Abstract:
A circuit arrangement for detecting a zero crossing of an alternating current is provided. The circuit arrangement includes a load through which the alternating current can flow, and an evaluation unit connected to the load and configured to evaluate an electrical voltage formed at the load, and to determining zero currents. The load comprises a capacitor through which the alternating current or a current to be measured derived therefrom can flow, and the evaluation unit is configured to account for a phase shift caused by the capacitor for determining the zero currents.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2014/069586 filed Sep. 15, 2014, which designates the United States of America, and claims priority to DE Application No. 10 2013 219 530.3 filed Sep. 27, 2013, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a circuit arrangement for identifying a zero-current of an alternating current, with a load through which the alternating current can flow, and an evaluation unit which is connected to the load in order to evaluate an electric voltage generated on the load through which the alternating current flows, and for the identification of zero-currents. The invention also relates to an inverter with an intermediate circuit capacitor, with at least one electronic half-bridge module connected to the intermediate circuit capacitor, and constituting a terminal for the connection of a coil for the generation of an alternating magnetic field, with a clock-pulse generator for the control of the half-bridge module and with a circuit arrangement for identifying a zero-current of an alternating current. The invention also relates to a charging station for an electrically-powered vehicle, with a terminal for an electric energy source and a charging unit for a wireless power coupling of the electrically-powered vehicle by means of an alternating magnetic field. Finally, the invention also relates to a method for identifying a zero-current of an alternating current, wherein an electric voltage generated on a load through which the alternating current flows is evaluated by means of an evaluation unit, and zero-currents are identified. 
       BACKGROUND 
       [0003]    Circuit arrangements of this generic type are known in principle, and are frequently used in converters of any type, specifically inverters, in order to permit the operation of the converter or inverter for its intended purpose. To this end, it is in fact necessary to identify current characteristics, specifically for output currents of the converter or inverter. A key parameter is the identification of a zero-current in the output current of the converter or inverter. 
         [0004]    Inverters of this generic type and methods for the operation thereof are also known in principle, for example from DE 10 2008 027 126 A1. Inverters are a form of energy converter, by means of which a d.c. voltage can be converted into an alternating voltage, specifically a single-phase or a three-phase alternating voltage. Inverters are now used in the form of “static” energy converters, meaning that, unlike dynamic energy converters, they feature no mechanical moving components, specifically rotating components, for the purposes of energy conversion. Inverters of this generic type in the form of static energy converters are generally configured as switched-mode electronic energy converters and, to this end, are provided with at least one half-bridge module, by means of which a d.c. voltage delivered to an intermediate circuit capacitor on an intermediate circuit may be converted into an alternating electric voltage. To this end, the half-bridge module is provided with two series-connected semiconductor switches, which can operate in cyclic mode by means of a clock-pulse generator such that, in the required manner, the alternating voltage is delivered on a center terminal of the half-bridge module, which is constituted by an electrical interconnection point of the two semiconductor switches in the half-bridge module. A circuit topology of this type is also described as a half-bridge circuit. Accordingly, an inverter of this type is designed for “single-phase” operation. 
         [0005]    A high-capacity variant of an inverter of this type is provided with two parallel-connected half-bridge modules, whereby the magnitude of the alternating voltage generated is essentially doubled. To this end, the half-bridge modules are controlled in a correspondingly complementary arrangement. A circuit topology of this type is also described as a full-bridge circuit. 
         [0006]    Inverters are also used, specifically for the constitution of a three-phase alternating voltage system, wherein at least one half-bridge module is provided for each of the alternating voltage phases. A circuit topology of this type is also described as a half-bridge circuit. Naturally, the inverter may also be provided with a pair of half-bridge modules for each phase, such that the circuit topology of a full-bridge circuit is constituted for each phase. 
         [0007]    Inverters of this generic type are frequently used in charging stations, which are used for the wireless power coupling of an electrically-powered vehicle, for the purposes of the charging of an electric energy storage device in said electrically-powered vehicle. A wireless power coupling is formed, which obviates the need for a complex mechanical connection by means of a cable. To this end, on both the charging station side and the vehicle side, coil arrangements are generally provided respectively which, during the charging process, are positioned opposite each other, and permit the formation of a power coupling by the use of an alternating magnetic field. An arrangement of this type is known, for example, from KR 10 2012 0 016 521 A. 
         [0008]    In order to be able to generate an appropriate alternating magnetic field in the charging station, the latter, in addition to an appropriate coil for this purpose, is also provided with an inverter which is connected to the coil and which supplies a corresponding alternating current to the coil. For the reliable operation of the charging station, and specifically of the inverter, it is necessary to identify the zero-currents. To this end, DE 10 2008 027 126 A1, for example, discloses a current sensor which is connected to a circuit arrangement which permits the evaluation of current via a differentiating circuit element. The circuit disclosed therein detects the falling current ramp by means of a differentiator, and compares the latter with a tripping signal for a semiconductor switch in a half-bridge module of the inverter. In order to permit the reliable operation of this circuit, a sinusoidal current characteristic is required which, insofar as possible, is free of harmonics. Specifically, there must be no superimposed interference. In practical operation, this results in problems, in that zero-currents cannot be detected with sufficient accuracy, thereby resulting in the inadequate control of the inverter. 
       SUMMARY 
       [0009]    One embodiment provides a circuit arrangement for identifying a zero-current of an alternating current, with a load through which the alternating current can flow, and an evaluation unit which is connected to the load in order to evaluate an electric voltage generated on the load through which the alternating current flows, and for the identifying of zero-currents, wherein the load comprises a capacitor through which the alternating current or a measuring current derived therefrom can flow, and that the evaluation unit is designed to take account of a phase displacement effected by the capacitor in the identification of zero-currents. 
         [0010]    In a further embodiment, the load is provided with a current converter through which, on the primary side, an alternating current can flow and, on the secondary side, is connected to the capacitor. 
         [0011]    In a further embodiment, the evaluation circuit is provided with an amplification unit, specifically a comparator. 
         [0012]    In a further embodiment, the amplification unit is provided with a low-pass filter on its output side. 
         [0013]    In a further embodiment, the evaluation unit incorporates a digital processing unit for the digitization and evaluation of the electric voltage generated on the load. 
         [0014]    Another embodiment provides an inverter with an intermediate circuit capacitor, with at least one electronic half-bridge module connected to the intermediate circuit capacitor, and constituting a terminal for the connection of a coil for the generation of an alternating magnetic field, with a clock-pulse generator for the control of the half-bridge module and with a circuit arrangement for identifying a zero-current of an alternating current, wherein the circuit arrangement is configured as claimed in one of the preceding claims. 
         [0015]    Another embodiment provides a charging station for an electrically-powered vehicle, with a terminal for an electric energy source and a charging unit for a wireless power coupling of the electrically-powered vehicle by means of an alternating magnetic field, wherein the charging unit is provided with an inverter as claimed in claim  6  and with a coil, for the generation of the alternating magnetic field, connected to the inverter. 
         [0016]    Another embodiment provides a method for identifying a zero-current of an alternating current, wherein an electric voltage generated on a load through which the alternating current flows is evaluated by means of an evaluation unit, and zero-currents are identified, wherein a capacitor is employed as the load and in that, by means of the evaluation unit, a phase displacement effected by the capacitor is considered in the identification of zero-currents. 
         [0017]    In a further embodiment, the electric voltage generated on the capacitor is digitized, and the phase displacement is digitally eliminated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Example embodiments of the invention are discussed in detail below with reference to the figures, in which: 
           [0019]      FIG. 1  shows a diagram of a schematic and standardized representation of an output voltage of an inverter, and of a coil current in a coil which is connected to the inverter. 
           [0020]      FIG. 2  shows a diagram including a schematic representation of curves, in which one curve represents the coil current in accordance with  FIG. 1 , a second curve represents an alternating voltage on a load according to the invention comprising a capacitor, and a third curve represents a resulting square-wave voltage; 
           [0021]      FIG. 3  shows a schematic single-line circuit diagram of a circuit arrangement according to the invention; 
           [0022]      FIG. 4  shows a diagram of a series of signal-time characteristics, which represent different signal processing steps in a digital signal processing function, in accordance with a flow diagram represented in  FIG. 5 , and 
           [0023]      FIG. 5  shows a schematic flow diagram of a digital signal processing function according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Embodiments of the invention provide an improved control system for an inverter. 
         [0025]    Within the context of the invention, power coupling is a coupling for the purposes of energy transmission, which makes it possible for energy to be transmitted, at least unidirectionally, from an energy source to an energy sink. The energy source may be, for example, a public electricity grid system, an electric generator, a solar cell, a fuel cell, combinations thereof and/or similar. The energy sink may be, for example, a drive of the electrically-powered vehicle, specifically an electric motor in the drive system and/or an electricity storage device for the drive system, for example an accumulator or similar. However, provision may also be made for bidirectional energy transmission, such that energy may be transmitted in both directions in an alternating arrangement. This function is fulfilled, among other elements, by the charging station, which sources energy from the energy source to which it is electrically connected, and which energy is then to be transmitted to the electrically-powered vehicle. However, provision may also be made for bidirectional energy transmission, i.e. alternating energy transmission in both directions. This function is fulfilled, among other elements, by the charging station, which sources energy from the energy source to which it is electrically connected, and which energy is then to be transmitted to the electrically-powered vehicle. 
         [0026]    Within the context of the invention, wireless power coupling means that there is no requirement for a mechanical connection between the charging station and the electrically-powered vehicle in order to form an electrical coupling. Specifically, the formation of an electrical connection by means of a cable can be obviated. Instead, the power coupling is essentially formed by an energy field only, e.g., an alternating magnetic field. 
         [0027]    The charging station is therefore designed for the generation of a corresponding energy field, specifically an alternating magnetic field. On the vehicle, corresponding provision is made for the capture of such an energy field or alternating magnetic field, and for the sourcing therefrom of energy for the operation of the electrically-powered vehicle in accordance with its intended purpose. Energy is converted by means of a charging device into electrical energy, which may be stored in an energy storage device on the vehicle, for the operation thereof in accordance with its intended purpose. The power coupling is therefore essentially intended for the transmission of energy, and not primarily for the transmission of information. Accordingly, means for the embodiment of the invention are designed for a correspondingly high power throughput, in comparison with a wireless communication link. 
         [0028]    In a generic circuit arrangement, it is specifically proposed under the invention that the load should comprise a capacitor through which the alternating current or a measuring current derived therefrom can flow, and that the evaluation unit is designed to take account of a phase displacement effected by the capacitor in the identification of zero-currents. Accordingly, for the generic method, it is specifically proposed that a capacitor should be employed as the load and that, by means of the evaluation unit, a phase displacement effected by the capacitor should be considered in the identification of zero-currents. 
         [0029]    As a load, the invention employs the capacitor which—in contrast with DE 10 2008 027 126 A1—delivers an integrating property for the alternating current such that interference, which is generally a highly frequent occurrence, can be suppressed. Simultaneously, the capacitor effects a phase displacement between the alternating current flowing therein and a resulting electric voltage generated on the capacitor, which is then eliminated during further processing by the evaluation unit. 
         [0030]    The capacitor may be an instrument capacitor with a very limited tolerance and as small a loss factor as possible, in order to permit the accurate identification of zero-currents in the alternating current. As a result of these properties, the circuit arrangement according to the invention permits the substantially more accurate and interference-free identification of zero-currents in the alternating current. Specifically, the amplification of interference associated with the use of a differentiating component according to the prior art can be avoided. The capacitor permits the smoothing of the alternating current, with interference signals superimposed. In contrast with other conventional filter circuits based upon resistance/capacitance filtering components, the circuit arrangement according to the invention delivers a constant phase displacement between the current and the measuring voltage, essentially to a value of 90 degrees. As this voltage displacement is constant, the identification of zero-currents in the alternating current can proceed with a high degree of accuracy in consideration of this circumstance. To this end, an appropriate compensation function is provided by the evaluation unit. 
         [0031]    According to a further aspect of the invention, the load is provided with a current converter through which, on the primary side, an alternating current can flow and, on the secondary side, is connected to the capacitor. The current converter may be configured as a conventional transformer, for the purposes of current conversion. On its secondary-side terminals, it delivers a current which corresponds to the primary-side alternating current, in accordance with the ratio of the number of turns. Accordingly, in respect of requirements for power throughput, the measuring structure can be downsized, thereby permitting the use of high-precision components. In general, components of this type are not designed for high power ratings. On its secondary side, the current converter therefore delivers the measuring current which is essentially proportional to the alternating current, and which flows through the capacitor. The resulting alternating voltage on the capacitor is then fed back—as previously—to the evaluation unit for the identification of zero-currents. 
         [0032]    According to a further development, the evaluation circuit is provided with an amplification unit, specifically a comparator. Using the linear amplification unit, the voltage signal generated on the capacitor in response to the alternating current or the measuring current can be amplified as required, and brought up to a requisite electrical potential. For example, the amplification unit may be designed for the adjustment of the voltage signal on the capacitor to a conversion range of an analog-digital converter. A high degree of resolution in analog-digital conversion can be achieved accordingly. The amplification unit may be configured, for example, as a semiconductor amplifier based upon a transistor circuit, or as a corresponding and appropriately-connected operational amplifier. For the adjustment of potential, it may be provided that the electric voltage delivered on the capacitor is routed to the amplification unit via a coupling capacitor. In respect of its capacity, the coupling capacitor may be selected such that it does not essentially affect the evaluation process. The configuration of the amplifier as a comparator is particularly advantageous. By this arrangement, the alternating electric voltage delivered by the load can be converted directly into a digital signal, which can then be routed to a digital control system. In this case, analog-digital conversion is not required. 
         [0033]    In a further configuration, it is proposed that the amplification unit is provided with a low-pass filter on its output side. A low-pass filter of this type permits the filtering-out of further unwanted interference signals. Evaluation by the evaluation unit is further improved accordingly. 
         [0034]    According to a further aspect of the invention, the evaluation unit incorporates a digital processing unit for the digitization and evaluation of the electric voltage generated on the load. To this end, the digital processing unit may incorporate, for example, the above-mentioned analog-digital converter. The digital processing unit may also be provided with a computing unit, a gate array (ASIC), combinations thereof or similar, designed for the consideration of the phase displacement effected by the capacitor in the identification of zero-currents, and specifically for the elimination thereof. 
         [0035]    Another embodiment provides a generic inverter, wherein the circuit arrangement is configured as disclosed herein. This permits the advantages associated with the circuit arrangement to be incorporated in the inverter, thereby permitting the achievement of greater operational reliability. Moreover, the employment of this circuit arrangement permits the reliable operation of the inverter, even at a high operating frequency. 
         [0036]    Another embodiment provides a generic charging station, wherein the charging unit is provided with an inverter according to the invention and with a coil, for the generation of the alternating magnetic field, connected to the inverter. By this arrangement, the advantages associated with the inverter can also be achieved in the charging station, such that more reliable operation of the latter will also be possible. 
         [0037]    Another embodiment provides a method whereby the electric voltage generated on the capacitor is digitized, and the phase displacement is digitally eliminated. This permits the achievement of the reliable and highly accurate identification of zero-currents. For this purpose, the evaluation unit correspondingly incorporates a computing unit, by means of which the corresponding measures can be executed. Accordingly, the computing unit is provided with a computer software product incorporating program code segments which permit the execution by the computing unit of the method according to the invention. 
         [0038]      FIG. 1  shows a diagram of a schematic signal characteristic of an inverter output voltage  16  and of a coil current  14  on a coil which is connected to the inverter. These elements are not represented in  FIG. 1 . The diagram shows a time axis—the x-axis—which is identified by the reference number  12 , and a signal axis—the y-axis—which is represented by the reference number  10 . The time axis  12  indicates the time in ps. The y-axis  10  indicates standardized values for inverter voltage  16  and for the coil current  14 . From the diagram shown in  FIG. 1 , it will be seen that the coil current  14  shows a temporal phase displacement in relation to the alternating voltage  16  on the inverter. 
         [0039]      FIG. 2  shows a second diagram, again with the x-axis  12  as the time axis and a y-axis  10 , with standardized values as per  FIG. 1 . In the diagram shown in  FIG. 2 , the coil current  14  is again represented by a curve. A second curve, with a phase displacement in relation to the curve  14 , shows an alternating voltage  18  on a capacitor  30  according to the invention, which is a constituent element of a load  26  ( FIG. 3 ). It will be seen that there is a phase displacement between the electric voltage  18  and the coil current  14  of approximately 90 degrees. It will also be seen that both the coil current  14  and the electric voltage  18  essentially show a sinusoidal profile whereby, however, interference is superimposed on the coil current  14 . The electric voltage  18  is essentially unaffected by this interference. The curve  20  also represents a square-wave voltage generated by the electric voltage  18 . Thereafter, this curve permits the identification of zero-currents or the phase difference between the coil current  14  and the inverter output voltage  16 . 
         [0040]      FIG. 3  shows a schematic single-line circuit diagram of a circuit arrangement  22  according to the invention. It will be seen than the circuit arrangement  22  is connected to an alternating current generator  24 , which supplies the alternating current  14 . In the present case, the alternating current generator is constituted by the inverter, which delivers the coil current  14 . 
         [0041]    The coil current  14  flows through a load  26  which, in turn, is connected to an evaluation unit  58 . To this end, the load  26  is provided with a current converter  28  through which, on the primary side, the coil current  14  can flow. On the secondary side, the current converter  28  is connected to a capacitor  30  according to the invention, on which an electric voltage  18  is generated. The electric voltage  18  is then delivered to the evaluation unit  58  for the purposes of further signal processing, specifically for the identification of zero-currents. 
         [0042]    To this end, the evaluation unit  58  is provided with an amplification unit  48 , formed by an operational amplifier, which is connected with corresponding electronic components, namely, a coupling capacitor  34  and electrical resistances  40  to  46 , in order to achieve the requisite amplification and signal formation. In the present case, it is provided that the amplification unit  48  should operate at maximum amplification, i.e. as a comparator, such that, from the essentially sinusoidal electric voltage  18 , a square-wave voltage is generated as a digital signal, from which a further electric voltage  20  is constituted thereafter. This is essentially the function of the electric resistances  40  to  46 . Two series-connected diodes  36 ,  38  are also provided, which limit the signal input connected via the coupling capacitor  34  to the positive or negative operating voltage potential of the amplification unit  48 . 
         [0043]    On the output side, the amplification unit  48  is connected to a down-circuit filter formed of electrical resistances  50 ,  52  and a capacitor  54  which, in the present case, executes a low-pass function and delivers the above-mentioned filtered square-wave voltage  20  as an output voltage. This is routed to a digital processing unit  56  in the evaluation unit  58  which, in the present case, is configured as a Field Programmable Gate Array (FPGA). This uses the electric voltage  20  for the identification of zero-currents or the phase displacement between the inverter output voltage  16  and the coil current  14 . 
         [0044]      FIG. 5  shows a schematic view of a section of a signal flow diagram for the digital processing unit  56 .  FIG. 4  shows the corresponding signal characteristics associated with the flow diagram in  FIG. 5 . 
         [0045]    The uppermost diagram in  FIG. 4 , designated as 1, represents the time characteristics of the inverter voltage  16 , the coil current  14  and the electric voltage  18  on the capacitor  30 . 
         [0046]    The three subsequent diagrams in  FIG. 4  show the corresponding signal-time characteristics for the signals represented in the flow diagram in  FIG. 5 . 
         [0047]    It will be seen that, in the second diagram in  FIG. 4 , the signals derived from the corresponding zero-crossings of the electric voltages represented in 1, and their corresponding associations for the evaluation unit  58 , are represented. The signals are square-wave signals. The areas under the square-wave signals represent the transit times of the inverter voltage  16  and the coil current  14 , and the association of the inverter voltage  16  and the electric voltage  18  on the capacitor  30 . 
         [0048]    One option for the evaluation of these areas involves counting by means of an auxiliary clock pulse. In the third diagram in  FIG. 4 , a counter reading is therefore represented for the identification of the interval between a zero-crossing of the inverter voltage  16  and the zero-crossing of the coil current  14 , displaced through ninety degrees. The fourth diagram in  FIG. 4 , i.e. the bottom diagram, shows a counter reading for the inverter output voltage  16 . From this variable, it is possible to deduce the current inverter frequency and the existing phase displacement between the inverter voltage  16  and the coil current  14 . From the variables tZCC90 and T/2, it is possible to identify the current phase relation, independently of the frequency. This is achieved by means of signal processing, in accordance with  FIG. 5 . 
         [0049]    The exemplary embodiment represented with reference to the figures is intended for the clarification of the invention only, and is not provided by way of limitation. As required, an expert will naturally be able to infer corresponding variations, without departing from the principle of the invention. 
         [0050]    Individual characteristics, specifically characteristics in the dependent claims, may also naturally be subject to any desired combination, in accordance with requirements. Moreover, the characteristics of devices may also naturally be indicated by corresponding process steps, and vice versa.