Abstract:
A circuit arrangement and method for overload protection for a control element is provided, wherein the voltage across a control path of the control element is monitored and utilized as a criterion for shutting off the control element. The actual shutting off of the control element is effected through the aid of a protection circuit that inhibits the control element above a critical current level.

Description:
BACKGROUND OF THE INVENTION 
     For monitoring a control element, it was previously standard to measure the current flow through a precision resistor arranged at the input of the control path of the control element and, when an upper limit of a predetermined current value was exceeded, the control element was driven such that it was shut off. 
     SUMMARY OF THE INVENTION 
     The present invention is, in part, based on an object of specifying a circuit arrangement, as well as a method, for overload protection for a control element. 
     This and other objects are achieved by a circuit arrangement for overload protection of a first control element including a voltage source and a load. In addition, the first control element includes a first control path that selectively connects the voltage source to the load. The first control element also has at least one control input for selectively controlling the first control path and at least one output terminal connected to the load and at least one input terminal connected to the voltage source. The at least one output terminal connected to the load comprises a first control path output and the at least one input terminal connected to the voltage source comprises a first control path input. In addition, the circuit arrangement includes a protective circuit arranged in parallel to the first control path of the first control element. The protective circuit is configured to monitor voltage across the first control path and drive the control input of the first control element when a short in the load occurs such that the load is separated from the voltage source. 
     According to another aspect of the present invention, a method for overload protection of a first control element having a control path that connects a load to a voltage source includes first monitoring a voltage across the control path of the first control element. A control input of the first control element is then driven when a short circuit in the load occurs wherein the control path of a first control element is interrupted and the load is separated from the voltage source. 
     The present invention is advantageous in that due to an evaluation of the voltage potential of the control path of the control element, the control element is dependably and reliably protected against overload. 
     The invention yields the further advantage that an input resistor of a control element can be dimensioned smaller and, as a result, a higher voltage potential can be taken at the output of the control element. 
     Additional advantages and novel features of the invention will be set forth, in part, in the description that follows and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the present invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is made to the attached drawings wherein: 
     FIG. 1 illustrates a block circuit diagram of a control unit with an overload protection; 
     FIG. 2 illustrates a current diagram of the control unit with an overload protection; and 
     FIG. 3 illustrates a current/voltage characteristic. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a block circuit diagram of a circuit arrangement SO with a control unit R, a protective circuit ULS as well as a drive unit A. At its input side, the circuit arrangement SO is connected to a voltage source UE. 
     The control unit R is driven by a control unit A in a normal operating case such that a constant output voltage at the output of the circuit arrangement SO is delivered to a user L located at the output of the circuit arrangement SO given an increase in flow of current through said user L. The protective circuit ULS has a first input E 1  connected to an output of the control unit R. An input for driving the drive unit A is connected to a measurement sensor M arranged, for example, parallel to the load L. An output of the protective circuit ULS is connected to an input of the drive unit A. Given a short-circuit current through the load L, the short-circuit current is registered by the protective circuit ULS and effects a shut-off of the control unit R. 
     FIG. 2 shows a circuit of the block circuit diagram of the control unit R shown in FIG. 1 with the protective circuit ULS. An input resistor R 1 , a first protection resistor R 2  and resistor R 7  as well as a first control element M 1  are contained in the control unit R. The first control element M 1  in this embodiment of the circuit is a MOS switching transistor. 
     A first input El of the control unit R is connected to a terminal of the input resistor R 1  that forms a series circuit with a control path from S to D of the first control element M 1 . The control input G of the first control element M 1  is connected via a first protective resistor R 2  to a second input E 2  of the control unit R. The first input E 1  and the second input E 2  are connected via the resistor R 7 . The second input E 2  of the control unit R is connected to a resistor R 6  arranged at an output of the drive unit A. 
     A control input of the drive unit A is connected to a measurement sensor M formed of the resistors R 8  and R 9  arranged in series. The control A contains an operational amplifier OP 1  whose first input is connected to a tap point P that lies between the resistors R 8  and R 9  of the measurement sensor M and whose second input is connected to a reference voltage source UREF. The output of the operational amplifier OP 1  is connected to the second input E 2  of the control unit R. 
     The protective circuit ULS contains a second protective resistor R 3 , an input resistor R 4 , a resistor R 5  and a second control element M 2  as well as a capacitor C 1 . The second control element M 2  is preferably a MOS switching transistor. The input S of the control path from S to D of the second control element M 2  and its control input G are connected to one another via the capacitor C 1 . The input resistor R 4  is connected to the input S of the second control element M 2  and to the terminal of the protective resistor R 1  connected with the first input E 1  of the first control element M 1 . The control element G of the second control element M 2  is connected to the output D of the control path from S to D of the first control element M 1  via the second protective resistor R 3 . The output D of the control path from S to D of the second control element M 2  is connected via the resistor R 5  to the second input E 2  of the control unit R. A capacitor C 2  is arranged parallel to the input of the circuit arrangement SO, and a further capacitor C 3  is arranged parallel to the output of the circuit arrangement SO. 
     Given an activation of the circuit arrangement SO, the second control element M 2 ,—at the turn-on time,—is prevented from a through-connect by the capacitor C 1  arranged between the control input G and the input S of the control path from S to D of the second control element M 2 . At the moment when the circuit turns on, the second control element M 2  has no influence on the circuit arrangement SO. A voltage UDS=UE−UA builds up via the control path S to D of the first control element M 1 . The output voltage UA of the circuit arrangement SO can rise from 0 volts to a predetermined nominal voltage UL. The current monitoring becomes active after the expiration of a time period that can be set with a RC element formed of the second protective resistor R 3  and the capacitor C 1 . 
     In normal operation, the first control element M 1  in the control unit R is driven by the control unit A via the first protective resistor R 2  and the resistor R 6  such that an input voltage UE adjacent at the input of the circuit arrangement SO is regulated onto a constant output voltage UA up to a maximally allowed value of current. The first control element M 1  is driven dependent on the drive of the control input G of the first control element M 1  by the operational amplifier OP 1  of the drive unit A. A voltage adjacent at the input S of the control path from S to D of the first switch element M 1  is regulated onto a load output voltage UL. When, for example, due to a reduction of the resistance of the load L at the output of the circuit arrangement SO, the output current rises, the voltage across the input resistor R 1  rises and the voltage at the resistor R 6  is reduced at the output of the drive unit A. The reduction of the load output voltage is forwarded via the measurement sensor M to the drive unit A. A lower voltage at the first input of the operational amplifier OP 1  arranged in the drive unit A effects a linear drop of the voltage at the output of the operational amplifier OP 1 . The voltage between the input S of the control path from S to D and a control input G of the first control element M 1  is increased. An increase of the voltage between the control input G and the input S of the control path from S to D of the first control element M 1  effects a lowering of the voltage along the control path from S to D of the first control element M 1 . A reduction in the voltage at the control path from S to D of the first control element M 1  effects a corresponding rise of the voltage UL at the load L. 
     A lowering of the control voltage through the drive circuit A effects an increase of the current through the resistor R 7 . An increase of the flow of current through the resistor R 6  is likewise effected by the increase of current through resistor R 7 . The increased flow of current through the resistor R 6  effects an increase of the voltage across the resistor R 6 . This simultaneously leads to a reduction of the voltage between the control input G and the input S of the control path from S to D of the first control element M 1 . A reduction of the voltage between the control input G and the input S of the control path from S to D of the first control element M 1  simultaneously effects a re-adjustment of the output voltage. The control mechanism for current limitation is thereby such that the voltage at the output of the drive unit A is continuously reduced. Given a short occurring in the load L, the voltage at the protective resistor R 1 , via the control path from S to D of the first control element M 1  rises, to a maximum and drives the second control element M 2 . A current flows across the second control element M 2 , the input resistor R 4 , the resistor R 5  and the resistor R 6  at the output of the drive unit A. The series-connected resistors R 4 , R 5  are connected in parallel to the resistor R 7  in the driven condition of the second control element M 2 . In the case of short, the voltage across the resistor R 7  is thereby reduced and the voltage across the resistor R 6  is increased. The voltage between the control input G and the input S of the control path from S to D of the first control element M 1  drops below a through-connect or threshold voltage needed for the through-connect or conduction of the first switch element M 1 . The voltage UL adjacent at the output of the circuit arrangement SO amounts to nearly 0 volts. 
     When the value of resistance of the load L at the output of the circuit arrangement SO increases to such an extent that the capacitor C 3  is loaded more than it is discharged by the resistance of the load L, the voltage at the output of the circuit arrangement SO (i.e., UA) increases. An increase in the output voltage UA in turn effects a lowering of the voltage between the control input G and the input S of the second control element M 2 . Due to the lowering of the voltage between the control input G and the input S of the control path from S to D of the second control element M 2 , the voltage between the input S and the output D of the control path of the second control element M 2  is increased. Due to the increase of the voltage at the control path from S to D of the second control element M 2 , the flow of current in the resistors R 4 , R 5  and R 6  is reduced. A corresponding reduction in the voltage at the resistor R 6  results in an increase of the voltage between the control input G and the input S of the control path from S to D of the first control element M 1 . Due to the increase in the voltage between the control input G and the input S of the control path from S to D of the first control element M 1 , M 1  becomes in turn through-connected or conductive, and the voltage UL across the resistance of the load L increases. 
     FIG. 3 shows a current/voltage characteristic. In the section marked AB 1 , the drive unit A attempts to drive the first control element M 1  such that the output voltage UA remains constant despite an increased flow of current through the load L. In the section of the current/voltage characteristic marked AB 2 , the protective circuit ULS is active. The flow of current is thereby only slightly increased. The first control element M 1  is inhibited above a critical value of current. 
     While this invention has been described with what is presently considered to be the most practical preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.