Abstract:
A device ( 100 ) for activating an electrical consumer ( 105 ) includes a controllable current source ( 140 ) for providing a control current, a switching unit ( 115 ) for controlling a consumer current as a function of the control current, and a sampling unit ( 145 ) for determining a time delay between an activation of the current source ( 140 ) and the enabling or interruption of the current flow by the switching unit ( 115 ). Furthermore, a processing unit ( 135 ) is provided, which is configured to determine that the current source ( 140 ) is defective if the time delay lies outside a predetermined range.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a technology for activating an electrical consumer. In particular, the present invention relates to the determination of a degradation of a current source in an activation of the electrical consumer. 
       BACKGROUND INFORMATION 
       [0002]    A switching unit, which is activated with the aid of a driver circuit, is used to activate an electrical consumer, to enable a current flow between the consumer and a predetermined potential. To keep waste heat low and electromagnetic compatibility high, it is possible to activate the switching unit for controlling the consumer current using a current source instead of using a voltage source. If the current source is activated, it thus delivers a predetermined control current to the switching unit, which permits a current flow through the consumer as a function of the control current. If the switching unit is constructed with the aid of a bipolar transistor, a current flow may thus be controllable in an analog manner as a function of the control current; in a specific embodiment having a field-effect transistor, another relationship may exist. 
         [0003]    It is possible that the current source degrades on the basis of a thermal stress or an aging effect, that is to say, the control current provided thereby is excessively large or excessively small. It is then a concern that the switching unit will be overloaded, so that it may be damaged, or will not be sufficiently activated, so that it enables the current to move through the consumer excessively slowly or not at all. In a safety-critical application, for example, for activating an electric motor as a steering aid in a motor vehicle, the activation must meet specific reliability and safety requirements. Such requirements are stipulated, for example, in the specification ASIL (automotive safety integrity level) corresponding to ISO 26262. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to provide a device for activating an electrical consumer, a method for determining a defect on a current source in the device, and a computer program product, with the aid of which predefined reliability and safety criteria may be met or improved. The present invention achieves this object with the aid of a device, a method, and a computer program product having the features of the descriptions herein. The further descriptions herein provide specific embodiments. 
         [0005]    A device according to the present invention for activating an electrical consumer includes a controllable current source for providing a control current, a switching unit for controlling a consumer current as a function of the control current, and a sampling unit for determining a time delay between the activation of the current source and the enabling or interruption of a current flow by the switching unit. Furthermore, a processing unit is provided, which is configured to determine that the current source is defective if the time delay lies outside a predetermined range. 
         [0006]    It is thus possible to determine early that the current source degrades, i.e., in particular it operates outside its specification as a result of slow changes or effects, in that the provided control current is excessively large or excessively small. For example, if the switching unit includes a field-effect transistor, initially a gate-source capacitance thus has to be charged during a switching-on procedure, then a Miller capacitance has to be discharged, and finally a gate-source capacitance has to be fully charged to enable complete gating of the field-effect transistor. If the current source provides an excessively large control current, the field-effect transistor may thus be damaged during the gating. If the provided control current is excessively small, the gating may take place more slowly or incompletely. In both cases, a function of a consumer connected to the switching unit may not be ensured under certain circumstances. The early determination of a degradation of the current source may therefore contribute to already recognizing an imminent fault early, in order to output a fault signal or take suitable countermeasures. The reliability and the safety of the device may thus be increased. 
         [0007]    The direction of the control current is arbitrary; a negative control current may also be provided so that the current source practically operates as a current sink. The switching unit may alternatively enable or interrupt the current flow as a function of the control current. Hereafter, the case of a current source having a positive control current for enabling (turning on) a current by way of the control unit is presumed as an example. 
         [0008]    In one variant, an external fault may also be determinable with the aid of the device, for example, a parallel connection between a gate of the field-effect transistor and a ground potential or an incorrect capacitor, which is connected to the gate. 
         [0009]    If the switching unit includes a field-effect transistor, the sampling unit may thus be configured to determine a gate-source voltage of the field-effect transistor. The degree of the gating or the phase of the gating operation may be read off at the gate-source voltage. An integrated driver of the device may already include a sampling or processing unit for the gate-source voltage, for example, to detect a short-circuited switching unit during operation. An already existing device may thus be used to determine the control current of the current source via the described time delay. 
         [0010]    The sampling unit may be configured to determine when the gate-source voltage has exceeded the plateau voltage of the field-effect transistor. If the control current is applied to the field-effect transistor, the gate-source voltage initially does not increase above a specific plateau voltage, before the Miller capacitance of the field-effect transistor is discharged. If the gate-source voltage exceeds the plateau voltage, almost the entire current flow is thus already typically enabled by the switching unit. A sufficiently precise determination of the point in time of the enabling of the current flow may thus be carried out. 
         [0011]    If the switching unit includes a field-effect transistor, in an alternative specific embodiment, the sampling unit may thus be configured to determine a drain-source voltage of the field-effect transistor. If the drain-source voltage falls below a predetermined threshold value, the current flow is thus enabled by the switching unit. If the drain-source voltage exceeds the threshold value, the current flow is reliably interrupted by the switching unit. The comparison of the drain-source voltage to the threshold value may be carried out with the aid of the processing unit. 
         [0012]    In another specific embodiment, a digital counter may be provided for counting pulses of a clock signal between the activation of the current source and the enabling or interruption of the current flow by the switching unit. The digital counter may already be provided in an integrated control device, to be able to study chronological sequences on the control device. The time delay may thus be determined in a simple and efficient manner. 
         [0013]    A method according to the present invention for determining a defect on a current source for providing a control current for a switching unit includes steps of activating the current source, detecting a time delay until a current flow is enabled or interrupted by the switching unit, and determining that the current source is defective if the time delay lies outside a predetermined range. 
         [0014]    In particular, it may be determined that the current source provides an excessively large control current if the time delay is less than the predetermined range. In a corresponding way, it may be determined that the current source provides an excessively small control current if the time delay is greater than the predetermined range. 
         [0015]    A computer program product according to the present invention includes a program code arrangement for carrying out the described method when the computer program product is run on a processing unit or is stored on a computer-readable data carrier. 
         [0016]    The present invention will be described in greater detail with reference to the appended figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a circuit diagram of a device for activating a consumer. 
           [0018]      FIG. 2  shows time sequences on a switching unit of the device for  FIG. 1 . 
           [0019]      FIG. 3  shows a flow chart of a method for activating the consumer with the aid of the device from  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  shows a circuit diagram of a device  100  for activating a consumer  105 . Although the provided technology is usable for activating an arbitrary number of switching units  115 , the specific embodiment shown in  FIG. 1  includes a somewhat more complex example having three half-bridges  110 , each including two switching units  115 , for activating a consumer  105 , which is provided here as a three-phase electric motor in a delta connection. Illustrated switching units  115  are each provided as examples as N-channel field-effect transistors, in particular as MOSFETs. In  FIG. 1 , first switching units  115 , which are shown on top, are each configured to enable a current flow from a first potential  120  into consumer  105  and, shown further down, second switching units  115  are each configured to enable a current from the consumer  105  to a second potential  125 . A first switching unit  115  is also referred to as a high-side switch and a second switching unit  115  is also referred to as a low-side switch. In other specific embodiments, another configuration of switching units  115  is provided for enabling a current through consumer  105 . 
         [0021]    A driver  130 , which may be integrated into switching units  115  and/or a processing unit  135 , is typically provided for activating switching units  115 . For reasons of clarity, elements of driver  130  are only shown for one of switching units  115  in  FIG. 1 ; other switching units  115  may be activated in a corresponding way. 
         [0022]    A current source  140  is provided for making available a control current for switching unit  115 . If current source  140  is activated, in particular by control of processing unit  135 , it thus provides a control current at switching unit  115 , upon which this switching unit enables or interrupts the described current flow into consumer  105  or out of consumer  105 . To avoid a short-circuit between potentials  120  and  125 , it is to be ensured that both switching units  115  of a half bridge  110  do not enable a current flow simultaneously. The activation of both switching units  115  is typically carried out by processing unit  135 , which has to ensure that the activations are offset in time so that a short-circuit does not take place. Because switching units  115  turn on or off with a delay upon a corresponding control current, the activations of current sources  140  may nonetheless be simultaneously active. 
         [0023]    A sampling unit  145  is provided to determine a time delay between an activation of current source  140  and the enabling or interruption of the current flow by switching unit  115 . In the illustrated specific embodiment, sampling unit  145  includes a monitoring unit  150  for the gate-source voltage of switching unit  115 . In another alternative, the drain-source voltage of switching unit  115  may instead be monitored with the aid of monitoring unit  150 . Monitoring unit  150  provides the sampled voltage or a signal when the sampled voltage has exceeded a predetermined threshold value. In the case of the gate-source voltage, this threshold value, as will be explained in greater detail hereafter, may be a plateau voltage of switching unit  115 . Furthermore, sampling unit  145  includes a counter  155 , which is configured to count pulses of a clock signal, which are provided by a clock generator  160 . 
         [0024]    Counter  155  is connected to current source  140  and monitoring unit  150  in such a way that the counting of the pulses begins when current source  140  is activated and ends when monitoring unit  150  provides a signal which indicates that the current flow is enabled or interrupted by switching unit  115 . The counter content of counter  155  is subsequently an indication of the time delay between the activation of current source  140  and the enabling or interruption of the current flow by switching unit  115 . This delay is typically primarily dependent on an input capacitance of switching unit  115 , in particular a gate capacitance, if it is a field-effect transistor, a turning-on threshold of switching unit  115 , and the current strength of the control current provided by current source  140 . Because primarily the last-mentioned parameters of aging and temperature influences on current source  140  are interrupted, while the other parameters typically remain constant, a possible degradation of current source  140  may be determined on the basis of the determined time delay. 
         [0025]      FIG. 2  shows time sequences on a switching unit  115  of device  100  from  FIG. 1 . A time is plotted in the horizontal direction and a gate-source voltage of a field-effect transistor, which switching unit  115  is implemented as, is plotted in the vertical direction. A first sequence  205  begins when current source  140  applies the correct control current to switching unit  115 . Current source  140  is activated at a point in time T 0 . From there, the gate-source voltage rises essentially linearly until point in time T 3 , at which it reaches a plateau voltage  220 . During this interval, the gate-source capacitance of switching unit  115  is charged. The 
         [0026]    Miller capacitance of switching unit  115  is discharged between points in time T 3  and T 4 . The gate-source voltage cannot exceed plateau voltage  220  during this time. Subsequently, the gate-source voltage rises further, while the gate-source capacitance is completely charged and switching unit  115  switches through completely. 
         [0027]    If the control current of current source  140  is excessively large, a second sequence  210  thus results. Plateau voltage  220  is already reached at point in time T 1  and left again at point in time T 2 ; the gating takes place in an accelerated way. This operating mode is stressful for switching unit  115  and may result in defects, in addition, an emission of electromagnetic waves may thus be promoted, so that the device does not meet the requirements for electromagnetic compatibility (EMC) under certain circumstances. If the provided control current is excessively low, a third sequence  215  thus results. The provided control current is excessively low here. Plateau voltage  220  is only reached at point in time T 5  and cancelled again at point in time T 6 . Additional stress of switching units  115  may thus be caused. 
         [0028]    As already described above, it may be determined that switching unit  115  enables or interrupts a current flow through consumer  105  when the gate-source voltage exceeds plateau voltage  220 . The time delay between the activation of current source  140  and the enabling or interruption of the current flow by switching unit  115  is (T 4 +T 0 ) in first sequence  205 , (T 2 −T 0 ) in second sequence  210 , and (T 6 −T 0 ) in third sequence  215 . With reference to point in time T 0 , a time range  225  may be specified in which the point in time should lie at which the gate-source voltage exceeds plateau voltage  220 , to identify the control current provided by current source  140  as correct. If the precise point in time is before range  225 , the control current is thus excessively high; if it is after range  225 , it is thus excessively low. 
         [0029]      FIG. 3  shows a flow chart of a method  300  for activating consumer  105  with the aid of device  100  from  FIG. 1 . Method  300  begins in a step  305 . Subsequently, a step  310 , in which counter  155  is started, and a step  315 , in which current source  140  is activated, may be performed simultaneously. It is then determined in a step  320  whether switching unit  115  enables or interrupts the current flow through consumer  105 . If switching unit  115  is a field-effect transistor, in a first variant, it may be determined in a step  325  whether the gate-source voltage exceeds plateau voltage  220 . In a second variant, it may be determined in a step  330  whether the drain-source voltage is less than a predetermined threshold value. If one of the conditions applies, counter  155  is thus stopped in a step  335 . Presuming that counter  155  counts upward, it is checked in a step  340  whether the counter content exceeds an upper limit. If this is the case, the control current provided by current source  140  is excessively small and a degradation of current source  140  is determined in a step  345 . If the counter content is less than the upper limit, it may thus be determined in a step  350  whether the counter content is less than a lower limit. If this is the case, the control current of current source  140  is thus excessively large and a degradation of current source  140  is likewise determined in step  345 . Otherwise, the counter content is between the lower limit and the upper limit and it is determined in a step  355  that current source  140  does not degrade, i.e., is not defective.