limited thereto.

[0068] In addition, in FIG. 1 the magnetic field within the inner volume of the inductor coil L2 is indicated schematically by a number of magnetic field lines B.sub.L at one specific moment in time, since the magnetic field generated by the alternating current i.sub.L2 flowing through the inductor coil L2 is an alternating magnetic field changing its polarity at the frequency of the alternating current i.sub.L2 which may be in the range of about 1 MHz to about 30 MHz (including the range of 1 MHz to 30 MHz), and may in particular be in the range of about 1 MHz to about 10 MHz (including the range of 1 MHz to 10 MHz, and may especially be smaller than 10 MHz), and very particularly the frequency may be in the range of about 5 MHz to about 7 MHz (including the range of 5 MHz to 7 MHz, and may for example be 5 MHz). The two main mechanisms responsible for generating heat in the susceptor 21, the power losses P.sub.e caused by eddy currents (closed circle representing the eddy currents) and the power losses P.sub.h caused by hysteresis (closed hysteresis curve representing the hysteresis), are also schematically indicated in FIG. 1. With respect to these mechanisms it is referred to the more detailed discussion of these mechanisms above.

[0069] FIG. 3 shows an embodiment of an inductive heating device 1 according to the invention. The inductive heating device 1 comprises a device housing 10 which can be made of plastic and a DC power source 11 (see FIG. 2) comprising a rechargeable battery 110. Inductive heating device 1 further comprises a docking port 12 comprising a pin 120 for docking the inductive heating device to a charging station or charging device for recharging the rechargeable battery 110. Still further, inductive heating device 1 comprises a power supply electronics 13 which is configured to operate at the desired frequency, for example at a frequency of 5 MHz as mentioned above. Power supply electronics 13 is electrically connected to the rechargeable battery 110 through a suitable electrical connection 130. And while the power supply electronics 13 comprises additional components which cannot be seen in FIG. 3, it comprises in particular an LC load network (see FIG. 4) which in turn comprises an inductor L2, this being indicated by the dashed lines in FIG. 3. Inductor L2 is embedded in the device housing 10 at the proximal end of device housing 10 to surround a cavity 14 which is also arranged at the proximal end of the device housing 10. Inductor L2 may comprise a helically wound cylindrical inductor coil having an oblong shape, as shown in FIG. 5. The helically wound cylindrical inductor coil L2 may have a radius r in the range of about 5 mm to about 10 mm, and in particular the radius r may be about 7 mm. The length 1 of the helically wound cylindrical inductor coil may be in the range of about 8 mm to about 14 mm. The inner volume accordingly, may be in the range of about 0.15 cm.sup.3 to about 1.10 cm.sup.3.

[0070] Returning to FIG. 3, the tobacco-laden solid aerosol-forming substrate 20 comprising susceptor 21 is accommodated in cavity 14 at the proximal end of the device housing 10 such that during operation the inductor L2 (the helically wound cylindrical inductor coil) is inductively coupled to susceptor 21 of the tobacco-laden solid aerosol-forming substrate 20 of smoking article 2. A filter portion 22 of the smoking article 2 may be arranged outside the cavity 14 of the inductive heating device 1 so that during operation the consumer may draw the aerosol through the filter portion 22. Once the smoking article is removed from the cavity 14, the cavity 14 can be easily cleaned since except for the open distal end through which the aerosol-forming substrate 20 of the smoking article 2 is to be inserted the cavity is fully closed and surrounded by those inner walls of the plastic device housing 10 defining the cavity 14.

[0071] FIG. 2 shows a block diagram of an embodiment of the inductive heating device 1 according to the invention, however, with some optional aspects or components as will be discussed below. Inductive heating device 1 together with the aerosol-forming substrate 20 including the susceptor 21 forms an embodiment of the inductive heating system according to the invention. The block diagram shown in FIG. 2 is an illustration taking the manner of operation into account. As can be seen, the inductive heating device 1 comprises a DC power source 11 (in FIG. 3 comprising the rechargeable battery 110), a microprocessor control unit 131, a DC/AC inverter 132, a matching network 133 for adaptation to the load, and the inductor L2. Microprocessor controller unit 131, DC/AC inverter 132 and matching network 133 as well as inductor L2 are all part of the power supply electronics 13 (see FIG. 1). Two feed-back channels 134 and 135 are provided for providing feed-back signals indicating the voltage and current through inductor L2 allowing to control the further supply of power. For example, in case the temperature of the susceptor exceeds a desired temperature, a corresponding signal may be generated interrupting the further supply of power until the temperature of the susceptor is again below the desired temperature whereupon further supply of power may be resumed. Correspondingly, it is possible to control the frequency of the switching voltage for optimal transfer of power to the susceptor. A matching network 133 may be provided for optimum adaptation to the load but is not mandatory and is not contained in the embodiment described in more detail in the following.

[0072] FIG. 4 shows some essential components of the power supply electronics 13, more particularly of the DC/AC inverter 132. As can be seen from FIG. 4, the DC/AC inverter includes a Class-E power amplifier including a transistor switch 1320 comprising a Field Effect Transistor (FET) 1321, for example a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), a transistor switch supply circuit indicated by the arrow 1322 for supplying the switching signal (gate-source voltage) to the FET 1321, and an LC load network 1323 comprising a shunt capacitor C1 and a series connection of a capacitor C2 and inductor L2. In addition, the DC power source 11 including a choke L1 is shown for supplying a DC supply voltage +V.sub.CC. Also shown in FIG. 4 is the ohmic resistance R representing the total ohmic load 1324, which is the sum of the ohmic resistance R.sub.Coil of the inductor L2 and the ohmic resistance R.sub.Load of the susceptor 21, as this is shown in FIG. 6.

[0073] It is evident, that due to the very low number of components the volume of the power supply electronics 13 can be kept extremely small. For example, the volume of the power supply electronics may be equal or smaller than 2 cm.sup.3. This extremely small volume of the power supply electronics is possible due to the inductor L2 of the LC load network 1323 being directly used as the inductor for the inductive coupling to the susceptor 21 of aerosol-forming substrate 20, and this small volume allows to keep the overall dimensions of the entire inductive heating device 1 small. In case a separate inductor other than the inductor L2 is used for the inductive coupling to the susceptor 21, this would automatically increase the volume of the power supply electronics, this volume being also increased if a matching network 133 is included in the power supply electronics.

[0074] While the general operating principle of the Class-E power amplifier is known and described in detail in the already mentioned article Class-E RF Power Amplifiers, Nathan O. Sokal, published in the bimonthly magazine QEX, edition January/February 2001, pages 9-20, of the American Radio Relay League (ARRL), Newington, Conn., U.S.A., some general principles will be explained in the following.

[0075] Let us assume that the transistor switch supply circuit 1322 supplies a switching voltage (gate-source voltage of the FET) having a rectangular profile to FET 1321. As long as FET 1321 is conducting (on-state), it does essentially constitute a short circuit (low resistance) and the entire current flows through choke L1 and FET 1321. As FET 1321 is non-conducting (off-state), the entire current flows into the LC load network since FET 1321 essentially represents an open circuit (high resistance). Switching the transistor between these two states inverts the supplied DC voltage and DC current into an AC voltage and AC current.

[0076] For efficiently heating the susceptor 21, an as large as possible amount of the supplied DC power is to be transferred in the form of AC power to inductor L2 (helically wound cylindrical inductor coil) and subsequently to the susceptor 21 of aerosol-forming substrate 20 which is inductively coupled to inductor 2. The power dissipated in the susceptor 21 (eddy current losses, hysteresis losses) generates heat in the susceptor 21, as described further above. Or to say it in other words, power dissipation in FET 1321 must be minimized while maximizing power dissipation in susceptor 21.

[0077] The power dissipation in FET 1321 during one period of the AC voltage/current is the product of the transistor voltage and current at each point in time during that period of the alternating voltage/current, integrated over that period, and averaged over that period. Since the FET 1321 must sustain high voltage during a part of that period and conduct high current during a part of that period, it must be avoided that high voltage and high current exist at the same time, since this would lead to substantial power dissipation in FET 1321. In the on- state of FET 1321, the transistor voltage is nearly zero when high current is flowing through the FET 1321. In the off- state of FET 1321, the transistor voltage is high but the current through FET 1321 is nearly zero.

[0078] The switching transitions unavoidably also extend over some fractions of the period. Nevertheless, a high voltage-current product representing a high power loss in FET 1321 can be avoided by the following additional measures. Firstly, the rise of the transistor voltage is delayed until after the current through the transistor has reduced to zero. Secondly, the transistor voltage returns to zero before the current through the transistor begins to rise. This is achieved by load network 1323 comprising shunt capacitor C1 and the series connection of capacitor C2 and inductor L2, this load network being the network between FET 1321 and the load 1324. Thirdly, the transistor voltage at turn-on time is practically zero (for a bipolar-junction transistor BJT it is the saturation offset voltage V.sub.o). The turning-on transistor does not discharge the charged shunt capacitor C1, thus avoiding dissipating the shunt capacitor's stored energy. Fourthly, the slope of the transistor voltage is zero at turn-on time. Then, the current injected into the turning-on transistor by the load network rises smoothly from zero at a controlled moderate rate resulting in low power dissipation while the transistor conductance is building up from zero during the turn-on transition. As a result, the transistor voltage and current are never high simultaneously. The voltage and current switching transitions are time-displaced from each other.

[0079] For dimensioning the various components of the DC/AC inverter 132 shown in FIG. 4, the following equations have to be considered, which are generally known and have been described in detail in the afore-mentioned article Class-E RF Power Amplifiers, Nathan O. Sokal, published in the bimonthly magazine QEX, edition January/February 2001, pages 9-20, of the American Radio Relay League (ARRL), Newington, Conn., U.S.A.

[0080] Let Q.sub.L (quality factor of the LC load circuit) be a value which is in any event greater than 1.7879 but which is a value that can be chosen by the designer (see the afore-mentioned article) let further P be the output power delivered to the resistance R, and let f be the frequency, then the various components are numerically calculated from the following equations (V.sub.o being zero for FETs, and being the saturation offset voltage for BJTs, see above):

L2=Q.sub.L.Math.R/2f

R=((V.sub.CCV.sub.o).sup.2/P).Math.0.576801.Math.(1.00000860.414395/Q.sub.L0.557501/Q.sub.L.sup.2+0.205967/Q.sub.L.sup.3)

C1=(1/(34.2219.Math.f.Math.R)).Math.(0.99866+0.91424/Q.sub.L1.03175/Q.sub.L.sup.2)+0.6/(2f).sup.2.Math.(L1)

C2=(1/2fR).Math.(1/Q.sub.L0.104823).Math.(1.00121+(1.01468/Q.sub.L1.7879))(0.2/((2f).sup.2.Math.L1)))

[0081] This allows for a rapid heating up of a susceptor having an ohmic resistance of R=0.6 to deliver approximately 7 W of power in 5-6 seconds assuming that a current of approximately 3.4 A is available using a DC power source having a maximum output voltage of 2.8 V and a maximum output current of 3.4 A, a frequency of f=5 MHz (duty ratio=50%), an inductivity of inductor L2 of approximately 500 nH and an ohmic resistance of the inductor L2 of R.sub.Coil=0.1, an inductivity L1 of about 1 H, and capacitances of 7 nF for capacitor C1 and of 2.2 nF for capacitor C2. The effective resistance of R.sub.Coil and R.sub.Load is approximately 0.6. An efficiency (Power dissipated in the susceptor 21/maximum power of DC power source 11) of about 83.5% may be obtained which is very effective.

[0082] As has been mentioned already, the susceptor 21 can be made of a material or of a combination of materials having a Curie temperature which is close to the desired temperature to which the susceptor 21 should be heated. Once the temperature of the susceptor 21 exceeds this Curie temperature, the material changes its ferromagnetic properties to paramagnetic properties. Accordingly, the energy dissipation in the susceptor 21 is significantly reduced since the hysteresis losses of the material having paramagnetic properties are much lower than those of the material having the ferromagnetic properties. This reduced power dissipation in the susceptor 21 can be detected and, for example, the generation of AC power by the DC/AC inverter may then be interrupted until the susceptor 21 has cooled down below the Curie temperature again and has regained its ferromagnetic properties. Generation of AC power by the DC/AC inverter may then be resumed again.

[0083] For operation, the smoking article 2 is inserted into the cavity 14 (see FIG. 2) of the inductive heating device 1 such that the aerosol-forming substrate 20 comprising the susceptor 21 is inductively coupled to inductor 2 (e.g. the helically wound cylindrical coil). Susceptor 21 is then heated for a few seconds as described above, and then the consumer may begin drawing the aerosol through the filter 22 (of course, the smoking article does not necessarily have to comprise a filter 22).

[0084] The inductive heating device and the smoking articles can generally be distributed separately or as a kit of parts. For example, it is possible to distribute a so-called starter kit comprising the inductive heating device as well as a plurality of smoking articles. Once the consumer has purchased such starter kit, in the future the consumer may only purchase smoking articles that can be used with this inductive heating device of the starter kit. The inductive heating device is easy to clean and in case of rechargeable batteries as the DC power source, these rechargeable batteries are easy to be recharged using a suitable charging device that is to be connected to the docking port 12 comprising pin 120 (or the inductive heating device is to be docked to a corresponding docking station of a charging device).

[0085] Having described embodiments of the invention with the aid of the drawings, it is clear that many changes and modifications are conceivable without departing from the general teaching underlying the instant invention. Therefore, the scope of protection is not intended to be limited to the specific embodiments, but rather is defined by the appended claims.