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The invention relates to a method for operating of a PTC heater with at least one PTC element. The invention also relates to the PTC heater with a control unit for carrying out the method.
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A PTC heater (PTC: Positive Temperature Coefficient) can be used, for example, in a vehicle to heat cabin air. During operation of the PTC heater, a constant voltage is applied to the PTC heater i.e. its PTC elements. A control signal - for example a square-wave pulse - is used to control the heating power i.e. the temperature of the PTC heater. To change the electrical resistance and thus the temperature of the PTC elements, the duty cycle of the control signal is varied. The PTC heater is usually controlled in a so-called PTC area of the resistance-temperature characteristic curve of its PTC elements. In the PTC area, the resistance-temperature characteristic curve is steep at higher temperatures, thus the control of the temperature of the PTC element is complex.
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It is therefore the object of the invention to provide an improved or at least alternative embodiment for a method of the type according to the invention, in which the disadvantages described above are overcome. The object of the invention is also to provide a PTC heater with a control unit for carrying out the method.
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According to the invention, this object is solved by the subject matter of the independent claims. Advantageous embodiments are the subject of the dependent claims.
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The method according to the invention is provided for operating of a PTC heater with at least one PTC element having a temperature-dependent electrical resistance. In the method, an input voltage is applied to the at least one PTC element of the PTC heater. The input voltage at the at least one PTC element is modulated with a control signal characterized by its duty cycle and its frequency. In particular, the control signal can be a square-wave signal. The frequency and the duty cycle of the control signal are adjusted to set the PTC heater to a required temperature.
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The method according to the invention makes use of the fact that the resistance-temperature characteristic curve of the at least one PTC element is frequency-dependent. The resistance-temperature characteristic curve is a curve that indicates the dependence of the electrical resistance of the PTC element on its temperature. The resistance-temperature characteristic curve is specified for the PTC element. If the frequency of the control signal and thus the frequency of the on-off-switching at the PTC element changes, the dependency between the electrical resistance and the temperature of the at least one PTC element and thus its resistance-temperature characteristic curve also changes. In the PTC range, which lies between the temperature corresponding to the minimum electrical resistance of the PTC element and the Curie temperature of the PTC element, the resistance-temperature characteristic curve get flatter with the increasing of the frequency. In other words, a slope of the resistance-temperature characteristic curve of the PTC element in the PTC range decreases with the increasing frequency of the control signal i.e. the frequency of the on-off-switching at the PTC element. Is the resistance-temperature characteristic curve flat, it is easier to regulate i.e. control the electrical resistance and correspondingly the temperature of the PTC element i.e. the PTC heater. In addition, the electromagnetic compatibility of the PTC heater is also influenced when changing i.e. adjusting the frequency of the control signal i.e. the on-off-switching at the PTC element. The perceived electrical heating power of the PTC heater can be reduced, while the thermal heating power of the PTC heater remains the same.
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In the method, the frequency of the control signal is adjusted to adjust the resistance-temperature characteristic curve of the at least one PTC element. The duty cycle of the control signal is adjusted to set the PTC heater to the required temperature in accordance with the adjusted resistance-temperature characteristic curve of the at least one PTC element. As a result, the resistance-temperature characteristic curve of the at least one PTC element can be flattened by adjusting the frequency of the control signal and the required temperature can be set more easily at the PTC heater when adjusting the duty cycle.
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The frequency of the control signal can be set depending on the required temperature of the PTC heater. The frequency of the control signal can be set depending on the electrical resistance of the at least one PTC element corresponding to the required temperature of the PTC heater. The frequency of the control signal can be set depending on a slope of the resistance-temperature characteristic curve of the at least one PTC element, which is not adjusted by the frequency of the control signal i.e. is original i.e. is available at frequencies of the control signal below 1 Hz (Hertz), at the required temperature of the at least one PTC element. In other words, the frequency of the control signal can vary for different required temperatures of the PTC heater. For example, the frequency of the control signal can be varied between 1 Hz and 100 kHz, preferably between 1 Hz and 50 kHz, and especially preferably between 10 Hz and 50 kHz. The maximum frequency of the control signal is depending of the electronic components of the control unit provided for the carrying out the method described and/or depending on the elements of the PTC heater itself. The maximum frequency of the control signal must not exceed the value of 100 kHz.
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The control signal can be, for example, a square-wave pulse or a sinusoidal signal or of another form. The form of the control pulse can be selected/adjusted depending on the elements of the control unit provided for the carrying out the method described and/or depending on the elements of the PTC heater itself.
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The frequency of the control signal can be adjusted in a working area of the resistance-temperature characteristic curve of the at least one PTC element between a temperature corresponding to the minimum electrical resistance and the Curie temperature of the at least one PTC element. In other words, the frequency of the control signal can be adjusted in particular in a so-called PTC range of the resistance-temperature characteristic curve of the at least one PTC element. In the working area i.e. in the PTC range of the resistance-temperature characteristic curve, the at least one PTC element has a PTC behavior.
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The frequency of the control signal can be adjusted over the whole working area in such a way that the resistance-temperature characteristic curve of the at least one PTC element is flattened, especially nearly linearized. This means that the slope of the resistance-temperature characteristic curve of the PTC element in the working area is reduced. This can simplify the control i.e. adjustment of temperature of the PTC element and therefore of the PTC heater.
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The invention also relates to a PTC heater with at least one PTC element and a control unit, wherein the control unit is provided to carry out the method described above. In order to avoid repetition, reference is made to the above explanations.
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Further important features and advantages of the invention are apparent from the sub-claims, from the drawings and from the associated description of the figures with reference to the drawings.
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It is understood that the features mentioned above and those to be explained below can be used not only in the combination indicated in each case, but also in other combinations or on their own, without departing from the scope of the present invention.
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Preferred embodiments of the invention are shown in the drawings and are explained in more detail in the following description, with identical reference signs referring to identical or similar or functionally identical components.
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It shows, schematically in each case
- Fig. 1
- a resistance-temperature characteristic curve of a PTC element of a PTC heater according to the invention;
- Fig. 2
- a time dependence of the control signal when carrying out a method according to the invention;
- Fig. 3
- a resistance-temperature characteristic curve of the PTC element of the PTC heater adjusted in the method according to the invention.
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Fig. 1 shows a resistance-temperature characteristic curve of a PTC element of a PTC heater according to the invention. In the resistance-temperature characteristic curve, the electrical resistance R of the PTC element is shown as a function of the temperature T of the PTC element. A working area 2 of the PTC element i.e. a PTC range of the resistance-temperature characteristic curve is also marked in Fig. 1. The working area i.e. the PTC range lies between a temperature T_R_MIN corresponding to a minimum electrical resistance R_MIN of the PTC element and a Curie temperature T_C of the PTC element.
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Fig. 2 shows a time dependence of a control signal PWM when carrying out a method 1 according to the invention. The control signal PWM shown here is a square-wave i.e. rectangular signal characterized by its duty cycle and its frequency. But also other signal forms, for example, sinusoidal signals, are possible. As shown in Fig. 2, the frequency of the control signal PWM is changed in the method 1 according to the invention. In the example shown here, the frequency of the control signal PWM decreases with time t. The duty cycle of the control signal PWM does not change here.
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In method 1, the PTC heater i.e. its PTC elements are set to a required temperature. For this purpose, an input voltage - for example 350 V - is applied to the PTC elements of the PTC heater. The input voltage is thereby modulated with the control signal PWM. If the control signal PWM changes, the resulting voltage at the PTC elements and thus the electrical resistance R of the PTC elements also change. With reference to Fig. 1, by changing the electrical resistance R, the temperature T of the PTC elements and correspondingly of the PTC heater change. Accordingly, by adjusting the control signal PWM, the required temperature of the PTC heater can be set.
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Fig. 3 shows the resistance-temperature characteristic curve of the PTC element adjusted in method 1. By adjusting the frequency of the control signal PWM over the whole working area 2 shown in Fig. 1, the resistance-temperature characteristic curve can be flattened i.e. nearly linearized. This can significantly simplify the adjustment of the temperature T of the PTC element and therefore the setting the PTC heater to the required temperature.
List of reference signs
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- 1
- Method
- 2
- Working area
- R
- Electrical resistance
- R_MIN
- Minimum electrical resistance
- t
- Time
- T
- Temperature
- T_C
- Curie temperature
- T_R_MIN
- Temperature at R_MIN
- PWM
- Control signal