Heating system for a motor vehicle

Abstract

Heating device for a motor vehicle (1), in particular for heating an airflow supplied to the vehicle interior (43), comprising a control device (20) and several heating elements (12-19), each of which is individually or in heating element groups (8-11) connected to a first power supply connection (7) of the heating device (4) via a first switching means (21-24) assigned to the heating element (12-19) or heating element group (8-11), wherein the control device (20) is configured to activate the first switching means (21-24) sequentially in successive switching cycles (29) at a respective switch-on time (35, 37, 38) in the respective switching cycle (29) in order to connect the assigned heating element (12-19) or the assigned heating element group (8-11) to the first power supply connection (7), and at a respective switch-off time (36) to be controlled in the respective switching cycle (29),to disconnect the respective assigned heating element (12 - 19) or the assigned heating element group (8 - 11) from the first power supply connection (7), characterized in that the control device (20) is configured, on the one hand, to control the first switching devices (21 - 24) such that the switch-on time (35, 37, 38) of a respective first switching device (21 - 24) has a fixed predetermined time interval (30) to the switch-on time (35, 37, 38) of the preceding switched first switching device (21 - 24), and, on the other hand, the time interval (28, 33) of the switch-off time (36) of the respective first switching device (21 - 24) to the preceding switch-on time (35, 37, 38) of this switching device (21 - 24) as a function of an operating voltage that is applied between the first and a second power supply connection (6, 7) the heating device (4) is located,and / or a measured actual current through the heating device (4) and / or a determined resistance of at least one of the heating elements (12 - 19) or a parallel connection of at least two of the heating elements (12 - 19).

Description

[0001] The invention relates to a heating device for a motor vehicle, in particular for heating an air flow supplied to the vehicle interior, comprising a control device and a plurality of heating elements, each of which is connected individually or in heating element groups to a first power supply connection of the heating device via a respective first switching means assigned to the heating element or the heating element group, wherein the control device is designed to control the first switching means in successive switching cycles in time succession at a respective switching-on time in the respective switching cycle in order to connect the assigned heating element or the assigned heating element group to the first power supply connection, and to control them at a respective switching-off time in the respective switching cycle in order to disconnect the respectively assigned heating element or the assigned heating element group from the first power supply connection.The invention also relates to a motor vehicle.

[0002] Electric heaters are used in motor vehicles for a variety of purposes, particularly for heating the air supplied to the interior, but also for heating batteries, fuel additives, and the like. It is known that heating systems designed to provide high heat outputs comprise several heating elements connected in parallel that can be powered together. High heat outputs can and must be provided, particularly in electric vehicles with high-voltage electrical systems, since interior heating using waste heat from an internal combustion engine is not possible in these vehicles.

[0003] To control the heating power without generating high power losses, power control is usually achieved by pulse-width modulation of control signals from semiconductor switches, which turn them on for a specific period of time in one switching cycle and off for the remaining switching cycle. If all heating elements were energized simultaneously and then simultaneously turned off again, the maximum power of the heating device would be required alternately with no power. At high switching frequencies, this can be compensated for, for example, by capacitors or similar means for smoothing the power demand. However, it is advantageous to stagger the power supply to the various heating elements to avoid power peaks.

[0004] One possibility for this is described in document EP 1 157 868 B1. There, the activation times for various heating elements in a respective switching cycle are determined depending on the pulse width, i.e., the operating time of the respective heating element in the switching cycle. Specifically, it is proposed that, assuming identical operating times or pulse widths for the individual heating elements, the non-operating time of a heating element in the respective switching cycle be divided by the number of heating elements reduced by one, and the time thus determined be used as the interval between the various activation processes.

[0005] This approach leads to significantly lower power peaks than simultaneous control of the heating elements, particularly at low power requirements, i.e. when very narrow pulse widths are used to control the individual semiconductor elements. However, this approach has the disadvantage that the maximum power peaks result even at medium heating power levels. In the exemplary embodiment in the printed document, three heating elements are used, which means that with the approach described, all three heating elements are temporarily active at the same time even when the heating elements are operating for 50% of the switching cycle, meaning that the heating device briefly requires its maximum power. However, short-term high power requirements of the heating device can, under certain circumstances, lead to brief voltage dips in the vehicle network, which should be avoided.

[0006] The problems described occur particularly when high-voltage on-board electrical systems are used, whose supplied voltage can vary by a factor of 3.5, for example, depending on the battery charge level, temperature, operation of a drive motor for propulsion or recuperation, and other factors. This voltage variation alone can lead to a variation in the maximum power required by the heating system when all heating elements are switched on by a factor of 12.25.

[0007] This effect is further amplified when heating elements with a positive temperature coefficient, such as PTC ceramic elements, are used. While such elements have the advantage that their resistance increases at high temperatures, thus reliably preventing overheating, the problem with regard to the maximum power drawn by the heating device is that the resistance of PTC ceramics decreases with increasing voltage. For the voltage ranges relevant to the high-voltage networks of electric vehicles, the resistance can vary by a factor of 4, for example. This means that, together with the factor mentioned above, the power requirement of the heating device can vary by a factor of 50 depending on the on-board network voltage. In other words, with high-voltage heating devices, a high peak load on the heating device can lead to very high short-term power requirements and thus, under certain circumstances, generate voltage dips.

[0008] EP 1 091 621 A1 discloses a method for controlling an electric heater with at least two heating resistors. Current pulses for energizing the heating resistors can be temporally shifted and overlapped.

[0009] The electric heater according to the document DE 101 09 734 A1 has several groups of parallel-connected heating elements, the current supply of which is controlled by transistors.

[0010] A heating device comprising resistance heaters with an operating voltage of at least 60 V or 200 V or 300 V is known from the document US 2011 / 0091 190 A1.

[0011] The invention is therefore based on the object of providing a heating device which is improved compared to the aforementioned and which, in particular, at least in some operating situations, requires lower peak powers for the same target heating power.

[0012] The object is achieved according to the invention by a heating device of the type mentioned at the outset, wherein the control device is designed, on the one hand, to control the first switching means in such a way that the switch-on time of a respective first switching means has a fixed predetermined time interval from the switch-on time of the previously switched first switching means, and, on the other hand, to specify the time interval of the switch-off time of the respective first switching means from the previous switch-on time of this switching means as a function of an operating voltage which is applied between the first and a second power supply connection of the heating device, and / or a detected actual current through the heating device and / or a determined resistance of at least one of the heating elements or a parallel connection of at least two of the heating elements.

[0013] Surprisingly, it was discovered that the use of fixed time intervals between the switch-on times, i.e., the use of control signals that are identical except for this time interval but with a fixed phase offset between the control signals, is very well suited to avoiding power peaks. For example, in the example mentioned above with three heating elements in use, a time interval between the switch-on times can be used that is one-third of the cycle time of the switching cycle. In this case, even if the individual heating elements are operated for, say, 65% of the cycle time, only two of the heating elements are operated simultaneously, so that the maximum power required by the heating device during the switching cycles is only two-thirds of the maximum power that would be required if three of the heating elements were operated.This results in a significant improvement compared to the approach explained at the beginning, especially for medium performances.

[0014] Heating elements can be, for example, PTC ceramic elements or parallel circuits of several PTC ceramic elements. The heating device can be configured, in particular, for operation with high voltages or high heating powers. Semiconductor switches, for example, transistors, particularly IGBTs or MOSFETs, can be used as switching devices. The heating elements can be connected to a second power supply terminal of the heating device directly or indirectly, for example, via additional switching devices.

[0015] The fixed time interval between the switch-on times can correspond to the quotient of the duration of the switching cycle and the number of first switching means of the heating device. If, for example, four first switching means are used to switch the power supply to four heating elements or four heating element groups, this time interval corresponds to a quarter of the duration of the switching cycle. In other words, switch-on times that are evenly distributed throughout the switching cycle are preferably used. This means that in cases in which the switching means are switched on for a time interval that is longer than the quotient of the duration of the switching cycle and the number of first switching means, at least one of the first switching means remains switched on beyond the cycle limit of the switching cycles, whereby the switch-off time of this first switching means lies before the switch-on time within the respective switching cycle.

[0016] The control device can be configured to additionally specify the time interval between the switch-off time of the respective first switching means and the preceding switch-on time of this switching means depending on a predetermined target heating output and / or a determined temperature of at least one component of the heating device. In particular, this time interval can be the same for all first switching means. However, different time intervals can also be used for different first switching means, for example, to provide different proportions of the total heating output through different heating elements.

[0017] If the current maximum output of the heating system, i.e., the output resulting from the operation of all heating elements, is known, the quotient of this time interval and the duration of the switching cycle can be equal to the quotient of the target heating output and the maximum heating output. The maximum heating output can be determined, for example, from the resistances of the heating elements or the resistance of the entire heating system when all heating elements are switched on, and the operating voltage.

[0018] However, since there is often no means available to measure the resistance of the individual heating elements, and simultaneous operation of all heating elements to determine the resistance is not desirable, and since it cannot often be assumed that all heating elements have the same resistance, this direct approach is often only possible with a certain degree of inaccuracy. For correction, the actual current or the temperature, for example, can be used to obtain additional information about the actual state of the heating device. However, it is also possible to determine the average actual power, for example by integrating the product of the actual current and the operating voltage, and to regulate this to the target heating power by adjusting the time interval, e.g. using a PI or PID controller, or similar.

[0019] Monitoring devices for voltages, currents, resistances, and temperatures are well known in the art and will therefore not be discussed in detail. Such monitoring devices can be part of the control device or be designed separately and provide data to it, for example, via a bus or similar.

[0020] In cases in which the heating elements in heating element groups are connected to the first power supply connection of the heating device via the respective first switching means assigned to the heating element group, the heating device can comprise second switching means which each connect a heating element of each heating element group to the second power supply connection of the heating device, wherein the control device is designed to control the second switching means when a low power condition dependent on the or a target heating power is met such that in each of the switching cycles only one of the heating elements of a heating element group or only a subgroup of these heating elements which does not comprise all of the heating elements of the heating element group is connected to the second power supply connection.In particular, the second switching means can be switched alternately in order to alternately operate different heating elements of the respective heating element group in each switching cycle.

[0021] For example, if two heating elements of a heating element group are used, only one of these heating elements can be operated in each switching cycle if the target heat output is sufficiently low. Compared to a setting in which both heating elements of the heating element group are operated, twice the time interval between the switch-on and switch-off times of the first switching device assigned to this heating element group can be used.

[0022] In other words, the procedure described means that the individual heating elements of a heating element group can be switched separately or in smaller subgroups, whereby the current power of the heating device can be regulated in smaller steps, which can further reduce peaks in power consumption.

[0023] The cycle length can be longer than 2 ms or longer than 5 ms. In other words, the switching frequency at which the individual first switching elements are switched can be a maximum of 500 Hz or a maximum of 200 Hz. In particular, switching frequencies of approximately 100 Hz are used, since very high switching frequencies, such as those used in low-voltage ranges, are hardly feasible for high-voltage heaters. At the relatively low switching rates mentioned, however, complete smoothing of the power supplied to the heating device is not reasonably possible. A power supply for the heating device, for example in the on-board electrical system, must therefore actually be designed to provide the peak power consumption that occurs, for example the power required when the maximum number of heating elements is operated simultaneously during the switching cycle.Therefore, approaches to reducing power peaks are particularly relevant for high-voltage heaters.

[0024] In addition to the heating device according to the invention, the invention relates to a motor vehicle comprising a heating device according to the invention. The motor vehicle can additionally have a power supply or an on-board electrical system connected to the first and the or a second power supply connection of the heating device.

[0025] A power supply for the heating device can provide a maximum operating voltage of at least 100 V or at least 200 V or at least 400 V and / or a maximum heating output of at least 4 kW or at least 7 kW or at least 10 kW. Corresponding voltages or outputs can be provided in motor vehicles, in particular, by high-voltage on-board electrical systems, which can also be used, for example, to supply current to a drive motor of an electric vehicle. Relatively strong fluctuations in the operating voltage can occur in such high-voltage on-board electrical systems, whereby the operating voltage can depend, for example, on the charge state of a motor vehicle's battery, operation of the drive motor for propulsion or recuperation, temperature, and / or consumption of other consumers in the on-board electrical system.

[0026] The minimum operating voltages occurring during normal operation of the motor vehicle can be a factor of 2 to 5, for example a factor of 3.5, lower than the maximum operating voltages occurring during normal operation, i.e., for example, the values ​​mentioned above. Thus, the maximum heating output of the heating device, particularly when PTC ceramics are used as heating elements, can vary by a factor of 50, for example, at different operating times, as explained above. However, this means that in some operating situations, for example at maximum operating voltages, the maximum power consumption of the heating device can be much higher, for example, a factor of 50, than in other operating situations in which, for example, the minimum vehicle electrical system voltage occurs.Accordingly, the heating system must be sufficiently large, and in some operating situations, high power peaks may occur when many of the heating elements are switched on. These can be largely avoided by the procedure described.

[0027] The motor vehicle may have a ventilation device for supplying an air flow into the interior of the motor vehicle, with the heating device serving to heat the air flow. In particular, the heating elements may be arranged in an air duct carrying the air flow such that they are in direct contact with the air flow. The air flow, or the air supplied to the interior, may be fresh air drawn in from the vehicle's surroundings or recirculated air.

[0028] The control device or a processing device of the motor vehicle can be configured to determine, depending on the or a predetermined target heating power, an upper limit for a power required by the heating device to achieve this target heating power and to operate the heating device and / or another device of the motor vehicle depending on the upper limit.

[0029] Typically, in addition to the heating device, a vehicle-mounted power supply, for example a high-voltage on-board network, also supplies other vehicle-mounted devices, for example a drive motor and, in particular via a voltage converter in a low-voltage network, other vehicle electronics, such as lighting devices, multimedia devices, etc. If the heating device requires such a high level of power at the time when the maximum number of heating elements is switched on in the switching cycle or when the heating elements with minimum resistance are switched on that the power reserves of the power supply are heavily stressed or overloaded, the operation of the other devices in the motor vehicle can be impaired, for example if brief voltage drops occur in the on-board network.

[0030] To avoid this, either the operation of the heating system can be adjusted, for example, by requiring a lower target heating output, or the operation of other devices can be modified, for example, by temporarily deactivating unnecessary devices or operating them at lower power. For this purpose, a fixed or situation-dependent prioritization of the various devices and the heating system can be provided. This situation-dependent prioritization can be specified, for example, depending on the interior temperature, the detection of fogged windows, or similar parameters that may be relevant to the operation of the heating system.

[0031] However, this approach is problematic because, for heating devices operated with high voltage, especially when using PTC ceramics as heating elements, they can require significantly different maximum power levels for the same specified target heating output depending on various parameters. As already explained above, for example, the power required by a single switched heating element can vary by a factor of 50, particularly depending on temperature and vehicle electrical system voltage. Depending on the operating situation, controlling or regulating the target heating output thus results in a different control pattern for the first switching devices, and if the second switching devices are used, possibly also for the second switching devices.These different switching patterns, combined with the different power ratings of the individual heating elements, result in different maximum required power levels for the heating system to provide the same target heat output. Therefore, it is proposed to determine and use an upper limit for the required power.

[0032] The control device or the processing device can be designed to - in a first alternative, to determine the upper limit as the maximum of, on the one hand, the power required to operate the heating element or group of heating elements with the minimum resistance and, on the other hand, a product of the target heating power and a predetermined scaling factor, or - in a second alternative, the upper limit is to be determined as the sum of the power required to operate the heating element or group of heating elements with the minimum resistance and the target heating power, or - in a third alternative, the upper limit is to be determined as the product of a scaling factor specified as a function of the target heating power and the power required to operate the heating element or group of heating elements with the minimum resistance.

[0033] The scaling factor can, in particular, depend on the maximum number of simultaneously switched on first or first and second switching devices occurring in a used switching pattern, or it can be fixed. In particular, depending on the target heating output, it can be determined how many of the first or second switching devices are switched on at the maximum simultaneously, or how many of the heating elements or groups of heating elements are active at the maximum simultaneously, and the scaling factor can be determined depending on this value.

[0034] The switching patterns used by the first and second switching devices can be determined, for example, as part of adjusting the target power. In principle, it would be possible to determine the upper limit for the required power for this specific switching pattern before changing the switching pattern, taking into account all parameters, in particular the exact resistances of the individual heating elements. However, since the available computing power is typically limited, and since not all parameters, such as the exact resistances of all heating elements, are necessarily available for an exact calculation, one of the two estimates mentioned above can be used.

[0035] The first of the above-mentioned estimates is based on the fact that a distinction can initially be made between two cases: the case in which the heating elements or groups of heating elements are not operated in an overlapping manner, i.e., each heating element or group of heating elements is switched on for a sufficiently short time, so that at any given time within the switching cycle, only one of the heating elements or group of heating elements is operated. In this case, the required power is maximum when the heating element or group of heating elements, in particular a group of heating elements connected in parallel, is operated with minimal resistance.The resistance of this heating element or this group of heating elements can, for example, be measured during operation, stored as a predetermined value, in particular as a predetermined value depending on an operating voltage and / or a temperature, or the like.

[0036] In the case where the operating times of several heating elements or groups of heating elements overlap, for example, in the case where several first switching devices are switched on simultaneously, it was recognized that the upper limit for the required power can be determined as the product of the target power and a specified scaling factor. This will first be explained using an example in which four separately switchable heating elements are used; however, this explanation can also be applied to four heating element groups connected in parallel.

[0037] The auxiliary variables used here are a maximum device power and a required maximum power, which are initially unknown. The maximum device power is the power the heating device would consume if all heating elements were operating simultaneously. The required maximum power is the power consumed by the heating device if the maximum number of heating elements is operating simultaneously for a given control pattern. For example, if a maximum of two heating element operating times overlap in a control pattern, i.e., a maximum of two first switching devices are switched simultaneously, the required maximum power is the power consumed when two heating elements are operating simultaneously.

[0038] Assuming that the switch-on times are evenly distributed throughout the switching cycle, as explained above, the operating times of two heating elements in the example overlap precisely when the individual heating element is operated for at least a quarter of the duration of the switching cycle. Assuming, for simplicity, that the heating elements all have the same resistance and thus the same power consumption, this is precisely the case when the target heating power is at least a quarter of the device's maximum power. Thus, for this case, an upper limit for the unknown device's maximum power can be specified, which corresponds to four times the target heating power.

[0039] However, since the required maximum power is exactly half of the maximum device power when exactly two of the heating elements are activated overlappingly, since a maximum of two of the four heating elements are operated, an upper limit can be specified for the required maximum power that corresponds to twice the target heating power, which means that in this case the scaling factor for determining the upper limit for the required power would be two.

[0040] With the control strategy explained above, overlapping activation of three of the four heating elements occurs when the target heating output reaches or exceeds half of the device's maximum output. Thus, the device's maximum output is less than or equal to twice the target heating output. Since a maximum of three of the four heating elements are operating, the required maximum output is equal to three-quarters of the device's maximum output, which means that the upper limit for the required output in this case can be specified as 1.5 times the target heating output.

[0041] With the control strategy described, all four heating elements are active simultaneously when the target heating output reaches three-quarters of the device's maximum output. Thus, the upper limit for the device's maximum output, which in this case is equal to the required maximum output, can be specified as four-thirds of the target heating output.

[0042] By comparing the upper limits for the required heating output explained above, it can be seen that the upper limit determined for the simultaneous operation of two heating elements is greater than the upper limit determined for the simultaneous operation of three or four heating elements. Thus, in general, for the case where more than one heating element is operated, an upper limit for the required output can be specified as twice the target heating output. This corresponds to the procedure explained above, with a fixed scaling factor of 2.

[0043] The above discussion can also be applied to cases with different numbers of heating elements or heating element groups that are controlled separately. In general, regardless of the number of heating elements or heating element groups present, the required maximum power for a case in which a maximum of a certain number of heating elements or heating element groups are active simultaneously is the product of the target heating power and the quotient of this number and the number reduced by one. For example, if a maximum of six heating elements or heating element groups are active simultaneously in a control sequence, the prefactor of the target heating power would be 6 / 5.

[0044] In this case too, an upper limit for the required maximum power results in the case in which exactly two of the heating elements or heating element groups are used simultaneously, so that again the scaling factor can be set to 2.

[0045] As explained above, the maximum power required when several heating elements or groups of heating elements are operated simultaneously in relation to the target heating power is at its maximum when a maximum of two heating elements are operated simultaneously in a control pattern. The factor of 2 for the upper limit described above assumes that each of the two heating elements or groups of heating elements requires a maximum of the target heating power. If heating elements or groups of heating elements with different resistances are used, the maximum required power occurs when the heating element or group of heating elements is operated with the minimum resistance. The upper limit of the required maximum power can therefore be specified as the sum of the power required to operate this heating element or group of heating elements and the target heating power.At the same time, however, this sum is always greater than the power required to operate this heating element or group of heating elements, so it represents an upper limit even if only one of the heating elements is operating. This consideration gives rise to the second approach explained above for determining the upper limit for the required power.

[0046] The two alternatives discussed above assume that at least approximately equal resistances of the heating elements or groups of heating elements can be assumed, or that at least all heating elements or groups except for the heating element or group with the minimum resistance require a maximum of the target heating power during operation. In cases where the resistances of the various heating elements or groups differ significantly, it can be advantageous to determine the upper limit as the product of a scaling factor specified as a function of the target heating power and the power required to operate the heating element or group of heating elements with the minimum resistance. In particular, the scaling factor can be equal to the maximum number of heating elements or groups that can be operated simultaneously. This corresponds to the assumption of a worst-case scenario in which several heating elements orGroups with minimal resistance are active simultaneously.

[0047] Further advantages and details of the invention will become apparent from the following exemplary embodiments and the accompanying drawings. These schematically show: Fig. 1 an embodiment of a motor vehicle according to the invention, Fig. 2 an embodiment of a heating device according to the invention, and Fig. 3 and Fig. 4 control patterns for controlling the Fig. 2 shown switching means of the heating device.

[0048] Fig. 1 shows a motor vehicle 1 comprising a heating device 4. The heating device 4 is part of a ventilation device 2, which draws in air through an intake opening 34 and supplies it to the interior 43 of the motor vehicle 1 through a conveying device 3, for example a fan, via an air outlet 5. For heating and in particular for drying the air, the ventilation device 2 comprises the heating device 4, the structure of which in Fig. 2 is shown schematically.

[0049] The heating device 4 comprises a plurality of heating elements 12 to 19, which are connected as a respective heating element group 8 to 11 via first switching means 21 to 24 to a first power supply connection 7 of the heating device 4. One of the heating elements 12 to 19 of each group 8 to 11 is connected to one of the second switching means 25, 26 and via this to a second power supply connection 6 of the heating device 4. In the example shown, the heating elements 12 to 19 are connected in series with a respective diode 44. This can prevent an additional current path via the heating elements 12, 13 and 15 from resulting, for example, when the switching means 21, 22 and 25 are closed when the switching means 26 is open.Particularly in the case of significant resistance differences between the heating elements 12 to 19, such undesired current paths could significantly reduce the power reduction when opening one of the switching devices 25, 26. This is prevented by the diodes 44.

[0050] The individual heating elements 12 to 19 can be PTC ceramic elements or groups of PTC ceramic elements connected in parallel. The heating elements 12 to 19 can, in particular, be arranged in the air duct 42 of the ventilation device 2 such that they directly contact an air flow passing through it.

[0051] A control device 20 of the heating device 4 controls the first and second switching means 21 to 26 according to a specific control pattern in order to selectively couple the heating elements 12 to 19 or the heating element groups 8 to 11 to the first and second power supply terminals 6, 7 or to disconnect them from them. By selecting the appropriate control pattern, in particular by specifying a pulse width of a control signal or a time interval between the respective times for switching on and off the respective switching means 21 to 26, the heating power of the heating device 4 can be set to a predetermined target heating power.

[0052] An example of the control of the switching means 21 to 24 is shown in Fig. 3. In the Fig. In the embodiment shown in Figure 3, it is assumed that both second switching means 25, 26 are closed. This can be advantageous, for example, if relatively high heating powers are to be provided. A corresponding control pattern can also be used if, in an alternative embodiment, the heating device 4 does not include second switching means 25, 26, but rather the individual heating elements 12 to 19 are connected directly to the second power supply connection 6.

[0053] In Fig. 3, time is represented on the X-axis 27, and four lines mark the times at which the respective first switching means 21 to 24 are closed, thus energizing the respective associated heating element group 8 to 11. Control occurs cyclically with a switching cycle 29. The respective time interval 28 between a switch-on time 35 and a switch-off time 36 of the respective first switching means 21 to 24 indicates for which portion of the switching cycle 29 the associated heating element group 8 to 11 is operated.

[0054] In the example shown, heating element groups 8 to 11 are operated for the same length of time, and it is approximately assumed that heating element groups 8 to 11 have the same resistance or require the same power during operation. Thus, the quotient of the time interval 28 and the duration of the switching cycle 29 corresponds to the proportion of the heating device's maximum power that is provided as the average heating power during the switching cycle 29.

[0055] In the simplest case, all four first switching elements 21 to 24 would be switched synchronously, so that for a short portion of the switching cycle 29, all four individual groups 8 to 11 would be operated. However, this would result in a very high maximum power consumption of the heating device, even if only a relatively low target heating power is to be provided. Therefore, the control device 20 is configured to space the switching times 35, 37, 38 of the various first switching means 21 to 24 apart by a fixed time interval 30. As shown in Fig. 3 is shown specifically for the first switching means 22 and 23, the switching-on time 38 of the first switching means 22 is spaced by a fixed predetermined time interval 30 from the switching-on time 37 of the previously switched first switching means 23.

[0056] The time interval 30 is advantageously selected such that it corresponds to the quotient of the duration of the switching cycle 29 and the number of first switching means 21 to 24, i.e. in the example shown, a quarter of the duration of the switching cycle 29. This ensures that in the example only two of the heating element groups 8 to 11 are energized simultaneously when the time interval 28 reaches a quarter of the duration of the switching cycle 29. Simultaneous operation of three of the heating element groups 8 to 11 only occurs when the time interval 28 reaches half the duration of the time interval 29, and simultaneous operation of all four of the heating element groups 8 to 11 only occurs when the time interval 28 reaches three-quarters of the duration of the time interval 29.In this way, it can be achieved that the maximum required power, which approximately corresponds to the product of the maximum number of simultaneously active heating element groups 8 to 11 and a quarter of the total heating power of the heating device, is kept as low as possible over a wide range of target heating powers.

[0057] The described use of a fixed time interval 30 between the switch-on times 35 to 37 results in the fact that, with larger time intervals 28 between the switch-on times 35, 37, 38 and the switch-off times 36 of the individual first switching means 21 to 24, at least one of the switching means, in the example the switching means 21, remains switched on beyond the cycle limit 39 of the switching cycle 29.

[0058] In particular, in order to achieve a significant reduction in the maximum power requirement of the heating device even at low target heating powers, the second switching elements 25, 26 can be used additionally. An example of a corresponding control pattern is shown in Fig. 4. Here, the control of the first switching elements 21 to 24 corresponds, apart from a changed time interval 33 between the respective switching-on time 35, 37, 38 and the respective switching-off time 36, which with reference to Fig. 3 discussed control.

[0059] In addition, the second switching means 25, 26 are controlled such that during a respective switching cycle 29 of the first switching means 21 to 24, only one of the first switching means 25, 26 is closed, and thus only one of the heating elements 12 to 19 of each heating element group 8 to 11 is connected to the second power supply terminal 6 and can thus be supplied with current by closing the first switching means 21 to 24. Thus, the duration 31 for which the respective second switching means 25, 26 remains closed corresponds to the duration of the cycle 29.

[0060] Preferably, the second switching means 25, 26 are switched alternately, so that for the second switching means 25, 26 a switching cycle 32 results that is twice as long as for the first switching means 21 to 24. By means of the described procedure, in cases in which the target heating power is less than half the maximum heating power of the heating device 4, it can be achieved that by switching on the individual heating element groups 8 to 11 by the first switching means 21 to 24, smaller power changes result, since only one of the heating elements 12 to 19 is switched at a time. The finer power gradation allows maximum power peaks to be further reduced.

[0061] An on-board electrical system of the motor vehicle 1 typically supplies not only the heating device 4 but also other devices 41 of the motor vehicle 1, for example a drive motor. Therefore, the control device 20 or a processing device 40 of the motor vehicle 1 can be configured to determine, as a function of a predetermined target heating power, an upper limit for the power required by the heating device 4 to achieve this target heating power and to control the heating device 4 or the other device 41 as a function thereof in order to avoid, for example, an overload of the on-board electrical system. For example, if the interior temperature is sufficient and the windows are not fogged up, the target heating power can be temporarily reduced until sufficient power is available again orIn situations where sufficient heating is necessary, for example, in the case of low interior temperatures, fogged windows, or the like, the additional device 41 can be activated to reduce its energy consumption. The determination of a corresponding upper limit for the required power of the heating device 4 has already been described in detail in the general part of the description. To avoid unnecessary repetition, reference is made to this explanation.

Claims

[1] Heating device for a motor vehicle (1), in particular for heating an air flow supplied to the vehicle interior (43), comprising a control device (20) and a plurality of heating elements (12 - 19), which are each connected individually or in heating element groups (8 - 11) via a respective first switching means (21 - 24) assigned to the heating element (12 - 19) or the heating element group (8 - 11) to a first power supply connection (7) of the heating device (4), wherein the control device (20) is designed to control the first switching means (21 - 24) in successive switching cycles (29) one after the other at a respective switching-on time (35, 37, 38) in the respective switching cycle (29) in order to connect the assigned heating element (12 - 19) or the assigned heating element group (8 - 11) to the first power supply connection (7), and at a respective Switch-off time (36) in the respective switching cycle (29),to disconnect the respective associated heating element (12 - 19) or the associated heating element group (8 - 11) from the first power supply connection (7), characterized bythat the control device (20) is configured, on the one hand, to control the first switching means (21 - 24) in such a way that the switch-on time (35, 37, 38) of a respective first switching means (21 - 24) has a fixed predetermined time interval (30) to the switch-on time (35, 37, 38) of the previously switched first switching means (21 - 24), and, on the other hand, to set the time interval (28, 33) of the switch-off time (36) of the respective first switching means (21 - 24) to the previous switch-on time (35, 37, 38) of this switching means (21 - 24) as a function of an operating voltage applied between the first and a second power supply connection (6, 7) of the heating device (4), and / or a detected actual current through the heating device (4) and / or a determined resistance of at least one of the Heating elements (12 - 19) or a parallel connection of at least two of the heating elements (12 - 19). [2] Heating device according to claim 1, characterized bythat the fixed time interval (30) between the switching-on times (35, 37, 38) corresponds to the quotient of the duration of the switching cycle (29) and the number of first switching means (21 - 24) of the heating device (4). [3] Heating device according to claim 1 or 2, characterized by in that the control device (20) is designed to additionally specify the time interval (28, 33) of the switch-off time (36) of the respective first switching means (21 - 24) to the preceding switch-on time (35, 37, 38) of this switching means (21 - 24) as a function of a predetermined target heating output and / or a determined temperature of at least one component of the heating device (4). [4] Heating device according to one of the preceding claims, wherein the heating elements (12 - 19) in heating element groups (8 - 11) are connected to the first power supply connection (7) of the heating device via the respective first switching means (21 - 24) assigned to the heating element group (8 - 11), characterized byin that the heating device (4) comprises second switching means (25, 26), each of which connects a heating element (12 - 19) of each heating element group (8 - 11) to the second power supply connection (6) of the heating device (4), wherein the control device (20) is designed to control the second switching means (25, 26) when a low-power condition dependent on the or a target heating power is fulfilled in such a way that in each of the switching cycles (29) only one of the heating elements (12 - 19) of a heating element group (8 - 11) or only a subgroup of these heating elements (12 - 19) which does not comprise all of the heating elements (12 - 19) of the heating element group (8 - 11) is connected to the second power supply connection (6). [5] Heating device according to one of the preceding claims, characterized by that a cycle length of the switching cycle (29) is longer than 2 ms or longer than 5 ms. [6] Motor vehicle, characterized bythat it comprises a heating device (4) according to one of the preceding claims. [7] Motor vehicle according to claim 6, characterized by that a power supply of the heating device (4) provides a maximum operating voltage of at least 100 V or at least 200 V or at least 400 V and / or a maximum heating power of at least 4 kW or at least 7 kW or at least 10 kW. [8] Motor vehicle according to claim 6 or 7, characterized by in that it has a ventilation device (2) for supplying an air flow into the interior (43) of the motor vehicle (1), wherein the heating device (4) serves to heat the air flow, wherein in particular the heating elements (12 - 19) are arranged in an air duct (42) carrying the air flow in such a way that they come into direct contact with the air flow. [9] Motor vehicle according to one of claims 6 to 8, characterized bythat the control device (20) or a processing device (40) of the motor vehicle (1) is designed to determine, as a function of the or a predetermined target heating power, an upper limit for a power required for the heating device (4) to achieve this target heating power and to operate the heating device (4) and / or a further device (41) of the motor vehicle as a function of the upper limit. [10] Motor vehicle according to claim 9, characterized by that the control device (20) or the processing device (40) is designed to - in a first alternative, to determine the upper limit as the maximum of, on the one hand, the power required to operate the heating element (12 - 19) or the group of heating elements (8 - 11) with the minimum resistance and, on the other hand, a product of the target heating power and a predetermined scaling factor, or - in a second alternative, to determine the upper limit as the sum of the power required to operate the heating element (12 - 19) or the group of heating elements (8 - 11) with the minimum resistance and the target heating power, or - in a third alternative, to determine the upper limit as the product of a scaling factor specified as a function of the target heating power and the power required to operate the heating element (12 - 19) or the group of heating elements (8 - 11) with the minimum resistance.

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