Device and method for monitoring a power semiconductor switch

A device for monitoring a power semiconductor switch includes a circuit section for applying to the power semiconductor switch an HF voltage having a frequency above a switching threshold of the power semiconductor switch, a shunt resistor for detecting an actual HF current resulting from application of the HF voltage to the power semiconductor switch, a monitoring circuit for comparing the actual HF current with an expected HF current that depends on a switching state of the power semiconductor switch when the HF voltage is applied to the power semiconductor switch, and a comparator for generating a power semiconductor status signal depending on a result of the comparison. A corresponding method for monitoring a power semiconductor switch of this type is also described.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is the U.S. National Stage of International Application No. PCT/EP2014/059283, filed May 7, 2014, which designated the United States and has been published as International Publication No. WO 2014/202274 and which claims the priority of German Patent Application, Serial No. 10 2013 211 411.7, filed Jun. 18, 2013, pursuant to 35 U.S.C. 119(a)-(d).

BACKGROUND OF THE INVENTION

The invention relates to a device and method for monitoring a power semiconductor switch to ensure the functional safety of the respective power semiconductor.

The invention addresses the problem that, for an item of equipment incorporating a switching element, a means of testing whether the switching element is available must exist if the equipment is to meet a heightened safety level. The same applies if a power semiconductor switch is to be used as the switching element. Accordingly, in the case of an item of equipment having a power semiconductor switch as the switching element, e.g. a power converter, it is necessary to be able to test the operation of the power semiconductor switch regularly in order to ensure that it is available.

There is currently no known device and method for such monitoring of a power semiconductor switch. Hitherto, mechanical relays have mainly been used for safety-relevant tasks. An additional mechanical contact unit in such a relay enables the relay to be monitored for correct operation. If power semiconductor switches are used for safety functions, it has hitherto only been possible to provide monitoring of the power semiconductor switch for operability by means of “trial operation”. However, this involves the power circuit containing the power semiconductor switch and is therefore somewhat undesirable.

SUMMARY OF THE INVENTION

The object of the invention is to therefore to specify a device and a method for monitoring a power semiconductor switch, said method and device not involving the power circuit of the power semiconductor switch.

According to one aspect of the invention, a device for monitoring a power semiconductor switch includes the following functional units: first, means of applying a high-frequency voltage (HF voltage; UHF) to the power semiconductor switch at a frequency above the switching threshold of the power semiconductor switch in addition to triggering the power semiconductor switch by means of an external triggering signal, wherein the external triggering signal produces a switching state of the power semiconductor switch corresponding to the triggering signal. Then, means of detecting an HF current (IHF,actual) resulting from the application of the HF voltage (UHF) to the power semiconductor switch. In addition, means of comparing the resulting HF current (IHF,actual) with an HF current (IHF,nom) which is expected according to the switching state of the power semiconductor switch as a result of the application of the HF voltage (UHF) to the power semiconductor switch. Lastly, means of generating a power semiconductor status signal depending on the result of the comparison.

According to another aspect of the invention, a method for monitoring a power semiconductor switch includes at least the following steps: the power semiconductor switch has applied to it a high-frequency voltage (HF voltage; UHF) having a frequency above a switching threshold of the power semiconductor switch, in addition to being triggered by means of an external triggering signal. The external triggering signal produces a switching state of the power semiconductor switch corresponding to the triggering signal. An HF current (IHF,actual) resulting from the application of the HF voltage (UHF) to the power semiconductor switch is detected. The resulting HF current (IHF,actual) is compared with an HF current (IHF,nom) which is expected according to the switching state of the power semiconductor switch as a result of the application of the HF voltage (UHF) to the power semiconductor switch. A power semiconductor status signal is generated depending on the result of this comparison.

The advantage of the invention is that the approach presented here specifies a device and a method for testing the operability of the power semiconductor switch by the triggering thereof, without involving the actual power circuit.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The representations inFIG. 1andFIG. 2show equivalent circuit diagrams of power semiconductor switches (power semiconductors)10that may currently be used for safety functions. The representation inFIG. 1shows the equivalent circuit diagram of a MOSFET12and the representation inFIG. 2shows the equivalent circuit diagram of an IGBT14.

For both power semiconductor switches10, the usual terminals are shown and denoted by the customary terminology in each case. Accordingly, a power semiconductor switch10in the form of a MOSFET12has a gate terminal (G), a source terminal (S) and a drain terminal (D). Correspondingly, a power semiconductor switch10in the form of an IGBT14has a gate terminal (G), a collector terminal (C) and an emitter terminal (E).

Marked between two of these terminals of a power semiconductor switch10in the equivalent circuit diagrams in each case is the capacitance resulting from the device characteristics, namely for a MOSFET12in the form of a capacitor having the capacitance CGSbetween the gate terminal (G) and the source terminal (S), of a capacitor having the capacitance CGDbetween the gate terminal (G) and the drain terminal (D), and a capacitor having the capacitance CDSbetween the drain terminal (D) and the source terminal (S). This applies correspondingly to the equivalent circuit diagram of the IGBT14. Accordingly, this shows a capacitor having the capacitance CGEbetween the gate terminal (G) and the emitter terminal (E), a capacitor having the capacitance CGCbetween the gate terminal (G) and the collector terminal (C), and a capacitor having the capacitance CCEbetween the collector terminal (C) and the emitter terminal (E).

InFIG. 1, for the representation of the equivalent circuit diagram shown there of a power semiconductor switch10in the form of a MOSFET12, the equivalent circuit diagram of an inverse diode16incorporated in such a power semiconductor switch10is also shown.

For the case that a power semiconductor switch10, in particular a power semiconductor switch10in the form of a MOSFET12or in the form of an IGBT14, is defective, this is reflected in a changed capacitive behavior. Defectiveness of the power semiconductor switch10is to be understood as meaning partial destruction of the respective component, but also loss of at least one contact. Destruction of cell regions of a power semiconductor switch10results, for example, in a reduction in the input capacitance, i.e. the gate capacitance CGSin the case of the MOSFET12or gate capacitance CGEin the case of the IGBT14. Even relatively minor defects can be detected in this way.

The representations inFIG. 3andFIG. 4show control circuits20for triggering a power semiconductor switch10. The power semiconductor switch10is shown together with a parallel-connected reverse diode16, as is the case, for example, in an input power converter or an output power converter of an AC/AC converter.

Depending on the setting of the switch22incorporated in the control circuit20, the power semiconductor switch10has applied to it, i.e. is triggered by, a positive or a negative control potential, here in the form of a first voltage source24supplying +15 V, for example, and a second voltage source26supplying −15 V, for example.

The control circuit20shown inFIG. 3corresponds to a control circuit20as is known in the prior art. In comparison to the control circuit20inFIG. 3, the control circuit20shown inFIG. 4comprises an additional circuit section30which injects a high-frequency voltage (HF voltage; UHF) into the gate circuit of the respective power semiconductor switch10. The circuit section30is accordingly an example of means30of applying to the power semiconductor switch.10an HF voltage (UHF) having a frequency above a switching threshold of the power semiconductor switch10. Said HF voltage is applied simultaneously with triggering of the power semiconductor switch10as a result of a respective setting of the switch22or of a corresponding signal source.

As the basis for the high-frequency voltage (UHF), an HF voltage source32is shown here. This is decoupled from the gate terminal of the respective power semiconductor switch10with respect to high-frequencies by means of a decoupling capacitor34of capacitance CHF connected in series with the HF voltage source32within the circuit section30. A current (IHF), hereinafter referred to as the HF current, resulting from the high-frequency voltage (UHF) is detected by means of a shunt resistor (RHF)36. However, instead of using a shunt resistor36, the HF current (IHF) can equally be detected inductively, for example. The shunt resistor (RHF)36or inductive detection of the HF current (IHF) are accordingly examples of means of detecting the HF current (IHF,actual) resulting from the application of the HF voltage (UHF) to the power semiconductor switch (10).

The frequency and amplitude of the high-frequency voltage (UHF) are selected such that the frequency far exceeds the switching frequency of the power semiconductor switch10. A possible frequency is accordingly a frequency higher than 10 MHz, for example. For the amplitude of the high-frequency voltage (UHF) it is provided that this is well below the normal voltage values used to trigger a power semiconductor switch10. A possible amplitude is accordingly an amplitude of about 1 V.

To monitor the respective power semiconductor switch10, the control circuit20is assigned a monitoring circuit40. This is shown in schematically simplified form inFIG. 5. The switching state of the switch22of the control circuit20is fed to the monitoring circuit40at a first input42. The current (IHF) resulting from the high-frequency voltage (UHF) is fed directly or indirectly to the monitoring circuit40at a second input44, e.g. in the form of a measure for the voltage that can be tapped off across the shunt resistor36.

If a measure for the voltage that can be tapped off across shunt resistor36is fed to the monitoring circuit40, this measure and the known resistance value of the shunt resistor36are used to determine a current (IHF,actual) actually resulting from the high-frequency voltage (UHF). For this purpose the monitoring circuit40comprises an actual HF current value determining device46. The functionality of the actual HF current value determining device46consists, for example, of forming the quotient of the measure fed to the second input44for the voltage that can be tapped off across the shunt resistor36and the known resistance value of the shunt resistor36. The actual HF current (IHF,actual) resulting from the high-frequency voltage (UHF) or a measure for the resulting actual HF current (IHF,actual) is present in any case at the output of the actual HF current value determining device46.

The actual HF current (IHF,actual) is compared by means of a comparator48with an HF current (IHF,nom) that is expected as a result of the high-frequency voltage (UHF). The expected (nominal) HF current (IHF,nom) or a measure for the expected HF current (IHF,nom) is provided by means of a nominal HF current value determining device50. As an input signal, this device processes the switching state of the switch22of the control circuit20, said state being fed to the monitoring circuit40at the first input42. The functionality of the nominal HF current value determining device50can be implemented, for example, in the form of a table which comprises, in a first table element, a measure for the expected HF current (IHF,nom1) for a first setting of the switch22and, in a second table element, a measure for the expected HF current (IHF,nom2) for a second setting of the switch22. Depending on the setting of the switch22fed to the first input42, the nominal HF current value determining device50accordingly outputs the HF current (IHF,nom=[IHF,nom1, IHF,nom2]) expected for the respective switch setting or a measure for the HF current (IHF,nom=[IHF,nom1) IHF,nom2]) expected and forwards it in each case to the comparator48. The comparator48performs the actual comparison between the expected HF current (IHF,nom) and the actual HF current (IHF,actual) resulting from the high-frequency voltage (UHF).

The monitoring circuit40and the comparator48incorporated therein are accordingly an example of means of comparing the resulting HF current (IHF,actual) with the HF current (IHF,nom) expected from the application of the HF voltage (UHF) to the power semiconductor switch10depending on the switching state of the power semiconductor switch10.

If the comparator48detects parity or parity within a predefined or predefinable tolerance range, the comparator48outputs an OK signal at its output serving as output52of the monitoring circuit40. In the event of no parity or insufficient parity of the two currents or current values compared by means of the comparator48, the comparator48accordingly outputs a fault signal. Possibilities for an OK signal and a fault signal are, for example, a first defined signal level and a second defined signal level, so that the signal produced at the output52of the monitoring circuit40can be processed as a binary signal and the status of the monitored power semiconductor switch10indicated.

The monitoring circuit40can be implemented and operate on an analog or digital basis. The advantage of the proposed solution is that the operation of the power semiconductor switch10is monitored without involving the respective power circuit56,58(FIG. 6). The monitoring circuit40can be incorporated together with the circuit section30in an “intelligent” semiconductor module60, as shown in schematically simplified form inFIG. 6.

FIG. 6shows a schematically simplified block diagram of a semiconductor module60operating according to the approach described here. The semiconductor module60is accordingly an example of a device for monitoring a power semiconductor switch10, said device comprising, in particular in integrated form, all the hitherto described functional units. The semiconductor module60accordingly comprises at least the respective power semiconductor switch10, possibly the power semiconductor switch10and the reverse diode16, the circuit section30explained the reference toFIG. 4and the monitoring circuit40explained with reference toFIG. 5. Such a semiconductor module60can be controlled by means of a switch22or the like. A resulting triggering signal62is fed to the power semiconductor switch10and the monitoring circuit40. The power semiconductor switch10is also triggered by means of the circuit section30and the high-frequency voltage (UHF) generated there using an HF signal64having a frequency above the switching threshold of the power semiconductor switch10(arrow pointing horizontally to the right from the circuit section30to the power semiconductor switch10). An HF current (IHF) flowing, which is dependent on the input capacitance of the power semiconductor switch10, is detected by means of the circuit section30, e.g. by means of the shunt resistor36there (arrow pointing horizontally to the left from the power semiconductor switch10to the circuit section30) and forwarded to the monitoring circuit40(arrow pointing vertically downward from the circuit section30to the monitoring circuit40). The triggering signal62is applied to the power semiconductor switch10and also at the same time to the monitoring circuit40. Depending on the status of the triggering signal62, this results in a status-dependent expected HF current or a status-dependent measure for an expected HF current within the monitoring circuit40. This is compared by means of the monitoring circuit40with the actual HF current received from the circuit section30or the measure received from the circuit section30for the actual HF current. Present at the output52of the monitoring circuit40and therefore at the coinciding output of the semiconductor module60is a power semiconductor status signal66dependent on the result of the comparison66, which signal can be evaluated for monitoring the power semiconductor switch10and is evaluated during operation. The comparator48incorporated in the monitoring circuit40is accordingly an example of means of generating a power semiconductor status signal66depending on the result of the comparison of the actual and expected HF current.

The respective power circuit56,58is connected to the drain and source terminal or to the collector and emitter terminal of the respective power semiconductor switch10. The power semiconductor status signal66that can be tapped off at the output52of the monitoring circuit40of such a semiconductor module60indicates the operability of the respective power semiconductor switch10. As long as a signal is present there which indicates parity or sufficient parity of the expected and actual HF current, the power semiconductor switch10incorporated in the semiconductor module60can be deemed to be certified as safe.

In order to also detect the capacitance between drain and source or collector and emitter in addition or alternatively to the gate capacitance of the power semiconductor switch10, the HF voltage can also be applied there. For this purposeFIG. 7shows a corresponding circuit section30′. This comprises a decoupling diode68and capacitance connected in parallel with the decoupling diode68(the control circuit20according toFIG. 3is shown only as a functional block inFIG. 7). The functionality implemented by means of the circuit section30′ can also be regarded as a decoupling circuit. The capacitances comprised thereby must not be significantly below the power semiconductor switch capacitance to be measured, so that sufficient measurement accuracy can be achieved.

Although the invention has been illustrated and described in detail by advantageous embodiments, the invention is not limited by the examples disclosed and variations for other components having capacitive behavior may be inferred therefrom by a person skilled in the art without departing from the scope of protection sought for the invention. For example, power semiconductors in the form of so-called JFETs can also be tested using the approach presented here.

Individual salient aspects of the description submitted here may be briefly summarized as follows:

Specified are a device for monitoring a power semiconductor switch10, wherein the device has means30of applying to the power semiconductor switch10an HF voltage (UHF) having a frequency above the switching frequency of the power semiconductor switch10, means36of detecting an HF current (IHF,actual) resulting from the application of the HF voltage (UHF) to the power semiconductor switch10, means (40,48) of comparing the resulting HF current (IHF,actual) with an HF current (IHF,nom) expected as a result of the application of the HF voltage (UHF) to the power semiconductor switch10according to the switching state of the power semiconductor switch10, and means48of generating a power semiconductor status signal66depending on the result of the comparison, as well as a corresponding method for monitoring a power semiconductor switch10.