Method and device for monitoring multiple parallel-connected semiconductor switches

Abstract

To provide a method and device for monitoring multiple parallel-connected semiconductor switches (10).SOLUTION: The method includes subjecting the multiple semiconductor switches (10) to a heating pulse, in which a predefined load current flows for a predefined period of time in order to achieve a predefined temperature change of the semiconductor switches (10), where a quantity of a semiconductor switch to be monitored (10') among the multiple semiconductor switches (10) is captured both before and after the application of the heating pulse, and where a state of the semiconductor switch to be monitored (10') and / or a packaging technique corresponding to the semiconductor switch to be monitored is ascertained on the basis of a deviation of the change of the at least one quantity from a predefined reference value. The method enables measurement of individual semiconductor switches to be monitored (10') among the multiple semiconductor switches (10).SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method and an apparatus for monitoring semiconductor switches, which are connected in parallel, especially formed as power semiconductor switches.

Background Art

[0002] From the prior art, a test method for parallel-connected power semiconductors is known, which determines changes in parameters, especially parameters of the packaging technology corresponding to the power semiconductor, based on sum temperature measurement (Summentemperaturmessung). Such a test method is known, for example, as the method defined in test guidelines AQG324 or LV324 (see also the measurement methods based on the standards DIN EN 60747-15 and JEDEC standard No. 51-1).

[0003] Based on such a test method, a lifetime model for each tested power semiconductor can be derived. However, due to the applied sum temperature measurement, this lifetime model may contain corresponding defects because drifts or failures of individual semiconductors within the parallel connection cannot be detected.

[0004] Especially in a power module structure having a plurality of power semiconductors of the same type, due to lateral heating, there is a possibility of different temperature rises within these power semiconductors during active operation or within the corresponding packaging technology, and these different temperature rises cannot be individually captured by the above-mentioned test methods.

Summary of the Invention

[0005] A method for monitoring a plurality of parallel-connected semiconductor switches is proposed according to a first aspect of the present invention, wherein the semiconductor switches are formed, in particular, as power semiconductor switches, and preferably as topological switches. The number of parallel-connected semiconductor switches can be selected arbitrarily, and for example, 2 to 8 or a different number of parallel-connected semiconductor switches can be monitored using the method according to the present invention. It is advantageous that this number be guided by the total output to be switched by the parallel-connected semiconductor switches.

[0006] The method proposed here is advantageous because it allows for monitoring of lateral heating between individual semiconductor switches, and thus can assume thermal coupling between at least some of the semiconductor switches. Furthermore, the proposed method also allows for monitoring of the packaging technology corresponding to each semiconductor switch.

[0007] In the first step of the method according to the present invention, the semiconductor switch of a plurality of parallel-connected semiconductor switches that is to be monitored at that time is turned on, while the other semiconductor switches of the plurality of parallel-connected semiconductor switches (i.e., semiconductor switches that are not to be monitored at that time) are turned off or turned off.

[0008] For this purpose, it is preferable that each control input of the semiconductor switch is electrically connected to its corresponding driver (driver circuit), so that each semiconductor switch can be turned on and off independently of other semiconductor switches. The control for turning each semiconductor switch on and off is performed, for example, using an evaluation unit, which is adapted to control each driver based on the method according to the present invention.

[0009] In a second step of the method according to the present invention, a first predetermined load current is applied to the semiconductor switch to be monitored for a period of a first predetermined time. The requirement to provide the first load current is generated, for example, by an evaluation unit according to the same invention and / or by a different component. In the case of advantageous use of power semiconductors, the first load current corresponds to a value of, for example, 50A to 200A, but this does not limit the usable height of the first load current. The specification of the height of the first load current is preferably guided by the respective characteristic values ​​of the semiconductor switches used and / or a current value that allows for the most reliable determination of the temperature load of the semiconductor switches.

[0010] In a third step of the method according to the present invention, a first voltage drop is captured in a parallel-connected load section of multiple semiconductor switches within a first time period. During this time, only the semiconductor switch to be monitored is turned on, so the first voltage drop is substantially influenced by the first semiconductor switch. The first time period may be, for example, a few milliseconds, but is not limited to such a period. Furthermore, the timing of the capture within the first time period is not fundamentally defined, as long as reliable measurement results are guaranteed after the semiconductor switch to be monitored is turned on (e.g., after a steady state is achieved).

[0011] In the fourth step of the method according to the present invention, after the end of the first time (preferably immediately in response to the end of the first time), all semiconductor switches of the plurality of parallel-connected semiconductor switches are turned on.

[0012] In the fifth step of the method according to the present invention, a predetermined second load current is applied to a plurality of parallel-connected semiconductor switches for a predetermined second time period, wherein the second load current is higher than the first load current, and the definition of the height of the second load current and the definition of the second time period ensures a predetermined temperature rise to be achieved during the second time (for example, in the range of 10K to 160K, preferably between 30K and 140K, and particularly preferably between 60K and 110K, or a different value). The second time period is, for example, 3s to 15s, but is not limited to such a period. The plurality of semiconductor switches are heated together by the second load current, also hereafter referred to as the heating current, and therefore, any lateral heating present in some cases may also act on the individual semiconductor switches.

[0013] In the sixth step of the method according to the present invention, after the end of the second time period, all semiconductor switches that are not to be monitored at that time are turned off.

[0014] In the seventh step of the method according to the present invention, the first load current is again applied to the semiconductor switch to be monitored for a third predetermined period of time, that is, the first load current is reused to create current conditions equivalent to those for measuring the voltage drop during the first time period.

[0015] In the eighth step of the method according to the present invention, a second voltage drop is captured in a parallel-connected load section of multiple semiconductor switches within a third time, preferably immediately before the end of the third time, and the selection of this third time is advantageous such that the second voltage drop is captured when the semiconductor switches that are not to be monitored at that time have cooled significantly, while the semiconductor switches that are to be monitored at that time have shown only a small temperature drop relative to the temperature at the end of the second time due to lateral heating. It is desirable that this small temperature drop be within the range of measurement accuracy required and / or possible when carrying out the method according to the present invention.

[0016] In the ninth step of the method according to the present invention, a change in at least one quantity of the semiconductor switch to be monitored (e.g., temperature and / or temperature coefficient and / or internal resistance) between the capture of a first voltage drop and the capture of a second voltage drop is determined, and this change is determined based on a first load current, a first voltage drop, a second load current, and a second voltage drop. For example, a change in the internal resistance of the semiconductor switch to be monitored may be determined from the quotients of the first voltage drop and the first load current, and from the second voltage drop and the second load current, respectively. Accordingly, a temperature change between both capture points may be derived based on the change in internal resistance.

[0017] In the tenth step of the method according to the present invention, the state of the semiconductor switch to be monitored and / or the packaging technology corresponding to the semiconductor switch to be monitored is determined based on the difference of a change in at least one quantity from a predetermined reference value.

[0018] If the difference of this change from the baseline exceeds a predetermined maximum allowable difference, there is a high probability that an anomaly exists in the semiconductor switch and / or packaging technology. The latter occurs, for example, when the bonding material for the thermal contact between the semiconductor switch and the heat dissipation element to be monitored has deteriorated, and therefore the temperature change between both capture points is higher than what would be expected under normal circumstances for the packaging technology.

[0019] To continuously monitor for unfavorable temperature changes that could indicate an abnormal state in each semiconductor switch, it is advantageous to perform the above method alternately and repeatedly on each semiconductor switch of a plurality of parallel-connected semiconductor switches.

[0020] Generally, the order of the method steps used in the above description is one preferred order of execution of this method, but it should be noted that this order does not need to be strictly adhered to. In particular, the first load current and / or the second load current may be adjusted and / or provided simultaneously with, immediately before, or immediately after the respective switching time of the semiconductor switch. In addition, the semiconductor switch to be monitored at that time may be replaced during the execution of the method steps in order to match the measurements of different semiconductor switches with each other, and / or to temporarily store the measurement results of each semiconductor switch and match these measurement results with the measurement results of the same semiconductor switch from different processes of the method according to the present invention.

[0021] The dependent claims illustrate preferred modifications of the present invention. In one advantageous embodiment of the present invention, the reference value is a value determined based on previous monitoring of the semiconductor switch to be monitored at that time (e.g., from the immediately preceding monitoring cycle or an earlier monitoring cycle), and / or based on previous monitoring of at least one semiconductor switch different from the semiconductor switch to be monitored at that time, and / or based on the datasheet of the semiconductor switch to be monitored, and / or based on measurements of the semiconductor switch to be monitored under predetermined temperature conditions, and / or based on a sum temperature measurement of all semiconductor switches of a plurality of parallel-connected semiconductor switches. It should be noted that the reference value determined from previous monitoring may be determined based on a plurality of individual values, for example, by combining the individual values ​​(result values) of successive monitoring of each semiconductor switch into a single reference value. The combination may be performed, for example, by taking the average of each of these individual values, or by a different calculation rule.

[0022] It is advantageous that the semiconductor switches of a plurality of parallel-connected semiconductor switches are each MOSFET and / or IGBT and / or JFET and / or HEMT and are formed as, for example, Si-based, SiC-based, or GaN-based ones. Alternatively or in addition, it is advantageous that each semiconductor switch is incorporated into a semiconductor module and / or located within an inverter, particularly within an automotive inverter.

[0023] This method is particularly advantageous when it is repeatedly performed in a predetermined order for all semiconductor switches in a plurality of parallel-connected semiconductor switches. The repeated execution of this method continues, for example, until one or more predetermined interruption conditions are met. As interruption conditions, for example, an exceedance of the maximum allowable temperature rise due to the temperature rise determined between the capture of the first voltage drop and the capture of the second voltage drop may be considered. Alternatively, or in addition to this, the order for monitoring each semiconductor switch may be adapted depending on predetermined constraints.

[0024] In a particularly advantageous embodiment of the present invention, the method according to the present invention is incorporated into an existing test method for semiconductor switches, in which, respectively, the sum temperature measurement of all parallel-connected semiconductors is performed by heating pulses between each application time of the semiconductor switches, the sum temperature measurement is performed while the semiconductor switches are applied at a third predetermined load current (e.g., 100 mA or less) smaller than the first load current, and / or the existing test method is performed in parallel as is, and / or the heating pulses of the existing test method correspond to the application of the semiconductor switches at a second load current within a second time period. Examples of such existing test methods include the test guidelines AQG324 or LV324, which are known and can be used in combination with the method according to the present invention.

[0025] The load currents used, i.e., the first load current, the second load current, and optionally the third load current mentioned above, can be generated by the respective control of one variable current source and / or by separate current sources that can be switched to provide each one of the load currents. Instead or in addition, the second load current substantially corresponds to a value obtained by multiplying the first load current by the number of semiconductor switches connected in parallel. Then, the load current flowing through the semiconductor switches to be monitored at that time is the same or substantially the same within the first time period, the second time period, and the third time period, respectively.

[0026] It is advantageous for each semiconductor switch of a plurality of semiconductor switches connected in parallel to be thermally coupled to a cooling element (cooling body) for dissipating the heat generated by the semiconductor switch. The cooling element is, for example, a cooling element provided separately for each semiconductor switch. Instead, it is also conceivable that one cooling element is used jointly for a partial amount of the semiconductor switches or for all of the semiconductor switches.

[0027] In a further advantageous embodiment of the invention, the method according to the invention is used in an inspection stand for semiconductor switches (e.g., during the development stage of semiconductor switches or modules enclosing these semiconductor switches) and / or during the production launch of semiconductor switches (e.g., in a vehicle, for example for diagnostic functions).

[0028] Preferably, a lifetime model is determined and / or adapted for each semiconductor switch and / or for a higher-level component (e.g., a semiconductor module) having the semiconductor switch, based on the determined state of each semiconductor switch. Alternatively, or in addition to this, a load is adapted for each semiconductor based on the determined state of each semiconductor switch. The latter can be advantageously used, particularly in the production input of the method according to the present invention, by appropriately controlling semiconductor switches to reduce the load on those switches whose heat dissipation capacity is reduced due to abnormalities in the packaging technology. This ensures, at least for a certain period of time, that a component (e.g., a vehicle inverter) using the method according to the present invention can continue to be used even if degradation of one or more semiconductor switches is already present.

[0029] According to a second aspect of the present invention, an apparatus for monitoring a plurality of parallel-connected semiconductor switches is proposed. This apparatus comprises at least one current source, a voltage sensor, and an evaluation unit. The evaluation unit is configured as, for example, an ASIC, FPGA, processor, digital signal processor, microcontroller, etc., and is adapted to turn on the semiconductor switches of the plurality of parallel-connected semiconductor switches that are to be monitored at that time, while keeping the other semiconductor switches of the plurality of parallel-connected semiconductor switches off or off. To control each semiconductor switch, for example, each gate driver corresponding to the semiconductor switch is incorporated into the evaluation unit. Alternatively, each gate driver may be configured separately from the evaluation unit and electrically connected to the evaluation unit for control by the evaluation unit. The evaluation unit is further adapted to apply a first predetermined load current, provided by at least one current source, to the semiconductor switch to be monitored at that time for a period of a first predetermined time, to capture a first voltage drop in the parallel-connected load section of the plurality of semiconductor switches using a voltage sensor during the first time, to turn on all semiconductor switches of the plurality of parallel-connected semiconductor switches after the end of the first time, and to apply a predetermined second load current, provided by at least one current source, to the plurality of parallel-connected semiconductor switches for a period of a predetermined second time, wherein the second load current is higher than the first load current, and the definition of the height of the second load current and the definition of the period of the second time ensure a predetermined temperature rise to be reached during the second time.After the end of the second time, the evaluation unit further turns off all the semiconductor switches that should not be monitored at that time among the plurality of parallel-connected semiconductor switches, applies a first predetermined load current to the semiconductor switches to be monitored during a period of a third predetermined time, captures a second voltage drop in the load section of the parallel connection of the plurality of semiconductor switches within the third time, and is adapted to determine at least one amount of change of the semiconductor switches to be monitored between the capture of the first voltage drop and the capture of the second voltage drop, and this change is determined based on the first load current, the first voltage drop, the second load current, and the second voltage drop. Finally, the evaluation unit is adapted to determine the state of the semiconductor switches to be monitored and / or the packaging technology corresponding to the semiconductor switches to be monitored based on the difference between at least one amount of change and a predetermined reference value.

[0030] Exemplary embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Brief Description of the Drawings

[0031] [Figure 1] FIG. shows one exemplary embodiment of a circuit diagram of a monitoring device according to the present invention connected to a power semiconductor module to be monitored. [Figure 2] FIG. shows an exemplary current transition in a plurality of parallel-connected semiconductor switches.

Mode for Carrying Out the Invention

[0032] Figure 1 shows one exemplary embodiment of a circuit diagram of a monitoring device 60 according to the present invention connected to a power semiconductor module 80 to be monitored, the power semiconductor module 80 having four parallel-connected high-side power semiconductor switches 10', 10”, 10”, 10”, (hereinafter also referred to as "10") each having a corresponding gate driver 70', 70”, 70'', 70''" (hereinafter also referred to as "70"), the gate driver 70 being electrically connected to an evaluation unit 50, which in this case is formed as a microcontroller of the monitoring device 60. The power semiconductor switches 10 are each formed as SiC-MOSFETs, and the power semiconductor module 80 is, here exemplary, located within an inverter provided for the drive system of a vehicle.

[0033] Furthermore, the power semiconductor module 80 to be monitored has four parallel-connected low-side power semiconductor switches, which are not labeled here for the sake of easier viewing. The gate drivers corresponding to the low-side power semiconductor switches are also not labeled, and in addition, the electrical connections of the gated drivers, which follow the high-side connections, are not shown.

[0034] The evaluation unit 50 is further electrically connected to the variable current source 30, and is therefore adapted to adjust different output currents for the variable current source 30 by controlling the variable current source 30.

[0035] The electrical connection of the monitoring device 60 to the power semiconductor module 80 to be monitored is made via an interface 90 formed by the electrical contacts of the monitoring device 60 and the power semiconductor module 80.

[0036] The evaluation unit 50 is adapted to capture the voltage in the load section of the high-side power semiconductor switch 10 using the first voltage sensor 40, while the evaluation unit 50 is adapted to capture the voltage in the load section of the low-side power semiconductor switch using the second voltage sensor 45.

[0037] The evaluation unit 50 is adapted to perform the method according to the present invention described above, based on the above configuration and a computer program executable by the evaluation unit 50.

[0038] Figure 2 shows exemplary current transitions I10', I10”, I10”', I10”” within multiple parallel-connected semiconductor switches 10', 10”, 10”', 10””, and the total current transition IG for all parallel-connected semiconductor switches 10', 10”, 10”', 10””. Current transitions I10', I10”, I10”', I10””, correspond to, for example, the high-side power semiconductor switch 10 shown in Figure 1.

[0039] I10' here represents the current transition through the semiconductor switch 10' to be monitored at that time based on the method according to the present invention. From Figure 2, a first time T1 is evident, during which only the semiconductor switch 10' to be monitored at that time is turned on and is applied with a first load current I1. At the end of the first time T1, a first voltage drop is captured in the parallel-connected load section of the semiconductor switches 10. The point of capture is characterized by the left arrow of both arrows in the current transition I10'.

[0040] At the beginning of the second time period T2, which immediately follows the first time period T1, all semiconductor switches 10', 10'', 10''', and 10'''' are turned on and simultaneously applied with a second load current I2 that is substantially four times higher than the first load current I1.

[0041] At the beginning of the third time period T3, which immediately follows the second time period T2, all semiconductor switches 10'', 10'', and 10'', which should not be monitored at that time, are turned off, while the first load current I1 is again applied to the semiconductor switch 10'', which should be monitored. At the end of the third time period T3, the second voltage drop is captured, and this capture point is characterized by the right-hand arrow of both arrows in the current transition I10'',

[0042] Based on the captured voltage drop and the respective load currents I1 and I2, the difference between the predicted temperature behavior of the semiconductor switch 10' to be monitored and the actual temperature behavior is determined according to the method of the present invention.

[0043] In a later step of the method according to the present invention, semiconductor switch 10'' is defined as the semiconductor switch to be monitored at that time, and in a further later step, semiconductor switches 10''' and 10'''' are defined accordingly as the semiconductor switches to be monitored at that time.

[0044] This sequence for monitoring each of the semiconductor switches 10', 10'', 10''', and 10'''' continues to be repeated until a predetermined interruption condition is met. The total current transition IG shown in Figure 2 is composed of the individual currents passing through each semiconductor switch 10', 10'', 10''', and 10''''. It should be noted that the vertical scaling in the display of the total current transition IG is not the same as the vertical scaling in the display of the individual current transitions I10', I10'', I10''', and I10'''', for the sake of simplification.

[0045] It is advantageous that test methods known from the existing art (e.g., based on AQG324) are performed in parallel with the method according to the present invention. For this purpose, a third load current I3 is applied to the semiconductor switches 10', 10'', 10''', and 10''' during the time interval between each third time T3 and the time T1 immediately following these third times T3, where the third load current I3 corresponds here to approximately 100 mA, and the third load current I3 is the forward current flowing through each body diode of the semiconductor switch 10. Thus, based on successive, joint voltage measurements across each body diode, the average temperature rise of all semiconductor switches 10', 10'', 10''', and 10'' can be derived together. [Explanation of Symbols]

[0046] 10, 10', 10”, 10'', 10''" semiconductor switches 20 Heating pulse 30 current source 40 Voltage Sensor 50 evaluation units 60 Monitoring equipment I1 First load current I2 Second load current I3 Third load current T1 First period T2 Second Period T3 Third Time

Claims

1. A method for monitoring multiple semiconductor switches (10) connected in parallel, - A first step of turning on the semiconductor switch (10') of the parallel-connected semiconductor switches (10) that should be monitored at that time, while the other semiconductor switches (10) of the parallel-connected semiconductor switches (10) are turned off or to be turned off, - A second step of applying a predetermined first load current (I1) to the semiconductor switch (10') to be monitored for a predetermined first time period (T1), - A third step to capture a first voltage drop in the parallel-connected load section of the plurality of semiconductor switches (10) within the first time (T1), - A fourth step of turning on all of the parallel-connected semiconductor switches (10) after the end of the first time (T1), - A fifth step is to apply a predetermined second load current (I2) to the plurality of parallel-connected semiconductor switches (10) for a predetermined second time period (T2), The second load current (I2) is higher than the first load current (I1), and A fifth step in which the definition of the height of the second load current (I2) and the definition of the duration of the second time (T2) ensures a predetermined temperature rise to be achieved during the second time (T2), - A sixth step, after the end of the second time (T2), to turn off all of the parallel-connected semiconductor switches (10) that are not to be monitored at that time, - A seventh step of applying the predetermined first load current (I1) to the semiconductor switch (10') to be monitored for a predetermined third time period (T3), - An eighth step to capture a second voltage drop in the parallel-connected load section of the plurality of semiconductor switches (10) within the third time (T3), - A ninth step for determining a change in at least one quantity of the semiconductor switch (10') to be monitored between the capture of the first voltage drop and the capture of the second voltage drop, wherein the change is determined based on the first load current (I1), the first voltage drop, the second load current (I2), and the second voltage drop. A method comprising: a tenth step of determining the state of the semiconductor switch (10') to be monitored and / or the packaging technology corresponding to the semiconductor switch (10') to be monitored, based on the difference of the change of at least one quantity from a predetermined reference value.

2. The aforementioned reference value is, - Prior monitoring of the semiconductor switch (10') to be monitored at that time and / or - Prior monitoring and / or monitoring of at least one semiconductor switch (10) different from the semiconductor switch (10') that should be monitored at that time. - Datasheet and / or datasheet of the semiconductor switch (10') to be monitored. - Measurement of the semiconductor switch (10') to be monitored under predetermined temperature conditions and / or - Measurement of the total temperature of all semiconductor switches (10) of the parallel-connected plurality of semiconductor switches (10) The method according to claim 1, wherein the value is determined based on the above.

3. The semiconductor switches (10) of the plurality of parallel-connected semiconductor switches (10) - It is a MOSFET and / or - IGBT and / or - It is a JFET and / or - It is HEMT and / or - It is incorporated into a semiconductor module, and / or • Located inside the inverter, The method according to claim 1.

4. - The method described above is repeatedly performed on all semiconductor switches (10) of the parallel-connected plurality of semiconductor switches (10) in a predetermined order, and / or The method according to any one of claims 1 to 3, wherein the sequence for monitoring each of the semiconductor switches (10) is adapted depending on predetermined constraints.

5. It is incorporated into an existing test method for semiconductor switches (10), in which the total temperature measurement of all parallel-connected semiconductors (10) is performed by heating pulses (20) between the respective application times (T1, T2, T3) of the semiconductor switches (10). - The sum temperature measurement is performed while the semiconductor switch (10) is being subjected to a predetermined third load current (I3) that is smaller than the first load current (I1), and / or - The existing test method is carried out in parallel as is, and / or The method according to claim 1, wherein the heating pulse (20) of the existing test method corresponds to the application of the second load current (I2) to the semiconductor switch within the second time (T2).

6. - The first and second load currents (I1, I2) are Each of the variable current sources (30) is controlled by and / or Generated by separate, switchable current sources, and / or - The second load current (I2) substantially corresponds to a value obtained by multiplying the first load current (I1) by the number of parallel-connected semiconductor switches (10). The method according to claim 1.

7. The method according to claim 1, wherein each semiconductor switch (10) is thermally coupled to a cooling element to dissipate the heat generated.

8. The method according to claim 1, used in a test stand for a semiconductor switch (10) and / or in the production input of the semiconductor switch (10).

9. Based on the determined state of each of the semiconductor switches (10), - A lifetime model is determined and / or adapted for each of the semiconductor switches (10') and / or for the components placed higher up having the semiconductor switches (10), and / or A load is adapted for each of the aforementioned semiconductors (10'). The method according to claim 1.

10. A device (60) for monitoring a plurality of semiconductor switches (10) connected in parallel, - At least one current source (30) and - Voltage sensor (40), • Evaluation unit (50), - The semiconductor switch (10') of the parallel-connected semiconductor switches (10) that needs to be monitored at that time is turned on, while the other semiconductor switches (10) of the parallel-connected semiconductor switches (10) are turned off or are turned off. - To apply a predetermined first load current (I1) provided by the at least one current source (30) to the semiconductor switch (10') to be monitored for a predetermined first time period (T1), - Within the first time period (T1), the voltage sensor (40) is used to capture the first voltage drop in the parallel-connected load section of the plurality of semiconductor switches (10). - After the end of the first time (T1), turn on all of the parallel-connected semiconductor switches (10). - A predetermined second load current (I2) provided by at least one current source (30) is applied to the plurality of parallel-connected semiconductor switches (10) for a predetermined second time period (T2). The second load current (I2) is higher than the first load current (I1), and The specification of the height of the second load current (I2) and the specification of the duration of the second time (T2) are such that a predetermined temperature rise to be achieved during the second time (T2) is ensured. - After the end of the second time (T2), all semiconductor switches (10) of the parallel-connected plurality of semiconductor switches that should not be monitored at that time are turned off. - To apply the predetermined first load current (I1) to the semiconductor switch (10') to be monitored for a predetermined third time period (T3), - Within the third time (T3), capture the second voltage drop in the parallel-connected load section of the plurality of semiconductor switches (10), - Determine the change in at least one quantity of the semiconductor switch (10) to be monitored between the capture of the first voltage drop and the capture of the second voltage drop, such that the change is determined based on the first load current (I1), the first voltage drop, the second load current (I2), and the second voltage drop, and Apparatus (60) comprising: an evaluation unit (50) adapted to determine the state of the semiconductor switch (10') to be monitored and / or the state of the packaging technology corresponding to the semiconductor switch (10') to be monitored, based on the difference of the change of at least one quantity from a predetermined reference value.

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