Protective circuit for a field-effect transistor

A protective circuit includes a first field-effect transistor having a first drain terminal, a first source terminal and a first gate terminal, a control device by which an electrical first voltage between the first drain terminal and the first source terminal can be determined, and a first temperature sensor by which a first temperature of the first field-effect transistor can be detected, wherein a first resistance of the first field effect transistor and an electrical first current conducted via the first field-effect transistor can be determined by the control device based on the first temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage of, and claims priority to, Patent Cooperation Treaty Application No. PCT/EP2017/076970, filed on Oct. 23, 2017, which application is hereby incorporated herein by reference in its entirety.

BACKGROUND

Some circuits having field-effect transistors (so-called FETs) for driving pumps, servomotors or other units are already known. U.S. Pat. No. 7,759,891 B2 discloses that temperature monitoring of a FET is provided in such a circuit. The temperature monitoring is intended to protect a motor driven via the FET against damage. It is assumed that a limit value of the temperature is exceeded, e.g., in the case of an electrical short circuit. Therefore, the temperature of the FET is compared with an ambient temperature and, as soon as the difference temperature thus determined exceeds the defined limit value, forwarding of current via the FET is turned off. A more differentiated evaluation of the cause of the limit value being reached is not possible in this case.

SUMMARY

The present disclosure relates to a protective circuit for a first field-effect transistor, wherein the protective circuit is used in a motor vehicle. A field-effect transistor comprises a source terminal, a drain terminal and also a gate terminal. An electrically conductive connection between the source terminal and the drain terminal can be switched on and off via the gate terminal.

The present disclosure provides a protective circuit which enables an accurate evaluation of the electrical currents conducted via a field-effect transistor. A protective circuit that is as cost-effective as possible is provided in this case.

A protective circuit comprises at least a first field-effect transistor, a control device, and a first temperature sensor. The first field-effect transistor has a first source terminal, a first drain terminal and a first gate terminal. An electrical first voltage [volts] between the first source terminal and the first drain terminal is determinable by the control device. A variable first temperature [degrees Celsius] of the first field-effect transistor is detectable by the first temperature sensor. A first (electrical) resistance [ohms] of the first field-effect transistor and (thus) an electrical first current (current intensity in [amperes]) conducted via the first field-effect transistor are determinable by the control device based on the first temperature.

It is proposed here, in particular, to determine the (present) first temperature of the first field-effect transistor via sensors and/or computationally. The first temperature is, however, in particular not compared with an ambient temperature. Here, in particular, exclusively the first temperature is determined and a first resistance of the first field-effect transistor that changes depending on the first temperature is thus deduced. Based on the first voltage drop across the first field-effect transistor and the first resistance, an electrical first current conducted via the first field-effect transistor is then determinable (in particular computationally).

In particular, for this purpose, a characteristic of the first field-effect transistor, that is to say a variation of the first electrical resistance as a function of a first temperature of the first field-effect transistor, is known and stored in the control device.

In particular, a relationship between the first temperature and the first resistance that is linear (at least approximately in the operating range) can be predefined here. This linear relationship is preferably present at least in a range of the first resistance in which the absolute value of the first resistance varies between 50% and 200% of an average value of the first resistance. The average value of the first resistance is present at a predefined reference temperature of the first temperature, e.g., at 20 degrees Celsius.

The first field-effect transistor can be connected to a voltage source via the first drain terminal, wherein a second field-effect transistor is arranged between the voltage source and the first field-effect transistor. The second field-effect transistor has a second source terminal, a second drain terminal and a second gate terminal. An electrical second voltage between the second source terminal and the second drain terminal is determinable by the control device. A variable second temperature of the second field-effect transistor can be detectable by the first temperature sensor and/or by a second temperature sensor. A second electrical resistance of the second field-effect transistor and (thus) an electrical second current conducted via the second field-effect transistor are determinable based on the second temperature (as described above for the first field-effect transistor).

The voltage source provides in particular a voltage of at most 100 volts, e.g. 12 volts, 24 volts or 48 volts.

The first source terminal can be connected to at least one electrical machine and to an electrical ground via the electrical machine.

A third field-effect transistor can be arranged between the electrical machine and the ground, wherein an electrical measuring resistor for determining an electrical third current conducted via the third field-effect transistor is arranged between the third field-effect transistor and the ground.

The measuring resistor (also referred to as shunt) is, in particular, a low-resistance electrical resistor that is used (if appropriate substantially only) for measuring the electrical current intensity. The current that flows through a shunt causes a voltage drop proportional to its intensity; the voltage drop is measured.

In particular, the measuring resistor is also electrically conductively connected to the control device, such that a current conducted via the third field-effect transistor can be monitored by the control device.

The arrangement of such a measuring resistor or of an additional measuring resistor in the circuit or protective circuit (that is to say in the region of the first and second field-effect transistors) is costly. It is thus proposed here, at least as a supplement to one measuring resistor (and/or as a replacement of the measuring resistor), to provide at least one temperature sensor for detecting a change in the resistance of a field-effect transistor, such that the electrical current conducted by the field-effect transistor can be deduced based on the change in the resistance.

The electrical machine can have a rated power [watts] of less than 10 kilowatts, in particular of less than 5 kilowatts, preferably of less than 1 kilowatt.

At least the first electrical current (and/or the second current) can be ascertainable with an accuracy of less than 2.0 amperes, in particular less than 1.5 amperes, preferably less than 1.0 ampere, based on the variable first temperature (and/or second temperature).

At least the first temperature sensor (and/or the second temperature sensor) can be connected to the control device via an analog-to-digital convertor. Via the analog-to-digital convertor, analog signals of a temperature sensor can be converted into digital signals. The conversion can be affected with a required resolution, such that an electrical current is determinable with a required or demanded accuracy.

A motor vehicle comprising an electrical machine and comprising a protective circuit described above is proposed, wherein the first source terminal of the first field-effect transistor is connected to at least the electrical machine and to an electrical ground via the electrical machine.

The explanations concerning the protective circuit can be enlisted individually and in combination with one another for elucidation for the motor vehicle, and vice versa.

Moreover, a method for operating and/or protecting an electrical circuit is proposed, wherein the electrical circuit includes at least one protective circuit described above. The method comprises at least the following steps:a) determining an electrical first voltage between the first source terminal and the first drain terminal;b) detecting a first temperature of the first field-effect transistor;c) calculating an electrical first current conducted via the first field-effect transistor (via the control device).

The first source terminal can be connected to at least one electrical machine and to an electrical ground via the electrical machine, wherein a third field-effect transistor is arranged between the electrical machine and the electrical ground. A measuring resistor for determining an electrical third current conducted via the third field-effect transistor can be arranged between the third field-effect transistor and the electrical ground. In a process i) the third current can be determined, wherein in a process ii) the first current and the third current (and/or the second current) can be evaluated in the control device.

Via the evaluation of the individual electrical currents, a state of the protective circuit can be deduced. In particular, short circuits are thus identifiable and assessable. Based on the evaluation, a decision can be taken as to whether operation of the protective circuit can be continued, whether a warning message ought to be transmitted or whether operation of the protective circuit ought at least temporarily to be interrupted or continued in a restricted manner. In particular, limit values can be stored in the control device, such that specific measures can be initiated in the event of specific limit values being reached.

The explanations concerning the method can be enlisted individually and in combination with one another for elucidation for the protective circuit and/or the motor vehicle, and vice versa.

As a precaution it should be noted that the numerals used here (“first”, “second”, “third”, . . . ) serve primarily (only) for distinguishing a plurality of objects, variables or processes of identical type, that is to say in particular do not necessarily stipulate a dependence and/or order of said objects, variables or processes with respect to one another. Should a dependence and/or order be required, this is explicitly specified here or it is evident in an obvious manner to the person skilled in the art upon studying the embodiment specifically described.

DETAILED DESCRIPTION

FIG. 1shows a motor vehicle27comprising an electrical circuit28, which includes a protective circuit1. The electrical circuit28includes a voltage source12, an electrical machine22and a ground23. A control device6regulates and monitors the operation of the electrical machine22by way of the circuit28. The protective circuit1for the first field-effect transistor2includes the first field-effect transistor2, a control device6and also a first temperature sensor8. The first field-effect transistor2has a first drain terminal3, a first source terminal4and a first gate terminal5. An electrical first voltage7between the first drain terminal3and the first source terminal4is determinable by the control device6. A variable first temperature9of the first field-effect transistor2is detectable by the first temperature sensor8. A first resistance10of the first field-effect transistor2and thus an electrical first current11conducted via the first field-effect transistor2are determinable by the control device6based on the first temperature9.

Here, exclusively the first temperature9is determined and a first resistance10of the first field-effect transistor2that changes depending on the first temperature9is thus deduced. Based on the first voltage7drop across the first field-effect transistor2and the first resistance10, an electrical first current11conducted via the first field-effect transistor2is determinable.

The first field-effect transistor2is connected to the voltage source12via the first drain terminal3, wherein a second field-effect transistor13is arranged between the voltage source12and the first field-effect transistor2. The second field-effect transistor13has a second drain terminal14, a second source terminal15and a second gate terminal16. An electrical second voltage17between the second drain terminal14and the second source terminal15is determinable by the control device6. A variable second temperature19of the second field-effect transistor13is detectable by a second temperature sensor18. A second resistance20of the second field-effect transistor13and thus an electrical second current21conducted via the second field-effect transistor13are determinable based on the second temperature19.

A third field-effect transistor24is arranged between the electrical machine22and the ground23, wherein a measuring resistor25for determining an electrical third current26conducted via the third field-effect transistor24is arranged between the third field-effect transistor24and the ground23.

The measuring resistor25is connected to the control device6, such that a third current26conducted via the third field-effect transistor24can be monitored by the control device6.

The first temperature sensor8and the second temperature sensor18are connected to the control device6via an analog-to-digital convertor29. Via the analog-to-digital convertor29, analog signals of the temperature sensors8,18can be converted into digital signals. The conversion can be affected with a required resolution, such that an electrical current11,21is determinable with a required or demanded accuracy. Via the analog-to-digital convertor29, the electrical voltages7,17can also be detected and, in particular, converted into digital signals.

Via the evaluation of the individual electrical currents11,21,26, a state of the protective circuit1and/or of the circuit28can be deduced. In particular, short circuits are thus identifiable and assessable. Based on the evaluation, a decision can be taken as to whether operation of the protective circuit1and/or of the circuit28can be continued, whether a warning message ought to be transmitted or whether operation of the protective circuit1/circuit28ought at least temporarily to be interrupted or continued in a restricted manner. In particular, limit values for currents11,21,26and/or limit values for ratios of currents11,21,26etc. can be stored in the control device6, such that specific measures can be initiated upon specific limit values being reached. Via the control device6, control signals30,31,32can be communicated to the gate terminals5,16of the field-effect transistors2,13,24and corresponding switching of the field-effect transistors2,13,24can be performed.

LIST OF REFERENCE SIGNS