Patent Publication Number: US-6700428-B2

Title: Circuit configuration with a controllable current limiting circuit for driving a load

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a circuit configuration for driving a load. The circuit configuration contains a first connecting terminal for connecting to the load, a second connecting terminal, a first drive input for receiving a first drive signal, and a semiconductor switching element having a first load terminal connected to the first connecting terminal, a second load terminal connected to the second connecting terminal, and a drive terminal coupled to the first drive input. A voltage limiting circuit is connected between the first load terminal and the drive terminal of the semiconductor switching element. 
     U.S. Pat. No. 4,658,203 discloses a circuit configuration for driving a motor, which has a power MOSFET whose load path (drain-source path) is connected between a connecting terminal of the motor and ground. In this case, the gate terminal of the MOSFET is connected to a pulse width modulator for driving the MOSFET. In order to limit the voltage across the load path of the MOSFET, a reverse-biased zener diode is connected between the drain terminal and the gate terminal of the MOSFET. If the potential at the drain terminal of the MOSFET exceeds a predetermined value which is dependent on the breakdown voltage of the zener diode, then the zener diode turns on and charges the gate capacitance of the MOSFET, as a result of which the MOSFET turns on and as a result of which the voltage across the load path thereof is limited. In this case, the zener diode is dimensioned in such a way that it turns on in order to drive the MOSFET before the breakdown voltage of the transistor is reached. 
     For switching loads it is known, moreover, to use so-called smart power FETs. Components of this type contain, in addition to a power transistor, inter alia a protective circuit for the power transistor that is intended to protect the power transistor against, for example, an excessively large load current or an excessively high temperature. The protective circuit usually has a switch which is connected to the gate terminal of the power transistor and serves for discharging the gate capacitance of the transistor in order to turn off the transistor if, for example, the load current of the transistor or the temperature thereof assumes a value at which there is the risk of the transistor being damaged. 
     If a power transistor for driving a load contains a protective circuit with a switch for discharging the gate capacitance and also a zener diode for voltage limiting, then the situation can arise wherein the switch of the protective circuit turns on in order to turn off the power transistor, and wherein, at the same time, the zener diode turns on in order to drive the power transistor and thereby to limit the voltage across the load path thereof. Whereas only a short current pulse flows through the zener diode, in order to charge the gate capacitance, when the switch of the protective circuit is turned off, a current flows permanently through the zener diode when the switch of the protective circuit is turned on and the zener diode is turned on. Due to an unavoidable internal resistance of the zener diode or of another voltage limiting circuit, which may be not inconsiderable particularly in the case of integrated zener diodes, there is then the risk that, on account of the voltage drop which is additionally brought about across the zener diode and results from the product of the flowing current and the internal resistance, the drain potential of the power transistor will rise to a value at which the power transistor is in danger of destruction. 
     In addition, the MOSFET turns off rapidly if its gate capacitance is discharged by the protective circuit. In this case, inductive loads or else only the inductance of the leads can give rise to high induced voltages in the leads which can reach the level of the breakdown voltage of the MOSFET. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a circuit configuration with a controllable current limiting circuit for driving a load which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which it is possible to use a semiconductor switching element with a protective circuit, in particular with a protective circuit against an over temperature or for current limiting, and wherein the semiconductor switching element is additionally protected against an over-voltage on its load path. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for driving a load. The circuit configuration has a first connecting terminal for connecting to the load, a second connecting terminal, a first drive input for receiving a first drive signal, and a semiconductor switching element having a first load terminal connected to the first connecting terminal, a second load terminal connected to the second connecting terminal, and a drive terminal coupled to the first drive input. A voltage limiting circuit is connected between the first load terminal and the drive terminal of the semiconductor switching element. The voltage limiting circuit has a second drive input receiving a second drive signal, and a threshold voltage of the voltage limiting circuit is dependent on the second drive signal. 
     The second drive signal may be variable for example depending on the switching state of the first semiconductor switching element, a temperature in the region of the first semiconductor switching element or a current through the first semiconductor switching element. The voltage limiting circuit is configured in such a way that a threshold voltage of the voltage limiting circuit is reduced in the case of those states of the first semiconductor switching element in which the voltage limiting circuit can permanently take up a current or in which it is possible for the semiconductor switch to be switched off rapidly by a protective circuit. This prevents the situation in which, as a result of a rise in the voltage across the voltage limiting circuit which results from a voltage drop brought about by a permanent current at an internal resistance of the voltage limiting circuit, the potential at the first load terminal of the first semiconductor switching element rises to a value at which the first semiconductor switching element is in danger of destruction. A situation in which the voltage limiting circuit permanently takes up current can arise when a protective circuit is present which short-circuits the drive terminal and the second load terminal of the first semiconductor switching element in the overload case, in order to turn off the first semiconductor switching element. 
     If the switch is in a state in which a rapid switch-off by a protective circuit can occur, then the threshold voltage of the voltage limiting circuit is preferably likewise reduced. In this case, the voltage limiting circuit counteracts the protective circuit and, upon commencement of the protective circuit, from the point when the reduced threshold voltage is reached, prevents the gate capacitance from being discharged too rapidly and thereby prevents high induced voltages on the load path of the MOSFET. 
     One embodiment of the circuit configuration according to the invention provides for the voltage limiting circuit to have a series circuit containing a first and a second voltage limiting element, it being possible for one of the voltage limiting elements to be short-circuited according to the second drive signal. In order to short-circuit the one voltage limiting element, in one embodiment of the invention, a switch, in particular a semiconductor switching element, is connected in parallel with the voltage limiting element, the switch having a drive terminal to which the second drive signal is fed. The series-connected voltage limiting elements are preferably zener diodes, which have the advantage that they can be integrated in a simple manner in the same semiconductor body as the first semiconductor switching element. 
     Depending on the switch position of the second switch, the voltage limiting circuit according to the invention has a breakdown voltage that corresponds to the sum of the breakdown voltages of the two voltage limiting elements or the breakdown voltage of the voltage limiting element which cannot be short-circuited. 
     In one embodiment of the invention, the second drive signal is dependent on a switching state of the first semiconductor switching element. As a result, a high breakdown voltage of the voltage limiting circuit can be set in the case of a first switching state, in which the first semiconductor switching element turns off, whereas a lower breakdown voltage of the voltage limiting circuit is set in the case of a second switching state, in which the first semiconductor switching element turns on. In the case of the switching state mentioned last, given the presence of a protective circuit which serves for turning off the first semiconductor switching element in an overload case, the situation can arise wherein a current permanently flows through the voltage limiting circuit, since part of the current through the voltage limiting circuit which is intended to turn on the first semiconductor switching element is taken up by the protective circuit. The protective circuit is used to achieve an opposite aim, namely to turn off the first semiconductor switching element. Due to an internal resistance that is inevitably present in the voltage limiting circuit, in the event of a permanent current flow through the voltage limiting circuit, the voltage across the voltage limiting circuit rises. The voltage limiting circuit has a breakdown voltage that is reduced in this case, however, and this prevents the voltage across the load path of the first semiconductor switching element from rising to a value at which the first semiconductor switching element is in danger of destruction. 
     One embodiment of the invention provides a protective circuit with a third semiconductor switching element, the third semiconductor switching element is connected between the control terminal of the first semiconductor switching element and the second load path terminal thereof. The third semiconductor switching element serves for turning off the first semiconductor switching element according to a drive signal, in that it short-circuits the drive terminal and the second load path terminal. In this case, the drive signal of the third semiconductor switching element is preferably dependent on a temperature in the region of the first semiconductor switching element and/or on a load current of the first semiconductor switching element. 
     A second embodiment of the invention provides for the second drive signal to be dependent on a temperature in the region of the first semiconductor switching element. As a result, the breakdown voltage of the voltage limiting circuit can be reduced for example when the temperature in the region of the first semiconductor switching element exceeds a predetermined value at which a protective circuit reacts in order to turn off the first semiconductor switching element. In this case, too, the protective circuit pursues an opposite aim to that of the voltage limiting circuit, namely the aim of turning off the first semiconductor switching element, which results in a permanent current flow through the voltage limiting circuit. The reduced breakdown voltage of the voltage limiting circuit in this case prevents the breakdown voltage of the first semiconductor switching element from being reached, the first semiconductor element being in danger of destruction at the breakdown voltage. 
     In a corresponding manner, a further embodiment of the invention provides for the second drive signal to be dependent on a current through the first semiconductor switching element. In this case, if the current through the first semiconductor switching element exceeds a value at which a protective circuit switches on for the purpose of turning off the first semiconductor switching element, then, as in the over-temperature case described above, the breakdown voltage of the voltage limiting circuit is reduced in order to intercept the voltage rise across the voltage limiting circuit, the voltage rise being brought about in the event of a permanent current flow, and to prevent the breakdown voltage of the first semiconductor switching element from being reached. 
     The voltage limiting circuit preferably has a drive circuit for driving the second switch connected in parallel with the first voltage limiting element, the drive circuit being fed a current signal which is dependent on the current through the first semiconductor switching element, and/or a temperature signal which is dependent on the temperature in the region of the first semiconductor switching element. In this case, for the provision of the current signal, a current sensor is connected in series with the first semiconductor switching element and, for the provision of the temperature signal, a temperature sensor is disposed in the region of the first semiconductor switching element. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a circuit configuration with a controllable current limiting circuit for driving a load, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a circuit configuration having a voltage limiting circuit for driving a load in accordance with a first embodiment of the invention; 
     FIG. 2 is a circuit diagram of the circuit configuration according to the invention in accordance with a second embodiment, which has a further protective circuit in addition to a voltage limiting circuit; 
     FIG. 3 is a circuit diagram of the circuit configuration according to the invention in accordance with a third embodiment; 
     FIG. 4 is a circuit diagram of the circuit configuration according to the invention in accordance with a fourth embodiment; 
     FIG. 5 is a circuit diagram of the circuit configuration according to the invention in accordance with a fifth embodiment; and 
     FIG. 6 is a circuit diagram of the circuit diagram of a sixth exemplary embodiment of the circuit configuration according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a first exemplary embodiment of a circuit configuration  10  according to the invention for connecting a load Z to a supply potential V+, GND. The circuit configuration  10  has a first connecting terminal K 1 , which, in order to illustrate the function of the circuit configuration in FIG. 1, is connected to the load Z. A terminal of the load Z that is remote from the connecting terminal K 1  is connected to a first supply potential V+. The circuit configuration  10  has a second connecting terminal K 2 , which is connected to a second supply potential GND in FIG.  1 . In addition to the first and second connecting terminals K 1 , K 2 , a drive input K 3  is present, for feeding in a drive signal S 1 . 
     The circuit configuration  10  contains a first semiconductor switching element T 1 , which is configured as an n-channel power MOSFET in the exemplary embodiment. The transistor T 1  has a drain terminal as a first load path terminal, which is connected to the first connecting terminal K 1 , and the transistor T 1  has a source terminal as a second load path terminal, which is connected to the second connecting terminal K 2 . A gate terminal G as a drive terminal of the transistor T 1  is connected via a resistor R 1 , which limits the drive current of the transistor T 1 , to the drive input K 3  for feeding in the first drive signal S 1 . The transistor T 1  has a gate-source capacitance, which is depicted as a capacitor Cg between a gate G and a source S in FIG.  1 . 
     A voltage limiting circuit SB is connected between the drain terminal D and the gate terminal G of the transistor T 1  and, according to the invention, is fed a second drive signal S 2 . The voltage limiting circuit SB is configured to drive the transistor T 1  by charging the gate-source capacitance Cg if a potential at the drain terminal D reaches a value at which there is the risk of the transistor T 1  being destroyed. In this case, the transistor T 1  is turned on in a manner driven by the voltage limiting circuit SB, thereby preventing a further rise in the drain potential or the load path voltage Uds of the transistor T 1 . 
     A voltage Udg present between the drain terminal D and the gate terminal G of the transistor is present across the voltage limiting circuit SB. The value of the voltage Udg, at which a threshold voltage of the voltage limiting circuit SB is reached and the latter is activated in order to drive the transistor T 1 , is variable and dependent on the second input signal S 2 . Preferably, at least two different threshold voltages can be set depending on the second drive signal S 2 . 
     FIG. 2 shows an exemplary embodiment of the invention in which the voltage limiting circuit SB has a series circuit containing two voltage limiting elements Z 1 , Z 2 , a first zener diode Z 1  and a second zener diode Z 2  in the exemplary embodiment. A second transistor T 2 , which is a p-channel MOSFET, is connected in parallel with the second zener diode Z 2 , the second drive signal S 2  is fed to a gate terminal G of the second transistor T 2 . The two zener diodes Z 1 , Z 2  are connected in the reverse direction between the drain terminal D and the gate terminal G of the first transistor T 1 . A diode D 1 , which is connected in series with the first and second zener diodes Z 1 , Z 2  and is connected in the forward direction between the drain terminal D and the gate terminal G of the first transistor T 1 , serves for preventing a current flow from the gate terminal G to the drain terminal D of the first transistor T 1 . 
     The circuit configuration  10  in accordance with FIG. 2 furthermore has a protective circuit SC connected between the gate terminal G and the source terminal S of the first transistor T 1 . The protective circuit SC has a third transistor T 3 , whose drain-source path D-S is connected between the gate terminal G and the source terminal S of the first transistor T 1 . A third drive signal S 3  from a drive circuit A 3  of the protective circuit SC is fed to a gate terminal G of the third transistor T 3 . The drive circuit A 3  is fed a temperature signal S 4 , which is dependent on a temperature in a region of the first transistor T 1  and which, in the exemplary embodiment, is generated by a temperature sensor TS disposed in a manner thermally coupled to the first transistor T 1 . 
     If, in the circuit configuration  10  in accordance with FIG. 2, the second transistor T 2  is turned on by the second drive signal S 2 , then the second zener diode Z 2  is short-circuited and the first transistor T 1  is turned on by the voltage limiting circuit SB when the drain potential of the first transistor T 1  exceeds the value of the gate potential of the first transistor T 1  for instance by the value of the breakdown voltage of the first zener diode Z 1 . If the second transistor T 2  turns off in a manner driven by the second drive signal S 2 , then the first transistor T 1  is turned on by the voltage limiting circuit SB if the drain potential of the first transistor T 1  exceeds the gate potential for instance by a value which corresponds to the sum of the breakdown voltages of the first and second zener diodes Z 1 , Z 2 . 
     In this case, the switching state of the second transistor T 2  is preferably dependent on the switching state of the first transistor T 1 , the threshold voltage of the voltage limiting circuit SB at which the voltage limiting circuit SB drives the first transistor T 1  being lower when the first transistor T 1  is turned on than when the first transistor T 1  is turned off. 
     If the first transistor T 1  turns on and, in this case, an over temperature occurs which is registered by the temperature sensor TS and at which the first transistor T 1  is in danger of destruction, then the third transistor T 3  is turned on by the third drive signal S 3  generated by the drive circuit A 3  in order to discharge the gate-source capacitance Cg of the first transistor T 1  and thereby to turn off the first transistor T 1 . As a result, the load path voltage Uds of the first transistor T 1  rises, in which case, given the presence of an inductive load Z or else due to the inductance of the leads to the transistor T 1 , the load path voltage Uds can rise considerably in a manner due to induction. In this case, if the drain potential of the first transistor T 1  reaches a value which exceeds the gate potential of the first transistor T 1  by the value of the breakdown voltage of the first zener diode Z 1  then a current flows from the drain terminal D via the second transistor T 2 , the first zener diode Z 1  and the diode D 1  to the gate terminal G of the first transistor T 1 , part of the current flowing away to reference-ground potential GND via the turned-on third transistor T 3 . 
     The first zener diode Z 1  unavoidably has an internal resistance which, in the event of a current permanently flowing through the first zener diode Z 1 , results in an additional voltage drop across the zener diode Z 1 , which results from the product of the internal resistance and the flowing current. The additional voltage drop results in a rise in the drain potential or the load path voltage Uds of the first transistor T 1 . In this case, the breakdown voltage Z 1  of the zener diode is chosen such that a load path voltage Uds at which the first transistor T 1  is in danger of destruction does not occur even as a result of the additional voltage rise caused by the internal resistance and the permanent current. 
     In addition, the first zener diode Z 1  counteracts a rapid discharge of the gate capacitance Cg in order to avoid high induced voltages that can be caused by an inductive load or by the inductance of the leads. In this case, the breakdown voltage of the zener diode Z 1  is dimensioned such that a further voltage rise in the load path voltage due to inductive effects does not lead to the breakdown voltage of the transistor T 1  being reached. 
     The zener diode Z 1  and the third transistor T 3  are coordinated with one another in such a way that when the third transistor T 3  is turned on and the threshold voltage of the voltage limiting circuit is reached, the current taken up by the zener diode Z 1  is greater than the current taken up by the third transistor T 3 . 
     If the first transistor T 1  turns off, then it is usually not possible for an over temperature to occur at the first transistor T 1 , that is to say the protective circuit for discharging the gate capacitance Cg is deactivated. In this case, a current can flow only briefly via the voltage limiting circuit SB in order to charge the gate capacitance Cg. In this case, the second switch T 2  is open and the voltage limiting circuit SB turns on when the drain potential exceeds the gate potential by a value that corresponds to the sum of the breakdown voltages of the first and second zener diodes Z 1 , Z 2 . Since a permanent current flow through the voltage limiting circuit SB cannot take place in this case, there is no need to take account of an additional voltage rise due to the permanent current flow and the internal resistance of the zener diodes Z 1 , Z 2 . 
     FIG. 3 shows an exemplary embodiment of the invention in which the second drive signal S 2 , which is fed to the p-conducting second transistor T 2  in the exemplary embodiment, is formed by inversion of the first drive signal S 1  by an inverter INV. In this embodiment of the invention, it is ensured that the second transistor T 2  turns on in order to short-circuit the second zener diode Z 2  if the first transistor T 1  turns on. The second transistor T 2  turns off if the first transistor T 1  turns off. The drive circuit A 3  of the third transistor T 3  of the protective circuit SC is fed a drive signal S 5  which, in accordance with the exemplary embodiment as shown in FIG. 2, may be a signal which is dependent on the temperature in the region of the first transistor T 1 , or else a signal which is dependent on a load current I of the first transistor T 1 . 
     FIG. 4 shows a further exemplary embodiment of the circuit configuration according to the invention, in which a delay element T is connected upstream of the inverter INV. The second transistor T 2  then switches in each case in a time-delayed manner with respect to the first transistor T 1 . 
     FIG. 5 shows a further exemplary embodiment of the circuit configuration according to the invention, in which a drive circuit A 2  is provided for generating the second drive signal S 2  of the second transistor T 2 , which drive circuit A 2 , like the drive circuit A 3  of the third transistor T 3 , is fed a temperature signal S 4  dependent on the temperature in the region of the first transistor T 1 . The second drive circuit A 2  is configured to drive the second transistor T 2  if the temperature signal S 4  reaches a predetermined value, which is preferably less than the value at which the third switch T 3  switches on. The breakdown voltage of the voltage limiting circuit SB is reduced in this way before the third switch T 3  is activated in the event of a further rise in temperature. 
     A further embodiment of the invention, which embodiment is not specifically illustrated, provides for the second transistor T 2  and the third transistor T 3  to be driven simultaneously, the second drive signal S 2  resulting from the third drive signal S 3  by inversion by an inverter. The breakdown voltage of the voltage limiting circuit SB is thereby reduced only when the third transistor T 3  is activated. 
     Whereas in the exemplary embodiment in accordance with FIG. 5 the breakdown voltage of the voltage limiting circuit SB is dependent on the temperature in the region of the first transistor, FIG. 6 shows a further exemplary embodiment of the circuit configuration according to the invention, in which the drive circuit A 3  of the third transistor T 3  and the drive circuit A 2  of the second transistor T 2  are fed a signal S 6  dependent on the current through the transistor T 1 . The signal S 6  is generated by a current sensor IS connected in series with the first transistor T 1 .