Method for detecting overheating of a heating device and corresponding control unit

Abstract

The invention relates to a method for detecting overheating of an electric heating device, the electric heating device comprising a plurality of resistive elements configured to be powered by pulse width modulation using a control signal according to a set point, the method comprising the following steps: - detecting the set point, - detecting a duty cycle (PWM_system; PWM_subsystem), - defining a threshold for detecting the duty cycle according to the set point or the value of at least one parameter for monitoring the occurrence of overheating, - comparing the detected duty cycle value with the detection threshold, and - detecting the occurrence of overheating when the detected duty cycle value reaches the detection threshold. The invention also relates to a corresponding control unit.

Description

Technical Field

[0001] The present invention relates to the detection of overheating of an electric heating device for heating a fluid. Without limitation, the electric heating device may be configured to heat, for example, an air flow passing through the heating device. The present invention may be equally applicable to high voltage electric heating devices and low voltage electric heating devices.

[0002] The invention applies in particular to a motor vehicle heating and / or ventilation and / or air-conditioning device which comprises such a heating device. Background Art

[0003] Motor vehicles are usually equipped with a heating and / or ventilation and / or air conditioning device which is intended to adjust the air thermal parameters of an air flow for delivery to the passenger compartment, in particular to adjust the temperature of the air flow. To this end, the device usually comprises one or more heat treatment devices, in particular electric heating devices (also called electric radiators) for heating a fluid such as an air flow.

[0004] The electric heating device comprises an electric heating module. For example, the electric heating module can be arranged to be directly exposed to the air flow flowing through the electric heating device.

[0005] According to a known solution, the heating module comprises a resistive element, for example with a positive temperature coefficient (PTC), ie an element such as a PTC ceramic resistor.

[0006] This is a problem where the resistance of a component varies greatly with temperature. More precisely, the ohmic value of a PTC resistor element increases very rapidly when a preset temperature threshold is exceeded.

[0007] The resistance element can be powered by an on-board voltage source, i.e. a battery. An electrical connector can be connected to a voltage source located on the vehicle in order to allow the required electrical power to be supplied to the electric heating device, in particular the resistance element. Furthermore, the resistance element is controlled by an electronic control unit, which usually comprises a power supply circuit. The power supply circuit is mounted, for example, on a printed circuit board.

[0008] Particularly in the case of a high voltage electric heater, this may be a problem for the vehicle's main heater, which may therefore be very powerful.

[0009] In the event of overheating, the device may reach the temperature limit for normal operation of the system at least at one point. PTC ceramic resistors are used to prevent excessive overheating, such as a possible fire, thereby ensuring the safety of passengers.

[0010] However, certain components close to the electrical heating device, such as for example plastic parts of the heating and / or ventilation and / or air-conditioning device, may be more sensitive, in particular under certain conditions, for example at high temperatures when shutters of the heating and / or ventilation and / or air-conditioning device are closed intentionally or due to an undetected mechanical malfunction.

[0011] It is therefore advantageous to control the temperature of the electrical heating device to avoid degradation of surrounding components.

[0012] For this purpose, it is known to provide an additional sensor, such as a thermal probe, which is able to directly measure the temperature of the electrical heating device. Such a thermal probe can, for example, be arranged in contact with the heating module or in an electronic control unit, in particular on a printed circuit board. Depending on the recorded temperature, the electrical power may be switched off or limited.

[0013] However, providing such an additional sensor for directly measuring the temperature generates additional costs, requires additional space on the printed circuit board and increases the weight of the electrical heating device. In addition, the overheating detected in this way depends on the distance between the sensor and the resistance element and usually also on the inertia of the system. In addition, this increases the probability of additional malfunctions occurring in the event of, for example, a malfunction of the additional sensor. Summary of the invention

[0014] The object of the present invention is to at least partially alleviate these disadvantages of the prior art by providing an alternative solution allowing to detect overheating of an electrical heating device.

[0015] To this end, the present invention relates to a method for detecting overheating of an electric heating device, the electric heating device comprising a plurality of resistive elements configured to be powered by a voltage source, wherein the power supply of the resistive elements is driven by a pulse width modulated drive signal according to a power set point, or a temperature set point, or a resistance set point, or a set point of current amplitude. The method comprises the following steps:

[0016] Recording the set point,

[0017] recording the duty cycle of the pulse width modulated drive signal for a predefined number of resistive elements,

[0018] defining a detection threshold of the duty cycle of the pulse width modulated drive signal of the predefined number of resistive elements, the detection threshold representing overheating of the electric heating device, said detection threshold being defined as a function of the measured supply voltage and / or said set point or the value of at least one parameter for monitoring overheating of said electric heating device,

[0019] comparing the recorded value of the duty cycle to a detection threshold, and

[0020] Overheating is detected when the recorded value of the duty cycle reaches a defined duty cycle detection threshold.

[0021] The method may also include one or more of the following features, implemented independently or in combination:

[0022] According to a preferred embodiment, the method comprises the additional step of measuring the amplitude of the current flowing through a predefined number of resistive elements.

[0023] The at least one parameter for monitoring overheating of the electric heating device may be dependent on the amplitude of the electric current.The method may comprise the additional step of calculating the value of the at least one parameter when the at least one parameter differs from the amplitude of the electric current.

[0024] According to one embodiment, the method comprises the step of measuring the supply voltage.

[0025] According to this embodiment, the detection threshold of the duty cycle of the pulse width modulation drive signal of the predefined number of resistance elements may also be determined according to the measured power supply voltage.

[0026] Preferably, the electrical supply depends on a power set point.

[0027] The power set point itself may be dependent on the temperature set point.

[0028] The detection threshold may be defined in terms of a power set point or a temperature set point.

[0029] The method may comprise the additional step of calculating the value of said at least one parameter when said at least one parameter differs from the magnitude of the electric current.

[0030] According to one aspect of the invention, the value of the at least one parameter may be calculated from a measured amplitude of a current flowing through a predefined number of resistive elements and optionally from a measured supply voltage.

[0031] As a variant or in addition, the value of the at least one parameter is calculated from the recorded duty cycle.

[0032] The at least one parameter may be selected from the resistance of a predefined number of resistance elements, the amplitude of a current flowing through a predefined number of resistance elements, a multiple or power of the amplitude of the current flowing through a predefined number of resistance elements, and the electrical power of a predefined number of resistance elements.

[0033] According to one variant embodiment, the resistance element is an element with a positive temperature coefficient. According to another variant embodiment, the resistance element is an element with a negative temperature coefficient.

[0034] According to another aspect of the present invention, when the pulse width modulated driving signal is 100%, the measured current amplitude is the amplitude of the instantaneous current flowing through a predefined number of resistance elements.

[0035] The method may comprise the steps of verifying whether at least one criterion of the device represents a cold state of the device and, when the at least one criterion represents a cold state, at least inhibiting the step of detecting overheating.

[0036] According to another aspect of the invention, at least two subsets of independent resistive elements are independently driven by pulse width modulation of the power supply. For each subsystem, the value of the at least one selected parameter can be calculated independently. As a variant or in addition, depending on the nature and / or number of the resistive elements of the subset, the detection threshold of the duty cycle of the drive signal can be independently defined.

[0037] The invention also relates to a control unit for an electric heating device, the control unit comprising a plurality of resistive elements configured to be powered by a voltage source, the control unit being configured to generate a pulse width modulated drive signal for driving the power supply of the resistive elements according to a power set point, or a temperature set point, or a resistance set point or a set point of current amplitude. The control unit comprises at least one processing device for:

[0038] Recording the set point,

[0039] recording the duty cycle of the pulse width modulated drive signal for a predefined number of resistive elements,

[0040] defining a detection threshold of the duty cycle of the pulse width modulated drive signal of the predefined number of resistive elements, which detection threshold is representative of overheating of the electric heating device and which detection threshold depends on the measured supply voltage and / or said set point or on the value of at least one parameter for monitoring overheating of the electric heating device, and comparing the recorded value of said duty cycle with the detection threshold, and

[0041] Overheating is detected when the recorded value of the duty cycle reaches a defined detection threshold of the duty cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Other characteristics and advantages of the invention will become more apparent on reading the following description, given by way of non-limiting illustrative examples, and from the accompanying drawings, in which:

[0043] FIG1A shows a flow chart of various steps of a detection method according to a first embodiment;

[0044] FIG1B shows a flow chart of various steps of the detection method according to the second embodiment;

[0045] Figure 2 is a graph schematically showing an example of changes in the electric power and the duty cycle of the pulse width modulation drive signal when the air flow rate decreases;

[0046] In these drawings, the same elements are designated by the same reference numerals. DETAILED DESCRIPTION

[0047] The following embodiments are examples. Although this specification refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment or that a feature applies to only one embodiment. Individual features of various embodiments may also be combined or interchanged to create other embodiments.

[0048] The invention relates to the field of heating and / or ventilation and / or air-conditioning devices with which motor vehicles are intended to be equipped in order to adjust the air thermal parameters of an air flow delivered to one or more regions of the vehicle's passenger compartment.

[0049] The invention relates more particularly to an electric heating device for a motor vehicle (also called an electric radiator), in particular equipped with such a device. This is a problem of an electrical device for heating a fluid. Without limitation, this may be a problem of a device for heating an air flow. In the following, the description is given with reference to an air flow, but the invention can be applied to another fluid.

[0050] Specifically, this may be a problem with a high voltage radiator or electrical heating device. Here, high voltage is defined as a voltage above 90 V or 120 V. As a variant, it may be a problem with a low voltage radiator.

[0051] The electric heating device is able to convert electrical energy, for example extracted from the vehicle, into thermal energy which is transferred to the air flowing through the heating and / or ventilation and / or air conditioning device 1 .

[0052] The electric heating device may comprise a predefined number of heating modules. These heating modules may be arranged to be directly exposed to the air flow flowing through the electric heating device.

[0053] More precisely, the heating modules can each include a resistance element with a positive temperature coefficient (PTC). For example, the resistance element takes the form of a PTC ceramic resistor. As a variant, this can be a problem of a resistance element with a negative temperature coefficient (NTC).

[0054] The electric heating device also usually comprises an electronic control unit for controlling the heating module. Such a control unit comprises one or more electronic and / or electrical components. The control unit comprises in particular a circuit (not shown) for supplying the resistance element. The supply circuit is for example mounted on a circuit board such as a printed circuit board (using the well-known abbreviation PCB).

[0055] By way of example, the power supply circuit comprises transistors (not shown), each of which allows or does not allow current to pass through a predefined number of heating modules.

[0056] The resistance element is intended to be powered by a power source (not shown), such as a battery of a vehicle, for example. The power supply of the resistance element is driven by pulse width modulation (or PWM using the well-known acronym). The control unit is configured to generate a pulse width modulation drive signal for driving the power supply of the resistance element. At least two independent subsets of the resistance element can be driven independently by pulse width modulation. The power supply of the resistance element can be performed according to an electric power set point. The device is controlled in a closed loop mode. As a variant, the power supply of the resistance element can be performed according to a temperature set point, or an optional resistance set point, or indeed a set point of the amplitude of the current.

[0057] With reference to FIG. 1A or FIG. 1B , a method for detecting overheating of such an electric heating device will now be described, which method allows for real-time detection of potential overheating of the device.

[0058] All heating modules can be monitored together or each subset of heating modules controlled by one transistor or multiple transistors can be monitored individually. This allows in particular the detection of various hot spots, for example when the electric heating device is installed in a so-called multi-zone heating and / or ventilation and / or air conditioning device (in which case the heating modules can be dedicated to heating different zones of the passenger compartment).

[0059] According to a variant embodiment, provision may be made for a step E0 of activating or initializing the method.

[0060] Typically, the method comprises a step E1 'of recording a set point. Preferably, this is a matter of a power set point P_(sub)system_target. It may also be a matter of a temperature set point T_(sub)system_target, or a resistance set point R_(sub)system_target, or indeed an amplitude set point i_(sub)system_target. The prefix "sub" is written between brackets to indicate that a set point may concern one subsystem or all resistance elements.

[0061] The method may further comprise a step E1, in which the supply voltage U_battery is recorded or measured. This step E1 may be implemented using a sensor for measuring voltage. The supply voltage U_battery may be constant.

[0062] The method may comprise a step E2 in which the value of the amplitude i_system_max or i_subsystem_max of the current flowing through a predefined number of resistive elements or even all the resistive elements of the electric heating device is recorded or measured. It is a matter of recording the consumption of the current of the heating module of the subset of parameters of which it is desired to monitor. For example, the instantaneous current flowing through the resistive elements is measured. This step E2 can be implemented using a sensor for measuring the current.

[0063] For example, when the pulse width modulation drive signal is 100%, the measured current is the maximum instantaneous current or peak current.

[0064] In step E3, the duty cycle PWM_system or PWM_subsystem of the pulse width modulated drive signal of the predefined number of resistor elements is recorded. In the rest of this description, PWM_(sub)system with "sub" between brackets indicates that the duty cycle of the pulse width modulated drive signal may relate to one subsystem or all resistor elements.

[0065] The method may include step E4 (see Figure 1b ), wherein at least one parameter value for monitoring overheating of the electric heating device is calculated.

[0066] Advantageously, in order to monitor overheating of the electric heating device, the parameter depends on the amplitude i_subsystem_max of the current flowing through a predefined number of resistance elements, or even on the amplitude i_system_max of the current flowing through all the resistance elements. This step E4 can be implemented by processing means such as a computer. It may be a question of the actual value of the parameter.

[0067] The value of the parameter may be calculated from the amplitude i_subsystem_max; i_system_max of the current flowing through the predetermined number of resistance elements in step E2. When implementing this step E1, the supply voltage U_battery measured in step E1 may also be taken into account in the calculation of step E4.

[0068] As a variant or in addition, the parameter may depend on the recorded value of the duty cycle PWM_(sub)system of the pulse width modulated drive signal of the predefined number of resistance elements.

[0069] This step E4 can be carried out for one or more subsystems, ie for one or more groups of heating modules controlled by one or more transistors, or for the entire system, ie for all the resistive elements of all the heating modules.

[0070] The parameter may be the resistance R_system, R_subsystem of a predefined number of resistance elements; the electric power P_system, P_subsystem of a predefined number of resistance elements; the current amplitude i_system_max, i_subsystem_max flowing through a predefined number of resistance elements; or a multiple or power of the current amplitude flowing through a predefined number of resistance elements. Specifically, when the selected parameter is not the amplitude of the current, calculation step E4 is implemented.

[0071] Alternatively, the parameter may not depend on the magnitude of the current. For example, it may be a matter of the temperature of the resistor element.

[0072] Several parameters may be used in a complementary manner to monitor overheating of the electrical heating device during implementation of the method.

[0073] In step E5 , a detection threshold PWM_(sub)system_lim of the duty cycle of the pulse width modulated drive signal of the predefined number of resistance elements is defined, which detection threshold represents overheating of the electric heating device.

[0074] As in Figure 1a As schematically shown, the duty cycle detection threshold PWM_(sub)system_lim can be defined based on the set point recorded in step E1 ′, preferably the power set point P_(sub)system_target.

[0075] As a variant, when step E1 is implemented beforehand, the detection threshold PWM_(sub)system_lim can be defined as a pair consisting of the supply voltage U_battery measured in E1 and the set point (preferably the power set point P_(sub)system_target) recorded in step E1'. In this case, both step E1 and step E1' are implemented beforehand, as in Figure 1a The dashed arrow between E1 and E5 and the solid arrow between E1' and E5 are schematically shown.

[0076] Also as a variant, the duty cycle detection threshold PWM_(sub)system_lim can be defined according to the value of the selected parameter calculated in step E4, as in Figure 1b , or defined according to the value of the current amplitude recorded in step E2.

[0077] The detection threshold can also be defined as a pair consisting of the supply voltage U_battery measured in step E1 and the value of the selected parameter calculated in step E4. In this case, both step E1 and step E1' are pre-implemented, as in Figure 1b The dashed arrow between E1 and E5 and the solid arrow between E4 and E5 are schematically shown in FIG. The detection threshold can also be defined as a pair consisting of the value of the supply voltage U_battery measured in step E1 and the current amplitude recorded in step E2 .

[0078] In step E6 , the value of the duty cycle PWM_(sub)system recorded in step E3 is compared with the detection threshold PWM_(sub)system_lim defined or preset in step E5 .

[0079] This step E6 can be implemented by a processing device such as a comparator. According to the comparison result, an overheating can be detected. In other words, if the recorded value of the duty cycle has reached or exceeded a defined duty cycle detection threshold, this corresponds to an overheating of the device. According to the nature of this parameter and the nature of the resistive element, the recorded value of the duty cycle can cross the detection threshold by becoming higher or lower than the detection threshold. In this case, one or more actions for this overheating can be implemented, which actions will not be described in detail below. In the opposite case, the steps of the method can be repeated until an overheating is detected in step E6.

[0080] When the method takes into account one or more parameters, according to a first approach, the parameter can be the resistance of the heating module. In this case, in step E4, the resistance values ​​R_system, R_subsystem of a predefined number of resistance elements can be calculated based on the measured supply voltage U_battery and based on the measured current amplitudes i_system_max, i_subsystem_max. In the remainder of this specification, R_(sub)system is accompanied by "sub" between brackets to indicate that the resistance value can relate to one subsystem or all resistance elements. The duty cycle detection threshold PWM_(sub)system_lim is determined in step E5 based on the resistance R_(sub)system of the predefined number of resistance elements calculated in step E4, and possibly also based on the supply voltage U_battery measured in step E1.

[0081] This value may be determined for one or more subsystems, ie one or more groups of heating modules controlled by one or more transistors, or for the entire system, ie all resistive elements of all heating modules.

[0082] In step E6 , the value of the pulse duty factor PWM_(sub)system recorded in step E3 is compared with the detection threshold PWM_(sub)system_lim thus determined in step E5 .

[0083] According to a second approach, the parameter may be the electrical power of a predefined number of resistance elements. The second approach may be implemented as a variation or addition to the first approach.

[0084] Only the differences from the first approach are explained in detail below. In step E4, the electric power values ​​P_system, P_subsystem of the predefined number of resistance elements can be calculated from the measured supply voltage U_battery and from the measured current amplitudes i_system_max, i_subsystem_max. For this second approach, the duty cycle recorded in step E3 is also taken into account in the electric power calculation in step E4. In particular, the electric power can be calculated by calculating the product of the amplitude of the instantaneous current, the supply voltage and the duty cycle.

[0085] In the rest of this description, P_(sub)system with "sub" between brackets indicates that the electrical power value may relate to one subsystem or to all resistive elements.

[0086] The duty cycle detection threshold PWM_(sub)system_lim can be determined in step E5 from the value P_(sub)system of the electrical power of the predefined number of resistance elements calculated in step E4 and possibly from the supply voltage U_battery measured in step E1 .

[0087] In step E6 , the value of the pulse duty factor PWM_(sub)system recorded in step E3 is compared with the detection threshold PWM_(sub)system_lim thus determined in step E5 .

[0088] According to a third approach, the parameter may be the amplitude of the current flowing through the predefined number of resistance elements. The third approach may be implemented as a variation or addition to the first approach and / or the second approach.

[0089] The third method (not shown in the figure) differs from the second method in that there is no calculation step E4, but instead the value of the parameter is measured in step E2. The duty cycle detection threshold PWM_(sub)system_lim can be determined in step E5 according to the amplitude i_system_max or i_subsystem_max measured in step E2, and may be determined according to the power supply voltage U_battery measured in step E1.

[0090] In step E6 , the value of the pulse duty factor PWM_(sub)system recorded in step E3 is compared with the detection threshold PWM_(sub)system_lim thus determined in step E5 .

[0091] The parameter may also be a multiple or power of the amplitude of the current flowing through a predefined number of resistive elements. The square or cube of the current amplitude, twice the current amplitude or even the ratio of the current amplitude to the duty cycle of the pulse width modulated drive signal may be mentioned non-exhaustively.

[0092] Finally, according to another approach, when a parameter does not depend on the amplitude of the current, such as for example the temperature of a resistive element, it is possible to measure the value of such a parameter, for example.

[0093] The general principle of this method is Figure 2 , schematically illustrated in a simplified manner. In the figure, various phases of operation of an electric heating device comprising a predefined number of heating modules are illustrated, each of which, as shown, comprises a resistive element, for example, with a positive temperature coefficient (PTC). Curves for the electric power P, the duty cycle of the drive signal PWM_(sub)system and the air flow F are schematically illustrated. In phase A, the device operates without faults under normal conditions of use, in particular with regard to the air flow rate and the air flow temperature. Phase B corresponds to a first drop in the air flow rate, which is represented by curve F. During this phase B, the duty cycle PWM_(sub)system of the pulse width modulated drive signal is increased in order to avoid a power drop, but does not reach the duty cycle detection threshold PWM_(sub)system_lim defined in step E5 (see also Figure 1a or Figure 1b ). In the example shown, this compensation allows the power to be maintained during phase B, since the air flow rate is not too low.

[0094] The figure shows a second drop in the air flow at the end of phase B. Again, the duty cycle PWM_(sub)system of the pulse width modulated drive signal is increased to avoid a power drop. The duty cycle cannot be increased beyond the duty cycle detection threshold PWM_(sub)system_lim defined in step E5. When the duty cycle reaches this duty cycle detection threshold PWM_(sub)system_lim, this corresponds to the detection of an overheating of the device in step E6. In this example, the recorded value of the duty cycle crosses the defined duty cycle detection threshold from below, i.e. by becoming greater than the duty cycle detection threshold.

[0095] Furthermore, in the above description, steps E0 to E6 have been indexed as first step, second step, etc. This is a simple matter of indexing to distinguish and name the various steps of the method. Such indexing does not necessarily mean that one step takes precedence over another. The order of certain steps of the method may be reversed without departing from the scope of this specification. Such indexing does not imply a chronological order either. For example, some steps may be performed simultaneously.

[0096] The method according to one or another of the above mentioned variants may also comprise at least one verification step, in which it is verified whether the standard of the electrical heating device represents a cold state.

[0097] This can happen for example at start-up, especially when the resistance of the heating device is very high and the value of the duty cycle is so high that it exceeds the detection threshold. It would be incorrect to detect overheating at this time, while the device is still cold and the current is low, which would not allow the device to overheat.

[0098] The criterion is, for example, the temperature of a circuit board of the power supply circuit on which the resistor element is mounted.

[0099] During the verification step, the temperature of the circuit board is recorded, for example via a temperature sensor such as a thermal probe with a negative temperature coefficient.

[0100] When the recorded temperature reaches or exceeds a predefined threshold value representing minimum heating of the electrical heating device, this confirms that the device is ready to be tested.

[0101] In the opposite case, this represents a cold state or "overheating" of the device. The method may include the step of inhibiting at least the step of detecting overheating as long as a criterion such as the temperature of the circuit board represents the cold state.

[0102] In the case of a thermal probe with a negative temperature coefficient, the predefined threshold may be a minimum value below which no attempt is made to detect overheating.

[0103] This avoids false or untimely detection of overheating.

[0104] Such a verification can be performed, for example, in step E0 .

[0105] The method for detecting overheating as described above can be implemented by a control unit. Specifically, the method for detecting overheating can be implemented by a control unit that has been used to control a heating module of an electric heating device.

[0106] Therefore, the control unit is configured to monitor overheating according to the above-described detection method. To this end, the control unit comprises at least one processing device for implementing the steps of the above-described method.

[0107] In particular, the control unit comprises one or more processing means for recording a power set point P_(sub)system_target, or a temperature set point T_(sub)system_target, or a resistance set point R_(sub)system_target, or even a set point i_(sub)system_target of the amplitude of the current.

[0108] The control unit comprises, for example, a sensor for measuring a voltage in order to measure or record the supply voltage U_battery.

[0109] The control unit comprises, for example, a sensor for measuring current in order to measure or record the current i_(sub)system_max flowing through a predefined number of resistance elements or even all resistance elements.

[0110] The control unit comprises, for example, processing means for determining or recording a duty cycle PWM_(sub)system of a pulse width modulated drive signal for a predefined number of resistor elements.

[0111] The control unit may further comprise one or more calculation means, for example for calculating the value of at least one parameter for monitoring overheating of the electric heating device as a function of the amplitude i_system_max, i_subsystem_max of the current flowing through a predefined number of resistance elements, when this parameter differs from the current amplitude, in particular for calculating this parameter based on the measurement result of the current i_(sub)system_max and a possible supply voltage U_battery.

[0112] The calculation means may also be configured to define a detection threshold PWM_(sub)system_lim of the duty cycle of the pulse width modulated drive signal of the predefined number of resistive elements, which threshold represents overheating of the electric heating device and is defined as a function of a set point or as a function of the value of at least one parameter for monitoring overheating of the electric heating device depending on the current amplitude. This value may be precalculated or, as a variant, depend on a pair consisting of a supply voltage and a set point, or depend on the value of this parameter.

[0113] The control unit comprises, for example, at least one comparator for comparing the recorded value of the duty cycle PWM_(sub)system with a detection threshold PWM_(sub)system_lim.

[0114] The control unit may comprise a calculation device or a microprocessor for determining whether an overheating is present based on the comparison result. The microprocessor may in particular evaluate whether the recorded value of the duty cycle is higher than or equal to the defined duty cycle detection threshold.

[0115] The control unit may also comprise at least one processing means for verifying whether the standard of the electrical heating device represents a cold state of the device.

[0116] For example, an additional temperature sensor (not shown in the figure) may be provided. The control unit may include the additional temperature sensor. Such a temperature sensor may be placed on a printed circuit board (PCB) and may, for example, be welded, soldered or bonded to the printed circuit board (PCB). This may be a problem for negative temperature coefficient (NTC) thermoprobes, whose resistance decreases uniformly with temperature. Alternatively, this may be a problem for positive temperature coefficient (PTC) thermoprobes, whose resistance increases sharply with increasing temperature.

[0117] For example, the control unit may include a comparator for comparing the recorded circuit board temperature with a predefined threshold representing a minimum heating of the electrical heating device. As long as the recorded temperature does not reach this predetermined threshold, this represents a cold state of the device, and the control unit may include processing means for inhibiting overheating detection.

[0118] Thus, by actively defining a detection threshold for the duty cycle of the pulse width modulated drive signal, the method according to the invention enables overheating to be detected in an indirect manner in real time when the duty cycle reaches the detection threshold. This makes it possible to prevent the electrical heating device from becoming hot enough to risk damaging certain surrounding components even without ignition.

[0119] Furthermore, no additional sensors are required to monitor the temperature of the electrical heating device.

Claims

1. A method for detecting overheating of an electric heating device, the electric heating device comprising a plurality of resistive elements configured to be powered by a voltage source, wherein the power supply of the resistive elements is driven by a pulse width modulated drive signal depending on a power set point (P_(sub)system_target) or a temperature set point (T_(sub)system_target) or a resistance set point (R_(sub)system_target) or a set point for the amplitude of the current (i_(sub)system_target), characterized in that The method comprises the following steps: - recording the power set point (P_(sub)system_target), the temperature set point (T_(sub)system_target), the resistance set point (R_(sub)system_target) or the set point for the amplitude of the current (i_(sub)system_target), - recording the duty cycle of the pulse width modulated drive signal of a predefined number of resistor elements (PWM_system; PWM_subsystem), - defining a detection threshold (PWM_system_lim; PWM_subsystem_lim) of the duty cycle of the pulse width modulated drive signal of the predefined number of resistive elements, the detection threshold representing an overheating of the electric heating device, the detection threshold being defined as a function of the power set point (P_(sub)system_target), the temperature set point (T_(sub)system_target), the resistance set point (R_(sub)system_target) or the set point for the amplitude of the current (i_(sub)system_target) or the value of at least one parameter for monitoring overheating of the electric heating device, - comparing the recorded value of the duty cycle (PWM_system; PWM_subsystem) with the detection threshold (PWM_system_lim; PWM_subsystem_lim), - detecting overheating when said recorded value of said duty cycle reaches said defined duty cycle detection threshold, and - verifying whether at least one criterion of said device represents a cold state of said device and, when said at least one criterion represents a cold state, inhibiting at least the step of detecting overheating, wherein the resistance element is a positive temperature coefficient element, and Therein, the control unit for the electrical heating device comprises an additional temperature sensor in order to verify whether at least one criterion of the device represents a cold state of the device.

2. A method as claimed in claim 1, comprising the additional step of measuring the value (i_system_max; i_subsystem_max) of the amplitude of the current flowing through a predefined number of resistive elements.

3. The method of claim 2, wherein: The at least one parameter for monitoring overheating of the electric heating device depends on the magnitude of the electric current (i_(sub)system_max), the value of the at least one parameter being calculated when the at least one parameter differs from the magnitude of the electric current.

4. The method according to any one of claims 1 to 3, comprising the step of measuring the supply voltage (U_battery) of the voltage source.

5. The method of claim 4, wherein: The detection threshold (PWM_system_lim; PWM_subsystem_lim) of the duty cycle of the pulse width modulated drive signal of the predefined number of resistor elements is also determined based on the measured supply voltage (U_battery).

6. The method according to any one of claims 1 to 3, wherein: The power supply depends on a power set point (P_(sub)system_target), and wherein the detection threshold is defined according to the power set point (P_(sub)system_target).

7. The method of claim 6, wherein: The power set point (P_(sub)system_target) depends on the temperature set point (T_(sub)system_target).

8. The method of claim 4, wherein: When the at least one parameter is different from the amplitude of the current, the value of the at least one parameter is calculated from the measured supply voltage (U_battery) and from the amplitude (i_system_max; i_subsystem_max) measured from the current flowing through the predefined number of resistive elements.

9. The method according to any one of claims 1 to 3, wherein: The at least one parameter is selected from: the resistance of a predefined number of resistor elements (R_system; R_subsystem), the magnitude of the current flowing through the predefined number of resistance elements (i_system_max; i_subsystem_max), a multiple or power of the magnitude of the current flowing through the predefined number of resistance elements (i_system_max; i_subsystem_max), and The electrical power of the predefined number of resistance elements (P_system; P_subsystem).

10. The method according to any one of claims 1 to 3, wherein: When the pulse width modulated drive signal is 100%, the measured magnitude of the current (i_system_max; i_subsystem_max) is the magnitude of the instantaneous current flowing through the predefined number of resistance elements.

11. The method according to any one of claims 1 to 3, wherein: - at least two subsets of independent resistive elements are independently driven by pulse width modulation of the power supply, and wherein - For each subsystem, a detection threshold of the duty cycle of the drive signal is defined independently according to the nature and / or the number of resistive elements of said subset.

12. A control unit for an electric heating device, the electric heating device comprising a plurality of resistive elements configured to be powered by a voltage source, the control unit being configured to generate a pulse width modulated drive signal for driving the power supply of the resistive elements according to a power set point (P_(sub)system_target) or a temperature set point (T_(sub)system_target) or a resistance set point (R_(sub)system_target) or a set point for the amplitude of the current (i_(sub)system_target), characterized in that The control unit comprises at least one processing device for: - recording the power set point (P_(sub)system_target), the temperature set point (T_(sub)system_target), the resistance set point (R_(sub)system_target) or the set point for the amplitude of the current (i_(sub)system_target), - recording the duty cycle (PWM_system) of the pulse width modulated drive signal of a predefined number of resistor elements; PWM_Subsystem), - defining a detection threshold (PWM_system_lim) of the duty cycle of the pulse width modulated drive signal of the predefined number of resistive elements according to the set point or the value of at least one parameter for monitoring overheating of the electric heating device; PWM_subsystem_lim), the detection threshold represents overheating of the electric heating device, - Compare the recorded value of the duty cycle (PWM_system; PWM_subsystem) with the detection threshold (PWM_system_lim; PWM_subsystem_lim), and - detecting overheating when the recorded value of the duty cycle reaches a defined duty cycle detection threshold, and, wherein the resistance element is a positive temperature coefficient element, and The control unit comprises an additional temperature sensor for verifying whether the standard of the electrical heating device represents a cold state of the device.

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