Abstract:
A power semiconductor module includes a power electronics circuit and a measuring circuit for measuring a physical parameter occurring in the power electronics circuit and for providing a corresponding measurement signal. A transmission circuit is coupled to a secondary side of a transfer unit, and an evaluation circuit is coupled to the primary side and galvanically isolated from the transmission circuit by the transfer unit. The evaluation circuit supplies an AC voltage to the primary side, causing primary current to flow on the primary side, which in turn results in secondary current on the secondary side, the secondary current being supplied to the transmission circuit. The transmission circuit receives the measurement signal and modulates the secondary current in accordance with the measurement signal, which results in a modulation of the primary current. The evaluation circuit evaluates the modulation of the primary current and generates an output signal dependent thereon.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority to German Patent Application No. 10 2013 211 386.2, filed on 18 Jun. 2013, the content of said German application incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The invention relates to a power semiconductor module, in particular an arrangement for measuring and transferring parameters of a power semiconductor module. 
       BACKGROUND 
       [0003]    In order to monitor in particular power semiconductors, various parameters are often measured in electronic circuits. A high operating temperature and thermal cycling can impair, for example, the component efficiency and the failsafety of the components to a not inconsiderable extent. In order to avoid failures of components, the temperature is therefore measured, for example, in order to be able to adopt safety measures in the event that a limit value is exceeded. However, other parameters such as currents or voltages can also be measured. 
         [0004]    In power converters (converters), for example, the temperature and the DC link voltage are measured. The power semiconductor components of a power converter are in this case supplied a high voltage (for example 0.3-1.7 kV), whereas the measuring circuit and the evaluation circuit, which implement the measurement and the measurement data processing, operate on a low supply voltage (for example 15 V). The measurement electronics are therefore usually galvanically isolated from the evaluation electronics. 
         [0005]    Generally, the control circuit for the converter is supplied via a reinforced insulation transformer. The data transfer from the measuring circuit, which can be part of the control circuit, to the evaluation circuit takes place via separate transfer elements or via optocouplers. For this purpose, a large number of components is required, which results in a high space requirement and high costs of the entire module. 
       SUMMARY 
       [0006]    Embodiments described herein provide a circuit arrangement which is improved over the prior art and which can be implemented more compactly and at lower cost. 
         [0007]    A power semiconductor module is described. In accordance with a first aspect of the present invention, the power semiconductor module comprises a power electronics circuit and a measuring circuit, which is designed to measure at least one physical parameter occurring in the power electronics circuit and to provide a measurement signal which represents the measured parameter (M X ). The power semiconductor module furthermore comprises a transfer unit with a primary side and a secondary side, a transmission circuit, which is coupled to the secondary side, and an evaluation circuit, which is coupled to the primary side and which is galvanically isolated from the transmission circuit by the transfer unit. The evaluation circuit is designed to supply an AC voltage to the primary side of the transfer unit, as a result of which a corresponding primary current flows on the primary side, which in turn results in a secondary current on the secondary side of the transfer unit, which secondary current is supplied to the transmission circuit. The transmission circuit is designed to receive the measurement signal from the measuring circuit and to modulate the secondary current in accordance with the measurement signal, which results in a corresponding modulation of the primary current. The evaluation circuit is furthermore designed to evaluate the modulation of the primary current and to generate an output signal dependent thereon. 
         [0008]    Furthermore, a method for measuring and transferring circuit parameters measured in a power semiconductor module is described. 
         [0009]    In accordance with a further aspect of the invention, the method comprises generating a primary current on the primary side of a transfer unit by virtue of supplying an AC voltage to the primary side, wherein the primary current results in a secondary current on a secondary side of the transfer unit, which secondary side is galvanically isolated from the primary side. The method further comprises measuring at least one parameter of a power electronics circuit and providing a measurement signal which represents the measured parameter. The secondary current is modulated in accordance with the measurement signal, which results in a corresponding modulation of the primary current. The resultant modulation of the primary current is evaluated and an output signal dependent on the evaluation is generated. 
         [0010]    Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]    The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. The features of the various illustrated embodiments can be combined unless they exclude each other. Embodiments are depicted in the drawings and are detailed in the description which follows. 
           [0012]      FIG. 1  shows a power semiconductor module in accordance with one example of the invention. 
           [0013]      FIG. 2  shows, by way of example, using time characteristics, a principle of the data transfer between a measuring circuit and an evaluation circuit. 
           [0014]      FIG. 3  shows a circuit arrangement for modulating measurement data. 
           [0015]      FIG. 4  shows an evaluation circuit for evaluating transferred measurement data. 
           [0016]      FIG. 5  shows the time sequence of various operations in an arrangement for transferring data between a measuring circuit and an evaluation circuit. 
           [0017]      FIG. 6  shows the same time sequences as in  FIG. 5 , but with start and stop bits being transferred. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows a power semiconductor module in accordance with one example of the invention. The power semiconductor module comprises a power electronics circuit  4 , for example a power converter circuit comprising three transistor half-bridges for generating a three-phase alternating current (three-phase inverter circuit). In the present example, these three transistor half-bridges are constructed from six IGBTs (Insulated-Gate Bipolar Transistors). Alternatively, MOSFETs (Metal-Oxide Semiconductor Field-Effect Transistors), bipolar transistors or the like could also be used. In the example in  FIG. 1 , the half-bridges are denoted by  4   1 ,  4   2 , and  4   3 . Each half-bridge  4   1 ,  4   2 ,  4   3  can generate a corresponding AC output voltage from a DC voltage V DClink . For this, the individual semiconductor switches  41   1 ,  42   1 ,  41   2 ,  42   2 ,  41   3 ,  42   3  from which the half-bridges are constructed are driven correspondingly at their control connections. A three-phase power converter having three half-bridges  4   1 ,  4   2 ,  4   3  connected in parallel can provide, for example, three AC output voltages for a three-phase AC system (not illustrated). 
         [0019]    Each of the half-bridges  4   1 ,  4   2 ,  4   3  in this case comprises, for example, two series-connected power switches  41   1  and  42   1 ,  41   2  and  42   2 ,  41   3  and  42   3 , which are each connected in series between a first and a second potential of the DC link V DClink . Such power converters and their function have already long been known and are not described in detail here. The application of the invention is restricted to an application with power converters, but can be used for any power electronics applications. The power electronics circuit  4  can also have other components in addition to a number of power switches, for example diodes or the like. 
         [0020]    In order to protect the components of the power electronics circuit  4  or for the purpose of controlling or monitoring the power converter, measurements of various parameters of the circuit can take place. For example, the current in the DC link or the output currents can be measured. In order to protect against excessively high temperatures, the temperature of the power semiconductor components  41 ,  42  can be measured. In this case, power semiconductor components with an integrated temperature sensor (for example NTC thermistors) are often used (NTC=Negative Temperature Coefficient). A measurement of the DC link voltage V DClink  is often desired as well. 
         [0021]    The block circuit diagram illustrated in  FIG. 1  shows a power converter module as an example of a power semiconductor module. The power converter module comprises, in addition to the abovementioned power electronics circuit (three-phase inverter circuit  4 ), a measuring circuit  50  for measuring one or more physical parameters M X  occurring in the power electronics circuit  4 . The parameter(s) M X  can include the following, for example: temperatures occurring in the module, currents, the DC link voltage (or voltages occurring at another point), phase angle, etc. For this, the measuring circuit  50  can have in each case suitable sensors, such as, for example, shunt resistors, Hall sensors or similar current sensors for measuring currents, temperature measuring resistors (for example NTC thermistors), etc. 
         [0022]    The measuring circuit  50  detects and processes the parameter(s) M X  to be measured and provides a measurement signal S meas . This measurement signal S meas  represents the measured parameter(s) M X  and can be in digital form, for example, i.e. as a sequence of binary data words. Evaluation or further-processing of the measurement signal S meas  generally does not take place in the measuring circuit  50  itself, but in a separate evaluation circuit  1 . This evaluation circuit is generally galvanically isolated from the power electronics circuit  4  and the measuring circuit  50 . Depending on the application, the mentioned galvanic isolation can be desirable or necessary, since the power electronics circuit  4  is operated on a high voltage (for example 0.3-1.7 kV), while the circuit components for evaluating the measurement signals or for the measurement signal processing operate on a low voltage (for example 0-15 V). A transfer unit  30 , which ensures the galvanic isolation of the power electronics circuit  4  and the measuring circuit  50  from the evaluation circuit  1 , is provided for the transfer of the measurement data (signal S meas ) to the evaluation unit  1 . The transfer unit  30  has a primary side comprising a primary winding  10  and a secondary side comprising a secondary winding  20 . The transfer unit  30  is a magnetic transfer element, for example a transformer. 
         [0023]    The evaluation circuit  1  (reception circuit) is connected to the primary side  10  of the transfer unit  30 . A transmission circuit  2  (transmission circuit) is connected to the secondary side  20  of the transfer unit  30 . The evaluation circuit  1  and the transmission circuit  2  are therefore galvanically isolated from one another. In addition to data transfer, the transformer  30  is also used for transferring electrical energy. The transformer  30  can therefore be used for supplying energy (voltage) to the transmission circuit and the measuring circuit. 
         [0024]    If the transfer unit  30  is connected to an AC voltage V PRIM  on the primary side, for example, which AC voltage is generated in the present example with the aid of a transistor output stage in the evaluation circuit  1  from a DC supply voltage V+. As a result, a corresponding voltage is induced on the secondary side  20 . This voltage can be used to supply energy to the transmission circuit  2  and the measuring circuit. The AC voltage V PRIM  which is supplied to the primary side  10  of the transfer unit  30  can be provided by the evaluation circuit  1 , for example. In this case, the evaluation circuit also provides for the supply of energy to the circuits connected to the secondary side of the transformer  30  (transmission circuit  2 , measuring circuit  50 ). If the primary side  10  of the transformer  30  is supplied a square-wave voltage (as AC voltage V PRIM ), for example, which has 0V during a switch-off phase and a maximum voltage value V max  during a switch-on phase, energy is transferred to the secondary side  20  in each case during the switch-off process (i.e. during the transition from V max  to zero). 
         [0025]    In order to transfer the measurement signal S meas  to the evaluation circuit  1 , the transmission circuit  2  receives the measurement S meas  from the measuring circuit  50 . Owing to the AC voltage V PRIM , which is supplied on the primary side to the transfer unit  30 , a primary current I prim  flows on the primary side  10  of the transfer unit  30 . This primary current I prim  on the primary side  10  results in a corresponding secondary current I sec , which is used inter alia for supplying the transmission circuit  2  and the measuring circuit  50 . The transmission circuit  2  is designed to modulate the secondary current I sec  in accordance with the measurement signal S meas . The modulation can consist in varying (increasing or decreasing) the secondary current I sec  in a targeted manner by a known value ΔI sec . A modulation of the secondary current I sec  results in a corresponding modulation of the primary current I prim . By virtue of the modulation of the secondary current I sec , therefore, information can be transferred from the secondary side  20  of the transformer  30  to the primary side  10  thereof, i.e. from the transmission circuit  2  to the evaluation circuit  1 . By virtue of an evaluation (demodulation) of the primary current I prim , the transferred data can be retrieved. The evaluation circuit  1  can provide, for example, a signal S I  for further evaluation and processing, which signal represents the measured primary current I prim  or the measured change in the primary current ΔI prim . The principle of data transfer will be described in more detail below. 
         [0026]    If the secondary current I sec  is being modulated, this also results in a corresponding modulation of the primary current I prim . The primary current I prim  is dependent on the secondary current I sec  and a linearly rising magnetization current of the transformer  30 . This is illustrated in the first timing diagram in  FIG. 2 . Said diagram shows the primary current I prim  measured on the primary side at different time intervals T 0 , T 1 , T 2 , T 3 , during which data transfer (i.e. modulation of the secondary current) takes place. 
         [0027]    The measurement data can be transferred as a bit sequence, for example. In this case, for example, a word length of 12 bits per measurement value can be provided. However, other word lengths are also possible. In the case illustrated in  FIG. 2 , eight bits are transferred within the four time intervals T 0 , T 1 , T 2 , T 3 , wherein, in each interval, two bits (“bit  0 ” and “bit  1 ”) are transferred. The time intervals are also referred to as frames. In the example in  FIG. 2 , only one symbol which represents two bits is transferred in each frame (serial multi-level transfer). The further timing diagrams illustrated in  FIG. 2  show the state of the transferred bits (“bit  0 ” and “bit  1 ”) during the respective transfer time periods T 0 , T 1 , T 2 , T 3 . A single bit can only assume one of two possible states (logic 0 or logic 1 represented, for example, by a high level H and a low level L). No differential current ΔI prim  is modulated onto the characteristic of the current I prim  in the time interval T 0 , and the linear rise in the primary current during this time interval T 0  represents the magnetization current of the transformer  30  rising during this time. A lack of change in current in a time interval (in this case T 0 ) is represented by a 0. A differential current ΔI prim =ΔI 1  is superimposed on the characteristic of the current I prim  in the time interval T 1 , which corresponds to a logic 1. The corresponding bit  0  has a high level. In this way, a bit can be transferred in each time interval T 0 , T 1 , T 2 , T 3  (i.e. in each time increment). 
         [0028]    The example illustrated in  FIG. 2  shows a multi-level transfer, in which two bits (i.e. a 2-bit word) are transferred by in each case one symbol in each time interval. The bit combinations (symbols) which can be transferred are therefore “00”, “01”, “10” and “11” and are represented by the lower two timing diagrams for “bit  0 ” and “bit  1 ”. A bit combination “00” to be transferred corresponds to a current difference ΔI prim =0 (see time interval T 0 ), a bit combination “01” corresponds to a current difference ΔI prim =ΔI 1  (see time interval T 1 ), a bit combination “10” corresponds to a current difference ΔI prim =ΔI 2  (see time interval T 2 ) and bit combination “11” corresponds to a current difference ΔI prim =ΔI 3  (see time interval T 3 ). Said bit combinations can be decoded by a determination of the level of the modulated current difference ΔI prim  in each time increment T 0 , T 1 , T 2 , T 3 . 
         [0029]    A change in the secondary current ΔI sec  and therefore in the primary current ΔI prim  does not need to be present over the entire transfer time period T 0 , T 1 , T 2 , T 3 . The change can also only be present over a relatively short time period within the transfer time period T 0 , T 1 , T 2 , T 3 , as illustrated in  FIG. 2 . In accordance with the same principle, it is also possible for more than two bits to be transferred during a transfer time period T 0 , T 1 , T 2 , T 3 . The change in the secondary current ΔI sec  and the resultant change in the primary current ΔI prim  can represent both an increase and a reduction in the currents. 
         [0030]      FIG. 3  shows an exemplary embodiment of a transmission circuit  2 , which is connected to the secondary side  20  of the transfer unit  30 . As mentioned above, the transmission circuit  2  firstly provides the supply of energy to the secondary-side circuit components and secondly is used for modulating the secondary current in order to transfer data to the primary side, as explained above. Those parts of the transmission circuit which are used for the supply of energy can substantially correspond to the output circuit of a flux converter. That is to say that, in respect of the supply of energy, the evaluation circuit  1 , the transformer  30  and the transmission circuit  2  operate as a switching converter with galvanic isolation (for example two-transistor flux converter, push-pull converter as illustrated in  FIG. 4  or else a full-bridge converter). 
         [0031]    In such a flux converter, during a first phase (on phase), a current I prim  on the primary side  10  (not illustrated in  FIG. 3 ) of the transfer unit  30  rises linearly. During this phase, a current flows through the diodes  21   1  or  21   2 . During a second phase (freewheeling phase), the diodes  21   1 ,  21   2  are off and are therefore not energized. 
         [0032]    A modulation of the secondary current I sec  can be achieved, for example, by a secondary-side load resistance being varied correspondingly. By virtue of a change in the load resistance, the current consumption of the transmission circuit  2  varies, i.e. more or less current is “drawn”. For this purpose, resistors  22   1 ,  22   2 ,  22   3  are provided in the second circuit  2 . The resistors  22   1 ,  22   2 ,  22   3  are connected in parallel with one another between two connections X, Y.  FIG. 3  shows three resistors, but it is possible for more or fewer resistors to be provided, for example corresponding to the number of bits to be transferred simultaneously. The transmission circuit  2  furthermore has switches  23   1 ,  23   2 , wherein in each case one switch  23   1 ,  23   2  is connected in series with a resistor  22   1 ,  22   2 . If a switch  23   1 ,  23   2  is open, no current can flow via the respective resistor  22   1 ,  22   2 . The “basic load” is represented by the resistance value of the resistor  22   3 . By opening or closing of specific switches  23   1 ,  23   2 , further resistors  22   1 ,  22   2  are connected in parallel with the resistor  22   3 , which effectively results in a reduction in the total load resistance (i.e. an increase in the load) and therefore in a correspondingly higher secondary current I sec . Therefore, the desired modulation of the secondary current I sec  can be achieved by virtue of the total load resistance of the second circuit  2  being varied. It is possible in this case for a switch to be provided in series with each of the resistors. However, it is also possible for resistors to be provided which do not have a switch connected in series (i.e. which are always “switched on”). 
         [0033]    In order to drive the switches  23   1 ,  23   2 , the transmission circuit can have a driver circuit  24 . This driver circuit  24  is designed to provide driver signals S 2   1 , S 2   2  for driving the switches  23   1  and  23   2 . The driver signals S 2   1 , S 2   2  can assume two states, for example. If a drive signal S 2   1 , S 2   2  assumes a first state (S 2   1 =1 or S 2   2 =1), the corresponding switch  23   1 ,  23   2  is closed, for example. If a drive signal S 2   1 , S 2   2  assumes a second state (S 2   1 =0 or S 2   2 =0), the corresponding switch  23   1 ,  23   2  is open, for example. The driver circuit  24  is therefore designed to adjust the state of the driver signals S 2   1 , S 2   2  on the basis of the measurement signal S meas  to be transferred (and therefore on the basis of the data to be transferred). In this case, the switch-on and switch-off times of the switches  23   1 ,  23   2  are synchronized with the switching edges in the secondary-side voltage V sec  and therefore synchronized with the switching edges in the primary-side AC voltage V PRIM . The synchronization takes place with the aid of the driver circuit  24 , which controls the timing of the signals illustrated in  FIG. 2 . 
         [0034]      FIG. 4  shows an exemplary embodiment of the evaluation circuit  1 , which is connected to the primary side  10  of the transfer unit  30 . As already mentioned, the evaluation circuit  1  ensures both reception and evaluation of the transmitted data and provides a voltage supply to the circuits connected to the secondary side (transmission circuit  2 , measuring circuit  50 ). The evaluation circuit  1  has a switching unit  12  comprising a first power transistor  12   1  and a second power transistor  12   2 . The power transistors  12   1 ,  12   2  operate as switches. By driving of the power transistors  12   1 ,  12   2  with suitable driver signals, said power transistors can be switched on (on phase) and off (off phase). During the on phase, the primary current rises approximately linearly and energy is transferred to the secondary side via the coils of the primary side  10  of the transformer  30 . If the power transistors  12   1 ,  12   2  are turned off, the current through the respective transistor  12   1 ,  12   2  is interrupted. 
         [0035]    The driver signals S 1   1 , S 1   2  for driving the power transistors  12   1 ,  12   2  can be provided by a microcontroller  13 , for example. Said microcontroller can have a PWM modulator  13   2  for this purpose, for example. In the present example, the switching unit  12  is connected to a connection for a positive potential V+ via the primary side  10  of the transfer unit  30  and to a connection for a negative potential V− via an (optional) resistor  14 . The resistor  14  can be used, for example, as measuring resistor for measuring the primary current I prim . The primary current I prim  causes a voltage drop in the measuring resistor  14  which is proportional to the primary current I prim . 
         [0036]    A change in the secondary current I sec , as already described, does not need to take place over an entire transfer time period (frame, cf. time intervals T 0  to T 1  in  FIG. 2 ), during which one of the transistors  12   1 ,  12   2  is on, but can also only take place during a specific time segment within the transfer time periods. In order to be able to detect such short-term changes in current during a transfer time period, the primary current can be sampled at specific sampling times, for example. For this purpose, the microcontroller can have an analog-to-digital converter (ADC)  13   1 , for example, which implements the sampling and is triggered with the aid of the PWM modulator  13   2 , for example. 
         [0037]      FIG. 5  shows various transfer time periods. During a first transfer time period (time t 1  to t 2 ), the first power transistor  12   1  is in the switched-on state. During a second transfer time period (time t 3  to t 4 ), the second power transistor  12   2  is in the switched-on state. During these frames or transfer time periods (t 1  to t 2  and t 3  to t 4 ), a primary current I prim  flows through the transformer and via the resistor  14 . During the switch-on phase of a power transistor  12   1 ,  12   2  which lasts up to time t 2  (end of the switch-on phase of the first power transistor  12   1 ) or t 4  (end of the switch-on phase of the second power transistor  12   2 ), the secondary current I sec  is modulated as explained above, which results in a corresponding change in the primary current In the present example, the secondary current I sec  is not only changed once during a frame, as described in relation to  FIG. 2 . Instead, two changes in the current during a frame (transfer time period) are provided for the transfer of two symbols (i.e. bits or, in the case of multi-level transfer, bit groups). In the case of multi-level transfer, as shown in  FIG. 2  with four different levels ( 0 , Δi 1 , Δi 2 , Δi 3 ), four bits (in the form of two symbols) can be transferred in one frame (t 1  to t 2  or t 3  to t 4 ). If the primary current I prim  is sampled at corresponding sampling times A 2  and A 3 , these changes can be detected. As already described in relation to  FIG. 2 , one or more bits can be transferred simultaneously at a sampling time. For example, two bits (first symbol: bits  0 + 1 ) are transferred at sampling time A 2  and two further bits (second symbol: bits  2 + 3 ) are transferred at sampling time A 3 . By virtue of transferring more than two bits at a sampling time or by virtue of the provision of further transfer levels and corresponding sampling times during the transfer time period, it is also possible for more than four bits to be transferred per frame (transfer time period). If necessary, however, it is also possible for fewer than four bits to be transferred per transfer time period. It is also possible for more than two symbols to be transferred per frame (sequentially). 
         [0038]    In some applications it is necessary, for example, to detect and transfer the DC link voltage of a secondary-side converter in intervals of less than 100 microseconds (μs). If, for example, data transfer with 12 bits per measurement value including a start bit and a stop bit is provided, there is a minimum data rate of 120 kbits/s (10 000·12 bits/measurement value). At a switching frequency of 200 kHz (frequency of the secondary-side voltage V sec ), a data rate of 800 kbits/s can be achieved, for example. In the example shown in  FIG. 5 , the switching frequency would be f S =(t 3 −t 1 ) −1 . 
         [0039]    In order that the changes in the primary current I prim  can be identified reliably, provision can be made for the times at which the modulation is implemented on a secondary side to be synchronized with the sampling times A 2 , A 3 . The voltage V sec  on the secondary side  20  of the transfer device  30  is determined by the voltage on the primary side  10 . If the primary side  10  is driven by a square-wave voltage, a square-wave voltage V sec  also results on the secondary side. The rising edges of this secondary voltage V sec  can be detected, for example. If a rising edge is detected at a time t 1 , for example, the secondary-side current I sec  can be modulated correspondingly after elapse of a synchronization time t sync  in order to transfer the first two bits (bits  0 + 1 ). Thereafter, a further modulation can be implemented in order to transfer two further bits (bits  2 + 3 ). The sampling times A 2  and A 3  are then in the center (in time) of a bit or symbol to be sampled, for example. 
         [0040]    As already described above, the primary current I prim  comprises the secondary current I sec  converted by means of the transfer unit and a linearly rising magnetization current. This magnetization current can falsify the measurement on the primary side. For this reason, further sampling times can be provided. As shown in  FIG. 5  for the second transfer time period (switch-on phase of the second power transistor  12   2 ), first sampling A 1  can take place at the beginning of the transfer time period, for example even before sampling time A 2 . The first sampling A 1  can take place, for example, even during the synchronization time t sync . A further additional sampling A 4  can take place at the end of the transfer time period, after sampling time A 3 . If the secondary current I sec  does not change both at the first sampling time A 1  and at the last sampling time A 4 , or changes by the same magnitude at both sampling times, the gradient m of the magnetization current can be determined as follows, for example: 
         [0000]        m =( I   prim ( A 4)− I   prim ( A 1))/( A 4− A 1).
 
         [0041]    This gradient m represents a systematic error in the measurements at the sampling times A 2  and A 3 . Then, the current values measured at the sampling times A 2  and A 3  can be corrected in the microcontroller  13  corresponding to the gradient m determined. 
         [0042]    A transfer time period can have, for example, a specific minimum duration in order to ensure safe measurement at all four positions A 1 , A 2 , A 3 , A 4 . In this case, a jitter can also be included in the calculations by virtue of the minimum duration being selected to be so long that the last measurement at the sampling time A 4  can also be implemented despite possible jitter safely during the transfer time period. 
         [0043]    As mentioned above, start and stop bits (or start and stop symbols) can also be transmitted for synchronization of frame (t 1  to t 2 , t 3  to t 4 ) and sampling times (A 1  to A 4 ). The start and stop bits (or symbols) are in this case transmitted at the beginning or at the end of a frame (transfer time period). If a data word to be transferred comprises 12 bits, for example, three frames of in each case four bits are necessary for the transfer of said data word in the present example (multi-level transfer with two symbols or four bits per frame) (three frames with in each case two symbols, two bits per symbol). In order to mark the beginning and the end of a data word, a start symbol is introduced in the first frame and a stop symbol is introduced in the last frame of a data word. 
         [0044]    Transfer with start and stop symbols is illustrated in  FIG. 6 . Apart from the start and stop symbols, this transfer is identical to that shown in  FIG. 5 . In the present example, the start symbol is not transmitted instead of a “normal” symbol, but is transmitted prior to the time interval in which the first bit of a frame is normally transmitted. A start symbol is therefore sampled at the sampling time A 1 . Similarly, the stop symbol is not transmitted instead of a “normal” symbol, but is transmitted after the time interval in which the last bit of a frame is normally transmitted. A stop symbol is therefore sampled at the sampling time A 4 . 
         [0045]    Terms such as “first”, “second”, and the like, are used to describe various elements, regions, sections, etc. and are also not intended to be limiting. Like terms refer to like elements throughout the description. 
         [0046]    As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. 
         [0047]    It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise. 
         [0048]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.