Method for determining the temperature of a power semiconductor

A method for determining the temperature of a power semiconductor, wherein a first control contact is connected to a first pole of a series resistor integrated in the power semiconductor. A second pole—which continues to the power semiconductor—of the series resistor is connected to a second control contact. A first control contact and a second control contact are connected to a first connection terminal and second connection terminal via respective bonding wires. The resistance value of the series resistor is determined by an electrical measurement between the two connection terminals. On the basis of the resistance value and a temperature-resistance characteristic curve of the series resistor, the temperature of the power semiconductor is determined based on the temperature of the series resistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for determining the temperature of a power semiconductor.

2. Description of the Related Art

Integrated semiconductor components are produced as so-called semiconductor chips, or chips for short, in general from a base substrate, e.g., a silicon wafer. Power semiconductors, e.g., IGBTs, are also produced as chips of this type.

A power semiconductor generally has a control contact, via which the power semiconductor is driven. In the case of an IGBT, for example, this is the gate, via which a switching operation is initiated in the IGBT. Power semiconductors with an integrated series resistor are known. The series resistor is then connected in series with the control contact within the power semiconductor.

In this case, the driving is still effected via the control contact. The internal gate resistor co-ordinated with the respective power semiconductor is intended, e.g., to compensate for differently formed or pronounced Miller plateaus and to bring about more uniform switching of the power semiconductor.

A more recent generation of power semiconductors is also known which have, alongside the first control contact mentioned, a second contact, i.e., there is a usually smaller contact window or so-called auxiliary window on the chip. This auxiliary window is in direct contact with the actual internal control contact or control connection in the power semiconductor and thereby bypasses the series resistor.

In other words, the resistor integrated in the chip is situated between the two control contacts. The first pole of the resistor is therefore connected to the first control contact, and its second pole is connected to the second control contact. The internal gate resistor has, under normal conditions, for example a resistance value of approximately 10Ω. Usually, only the contact window—designated as the main window—of the first control contact is ever bonded or contact-connected on such a chip. The series resistor itself is inaccessible for contact-connection since it is situated internally in the power semiconductor.

The service life of such a power semiconductor depends largely upon the temperature stress or the temperature loading experienced by the power semiconductor in the course of its operating. For this reason, the so-called junction temperature is crucial, primarily in power semiconductor switches. It is therefore desirable to track the junction temperature of a power semiconductor as accurately as possible during the operation of the power semiconductor; e.g., a rough estimation of the expected remaining service life of the chip thus becomes possible. Replacement can then take place in a timely and planned fashion before failure of the chip is expected.

SUMMARY OF THE INVENTION

It is known to arrange a semiconductor chip on a substrate and to apply a conventional temperature sensor, e.g., in the form of an NTC (negative temperature coefficient) thermistor, as near as possible to the semiconductor chip. The sensor detects the temperature of the substrate and thus indirectly, or very approximately, the temperature of the chip and thus the junction temperature. However, temperature differences of about up to 80° C. between the actual junction temperature and measured temperature are to be expected with this method.

It is also known to apply a thermosensor, e.g., in the form of a thermoelement, on the chip itself, e.g., on the potting housing. Under this procedure, the temperature detection takes place nearer to the actual chip or the depletion layer. In this case as well, however, the junction temperature is detected only inaccurately with differences of about up to 20° C. The detection of the temperature by means of a thermoelement is known, e.g., from DE 10 2009 045 068.8.

Overall, therefore, the present temperature or junction temperature of a power semiconductor is not detected satisfactorily. Thus hitherto, there exists no sufficiently accurate device or method for determining the stress state and hence service life estimates or prognoses or the like for a power semiconductor.

It is an object of the invention to provide an improved method for determining the temperature of a power semiconductor.

The invention is based on the recognition that the integrated gate resistor lies particularly near the depletion layer and is temperature-dependent. The resistance value of the gate resistor is thus dependent on the actual internal temperature inside the chip.

The invention is based on the concept that, in the case of a power semiconductor embodied as a chip, the second control contact can also be connected via a bonding wire to provide, together with a bonding of the first control contact, direct electrical access to the internal gate resistor on both sides. Both connections of the internal resistor can therefore be made accessible by means of electrical contacts. Since the internal gate resistor is temperature-dependent, the temperature can be measured indirectly here via the gate resistor. The so-called auxiliary window on the chip, which forms the second control contact, is generally sufficient in terms of its geometry to be able to be bonded, e.g., by means of a bonding wire having a diameter of about 75 μm.

According to the invention, therefore, the first and second control contacts of the chip are connected, that is to say bonded, to a first connection terminal and a second connection terminal via a respective bonding wire. Only the connection terminals are then externally accessible by further electrical circuitry or contact-connection. According to the invention, the resistance value of the series resistor is determined by an electrical measurement between the two connection terminals. On the basis of the resistance value and a temperature-resistance characteristic curve of the series resistor, the temperature of the series resistor is determined to be the temperature of the power semiconductor.

According to the invention, electrical access to the resistor on both sides is provided via the connection wires and the connection terminals. The temperature dependence of the on-chip integrated series resistor is proportional to the reciprocal mobility of the charge carriers. The mobility decreases as the temperature rises, and so the resistance increases as the temperature rises. In order to evaluate the resistance, it is necessary to measure current and voltage at the internal gate resistor. Current and voltage can then be converted in a simple manner into an equivalent resistance value and, by way of the characteristic curve, into a temperature value.

A measurable current flow is present in the series resistor only when a switching current is fed to or removed from the first control contact; that is to say, e.g., only during the switch-on or switch-off operation of an IGBT. In a first embodiment of the invention, therefore, the measurement is effected during the operationally governed energization of the first control contact with an operating current. In other words, the operating current is used as a measurement current. In the first embodiment of the invention, therefore, the operating current is measured and, at the same time, the voltage across the series resistor is determined.

In an alternative embodiment, the measurement is carried out by means of a measurement current explicitly fed via the two control contacts. For this purpose, it is also possible, e.g., for a further resistor, generally having a higher resistance, to be connected to the second control contact, for example for a corresponding resistor to be contact-connected to the auxiliary window. By means of a small potential difference, a small auxiliary current or measurement current can then be generated at any desired point in time in the series resistor and the instantaneous resistance value can be measured here. The temperature measurement is then, e.g., not coupled to the switching operations in the IGBT.

In a further embodiment of the invention, the characteristic curve is determined on the basis of a calibration procedure at the power semiconductor. This can be effected, e.g., for each individual chip, a chip series or a chip type. By way of example, before the first start-up of a power semiconductor, an automatic calibration can take place, during which a specific characteristic curve is established. The characteristic curve can then be stored fixedly and permanently for said chip in order to be able to use the characteristic curve in the method.

In one preferred embodiment, the chip is mounted on a carrier circuit board (by, for example, DCB—Direct Copper Bonding), which additionally has a dedicated temperature sensor mentioned above. The characteristic curve can then be determined during no-load operation of the power semiconductor with the aid of the temperature sensor. Since, in the context of the calibration procedure, a temperature equilibrium between chip and temperature sensor can be ensured, or a differential temperature which is known in this case prevails here, under certain circumstances, an accurate temperature calibration can take place by means of the temperature sensor of the carrier circuit board. Since, in particular, the chip is in no-load operation, no thermal local disturbances arise such as, e.g., during the operation of the chip. That is to say if the latter produces waste heat as a result of a load current, for example, the waste heat is reflected locally only in the chip and not at the temperature sensor.

In other words: If the chip is mounted on a circuit board and if the latter has a conventional temperature sensor, e.g., the NTC mentioned above, the NTC temperature can be used as a reference during no-load operation of the chip for the actual resistor temperature.

Known chips in the form of IGBTs nowadays have a single internal gate resistor or series resistor. With regard to recent developments, however, chips having a plurality of internal series resistors are also conceivable. Alternatively, a parallel circuit formed by a plurality of IGBTs is conceivable; an arrangement having a plurality of internal series resistors may also be formed in this way. In these and other comparable situations, a structure arises having a plurality of series resistors which are thermally coupled among one another and each individual one of which can be used for the temperature measurement according to the invention.

In one advantageous embodiment, one or more of the power semiconductors or chips just mentioned has/have a plurality of series resistors with two respective control contacts. The method is then carried out at that one of the series resistors which is known to offer the best initial position for the desired measurement: by way of example, for determining service life, that series resistor is chosen which is known to be subjected to the most thermal stress as a result of its positioning. In other words, therefore, for a chip having a plurality of series resistors, only the series resistor subjected to the most thermal loading is evaluated to determine the maximum temperature loading of the chip. Thus, e.g., a “worst case” evaluation of the temperature with regard to service life estimation, etc., becomes possible.

In an alternative embodiment, wherein the power semiconductor as just explained likewise has at least some series resistors with respective control contacts, the method is carried out on at least some of the series resistors. By way of example, by means of a series circuit formed by a plurality or all of the series resistors, an average temperature of all the corresponding measurement locations could be determined. Alternatively, by way of example, it is also possible for a respective individual measurement to take place and for a subsequent selection or averaging of different determined temperatures of individual series resistors to take place.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1shows a power semiconductor in the form of an IGBT, which is embodied in integrated fashion as a chip2.FIG. 1shows chip2in plan view, i.e., an excerpt from the top side4of chip2. On top side4, windows to be contact-connected in the form of metallization surfaces arranged on the processed silicon substrate can be seen, namely four emitter windows6, one gate window8and one auxiliary window10.

The on-chip circuitry is indicated by dashed lines inFIG. 1: a power semiconductor12, that is to say the IGBT as an integrated semiconductor structure, is connected in parallel by its emitter E with the four emitter windows6. Collector C leads to the chip underside (not visible inFIG. 1) in the form of a whole-area metallization as collector connection11(seeFIG. 3). Gate G of power semiconductor12is directly contact-connected to the auxiliary window10. Gate G is connected to gate window8via an on-chip series resistor14having the resistance value RV.

Chip2per se cannot be directly contact-connected, e.g., soldered. For this purpose, so-called bonding firstly has to take place: for the purpose of contact-connecting power semiconductor12, emitter windows6are connected to an emitter connection terminal20with the aid of bonding wires16. Collector C can be soldered directly via collector connection11. By way of example, the chip underside is soldered in a planar fashion. Gate window8as first control contact22aof chip2is likewise connected to a first gate connection terminal24avia a bonding wire16. The circuitry interconnection or electrical driving of power semiconductor12during operation is thus effected via collector connection11and also connection terminals20and24a. Auxiliary window10is not used in accordance with the prior art.

According to the invention, auxiliary window10as second control contact22bis connected to a second gate connection terminal24b. According to the invention, the voltage URdrop across series resistor14and the current IRflowing through the latter are measured at connection terminals24a, b. Value RVof series resistor14can be determined from these two variables.

FIG. 2shows a characteristic curve26which is applicable to series resistor14and which reproduces the relationship thereof between resistance value RV, expressed in ohms, and the temperature, expressed in ° C. of series resistor14. In this case, the temperature of series resistor14can be equated with the temperature of chip2or of power semiconductor12, particularly in the case of an IGBT with the junction temperature thereof, since series resistor14is likewise situated in an integrated fashion in the same semiconductor substrate and, consequently, it is possible to assume that these respective components of chip2are at the same temperature. According to the invention, therefore, the temperature of power semiconductor12is determined by means of the characteristic curve26resistance value RVdetermined from the voltage URand the current IR.

In a first embodiment, in this case, the current IRis that operating current IBwhich flows during a switching operation at power semiconductor12. Current IRthus flows exclusively via the connection terminal24ainto series resistor14and from there into gate G of power semiconductor12. At the connection terminal24b, no current flow is measured, rather only voltage URwithout a current (apart from a possible minimal measuring device current).

In an alternative embodiment, by contrast, no current flows into gate G of power semiconductor12since the latter is not presently in a switching operation. As an alternative embodiment, an electric circuit is then formed via connection terminal24a, series resistor14and connection terminal24b, the current IRflowing through said electric circuit. In contrast to the switching current mentioned above, IRis then a pure measurement current IM. In this case, the voltage URis determined in the same way between the connection terminals24a, b.

In a further embodiment, characteristic curve26is determined beforehand in a calibration procedure, wherein chip2is heated to specific temperatures in a thermal cabinet, for example, the temperature T being known, e.g., on account of a measurement in the thermal cabinet, and then resistance value RVis determined with respect to known temperature T.

FIG. 3shows an alternative embodiment, wherein chip2is mounted on a carrier circuit board28by virtue of collector connection11of chip2being soldered to said carrier circuit board. A temperature sensor30is additionally mounted on carrier circuit board28directly alongside chip2. Characteristic curve26is then alternatively determined in such a way that the entire arrangement composed of carrier circuit board28and chip2is heated, the temperature being determined in each case by temperature sensor30. The associated resistance RVis then determined again by means of the above-described measurement of voltage URand current IR. This measurement takes place, in particular, during no-load operation of power semiconductor12, that is to say when collector C and emitter E thereof are not energized and are free of voltage. This ensures that no additionally generated waste heat arises internally in chip2and the temperature measurement of temperature sensor30yields accurate results.

FIG. 3additionally shows an embodiment of a chip2or a power semiconductor12which internally has a plurality of series resistors14and, in an accessible fashion, a plurality of control contacts22a, b(in part not illustrated) respectively connected thereto, and also connection terminals24a, b(in part not illustrated). In a first embodiment, the method described above is carried out only on that series resistor from among series resistors14shown which is subjected to the most thermal stress.

In an alternative embodiment of the method, the latter is carried out on some or all of series resistors14, a respective selection or communication of the possibly different temperatures taking place in order to determine the respective temperature of power semiconductor12as accurately as possible.