TEMPERATURE MEASUREMENT OF A POWER SEMICONDUCTOR SWITCHING ELEMENT

Abstract

A device for determining a temperature of a semiconductor power switch with a built-in temperature-dependent gate resistor may include a non-inverting amplifier circuit comprising an operational amplifier and a feedback resistor. Inverting input of the operational amplifier may be connected to the semiconductor power switch such that a gain of the non-inverting amplifier circuit in a predefined frequency range of an input signal depends on the built-in temperature-dependent gate resistor and the feedback resistor and is a measure of the temperature of the semiconductor power switch. The feedback resistor may be disposed between a negative input and an output of the operational amplifier.

Claims

1.-13. (canceled)

14. A device for determining a temperature of a semiconductor power switch with a built-in temperature-dependent gate resistor, the device comprising a non-inverting amplifier circuit including an operational amplifier and a feedback resistor, wherein an inverting input of the operational amplifier is connected to the semiconductor power switch such that a gain of the non-inverting amplifier circuit in a predefined frequency range of an input signal depends on the built-in temperature-dependent gate resistor and the feedback resistor and is a measure of the temperature of the semiconductor power switch.

15. The device of claim 14 wherein the feedback resistor is disposed between a negative input and an output of the operational amplifier.

16. The device of claim 14 comprising an input, wherein the built-in temperature-dependent gate resistor is disposed between a negative input of the operational amplifier and the input of the device.

17. The device of claim 14 wherein in the predefined frequency range of the input signal the built-in temperature-dependent gate resistor forms a substantial part of input impedance of the semiconductor power switch.

18. The device of claim 14 wherein in the predefined frequency range of the input signal the built-in temperature-dependent gate resistor forms a majority of input impedance of the semiconductor power switch.

19. The device of claim 14 wherein the predefined frequency range is above a frequency of a pole of a transfer function of an ideal non-inverting amplifier circuit.

20. The device of claim 14 wherein the semiconductor power switch is a power MOSFET or an IGBT.

21. An electromechanical steering system of a motor vehicle having a multiphase permanently excited electric motor that is controllable via an electronic control unit, wherein the electronic control unit includes semiconductor power switches that are part of an inverter and/or disposed as a semiconductor relay in each phase, wherein each of the semiconductor power switches includes the device of claim 14 for determining a temperature of the respective semiconductor power switch.

22. The electromechanical steering system of claim 21 wherein the multiphase permanently excited electric motor is three-phase and includes two of the semiconductor power switches for each phase in a half-bridge circuit, wherein the semiconductor power switches are controllable by way of pulse width modulation.

23. A method for measuring a temperature of a semiconductor power switch having a built-in temperature-dependent gate resistor via a circuit comprising a non-inverting amplifier circuit having an operational amplifier and a feedback resistor that is disposed between a negative input and an output of the operational amplifier, wherein the built-in temperature-dependent gate resistor is disposed between the negative input of the operational amplifier and an input of the circuit, the method comprising: operating the circuit with an input signal having a frequency that is in a predefined frequency range such that the built-in temperature-dependent gate resistor forms a substantial part of input impedance of the semiconductor power switch; measuring a gain of the non-inverting amplifier circuit; and calculating a resistance of the built-in temperature-dependent gate resistor by way of the measured gain and determining the temperature of the semiconductor power switch.

24. The method of claim 23 wherein the determined temperature is a measure of a junction temperature of the semiconductor power switch.

25. The method of claim 23 wherein the semiconductor power switch is a power MOSFET or an IGBT.

26. The method of claim 23 wherein the predefined frequency range is above a frequency of a pole of a transfer function of an ideal non-inverting amplifier circuit.

27. A method for measuring a temperature of a semiconductor power switch having a built-in temperature-dependent gate resistor via a circuit comprising a non-inverting amplifier circuit having an operational amplifier and a feedback resistor that is disposed between a negative input and an output of the operational amplifier, wherein the built-in temperature-dependent gate resistor is disposed between the negative input of the operational amplifier and an input of the circuit, the method comprising: operating the circuit with an input signal having a predefined frequency that is in a predetermined frequency range such that the built-in temperature-dependent gate resistor forms an integral part of input impedance of the semiconductor power switch; measuring an output signal from the non-inverting amplifier circuit for at least two temperature settings; calculating a temperature dependence of the output signal; and determining the temperature of the semiconductor power switching element based on the output signal and the calculated temperature dependence.

28. The method of claim 27 wherein the determined temperature is a measure of a junction temperature of the semiconductor power switch.

29. The method of claim 27 wherein the semiconductor power switch is a power MOSFET or an IGBT.

30. The method of claim 27 wherein the predetermined frequency range is above a frequency of a pole of a transfer function of an ideal non-inverting amplifier circuit.

Description

[0027] Two preferred embodiments of the invention are explained in more detail below on the basis of the drawings. Identical or functionally identical components are provided in this case with the same reference signs throughout the figures, in which:

[0028] FIG. 1: shows a circuit diagram of a device for determining a temperature of a power switching element having a power switch and a non-inverting amplifier circuit,

[0029] FIG. 2: shows a circuit diagram of a device for determining a temperature of a power MOSFET, and

[0030] FIG. 3: shows a graph with a transfer function of an ideal and a real non-inverting amplifier circuit.

[0031] FIG. 1 illustrates a circuit having a power MOSFET 5, which acts as a semiconductor power switching element 1, with a built-in gate resistor 3.

[0032] The semiconductor power switch 1 has a parallel circuit (not illustrated) of a multiplicity of individual semiconductor switches arranged on a common chip. A significant advantage of the semiconductor power switches 1 is the high possible switching frequency which is advantageous, for example, for pulse width modulation in an motor controller. The power MOSFET 5 has a built-in gate resistor 3 which is provided for the purpose of balancing the current distribution between the individual semiconductor switches on a chip in order to avoid parasitic oscillations and to reduce the Q factor of a possible RLC series circuit at the input. The built-in gate resistor 3 is part of an input impedance of the power MOSFET 5. The built-in gate resistor 3 has a known temperature dependence which is a measure of the temperature of the depletion layer of the semiconductor power switching element 1. The temperature-dependent change in the resistance of the gate resistor 3 is detected by means of a non-inverting amplifier circuit 200 comprising an operational amplifier 2, a feedback resistor 4 and the built-in gate resistor 3. The semiconductor power switch 1, with its input impedance, is arranged between the negative input of the operational amplifier 2 and a setpoint input of the complete circuit V.sub.IN−.

[0033] The voltage V.sub.IN+ to be amplified of the setpoint input of the operational amplifier 2 is applied to the non-inverting, positive input of the operational amplifier 2. A fraction of the output voltage V.sub.OUT+ from the operational amplifier 2 is fed back to the inverting, negative input as negative feedback by means of voltage division using two resistors. The feedback resistor 4 is arranged between the negative input of the operational amplifier and the output.

[0034] The input impedance of the semiconductor power switch 1 can be modeled using an RC series circuit (see FIG. 2) having an input capacitor and an input resistor. In the case of a particular input signal, the series capacitance of the RC series circuit can be considered to be a short circuit, with the result that the input impedance is formed only by the built-in gate resistor 3. The gain of the non-inverting amplifier circuit 200 depends on a frequency f. In the case of frequencies above a cut-off frequency f.sub.P, the input impedance is formed by the built-in gate resistor 3. The impedance of the capacitor 10 is reduced and can be considered to be a shortcut. This effect can be observed to an increasing extent with increasing frequency.

[0035] The cut-off frequency f.sub.P is calculated using the following formula:

[00001]$f_P=\frac{1}{2\pi C_{GS}{R}_G}$

[0036] where C.sub.is is the input capacitor 10, R.sub.G is the gate resistor 3 (see FIG. 2).

[0037] In this case, the output of the operational amplifier 2 must adjust a ratio of the feedback resistor 4 and the gate resistor 3 in order to control the voltage at the negative input to that of the positive input V.sub.IN+. The terminal gain of the operational amplifier between the input and output terminals is provided solely by the feedback resistor 4 and the gate resistor 3.

[0038] On account of the temperature dependence of the gate resistor 3, the gain of the non-inverting amplifier circuit 200 is a measure of the temperature of the depletion layer of the semiconductor power switch.

[0039] FIG. 2 shows a simplified model of a power MOSFET 5 which has a drain connection 6, a source connection 7 and a gate connection 8. The power MOSFET 5 is used in the above-described circuit for measuring the temperature of the depletion layer. In addition to the built-in gate resistor 3, the RC series circuit 9 for modeling the input impedances at the source connection 7 and drain connection 6 is also illustrated. An input capacitor 10 of the RC series circuit 9 symbolizes the capacitance of the gate, which is an intrinsic property of any MOSFET. The resistors 3 connected in series are installed in order to ensure a uniform distribution of voltage over the respective MOSFETs on the chip. Furthermore, a common input resistor 3 is provided and is intended to prevent the presence of high-frequency oscillations. The non-inverting amplifier circuit 200 operates as an amplifier with a gain factor of 1 for DC voltage signals. In this case, the capacitor 10 of the power MOSFET is treated as idling and the remaining circuit represents a gain of 1. The circuit begins to operate as an amplifier with an increase in the frequency of the voltage signal.

[0040] FIG. 3 illustrates the transfer function of the non-inverting amplifier circuit 14. In this case, the gain of the non-inverting amplifier circuit is plotted against the frequency.

[0041] The asymptotically approximated transfer function of an ideal non-inverting amplifier circuit 12 is illustrated as a dashed line.

[0042] The transfer function of the non-inverting amplifier circuit 14 comprises a zero at f.sub.Z and a pole at f.sub.P, wherein the frequency of the zero f.sub.Z is always less than the frequency of the pole f.sub.P. The transfer function therefore exhibits the characteristics of a high-pass filter. If a true operational amplifier is used, the characteristics exhibit an additional pole and passband-like filtering. The horizontal arrow 13 indicates the frequency range in which the signal is amplified by the filter. The gain increases in the range between the zero and the pole, but the capacitor of the RC series circuit still exhibits enough impedance to superpose the slight temperature-related changes in the gate resistance, with the result that they are not visible in the gain. For frequencies above the pole, the impedance of the capacitor has slightly more influence on the input impedance and the capacitor behaves as if it were short-circuited. The gain is dependent on the temperature-dependent resistance. The vertical arrow 15 indicates the temperature-related fluctuation in the measured gain.

[0043] For frequencies above the pole f>f.sub.P, the gain v is only dependent on the gate resistor R.sub.G(T) and the feedback resistor RF, like in a normal non-inverting amplifier:

[00002]$v\left(T\right)=1+\frac{R_F}{R_G\left(T\right)}.$

[0044] The invention is not limited to MOSFETs. It is also possible to use other semiconductor power switching elements which have a temperature-dependent resistor at a control input.

[0045] Semiconductor power switching elements are used, for example, in the phase winding of an electric motor of a steering system of a motor vehicle, preferably in the form of half-bridges, in particular a triple half-bridge for controlling a three-phase motor. The choice of a suitable semiconductor component results from the desired switching behavior. Power MOSFETs are preferably used as semiconductor components, but other components, for example IGBTs, can also be used. The temperature information determined using the apparatus according to the invention can be used, for example, to protect the MOSFETs from thermal overloading. If a critical junction temperature is reached, for example, steering assistance of an electromechanical steering system of a motor vehicle can be reduced and the power loss can therefore be reduced.