Abstract:
The invention relates to a control circuit which serves for security-critical control of a consumer with an inductive load portion, to be connected to a direct voltage source, and a method for failure control. It is in this case assumed that the control circuit has a power driving assembly, a free-wheeling assembly and a reverse-connection protected assembly. In order to increase the probability of failure recognition, this control circuit is extended by a method for failure control. For this purpose the semiconductor switches of the assemblies, each formed by a MOSFET, are individually driven. The different switching statuses are checked by a diagnostic device which processes voltage values to be read out at outputs of the control circuit. In this way failure-free functionality and also possible causes of failure in the control circuit can be diagnosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Application No. PCT/EP2006/012203 filed Dec. 18, 2006, the disclosures of which are incorporated herein by reference in their entirety, and which claimed priority to German Patent Application No. 10 2005 061 215.6 filed Dec. 21, 2005, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a control circuit which serves for security-critical driving of an electric consumer with an inductive load portion (e.g. direct current motor) to be connected to a direct voltage source. 
     Direct current motors are often controlled today with the aid of a power driving assembly integrated in the control circuit. The power driving assembly disconnects or connects the direct current motor electrically from/to the direct voltage source. A circuit construction suitable for this and known from the applicant&#39;s operational practice provides switching the power driving assembly as connecting link between a feed voltage source and the direct current motor.  FIG. 1  illustrates an arrangement of the power driving assembly of this kind. 
     Citation (1) (DE 100 50 287) describes a protective device for the drive for unipolar direct current motors, which enables compact design of the drive circuit components and prevents thermal overload of the components. 
     A direct current drive device is known from citation (2) (DE 101 18 401, and corresponding U.S. Pat. No. 6,512,346 B2, both of which are incorporated by reference herein). The direct current drive device has a switching device on a first current path between a direct current supply and a direct current motor and a detection device which detects a voltage on a second current path between the direct current motor and the switching device. 
     The switching device has various switching elements and the detection device has various detection devices. Each switching element is provided on the first current path. Each detection element detects the voltage on the second current path. Each of the first current paths contains one of the second current paths in each case. An assessment device judges that there is a failure on a third current path from the direct current supply to the switching devices via the direct current motors if one of the voltages of the detection device does not change. 
     The control circuit  2  illustrated in  FIG. 1  has, apart from the power driving assembly  4 , a free-wheeling diode  6 . The power driving assembly  4  has a power MOSFET ( 8 ) with three terminals (drain  14 , source  16 , gate  18 ). The drain terminal  14  is connected to the feed voltage source  20 . A control assembly  22  serves to drive the MOSFET  8 . It contains a charge pump ( 24 ), which delivers the gate voltage ( 18 ) of the MOSFET  8 , and a microcontroller  26  to drive the MOSFET  8 . The control assembly  22  is connected to the gate terminal  18  of the MOSFET  8  and via two further terminals returned to differently designed earths (GND_P  28  and GND  30 ). An ohmic resistor  66  is switched parallel to the gate source path of MOSFET  8  of the power driving assembly  4 . The series circuit consisting of two breakdown diodes  68 ,  70  is likewise switched parallel to the gate source path of MOSFET  8  of the power driving assembly. Since the anodes of the breakdown diodes  68 ,  70  are switched together, their effect is comparable to that of an electric resistor or a bi-directionally operating limiting diode. The MOSFET  8  serves as semi-conductor switch, the respective switching status of which is fixed by the microcontroller  26 . When the direct current motor  32  is electrically disconnected from the feed voltage source  20  by the power driving assembly  4 , the inductive load portion of the direct current motor  32  generates an undesired voltage peak, among other things, on the basis of self-induction. The energy stored in the motor inductance and also energy from the feed voltage source  20  are in this case reduced via MOSFET  8  of the power driving assembly  4 . To protect MOSFET  8  of the power driving assembly  4  a power diode, acting as free-wheeling diode  6 , is switched parallel to the direct current motor  32 . This is switched in the blocking direction in respect of the feed voltage source  20  and has the task of reducing the voltage peak occurring when the feed voltage source  20  is electrically separated from the direct current motor  32 . The parallel switching consisting of direct current motor  32  and free-wheeling diode  6  is connected to the source terminal  16  of MOSFET  8 . The second terminal of this parallel circuit is returned to earth  28 . 
     A further development of the control circuit  2  illustrated in  FIG. 1  provides for expanding the control circuit  2  by a reverse-connection protected MOSFET  12 . At the same time the free-wheeling diode  6  from  FIG. 1  is replaced by a free-wheeling MOSFET  10 , the intrinsic diode  62  of which acts as a free-wheeling diode in blocking operation of the free-wheeling MOSFET  10 . The resulting control circuit  34  with power driving assembly  4 , free-wheeling MOSFET  10  and reverse-connection protected MOSFET  12  is illustrated in  FIG. 2 , as known from DE 10050287 A1. 
     The control circuit  34  illustrated in  FIG. 2  has a power driving assembly  4  equivalent to the control circuit  2  in  FIG. 1 . The drain terminal  14  of MOSFET  8  of the power driving assembly  4  is connected to the feed voltage source  20 . Driving of MOSFET  8  is done via the gate terminal  18  by a PCU  36  power control unit. A further terminal of the PCU  36  is returned to earth  28 . The source terminal  16  of MOSFET  8  is connected to the direct current motor  32 , a zero-point comparator  38  and the drain terminal  14  of the free-wheeling MOSFET  10 . The direct current motor  32  and the zero-point comparator  38  are returned to earth  28  with a second terminal. To drive the free-wheeling MOSFET  1 . 0  its gate terminal  18  is connected to the zero-point comparator  38 . The source terminal  16  of the free-wheeling MOSFET  10  and that of the reverse-connection protected MOSFET  12  form a direct connection. Drain  14  of the reverse-connection protected MOSFET  12  is returned to earth  28 . To drive the reverse-connection protected MOSFET  12  its gate terminal  18  is connected to the feed voltage source  20  via an ohmic resistor  84 . The zero-point comparator  38  is likewise connected to the feed voltage source  20 . MOSFETS  8 ,  10 ,  12  of the control circuit  34  have in each case a series circuit, switched parallel to the gate source path, consisting of two breakdown diodes. Since the anodes of the two breakdown diodes of each of the three pairs of breakdown diodes  72 ,  74 ;  76 ,  78  and  80 ,  82  are switched together, the effect of each pair of breakdown diodes is comparable to that of an electric resistor or a bi-directionally operating limiting diode. MOSFETS  8 ,  10 ,  12  of the control circuit  34  further behave in blocking operation like a diode switched parallel to the MOSFET (intrinsic diode), the cathode of which is led through at the drain terminal  14  and the anode of which is led through at the source-terminal  16  of the MOSFET. 
     The mode of operation of the control circuit  34  illustrated in  FIG. 2  is examined below. MOSFET  8  of the power driving assembly  4  here acts as semi-conductor switch. Controlled by a PCU  36 , it connects the direct current motor  32  to the feed voltage source  20 . 
     Functioning of the free-wheeling MOSFET  10  is controlled in the switched-off phases by the zero-point comparator  38 . This identifies an electrical disconnection between the feed voltage source  20  and the direct current motor  32  with the aid of the negative potential at the source terminal  16  of MOSFET  8  of the power driving assembly  4 . As a result of this the zero-point comparator  38  feeds the free-wheeling MOSFET  10  with a gate voltage. The free-wheeling MOSFET  10  now remains switched on during the entire switching off process. Compared with the free-wheeling diode  6  in  FIG. 1 , a smaller drop in voltage occurs via the switched-on free-wheeling MOSFET  10 . Besides the reduction in losses in the free-wheeling MOSFET  10 , at the same time a tail current in MOSFET  8  of the power driving assembly  4  during the switching off process is avoided. At the end of the switching off process the potential at the source terminal  16  of MOSFET  8  of the power driving assembly  4  again approaches that of the earth  28 . The zero-point comparator  38  identifies this status and reduces the gate potential of the free-wheeling MOSFET  10  until it is operating purely as a diode. This avoids an undesired braking effect of the direct current motor  32  because of a negative current influence. 
     The reverse-connection protected MOSFET  12  has the task of protecting the free-wheeling MOSFET  10  from overload in the event of reversed polarity. This is achieved by the blocking behaviour of the reverse-connection protected MOSFET  12 , which causes disconnection of the free-wheeling MOSFET  10  from the voltage supply. 
     Driving of the power driving assembly by the PCU is frequently based on a specific clock rate. The power driving assembly here disconnects and connects the direct voltage source electrically from/to the control circuit within a clock period. 
     Reducing the losses during the switching process by using a free-wheeling diode or a free-wheeling MOSFET is of great significance at low-frequency clock rates. The presence and failure-free functional ability of a free-wheeling diode or a free-wheeling MOSFET are an important precondition for the operation of the control circuit in security-critical applications. These include, for example, the ABS (anti-lock brake system) or the FDR (driving dynamics regulation) in automobile technology. The above-described control circuits have no facility for failure control. As a result they have a high risk priority number (RPN) in a failure mode and effects analysis (FMEA). 
     The probability of failure occurring is currently dependent exclusively on the production options and the failures occurring in practical use. However, an improvement in the production options is often not possible on technological or economic grounds. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to demonstrate a control circuit which serves for security-critical driving of an electric consumer with an inductive load portion (e.g. direct current motor) to be connected to a direct voltage source for use in security-critical applications and which has a reduced risk priority number in a failure mode and effects analysis. The invention additionally relates to a method for failure control in a control circuit of this kind. In order to identify malfunctions of the drive circuit in good time, a control circuit with additional failure control is proposed by the present invention. 
     To achieve the above object the control circuit has a control mechanism which serves for failure control. The failure control of the control circuit is set up in such a way that control signals are to be fed in at the inputs of the assemblies. Different operating statuses of the control circuit are to be set by the driving of the assemblies. Further provided are outputs at which voltage values are to be read out. Using these values, a diagnostic device determines the switching statuses of the assemblies. 
     This makes checking of the functionality of the assembly possible. As well as ascertaining a failure-free mode of operation, defective functions of the control circuit are recognisable. Further, by comparing with reference values concrete reasons for recognised malfunctions can be identified. 
     By the introduction of the failure control two central aims can be achieved.
         1. The use of failure control in the operating period of the control circuit leads to an improvement in the probability of failure recognition.   2. By the use of failure control, for example in a test run at the manufacturer&#39;s, the probability of failure occurrence can be reduced even before delivery of the control circuit to a customer.       

     The failure control of the control circuit serves for security-critical control of an electric consumer to be connected to a direct voltage source. The electric consumer may in this case be an electric motor to be operated with direct current (direct current motor). 
     In order to disconnect the feed voltage source electrically from the control circuit or to connect it thereto, the control circuit has a power driving assembly. This contains a driveable first semi-conductor switch. This is preferably a MOSFET, which by its conductive and blocking behaviour generates two switching statuses and is to be driven via its gate terminal. Control of the first semi-conductor switch can be done in this case with the aid of a microcontroller. The microcontroller here generates control signals which set the conductive and blocking behaviour of the first semi-conductor switch. 
     Furthermore, the control circuit has a free-wheeling assembly which has a free-wheeling diode for reducing the electric voltage peak, which arises because of the electric disconnection of the feed voltage source from the consumer by self-induction. Depending on the configuration of the free-wheeling assembly, the intrinsic diode of a MOSFET operates like a free-wheeling diode. 
     The free-wheeling assembly has a driveable second semi-conductor switch. This is a MOSFET, which by its conductive and blocking behaviour generates two switching statuses and is to be driven via its gate terminal. Control of the second semi-conductor switch can be done with the aid of a microcontroller. The microcontroller here generates control signals which set the conductive and blocking behaviour of the second semi-conductor switch. 
     Furthermore, the control circuit has a reverse-connection protected assembly which has a driveable third semi-conductor switch. This is preferably a MOSFET, which by its conductive and blocking behaviour generates two switching statuses and is to be driven via its gate terminal. Control of the third semi-conductor switch can be done with the aid of a microcontroller. The microcontroller here generates control signals which set the conductive and blocking behaviour of the third semi-conductor switch. 
     To guarantee the reverse-connection protection the third semi-conductor switch has an electric resistor which avoids a short circuit current between the poles of the feed voltage source when the feed voltage source is connected to the control circuit with reversed polarity. 
     The invention provides that the semi-conductor switch of at least one of the assemblies is to be driven with a control signal. The assembly to be driven in this case has at least one input at which the control signal is to be fed in. 
     At the same time the semi-conductor switch of the assembly to be driven is to be transferred by the drive into at least two switching statuses. Depending on the configuration of the assembly the semi-conductor switch may be transferred into at least two different switching statuses. At least one first and one second switching status of the respective semi-conductor switch are to be set by a control signal being fed in at the first input of the respective assembly. 
     In the first switching status to be set the semi-conductor switch is switched into blocking operation. In the second switching status to be set the semi-conductor switch is switched into conductive operation. 
     Furthermore, different operating statuses of the control circuit are to be set by driving the semi-conductor switch. In particular, at least two different operating statuses are to be set by driving at least one semi-conductor switch. 
     In a first operating status (normal operating status) of the control circuit the semi-conductor switch of the free-wheeling assembly has the first switching status (blocking operation) and the semi-conductor switch of the reverse-connection protected assembly the second switching status (conductive operation). At the same time, at least one first output of the control circuit an output signal is provided, which is to be supplied to a diagnostic device. The diagnostic device can recognise the switching statuses of the first operating status and deviations therefrom. 
     In a second operating status (reversed polarity operating status) of the control circuit the semi-conductor switch of the reverse-connection protected assembly has the first switching status (blocking operation). At the same time, at least one first output of the control circuit an output signal is provided which is to be supplied to a diagnostic device. The diagnostic device can recognise the switching statuses of the first operating status and deviations therefrom. 
     In a third operating status (test case 1 operating status) of the control circuit the semi-conductor switch of all the assemblies has the first switching status (blocking operation). At the same time, at least one first output of the control circuit an output signal is provided which is to be supplied to a diagnostic device. The diagnostic device can recognise the switching statuses of the third operating status and deviations therefrom. 
     In a fourth operating status (test case 2 operating status) of the control circuit the semi-conductor switch of the power driving assembly and the reverse-connection protected assembly has the first switching status (blocking operation) and the semi-conductor switch of the free-wheeling assembly the second switching status (conductive operation). At the same time, at least one first output of the control circuit an output signal is provided, which is to be supplied to a diagnostic device. The diagnostic device can recognise the switching statuses of the fourth operating status and deviations therefrom. 
     In a fifth operating status (test case 3 operating status) of the control circuit the semi-conductor switch of the power driving assembly and the free-wheeling assembly has the first switching status (blocking operation) and the semi-conductor switch of the reverse-connection protected assembly the second switching status (conductive operation). At the same time, at least one first output of the control circuit an output signal is provided, which is to be supplied to a diagnostic device. The diagnostic device can recognise the switching statuses of the fifth operating status and deviations therefrom. 
     According to the invention the inputs of the assemblies can be impinged with control signals in such a way that a demultiplexer is used for this internally. In this case, after a switching function the control signals from a serial signal sequence are supplied to the inputs. In this way different switching statuses can be generated sequentially after a switching function. The use of different switching sequences for automated failure control is in this way possible, wherein a control programme can be used. 
     Finally, the invention relates to a diagnostic device which performs a comparison between the output signal provided at the first output and reference values to determine failures in the control circuit. The comparisons are here performed by means of threshold value decisions. 
     Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a known control circuit consisting of a power driving assembly and a free-wheeling diode. 
         FIG. 2  shows an extended form of the known control circuit, which reproduces the prior art, consisting of a power driving assembly, a free-wheeling assembly and a reverse-connection protected assembly. 
         FIG. 3  shows the control circuit according to the invention which has failure control and has a power driving assembly, a free-wheeling assembly and a reverse-connection protected assembly. 
         FIG. 4  shows a matrix in which the voltage values to be read out at two outputs of the control circuit are assigned to five different operating statuses of the control circuit. Each voltage value listed in the matrix is explained by the legend. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  shows a control circuit  40 , which has a power driving assembly  4 , a free-wheeling assembly  42  and a reverse-connection protected assembly  44 . The power driving assembly  4  is here equivalent to the power driving assembly  4  in  FIG. 2 . Inside the control circuit  40  the power driving assembly  4  is switched in series with a parallel circuit part. The direct current motor  32  forms the first branch of the parallel circuit part. The second branch of the parallel circuit part contains the series circuit of free-wheeling assembly  42  and reverse-connection protected assembly  44 . 
     The assemblies contained in the control circuit  40 , power driving assembly  4 , reverse-connection protected assembly  44  and free-wheeling assembly  42 , each have a MOSFET  8 ,  10 ,  12  with three terminals (drain  14 , source  16 , gate  18 ). In blocking operation each MOSFET has the behaviour of a diode switched parallel to the MOSFET (intrinsic diode), the cathode of which is led through at the drain terminal  14  and the anode of which is led through at the source terminal  16  of the respective MOSFET. 
     The drain terminal  14  of MOSFET  8  of the power driving assembly  4  is connected to the feed voltage source  20 . The control circuit  40  is returned to earth  28  by the drain terminal  14  of MOSFET  12  of the reverse-connection protected assembly  44 . The drain  14  and source  16  terminals of MOSFET  8  of the power driving assembly  4  and MOSFET  10  of the free-wheeling assembly  42  are switched in such a way that the intrinsic diodes  60 ,  62  of MOSFETS  8 ,  10  are loaded in the conductive direction by the feed voltage source  20 . Drain  14  and source  16  of MOSFET  12  of the reverse-connection protected assembly  44 , on the other hand, are switched in such a way that MOSFET  12  is in the blocking direction in respect of the feed voltage source. The source terminals  16  of the free-wheeling assembly  42  and the reverse-connection protected assembly  44  are therefore connected to one another. MOSFETS  10 ,  12  of the free-wheeling assembly  42  and the reverse-connection protected assembly  44  are in each case connected by gate terminal  18  via an ohmic resistor  92 ,  98  to separate level converters  46 ,  98 , which are fed with a feed voltage  48 . MOSFETS  10 ,  12  of the free-wheeling assembly  42  and the reverse-connection protected assembly  44  have in each case an ohmic resistor  96 ,  102  parallel to their gate source path. Likewise switched parallel to the gate source path of the two MOSFETs  10 ,  12  is in each case a breakdown diode  94 ,  100 , the anodes of which are connected in each case to the source terminals  16  of MOSFETs  10 ,  12 . Driving of the power driving assembly  4  is achieved by a control assembly  22 . This contains a charge pump  24 , which delivers the gate voltage for MOSFET  8 , which is to be driven, and a microcontroller  26  to control MOSFET  8 . Further switched parallel to the gate source path of MOSFET  8  of the power driving assembly  4  is an ohmic resistor  86 . Likewise switched parallel to the gate source path of MOSFET  8  of the power driving assembly  4  is the series circuit consisting of two breakdown diodes  88 ,  90 . Since the anodes of the breakdown diodes  88 ,  90  are switched together, their effect is comparable to that of an electric resistor or a bi-directionally operating limiting diode. 
     One aspect of the invention is to integrate failure control into the control circuit  40 . A driving option for the free-wheeling assembly  42  and the reverse-connection protected assembly  44  is provided in each case for the failure control. The level converter  46  of the two assemblies is impinged for this purpose with a control signal generated by an external microprocessor. In  FIG. 3  the inputs for the control signals are designated by Motor_Diode  50  and Motor_Prot_FET  52 . The MOSFETs of the free-wheeling assembly  42  and the reverse-connection protected assembly  44  are to be transferred by the control signals into two different switching statuses in each case. 
     Voltage values are further to be read out at two outputs of the control circuit  40 . A first output is for this purpose connected to the source terminal  16  of MOSFET  10  of the free-wheeling assembly  42 . This output is supplied with a feed voltage  48  via an ohmic resistor  104 . In  FIG. 3  the first output bears the designation Diode_FBK  56 . The second output is connected to the source terminal  16  of MOSFET  8  of the power driving assembly  4 . This output is supplied with a feed voltage  48  via an ohmic resistor  106 . In  FIG. 3  the second output bears the designation Motor_FBK  54 . Both outputs are connected to a diagnostic device  58 . 
     The mode of operation of the control circuit  40  illustrated in  FIG. 3  is described below. If there is no reversed polarity present, the feed voltage source  20  has a positive potential in respect of earth  28 . In this case MOSFET  8  of the power driving assembly  4  acts as semi-conductor switch which, controlled by the microcontroller  26 , connects the direct current motor  32  to the feed voltage source  20 . 
     If the feed voltage source  20  has a positive potential (not reverse-connected) in respect of earth  28 , the level converter  46  of the reverse-connection protected assembly  44  delivers a signal which serves to drive MOSFET  12  of the reverse-connection protected assembly  44 . Owing to the effect of the intrinsic diode  64  of MOSFET  12  of the reverse-connection protected assembly  44 , the source terminal  16  of MOSFET  12  initially has a potential of 0.7V. The voltage U GS , resulting from the difference in potential between gate  18  and source  16  of MOSFET  12  of the reverse-connection protected assembly  44  causes MOSFET  12  of the reverse-connection protected assembly  44  to have conductive behaviour. The decreasing electric resistance R DS  between drain  14  and source terminal  16  of MOSFET  12  allows the source potential to tend towards the value zero. At the same time the difference in potential U GS  between source  14  and gate  16  of MOSFET  12  is further raised, whereby the electric resistance R DS  between drain  14  and source  16  is further reduced. The source terminal  16  of MOSFET  10  of the free-wheeling assembly  42  is therefore approximately at the earth potential. If, in the meantime, no driving of the gate terminal  18  of MOSFET  10  of the free-wheeling assembly  42  takes place, the intrinsic diode  62  of MOSFET  10  acts like the free-wheeling diode  6  from  FIG. 1 . 
     If the feed voltage source  20  has been connected to the control circuit  40  with reversed polarity, the feed voltage source  20  has a negative potential in respect of earth  28 . In this status of the control circuit  40  no driving of MOSFET  12  of the reverse-connection protected assembly  44  by an output signal of the level converter  46  takes place. As a result, the difference in potential between gate  18  and source  16  of MOSFET  12  of the reverse-connection protected assembly  44  has a value U GS  close to zero. This in turn causes an electric resistance R DS  between source  16  and drain  18  of MOSFET  12  of the reverse-connection protected assembly  44  which goes towards infinity. Since the intrinsic diode  64  of MOSFET  12  is also switched in the blocking direction, electrical disconnection of the feed voltage source  20  connected to the control circuit  40  with reversed polarity from the free-wheeling assembly  42  takes place. The intrinsic diode  60  of the power driving assembly  4 , switched in the conductive direction, causes the feed voltage with reversed polarity to be charged exclusively with the direct current motor  32 . The direct current motor  32  operates in this situation at maximum speed, wherein there is an opposite direction of rotation in respect of normal operation. 
     The failure control of the control circuit  40  includes bringing about different switching statuses of MOSFETs  8 ,  10 ,  12  and verification of voltage values to be read out. 
     For this purpose, in stand-by operation or when the direct current motor  32  is stationary, the MOSFETs of the free-wheeling assembly  42  and the reverse-connection protected assembly  44  are to be set by control signals. These control signals are to be fed in at the inputs Motor_Diode  50  and Motor_Prot_FET  52 . In this way different operating statuses of the control circuit  40  are set. At the same time it is possible to read out voltage values at the outputs Motor_FBK  54  and Diode_FBK  56  of the control circuit  40 . These voltage values are to be supplied to a diagnostic device  58 . The diagnostic device  58  now recognises, using threshold value decisions, whether the operating status set by feeding in at the inputs Motor_Diode  50  and Motor_Prot_FET  52  has been reached. In the event of failure, using the voltage values read out at the outputs Motor_FBK  54  and Diode_FBK  56 , the diagnostic device  58  determines which assembly has a defective, because unexpected, switching status. In this way it is possible to establish whether and which assembly has a defect or defective driving. 
     Various different operating statuses of the control circuit  40  can be set by individual driving of the MOSFETs of the free-wheeling assembly  42 , the reverse-connection protected assembly  44  and the power driving assembly  4 . 
     Different causes of failure are to be discovered for the operating statuses looked at below. If the control circuit has a failure-free operating status, this is likewise recognised. 
     Normal Operating Status 
     In normal operating status no driving takes place via the gate terminal  18  of MOSFET  10  of the free-wheeling assembly  42 . The electric resistance R DS  between drain  14  and source  16  of MOSFET  10  therefore has a value tending towards infinity. Since MOSFET  12  of the reverse-connection protected assembly  44  is supplied with a feed voltage via its gate terminal  18  in this operating status, the gate source voltage U GS  causes the electric resistance R DS  between drain  14  and source  16  of MOSFET  12  to adopt a minimum value. The voltage value to be read out at output Diode_FBK  56  in this case has the value zero owing to the lack of difference from the earth potential. 
     If, on the other hand, a value V F     —     62  is diagnosed, which corresponds to the value of the conducting state voltage of the intrinsic diode  62  of MOSFET  10  of the free-wheeling assembly  42 , there is no defective function of the reverse-connection protected assembly  44 . In this case there is a defective reverse-connection protected assembly  44  or defective driving of the reverse-connection protected assembly  44 . 
     Reversed Polarity Operating Status 
     In the reversed polarity operating status the feed voltage source  20  has a negative potential in respect of earth  28 . The gate terminal  18  of MOSFET  12  of the reverse-connection protected assembly  44  is not driven in this operating status. The lack of driving causes the electric resistance R DS  between drain  14  and source  16  of MOSFET  12  to adopt a value towards infinity. The intrinsic diode  64  of MOSFET  12  is also switched into the blocking direction. 
     The voltage value to be read out at the output Diode_FBK  56  then results from the sum of the negative feed voltage U BAT , the conducting state voltage V F     —     60  of the intrinsic diode  60  of the power driving assembly  4  and the conducting state voltage V F     —     62  of the intrinsic diode  62  of the free-wheeling assembly  42 , as the following formula clarifies. This voltage value can be determined in the diagnostic device with the aid of threshold value decisions.
 
 U   Diode     —     FBK   =−U   BAT   +V   F     —     60   +V   F     —     62  
 
Test Case 1 Operating Status
 
     In test case 1 operating status none of MOSFETs  8 ,  10 ,  12  are driven. The electric resistance R DS  between drain  14  and source  16  of the MOSFETs has an infinite resistance. The voltage to be read out at output Diode_FBK  56  in this operating status adopts the value of the conducting state voltage of one of the two intrinsic diodes  64  of MOSFET  12  or  62  of MOSFET  10 , for example the value V F     —     64 . 
     However, if the diagnostic device  58  shows the value zero, this indicates that there is a short circuit of the free-wheeling assembly  42  or the reverse-connection protected assembly  44 . In this case there is defective switching behaviour or defective driving of the assemblies. 
     If, on the other hand, the value V F     —     62  is diagnosed, this means that the intrinsic diode  64  of MOSFET  12  of the reverse-connection protected assembly  44  does not have conductive behaviour, indicating a defect in MOSFET  12  of the reverse-connection protected assembly  44 . 
     The second voltage value to be read out at output Motor_FBK  54  has the value zero in failure-free switching behaviour. If, on the other hand, the voltage value U BAT  is recognised, this indicates that there is an interruption of the electrical connection between the direct current motor  32  and the control circuit  40 . 
     Test Case 2 Operating Status 
     In test case 2 operating status only MOSFET  10  of the free-wheeling assembly  42  is driven. The electric resistance R DS  between drain  14  and source  16  of the MOSFETs of the power driving assembly  4  and the reverse-connection protected assembly  44  tends towards the value zero. In this operating status the voltage values to be read out at the outputs Motor_FBK  54  and Diode_FBK  56  have the value zero. 
     If, on the other hand, the diagnostic device  58  recognises the conducting state voltage V F     —     64  of the intrinsic diode  64  of the reverse-connection protected assembly  44  at the output Diode_FBK  56 , this indicates that MOSFET  10  of the free-wheeling assembly  42  has blocking behaviour. The reason for this is a defect in MOSFET  10  of the free-wheeling assembly  42  or defective driving thereof. 
     If the voltage to be read out at output Motor_FBK  54  has the value of the conducting state voltage V F     —     64  of the intrinsic diode  64  of the reverse-connection protected assembly  44 , an interruption of the electrical connection between direct current motor  32  and control circuit  40  is to be concluded. 
     Test Case 3 Operating Status 
     In test case 3 operating status only MOSFET  12  of the reverse-connection protected assembly  44  is driven via the gate terminal  18 . The electric resistance R DS  between drain  14  and source  16  of the MOSFETs of the power driving assembly  4  and the free-wheeling assembly  42  tends towards the value infinity. The voltage to be read out at the output Diode_FBK  56  has the value zero, since there is no difference in potential between earth  28  and the output Diode_FBK  56 . 
     However, if the diagnostic device recognises the value of the conducting state voltage of V F     —     62  of the intrinsic diode  62  of MOSFET  10  of the free-wheeling assembly  42 , this means that the driven MOSFET  12  of the reverse-connection protected assembly  44  and its intrinsic diode  64  have blocking behaviour. This in turn signifies a defect in the reverse-connection protected assembly  44  or defective driving thereof. 
     If the voltage to be read out at the output Motor_FBK  54  has the value U BAT , an interruption of the electrical connection between direct current motor  32  and control circuit  30  is to be concluded. 
     For effective control of the functionality of the control circuit  40  the different operating statuses are to be set in succession. The driving of the assemblies required for this is to be performed with the aid of a microprocessor. Driving may also further be performed with the aid of a demultiplexer. This solution provides that the switching function of the demultiplexer assigns control signals to the inputs of the assemblies in succession. The sequential setting of different operating statuses with the aid of a demultiplxer can be achieved in that successive operating statuses can be transferred into one another only by alteration of a control signal to be read in. 
     In  FIG. 4  voltage values to be read out at output Motor_FBK  54  and Diode FBK  56  of five different operating statuses are combined in a matrix. Illustrated are the voltage values for the normal operating status, reversed polarity operating status, test case 1 operating status, test case 2 operating status and test case 3 operating status. The switching status of MOSFET  8  of the power driving assembly  4 , MOSFET  10  of the free-wheeling assembly  42  and MOSFET  12  of the reverse-connection protected assembly  44  is described for each of the five operating statuses by the designations On, Off and X (=either). If these switching statuses of MOSFETs  8 ,  10 ,  12  are achieved failure-free, the voltage values shown in grey in the matrix are to be read out at outputs Motor_FBK  54  and Diode_FBK  56  and to be recognised by the diagnostic device  58 . If in an operating status a voltage value deviating from the voltage value shown in grey is recognised, there is defective switching behaviour of MOSFETS  8 ,  10 ,  12 . The deviating voltage values are listed for each of the five operating statuses below and above the voltage value shown in grey. With the legend in  FIG. 4 , using the position of the deviating voltage values in the matrix, the cause of the deviation can be determined. A possible cause for this is a short circuit (R DS →0) of the drain source path of MOSFET  10  of the free-wheeling assembly  42  or of MOSFET  12  of the reverse-connection protected assembly  44 . A further cause is an infinitely large-resistance (R DS →∞) of the drain source path of MOSFET  10  of the free-wheeling assembly  42  or of MOSFET  12  of the reverse-connection protected assembly  44 . Disconnection of the direct current motor  32  from the control circuit  40  is a further cause. The causes mentioned are attributable to a defect or to defective driving of one of the assemblies. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.