Method for determining current temperature of PTC heater

Abstract

The invention relates to a method for determining a current temperature of a PTC heater, in particular to a method (1) for determining a current temperature (T) of a PTC heater (2) having at least one PTC element (3) by means of a control unit (4) during operation of the PTC heater (2). A capacitance (C) of the PTC element (3) is determined from the measured characteristic value of the PTC element (3), and a capacitance determination value of a current temperature (T) of the PTC element (3) is determined from the capacitance (C). The invention also relates to a PTC heater (2) having at least one PTC element (3) and a control unit (4) for carrying out the method (1).

Description

Technical Field

[0001] This invention relates to a method for determining the current temperature of a PTC heater during operation by means of a control unit using the general terminology of claim 1. The invention also relates to a PTC heater having at least one PTC element and a control unit for performing the method. Background Technology

[0002] PTC heaters (PTC: Positive Temperature Coefficient) typically consist of several PTC elements and can be used, for example, in motor vehicles to heat passenger cabin air. If a PTC heater operates at high power, it heats up, and materials around the heater may be damaged. Furthermore, the heated passenger cabin air may harm passengers. To avoid this, the PTC heater is typically controlled by measuring or calculating its actual temperature. Measuring the actual temperature of the PTC heater usually involves using a temperature sensor. Temperature sensors are disadvantageously expensive and have a certain degree of thermal inertia. Calculating the actual temperature of the PTC heater involves measuring its temperature-dependent resistance and using a resistance-temperature characteristic curve from this resistance. Disadvantageously, the resistance-temperature characteristic curve is not linear. Specifically, the resistance-temperature characteristic curve can be flat within a certain range, meaning the resistance changes slightly with increasing temperature. Within this range, the temperature of the PTC heater calculated based on the resistance can have a high tolerance. Summary of the Invention

[0003] Therefore, an object of the present invention is to provide improved or at least alternative embodiments for methods of the aforementioned type, which overcome the aforementioned disadvantages. A further object of the present invention is to provide a PTC heater having a control unit for performing the method.

[0004] According to the invention, this problem is solved by the subject matter of the independent claim. Advantageous embodiments are the subject matter of the dependent claims.

[0005] The present invention is based on the basic idea of ​​calculating the temperature of a PTC element and thus the PTC heater by using the temperature-dependent permittivity of the PTC element with a PTC heater.

[0006] The method according to the invention is provided for determining the current temperature of a PTC heater having at least one PTC element by means of a control unit during operation of the PTC heater. During operation of the PTC heater, a pulse width modulation input voltage is applied to the PTC heater, and characteristic values ​​of one of the PTC elements of the PTC heater are measured. According to the invention, the capacitance of the PTC element is also calculated from at least some characteristic values. The capacitance value of the PTC element, and thus the current temperature of the PTC heater, is then determined from the calculated capacitance.

[0007] In the context of this invention, the terms "resistance determination" and "capacitance determination" are used only to distinguish the corresponding values ​​at the current temperature from each other. The resistance determination value at the current temperature may be referred to as the "first value," and the capacitance determination value at the current temperature may be referred to as the "second value."

[0008] The corresponding PTC element of the PTC heater has not only a temperature-dependent resistance but also a temperature-dependent permittivity. Since the permittivity is related to the capacitance of the PTC element, the current temperature of the PTC element can be determined based on its capacitance. In other words, the PTC element of the PTC heater is characterized not only by a unique resistance-temperature characteristic curve but also by a unique capacitance-temperature characteristic curve. In the method according to the invention, the current temperature of the PTC element is determined based on the capacitance, i.e., the capacitance-temperature characteristic curve. As a result, the current temperature of the PTC element can be determined with high accuracy over the entire operating temperature range of the PTC heater.

[0009] When determining the capacitance value for the current temperature, the capacitance value can be read from a table of values ​​determined for the PTC heater as a function of capacitance. Before installing and operating the PTC heater, a table of values ​​can be created for the PTC heater with its specific PTC element. The capacitance value related to the current temperature can be stored in the table of values. To create the table of values, before installing and operating the PTC heater, the capacitance value related to the current temperature can be measured, for example, using a measuring unit. By using the table of values, it can be ensured that the capacitance value for the current temperature can be determined and read with sufficient accuracy.

[0010] Furthermore, the resistance of the PTC element can be calculated from at least some of its characteristic values, and the resistance of the PTC element, and thus the resistance of the PTC heater at its current temperature, is determined from the calculated resistance. In this method, the current temperature of the PTC element can be determined based on both the resistance (resistance-temperature characteristic curve) and the capacitance (capacitance-temperature characteristic curve). In this case, the current temperature of the PTC element can be determined based on both values, thus achieving high accuracy across the entire operating temperature range of the PTC heater.

[0011] The resistance value at the current temperature can be determined within a first range of the operating temperature of the PTC heater, and the capacitance value at the current temperature can be determined within a second range of the operating temperature of the PTC heater. In this second range, the first and second operating temperature ranges of the PTC heater can overlap. In other words, the capacitance and resistance values ​​at the current temperature can be determined within different ranges. For example, in a range where the resistance-temperature characteristic curve is flat and the resistance value can only be determined with a high tolerance, the capacitance value can be determined in addition to the resistance value. Since the resistance-temperature characteristic curve and capacitance-temperature characteristic curve of the same PTC element are different, the capacitance value within this range can be determined with a tolerance relatively smaller than the resistance value, and therefore the overall tolerance can be reduced by determining the current temperature within this range. The first and second ranges can be defined before the installation and use of the PTC heater and depend on the characteristics of the PTC heater, i.e., the PTC element.

[0012] The resistance determination value at the current temperature can be determined across the entire operating temperature range of the PTC heater. Then, the capacitance determination value at the current temperature can be determined within a range where the tolerance of the resistance determination value at the current temperature exceeds a predefined tolerance value. Depending on the characteristics of the PTC heater (i.e., the PTC element), this range can be defined before the PTC heater is installed and put into use. Therefore, the capacitance determination value at the current temperature can be determined within a range where the resistance-temperature characteristic is flat and the resistance determination value can only be determined with a high tolerance. Since the resistance-temperature characteristic curve and capacitance-temperature characteristic curve of the same PTC element are different, the capacitance determination value within this range can be determined with a relatively smaller tolerance than the resistance determination value, and therefore the overall tolerance can be reduced by determining the current temperature within this range.

[0013] The operating temperature of a PTC heater is the temperature that the PTC heater (i.e., its PTC element) can maintain during the operation of the PTC heater. For example, the operating temperature can vary between -25°C (by starting the operation of the PTC heater) and 200°C (by operating the PTC heater at maximum heating power).

[0014] When the resistance at the current temperature is determined, the resistance value can be read from a table of values ​​for the PTC heater as a function of resistance. Before installing and operating the PTC heater, a table of values ​​can be created for the PTC heater with its specific PTC element. The resistance at the current temperature and related resistance values ​​can be stored in the table of values. To create the table of values, the resistance at the current temperature and related resistance values ​​can be measured, for example, using a measuring unit before installing and operating the PTC heater. By using the table of values, it can be ensured that the resistance value at the current temperature can be determined and read with sufficient accuracy.

[0015] In this method, a pulse-width modulated input voltage is provided to the PTC heater and correspondingly to its PTC element. The input voltage is a DC voltage pulse-width modulated using a PWM signal. Specifically, the PWM signal can be a square wave signal. The heating power of the PTC heater can then be controlled by the duty cycle of the PWM signal. The control unit can have a corresponding switch, such as a semiconductor switch like an IGBT—which turns the input voltage on and off according to the PWM signal.

[0016] When the PTC heater is operating with a PWM signal duty cycle below 100%, individual voltage pulses of the pulse width modulated input voltage are applied to the PTC heater and correspondingly to its PTC element. The voltage level of each voltage pulse corresponds to the input voltage, and the duration and frequency of each voltage pulse correspond to the duration and frequency of the PWM signal. At the PTC heater and its PTC element, the rise of the pulse width modulated input voltage and the application of the input voltage and the fall of the pulse width modulated input voltage follow each other with each voltage pulse.

[0017] When a pulse-width modulated input voltage is applied to a PTC element, the resistance of the PTC element can be calculated using Ohm's equation. Specifically, when a pulse-width modulated input voltage is applied to a PTC element, both voltage and current can be measured simultaneously with the characteristic values ​​of the PTC element. The resistance of the PTC element can then be calculated using Ohm's equation based on the measured current and voltage. According to Ohm's equation, resistance corresponds to the ratio of voltage to current, thus allowing for easy calculation of the resistance. As described above, the calculated resistance can then be used to determine the resistance value at the current temperature.

[0018] During the charging of a PTC element when the pulse-width modulated input voltage at the PTC element increases, the capacitance of the PTC element can be calculated using a capacitance-related current-charge curve. Specifically, when the pulse-width modulated input voltage at the PTC element increases, the peak current and peak voltage, as well as the charging time, can be measured as characteristic measurements of the PTC element. Then, the capacitance of the PTC element can be calculated using the capacitance-related current-charge curve based on the peak current and the voltage change over a defined time interval. The voltage change can be calculated based on the measured peak voltage, and the defined time interval dt can be calculated based on the measured, i.e., detected charging time. According to the capacitance-related current-charge curve, the following applies:

[0019] ,

[0020] Where I is the peak current, C is the capacitance of the PTC element, dt is the defined time interval, and dV is the voltage change during the defined time interval dt. The capacitance can be easily calculated from the capacitance-related current-charge curve. As described above, the calculated capacitance can then be used to determine the capacitance value at the current temperature.

[0021] When the pulse width modulation input voltage at the PTC element decreases, the capacitance of the PTC element can be calculated using a capacitance-dependent voltage-discharge curve during the discharge period of the PTC element. Specifically, when the pulse width modulation input voltage at the PTC element decreases, a first voltage can be measured at a first time, and a second voltage can be measured at a second time. The capacitance of the PTC element can then be calculated based on the first and second voltages using the capacitance-dependent voltage-discharge curve. According to the capacitance-dependent voltage-discharge curve, the following applies:

[0022] ,

[0023] Where U(t) is the first / second voltage, U0 is the input voltage, R is the resistance of the PTC element, C is the capacitance of the PTC element, and t is the first / second time. Therefore, the resistance R can be a previously measured value or a known value. The capacitance can be easily calculated using the voltage-discharge curve associated with the capacitance. As described above, the calculated capacitance can then be used to determine the capacitance value at the current temperature. To simplify the determination of the capacitance, the first voltage can be measured at the PTC element at the start of the pulse width modulation input voltage decrease.

[0024] The present invention also relates to a PTC heater having at least one PTC element and a control unit. A control unit is provided to perform the above-described method. In particular, the control unit may include software and / or hardware capable of enabling the method to be executed.

[0025] In a first embodiment of the control unit, the control unit may include a first measuring circuit for detecting current values ​​and a second measuring circuit for detecting peak values ​​of current and / or voltage at one of the PTC elements. The measuring circuits can be designed such that, as the pulse-width modulated input voltage at the PTC element increases, the peak values ​​of the current and voltage, as well as the charging time, can be measured. The control unit may also include a microcontroller for calculating the capacitance of the PTC element. The microcontroller can be designed such that, based on the peak values ​​of the current and voltage and the charging time, it can use a capacitance-related current-charge curve to calculate the capacitance of the PTC element, and from the capacitance, determine a capacitance value for the PTC element and therefore the current temperature of the PTC heater.

[0026] In a second embodiment of the control unit, the control unit can include a measurement circuit for detecting the voltage at one of the PTC elements and a switch for turning off the measurement circuit. The measurement circuit and switch can be designed such that a first voltage can be measured at a first time when the pulse width modulation input voltage at the PTC element decreases, and a second voltage can be measured at a second time. Furthermore, the control unit can include a microcontroller for calculating the capacitance of the PTC element. The microcontroller can be designed such that the capacitance of the PTC element can be calculated based on the first and second voltages using a capacitance-discharge curve, and a capacitance determination value for the PTC element and therefore the current temperature of the PTC heater can be determined from the capacitance.

[0027] In a third embodiment of the control unit, the control unit may include measurement circuitry for detecting current and / or voltage at one of the PTC elements. The measurement circuitry can be designed such that when a pulse-width modulated input voltage is applied to the PTC element, both voltage and current can be measured simultaneously at the PTC element. Furthermore, the control unit may include a microcontroller for calculating the resistance of the PTC element. The microcontroller can be designed such that it can calculate the resistance of the PTC element using Ohm's equation based on the current and voltage, and determine the resistance of the PTC element and thus the current temperature of the PTC heater from the resistance.

[0028] The various embodiments of the control unit can be combined with each other. The measurement circuits of the various embodiments can be combined with and / or integrated with each other, as long as this is possible and / or useful. The microcontrollers of the various embodiments can be combined with and / or integrated with each other, as long as this is possible and / or useful. In particular, a single microcontroller can be provided for all embodiments present in the control unit.

[0029] Other important features and advantages of the invention are apparent from the dependent claims, the accompanying drawings, and the relevant description of the drawings with reference to the drawings.

[0030] It should be understood that, without departing from the scope of the invention, the above features and those features which will be explained below can be used not only in the combinations indicated in each case, but also in other combinations or individually. Attached Figure Description

[0031] Preferred embodiments of the invention are shown in the accompanying drawings and explained in more detail in the following description, wherein the same reference numerals refer to the same or similar or functionally identical parts.

[0032] It is illustrated schematically in each case:

[0033] Figure 1This is an exemplary resistance-temperature characteristic curve of the PTC element in the PTC heater according to the method of the present invention;

[0034] Figure 2 This is an exemplary capacitance-temperature characteristic curve of the PTC element in the PTC heater according to the method of the present invention;

[0035] Figure 3 This is a view of a PTC heater having a control unit according to the first embodiment of the present invention;

[0036] Figure 4 This is a view of a PTC heater having a control unit according to the second embodiment of the present invention. Detailed Implementation

[0037] Figure 1 An exemplary resistance-temperature characteristic curve of the PTC element of the PTC heater according to method 1 of the present invention is shown. Figure 2 An exemplary capacitance-temperature characteristic curve of the PTC element of the PTC heater according to Method 1 of the present invention is shown. In Method 1 of the present invention, a pulse width modulated input voltage U0 is provided to the PTC heater during operation, and characteristic values ​​of the PTC element are measured.

[0038] refer to Figure 1 The resistance R of the PTC element is calculated from at least some measured characteristic values, and the determined resistance value of the PTC element at its current temperature T is determined by the calculated resistance R. The resistance R can be calculated, for example, from voltage and current using Ohm's equation. The determined resistance value of the PTC element at its current temperature T can then be read from a table reflecting previously determined values ​​of the PTC element's resistance-temperature characteristic curve. The determined resistance value of the PTC element at its current temperature T is determined within a first range A between -25°C and 200°C (here, the entire range of operating temperatures for the PTC heater).

[0039] refer to Figure 2 The capacitance C of the PTC element is calculated from at least some measured characteristic values, and the capacitance of the PTC element at its current temperature T is determined by the calculated capacitance C. Based on Figure 3 and Figure 4 The calculation of capacitance C is explained in more detail. The determined capacitance value of the PTC element at the current temperature T can then be read from a table reflecting previously determined values ​​on the capacitance-temperature characteristic curve of the PTC element. The determined capacitance value of the PTC element at the current temperature T is determined within a second range B between 100°C and 160°C (in this case, a portion of the entire operating temperature range of the PTC heater).

[0040] As in Figure 1 and Figure 2As can be seen, the resistance-temperature characteristic curve and the capacitance-temperature characteristic curve of the PTC element differ significantly. In region B, the resistance-temperature characteristic curve of the PTC element is flat, allowing only a high tolerance to determine the resistance value at the current temperature T. In contrast, the capacitance-temperature characteristic curve of the PTC element in range B is steep and linear, allowing a low tolerance to determine the capacitance value at the current temperature T. As a result, the capacitance value of the PTC element at the current temperature T can be determined more accurately than the resistance value. Therefore, the capacitance value at the current temperature T can be used in range B, and the resistance value at the current temperature T can be used in the remaining range A to control the PTC heater. Thus, throughout range A, the current temperature T can be determined with a sufficiently low tolerance, and the PTC heater can be precisely controlled.

[0041] Figure 3 A view of a PTC heater 2 according to the invention in a first embodiment is shown. The PTC heater includes at least one PTC element 3 (only one is shown here) and a control unit 4 for performing method 1 described above. In the first embodiment of the PTC heater 2, the control unit 4 includes a first measuring circuit 5 for detecting current values ​​and a second measuring circuit 6 for detecting peak values ​​of current and / or voltage at the PTC element 3. Furthermore, the control unit 4 includes a microcontroller 7 for calculating the capacitance of the PTC element 3. The first measuring circuit 5, the second measuring circuit 6, and the microcontroller 7 can be arranged on a PCB (Printed Circuit Board).

[0042] In method 1, a pulse-width modulated (PWM) input voltage U0 is provided to the PTC heater 2 and corresponds to the PTC element 3. The input voltage U0 is a DC voltage pulse-width modulated by a PWM signal. Specifically, the PWM signal can be a square wave signal. During the operation of the PTC heater 2 when the duty cycle of the PWM signal is less than 100%, individual voltage pulses of the PWM input voltage U0 are applied to the PTC heater 2, i.e., its PTC element 3. At the PTC heater 2, i.e., at its PTC element 3, the increase and application of the input voltage U0, as well as the decrease of the input voltage U0, follow each voltage pulse.

[0043] As the pulse width modulation input voltage U0 increases, the current at PTC element 3 follows a capacitance-dependent current-charge curve:

[0044] ,

[0045] Where I is the peak current, C is the capacitance of the PTC element, dt is the defined time interval, and dV is the voltage change during the defined time interval dt. When the pulse width modulation input voltage U0 at the PTC element 3 increases, the control unit 4 can measure the peak current I, the voltage change dV, and the time t. Furthermore, the control unit 4 can calculate the capacitance C of the PTC element 3 based on the measured values ​​using a capacitance-related current-charge curve, and determine the capacitance value of the PTC element 3 at the current temperature T.

[0046] For this purpose, the first measurement circuit 5 of the control unit 4 can be, for example, a shunt or Hall sensor that converts current into output voltage. The second measurement circuit 6 of the control unit 4 can include a capacitor charged by the output voltage generated at the first measurement circuit 5. The peak voltage present at the capacitor in the second measurement circuit 6 can then be read by the microcontroller 7. The second measurement circuit 6 can be designed such that the discharge time of its capacitor is synchronized with the time read by the microcontroller 7. Alternatively, the second measurement circuit 6 can include a discharge switch, which is triggered by the microcontroller 7 after the charging state of the capacitor in the second measurement circuit 6 has been read. Furthermore, the second measurement circuit 6 is capable of detecting the peak current I. Since the peak current I occurs within a defined time window, this window can be synchronized with the current measurement, and the peak current I can be measured directly. This measurement occurs within a precise time window, and the first measurement circuit 5 is applicable to this time window.

[0047] Based on the peak current I, the peak voltage, and the charging time, the microcontroller 7 can calculate the capacitance C of the PTC element 3 according to the current-charging curve specified above. Then, the microcontroller 7 can read the capacitance value at the current temperature T from a table of values ​​previously determined for the PTC heater 2, depending on the calculated capacitance C.

[0048] Figure 4 A view of a PTC heater 2 with a control unit 4 according to a second embodiment of the present invention is shown. In the second embodiment of the PTC heater 2, the control unit 4 includes a measuring circuit 8 for detecting the voltage at the PTC element 3 and a switch 9 for turning off the measuring circuit 8. Furthermore, the control unit 4 includes a microcontroller 7. The measuring circuit 8 is arranged between the switch 9 and the PTC element 3 to ensure that other capacitances, such as another capacitor or battery (e.g., an HV battery in a vehicle), do not interfere with the measurement. The measuring circuit 8, the switch 9, and the microcontroller 7 can be arranged on a PCB.

[0049] In method 1, a pulse width modulated input voltage U0 is provided to the PTC heater 2 and corresponds to the PTC element 3. As described above, the increase and application of the input voltage U0, as well as the decrease of the input voltage U0, follow each other with each voltage pulse.

[0050] When the pulse width modulation input voltage U0 decreases, the voltage at PTC element 3 follows a capacitance-dependent voltage-discharge curve:

[0051] ,

[0052] Where U(t) is the current voltage, U0 is the input voltage, R is the resistance of the PTC element, C is the capacitance of the PTC element, and t is the current time. When the pulse width modulation input voltage U0 decreases at the PTC element 3, the control unit 4 can measure a first voltage at a first time and a second voltage at a second time. Furthermore, the control unit 4 can calculate the current capacitance C of the PTC element 3 using a capacitance-related voltage-discharge curve based on these values, and determine the capacitance value of the PTC element 3 at its current temperature T.

[0053] For this purpose, the measurement circuit 8 of the control unit 4 is capable of measuring a first voltage at a first time and a second voltage at a second time. The difference between the two voltages can be used by the microcontroller 7 to calculate the capacitance C of the PTC element 3 using the voltage-discharge curve described above. Then, the microcontroller 7 can read the capacitance determination value at the current temperature T from a table of values ​​previously determined for the PTC heater 2, depending on the calculated capacitance C.

Claims

1. A method (1) for determining the current temperature (T) of a PTC heater (2) during operation of the PTC heater (2) by means of a control unit (4), the PTC heater having at least one PTC element (3). During the operation of the PTC heater (2), a pulse width modulation input voltage is applied to the PTC heater (2). During the operation of the PTC heater (2), a characteristic value of one of the PTC elements (3) of the PTC heater (2) is measured. Its features are, The capacitance (C) of the PTC element (3) is calculated from at least some of the measured characteristic values ​​of the PTC element (3), and The capacitance of the PTC element (3) is determined from the calculated capacitance (C), and thus the capacitance of the PTC heater (2) at its current temperature (T).

2. The method (1) according to claim 1, characterized in that, When the capacitance determination value for the current temperature (T) is determined, the capacitance determination value is read from a table of values ​​determined for the PTC heater (2) as a function of the capacitance (C).

3. The method (1) according to claim 1 or 2, characterized in that, The resistance (R) of the PTC element (3) is calculated from at least some of the characteristic values ​​measured from the PTC element (3), and the resistance of the PTC element (3) and thus the current temperature (T) of the PTC heater (2) is determined from the calculated resistance (R).

4. The method (1) according to claim 3, characterized in that, The resistance value at the current temperature (T) is determined within a first range (A) of the operating temperature of the PTC heater (2), and the capacitance value at the current temperature (T) is determined within a second range (B) of the operating temperature of the PTC heater (2). The first range (A) and the second range (B) of the operating temperature of the PTC heater (2) overlap.

5. The method (1) according to claim 3 or 4, characterized in that, The resistance value at the current temperature (T) is determined over the entire operating temperature range (A) of the PTC heater (2), and The capacitance determination value at the current temperature (T) is determined within the range (B) of the operating temperature of the PTC heater (2), in which the tolerance of the resistance determination value at the current temperature (T) exceeds a predetermined tolerance value.

6. The method (1) according to any one of claims 3 to 5, characterized in that, When the resistance determination value for the current temperature (T) is determined, the resistance determination value is read from a table of values ​​determined for the PTC heater (2) as a function of the resistance (R), and / or.

7. The method (1) according to any one of claims 3 to 6, characterized in that, When the pulse width modulation input voltage is applied to the PTC element (3), the resistance (R) of the PTC element (3) is calculated using Ohm's equation.

8. The method (1) according to claim 7, characterized in that, When the pulse width modulation input voltage is applied to the PTC element (3), voltage and current are simultaneously measured at the PTC element (3) as characteristic values, and The resistance (R) of the PTC element (3) is calculated using the Ohm equation based on the measured current and voltage.

9. The method (1) according to any one of the preceding claims, characterized in that, During the charging of the PTC element (3) as the pulse width modulation input voltage at the PTC element (3) increases, the capacitance (C) of the PTC element (3) is calculated using a capacitance-dependent current-charge curve.

10. The method (1) according to claim 9, characterized in that, When the pulse width modulation input voltage at the PTC element (3) increases, the peak current and peak voltage, as well as the charging time, are measured at the PTC element (3) as characteristic values, and The capacitance (C) of the PTC element (3) is calculated using the current-charge curve associated with the capacitance, based on the peak value of the current and the peak value of the voltage and the charging time.

11. The method (1) according to any one of the preceding claims, characterized in that, During the discharge of the PTC element (3) when the pulse width modulation input voltage at the PTC element (3) decreases, the capacitance (C) of the PTC element (3) is calculated using a capacitance-dependent voltage-discharge curve.

12. The method (1) according to claim 11, characterized in that, When the pulse width modulation input voltage at the PTC element (3) decreases, a first voltage at a first time and a second voltage at a second time are measured at the PTC element (3) as characteristic values, and Based on the first voltage and the second voltage, the capacitance (C) of the PTC element (3) is calculated using the voltage-discharge curve related to the capacitance.

13. The method (1) according to claim 9, characterized in that, The first voltage is measured at the PTC element (3) when the pulse width modulation input voltage begins to decrease.

14. A PTC heater (2) having at least one PTC element (3) and a control unit (4), wherein the control unit (4) is configured to perform the method (1) according to any one of the preceding claims.

15. The PTC heater (2) according to claim 13, characterized in that, The control unit (4) includes a first measuring circuit (5) for detecting the current value and a second measuring circuit (6) for detecting the peak value of the current and / or the peak value of the voltage at one of the PTC elements (3), and The first measurement circuit (5) and the second measurement circuit (6) are designed to measure the peak value of the current and the peak value of the voltage, as well as the charging time, when the pulse width modulation input voltage at the PTC element (3) increases. The control unit (4) includes a microcontroller (7) for calculating the capacitance (C) of the PTC element (3), and The microcontroller (7) is designed to be able to calculate the capacitance (C) of the PTC element (3) using the capacitance-related current-charge curve based on the peak value of the current and the peak value of the voltage and the charging time, and to determine the capacitance of the PTC element (3) and thus the current temperature (T) of the PTC heater (2) from the capacitance (C).

16. The PTC heater (2) according to claim 14 or 15, characterized in that, The control unit (4) includes a measuring circuit (8) for detecting the voltage at one of the PTC elements (3) and a switch (9) for turning off the measuring circuit (8). The measurement circuit (8) and the switch (9) are designed such that when the pulse width modulation input voltage at the PTC element (3) decreases, a first voltage can be measured at a first time and a second voltage can be measured at a second time. The control unit (4) includes a microcontroller (7) for calculating the capacitance (C) of the PTC element (3), and The microcontroller (7) is designed to be able to calculate the capacitance (C) of the PTC element (3) based on the first voltage and the second voltage using the voltage-discharge curve associated with the capacitance, and to determine the capacitance of the PTC element (3) and thus the current temperature (T) of the PTC heater (2) from the capacitance (C).

17. The PTC heater (2) according to any one of claims 14 to 16, characterized in that, The control unit (4) includes a measuring circuit for detecting the current and / or voltage at one of the PTC elements (3). The measurement circuit is designed to measure voltage and current at the PTC element (3) simultaneously with the pulse width modulation input voltage being applied to the PTC element (3). The control unit (4) includes a microcontroller for calculating the resistance (R) of the PTC element (3), and The microcontroller is designed to be able to calculate the resistance (R) of the PTC element (3) using Ohm's equation based on the current and the voltage, and to determine the resistance of the PTC element (3) and thus the resistance of the PTC heater (2) at the current temperature (T) from the resistance (R).

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