Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2001-12-10
2004-03-02
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S320000, C327S322000, C327S324000
Reexamination Certificate
active
06700428
ABSTRACT:
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 th
Greenberg Laurence A.
Mayback Gregory L.
Stemer Werner H.
Wells Kenneth B.
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