Method for estimating the intrinsic temperature of at least one power transistor of power electronics

Abstract

The invention relates to a method for estimating the intrinsic temperature (TJ) of at least one power transistor (76) of power electronics (34), comprising the following steps (114, 116, 118, 120): • measuring a load current (IU, IV) for supplying an electrical load (74), able to be controlled by the at least one power transistor (76), when the power transistor (76) is fully on, • measuring a voltage drop (UDS) over the fully-on power transistor (76), • determining a forward resistance (RDSon) of the power transistor (76) on the basis of the measured load current (IU, IV) and of the measured voltage drop (UDS) and • determining the intrinsic temperature (TJ) of the power transistor (76) on the basis of the forward resistance (RDSon) by means of a look-up table in which a relationship between the forward resistance (RDSon) and the intrinsic temperature (TJ) for the power transistor (UDS) is stored. The invention also relates to a drive unit (26) comprising an electric motor (30) and an electronic unit (32) having power electronics (34) for controlling the electric motor (30) and having open-loop or closed-loop control electronics (36) for carrying out the method, and to an electric motor-driven vehicle (10), in particular a one- or two-wheeled vehicle (12), having a corresponding drive unit (26).

Description

[0001] R. 416931

[0002] - 1 -

[0003] Description

[0004] title

[0005] Method for estimating the intrinsic temperature of at least one power transistor in a power electronics system

[0006] Description

[0007] The invention relates to a method for estimating the intrinsic temperature of at least one power transistor of a power electronics unit, as well as a drive unit comprising an electric motor and an electronics unit with power electronics for controlling the electric motor and with control electronics for carrying out the method. Furthermore, the invention relates to an electrically driven vehicle, in particular a one- or two-wheeler, with a corresponding drive unit.

[0008] State of the art

[0009] In power electronics with at least one power transistor, particularly for driving electric motors, the printed circuit board temperature (also known as PCB temperature) is often measured using a temperature-dependent resistor located near the power transistor, according to current best practices. Such a temperature-dependent resistor usually has a negative temperature coefficient (NTC), meaning its resistance decreases as the PCB temperature increases. Alternatively, PTC resistors with a positive temperature coefficient can also be used. This is especially relevant when multiple power transistors are present, as in a B6 or H-bridge for driving a brushless or electrically commutated three-phase electric motor (BLDC or DC).When EC motors are used, such a point measurement at a measuring point that is more or less far away from the power loss-generating components only allows for a relatively inaccurate conclusion. R. 416931.

[0010] - 2 - to the internal temperature (junction temperature) of the individual power transistors. This necessitates maintaining a conservative safety margin to protect the power components from potential damage under all operating conditions. In some operating conditions, the power output must therefore be reduced even at non-critical temperatures because precise knowledge of the temperature of the critical power components is lacking.

[0011] The object of the invention is to provide an improved temperature estimation of the power components of a power electronics system, in particular for controlling an electric motor for driving an electrically driven vehicle, which is both cost-effective and particularly reliable.

[0012] Advantages of the invention

[0013] A procedure with the following steps is provided to solve the problem:

[0014] • Measuring a load current to supply a load controllable by at least one power transistor when the power transistor is switched on,

[0015] • Measuring a voltage drop across the switched-on power transistor,

[0016] • Determining the on-resistance of the power transistor based on the measured load current and voltage drop, and • Determining the intrinsic temperature of the power transistor based on the on-resistance using a look-up table that contains a relationship between the on-resistance and the intrinsic temperature for the power transistor.

[0017] A particular advantage of the method according to the invention is that it enables a reliable estimation of the intrinsic temperature of the power transistor without the need for additional sensor components for temperature measurement. This not only saves costs, but also eliminates the need for additional installation space on the power electronics circuit board. Therefore, it is not necessary, for example, to use low-resistance MOSFETs with higher resistance. 416931

[0018] - 3 - to use a silicon area to achieve the same effect even with less precise knowledge of the intrinsic temperatures. By measuring the load current and the voltage drop across the switched-on power transistor and subsequently determining the on-resistance, the intrinsic temperature of at least one power transistor can be estimated very accurately. This is particularly advantageous because the intrinsic temperature has a significant influence on the performance and lifetime of the power transistor. The method according to the invention also enables continuous monitoring of the intrinsic temperature of the power transistor in real time. Thus, a rapid response to temperature changes can be achieved to prevent damage to the power transistor and / or the power electronics.Precise and continuous monitoring of the intrinsic temperature allows the control electronics to react early to overheating of the power electronics and initiate appropriate protective measures, such as limiting the load current, motor speed, or motor torque of the electric motor controlled by the power electronics. This can lead to improved efficiency and a longer service life for the power electronics. The method can be adapted to different types of power transistors and various applications using appropriate look-up tables (LUTs). Therefore, the method according to the invention is versatile.

[0019] The electrically powered vehicle can be, for example, an electric bicycle (e.g., EPAC - Electrically Power Assisted Cycle, e-bike, pedelec, e-cargo bike, etc.), an electric motorcycle, a one- or two-wheeled e-scooter, an e-moped, or the like, equipped with a suitable drive unit. The performance of the drive unit and the electrically powered vehicle can thus be improved without increasing the amount of material used. More precise temperature estimation makes it possible to approach the permissible component limits with the intrinsic temperatures, thereby providing electric power assistance to a cyclist for longer periods or over greater elevation gains. The invention is also applicable to other micromobility applications, such as...E-kick scooters, monowheels, tricycles, or other non-type-approved vehicles with permanently or interchangeably installed energy storage units. Therefore, an electrically powered vehicle should also include R. 416931.

[0020] - 4 -a vehicle shall be understood to have a drive unit to assist the driver or an electric motor partial drive.

[0021] The energy storage unit can be permanently integrated into the electrically powered vehicle or designed as a removable battery pack that can be detached without tools. In the case of a removable battery pack, it can also be provided that it can be charged both when connected to and disconnected from the electrically powered vehicle. The battery voltage of the energy storage unit is generally a multiple of the voltage of a single energy storage cell within the unit and results from the connection (parallel and / or series) of the individual energy storage cells. Preferably, the energy storage cells are lithium-based, e.g., Li-ion, Li-polymer, Li-metal, Na-ion, or the like. However, the invention is also applicable to energy storage units with Ni-Cd, Ni-MH cells, or other suitable cell types.For common Li-ion energy storage cells with a cell voltage of 3.6 V, nominal battery voltages of 3.6 V, 18 V, 36 V, 54 V, etc., are possible. However, the invention is not dependent on the type and design of the energy storage cells and the energy storage unit used, but can be applied to any electrochemical energy storage units and energy storage cells, e.g., in addition to cylindrical cells, also pouch cells or the like, with battery voltages of 36 V, 48 V, 52 V or the like.

[0022] In order to derive a clear relationship between the on-resistance and the intrinsic temperature of at least one power transistor that can be derived from it via the LUT, a further development of the invention provides that the load current and the voltage drop are measured simultaneously - i.e. synchronously at the same sampling time - to determine the on-resistance.

[0023] Since BLDC and EC motors, as well as other electrical loads in electrically powered vehicles, are generally controlled by means of a pulse-width modulated voltage signal, it is further stipulated that the power electronics include at least one bridge branch with a low-side power transistor and a high-side power transistor, as per R. 416931

[0024] - 5 - The load current and the voltage drop with the low-side power transistor switched on can be measured. The method according to the invention can thus be seamlessly integrated into existing circuit designs for PWM control without requiring extensive modifications. Furthermore, measuring with the low-side power transistor switched on ensures that the load current measured with a motor phase current sensor, in particular a shunt resistor, corresponds to the current flowing through the low-side power transistor. This simplifies the circuit arrangement and signal processing. Moreover, precise knowledge of the intrinsic temperature of the low-side power transistor enables better control and optimization of the entire bridge circuit to increase the efficiency and performance of the power electronics.

[0025] In the case of a three-phase load, particularly a three-phase BLDC or EC motor, the power electronics are preferably configured as a B6 bridge with three bridge arms, each comprising a low-side power transistor and a high-side power transistor for one phase of the electrical load. The load current is measured for only two of the bridge arms at a time when the low-side power transistor is switched on, and a load current for the third bridge arm is calculated based on these two measured load currents. Thus, three temperature readings can be provided for a potential power limiting algorithm based on only two low-side power transistors. These readings are generally more accurate than the PCB temperature measured by a single temperature sensor, as the latter is typically measured at a distance from the individual power-loss-generating components.

[0026] The use of a temperature sensor already present on the circuit board is nevertheless advantageous, since it allows the circuit board temperature to be measured in a preliminary step. This measurement then serves, in a subsequent step, particularly after a defined time window, to calibrate the estimated intrinsic temperature of the at least one power transistor. For this purpose, a load current test pulse is generated to measure the load current and the voltage drop of the at least one switched-on power transistor. The defined time window can preferably be selected such that, on the one hand, a steady state is reached after measuring the conductor resistance. 416931

[0027] - 6 - plate temperature is present and, on the other hand, there is no excessively large difference between the estimate of the intrinsic temperature and the measurement of the printed circuit board temperature, which could lead to a distortion of the calibration in the case of rapidly changing temperature conditions.

[0028] Additionally or alternatively, the circuit board temperature measured in the preceding step can be stored in the subsequent step as a reference temperature for calibrating the estimated intrinsic temperature of the at least one power transistor. This advantageously allows timestamps to be generated that permit inferences about previous operating states.

[0029] A particular advantage is that the process is repeated every time at least one power transistor is switched on. This ensures a timely estimate of the intrinsic temperature for detecting dynamic temperature changes.

[0030] Examples of implementation

[0031] drawing

[0032] The invention is explained below by way of example with reference to Figures 1 to 20, where identical reference numerals in the figures indicate identical components with the same function.

[0033] They show

[0034] Fig. 1 : a schematic representation of an electrically driven vehicle designed as a two-wheeled electric bicycle in a first variant,

[0035] Fig. 2: a schematic representation of the electrically powered vehicle designed as a two-wheeled electric bicycle in a second variant, R. 416931

[0036] - 7 - Fig. 3: a block diagram for the energy supply of an electrical load of the electric vehicle designed as a three-phase EC motor according to Figures 1 or 2,

[0037] Fig. 4: a circuit diagram of a power electronics unit designed as a B6 bridge for controlling the three-phase EC or BLDC motor according to Figure 3,

[0038] Fig. 5: A look-up table in the form of a diagram to illustrate the relationship between the on-resistance of a power transistor and its intrinsic temperature and

[0039] Fig. 6: a flowchart of the inventive method for estimating the intrinsic temperature of the low-side power transistors of the power electronics.

[0040] Description of the exemplary implementations

[0041] Figure 1 shows a schematic representation of an electrically powered vehicle 10, designed as a two-wheeled electric bicycle 12. The electric bicycle 12 is powered by an energy storage unit 16 designed as a replaceable battery pack 14 and can be, for example, a pedelec, an e-bike, or the like. The electric bicycle 12 has a housing in the form of a predominantly hollow cylindrical frame 18 with two wheels 20 mounted in the frame 18. The replaceable battery pack 14 is detachably connected via a connecting device 22 provided on the frame 18, which interacts electromechanically with an interface (not shown) attached to an outer housing 24 of the replaceable battery pack 14. Alternatively, the electric bicycle 12 can also be designed as a three-wheeled vehicle with two wheels 20 arranged on a rear axle.

[0042] The electric bicycle 12 has a drive unit 26 supplied with energy via the interchangeable battery pack 14, comprising an electric motor 30 arranged in a rear wheel hub 28 of the rear wheel 20 and an electronic unit 32 located at a distance from it and arranged in the frame 18. The electronic unit 32 is designed to control the electric motor 30 in such a way that a rider of the R. 416931

[0043] - 8 - The electric bicycle 12 is assisted while pedaling. Preferably, the electronic unit 32 is designed to be controllable by the rider, so that the rider can adjust the level of assistance. The electric motor 30 is preferably designed as a three-phase, electrically commutated (EC) or brushless (BLDC) DC motor, which is controlled by a power electronics unit 34 of the electronic unit 32 via pulse width modulation (PWM). The electronic unit 32 comprises a control electronics unit 36 ​​for controlling or regulating the electric bicycle 12, in particular the electric motor 30, by means of the power electronics unit 34. The electric bicycle 12 is further equipped with a crank 38 with a crank axle 40 and a chainring assembly 42, via which the rider can perform a pedaling motion to drive the rear wheel 20 in the manner of a conventional bicycle drive.

[0044] The pedaling motion of the crank 38 is transmitted via the chainring assembly 42 and a chain 44 to the electric motor 30 located in the rear wheel hub 28 of the rear wheel 20. Using a derailleur 46 and a cassette (not shown) located on the rear wheel hub 28, the rider can select different gears depending on the desired speed and / or gradient. The intensity of the assistance provided by the drive unit 26 can be controlled or regulated by the electronic unit 32. For this purpose, the control electronics 36 of the electronic unit 32 is connected to a sensor unit 48, which preferably includes a cadence sensor 50 for measuring the pedaling frequency of the crank 38 or the crank axle 40 and a speed sensor 52 for determining the rotational speed of the rear wheel 20.Alternatively or additionally, the sensor unit 48 can also include a torque sensor for measuring the torque of the crank axle 40 and / or the electric motor 30 and / or a bearing force sensor for measuring a force acting on a bearing of the crank axle 40 and / or the electric motor 30. Instead of the derailleur 46 and the cassette, an electric motor 30 with an integrated planetary gear in the rear hub 28 can also be used.

[0045] An on-board computer 56 is arranged on a handlebar 54 of the electric bicycle 12. This computer can be detachably connected to the handlebar 54 and provides a human-machine interface (HMI) for displaying information and controlling the drive unit 26 of the electric bicycle 12 in the manner described above. R. 416931

[0046] - 9 - The HMI is preferably designed as a touchscreen or the like. Alternatively or additionally, the on-board computer 56 may have hardware buttons for operation, which are not shown in detail. The on-board computer 56 is preferably connected to the drive unit 26 via a bus system for exchanging information and commands. For example, the HMI can display and / or set a speed determined by the sensor unit 48, a set level of support for the electric motor 30, a selected gear ratio between a sprocket of the chainring set 42 and a pinion of the cassette, route information from a navigation unit integrated into the on-board computer 56, a charge level of the removable battery pack 14, or the like, whereby the list is to be understood as merely exemplary and not exhaustive.

[0047] Figure 2 shows an alternative embodiment of the electric bicycle 12. In contrast to Figure 1, the energy storage unit 16, including its electromechanical interfaces for energy and / or data transmission between the electric bicycle 12 and the energy storage unit 16, is fully integrated into the frame 18 of the electric bicycle 12. For easier replacement of the energy storage unit 16, it also has an outer housing 24. Furthermore, the drive unit 26 of the electric bicycle 12 comprises an electric motor 30 acting on the pedal crank shaft 40, which is designed as a mid-drive motor arranged in the frame 18. The drive unit 26 is preferably replaceable by a service center and is therefore arranged in a drive housing 58 that is detachably connected to the frame 18. In addition to the electric motor 30, it also includes the electronics unit 32 and the sensor unit 48.However, it is also conceivable that the electronic unit 32 and / or the sensor unit 48 are at least partially housed in other locations within the frame 18. A charging and / or discharging interface 60 is provided in the frame 18 of the electric bicycle 12 for charging and / or discharging the energy storage unit 16. This interface is shown as a USB-C interface by way of example. However, other interfaces for wired or wireless energy and / or data transmission are also conceivable. Furthermore, several such interfaces 60 may be provided on the electric bicycle 12.

[0048] Figure 3 shows a simplified block diagram for the power supply of the drive unit 26R integrated into the electric bicycle 12 according to Figures 1 or 2. 416931

[0049] - 10 - via the energy storage unit 16 with the battery voltage Ußatt shown. As previously described, the electronics unit 32 of the drive unit 26 comprises the control electronics 36, the power electronics 34 and an intermediate circuit electronics 62. The power electronics 34 are used to adjust a speed n and / or a torque T. qThe rotor 64 of the electric motor 30 is controlled by the control electronics 36 such that a pulse-width modulated motor voltage signal UM generated by the control electronics for the three phases U, V, W of a stator winding 68 wound on a stator or stator core 66 sets the rotor 64, which is rigidly connected to a motor shaft 70 and equipped with permanent magnets, into a rotational movement. A gearbox of the drive unit 26 (not shown) for assisting the propulsion of the electric bicycle 12 is driven via the motor shaft 70. The electric motor 30 also has sensors (not shown) for determining the speed, torque, and / or position of the rotor 64, which provide the corresponding sensor signals to the control electronics 36 via the bus system 72. The invention is not explicitly limited to three-phase, brushless DC motors, but can also be applied to other types of electric motors 30, such as...Synchronous motors or universal motors are used.

[0050] The DC link electronics 62 convert the battery voltage Usatt into a DC link voltage Uic, which in the case of the electric bicycle 12 is typically between 20 and 60 VDC. The DC link electronics 62 are connected to the control unit 36 ​​via the bus system 72 such that, firstly, the discharge current I of the energy storage unit 16 is detected and, secondly, its maximum value l is measured. maxand a maximum motor torque Tq.max can be specified. By means of an inverter circuit of the downstream power electronics 34 (not shown in detail), which can be implemented, for example, as an H-bridge or B6-bridge, the intermediate circuit voltage Uic is converted into the pulse-width modulated motor voltage UM for controlling the electric motor 30, whereby the control unit 36 ​​is controlled via the bus system 72 to vary the motor speed n and / or the motor torque T. q a corresponding duty cycle is specified. The discharge current I of the energy storage unit 16 is typically approximately 20 to 25 A and can briefly reach a maximum value I-max of up to 40 A. R. 416931

[0051] - 11 - Figure 4 shows a circuit diagram of the power electronics 34 of the electronic unit 32, configured as a B6 bridge, for controlling an electrical load 74 configured as a three-phase EC or BLDC motor 30, wherein the three phases U, V, W of the stator winding 68 of the electric motor 30 are each assigned to at least two power transistors 76 of a bridge branch of the B6 bridge. The stator winding 68 of a phase U, V, W can be distributed over several stator teeth (not shown) of the stator 66 of the EC motor 30, with each stator tooth of a phase U, V, W forming a stator pole. The individual windings 68 of the EC motor 30, connected in a delta configuration, are switched for each phase U, V, W by means of a high-side power transistor 78 and a low-side power transistor 80.The power transistors 76 each have control electrodes 82 for generating the pulse-width modulated motor voltage signal UM via their power electrodes 84, 86 by means of the control electronics 36 of the electronic unit 32. The control electronics 36 can be a microprocessor (pP), a digital signal processor (DSP), an application-specific integrated circuit (ASIC), or the like. A discrete design is also conceivable. The PWM control of the individual windings 68 of the stator poles of the EC motor 30 is carried out in a known manner via corresponding power nodes 88 between the high-side and low-side power transistors 78, 80 of each bridge branch of the B6 bridge such that the high-side and low-side power transistors 78, 80 of a bridge branch are switched on and off alternately, whereby the transition from one to the next phase U, V, W (commutation) has a phase shift of 120° el.exhibits such that the energizing of the individual windings 68 results in a corresponding rotational movement of the rotor 64 of the EC motor 30. Since the PWM control of an EC motor 30 using a B6 bridge is well known to those skilled in the art, it will not be discussed further here.

[0052] The power electronics 34 and the control electronics 66 are supplied with energy via a first reference potential Vi, in particular the DC link voltage Uic, and a second reference potential V2, in particular a ground potential GND. High-frequency interference can be filtered out by means of at least one filter capacitor 90 of the DC link electronics 62 connected between the first reference potential V1 and the second reference potential V2. R. 416931

[0053] - 12 - In addition to a B6 bridge, depending on the electrical load 74 to be driven, other circuit topologies, such as an H-bridge or a single power transistor per phase U, V, W, are also possible. It is also conceivable that the following invention is applicable without limitation to a star connection of the phases U, V, W. Furthermore, the electronic unit 32 can comprise power electronics 34 configured as a half-bridge or a single power transistor 76 for driving a single-phase electric motor 30, in which case the control electronics 36 can also be designed accordingly simply. The various circuit topologies and types of electric motors 30 are known to those skilled in the art, so it does not seem necessary to go into further detail here.

[0054] As mentioned at the outset, each power transistor 76 of the power electronics 34 can be configured as a MOSFET. In this case, the control electrode 82 is configured as a gate electrode, and the two power electrodes 84 and 86 are configured as a drain electrode and a source electrode, respectively. However, power transistors in the form of bipolar transistors (BJTs - Bipolar Junction Transistors), IGBTs (Insulated Gate Bipolar Transistors), or similar devices are also possible. In the case of a bipolar transistor, the control electrode 82 and the two power electrodes 84 and 86 are each configured as a base electrode, a collector electrode, and an emitter electrode, respectively. In the case of an IGBT, each is configured as a gate electrode, a collector electrode, and an emitter electrode. The various configurations of power transistors 76 are known to those skilled in the art, so they will not be discussed in further detail here.

[0055] To control the EC motor 30, knowledge of the load currents lu, lv, Iw flowing through the stator winding 68 for each phase U, V, W is required. These are measured for phases U, V using appropriate motor phase current sensors designed as shunt resistors 92 and transmitted as analog voltage values ​​via a downstream differential measuring amplifier 94 to the control electronics 36, which converts and evaluates these values ​​into digital values ​​using integrated analog-to-digital converters. It is sufficient to measure the load currents lu, lv for only two of the three phases U, V and to measure the load current Iw for the third phase W according to the relationship R. 416931.

[0056] - 13 - Iw = "lu ■ lv

[0057] to calculate.

[0058] With the low-side power transistor 80 activated, it is ensured that the load current lu, lv measured with the shunt resistor 92 has the same current magnitude as the current flowing through the low-side power transistor 80. Therefore, the load current lu, lv can be used directly to calculate the on-resistance Ruson of the low-side power transistor 80. The same applies to the calculated load current Iw of the third phases W for calculating the on-resistance RDSOH of the associated low-side power transistor 80.

[0059] Simultaneously with the load currents lu, lv, Iw (for simplicity also written as lu.vw below), a voltage drop UDS across the power electrodes 84, 86 is measured for each associated low-side power transistor 80 of a bridge branch by means of further differential measuring amplifiers 94 and evaluated by the control electronics 36. The resulting on-resistance RDSOH is then calculated as

[0060] RDSOFI = UDS / lu,v,w.

[0061] Figure 5 shows a diagram illustrating the relationship between the on-resistance RDSOH of the power transistor 76, specifically the low-side power transistor 80, and its intrinsic temperature Tj for two different characteristic curves 96 and 98. The upper characteristic curve 96 refers to a control voltage UGS of 6.5 V required to switch on the power transistor 76, and the lower characteristic curve 98 to a control voltage UGS of 10 V. The characteristic curves are stored as a LUT in the control electronics 36. For example, if the control electronics 36 calculates an on-resistance RDSOH of 2.7 mΩ at UGS = 6.5 V, an intrinsic temperature Tj of the power transistor 76 of T = 30 °C can be deduced.

[0062] To account for any variations in the on-resistance RDSOH within a batch of power transistors 76, an initial adjustment can be performed at R. 416931.

[0063] - 14 - Room temperature To. A temperature sensor 100, which is generally used anyway, can serve as a reference sensor for this purpose. Immediately after switching on the drive unit 26, it can be assumed that a circuit board temperature TPCB measured with the temperature sensor 100 corresponds to the intrinsic temperature Tj, because the power electronics 34 have not yet generated any significant power loss.

[0064] Figure 6 shows a flowchart of the inventive method for estimating the intrinsic temperature Tj of the low-side power transistors 80 of the power electronics 34. The method is started in step 102, for example, by switching on the on-board computer 56 of the electric bicycle 12. In step 104, the circuit board temperature TPCB is measured using the temperature sensor 100. If the circuit board temperature TPCB is, for example, greater than or equal to 40 °C, it can be assumed that the drive unit 12 or its power electronics 34 was in operation very recently.Thus, in step 106, the low-side power transistors 80 of phases U, V are subjected to a short load current test pulse lp(u,v) after a defined time window P to measure the load currents lu,v and the respective voltage drop UDS. Based on these measurements, the intrinsic temperature Tj of the switched-on low-side power transistors 80 is calculated as described above. Subsequently, in step 108, the measured circuit board temperature TPCB is stored as a reference temperature TR in the control unit 36, and a calibration factor CF is calculated according to the relationship.

[0065] Cp = TR / TJ

[0066] for the individual low-side power transistors 80. If the circuit board temperature TPCB measured in step 104 is less than 40 °C, it is assumed that the drive unit 12 or its power electronics 34 has not been in operation for a certain period of time and, consequently, all power transistors 76 have an intrinsic temperature Tj that corresponds to the reference temperature TR or the circuit board temperature TPCB, so that in step 106 the calibration factor CF can be set to the value 1 or a correspondingly conservative value > 1. This calibration process makes it possible to compensate for any component variations. In addition, it is conceivable to generate defined timestamps in conjunction with the calibration process, which provide a feedback reference. 416931

[0067] - 15 - This allows conclusions to be drawn about previous operating states. Likewise, the calibration process can also be performed only at the end of the method according to the invention or several times in between. The defined time window P can preferably be selected such that, on the one hand, a steady state exists after measurement of the printed circuit board temperature TPCB, and on the other hand, there is not too great a gap between the estimation of the intrinsic temperature Tj and the measurement of the printed circuit board temperature TPCB, in order to avoid distortion of the calibration in the case of rapidly changing temperature conditions. Preferably, the defined time window P can be selected between 0 and 10 seconds.

[0068] After completion of the calibration process in steps 104 to 110, step 112 checks whether the corresponding low-side power transistor 80 of the power electronics 34 is switched on or off during the control of the EC motor 30. If it is not switched on, the procedure is terminated in step 114 and, if necessary, repeated with steps 112 or 104. If the low-side power transistor 80 of a bridge branch is switched on, the load currents λi, λv of phases U and V and the respective voltage drop UDS are measured in step 116 according to the description in Figure 4 in order to calculate the on-resistance RDSOHZU in the subsequent step 118. The load current Iw of the third phase W is calculated from the two measured load currents λi, λv of phases U and V as described above.In step 120, the intrinsic temperature Tj of the switched-on low-side power transistor 80 is determined using the LUT according to Figure 5, based on the on-resistance RDSOH calculated in step 118. Finally, in step 122, the intrinsic temperature Tj is corrected using the values ​​determined in steps 108 and 118.

[0069] 110 determined the calibration factor CF via the relationship

[0070] Tj = Tj * CF.

[0071] The method according to the invention is then repeated from step 114 each time step 112 is switched on the low-side power transistor 80.

[0072] Finally, it should be noted that the exemplary embodiments shown do not refer to Figures 1 to 6 or to the forms shown therein, R. 416931

[0073] - 16 - The sizes and quantities of the elements, particularly those not essential to the invention, are limited. These are to be understood as examples only.

Claims

R. 416931 - 17 - Claims 1. Method for estimating the intrinsic temperature (Tj) of at least one power transistor (76) of a power electronics (34) comprising the following steps (114, 116, 118, 120): • Measuring a load current (I u, lv) to supply an electrical load (74) controllable by the at least one power transistor (76) when the power transistor (76) is switched on, • Measuring a voltage drop (UDS) across the switched-on power transistor (76), • Determining a forward resistance (Roson) of the power transistor (76) based on the measured load current (lu, lv) and the measured voltage drop (UDS) and • Determining the intrinsic temperature (Tj) of the power transistor (76) based on the on-resistance (Roson) using a look-up table that contains a relationship between the on-resistance (RDSOH) and the intrinsic temperature (Tj) for the power transistor (UDS).

2. Method according to claim 1, characterized in that the load current (lu, lv) and the voltage drop (UDS) are measured simultaneously.

3. Method according to one of the preceding claims, characterized in that the power electronics (34) comprises at least one bridge branch with a low-side power transistor (80) and a high-side power transistor (78), wherein the load current (lu, lv) and the voltage drop (UDS) are measured when the low-side power transistor (80) is switched on.

4. Method according to one of the preceding claims, characterized in that the power electronics (34) is a B6 bridge with three bridge branches, each comprising a low-side power transistor (80) and an ei-R. 416931 - 18 - comprising a high-side power transistor (78) for each phase (U, V, W) of the electrical load (74), wherein the load current (lu, lv) is measured for only two of the bridge branches with the low-side power transistor (80) switched on and a load current (Iw) for the third bridge branch is calculated on the basis of the two measured load currents (lu, lv).

5. Method according to one of the preceding claims, characterized in that in a preceding step (104) a printed circuit board temperature (TPCB) is measured by means of a temperature sensor (100), which in a subsequent step (108, 110), in particular after the expiry of a defined time window (P), serves to calibrate the estimated intrinsic temperature (Tj) of the at least one power transistor (76, 80).

6. Method according to claim 5, characterized in that in a step (106) after measuring the printed circuit board temperature (TPCB) a load current test pulse (IP(U), lp(v>) is generated to measure the load current (lu, lv) and the voltage drop (UDS) of the at least one switched-on power transistor (76, 80).

7. Method according to one of the preceding claims, characterized in that the printed circuit board temperature (TPCB) measured in the preceding step (104) is stored in the subsequent step (108) as a reference temperature (TR) for calibrating the estimated intrinsic temperature (Tj) of the at least one power transistor (76).

8. Method according to one of the preceding claims, characterized in that the method is repeated each time the at least one power transistor (76, 80) is switched on.

9. Drive unit (26) comprising an electric motor (30) and an electronic unit (32) with power electronics (34) for controlling the electric motor (30) and with control electronics (36) for carrying out the method according to one of the preceding claims. R. 416931 - 19 - 10. Drive unit (26) according to claim 9, characterized in that the power electronics (34) is mounted on a printed circuit board, and the printed circuit board has a temperature sensor (100) for measuring a printed circuit board temperature (TPCB).

11. Electrically powered vehicle (10), in particular a one- or two-wheeler (12), with a drive unit (26) according to one of the preceding claims 9 or 10.

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