SWITCH FAILURE MONITORING IN AN ELECTRICALLY HEATED SMOKING SYSTEM

Abstract

A method of controlling an electric heater in an electrically heated smoking system is provided, including providing electrical power to the heater in pulses such that during active periods power is supplied to the heater and during inactive periods power is not supplied to the heater; charging a capacitor in an RC circuit during the inactive periods and allowing the capacitor to discharge during the active periods; and monitoring a discharge voltage of the capacitor and if the discharge voltage of the capacitor drops below a threshold voltage level, then stopping further supply of electrical power to the heater.

Claims

1.-14. (canceled)

15. A method of controlling an electric heater in an electrically heated smoking system, comprising: providing electrical power to the heater in pulses such that during active periods power is supplied to the heater, and during inactive periods, power is not supplied to the heater; charging a capacitor in an RC circuit during the inactive periods and allowing the capacitor to discharge during the active periods; and monitoring a discharge voltage of the capacitor and stopping further supply of electrical power to the heater if the discharge voltage drops below a threshold voltage level.

16. The method according to claim 15, wherein the electrical power is provided to the heater by regularly switching a first switch, and wherein the stopping further supply of electrical power to the heater comprises switching a second switch.

17. The method according to claim 15, wherein a time constant of the RC circuit is greater than twice a time width of the pulses of the electrical power provided to the heater.

18. An electrically heated smoking system, comprising: a power supply; an electric heater; a first switch connected between the electric heater and electrical ground; a second switch connected between the power supply and the electric heater; an RC circuit comprising a capacitor and being connected to the power supply, configured such that the capacitor charges when the first switch is open and discharges when the first switch is closed; and control circuitry connected to the RC circuit and being configured to monitor a discharge voltage of the RC circuit and to open the second switch when the discharge voltage of the RC circuit falls below a threshold voltage value.

19. The electrically heated smoking system according to claim 18, wherein the control circuitry comprises a Schmitt trigger connected between the RC circuit and second switch, the Schmitt trigger being configured to open the second switch when the discharge voltage of the RC circuit falls below the threshold voltage value.

20. The electrically heated smoking system according to claim 18, wherein the first switch is a MOSFET.

21. The electrically heated smoking system according to claim 18, wherein the second switch is a MOSFET.

22. The electrically heated smoking system according to claim 18, further comprising a diode configured to prevent discharge of the RC circuit through the first switch when the first switch is closed.

23. The electrically heated smoking system according to claim 18, wherein the RC circuit has a time constant greater than twice a longest period for which the first switch is closed during normal operation of the system.

24. The electrically heated smoking system according to claim 18, further comprising an inverter connected between the RC circuit and the second switch.

25. The electrically heated smoking system according to claim 18, further comprising a controller configured to control operation of the first switch to maintain the electric heater at a target temperature.

26. The electrically heated smoking system according to claim 18, wherein the power supply is a battery.

27. The electrically heated smoking system according to claim 18, wherein the system is a handheld electrically heated smoking system.

28. The electrically heated smoking system according to claim 18, wherein the system is a heated tobacco smoking system.

Description

[0046] Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

[0047] FIG. 1 is a schematic illustration of an electrically heated smoking system;

[0048] FIG. 2 is a schematic cross-section of the front end of a first embodiment of a device of the type shown in FIG. 1;

[0049] FIG. 3 is a schematic illustration of a switch failure monitoring circuit in accordance with the invention; and

[0050] FIG. 4 is an embodiment of a circuit of the type shown in FIG. 2 showing circuit components in greater detail.

[0051] In FIG. 1, the components of an embodiment of an electrically heated aerosol-generating device 100 are shown in a simplified manner. Particularly, the elements of the electrically heated aerosol-generating device 100 are not drawn to scale in FIG. 1. Elements that are not relevant for the understanding of this embodiment have been omitted to simplify FIG. 1.

[0052] The electrically heated aerosol-generating device 100 comprises a housing 10 and an aerosol-forming substrate 12, for example a cigarette. The aerosol-forming substrate 12 is pushed inside the housing 10 to come into thermal proximity with the heating element 14. The aerosol-forming substrate 12 will release a range of volatile compounds at different temperatures. By controlling the operation temperature of the electrically heated aerosol-generating device 100 to be below the release temperature of some of the volatile compounds, the release or formation of these smoke constituents can be avoided.

[0053] Within the housing 10 there is an electrical energy supply 16, for example a rechargeable lithium ion battery. A controller 18 is connected to the heating element 14, the electrical energy supply 16, and a user interface 20, for example a button or display. The controller 18 controls the power supplied to the heating element 14 in order to regulate its temperature. Typically the aerosol-forming substrate is heated to a temperature of between 250 and 450 degrees centigrade.

[0054] In the described embodiment the heating element 14 is an electrically resistive track or tracks deposited on a ceramic substrate. The ceramic substrate is in the form of a blade and is inserted into the aerosol-forming substrate 12 in use. FIG. 2 is a schematic representation of the front end of the device and illustrates the air flow through the device. It is noted that FIG. 2 does not accurately depict the relative scale of elements of the device. A smoking article 102, including an aerosol forming substrate 12 is received within the cavity 22 of the device 100. Air is drawn into the device by the action of a user sucking on a mouthpiece 24 of the smoking article 102. The air is drawn in through inlets 26 forming in a proximal face of the housing 10. The air drawn into the device passes through an air channel 28 around the outside of the cavity 22. The drawn air enters the aerosol-forming substrate 12 at the distal end of the smoking article 102 adjacent a proximal end of a blade shaped heating element 14 provided in the cavity 22. The drawn air proceeds through the aerosol-forming substrate 12, entraining the aerosol, and then to the mouth end of the smoking article 102. The aerosol-forming substrate 12 is a cylindrical plug of tobacco based material.

[0055] FIG. 3 is a schematic illustration of a switch failure monitoring circuit in accordance with the invention. As shown in FIG. 3, the heater 14 is connected to electrical ground through a low side switch 32, also referred to as the first switch herein. The heater 14 is connected to the battery voltage through a high side switch 34, herein referred to as the second switch.

[0056] The first switch 32 is an n channel MOSFET. The second switch is a p channel MOSFET. During normal operation of the system, the second MOSFET 34 is maintained on, corresponding to the second switch being in a closed position, allowing current to flow from the battery to the heater. The first MOSFET 32 is switched on and off by the controller 18 in accordance with a particular duty cycle to control the temperature of the heater 14. When the first MOSFET 32 is on, corresponding to the switch being closed, current is allowed to flow from the heater to ground and the MOSFET 32 has a very low electrical resistance. Almost all of the battery voltage is then dropped across the heater and the heater heats up as a result of the Joule effect. When the first MOSFET is off it presents a very high electrical resistance. In this case very little voltage is dropped across the heater and there is almost no heating of the heater as a result of the Joule effect.

[0057] If there is a fault with the first switch and it stays on allowing current to flow through the heater continuously the temperature of the heater will rise in an uncontrolled manner. To detect a fault with the first switch a monitoring system is provided. The monitoring system comprises an RC circuit 36 connected to the heater through a diode 40, and a trigger component 38 connected between the RC circuit and a control input of the second switch 34.

[0058] When the first switch 32 is off and so has very high resistance, the RC circuit 36 is allowed to quickly charge up as a result of the battery voltage. When the first switch 32 is on, the voltage at the low side switch is very close to ground and the RC circuit discharges. The diode 40 prevents the RC circuit discharging through the heater. The trigger component 38 receives the discharge voltage of the RC circuit and is configured to switch second switch off when the discharge voltage falls below a predetermined threshold.

[0059] During normal operation the first switch is on for a consistent time period (the active phase), for example 1 millisecond, and is off (the inactive phase) for periods between. It is possible to charge the RC circuit quickly during the inactive phase and allow it to discharge only slowly during the active phase by making the discharge path have a greater resistance than the charging path. So even at a maximum duty cycle, in which the first switch may be on for 99% of the time and off for only 1% of the time in order to increase the heater temperature, it can be ensured that the trigger only operates the second switch if the active phase lasts significantly longer than the expected 1 millisecond.

[0060] If the discharge voltage of the RC circuit falls below the triggering threshold of the trigger component, the second switch is switched to an off state and so power to the heater is stopped. At the same time the trigger component is configured to provide a reset signal to the controller 18 so that the controller can then reset the first switch to an off state, allowing the RC circuit to recharge, which in turn switched the trigger component 38 off allowing the second switch 34 to be reset to an on state.

[0061] By using the predictable timing of the discharge of an RC circuit and selecting the resistance and capacitance values of the components carefully, this arrangement can be used to ensure that the second switch is always turned off before the heater is able to reach a dangerous or even undesirable temperature. The monitoring system can be implemented in a small package that consumes very little power.

[0062] FIG. 4 is an embodiment of a circuit of the type shown in FIG. 2 showing circuit components in greater detail. It can be seen in FIG. 4 that the first switch 32 is an n-channel MOSFET with the source connected to ground and the drain connected to the heater. The gate is connected to the controller through connection G1. A gate series resistor 62 is used to limit the current into the gate when the controller switches the gate. A pull-down resistor 64 is provided to hold the gate near the source voltage when the controller is resetting and the G1 input is not being driven.

[0063] Diode 40 is a Schottky diode that allows the RC circuit to charge during the inactive phase while not allowing it to discharge through the first switch in the active phase. A diode series resistor 42 is provided to limit the peak current through the diode 40 when charging the RC circuit, especially at start-up.

[0064] The RC circuit 36 comprises a timing network resistor 54 and a timing network capacitor 52, each connected to ground.

[0065] The trigger component 38 is a Schmitt trigger that has a negative going threshold for the input voltage from the RC network, below which it will provide a switching output to inverter 56. The inverter 56, powered by the battery voltage, is then used to pull the input to the gate of the second switch, which is a p-channel MOSFET, to the source voltage, blocking the second switch. In normal operation the inverter ensures the gate is provided with an inverted battery voltage (−V.sub.batt) so the second switch is on.

[0066] The controller is connected to the “Pwr ok” line 70. This allows the controller to monitor the output of the Schmitt trigger 38 and also allows the controller to disable the second switch by pulling the input to the inverter low through diode 72. A resistor 60 is provided for this purpose. Resistor 58 is a pull-down resistor ensuring that the input to the inverter 56 is low in case of a logic power supply failure.

[0067] Resistor 68 is a pull-up resistor ensuring that the gate of the second switch is pulled to the source voltage and keeps the switch blocked if the inverter 56 fails. Resistor 66 is a gate series resistor that limits the output current from the inverter 56.

[0068] It should be clear that, the exemplary embodiments described above illustrate but are not limiting. In view of the above discussed exemplary embodiments, other embodiments consistent with the above exemplary embodiments will now be apparent to one of ordinary skill in the art.