WO2025026926A1 - Electric heater - Google Patents

Abstract

The invention relates to an electric heater (100), comprising: a first connection which conducts a voltage potential (HV+) and a second connection which conducts a reference potential (HV-); and a number of at least two strands (10, 20, 30) which are connected in parallel between the first connection and the second connection. At least one heat resistor (R1, R2, R3) is arranged in each strand (10, 20, 30), and in each strand (10, 20, 30), a high-side switch device (S1, S2, S3) is arranged between the heat resistor (R1, R2, R3) and the first connection (HV+) and a low-side switch device (S4, S5, S6) is arranged between the heat resistor (R1, R2, R3) and the second connection (HV-). A first node (N4, N5, N6) is arranged between the high-side switch device (S1, S2, S3) and the low-side switch device (S4, S5, S6) in a first strand of the strands (10, 20, 30), and a second node (N1, N2, N3) which is electrically connected to the first node (N4, N5, N6) is arranged between the high-side switch device (S1, S2, S3) and the low-side switch device (S4, S5, S6) in a second strand of the strands (10, 20, 30). The corresponding heat resistors (R1, R2, R3) can be connected by the switch devices (S1 – S6) of the first strand and the second strand in a selective manner in series via the connected nodes (N1 – N6) or individually between the voltage and reference potential in each case.

Description

ELECTRIC HEATING

DESCRIPTION

technical field

Various aspects relate to an electric heater, in particular an electric heater for a vehicle, for example a motor vehicle, and a method for controlling or operating the electric heater.

Technical Background

Vehicles are increasingly being equipped with electric heaters, not only because of the current rise of electromobility. This has also been the case for some time now, at least for auxiliary or auxiliary heaters in vehicles powered by combustion engines, for example, when fuel supply lines can be saved.

Electric heaters, which are intended for electric vehicles, for example, convert electrical energy into thermal energy in one or more heating resistors. To regulate the power, these heating resistors are often operated in a pulsed manner using pulse width modulation (PWM). More precisely, assigned power switches are controlled in a pulsed manner, which, due to their wiring, supply the heating resistor(s) as a load with a corresponding voltage, whereby the desired heating power can be set using a duty cycle.

In the case of electric vehicles in particular, the input voltages to be used are in the high-voltage range, at 50 V or more, which is at least a factor of 2 higher than the input voltages used in the classic case, e.g. vehicles with combustion engines. In practice, these high input voltages mean that a wide range of voltages must be covered, and they can therefore be used with comparatively unfavourable This can be accompanied by a reduction in duty cycles, particularly to extremely low duty cycles when small heating outputs are required, but this has the disadvantageous effect that high ripple currents can arise due to the resistances involved. These are undesirable and in the worst case can spread through an electrical network to which the heater is connected, e.g. a vehicle's electrical system. The ripple currents can be filtered out again by further measures, but this increases overall costs and effort.

An approach described in WO 2017/186958 A1, for example, to reduce ripple currents that are based on common-mode currents caused by capacitances can consist of dividing a total resistance connected between the poles of the input voltage into several individual or partial heating resistors. A switching device set up on the high side (with respect to the input voltage) and a switching device set up on the low side are connected in series with the partial heating resistors arranged between them and are switched on and off synchronously using PWM. Optionally, a node that is at an intermediate potential can also be set up between two of the partial heating resistors connected in series.

Another approach, described in DE 10 2011 009 672 A1 or DE 10 2011 007 817 A1, for example, involves setting up several heating resistors, which can then be connected in parallel or in series using power switches in order to further reduce the ripple current, among other things. The aim is to select a total resistance for a given voltage and a desired heating power using a suitable parallel or series connection, which is dimensioned in such a way that the highest possible duty cycle is required for the pulse width modulation.

However, these solutions can have a disadvantage in that a series connection only reduces a ripple current in the lower power range. Furthermore, a switching transition between the series and parallel connection is not immediately useful. In any case, in such a case, exact timing of the switching times for the circuit breakers would be critical. A Inaccurate coordination of the switching processes between the circuit breakers or excessive tolerances can theoretically even lead to current flows that damage the components. It can also happen that the heating power is not evenly distributed across the individual (partial) heating resistors. Such a situation can make it necessary to over-dimension the heating resistors in terms of power, which in turn results in an unnecessary increase in effort and costs.

DE 10 2016 109 039 A1 discloses a heater for motor vehicles, among other things. This comprises three parallel strands, each with a heating resistor consisting of an ohmic resistance and an inductance. The individual strands are each set up to be controllable by a low-side PWM control. For safety reasons, a diode can be connected anti-parallel to the series connection of resistance and inductance in the heating resistor in order to prevent or reduce voltage peaks in the negative direction.

DE 31 33 325 A1 discloses a power supply circuit for an electrical consumer with heating resistors connected in two parallel strands and a fan motor. The fan motor is connected in the middle branch of a diode bridge, which is connected on the one hand to taps of the two heating resistors and on the other hand to a pole of the supplying AC voltage source.

DE 10 2007 006 438 A1 discloses a circuit for simultaneously controlling an arrangement of similar consumers. Several consumers are connected in series and form a string. Several such strings can be operated in parallel. The number of consumers connected in series in a string can be selected depending on the operating voltage. Connecting lines are set up between the relevant string lines, in which switches and/or consumers are arranged. One switch is provided for each string. The consumers are primarily light-emitting diodes with a forward voltage, whereby the switches are operated in such a way that a number of light-emitting diodes adapted to the operating voltage are connected in series in order to achieve maximum efficiency. The LEDs are operated at a constant luminosity by means of current regulation. Pulse width modulation is rejected due to the EMC emissions caused by ripple currents. Resistive heating elements are also considered as consumers, but if these are ohmic resistors, the lack of forward voltage means that it remains unclear how the number of heating elements connected in series is selected in order to improve efficiency.

There is therefore a need for an electric heater and/or a method for controlling the same, in which the disadvantages described above can be overcome and, in particular, ripple currents can be further reduced.

presentation of various aspects

According to aspects of the invention that can take such needs into account, an electric heater is proposed that comprises a first terminal carrying a voltage potential HV+ and a second terminal carrying a reference potential HV-, as well as a number of at least two strands that are connected in parallel between the first terminal and the second terminal. At least one heating resistor is installed in each of the strands. Furthermore, a high-side switching device is arranged in each of the strands between the heating resistor and the first terminal (voltage potential HV+) and a low-side switching device is arranged between the heating resistor in question and the second terminal (voltage potential HV-). Two switching devices are therefore provided in each strand. The heating resistor can be composed of several individual or partial heating resistors connected in series. Additional components, in particular additional heating resistors, can optionally be provided between the two switching devices and the respective terminals. However, these can also be explicitly excluded.

In a first of the strands, a first node is set up between the high-side switching device and the low-side switching device. In a second of the A second node electrically connected to the first node is set up between the high-side switching device and the low-side switching device. The nodes are arranged in their strands in such a way that the corresponding heating resistors can be switched by the switching devices of the first strand and the second strand either in series via the connected nodes or individually between the voltage and reference potential.

This can be achieved according to one embodiment in that the first node is connected between the relevant heating resistor and the relevant low-side switching device of the first strand, and the second node is connected between the relevant high-side switching device and the relevant heating resistor of the second strand. In the case of several partial heating resistors, the node can also be located between two of these partial heating resistors. It is important that at least one partial heating resistor in the second strand is connected between the relevant node and the low-side switching device.

The switching devices can be power switches, in particular semiconductor switches such as bipolar transistors, field effect transistors, or IGBTs (Insulated Gate Bipolar Transistors), or power MOSFETs (metal oxide semiconductor field effect transistors), e.g. silicon carbide MOSFETs.

The starting point of embodiments of the invention is therefore a structure of parallel strands, each with at least one heating resistor, which can be switched by high- and low-side switching devices, in particular by pulse width modulation, as described in the prior art. The special feature of the present embodiments is that, due to the special arrangement, switching between parallel operation of the heating resistors and corresponding serial operation does not have to be carried out synchronously by two switching devices, but instead sequential switching is made possible, with only one switching device opening or closing at a given time. This alone leads to an improvement in the situation, because with synchronous When two switching devices are connected in series, not quite exact timing of the pulses can lead to common mode ripple.

Furthermore, the circuit arrangement of the electric heater can be set up to limit both the change in the current flow and the voltage applied to it (when the switching device is open) to a fraction of the maximum value, e.g. to half, during such a switching process of a respective switching device. In any case, this already significantly reduces the ripple current.

A corresponding method for controlling the electric heater accordingly provides, according to aspects of the invention, that after providing the voltage between the first terminal with the (operating) voltage potential and the second terminal with the reference voltage potential, a state is first set in which one of the two or more strands is activated, i.e. the high- and low-side switching devices are switched on, so that the relevant first heating resistor of the first strand is exposed to the full input voltage and a maximum current flows through it, and another second strand assigned via the nodes is inactive, i.e. the high- and low-side switching devices are switched off and therefore no current flows through the relevant second heating resistor in the second strand.

After a first period of time has elapsed after this state has been set, the low-side switching device of the second strand can now be switched on - alone or asynchronously. This opens an electrical connection between the first and second connections providing the input voltage, not only via the two switching devices of the first strand but also via the high-side switching device of the first strand and - via a connecting line between the two nodes - the low-side switching device of the second strand. However, no current flows through the latter connection in this switching state. After a second period of time has elapsed after the low-side switching device of the second strand is switched on, the low-side switching device of the first strand is then switched off - also alone or asynchronously. As a result, only one current now flows through the high-side switching device and the first heating resistor of the first strand, the connecting line between the two nodes, and the second heating resistor and the low-side switching device of the second strand. If the heating resistors of the two strands are designed identically, which corresponds to a preferred embodiment, then the current flow through the circuit arrangement is halved, while the voltage applied to the low-side switching device that has just been opened increases to half the value of the input voltage. During this switching process, the ripple current is also comparatively reduced in the same way as before when switching the low-side switching device of the second strand.

The described sequence of controlling the electric heater can be continued according to specific embodiments by switching on the high-side switching device of the second strand next (after a third period of time has elapsed after the low-side switching device of the first strand has been switched off). The first strand is now deactivated and the second strand activated, since current only flows through the latter. This increases again (due to the fact that there is now only one heating resistor) from, for example, a halved value to a maximum value. In this step, the system switches back from the serial or series connection of the heating resistors of the second strand to the individual, non-serial loading of a heating resistor, in contrast to the initial state set at the beginning, now in the second instead of the first strand.

After a fourth time period has elapsed after the high-side switching device of the second string has been switched on, the high-side switching device of the first string is switched off.

In this operation, the heating operation is switched from the first to a second line by individualized switching devices that are not simultaneous, temporally decoupled switching processes. In addition, the proposed electric heating and the corresponding control method make it possible to go through an intermediate step with series-connected heating resistors, in which the heating power only takes up a fraction of the maximum heating power (achievable by one string alone), e.g. , etc., but does not drop directly to zero. Overall, the voltage and current jumps during the switching processes are therefore smaller compared to the state of the art.

At the same time, however, control of the switching devices by pulse width modulation (PWM) is retained, because the duty cycle can still be set by appropriately selecting the window widths of the time periods mentioned above. By switching the switching devices on and off (or closing and opening) in a coordinated manner, they are each controlled directly in a pulse width modulated manner. Depending on the current path resulting from this in the circuit arrangement, this also results in pulse width modulation with regard to the current supply to the heating resistors and the voltage drop across them, and ultimately the heating power. However, precisely because, as described in the special embodiment, in the latter case the two signal levels of the pulse width modulation are different from zero (see, for example, Fig. 2B below for the special embodiment), the PWM can provide a significantly reduced ripple power supply, particularly in the high power range.

As mentioned above, the first node of the first strand and the second node of the second strand can be electrically connected by a connecting line. According to embodiments, an electrical component can be connected in the connecting line, which prevents a current flow from the second node to the first node. This is preferably a diode. In principle, however, it would also be possible to have another switch in the connecting line instead of a diode, which is opened or closed at a suitable time, in particular to prevent a short circuit between the connections. According to a further, particularly advantageous embodiment, each of the strands has a first node and a second node. In the process described, each strand is assigned a first adjacent strand to which the activation of the respective heating resistor can be transferred according to, for example, four steps (corresponding to four time periods), and a second adjacent strand is assigned from which the activation is transferred in an analogous manner. In the case of only two strands in total, the activation of the heating resistors can be transferred alternately, ie the arrangement of switching devices, heating resistors, nodes, connecting lines and diodes is completely symmetrical.

In embodiments with at least three strands, the first node of the second strand can also be connected to the second node of a third strand that is different from the first strand. If a fourth strand is present, a first node of the third strand can be connected to a second node of the fourth strand. In this way, all strands can be connected to one another in sequence via connecting lines between the respective first and second nodes involved, with the last of the strands with its first node being connected again to the second node of the first strand via the corresponding connecting line, thereby closing the ring. In the process sequence described above, the heating resistors are then advantageously activated in sequence. With four or more strands, two strands can even be activated at the same time, and with six or more strands, three strands can be activated at the same time. The activations rotate in unison.

With low input voltages and high power, it is even possible to operate six strings simultaneously. Power control can also be carried out using classic PWM, with only one of the strings being switched off at a time if possible.

The electric heater according to the above aspects, embodiments and embodiments as well as the corresponding method bring further advantages and open up interesting designs: for example, the high-side switching devices in the strings can be set up as thyristors, preferably as SCRs (silicon controlled rectifiers), while the low-side switching devices are set up as transistors (MOSFETS, IGBTs, etc. as described above). This is possible because the high-side switching devices can be switched off without power (see process sequence above and in the embodiments described below). Therefore, if a safety shutdown in the event of a fault is not required or is implemented in another way, the switching functionality of the high-side switching device can also be implemented with more cost-effective thyristors or SCR components.

These or other embodiments can also provide that such drivers configured for switching the high-side switching devices are supplied with power from a simple bootstrap circuit. This can then consist, for example, of just a capacitor and a diode. The reason is that in the described process sequence, both a voltage of 0 V and the full input voltage are present at the low-side switching device provided in the same strand, albeit only for a short period of time (in the above explanations, for example, the fourth period of time in relation to the low-side switching device of the first strand). If this (e.g. fourth) period of time is chosen to be sufficiently long, this is sufficient for the power supply of the relevant driver of the high-side switching device via a simple bootstrap circuit, which saves a complex structure for the level shift and thus costs.

According to further developments, the electric heater can have a control device that is configured to open and close the switching devices separately from one another and individually. This control device can preferably be configured to adjust the heating power by actuating the switching devices using pulse width modulation, as already mentioned above. As described, within a period of pulse width modulation, the switching devices can only be actuated sequentially with an intermediate time interval. Overall, aspects of the invention offer advantages in that, for example, the power can be distributed evenly across the heating resistors. In the prior art, e.g. DE 10 2011 009 672 A1, this load distribution is partially asymmetrical depending on the operating mode and circuit. Furthermore, the state transitions occur directly, without the need for an intermediate shutdown. Furthermore, a reduction in the ripple current by a factor of 2 to 6 or more can be achieved with a correspondingly larger number of heating resistors. This is accompanied, for example, by half the voltage load for the switching devices, which increases the durability of the electric heater. Furthermore, a significant reduction in switching losses can be observed, since during operation the high-side switching devices are switched off without loss and the low-side switching devices are switched on without loss. In addition, only half or a third of the voltage is switched during the lossy switching processes, which also significantly reduces the losses here.

In addition, the requirements for precise switching of the switching devices are not critical, so that the effort can be reduced here too. In addition, the individual signals for controlling the switching devices each represent normal PWM signals, so that despite the seemingly complex process sequence, a simple PWM-modulated heating power curve results.

Furthermore, as described, simplifications can be made in the components used, such as supplying the high-side switch control (driver) with simple bootstrap circuits, or using thyristors at this point. Furthermore, negative effects due to possible parasitic inductances of the resistors are significantly reduced, since the current changes are smaller and correspondingly smaller switch-off peaks occur at the switches.

Further advantages, features and details of the various aspects emerge from the claims, the following description of preferred embodiments and from the drawings. In the figures, the same reference symbols designate the same features and functions. Short description of the drawings

They show:

Fig.1 is a circuit diagram of an electric heater according to an embodiment;

Fig. 2A shows a schematic representation of a time diagram with the temporal course of the current flows resulting in the three heating resistors from Fig. 1 during operation during a PWM period;

Fig. 2B as Fig. 2A, but showing only the total current flow;

Fig. 3A like Fig. 2A, but only for the current flow through one of the heating resistors;

Fig. 3B shows a schematic representation of a timing diagram with the temporal course of the voltage drop at the low-side switching device of the first phase during a PWM period;

Fig. 4A shows the circuit diagram from Fig. 1 for a concrete switching state in which one third of the maximum heating power of a strand is achieved;

Fig. 4B shows the circuit diagram from Fig. 1 for a concrete switching state in which half of the maximum heating power of a strand is achieved;

Fig. 4C shows the circuit diagram from Fig. 1 for a concrete switching state in which one and a half times the maximum heating power of a strand is achieved.

Preferred embodiment(s) of the invention In the following description of preferred embodiments, it should be noted that the present disclosure of the various aspects is not limited to the details of the construction and arrangement of the components as shown in the following description and in the figures. All embodiments, even those not shown in the figures, can be implemented or carried out in various ways. It should also be noted that the expression and terminology used herein is used only for the purpose of concrete description and should not be interpreted as such in a limiting manner by those skilled in the art. Furthermore, in the following description, like reference numerals in the figures designate like or similar features or objects, so that in some cases a repeated detailed description of them is omitted in order to maintain the compactness and clarity of the illustration.

Embodiments relate to an electric heater 100 that converts electrical energy into heat in order to heat a medium or a volume directly or indirectly. Such a medium can be a fluid, for example a gas such as air or a liquid such as water. The electric heater is intended in particular for mobile use, for example for a vehicle, preferably a hybrid or purely electrically operated motor vehicle.

Fig. 1 shows a circuit diagram of an electric heater 100 for a motor vehicle, according to an embodiment. An (operating) voltage potential HV+ is provided via a line connected to a first connection 1, e.g. directly or indirectly from a vehicle battery (not shown). A reference or reference potential HV-, e.g. a ground or mass potential, is provided via a line connected to a second connection 2. Here, the two lines are considered to be part of the two connections 1, 2. The voltage determined by the potential difference between the voltage potential HV+ and the reference potential HV- is 50 volts or more (high-voltage range) in modern electrically powered motor vehicles. Three strands 10, 20, 30 are interposed in a parallel arrangement between the two connections 1, 2. In alternative embodiments, 2 or 4 or more of the strands shown in Fig. 1 can also be provided.

All three strands 10, 20, 30 are identical. Each strand 10, 20, 30 has a heating resistor R1, R2, R3, which according to optional embodiments can be divided into various partial resistors (not shown). The heating resistors R1, R2, R3 are each arranged between switching devices S1 - S6 set up within each strand. For example, a high-side switching device S1, a heating resistor R1 and a low-side switching device S4 are connected in series in the first strand 10. Similarly, a high-side switching device S2, a heating resistor R2 and a low-side switching device S5 are connected in series in the second strand 20, and a high-side switching device S3, a heating resistor R3 and a low-side switching device S6 are connected in series in the third strand 30.

The heating resistors can be equipped with a high resistance value compared to line resistors in order to convert electrical energy into heat when current flows and to release this heat into a fluid. Such a heating resistor can, for example, be designed in the form of a line wire or heating wire wound into a coil. Possible materials include cold or hot conductors, ceramics and metals, and all of the embodiments listed here are not limited to specific materials and functions.

The switching devices S4 - S6, which are identical in the embodiment, can be power switches, e.g. power MOSFETs or IGBTs etc. The switching devices S1 - S3, which are identical in design, can also be power switches of the type mentioned, or thyristors. The drivers (not shown) of the high-side switching devices can have a bootstrap circuit with a simple structure, e.g. in a known manner from just one capacitor and one diode. The switching devices S1 - S6 are controlled or opened or closed by means of drivers by a control device 200. For this purpose, the control device 200 comprises a pulse width modulation device (not shown) which individually sets an opening and closing time for each switching device. None of the switching devices S1 to S6 shown in Figure 1 opens or closes at the same time as another of the switching devices S1 to S6, i.e. the opening and closing times are coordinated with one another, in particular the cycle or period length is identical for each of the switching devices S1 - S6, but the individual pulses are not switched at the same time but rather decoupled from one another in time, although they can overlap one another in time. The time interval between a closing time and an opening time determines the pulse duration. The pulse duration is determined depending on the desired heating output. The pulse duration is calculated from a predetermined heating power in the control device 200. For this purpose, the pulse duration can be variably set by the control device 200 for each of the switching devices S1 - S6.

Each of the strands 10, 20, 30 comprises two nodes which are arranged on a strand length between the respective switching devices. Each node represents a tapping point for a connecting line to a different one of the strands in the electric heater 100.

In the first strand 10, a first node N4 is set up between the low-side switching device S4 and the heating resistor R1. Furthermore, a second node N1 is set up between the high-side switching device S1 and the heating resistor R1. In the same way, in the second strand 20, a first node N5 is set up between the low-side switching device S5 and the heating resistor R2. Furthermore, a second node N2 is set up between the high-side switching device S2 and the heating resistor R2. Finally, in the third strand 30, a first node N6 is also set up between the low-side switching device S6 and the heating resistor R3. Furthermore, a second node N3 is set up between the high-side switching device S3 and the heating resistor R3. In each of the strands 10, 20, 30, the first node N4, N5, N6 is connected via a connecting line to the second node N1, N2, N3 of an adjacent strand. In each of the connecting lines, a diode D1, D2, D3 is provided, the anode connection of which is connected to the corresponding first node N4, N5, N6 and the cathode connection of which is connected to the second node N1, N2, N3 assigned via the connecting line.

A method for controlling the switching devices S1 to S6 or for operating the electric heater 100 with the aid of the control device 200 is explained with reference to Figures 2A to 3B.

First, an initial state is arbitrarily set in which only the first branch 10 is activated, i.e. the switching devices S1 and S4 are switched on, while the switching devices S2, S3, S5 and S6 are switched off and thus the branches 20, 30 are deactivated. Figure 2A shows a schematic representation of a time diagram with the temporal progression of the current flows resulting in the 3 heating resistors from Fig. 1 during operation during a PWM period for the switching cycle of the switching devices S1 - S6, whereby this PWM period begins, for example, at time t0 and ends at time t12, at which the same state as time t0 is reached. Each of the switching devices S1 - S6 is operated individually in a pulse width modulated manner according to a corresponding period as shown in Fig. 2A, whereby the switching times can be variably set and the resulting window widths determine the heating power. Fig. 2B shows the overall current flow resulting in the circuit arrangement. The unit for the current flow selected on the coordinate axis is chosen so that the maximum current flowing through a single strand has the value "1". It is ideally assumed that the voltage provided is essentially constant over time.

The "activation" of a strand here refers to the state of the first strand 10 described above. In this state, the maximum current flows through the first strand 10, with only the heating resistor R1 of the first strand 10 being loaded. At switching time t1, the low-side switching device S5 of the second string 20 is switched on by the control device 200, as can be seen by the upward arrow behind the switch designation in Fig. 2A.- 3B - switching off is indicated in the figures by the downward arrow behind the switch designation. The current path does not change due to the additional heating resistor R2 seen from the first node N4 of the first string 10 when a current path via the connecting line to the second node N2 of the second string 20 is considered.

At switching time t2, however, the low-side switching device S4 of the first branch 10 is now switched off. In this case, the current path changes and includes the high-side switching devices S1, the heating resistor R1 and the first node N4 of the first branch 10, the diode D2 of the connecting line as well as the second node N2, the heating resistor R2 and the low-side switching device S5. This state is shown in Fig. 4B. The heating resistors R1 and R2 are connected in series. The corresponding current flows IRI and IR2 are now at a factor of 0.5, as can be seen in Fig. 2a, as is the total current flow ISUM, as can be seen in Fig. 2B, which reflects the heating power.

While Figure 3A once again illustrates the current flow through the heating resistor R2 of the second phase 20, Figure 3B specifically shows the voltage drop across the low-side switching device S4 of the first phase 10 during the illustrated PWM period for switching the switching devices S1 - S6. At switching time t2 (the switching device S4 opens), there is indeed a jump in the voltage at this switching device S4, but only by half the value of the total input voltage; the same applies to the current that does not flow from this point onwards, which falls from 0.5 to 0 (and not from 1 to 0). Although the switching process is not load-free, the load on the switch S4 is reduced.

At the next switching time t3, the high-side

Switching device S2 is switched on. This activates the second line 20. A Current now only flows through the heating resistor R2 of the second strand 20. Accordingly, the current flow IR2 through the heating resistor, which is now only loaded individually (and no longer in series), increases to the value "1" (for the heating resistor R1, this value drops to "0" because the same voltage potential is present at the nodes N1 and N2).

At the next switching time t4, the high-side switching device S1 in the first string 10 is switched off. The activation is now completely transferred from the first string 10 to the second string 20. The voltage potential dropping at the switching device S4 from this switching time t4 onwards can no longer be precisely determined, since the collector or drain side of the switching device S4 is no longer connected to the voltage supply and "floats", so to speak. This state lasts until the switching time t9, at which the switching device S4 closes. Until then, with regard to the high-frequency switching operation, the actual voltage drop can be determined by parasitic capacitances and resistances. This is shown schematically in Fig. 3B by a dotted line.

In the next 4 switching times t5 - 18, the same steps are repeated in an analogous order for the transition of activation from the second strand 20 to the third strand 30.

Furthermore, in the subsequent 4 switching times t9 - 112, the same steps are repeated in an analogous order for the transition of the activation from the third strand 30 back to the first strand 10, whereby the initial state is reached. The low-side switching device S4 of the first strand 10 is switched on again at switching time t9, the high-side switching device S1 at switching time t11. It should be noted here that when the low-side switching device S4 is switched on, the voltage drops from 0.5 to 0, but in this state no current still flows through the low-side switching device S4, as can be seen in Fig. 2A. This switching on is therefore completely load-free. It should be noted that the initial state chosen here is arbitrary and can instead be any other according to the time intervals shown in Figs. 2A - 3B. It should also be noted that the time intervals or

Time periods (t1 -tO, t2-t1, t3-t2, t4-t3, t5-t4 etc.) are significantly longer than unavoidable delays occurring within error tolerances in switching events controlled by the control device. In any case, the time periods significantly exceed a period within which a current flow stabilizes after a switch is switched on.

The described process illustrates the sequential transition of activation from one strand to the next by way of example. Figure 2B shows the resulting total current flow through the electric heater 100, which corresponds to the heating power and is set by the pulse width modulation of the individual switching devices. The width of the pulses can be set by a suitable choice of the times t0 to t12. Each switching device S1 - S6 is switched on and off once during the PWM period, with the switching times being set separately from one another.

The current flow made up of all the PWM signals takes on two levels in the embodiment shown: 1.0 and 0.5. However, other constellations and embodiments are also possible. For example, a cycle can be selected in which - as shown in Fig. 4A - not only is switching alternately carried out between individual and two series-connected heating resistors R1 - R3, but also a state is included in which three series-connected heating resistors are present. Here, the value 0.33 (one third) is then achieved for the current flow. By setting up additional strands (e.g. 4, 5 or 6 strands, etc.), this value can be reduced further. Analogously - as shown in Fig. 4C - the strand not currently affected by the transfer process described above can also be switched to active instead of inactive. Here, a value of 1.5 is achieved.

It should also be noted that the diodes D1, D2 and D3 serve to prevent a short circuit. In the very specific sequence of the procedure, as described above in connection with an embodiment with 3 strands, however, the selected switching sequence does not result in the case where, for example, the switching devices S2 and S4 are switched on at the same time. As long as this switching sequence is ensured, the diodes could conceivably be dispensed with according to further embodiments. However, in this case there are considerable restrictions in that, for example, the state in which one strand is to be shaded and two others allow a series connection (factor 1.5, see above) cannot be implemented. The diodes therefore represent a reliable safety device and allow the greatest flexibility of the circuit arrangement.

REFERENCE NUMERALS LIST:

1 first connection

2 second connection

10 first strand

20 second strand

30 third strand

100 electric heater

200 control device

D1,D2,D3 diode

HV+,HV- voltage potential or reference potential

N1,N2,N3 second node

N4,N5,N6 first node

R1,R2,R3 heating resistor

S1,S2,S3 high-side switching device

S4,S5,S6 low-side switching device

Claims

Claims: 1. Electric heater (100), comprising: a first terminal carrying a voltage potential (HV+) and a second terminal carrying a reference potential (HV-); a number of at least two strands (10, 20, 30) which are connected in parallel to one another between the first terminal and the second terminal; wherein at least one heating resistor (R1, R2, R3) is arranged in each of the strands (10, 20, 30); wherein in each of the strands (10, 20, 30) a high-side switching device (S1, S2, S3) is arranged between the heating resistor (R1, R2, R3) and the first terminal (HV+) and a low-side switching device (S4, S5, S6) is arranged between the heating resistor (R1, R2, R3) and the second terminal (HV-); wherein in a first of the strands (10, 20, 30) a first node (N4, N5, N6) is set up between the high-side switching device (S1, S2, S3) and the low-side switching device (S4, S5, S6), and in a second of the strands (10, 20, 30) a second node (N1, N2, N3) electrically connected to the first node (N4, N5, N6) is set up between the high-side switching device (S1, S2, S3) and the low-side switching device (S4, S5, S6), wherein the corresponding heating resistors (R1, R2, R3) can be switched by the switching devices (S1 - S6) of the first strand and the second strand either in series via the connected nodes (N1 - N6) or individually between the voltage and the reference potential. 2. Electric heater (100) according to claim 1, wherein the first node (N4, N5, N6) is connected between the respective heating resistor (R1, R2, R3) and the respective low-side switching device (S4, S5, S6) of the first strand (10, 20, 30), and the second node (N1, N2, N3) is connected between the respective high-side switching device (S1, S2, S3) and the respective heating resistor (R1, R2, R3) of the second strand (10, 20, 30). 3. Electric heater (100) according to claim 2, wherein the first node (N4, N5, N6) and the second node (N1, N2, N3) are electrically connected by a connecting line, wherein an electrical component is connected in the connecting line which prevents a current flow from the second node to the first node, preferably a diode. 4. Electric heater (100) according to one of claims 1 to 3, wherein each of the strands (10, 20, 30) has a first node (N4, N5, N6) and a second node (N1, N2, N3). 5. Electric heater (100) according to claim 4, wherein at least three strands (10, 20, 30) are arranged, wherein the first node (N4, N5, N6) of the second strand is connected to the second node (N3, N4, N5) of a third strand (10, 20, 30) different from the first strand. 6. Electric heater (100) according to claim 5, wherein all strands (10, 20, 30) are connected to one another in sequence via connecting lines between the first nodes (N4, N5, N6) of a strand and the second nodes (N1, N2, N3) of a next strand, wherein the last of the strands (10, 20, 30) in the series is connected again to the first strand (10, 20, 30) via the corresponding connecting line. 7. Electric heater (100) according to one of claims 1 to 6, wherein the heating resistors (R1, R2, R3) are equal. 8. Electric heater (100) according to one of claims 1 to 7, wherein the high- and low-side switching devices are configured as semiconductor switches. 9. Electric heater (100) according to claim 9, wherein the high-side switching devices are designed as thyristors, preferably as SCRs, and/or the low-side switching devices are designed as transistors. 10. Electric heater (100) according to one of claims 1 to 9, comprising a control device (200) configured to control the switching devices (S1 - S6) to open and close at different times. 11. Electric heater (100) according to claim 10, wherein the control device (200) is configured to adjust the heating power by pulse width modulation by actuating the switching devices (S1 - S6). 12. Electric heater (100) according to claim 11, wherein the control device is configured to operate the switching devices (S1 - S6) exclusively sequentially with a mutual time interval therebetween. 13. A method for operating an electric heater (100) according to any one of the preceding claims, comprising: Providing the voltage between the first terminal (1) and the second terminal (2); Setting a state of the high- and low-side switching devices in which in a first of the strands the two high- and low-side switching devices are switched on and in a second of the strands the two high- and low-side switching devices are switched off, so that a current flows through the first heating resistor, but not through the second heating resistor; after a first time period (t1 -t0) has elapsed after setting the state, switching on the low-side switching device of the second strand; after a second time period (t2-t1) has elapsed after switching on the low-side switching device of the second strand, switching off the low-side Switching device of the first strand so that a current flows through the first and second heating resistors. 14. The method according to claim 13, further comprising: after a third time period (t3-t2) has elapsed after the low-side switching device of the first strand has been switched off, switching on the high-side switching device of the second strand; after a fourth time period (t4-t3) has elapsed after the high-side switching device of the second strand has been switched on, switching off the high-side switching device of the first strand so that a current flows through the second heating resistor, but no longer through the first heating resistor.