Circuit arrangement and electric heater for use in a vehicle

Abstract

A circuit arrangement (1) for an electric heater, in particular an electric liquid heater (12), for use in a vehicle, comprises a terminal for providing a voltage, in particular a high-voltage voltage, wherein the terminal has a first terminal pole (HV+) for a first voltage potential and a second terminal pole (HV-) for a second voltage potential, a heating resistor (RH) configured to convert a current flowing through it when the voltage is applied into heat, an electronic switch (S1) connected in series with the heating resistor (RH) between the first terminal pole (HV+) and the second terminal pole (HV-), a control device (42) connected to the electronic switch (S1) and configured to operate the switch (S1) in a pulsed manner in order to adjust a heating power of the electric liquid heater (12), and an input capacitor (C1),which is connected in parallel to the series-connected heating resistor (RH) and switch (S1) between the first terminal (HV+) and the second terminal (HV-). The heating resistor (RH) comprises a first and a second partial resistor (R1, R2) connected in series and defining a center tap (34, 36, 38) between them. The circuit arrangement (1) further comprises an additional capacitor (C2) connected between the center tap and the second terminal (HV-). The input capacitor (C1), together with the first and second partial resistors (R1, R2), forms a first low-pass filter, and the additional capacitor (C2), together with the first partial resistor (R1), forms a second low-pass filter, so that the circuit arrangement as a whole forms a second-order low-pass filter.

Description

Technical area:

[0001] The present invention relates to a circuit arrangement and an electric heater, in particular an electric liquid heater, with the circuit arrangement for use in a vehicle. Technical background:

[0002] It is known that electric heaters for vehicles, in particular electric liquid heaters, can have one or more heating elements, each comprising a heating conductor layer arranged on a carrier element. The heating conductor layer has a heating conductor track and connection regions for electrically contacting the heating conductor track. The heat required for heating operation can be generated in the heating conductor tracks of the heating conductor layer by applying a voltage, with the heating conductor layer acting as a heating resistor. The heating output can be adjusted, for example, by pulsed operation, in particular by pulse width modulation (PWM), pulse frequency modulation (PFM), constant-on-time control (COT) or similar methods that are known as such.

[0003] Pulsed operation is associated with abrupt current and voltage surges. Such current and voltage surges can lead to conducted interference. To meet generally applicable EMC requirements, these current and voltage surges must be filtered out. Otherwise, there is a risk of damaging electronic components in the circuitry involved or connected devices, causing unwanted electromagnetic emissions in relevant frequency ranges, or generating unpleasant noise.

[0004] For this purpose, electronic filter components, such as an input capacitor, also known as a DC link capacitor, or a common-mode choke, are generally provided in the relevant circuit arrangements, each of which fulfils specific functions and is dimensioned accordingly.

[0005] High-voltage heaters, especially high-voltage liquid heaters, for vehicles, for example, are operated with a voltage greater than or equal to 250 volts, for example in a range from 250 V to 490 V in the case of a so-called 400 V heater, and in particular with a voltage greater than or equal to 500 V, greater than or equal to 700 V, 800 V, or 1,000 V, etc. This entails very high heating outputs of, for example, 5 kW, 8 kW, 10 kW, or more. The corresponding high-frequency switched voltages and currents require dimensions or sizes, for example in the case of the input capacitor, that already occupy a considerable volume of the control housing in the heater in question, with the general need to save space. Even more serious, however, are the considerable costs for the component due to the increased capacitance that the input capacitor must provide under these conditions.The cost and size of the capacitor module are both significant factors in the design and manufacture of high-voltage electric heaters for use in vehicles.

[0006] DE 10 2016 108 005 A1 discloses a circuit arrangement with two heat-dissipating resistors connected in series, a control device for switching the resistors, and a grounded housing, wherein the resistors are arranged in close proximity to the housing. The control device has two switching devices connected in series with the resistors, located between the switching devices. A center tap is arranged between the internal switching devices. This center tap is connected via capacitors in parallel to the respective resistor and the subsequent switching device, to a respective high-side potential terminal and a respective low-side potential terminal of the circuit arrangement.

[0007] DE 10 2017 114 714 A1 discloses a similar circuit arrangement as in DE 10 2016 108 005 A1, but without the center tap and the capacitors connected in parallel.

[0008] DE 10 2019 103 832 A1 discloses a device for suppressing EMC common-mode interference in automotive high-voltage applications.

[0009] DE 10 2014 225 449 A1 discloses an electric heating device. Description of the invention:

[0010] The present invention is therefore based on the object of providing a circuit arrangement for an electric heater, in particular an electric liquid heater, as well as such an electric heater, in particular an electric liquid heater for a vehicle, which in comparison with known heaters entails lower costs and, moreover, is also designed to be more space-saving.

[0011] According to various aspects of the invention, a circuit arrangement for an electric heater, in particular an electric liquid heater for use in a vehicle, is provided. This circuit arrangement comprises a terminal for providing a voltage, in particular a high-voltage voltage, wherein the terminal has a first terminal pole for a first voltage potential and a second terminal pole for a second voltage potential (e.g., ground or earth potential). The power supply can be provided at the terminal, for example, by a vehicle battery. The provided voltage is a direct voltage.

[0012] The circuit arrangement further comprises a heating resistor configured to convert a current flowing through it into heat when the voltage is applied, as well as an electronic switch connected in series with the heating resistor between the first terminal and the second terminal. The electronic switch can be a power switch, in particular a power MOSFET or an IGBT, etc. The circuit arrangement further comprises a control device connected to the electronic switch and configured to operate the switch in a pulsed manner, for example, pulse width modulated (PWM) or pulse frequency modulated (PFM), in order to adjust the heating power of the electric heater. Other pulsed operating methods such as COT (Constant-On-Time Control), etc., are also possible.

[0013] In addition, the circuit arrangement also includes an input capacitor connected in parallel with the heating resistor connected in series with the switch and between the first and second terminals. This input capacitor serves as a filter capacitor for smoothing voltage surges, which are caused by the switch, particularly during the switching process. The input capacitor is also referred to as a DC link capacitor.

[0014] A special feature of the present aspects is that, while the heating resistor comprises a first partial resistor and a second partial resistor which are connected in series and define a center tap between them, the circuit arrangement further comprises an additional capacitor which is connected between this center tap and the second connection pole - i.e. parallel to the second partial resistor but in series with the first partial resistor.

[0015] The circuit arrangement according to the present invention therefore provides that the input capacitor, together with the first and second partial resistors, or the entire heating resistor, forms a first low-pass filter, and the additional capacitor, together (only) with the first partial resistor of the heating resistor, forms a second low-pass filter, so that the circuit arrangement as a whole forms a second-order low-pass filter. This design alone increases the filter attenuation from, for example, 20 dB per decade (frequency) to 40 dB compared to a conventional arrangement in which only the input capacitor is provided, but not an additional capacitor as described above.

[0016] However, the decisive factor for the advantage to be achieved is not so much increased attenuation but rather the ability to make the input capacitor smaller in terms of its size (both dimensions and capacitance) and at the same time meet the requirements in terms of EMC (electromagnetic compatibility) by adding the additional capacitor to the circuit arrangement as described.

[0017] According to embodiments, the additional capacitor can have a (second) capacitance value that is lower than the corresponding (first) capacitance value of the input capacitor. It has been found that a value for the additional capacitor that is several orders of magnitude lower is sufficient to allow a reduction of, for example, half or more of the first capacitance value of the input capacitor for the same filtering effect (from a certain frequency range onward) – in comparison to the conventional case, while still meeting the same EMC requirements.

[0018] In particular, the sum of the two capacitance values is less than the single capacitance value according to the conventional case, again provided that the same EMC requirements are met. The costs for the additional capacitor are low due to the low capacitance required. This also applies to its size. Furthermore, it can also be attached to a circuit board in a conventional manner, e.g., using THT technology, so that the additional assembly effort remains manageable. The large input capacitor, on the other hand, can be designed considerably more cheaply, e.g., entailing only half the BOM (bill of material) costs. The same applies to its dimensions, so that considerable space can be saved in the housing. Overall, the aspects of the invention can therefore achieve a reduction in costs and space savings.

[0019] For example, the first capacitance value of the input capacitor may be at least a factor of 10 greater than the second capacitance value of the additional capacitor, preferably by at least a factor of 100, more preferably by at least a factor of 1000.

[0020] The above aspects also reduce the voltage slopes, particularly on the high-side part of the heating conductor, and thus also the generated common-mode interference. This can also indirectly relax the requirements for any common-mode choke.

[0021] Furthermore, a second-order filter, as achieved here, cannot be created simply by adding a resistor, which at first glance would seem obvious if one were to realize that action is necessary. This would result in significant power loss that cannot be utilized. Instead, part of the heating resistor is used in conjunction with another, additional capacitor to improve the filtering effect or, while maintaining the same filtering effect, to reduce the load on the input capacitor, allowing it to be designed smaller.

[0022] According to a special refinement of aspects of the circuit arrangement, the first and second partial resistors have the same ohmic resistance. The voltage divider in the heating resistor designed for the additional capacitor is thus implemented with a voltage drop at the center tap that is half the maximum or provided voltage. On the one hand, this has been shown to achieve good, if not optimal, decoupling of potentially resonant circuit components; on the other hand, it offers almost optimal opportunities for component size reduction with regard to the input capacitor.

[0023] However, other ratios between the values of the first partial resistance and the second partial resistance are also possible in principle, preferably in a ratio range of 1 / 4 to 4, more preferably in a ratio range of 2 / 3 to 3 / 2. If the center tap is placed too close to the high-side end of the entire heating resistor, the additional capacitor approximates a mere parallel connection to the input capacitor. Its function is then reduced to a mere additional contribution to the total capacitance. If, on the other hand, the center tap is placed too close to the low-side end of the entire heating resistor, the additional capacitor loses considerable effectiveness due to the decreasing resistance, so that its contribution to the size reduction of the input capacitor disappears.

[0024] It should be noted that in the circuit arrangement according to the above aspects, developments and embodiments, both the input capacitor and the additional capacitor each form electronic components, i.e. they are designed as discrete components, for example on the circuit board in question, or as specifically structured capacitor elements on the substrate in question (in the case of structures formed on a ceramic substrate using thick-film technology), and are not provided as parasitic structures, for example as outer housing parts that randomly interact with the conductor tracks of the arrangement, which cannot always be avoided.

[0025] Further aspects of the invention provide an electric liquid heater for a vehicle having a circuit arrangement as described above.

[0026] In this case, an electric liquid heater is understood to be a heater in which heat is transferred to a liquid heat transfer medium of a heat transfer circuit flowing through the heater. The heat transfer medium can in particular be a vehicle's liquid coolant, which transports heat within the vehicle and can release it at various points. Alternatively, the liquid heater can also be part of a vehicle's heat pump, for example, so that the heat transfer medium can be a coolant in a heat pump, for example. In this case, the coolant may only be in completely liquid form under certain conditions and only temporarily, or perhaps never, and may otherwise be partially or completely gaseous. Nevertheless, this is also understood to be a liquid heater.

[0027] The electric liquid heater is intended for a vehicle. A vehicle is generally understood to include all possible mobile applications, in particular passenger cars, trucks or commercial vehicles, construction machinery, aircraft, and watercraft. This also includes, for example, construction machinery or cranes, as well as trailers such as caravans that can be towed and transported by other vehicles.

[0028] The electric liquid heater preferably has a heating output of at least 3 kW, preferably at least 5 kW, more preferably at least 7 kW, for example at least 9 kW. The heating output is preferably less than or equal to 13 kW. The operating voltage at which the vehicle heater is operated, i.e. the voltage or high voltage applied between the connection poles of the circuit arrangement connection, which can be the same as the on-board voltage of an electrically powered vehicle, is greater than or equal to 250 V. For example, a heater referred to as a 400 V heater can typically cover a voltage range from 250 V to 490 V. Higher voltage ranges are also included, e.g. greater than or equal to 500 V, for example over 800 V, 900 V or 1000 V. The resulting value of the ohmic resistance of the heating resistor is, for example, approximately in a range from 10 to 200 ohms.

[0029] The liquid heater comprises at least one heating element and at least one heating conductor layer. The heating conductor layer is arranged on a carrier element, which may be a ceramic substrate, in particular made of Al2O3. The heating conductor layer has, in a heating conductor layer plane, which may also be curved, a heating conductor track which is delimited or fixed by at least one insulation interruption in the heating conductor layer plane, whereby the heating conductor track forms the heating resistance of the circuit arrangement. The heating conductor layers and heating conductor tracks can be arranged together on a single carrier element, or else on two or three different ones. Each heating conductor layer or each heating conductor track is preferably applied to its own, separate carrier element. The heating conductor layer or the heating conductor tracks can be produced on the thermally conductive ceramic substrate using a screen printing process or thick-film technology.

[0030] The liquid heater also features a heat exchanger, which is in thermal communication with the heating element. The heat converted by the heating resistor is transferred via the ceramic substrate and, if necessary, an adhesion promoter to a fluid flowing through the heat exchanger.

[0031] Such a design enables a particularly advantageous application of the circuit arrangement, because the heating resistor, designed as a heating conductor, is relatively easily accessible on the substrate. A voltage divider can be implemented at any location by means of a tap, for example, via a bonding conductor. The additional capacitor can be implemented as described. Intervention in the heating element itself is not necessary for this; the two partial resistors are then located directly on either side of the tap.

[0032] According to a corresponding development, the heating conductor extends between a first connection area and a second connection area, in each of which the heating conductor is electrically connected to at least one electrical connection conductor, which establishes an electrical connection with the switch or with one of the two connection poles. The center tap is located in a section of the heating conductor between the first connection area and the second connection area.

[0033] The center tap itself can define a third connection area which is connected to an electrical conductor, e.g. the bond conductor mentioned above, which establishes an electrical connection to the additional capacitor. In a special embodiment, a third connection point already present on the heating element is used for this purpose. This third connection point is characterized by a surface that is locally widened or laterally protruded in the conductor track. This can be used to provide an alternative circuit for the same heating element in which the two strands of the conductor track on either side of the connection point are operated in parallel instead of in series with the appropriate voltage. This makes higher power possible. If this higher power is not required, this center tap could instead be used for the voltage divider proposed according to the invention with an additional capacitor connected.An example implementation is described below. Short description of the drawings:

[0034] The invention is explained below by way of example with reference to the following figures.

[0035] They show: Fig. 1 is a schematic circuit diagram of a circuit arrangement for an electric liquid heater according to a comparative example; Fig. 2 a plan view of a comparison example from Fig. 1 or the embodiment from Fig. 4 usable heating elements; Fig. 3 is a schematic circuit diagram of a circuit arrangement for an electric liquid heater according to an alternative comparative example, but using the same heating element as in Fig. 1; Fig. 4 is a schematic circuit diagram of a circuit arrangement for an electric liquid heater according to the embodiment; Fig. 5 an equivalent circuit diagram for the schematic circuit diagram from Fig. 4; Fig. 6 is a schematic circuit diagram of a circuit arrangement for an electric liquid heater in a modification according to a second embodiment; and Fig. 7 a simplified representation of an electric liquid vehicle heater. Detailed description of preferred embodiments:

[0036] In the following description of the drawings, the same reference symbols refer to the same or comparable components.

[0037] In the Fig. 1 shows a schematic circuit diagram of a circuit arrangement 1 for an electric liquid heater according to a comparative example.

[0038] The circuit arrangement 1 has a connection for a high-voltage voltage, for example, 400 V or 800 V, etc., which comprises a first connection terminal HV+ for the actual voltage potential and a second connection terminal HV- for the reference or ground potential. Also symbolically shown on the side of the first connection terminal HV+ is an ohmic resistance referred to as input resistance Re, which reflects, for example, line resistances or the internal resistance of the vehicle battery. Corresponding impedances can also be included. A push-pull choke, which is optionally present in the circuit arrangement, has been omitted for the sake of simplicity. This also applies to the other figures of the application.

[0039] An input capacitor C1 is connected between the two connection terminals. This capacitor, also known as a DC link capacitor, has a filter function that smooths out fluctuations in the voltage provided by the vehicle's electrical system and filters out voltage and current surges caused by a switching operation by a switch S1, which will be described below.

[0040] The circuit arrangement 1 further comprises - connected in parallel to the input capacitor C1 - a heating resistor R H which is caused by a Fig. 2 shown heating conductor tracks 20, which will be discussed below. The heating resistor R His connected in series with switch S1, which is operated in a pulse-width modulated manner by a control device 42 to set a desired heating output. Switch S1 can be an IGBT or a power MOSFET, etc. The control device 42 can be connected to an input device (not shown), via which, for example, a target temperature can be set, wherein either the input device or the control device 42 or another device sets the duty cycle for the pulse-width modulation in a known manner depending on the temperature at which switch S1 is operated.

[0041] The Fig. 2 shows a top view of a comparison example from Fig. 1 (or the embodiment described below from Fig. 4) usable heating element 10. The heating element 10 comprises a carrier element 14 formed as a ceramic substrate and a heat conductor layer 16 formed thereon. In the illustration, the carrier element 14 is almost completely covered by the heat conductor layer 16. The heat conductor layer 16 has been structured, for example, in a screen printing process, so that by means of a suitable arrangement of insulation interruptions 22, it Fig. 1. The heating conductor layer 16 can be formed on the carrier element 14 in a heating conductor layer plane 18.

[0042] The carrier element 14 has a rectangular shape. The heating conductor track 20 is defined by two end points, which form a first connection region 32 and a second connection region 33. The heating conductor track 20 can be formed from a copper alloy and, without restriction of generality, have a thickness of, for example, 12 µm. The two connection regions 32, 33 can be formed from the same material or supplemented by an additional material to enable the connection, for example, of a bonding wire or another type of electrical connection conductor 35. In a third connection region 34 arranged symmetrically in the center of the heating conductor tracks 20, contacting by an electrical connection conductor 35 can also be provided, as is also shown in Fig. 1 in the middle of the heating resistor R His indicated schematically. Electrical energy can be supplied to the heating conductor track 20 through the electrical connecting conductors 35, which may be bonding wires or conductors. This energy is converted into heat in the heating conductor track 20 and transferred to the carrier element 14.

[0043] The electrical connection conductors 35 can connect the connection areas 32, 33 (and possibly 34) to a connection electronics that is not shown in the figures. This can be a power board, a control unit that includes power electronics, or the like. The switch S1 can be provided in the area of this connection electronics. Each connection area 32, 33 (and possibly 34) can be electrically connected to more than one electrical connection conductor 35, whereby the number can differ between the two connection conductors. The connection conductors 35 extend from the connection areas 32, 33 (and possibly 34) towards a connection side that is shown in the Fig. 2 to the right, and beyond the edge of the heating conductor layer 16 running along the connection side. As described, these can each be bond wires.

[0044] The Fig. 3 shows an alternative circuit arrangement 1 according to a modified comparative example, which has the same heating element 12 as in Fig. 2. In this modification, the two connection areas 32 and 33 are connected to the first connection terminal HV+ carrying the voltage potential, while the third connection area 34 in the middle of the heating conductor track 20 is connected to the switch S1, which is connected to the second connection terminal HV-. Furthermore, the Fig. 3 shown circuit arrangement 1 is identical to that in Fig. 1. The third connection area 34 used as a center tap allows the heating resistor R to be divided into two parts. H , whereby the resulting partial resistors are thus operated in parallel. The circuit arrangement 1 according to the Fig. 3 therefore allows for greater heating power, as long as the voltage provided is the same as in Fig. 1. The Fig. The heating element 12 shown in Figure 2 therefore allows flexible use, for example, in liquid heaters with different specified maximum outputs, for example 400 W or 800 W devices.

[0045] The Fig. Figure 4 shows a circuit arrangement 1 according to a first embodiment of the present invention. As far as the elements contained therein are identical to those in Fig. 1 or Fig. 3, repetition is avoided and reference is made to the relevant description above. In particular, the circuit arrangement 1 of the Fig. 4 an input capacitor C1, a heating resistor R H , a switch S1 and a control device 42 as described above. In the present, specific, non-limiting embodiment, for example, the one for the higher power variant according to Fig. 3, the center tap or connection point 34 is used to create a voltage divider. Because the center tap or connection point 34 is provided centrally in the conductor track 20, the same conductor track lengths lie between it and the first connection pole and the second connection pole, so that the resulting partial resistances R1, R2, which exist between the center tap and the first connection pole and between the center tap and the second connection pole, are essentially identical. Therefore, half the voltage drops between the center tap or the third connection point 34 and the second connection pole HV-.

[0046] In particular, an additional capacitor C2 is connected between the center tap or the third connection point 34 and the second connection pole HV-. The strand containing the additional capacitor C2 is connected in parallel with the second partial resistor R2, which is connected in series with the switch S1.

[0047] The capacitance values of the input capacitor C1 and the additional capacitor C2, which together form a second-order low-pass filter, are matched to achieve a specified cutoff frequency and a specified damping behavior. If the capacitance of the capacitor C1 is significantly larger than that of the additional capacitor C2, the cutoff frequency of the first low-pass filter is 1 / (2 π R C1) and the cutoff frequency of the second low-pass filter is 2 / (π R C2), where R denotes the total resistance. If the center tap is not located in the middle of the heating element, the second frequency is scaled accordingly. This allows the cutoff frequencies to be adjusted to the desired damping behavior. The specified cutoff frequency and the specified damping behavior correspond to the same requirements as for the Fig. 1 and Fig. 3 comparison examples shown.

[0048] Determining the exact capacitance values required for the two capacitors C1 and C2 in the example is a complex task. However, the result shows that the capacitance value of the input capacitor C1 is higher than the capacitance value of the respective input capacitor C1 according to the Fig. 1 or Fig. 3, which saves costs and component size. At the same time, the additional capacitor C2 only needs to contribute a very small amount of capacitance to achieve the desired overall filtering effect.

[0049] In the Fig. 6 shows a modification according to a second embodiment. In contrast to the first embodiment according to the Fig. 4, the center tap is not provided at the third connection point 34 provided in the heating element 12, but at a first freely placed connection point 36 along the heating conductor track 20, which, viewed from the third connection point 34, is positioned in the direction of the second connection point 33, i.e., opposite the technical current direction. Accordingly, the first partial resistance R1 is now smaller than the second partial resistance R2. The center tap can also be achieved here by the targeted placement of a bonding conductor along the heating conductor track 20.

[0050] As also in the Fig. 6, namely by the dashed line to the second placed connection point 38 along the heating conductor track 20 and the associated double arrow, it is possible to set up an optimal ratio of R1 to R2 within the scope of the design during the manufacture of the circuit arrangement 1, which in interaction with the capacitance values of C1 and C2 achieves an optimal filter effect.

[0051] The Fig. 7 shows a simplified representation of an electric liquid heater 12 for a vehicle. In addition to the electric heating element 10, the electric liquid heater 12 comprises a liquid heat exchanger 44, on which the electric heating element 10 is arranged according to the exemplary embodiments and to which the heat generated by the heating element 10 during heating operation is transferred, as well as a control unit 46 for controlling the electric heating element 10. The control unit 46 can comprise one or more printed circuit boards on which the input capacitor C1, the additional capacitor C2, and the control device 42 are arranged. For this purpose, the control unit 46 is connected to the electric heating element 10 via electrical connecting lines 50. According to specific exemplary embodiments that do not restrict the general scope, the heating element 10 can be assigned a further printed circuit board that has one or more power switching elements forming the switch S1.The connecting lines 50 connect this circuit board to that of the control unit. The connecting lines 50 can be designed as lead frames. The connection lines shown in . Fig. 7 not shown separately printed circuit board with the power switching elements can be connected to the heating element via the connecting conductors 35. Further in Fig. 7 not shown components of the electric vehicle heater 12, which for the sake of simplicity are Fig. 7 are not explicitly shown, are well known to the person skilled in the art and are supplemented by this to ensure the functionality of the electric vehicle heater 12.

[0052] The features of the invention disclosed in the above description, in the drawings and in the claims may be essential for the realization of the invention both individually and in any combination. List of reference symbols 1 Circuit arrangement 2 LISN (line impedance stability network) 10 Heating element 12 electric liquid heaters 14 support element 16 Heating conductor layer 18 Heating conductor layer level 20 heating conductor track 22 Insulation break 32 first connection area 33 second connection area 34 third connection area 35 electrical connecting conductor 36 first freely placed junction 38 second freely placed junction 42 Control device 44 heat exchangers 46 Control unit (control board) 50 electrical connection cables S1 electronic switch R H Heating resistor R1 first partial resistor R2 second partial resistor Re input resistance (mains power, internal resistance of the battery, etc.) C1 Input capacitor, DC link capacitor C2 additional capacitor

Claims

[1] Circuit arrangement (1) for an electric heater, in particular an electric liquid heater (12), for use in a vehicle, comprising: - a connection for providing a voltage, in particular a high-voltage voltage, wherein the connection has a first connection pole (HV+) for a first voltage potential and a second connection pole (HV-) for a second voltage potential; - a heating resistor (R H ) which is designed to convert a current flowing through it when the voltage is applied into heat; - an electronic switch (S1) connected to the heating resistor (R H ) is connected in series between the first terminal (HV+) and the second terminal (HV-); - a control device (42) connected to the electronic switch (S1) and configured to operate the switch (S1) in a pulsed manner in order to adjust a heating power of the electric liquid heater (12); - an input capacitor (C1) connected in parallel to the series-connected heating resistor (R H ) and switch (S1) is connected between the first terminal (HV+) and the second terminal (HV-), - where the heating resistance (R H ) comprises a first partial resistor (R1) and a second partial resistor (R2) which are connected in series and define a center tap (34, 36, 38) between them; and wherein the circuit arrangement (1) further comprises an additional capacitor (C2) which is connected between the center tap and the second connection pole (HV-), characterized bythat the input capacitor (C1) together with the first partial resistor (R1) and the second partial resistor (R2) form a first low-pass filter and the additional capacitor (C2) together with the first partial resistor (R1) form a second low-pass filter, so that the circuit arrangement as a whole forms a second-order low-pass filter. [2] Circuit arrangement (1) according to claim 1, wherein the input capacitor (C1) has a first capacitance value and the additional capacitor (C2) has a second capacitance value, the first capacitance value being greater than the second capacitance value. [3] Circuit arrangement (1) according to claim 2, wherein the first capacitance value is greater than the second capacitance value by at least a factor of 10, preferably by at least a factor of 100, more preferably by at least a factor of 1000. [4] Circuit arrangement (1) according to one of claims 1 to 3, wherein the first partial resistor (R1) and the second partial resistor (R2) have the same value for the ohmic resistance. [5] Circuit arrangement (1) according to one of claims 1 to 4, wherein the input capacitor (C1) and the additional capacitor (C2) each form electronic components which are mounted on one or more printed circuit boards. [6] Electric heating, in particular electric liquid heating (12) comprising: - a circuit arrangement (1) according to one of claims 1 to 5; - a heating element (10) with a carrier element (14) and a heat conductor layer (16) arranged on the carrier element (14), wherein the heat conductor layer (16) has a heat conductor track (20) in a heat conductor layer plane (18), which is delimited in the heat conductor layer plane (18) by at least one insulation interruption (22), wherein the heat conductor track (20) has the heating resistance (R H ) and - a heat exchanger (44) which is in thermal connection with the heating element (10). [7] Electric heater (12) according to claim 6, wherein the heating conductor track (20) extends between a first connection region (32) and a second connection region (33), in each of which the heating conductor track (20) is electrically conductively connected to at least one electrical connection conductor (35) which establishes an electrical connection with the switch (S1) or with one of the two connection poles (HV+); wherein the center tap is located in a section of the heating conductor track (20) between the first connection region (32) and the second connection region (33). [8] Electric heater (12) according to claim 7, wherein the center tap defines a third terminal region (34) which is connected to an electrical conductor (35) which establishes an electrical connection with the additional capacitor (C2). [9] Electric heater (12) according to one of claims 6 to 8, wherein the carrier element (14) is a ceramic substrate and the heating conductor track (20) is a copper conductor track formed on the carrier element using thick-film technology.

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