阅读说明：本技术 检测加热设备过热的方法和相应的控制单元 (Method for detecting overheating of heating device and corresponding control unit ) 是由 E.戈格莫斯 B.普泽纳特 于 2019-11-05 设计创作，主要内容包括：本发明涉及用于检测电加热设备过热的方法,该电加热设备包括多个电阻元件,该电阻元件被配置为根据设定点使用控制信号通过脉宽调制来供电,该方法包括以下步骤：-检测设定点,-检测占空比(PWM-system；PWM-subsystem),-根据设定点或者用于监控过热的发生率的至少一个参数的值来定义用于检测占空比的阈值,-将检测到的占空比值与检测阈值进行比较,以及-当检测到的占空比值达到检测阈值时检测过热的发生。本发明还涉及相应的控制单元。(The present invention relates to a method for detecting overheating of an 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 steps of: -detecting a set point, -detecting a duty cycle (PWM _ system; PWM _ subsystem), -defining a threshold value for detecting the duty cycle depending on the set point or a value of at least one parameter for monitoring the occurrence of overheating, -comparing the detected duty cycle value with the detection threshold value, and-detecting the occurrence of overheating when the detected duty cycle value reaches the detection threshold value. The invention also relates to a corresponding control unit.)

1. A method for detecting overheating of an electric heating device comprising a plurality of resistive elements configured to be powered by a voltage source, wherein said powering of said resistive elements is driven by a pulse width modulated driving signal depending on a power setpoint (P _ system _ target) or a temperature setpoint (T _ system _ target) or a resistance setpoint (R _ system _ target) or a setpoint (i _ system _ target) for the amplitude of the current, characterized in that said method comprises the steps of:

-recording said set point (P _) system _ target, T _) system _ target, R _) system _ target, i _ (sub) system _ target),

-recording the duty cycle (PWM _ system; PWM _ subsystem) of the pulse width modulated drive signal for a predefined number of resistive elements,

-a detection threshold (PWM system lim; PWM subsystem lim) defining the duty cycle of the pulse width modulated drive signal for 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 set point or a value of at least one parameter for monitoring the 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), and

-detecting overheating when the recorded value of the duty cycle reaches the defined duty cycle detection threshold.

2. The method of claim 1, comprising the additional step of measuring the value of the amplitude of the current (i _ system _ max; i _ subsystem _ max) flowing through a predefined number of resistive elements.

3. A method according to claim 2, wherein the at least one parameter for monitoring the overheating of the electric heating device depends on the magnitude (i _) system max of the electric current, the value of which is calculated when the at least one parameter is different from the magnitude of the electric current.

4. The method according to any one of the preceding claims, comprising the step of measuring the supply voltage (U _ battery).

5. The method according to claim 4, wherein the detection threshold (PWM system lim; PWM subsystem lim) for the duty cycle of the pulse width modulated drive signal for the predefined number of resistive elements is determined also as a function of the measured supply voltage (U _ battery).

6. The method according to any one of the preceding claims, wherein said power supply depends on a power setpoint (P _) system _ target, and wherein said detection threshold is defined as a function of said power setpoint (P _) system _ target.

7. Method according to the preceding claim, wherein the power setpoint (P _ system _ target) depends on a temperature setpoint (T _ sub) system _ target).

8. A method as claimed in claim 4, when combined with any preceding claim, wherein, when the at least one parameter is different from the magnitude of the current, the value of the at least one parameter is calculated from the measured supply voltage (U _ battery) and the magnitude (i _ system _ max; i _ subsystem _ max) measured from the current flowing through the predefined number of resistive elements.

9. The method of any one of the preceding claims, wherein the at least one parameter is selected from:

the resistance of a predefined number of resistive elements (R _ system; R _ subsystem),

a multiple or power of a magnitude (i _ system _ max; i _ subsystem _ max) of a current flowing through the predefined number of resistive elements, and

the electrical power (P _ system; P _ subsystem) of the predefined number of resistive elements.

10. The method of any one of the preceding claims, wherein the resistive element is a positive or negative temperature coefficient element.

11. A method as claimed in any preceding claim, wherein 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 resistive elements when the pulse width modulated drive signal is 100%.

12. The method of any one of the preceding claims, 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, independently defining a detection threshold for the duty cycle of the drive signal according to the nature and/or number of resistive elements of said subset.

13. A control unit for an electric heating device comprising a plurality of resistive elements configured to be supplied by a voltage source, said control unit being configured to generate a pulse width modulated driving signal for driving the supply of electric resistance elements as a function of a power setpoint (P) system _ target) or a temperature setpoint (T) system _ target) or a resistance setpoint (R) system _ target) or a setpoint (i) system _ target) for the amplitude of the electric current, characterized in that said control unit comprises at least one processing means for:

-recording said set points (P _) system _ target, T _) system _ target, R _) system _ target, i _ (sub) system _ target),

-recording the duty cycle (PWM _ system; PWM _ Subsystem) of the pulse width modulated drive signal for the predefined number of resistive elements,

-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 as a function of the setpoint or a value of at least one parameter for monitoring the overheating of the electric heating device, the detection threshold representing the 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), and

-detecting overheating when the recorded value of the duty cycle reaches a defined duty cycle detection threshold.

Technical Field

The present invention relates to the detection of overheating of an electric heating device for heating a fluid. Without limitation, the electrical heating device may be configured to heat, for example, an air flow for flowing through the heating device. The invention can be equally applied to high-voltage electric heating equipment and low-voltage electric heating equipment.

The invention applies in particular to motor vehicle heating and/or ventilation and/or air conditioning devices comprising such a heating device.

Background

Motor vehicles are generally equipped with heating and/or ventilation and/or air-conditioning devices intended to regulate the air thermal parameters of the air flow for delivery to the passenger compartment, in particular to regulate the temperature of the air flow. To this end, the apparatus generally comprises one or more heat treatment devices, in particular electrical heating devices (also called electrical radiators) for heating a fluid such as an air flow.

The electric heating device comprises an electric heating module. For example, the electric heating module may be arranged to be directly exposed to the air flow through the electric heating device.

According to one known solution, the heating module comprises a resistive element, for example with a Positive Temperature Coefficient (PTC), i.e. an element such as a PTC ceramic resistor.

This is a problem in that the resistance of the element varies greatly with temperature. More precisely, the ohmic value of the PTC resistive element increases very rapidly in the event of exceeding a preset temperature threshold.

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

This can be a problem in the main heating devices of vehicles, especially in the case of high-voltage electric heating devices, which can therefore be very powerful (powerful).

In the event of overheating, the device may reach a temperature limit at least one point at which the system operates properly. The PTC ceramic resistor is used to prevent excessive overheating, such as a possible fire, thereby securing safety of passengers.

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

Therefore, it is advantageous to control the temperature of the electrical heating device to avoid degrading surrounding components.

To this end, it is known to provide additional sensors, such as thermal probes, capable of directly measuring the temperature of the electrical heating device. Such a heat probe may, 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.

However, providing such additional sensors that directly measure temperature incurs additional cost, requires additional space on the printed circuit board, and increases the weight of the electrical heating apparatus. In addition, the overheating detected in this way depends on the distance between the sensor and the resistive element and generally also on the inertia of the system. Furthermore, this increases the likelihood of additional faults in the event of, for example, additional sensors failing.

Disclosure of Invention

It is an object of the present invention to at least partly alleviate these drawbacks of the prior art by providing an alternative solution that allows detecting overheating of an electric heating device.

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

the set point is recorded and the set point is recorded,

the duty cycles of the pulse width modulated drive signals for a predefined number of resistive elements are recorded,

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

comparing the recorded value of the duty cycle with a detection threshold, an

Detecting overheating when the recorded value of the duty cycle reaches a defined duty cycle detection threshold.

The method may further comprise one or more of the following features, implemented independently or in combination:

according to a preferred embodiment, the method comprises the additional step of measuring the magnitude of the current flowing through a predefined number of resistive elements.

The at least one parameter for monitoring the overheating of the electric heating device may depend on the magnitude of the electric current. The method may comprise the additional step of calculating a value of the at least one parameter when the at least one parameter is different from the magnitude of the current.

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

According to this embodiment, the detection threshold for the duty cycle of the pulse width modulated drive signal for a predefined number of resistive elements may also be determined from the measured supply voltage.

Preferably, the electrical supply (electrical supply) is dependent on the power set point.

The power set point itself may depend on the temperature set point.

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

The method may comprise the additional step of calculating the value of said at least one parameter when said at least one parameter is different from the magnitude of the current.

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

Alternatively or additionally, a value of the at least one parameter is calculated from the recorded duty cycle.

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

According to a variant embodiment, the resistive element is a positive temperature coefficient element. According to another variant embodiment, the resistive element is a negative temperature coefficient element.

According to another aspect of the invention, the measured current amplitude is the amplitude of the instantaneous current flowing through a predefined number of resistive elements when the pulse width modulated drive signal is 100%.

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

According to another aspect of the invention, at least two subsets of the independent resistive elements are independently driven by pulse width modulation of the power supply. The value of the at least one selected parameter may be calculated independently for each subsystem. Alternatively or additionally, the detection threshold of the duty cycle of the drive signal may be defined independently, depending on the nature and/or number of resistive elements of the subset.

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 powering of the resistive elements in dependence on a power setpoint, or a temperature setpoint, or a resistance setpoint, or a setpoint of a current amplitude. The control unit comprises at least one processing means for:

the set point is recorded and the set point is recorded,

the duty cycles of the pulse width modulated drive signals for a predefined number of resistive elements are recorded,

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

detecting overheating when the recorded value of the duty cycle reaches a detection threshold of the defined duty cycle.

Drawings

Other features and advantages of the invention will become more apparent from reading the following description, given by way of non-limiting illustrative example, and from the accompanying drawings, in which:

FIG. 1A shows a flow chart of the steps of a detection method according to a first embodiment;

FIG. 1B shows a flow chart of the steps of a detection method according to a second embodiment;

fig. 2 is a graph schematically showing an example of the variation of the duty ratio of the electric power and the pulse width modulation drive signal in the case where the air flow rate is decreased;

in the drawings, like elements are designated with like reference numerals.

Detailed Description

The following embodiments are examples. While this specification refers to one or more embodiments, this does not necessarily mean that each reference refers to the same embodiment, or that a feature only applies to one embodiment. Various features of the various embodiments may also be combined or interchanged to create further embodiments.

The present invention relates to the field of heating and/or ventilation and/or air-conditioning devices, motor vehicles being intended to be equipped with such a device in order to regulate the air thermal parameters of an air flow delivered to one or more zones of the passenger compartment of the vehicle.

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

This may be a problem in particular with high voltage radiators or electrical heating devices. Here, the high voltage is defined as a voltage higher than 90V or 120V. As a variant, it may be a problem with low voltage heat sinks.

The electric heating device is able to convert electric 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.

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 through the electric heating device.

More precisely, the heating modules may each comprise a Positive Temperature Coefficient (PTC) resistive element. The resistive element takes the form of a PTC ceramic resistor, for example. As a variant, this may be a problem with Negative Temperature Coefficient (NTC) resistive elements.

The electric heating device usually further 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 resistive element. The power supply circuit is mounted, for example, on a circuit board such as a printed circuit board (PCB using the well-known abbreviation).

For example, the power supply circuit comprises transistors (not shown), each transistor allowing or not allowing a current through a predefined number of heating modules.

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

With reference to fig. 1A or 1B, a method for detecting overheating of such an electrically heated device will now be described, which allows real-time detection of potential overheating of the device.

All of the heating modules may be monitored together or each subset of heating modules controlled by a transistor or transistors may be monitored independently. This allows in particular to detect 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 module may be dedicated to heating different zones of the passenger compartment).

According to a variant embodiment, a step E0 for activating or initializing the method may be provided.

Generally, the method comprises a step E1' of recording the set point. Preferably, this is a matter of power set point P _ (sub) system _ target. This may also be a problem with the temperature setpoint T _, or with the resistance setpoint R _, or, in fact, with the amplitude setpoint i _, system _ target. The prefix "sub" is written between parentheses to indicate that the setpoint may relate to one subsystem or all resistive elements.

The method may further comprise a step E1, wherein the supply voltage U _ battery is recorded or measured. This step E1 may be implemented using a sensor for measuring voltage. The supply voltage ucotton may be constant.

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 resistive elements of the electric heating device is recorded or measured. This is a problem of logging the consumption of current by the heating module for which it is desired to monitor a subset of its parameters. For example, the instantaneous current flowing through the resistive element is measured. This step E2 may be implemented using a sensor for measuring the current.

For example, when the pulse width modulated drive signal is 100%, the measured current is the maximum instantaneous or peak current.

In step E3, the duty cycle PWM _ system or PWM _ subsystem of the pulse width modulated drive signal for a predefined number of resistive elements is recorded. In the remainder of this description, a PWM system with "sub" between brackets indicates that the duty cycle of the pulse width modulated drive signal may refer to one subsystem or all resistive elements.

The method may comprise a step E4 (see fig. 1b) wherein at least one parameter value for monitoring the overheating of the electric heating device is calculated.

Advantageously, in order to monitor the overheating of the electric heating device, this parameter depends on the amplitude i _ subsystem _ max of the electric current flowing through a predefined number of resistive elements, or even on the amplitude i _ system _ max of the electric current flowing through all resistive elements. This step E4 may be implemented by a processing means, such as a computer. This may be a problem with the actual values of the parameters.

May be based on the magnitude i _ subsystem _ max of the current flowing through the predetermined number of resistive elements in step E2; i _ system _ max to calculate the value of the parameter. When this step E1 is implemented, the supply voltage U _ battery measured in step E1 may also be taken into account in the calculation of step E4.

Alternatively or additionally, the parameter may depend on the recorded value of the duty cycle PWM _ system of the pulse width modulated drive signal for a predefined number of resistive elements.

This step E4 may be performed for one or more subsystems, i.e. for one or more sets of heating modules controlled by one or more transistors, or for the entire system, i.e. for all resistive elements of all heating modules.

The parameter may be the resistance R _ system, R _ subsystem of a predefined number of resistive elements; an electrical power P _ system, P _ subsystem of a predefined number of resistive elements; the current amplitude i _ system _ max, i _ subsystem _ max flowing through a predefined number of resistive elements; or a multiple or power of the magnitude of the current flowing through a predefined number of resistive elements. In particular, when the selected parameter is not the amplitude of the current, the calculation step E4 is implemented.

Alternatively, the parameter may not depend on the magnitude of the current. This may be a temperature problem of the resistive element, for example.

A plurality of parameters may be used in a complementary manner to monitor the overheating of the electric heating device during implementation of the method.

In step E5, a detection threshold PWM _ system _ lim of the duty cycle of the pulse width modulated drive signal for a predefined number of resistive elements is defined, which detection threshold represents an overheating of the electric heating device.

As schematically shown in fig. 1a, the detection threshold PWM _ system _ lim of the duty cycle may be defined according to the set point (preferably the power set point P _ system _ target) recorded in step E1'.

As a variant, when step E1 is implemented beforehand, the detection threshold PWM _ system _ lim can be defined according to a pair consisting of the supply voltage U _ battery measured in E1 and the setpoint recorded in step E1', preferably the power setpoint P _ system _ target. In this case, both step E1 and step E1 'are implemented in advance, as schematically illustrated by the dashed arrow between E1 and E5 and the solid arrow between E1' and E5 in fig. 1 a.

Also as a variant, the duty cycle detection threshold PWM _ (sub) system _ lim may be defined according to the value of the selected parameter calculated in step E4, as schematically shown in fig. 1b, or according to the value of the current amplitude recorded in step E2.

The detection threshold may also be defined according to 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 implemented in advance, as schematically illustrated by the dashed arrow between E1 and E5 and the solid arrow between E4 and E5 in fig. 1 b. The detection threshold may also be defined in terms of a pair consisting of the supply voltage U _ battery measured in step E1 and the value of the current amplitude recorded in step E2.

In step E6, the value of the duty ratio PWM _ system recorded in step E3 is compared with the detection threshold PWM _ system _ lim defined or preset in step E5.

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

When the method takes one or more parameters into account, according to a first mode (first approach), the parameter may be the resistance of the heating module. In this case, in step E4, the resistance values R _ system, R _ subsystem of the predefined number of resistive elements may be calculated from the measured supply voltage U _ battery and from the measured current amplitudes i _ system _ max, i _ subsystem _ max. In the remainder of this description, R _ system with "sub" between brackets indicates that the resistance value may relate to one subsystem or all resistive elements. The duty detection threshold PWM _ system _ lim is determined in step E5 from the resistance R _ system of the predefined number of resistive elements calculated in step E4 and possibly also from the supply voltage U _ battery measured in step E1.

This value may be determined for one or more subsystems, i.e., one or more sets of heating modules controlled by one or more transistors, or for the entire system, i.e., all of the resistive elements of all of the heating modules.

In step E6, the value of the duty ratio PWM _ system recorded in step E3 is compared with the detection threshold PWM _ system _ lim thus determined in step E5.

According to a second way, the parameter may be the electrical power of a predefined number of resistive elements. The second mode may be implemented as a modification or addition to the first mode.

Only the differences from the first mode will be described in detail below. In step E4, the electrical power value P _ system, P _ subsystem for a predefined number of resistive elements may be calculated from the measured supply voltage U _ battery and from the magnitude i _ system _ max, i _ subsystem _ max of the measured current. For this second approach, the duty cycle recorded in step E3 is also taken into account in the electric power calculation in step E4. Specifically, the electric power may be calculated by calculating the product of the amplitude of the instantaneous current, the power supply voltage, and the duty ratio.

In the remainder of this description, P _ system with "sub" between brackets indicates that the electrical power value may relate to one subsystem or all resistive elements.

The duty detection threshold PWM _ system _ lim may be determined in step E5 from this value P _ system of the electrical power of the predefined number of resistive elements calculated in step E4, and possibly from the supply voltage U _ battery measured in step E1.

In step E6, the value of the duty ratio PWM _ system recorded in step E3 is compared with the detection threshold PWM _ system _ lim thus determined in step E5.

According to a third way, the parameter may be the magnitude of the current flowing through a predefined number of resistive elements. This third mode may be implemented as a modification or addition to the first mode and/or the second mode.

The third way (not shown in the figure) differs from the second way in that step E4 is not calculated, but instead the value of the parameter is measured in step E2. The duty detection threshold PWM _ system _ lim may be determined in step E5 from the magnitude i _ system _ max or i _ subsystem _ max measured in step E2, and possibly from the supply voltage U _ battery measured in step E1.

In step E6, the value of the duty ratio PWM _ system recorded in step E3 is compared with the detection threshold PWM _ system _ lim thus determined in step E5.

The parameter may also be a multiple or power of the magnitude of the current flowing through a predefined number of resistive elements. Non-exhaustive mention may be made of 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.

Finally, according to another approach, the value of such a parameter can for example be measured when it does not depend on the amplitude of the current, such as for example in the case of the temperature of a resistive element.

The general principle of such a method is schematically illustrated in a simplified manner in fig. 2. In this figure, the various phases of operation of an electric heating device comprising a predefined number of heating modules are shown, each comprising, for example, a Positive Temperature Coefficient (PTC) resistive element, as shown. The curves of electric power P, drive signal duty cycle PWM system and air flow F are schematically shown. In phase a, the plant operates without failure under normal use conditions, in particular in terms of air flow and air flow temperature. Phase B corresponds to a first drop in the air flow, represented by curve F. During this phase B, the duty cycle PWM system of the PWM drive signal is increased to avoid power drop, but does not reach the duty cycle detection threshold PWM system lim defined in step E5 (see also fig. 1a or 1B). In the example shown, this compensation allows to maintain the power during phase B, since the air flow rate is not too low.

The figure shows a second drop in air flow at the end of phase B. Again, the duty cycle PWM _ system of the pulse width modulated drive signal is increased to avoid a power drop. The duty ratio cannot be increased beyond the duty ratio detection threshold PWM _ (sub) system _ lim defined in step E5. When the duty cycle reaches the duty cycle detection threshold PWM _ (sub) system _ lim, this corresponds to detection of 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 larger than the duty cycle detection threshold.

Further, in the above description, steps E0 through E6 have been indexed as a first step, a second step, and so on. This is a simple indexing problem to distinguish and name the various steps of the method. Such indexing does not necessarily imply that one step takes precedence over another. The order of certain steps of the method may be reversed without departing from the scope of the specification. Such indexing also does not imply a chronological order. For example, some steps may be performed simultaneously.

The method according to one or other of the above variants may further comprise at least one verification step, wherein it is verified whether the criterion of the electric heating device represents a cold state.

This may occur, for example, at start-up, particularly when the resistance of the heating device is very high and the value of the duty cycle is so high that the detection threshold is exceeded. At this point it is incorrect to detect overheating while the device is still cold and the current is low, which will not allow the device to overheat.

The criterion is, for example, the temperature of the circuit board on which the supply circuit of the resistive element is mounted.

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

When the recorded temperature reaches or exceeds a predefined threshold value representing a minimum heating of the electrically heated device, this confirms that the device is ready to be detected.

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

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.

This may avoid false or untimely detection of overheating.

Such verification may be performed, for example, in step E0.

The method for detecting overheating as described above may be implemented by a control unit. In particular, the method for detecting overheating may be implemented by a control unit already used for controlling the heating module of the electric heating device.

Thus, 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 means for implementing the steps of the method described above.

In particular, the control unit comprises one or more processing means for recording the power setpoint P _) system _ target, or the temperature setpoint T _) system _ target, or the resistance setpoint R _) system _ target, or even the setpoint i (sub) system _ target of the amplitude of the current.

The control unit for example comprises a sensor for measuring a voltage in order to measure or record the supply voltage U _ battery.

The control unit for example comprises a sensor for measuring the current in order to measure or record the current i _) system max flowing through a predefined number of resistive elements or even all resistive elements.

The control unit for example comprises processing means for determining or recording the duty cycle PWM system of the pulse width modulated drive signal for a predefined number of resistive elements.

The control unit may further comprise one or more calculation means, for example, for calculating the value of at least one parameter for monitoring the overheating of the electric heating device from the amplitudes i _ system _ max, i _ subsystem _ max of the currents flowing through a predefined number of resistive elements, when this parameter is different from the current amplitudes, in particular on the basis of the current measurement i (sub) system _ max and a possible supply voltage U _ battery.

The calculation means may be further configured to define a detection threshold PWM system lim for the duty cycle of the pulse width modulated drive signals of a predefined number of resistive elements, the threshold representing an overheating of the electric heating device and being defined according to a set point or according to a value of at least one parameter for monitoring the overheating of the electric heating device depending on the current amplitude. The value may be pre-calculated or, as a variant, the value depends on the pair consisting of supply voltage and set point or on the value of the parameter.

The control unit for example comprises at least one comparator for comparing the recorded value of the duty cycle PWM system with a detection threshold PWM system lim.

The control unit may comprise a computing means or microprocessor for determining whether there is overheating based on the comparison. The microprocessor may specifically evaluate whether the recorded value of the duty cycle is higher than or equal to the defined duty cycle detection threshold.

The control unit may further comprise at least one processing means for verifying whether the criterion of the electrically heated device represents a cold state of the device.

For example, an additional temperature sensor (not shown in the figures) may be provided. The control unit may comprise the additional temperature sensor. Such a temperature sensor may be placed on a Printed Circuit Board (PCB) and may, for example, be soldered, soldered or glued to the Printed Circuit Board (PCB). This can be a problem for Negative Temperature Coefficient (NTC) thermal probes, whose resistance drops uniformly with temperature. Alternatively, this may be a problem with Positive Temperature Coefficient (PTC) thermal probes, whose resistance increases sharply with increasing temperature.

For example, the control unit may comprise a comparator for comparing the recorded circuit board temperature with a predefined threshold value representing a minimum heating of the electric heating device. As long as the recorded temperature does not reach the predetermined threshold, this represents a cold state of the device, and the control unit may comprise processing means for disabling the overheat detection.

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 to detect overheating in real time in an indirect manner when the duty cycle reaches the detection threshold. This makes it possible to prevent the electric heating device from becoming hot enough to risk damaging some of the surrounding components, even if a fire does not occur.

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