Abstract:
Methods and apparatuses for detection of a presence of a load. A method may include, for example, applying and subsequently removing a supply voltage across the pair of nodes, comparing an electrical potential at one of the pair of nodes at a time after the supply voltage is removed with a reference value, and generating a signal having a value that depends upon an outcome of the comparison.

Description:
TECHNICAL BACKGROUND 
     Inductive loads, such as electric motors or solenoid valves, are used for widely differing purposes, for example in motor vehicles. Particularly in the case of applications which are safety-relevant, such as occupant protection systems, or which influence the emission of hazardous substances from the vehicle, there are stringent requirements for the reliability of the inductive loads and for the drive circuits which drive the loads. For example, an interruption in an electrical line connection between the load and the drive circuit or within the inductive load itself can lead to a failure of the inductive load. The presence of the load and its correct operation are in this case intended to be detected regularly, ideally during operation. 
     SUMMARY 
     Various aspects are described herein. For example, according to some aspects, methods and apparatuses are provided for detection of a presence of a load. A method may include, for example, applying and subsequently removing a supply voltage across a pair of nodes, comparing an electrical potential at one of the pair of nodes at a time after the supply voltage is removed with a reference value, and generating a signal having a value that depends upon an outcome of the comparison. 
     These and other aspects are described in further detail with reference to various illustrative embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Exemplary embodiments will be explained in more detail in the following text with reference to the figures. Unless stated to the contrary, the same reference symbols in the figures denote identical circuit components, and signals having the same meaning. 
         FIG. 1  shows a first example of a drive circuit for driving a load, having a circuit for identification of load interruption. 
         FIG. 2  shows, by way of example, time profiles of the signals which occur in the drive circuit shown in  FIG. 1 , in order to illustrate a method of operation. 
         FIG. 3  shows one example of a circuit for identification of a load interruption. 
         FIG. 4  shows a circuit arrangement that has been modified in comparison to the circuit arrangement shown in  FIG. 1 . 
         FIG. 5  shows a second example of a drive circuit for driving a load. 
         FIG. 6  shows time profiles of selected signals which occur in the drive circuit shown in  FIG. 5 , in order to illustrate a method of operation. 
         FIG. 7  shows a third example of a drive circuit for driving a load. 
         FIG. 8  shows a fourth example of a drive circuit for driving a load. 
         FIG. 9  shows time profiles of selected signals which occur in the drive circuit shown in  FIG. 8 , in order to illustrate a method of operation. 
         FIG. 10  shows a fifth example of a circuit arrangement for driving a load. 
         FIG. 11  shows a circuit arrangement, which has a bridge circuit, for driving a load. 
         FIG. 12  shows a circuit arrangement for driving a load, which has a clamping circuit in order to limit the voltage drop across the load. 
         FIG. 13  illustrates a method for determination of an evaluation time. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a first exemplary embodiment of a drive circuit for driving a load  11 , in particular an inductive load. By way of example, this inductive load  11  is a direct-current electric motor or a solenoid valve, and can be driven by cyclic application of a supply voltage. In the course of a cyclic drive process such as this, a drive voltage is applied to the inductive load during each of successive drive periods for a switched-on period which is followed by a switched-off period. The duty ratio (duty cycle) of the cyclic drive, that is to say the ratio between the switched-on period and the total duration of the drive period, governs the motor current and thus the torque, for example in the case of an electric motor. Indirectly, this also influences the rotation speed. 
     For cyclic application of a supply voltage between the connecting terminals  12 ,  13  and thus across the load  11 —if there is one—the drive circuit which is illustrated in  FIG. 1  has a voltage supply terminal for a first supply potential or positive supply potential V+, and a second voltage supply terminal for a second supply potential, or reference ground potential GND, for example ground. In the following text, V denotes a supply voltage which is applied between the voltage supply terminals. 
     The drive circuit also has a switching arrangement for cyclic application of the supply voltage V to the connecting terminals  12 ,  13 . This switching arrangement is in the form of a switch  15  in the example shown in  FIG. 1 , which is connected as a low-side switch between the second connecting terminal  13  for the load and the second voltage supply terminal. The first connecting terminal  12  for the load  11  in the drive circuit illustrated in the figure is connected to the first voltage supply terminal. The switching element  15  is, for example, a semiconductor switch, such as a metal-oxide-semiconductor field-effect transistor (MOSFET), an insulated-gate bipolar transistor (IGBT) or a bipolar transistor, and is driven by a pulse-width-modulated drive signal S 1  during operation of the drive circuit. In the case of this drive circuit, any load  11  which is present is connected in series with the switching element  15  between the supply voltage terminals, such that, when the switch  15  is switched on, the supply voltage V between the supply voltage terminals is applied virtually in its entirety across the load  11 . The duty ratio of the pulse-width-modulated signals S 1  in this case governs the duty ratio used to cyclically apply the supply voltage V to the inductive load  11 . 
     When the switching element  15  is closed, the inductive load  11  receives electrical energy. In order to allow the inductive load  11  to be commutated off after the switching element  15  has been opened, a freewheeling element  14  is provided, and in the illustrated example is connected between the connecting terminals  12 ,  13  and therefore in parallel with any load  11  that is present. This freewheeling element in the illustrated example is in the form of a diode, which is connected in the forward-biased direction between the second and first connecting terminals  13 ,  12 . 
       FIG. 2  uses time profiles of the drive signal S 1  of the switching element  15  and of an electrical potential V 20  at the second connecting terminal  13 , which is referred to in the following text as the evaluation potential, to illustrate the method of operation of the drive circuit illustrated in  FIG. 1 . For explanatory purposes, it is assumed in this case that the switch  15  is first of all closed prior to a first time t 1 . Approximately all of the supply voltage prior to this time is applied between the connecting terminals  12 ,  13  and thus across the inductive load  11 , provided that there is no short circuit in the load  11 , as will be assumed in the following explanation. The detection of a load short-circuit such as this is not the subject matter of the present example. When approximately all of the supply voltage is applied between the connecting terminals  12 ,  13 , the electrical potential V 20  at the second connecting terminal  13  when the switch  15  is closed, or switched on, corresponds to the reference ground potential GND, ignoring any resistance of the switch  15  when it is switched on. In this case, the voltage across the switch  15  is 0. 
     If the switch  15  is opened at the time t 1 , then the electrical potential at the second connecting terminal  13  starts to rise because of the energy that has previously been stored in the inductive load  11 , and a freewheeling current starts to flow via the freewheeling element  14  between the connecting terminals  12 ,  13 . During this process, the electrical potential at the second connecting terminal  13  is greater than the first supply potential V+, to be precise by a value which corresponds to the forward voltage of the freewheeling diode  14 . 
     The rate at which the electrical potential V 20  at the second connecting terminal  13  rises, starting from the reference ground potential GND, after the switch  15  is opened is in this case dependent on the electrical energy previously stored in the inductive load  11  when the switch  15  was closed, and, for any given switched-on period, is dependent on the inductance of the inductive load, and on the rate of change of the current flowing through the load. This rate of change of the current is in this case dependent on the switching behavior of the switch  15 , in particular on its switching rate. The stored energy for a given switched-on period is in this case greater, the greater the inductance of the inductive load  11 . In a corresponding manner, the current level of a freewheeling current I 11  which flows through the load  11  after the switch  15  has been opened is dependent on the electrical energy previously stored in the inductive load  11 , and is therefore dependent on the inductance of the load. As the duration of the freewheeling current increases, and therefore as the demagnetization of the inductive load  11  increases, the electrical potential at the second connecting terminal  13  falls again, although this is not illustrated explicitly in  FIG. 2 . The time scale in  FIG. 2  is chosen such that the time period during which the inductive load  11  is commutated off is very long in comparison to the rise time of the electrical potential V 20  after opening the switch  15 . 
     In order to explain this further, a fault situation will be considered in which an electrical connection between the two connecting terminals  12 ,  13  is completely interrupted, and in which the electrical impedance between the connecting terminals  12 ,  13  is exclusively resistive, with its value being dependent on the electrical insulation between the connecting terminals  12 ,  13 , and in the ideal case, tending to infinity. If the resistance between the connecting terminals  12 ,  13  is infinitely high, the electrical potential at the second connecting terminal  13  remains at the reference ground potential after the switch  15  has been opened. The expression “normal operating state” in the following text refers to an operating state in which a drivable inductive load  11  is connected between the connecting terminals  12 ,  13 . The expression “interruption state” refers to an operating state in which an electrical connection between the connecting terminals  12 ,  13  is interrupted. An interruption such as this can result from a defect in the inductive load or from an interruption in the supply lines to the load. In order to make it possible to distinguish between an interruption state such as this and a normal operating state, one exemplary embodiment provides for the electrical potential V 20  at the second connecting terminal  13  to be evaluated after the switch  15  has been opened, and for determination of whether the magnitude of the difference between the electrical potential V 20  after the switch has been opened and the electrical potential V 20  before the switch was opened, with this being referred to in the following text as the voltage shift or potential shift, is greater than a predetermined comparison value. In this case, use is made of the knowledge that a change in the electrical potential V 20  at the second connecting terminal  13  after the switch has been opened presupposes an inductive load between the connecting terminals  12 ,  13 , which received electrical energy when the switch  15  was previously closed and is commutated off via the freewheeling diode  14  after the switch  15  has been opened thus resulting in an increase in the electrical potential at the second connecting terminal  13 . 
     In order to detect the presence of an inductive load  11  and in order to detect an interruption in the inductive load, the drive circuit illustrated in  FIG. 1  has a circuit for detection/identification of a load interruption  20 , and this circuit is connected to the second connecting terminal  13  of the drive circuit. The illustrated circuit  20  has a comparator arrangement  21 , for example a comparator which compares the voltage shift of the evaluation potential V 20  with the comparison value. In the illustrated example, the comparator arrangement  21  for this purpose compares the evaluation potential with a reference voltage Vref which is produced by a reference voltage source  22 . This reference voltage V ref  in the example corresponds to the comparison value with which the voltage shift in the evaluation potential is compared. During the switched-on period, the electrical potential V 20  at the second connecting terminal  13  is the reference ground potential, as has already been explained above. In order to determine whether this potential V 20  rises by more than the value of the reference potential V ref  after the switch has been opened, the reference voltage source  22  in the illustrated example is connected between one of the inputs, in the example the negative input, of the comparator  21  and the reference ground potential GND. Another input, in the example the positive input, of the comparator  21  is connected to the second connecting terminal  13  of the drive circuit. A comparison signal S 21  is available at the output of the comparator  21 , and is dependent on comparison of the electrical potential V 21  at the second connecting terminal  13  and the reference potential V ref . This comparison signal S 21  in the illustrated example assumes a high level when the evaluation potential V 20  at the second connecting terminal  13  is greater than the reference voltage V ref . This is equivalent to the evaluation potential V 20  after the switch has been opened rising by more than the reference voltage V ref  above the value of the evaluation potential before the switch was opened, and the potential shift of the evaluation potential being greater than the reference voltage V ref  which corresponds to the comparison value. 
     In the case of the circuit illustrated in  FIG. 1  for identification of a load interruption  20 , the output of the comparator  21  is followed by an evaluation circuit  24  which is designed to evaluate the comparison signal S 21  at a predetermined evaluation time. The timing of the evaluation point is, for example, dependent on the pulse-width-modulated drive signal S 1  for the switch  15 . Provision is therefore made in one exemplary embodiment for the evaluation time to occur at a predetermined time after the time at which the drive signal S 1  assumes a level at which the switch  15  is switched off or on. 
     The reference voltage V ref  is chosen such that the evaluation potential V 20  when the inductive load  11  is connected correctly and is intact changes during the drive pause of the switch  15  by a potential value which is greater than the reference voltage V ref . For the drive circuit illustrated in  FIG. 1 , the potential shift in the evaluation potential V 20  after the switch  15  has been opened corresponds to the sum of the supply voltage V and the forward voltage of the freewheeling diode  14 . The reference voltage V ref  may be chosen within a wide range and may, for example, be dependent on the supply voltage V. For example, the reference voltage may be between 50% and 75% of the supply potential. A reference voltage such as this, which is dependent on the supply voltage, may, for example, be derived via a simple voltage divider (not illustrated) from the supply voltage V between the supply voltage terminals. 
     In a further exemplary embodiment, the reference voltage V ref  has a constant value which is independent of the supply voltage V and, for example, is between 3V and 5V for the exemplary embodiment shown in  FIG. 1 . A constant reference voltage such as this can be produced by a reference voltage source (not illustrated) such as a bandgap reference circuit. 
     The choice of the comparison value governs the disturbance sensitivity of the circuit for detection of the load interruption, as will be explained in the following text with reference to two extreme examples: if the value of the reference voltage V ref  is chosen to be very small, then even small inductances, such as parasitic inductances between the connecting terminals, will be sufficient to cause the evaluation potential to rise above the reference voltage V ref . If, in contrast, the comparison value and the reference value V ref  are chosen to be very large, then there is a risk of a spuriously detected load interruption, since the presence of a load will be detected only after a major rise in the evaluation potential. 
     The circuit for identification of a load interruption  20  optionally has a current source  23  which, in the illustrated example, is connected between the second connecting terminal  13  and the reference ground potential GND. This current source is in this case chosen such that its current is less than the freewheeling current which flows when the inductive load is connected correctly. The current source  23  thus has no influence, or only a minor influence, on the evaluation potential V 20  when the inductive load  11  is connected correctly and is intact. When no inductive load is connected, or an inductive load which is not intact is connected, the current source  23  results in any parasitic inductance which may be present between the connecting terminals  12 ,  13 , and/or any parasitic resistance, causing the evaluation potential at the second connecting terminal  13  not to rise above the value of the reference voltage V ref  after the switch  15  has been opened. Any current flowing as a result of parasitic effects such as these after the switch  15  has been opened is in this case less than or equal to the current received from the current source  23 , so that the evaluation potential V 20  in this case remains below the reference voltage V ref  after the switch  15  has been opened. If no inductive load is present, or the inductive load is not intact, and there is a pure resistance between the connecting terminals  12 ,  13 , current source  23  results in the majority of the supply voltage V definitely being dropped between the connecting terminals  12 ,  13 , so that the supply potential V 20  likewise remains below the reference voltage V ref . The profile of the evaluation potential V 20  is illustrated using dashed-dotted lines in  FIG. 2  for the situation in which a resistive load is provided between the connecting terminals  12 ,  13  and a current source  23  in the circuit for identification of a load interruption. The resistance of this resistive load is in this case less than infinity, so that, although the value of the evaluation potential V 20  is not equal to zero, it is less than the reference voltage or the reference potential V ref . The choice of the reference voltage V ref  in this case governs the resistance of the resistive load from which a load interruption will be detected. The greater the reference voltage V ref  is in the example shown in  FIG. 1 , the lower is the resistance from which a load interruption will be detected. 
     One possible circuitry implementation example of the evaluation circuit  24  is illustrated in  FIG. 3 . In this exemplary embodiment, the evaluation circuit  24  comprises a flip flop  241  with an inverting set input S to which the comparator signal S 21  is supplied, and with a clock input to which a timer signal S 242  is supplied. The timer  242  is driven by the drive signal S 1  for the switch  15  and is in each case set at a time at which the switch  15  is opened, that is to say for example on a falling edge of the drive signal S 1 . The output signal S 242  from this timer  242  changes its level from a first level value, for example a low level, to a second level value, for example a high level, after a waiting time, which is predetermined by the timer  242 , has elapsed. The flip flop  241  is designed to evaluate the comparator signal S 41  applied to the set input at the time of this level change, and to set the flip flop  241  when the comparison signal S 21  assumes a low level at this time. With reference to the exemplary embodiment shown in  FIGS. 1 and 2 , setting of the flip flop  241  is therefore equivalent to the evaluation potential V 20  not having exceeded the reference voltage V ref  at the evaluation time, which is therefore in turn equivalent to no intact inductive load being connected between the connecting terminals  12 ,  13 . An output signal from the flip flop  241  represents the state in which a load is connected between the connecting terminals  12 ,  13 , in which case, for example, a high level of this output signal S 20  indicates that no inductive load is present, or that an inductive load is present that is not intact. The output signal S 20  from the circuit for identification of a load interruption is therefore used as a load interruption signal. 
     In the circuit illustrated in  FIG. 1 , the reference voltage V ref  is related to a reference ground potential GND. In this example, the reference voltage V ref  corresponds to the comparison value with which the potential shift of the evaluation potential is compared. Alternatively, with reference to  FIG. 4 , it is possible to connect the reference voltage source between one of the inputs of the comparator  21  and the supply potential V+. In this case, the presence of a load between the connecting terminals  12 ,  13  is assumed when the evaluation potential rises after the switch has been opened to a value which is greater than the supply potential V+minus the reference voltage V ref . The comparison value with which the potential shift in the evaluation potential is compared in this case corresponds to the difference between the supply potential V+ and the reference voltage V ref , and is therefore dependent on the supply potential or the supply voltage V+. The dashed arrow annotated V ref  in  FIG. 2  illustrates the determination of the comparison value in this situation. 
       FIG. 5  shows a second exemplary embodiment of a drive circuit for driving a load. This drive circuit differs from the drive circuit illustrated in  FIG. 1  in that the switch  15  is in the form of a high-side switch, that is to say it is connected between the first supply potential terminal and the first connecting terminal  12 , while the second connecting terminal is connected to the supply potential terminal for the reference ground potential GND. The circuit for identification of a load interruption  20  in the case of this drive circuit is connected to the first connecting terminal  12 . The evaluation potential V 20  therefore corresponds to an electrical potential at this first connecting terminal  12 . 
       FIG. 6  illustrates time profiles of the drive signal S 1  for the switch  15 , of the evaluation potential V 20  and of the comparator signal S 21  at the output of the comparator  21  in the circuit for identification of a load interruption  20 . In the case of the drive circuit illustrated in  FIG. 5 , the evaluation potential V 20  during a period in which the switch  15  is switched on corresponds to the positive supply potential V+—ignoring line resistances and the resistance of the switch  15  when it is switched on. When an inductive load  11  is present between the connecting terminals  12 ,  13 , the evaluation potential V 20  falls after the switch  15  has been opened to a negative value, which is less than the reference ground potential GND by the value of the field voltage of the freewheeling diode  14 . Any voltage shift in the evaluation potential V 20  after the switch  15  has been opened corresponds, as in the case of the exemplary embodiment shown in  FIG. 1 , to the sum of the supply voltage V+ and the forward voltage of the freewheeling diode  14 . 
     In the case of the drive circuit illustrated in  FIG. 5 , it is assumed that an inductive load  11  is intact and correctly connected when the evaluation potential V 20  after the switch  15  has been opened falls by more than the value of the reference voltage V ref . The comparison value with which the potential shift of the evaluation potential is compared in this case corresponds to the reference voltage V ref . The reference voltage source which produces the reference voltage V ref  is for this purpose connected, with reference to  FIG. 5 , between the positive input of the comparator  21  and the terminal for the positive supply potential V+. The evaluation potential V 20  is in this case supplied to the negative (inverting) input of the comparator. In this case—as in the other examples as well—the reference voltage V ref  may be a constant voltage that is produced by a constant voltage source, or may be dependent on the supply voltage V+, and in this case may, in particular, be proportional to the supply voltage V+. 
     The current source  23  that is optionally provided for this drive circuit is connected between the first connecting terminal  12  and, in the same way, the terminal for the positive supply potential V+. The comparator signal V 21  at the output of the comparator  21  in this case assumes a high level when the evaluation potential V 20  has fallen by the value of the reference voltage V ref  below the positive supply potential V+, as is illustrated at a time t 2  in  FIG. 6 . 
       FIG. 7  shows a modification of the circuit arrangement illustrated in  FIG. 5 . In this circuit arrangement, the reference voltage V ref  is related to the reference ground potential GND. The reference voltage source  22  is for this purpose connected between one of the inputs of the comparator, in the example the positive input, and the reference ground potential GND. The voltage shift which the evaluation potential V 20  must at least reach in this circuit in order to detect that the load is present is in this case depends on supply voltage V+ and the reference voltage V ref . In the case of this circuit, the presence of a load is assumed when the evaluation potential V 20  after the switch has been opened falls below the value of the reference potential V ref . The voltage shift in the evaluation potential V 20  is in this case greater than the difference between the supply voltage V+ and the reference voltage V ref . The comparison value by which the evaluation potential V 20  must differ at least from the value before the switch was opened after the switch has been opened in order to detect the presence of a load corresponds in this case to this difference between the supply voltage V+ and the reference voltage V ref . In this case, as already explained above, the reference voltage V ref  may be dependent on the supply voltage V or may be independent of the supply voltage V. 
     In the case of the circuit arrangement illustrated in  FIG. 7 , the voltage shift of a voltage across the load  11  is evaluated. 
       FIG. 8  shows a drive circuit, modified from the drive circuit shown in  FIG. 1 , for an inductive load  11 . In the case of this drive circuit, a switchable freewheeling element  16  is connected between the connecting terminals  12 ,  13  and, in the illustrated example, is in the form of a MOSFET with an integrated body diode  161 . This freewheeling element  16  is driven by a second drive signal S 2  which, during operation of the drive circuit, is matched to the drive signal S 1  for the switch  15  such that the freewheeling element  16  and the switch  15  are not switched on at the same time, thus avoiding parallel currents. In the case of this circuit, the body diode of the MOSFET carries a freewheeling current during the delay time between the switch  15  being switched off and the MOSFET  16  being switched on. The circuit  20  for detection of the load interruption is illustrated just as a circuit block in  FIG. 8 , and is implemented, for example, in a corresponding manner to the circuit illustrated in  FIG. 1 . 
       FIG. 9  shows time profiles of the first and second drive signals S 1 , S 2  for the evaluation potential V 20  at the second connecting terminal  13 , and of the comparator signal S 21  which is produced in the circuit  20  for identification of a load interruption. In the case of the drive circuit illustrated in  FIG. 8 , the freewheeling element  16  is switched on with a time delay of the drive signal S 2  after the switch S 1  has been opened. In  FIG. 9 , t 1  denotes a time at which the switch S 1  is switched off, and t 3  denotes a later time, at which the freewheeling element  16  is switched on. Immediately after opening of the switch  15 , the drive circuit illustrated in  FIG. 8  operates in a corresponding manner to the drive circuit illustrated in  FIG. 1 . The integrated freewheeling diode  161  in the freewheeling element  16  in this case operates in a corresponding manner to the freewheeling diode  14  illustrated in  FIG. 1 . Once the switch  15  has been opened and when a correctly connected and intact inductive load is present, the evaluation potential V 20  in this case rises to a potential value which corresponds to the sum of the supply voltage V+ and the forward voltage of the integrated freewheeling diode  161 . After the freewheeling element has been switched on, a freewheeling current path arises between the connecting terminals  12 ,  13  and carries current in both directions, in contrast to the situation in the case of the freewheeling element  14  shown in  FIG. 1  or of the integrated freewheeling diode  161 . In consequence, when an inductive load is present, the evaluation potential V 20  falls to the value of the upper supply potential V+. 
       FIG. 9  uses dashed-dotted lines to show the time profile of the evaluation potential V 20  when no inductive load is present or when a resistive load is present between the connecting terminals  12 ,  13 . Starting from a low potential value, the evaluation potential rises in this case, when the freewheeling element  16  is being driven, to the value of the positive supply potential V+. The evaluation time at which the evaluation potential is evaluated in order to detect the presence of an inductive layer occurs, for example, between the time t 1  at which the switch  15  is switched off and the time t 3  at which the freewheeling element  16  is switched on. By way of example, the evaluation time occurs a predetermined time period before the time t 3  at which the freewheeling element  16  is switched on. 
     Since in this example the evaluation potential V 20  changes only after a time delay from the start of the driving of the freewheeling element, as a result of switching delays, a further exemplary embodiment provides for the evaluation potential V 20  to be evaluated at the start of the drive of the freewheeling element. 
       FIG. 10  shows a modification of the drive circuit illustrated in  FIG. 7 . In the case of the drive circuit illustrated in  FIG. 10 , a switchable freewheeling element  16  is connected, instead of a freewheeling diode, between the connecting terminals  12 ,  13 . This freewheeling element  16  is in the form of a MOSFET with an integrated freewheeling diode  161  in the illustrated example, and is driven by a second drive signal S 2 . In a corresponding manner to the drive circuit illustrated in  FIG. 8 , the freewheeling element  16  in the drive circuit shown in  FIG. 10  is switched on with a time delay after the switch  15  has been opened.  FIG. 5  uses dashed lines to show a time profile of the second drive signal S 2  of the freewheeling element  16  for the drive circuit illustrated in  FIG. 8 . In this case, t 3  denotes a time at which the freewheeling element  16  is switched on. The time profile of the evaluation potential V 20  for the drive circuit illustrated in  FIG. 10  corresponds, until the time t 3 , to the time profile of the evaluation potential V 20  for the drive circuit shown in  FIG. 7 . Once the freewheeling element  16  has been switched on, the evaluation potential V 20  changes in the direction of the reference ground potential GND. An evaluation time at which the evaluation potential V 20  is evaluated for detection of the presence of an inductive load occurs between the times t 1  and t 3  and, for example, occur a predetermined time period before the freewheeling element  16  is switched on. This evaluation time may, however, also correspond to the time t 3  at which the freewheeling element is switched on. 
     The circuit for identification of a load interruption  20  in the case of the drive circuit shown in  FIG. 9  is illustrated just as a circuit block. This circuit  20  may be, for example, implemented in a corresponding manner to the circuit for identification of a load interruption as shown in  FIG. 4 , that is to say it may have a reference voltage source which is connected between the terminal for the positive supply potential V+ and a comparator. 
       FIG. 11  shows a further exemplary embodiment of a drive circuit for driving an inductive load. This drive circuit is in the form of a bridge circuit and has two half-bridges, each having two series-connected switches  15 ,  16 , and  17 ,  18  respectively. The switches in the half-bridges in the illustrated example are in the form of semiconductor switches, specifically MOSFETs. A first of the half-bridges has a first semiconductor switch  15 , which is connected as a low-side switch, and a second semiconductor switch  16 , which is connected as a high-side switch. The semiconductor switches are in the form of MOSFETs and each have an integrated freewheeling diode, with these being annotated with the reference symbols  161  and  151 . The second half-bridge has a third semiconductor switch  17 , which is connected as a low-side switch, and a fourth semiconductor switch  18 , which is connected as a high-side switch. The reference symbol  181  denotes an integrated freewheeling diode for the fourth semiconductor switch  18 , and the reference symbol  171  denotes an integrated freewheeling diode for the third semiconductor switch  17 . 
     A first connecting terminal of the bridge circuit illustrated in  FIG. 11  is formed by a circuit node which is common to the third and fourth semiconductor switches  17 ,  18 . A second connecting terminal  13  of the drive circuit is formed by a circuit node which is common to the first and second semiconductor switches  15 ,  16 . 
     The semiconductor switches in the two half-bridges  15 ,  16  and  17 ,  18 , respectively, are each connected between a terminal for a first supply potential V+ and a terminal for a reference ground potential GND, between which a supply voltage V is applied. The bridge circuit illustrated in  FIG. 11  makes it possible to apply the supply voltage V between the first and the second connecting terminal  12 ,  13  with a first or a second polarity. The supply voltage which is applied between the first and second connecting terminals  12 ,  13  has a positive sign when the first and fourth switches  15 ,  18  in the bridge circuit are switched on, and the second and third switches  16 ,  17  are switched off. The supply voltage between the first and the second connecting terminals  12 ,  13  has a negative sign when the second and third switches  16 ,  17  are switched on, and the first and fourth switches  15 ,  18  are switched off. By way of example, the inductive load  11  may be an electric motor which rotates in a first rotation direction when the supply voltage is in the first polarity and in a second rotation direction when the supply voltage is in a second polarity. 
     For clocked application of a positive supply voltage between the connecting terminals  12 ,  13  and to the load  11  which is connected between the connecting terminals  12 ,  13 , the third switch  17  is permanently switched off by means of a third drive circuit S 3 , the fourth switch  18  is permanently switched on via a fourth drive signal S 4 , and the first switch  15  is driven in a pulse-width-modulated manner by means of a first drive signal S 1 . The first connecting terminal  12  is therefore permanently connected to the positive supply potential V+ via the switched-on fourth switch  18 . The bridge circuit illustrated in  FIG. 11  operates corresponding to the drive circuit illustrated in  FIG. 8  when in this operating state. The second switch  16  in the half-bridge in this case operates as a controlled freewheeling element which, after the first switch  15  has been opened, initially carries a freewheeling current, which is induced when the inductive load  11  is commutated off, via the integrated freewheeling element  161 . This freewheeling element is connected in parallel with the series circuit comprising the load and the permanently switched-on fourth switch  18 , and is therefore connected in parallel with the load, in a corresponding manner to the examples shown in  FIGS. 1 and 8 . 
     A first switch  20  is provided in order to detect a load interruption, is connected to the second connecting terminal  13 , and operates in a corresponding manner to the circuit  20 , as explained with reference to  FIGS. 1 and 8 , for identification of a load interruption. 
     For clocked application of a negative supply voltage between the first and second connecting terminals  12 ,  13 , the first semiconductor switch  15  is permanently switched off via the first drive signal S 1 , and the second switch  16  is permanently switched on via the second drive signal S 2 . For clocked application of a negative supply voltage, the third semiconductor switch  17  is in this case driven in a pulse-width-modulated manner via the third drive signal S 3 . The fourth semiconductor switch  18  together with the integrated freewheeling diode  181  in this case operates as a freewheeling element which, after the third semiconductor switch  17  has been switched off, carries a freewheeling current caused by the inductive load  11 . In order to detect a load interruption during this operating state, a further circuit  20 ′ is provided, is connected to the second connecting terminal  12 , and produces a second load interruption signal S 20 ′. This circuit for identification of a load interruption  20 ′ is provided in a corresponding manner to the circuit for identification of a load interruption  20 . It is optionally possible to provide only one circuit for identification of a load interruption, with this circuit being selectively, that is to say as a function of the operating state of the bridge circuit, connected to the first or the second connecting terminal  12 ,  13 . 
     In the case of the two methods of operation explained above, one of the connections  12 ,  13  of the load is permanently at the supply potential via one of the second and fourth switches  16 ,  18  of the bridge circuit, while the first or third switch  15 ,  17 , which form low-side switches in the bridge circuit, is driven in a clocked form. 
     Furthermore, the bridge circuit can also be operated such that in each case one of the first and third switches  15 ,  17 , that is to say one of the low-side switches, is permanently closed, while one of the second and fourth switches  16 ,  18 , that is to say one of the high-side switches, is driven in a clocked manner. In order to apply a positive voltage to the load  11 , the third switch  17  is switched on permanently, and the second switch  16  is driven in a clocked manner. The first switch  15  in this case operates as a switchable freewheeling element, which is switched on when the second switch  16  is switched off. In order to apply a negative voltage to the load  11 , the first switch  15  is switched on permanently, and the fourth switch  18  is driven in a clocked manner. In this case, the third switch  17  operates as a switchable freewheeling element which is switched on when the fourth switch  18  is switched off. During the two last-mentioned operating phases, the bridge circuit operates in a corresponding manner to the circuit shown in  FIG. 10 . 
     The already explained bridge circuit may, of course, be produced using any desired semiconductor switches, in particular IGBTs, and is not restricted to the use of MOSFETs. When using IGBTs, which do not have integrated freewheeling diodes, instead of MOSFETs, separate freewheeling elements can be provided for off-commutation of the inductive load  11 . 
     Instead of the already explained freewheeling elements which are connected in parallel with the load  11  and, in the explained examples, are in the form of freewheeling diodes, it is, of course, also possible to use any desired components or component arrangements which allow off-commutation of the load  11 . One example of a component arrangement such as this is a circuit for “active zenering”.  FIG. 12  shows one such circuit for active zenering in conjunction with a drive circuit as shown in  FIG. 1 . 
     The zener circuit  30  in the case of this drive circuit is connected between the second connecting terminal  13  of the load  11  and the reference ground potential GND, and has a first zener diode  32 . This first zener diode is connected in the reverse-biased direction between the second connecting terminal  13  and a drive connection of the switch  15  which, for example, is in the form of an MOS transistor. If the potential at the second connecting terminal  13  in this zener circuit  30  exceeds the breakdown voltage of the first zener diode after the switch  15  has been switched off and when an inductive load is present, then the switch is switched on by this zener diode  32 . The switch therefore limits the electrical potential at the second connecting terminal  13  in the upward direction, and allows off-commutation of the load  11 . 
     A second reverse-biased zener diode  33  is optionally connected between the drive connection of the switch  15  and the reference ground potential GND. This zener diode  33  is used to limit the drive voltage for the switch  15 , particularly during the operating phase in which the switch  15  is switched on by the zener circuit in order to commutate the load  11  off. 
     The zener circuit  30  that has been explained may be used for off-commutation of the load  11  in all of the exemplary embodiments explained above. The use of a zener circuit such as this instead of freewheeling diodes has no effect on the method of operation of the circuit  20  for identification of a load interruption. 
     In general, with regard to the drive circuits that have been explained above, it can be said that the circuit  20  for identification of a load interruption  20  is connected to a circuit node which is located between the load, or a connecting terminal for the load, and a switch which is used for clocked application of a supply voltage to the load and for this purpose is driven in a pulse-width-modulated manner. 
     By way of example, there is no need for the current source  23  which is optionally provided in the circuit for detection of a load interruption, in the case of the exemplary embodiment explained above, when the evaluation potential is evaluated at a time at which the switch which is used for application of the supply voltage has not yet been switched off completely after the start of a switched-off drive, that is to say a residual current is still flowing.  FIG. 13  schematically illustrates a current I through a switch as explained above. After a switching-off time t 1 , at which the switch is begun to be switched off, the current in this case decreases continuously. The evaluation time t 3  can in this case be defined on the basis of the current I flowing through the switch, by measuring the current through the switch and comparing this with a current reference value Iref. The evaluation time in this case corresponds to the time at which this current reaches the current reference value or falls below this current reference value.