Abstract:
The invention relates to a circuit arrangement ( 10 ) for a control device, and a method for operating said circuit arrangement ( 10 ). The circuit arrangement ( 10 ) comprises a first field-effect transistor ( 12 ) actuating the control device, and a comparator, which compares the voltage provided for actuating the control device with a threshold voltage, and which actuates a timed operation of the first field-effect transistor ( 12 ) via a control unit ( 20 ) if the threshold voltage is exceeded.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a circuit arrangement for a control device and a method for actuating a control device. 
         [0002]    Control devices, particularly for automotive applications, have to be protected against reverse polarity and electrical surge (for example: load dump). A so-called main relay, which is supposed to ensure an extremely small quiescent current consumption, is furthermore needed to operate these control devices. 
         [0003]    The functions: reverse polarity protection, electrical surge protection and main relay are known from prior art as individual, separate circuits. As a result, the electrical surge protection is usually implemented only by means of a transient voltage suppression diode; and all of the successive circuit parts have to be able to withstand the terminal voltages of the transient voltage suppression diode. 
       SUMMARY OF THE INVENTION 
       [0004]    The invention provides a circuit arrangement combining, at least in several embodiments, the individual functions: reverse polarity protection, electrical surge protection, plug contact protection and main relay. A type of “switching regulator” is achieved by the circuit arrangement, said regulator limiting the voltage by means of a timed operation if a switching threshold is exceeded. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  shows an embodiment of the circuit arrangement described according to the invention. 
           [0006]      FIG. 2  shows a further embodiment of the circuit arrangement presented according to the invention. 
           [0007]      FIG. 3  shows a simplified model of the circuit arrangement. 
           [0008]      FIG. 4  shows typical current and voltage profiles. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    A possible embodiment of the circuit arrangement presented, the totality thereof denoted by the reference numeral  10 , is depicted in  FIG. 1 . A first field-effect transistor T 1   12 , a second field-effect transistor T 2   14 , an optional inductor L 1   16 , a capacitor C 1   18 , a control unit or control logic  20 , an optional diode D 1   22  and an ignition switch  24  can be recognized in said circuit arrangement. 
         [0010]    If the ignition switch  24  is open, the transistor T 1   12  is cut off and the quiescent current consumption of the downstream circuit is approximately 0 A. Via the body diode of T 2   14 , the supply voltage less the diode voltage of T 2   14  is present at the drain terminal of T 1   12 . If the ignition switch  24  is now closed, first the transistor T 1   12  and subsequently T 2   14  are actuated via the control logic and thus with low impedance. If the voltage at C 1   18  (protected voltage) exceeds a threshold, T 1   12  is cut off by the control logic  20 . If the voltage at C 1   18  has again dropped below a second threshold C 1   18 , T 1   12  becomes conductive again and the process within the circuit starts again from the beginning. In so doing, the voltage at C 1   18  is limited. The capacitor C 1   18  and the optional inductor L 1   16  serve in this instance to store energy or to smooth the voltage at C 1   18 . 
         [0011]    The voltage at the drain terminal of T 1   12  can be limited to an intermediate voltage with the optional transient voltage suppression diode D 1   22  in order to reduce the power loss in T 1   12 . In the event of a polarity reversal, a cut off is made via the control logic  20  T 2   14  and the entire downstream circuit is consequently protected against reverse polarity. 
         [0012]    The circuit arrangement  10  depicted in  FIG. 1 , respectively input circuitry, fulfills the following functions:
   internal, electronic main relay   load dump protection   reverse polarity protection   plug contact protection when plugging into a live voltage source   
 
         [0017]      FIG. 2  shows a further embodiment of the circuit arrangement, which is denoted as a whole with the reference numeral  200 . A first field-effect transistor  202  and a second field-effect transistor  204 , which are depicted in a simplified manner as switches, are shown. Furthermore, a storage element  206 , in this case a capacitor, a control unit  208 , a Zener diode  210 , a booster capacitor  212  and a comparator  214  are depicted. 
         [0018]    The booster capacitor  212  provides energy for switching the first field-effect transistor  202  when turning on the circuit. The control unit  208  detects that the circuit has been turned on via a connection  216 , the so-called terminal  15 . In this way, said control unit detects whether an ignition key has been inserted. If this is not the case, the first field-effect transistor  202  inhibits current flow and said control unit is not supplied with voltage. 
         [0019]    The storage element  206  smoothes the voltage supplied to the control unit. The comparator  214  checks the voltage which has been provided to the control unit and compares said voltage with a threshold voltage. If said voltage provided to the control unit exceeds said threshold voltage, the first field-effect transistor is operated via said control unit in a clocked fashion. 
         [0020]    The second field-effect transistor  204  ensures the provision of reverse polarity protection. The first field-effect transistor  202  serves as a main relay and ensures the provision of surge protection and plug contact protection. 
         [0021]    In order to satisfy the demands of customers for low quiescent current when the control unit is switched off, no current paths may exist, which in total exceed a quiescent current of, for example, 100 :A. 
         [0022]    As a result of the selected circuit arrangement  10 , it is possible to connect the control unit to the wiring harness without a high contact current already arising upon making the connection, said contact current being due to the charging of the internal electrolytic capacitors. The circuit arrangement  30  is intended for use with standard MOSFETs. 
         [0023]    Load Dump Protection 
         [0024]    In order to protect against high voltage surges from the on-board power supply, a power Zener diode is disposed in the control unit downstream of the reverse polarity protection FET. The input voltage is, for example, limited to values &lt;60 V by this diode. 
         [0025]    When a load dump occurs, the diode has to absorb a high proportion of the energy arising from said load dump. The energy to be absorbed by the diode is dependent upon the customer specification or norm, with which the pulse is defined (amplitude, duration, internal resistance). 
         [0026]    A voltage reduction (e.g. &lt;50 V) is required for different circuit parts. If this reduction in voltage takes place entirely with a power Zener diode, said diode would possibly be overloaded. In order to achieve said reduction in voltage, the switch FET is intermittently switched on and off by a 2-point controller, which is contained in the control logic, and the output voltage of the switching module is controlled. 
         [0027]    A traditional switching regulator operation is not possible in the case of the relatively low frequency and low inductance of the EMC choke used as a storage choke. That means that the current is discontinuous during a timed operation. During power-up of the switch MOSFET, the rate of current rise is limited by the supply cable inductance and the storage choke inductance. The maximum pulse current value is determined by the differential voltage between generator voltage and electrolytic capacitor voltage and by the ohmic resistors included in the circuit. 
         [0028]    A satisfactory operation of the circuit is only possible if the internal resistance of the LD pulse is specified sufficiently high. Said resistance significantly determines the current amplitude. 
         [0029]      FIG. 3  shows a simplified model of the circuit arrangement, in which the relevant circuit components are included for an overview of the LD (load dump) transient voltage control system. The depiction shows a LD (load dump) generator  100 , a RI-LD generator  102 , an inductor supply cable (L)  104 , a reverse polarity protection FET  106 , a switch FET  108 , an LD (load dump) diode, a capacitor filter (C filter)  112 , a further C filter  114 , an EMC inductor (L)  116  (optional), an attenuator  118 , a diode D 8   120 , a buffer electrolytic capacitor (C)  122 , a resistor R 22   124  and a load resistor  126 .