Output frequency-dependent calculation of the junction temperature of power semiconductors in a traction converter

Abstract

The present disclosure relates to a set of power electronics (1) for an electric machine, comprising controllable power semiconductors (2) and a thermal control unit (3) for thermally monitoring the power semiconductors (2), wherein the thermal control unit (3) is designed to select a thermal model (M1, M2) for calculating a power loss of the power semiconductors (2) on the basis of a frequency (f, F1, F2) of an electrical output current of the set of power electronics (1). The present disclosure also relates to an electric machine having such a set of power electronics (1).

Description

[0001] Power electronics and electric machine The present disclosure relates to power electronics for an electric machine, in particular for an electric drive motor of a (motor) vehicle. The present disclosure also relates to an electric machine with such power electronics. In electric drive technology, power electronics are used to control or regulate electric machines or traction drives or for energy conversion. Modulation methods based on space vector modulated (SVPWM, space vector pulse width modulated) commutations are used, in which power semiconductor switches, such as metal oxide semiconductor field-effect transistors (MOSFETs) or bipolar transistors with an insulated gate electrode (IGBTs, insulated-gate bipolar transistors), are used, which are switched on or off depending on the position of the space vector.These commutation processes follow a mathematically predetermined pattern that is specified by a control algorithm and the modulation method, whereby the state of the power semiconductors, namely conductive state or non-conductive state, is controlled. Depending on the mechanical speed of the electrical machine or the frequency of an electrical output current of the power electronics, the residence time in a commutation pattern of the power semiconductors in a rotating machine changes. The higher the speed or frequency (i.e. the faster the machine rotates), the shorter the residence time, and the lower the speed or frequency (i.e. the slower the machine rotates). In a stationary machine, the commutation pattern remains the same.To date, the state of the art for thermal monitoring of power semiconductors in a thermal model is to assume either a short residence time (i.e., high electrical frequencies or speeds) or a long residence time (i.e., at low electrical frequencies or speeds). In this case, the power loss of the power semiconductors with a short residence time can be calculated using a current root mean square model, in which the power loss is calculated using the mean value of the current squared. This does not require a high amount of computing power and is easy to implement. While the power loss of the power semiconductors with a long residence time must be calculated using an instantaneous current model, in which the power loss is calculated using an instantaneous current value. This requires significantly higher computing power due to the continuous measurement of the instantaneous current value.The problem is that in power electronics, particularly in a small / low speed range, especially at 0 speed, thermal problems can arise due to slowly changing or static commutation patterns. This could lead to overheating of the power semiconductors, which in turn causes the achievable torque of the electric machine to be significantly lower in this speed range than at higher speeds. This is primarily due to the fact that at low speeds, the instantaneous value of the current through the power semiconductors becomes more dominant than the mean square of the current. This effect is particularly evident when an electrical period is below a thermal time constant of the power semiconductors.In particular, if the commutation pattern stops at the point in time at which a commutation branch of the power semiconductors is loaded with maximum current, overheating can occur. Such a condition can occur or be necessary for several seconds, such as when starting on a hill or when stationary and starting at a curb, whereby a requirement of a few seconds, such as 5 seconds, is assumed and whereby these operating points occur relatively rarely. To remedy such a condition, however, it is possible to dimension the power semiconductors larger or oversize them, which, however, requires increased silicon use (in the form of larger or multiple parallel chips), which in turn leads to increased costs and is therefore disadvantageous. Against this background, the object of the present disclosure is to avoid or at least reduce the disadvantages of the prior art.In particular, a power electronics system for an electrical machine or an electrical machine with such power electronics is to be provided in which the power semiconductors can be thermally monitored particularly reliably and easily and which can also be manufactured cost-effectively. This object is achieved by a power electronics system with the features of the independent patent claim and by an electrical machine with the features of the co-ordinate patent claim. Advantageous further developments are the subject of the subclaims. Accordingly, the present disclosure relates to a power electronics system for an electrical machine, in particular for an electric drive motor of a motor vehicle, with power semiconductors, the state of which, in particular as a function of a position of a space vector ora magnetic field of the electric machine, and a thermal control unit for thermally monitoring the power semiconductors. According to one aspect of the disclosure, the thermal control unit is configured to select a thermal model for calculating a power loss of the power semiconductors as a function of a frequency (of an electrical output current) of the power electronics. This means that, depending on the frequency, different thermal models are used for thermally monitoring the power semiconductors. This has the advantage that the thermal models can be used as needed and their use can be optimized, in particular with regard to their required computing power, which depends in particular on the commutation pattern or the residence time or frequency or the rotational speed.According to a preferred embodiment, the thermal control unit can be configured to calculate the power loss of the power semiconductors in a first frequency range using a first thermal model. According to a preferred embodiment, the thermal control unit can be configured to calculate the power loss of the power semiconductors in a second frequency range using a second thermal model. This means that the use of the thermal models depends in particular on the frequency. According to the preferred embodiment, the first thermal model can be a current root mean square model. This means that in the first thermal model, the power loss is calculated using an average of the square of the current of the power electronics. This ensures efficient thermal monitoring of the power semiconductors in this range.According to the preferred embodiment, the second thermal model can be an instantaneous current value model. This means that in the second thermal model, the power loss is calculated based on an instantaneous value of the power electronics current. This ensures efficient thermal monitoring of the power semiconductors in this range. According to the preferred embodiment, the first frequency range can contain higher frequencies than the second frequency range. This means that one application area of ​​the first thermal model is a high frequency or speed range (or a higher frequency range compared to the second frequency range) and one application area of ​​the second thermal model is a low frequency or speed range (or a lower frequency range compared to the first frequency range).According to a preferred embodiment, the first thermal model may require less computing power than the second thermal model. This means that the second thermal model is more computing-intensive than the first thermal model. In particular, if the application area of ​​the second thermal model is a low frequency or speed range, a higher computing power can be made available for the second thermal model. According to a preferred embodiment, the power electronics can have a safety functions control unit. Safety functions can be, for example, an ESP or a collision warning system. According to the preferred embodiment, the power electronics can be set up such that, in the second frequency range, computing power assigned to the safety functions control unit is made available to the thermal control unit. This means that the computing power is demand-oriented orcan be distributed depending on the frequency. According to a preferred embodiment, the first frequency range and / or the second frequency range can be defined depending on a thermal time constant of the power semiconductors. In particular, the second frequency range can be defined such that an electrical period is below the thermal time constant of the power semiconductors. According to a preferred embodiment, a loss frequency can be twice the current frequency. According to a preferred embodiment, a thermal cutoff frequency of the power semiconductors can be an inverse of the thermal time constant of the power semiconductors. According to the preferred embodiment, a boundary between the first frequency range and the second frequency range can be selected such that the loss frequency is 100 times greater than the thermal cutoff frequency.The present disclosure also relates to an electric machine, in particular an electric drive motor of a (motor) vehicle, having such power electronics. In other words, the present disclosure relates to a simple and non-computing-intensive methodology for detecting thermally critical operating states of the power semiconductors and for simulating the thermal behavior of the power semiconductors in these states using an appropriate simulation model. By detecting the thermally critical operating states, it is possible to effectively switch between a computationally intensive thermal model for low electrical frequencies and the less computationally intensive thermal model for high electrical frequencies. One use can then be a thermal offline or online simulation, the computational requirements of which can be adapted depending on the driving situation.This means that computing power can be shifted to the thermal model, particularly at low speeds and high currents, such as when starting on a hill or hitting a curb, where safety functions such as ESP, collision warning, etc. require less computing power than at high speeds. At high speeds, a thermal model with low computing requirements can be used to make computing power available to the vehicle's safety functions. Preferably, a thermal time constant of the power semiconductors can be used as a condition for switching between the two thermal models (current rms model and current instantaneous model), whose thermal behavior can essentially be viewed as a first-order delay element. The inverse of the thermal time constant is a thermal cutoff frequency of the power semiconductors.The power loss generated in power semiconductors is directly proportional to the square of the current. Particularly with a sinusoidal current, the fundamental power loss oscillation is twice the current frequency sin(x)²=1 / 2 * (1 – cos(2x)). Thus, for loss frequencies greater than the thermal fundamental frequency of the power semiconductors, these act as a mean value filter, and the mean power loss can be used for calculations. For example, a factor of 100 can be used as a limit. Thus, for a power loss frequency one hundred times greater than the cutoff frequency, the fundamental power loss oscillation is attenuated by 40 dB. This means that the thermal behavior of the power semiconductors / power modules (PM) is like a first-order low-pass filter with a thermal cutoff frequency ^^^^ ^^ℎ^^^^^^^^^^^^ℎ. The power modules act as a mean value filter for a frequency of the power loss >> ^^^^ ^^ℎ^^^^^^^^^^^^ℎ.In addition, the power loss is directly proportional to the square of the current, where the square of the current is: Î2sin2. ⋅ (1 − cos(2 ⋅ (2^^^^^^^^ ⋅ ^^))) → sin2(^^) =⋅ (1 − cos(2^^)). Consequently, a fundamental frequency of the power loss is 2 ⋅ ^^ . Thus, electrical1 ^^^^ frequencies >2⋅ ^^^^ ^^ℎ^^^^^^^^^^^^ℎ are significantly thermally damped → , where at very high attenuation ^^ > 40dB only the average value ^^ of the power loss ( ) ( )2 ^̅^ remains, namely ^^^^ ^^ = ^^^^^^^^^^ ⋅ (Î sin ^^^^ )= frequencies can be calculated using the mean square of the current (squared effective value). For low electrical frequencies ^^^^^^ ≤ 50 ⋅ ^^^^ ^^ℎ^^^^^^^^^^^^ℎ → ^^^^^^ ≤ 100 ⋅ ^^^^ ^^ℎ^^^^^^^^^^^^ℎ (attenuation < 40dB), the instantaneous power loss^^^^(^^) must be used in the calculation, and for high electrical frequencies ^^^^^^ > 50 ⋅ ^^^^ ^^ℎ^^^^^^^^^^^^ℎ →^^^^^^ > 100 ⋅ ^^^^ ^^ℎ^^^^^^^^^^^^^^ℎ (attenuation > 40dB), the average power loss ^^^^(^^) may be used in the calculation. The disclosure is explained below with the aid of drawings: Figure 1 shows a schematic representation of a power electronics system according to the present disclosure, and Figures 2 and 3 show different temporal profiles of a temperature of power semiconductors at various electrical frequencies of the power electronics. The figures are merely schematic in nature and serve exclusively to understand the present disclosure.Identical elements are identified by the same reference numerals. Fig. 1 shows a schematic representation of power electronics 1 according to the present disclosure. The power electronics 1 is for use in an electrical machine, in particular in an electric drive motor of a motor vehicle. The power electronics 1 has controllable power semiconductors 2 and a thermal control unit 3 for thermally monitoring the power semiconductors 2. The thermal control unit 3 is configured to select a thermal model M1, M2 for calculating a power loss of the power semiconductors 2 as a function of a frequency f of an electrical output current of the power electronics 1.Preferably, the thermal control unit 3 can be configured to calculate the power loss of the power semiconductors 2 in a first frequency range F1 using a first thermal model M1 and in a second frequency range F2 using a second thermal model M2. In particular, the first thermal model M1 can be a current rms model. This means that in the first thermal model M1, the power loss is calculated using an average value of the current squared in the power electronics 1. In particular, the second thermal model M2 can be an instantaneous current model. This means that in the second thermal model M2, the power loss is calculated using an instantaneous value of the current in the power electronics 1. In particular, the first frequency range F1 can contain higher frequencies f than the second frequency range F2.This means that one application area of ​​the first thermal model M1 is a higher frequency range than the second frequency range F2, and one application area of ​​the second thermal model M2 is a lower frequency range than the first frequency range F1. Figures 2 and 3 show different electrical frequencies f or their loss frequencies of a fundamental oscillation and the junction temperature T in °C of the power semiconductors over time t in seconds. The second thermal model M2 is applied at a first electrical frequency f1 of 0.05 rad / s or a loss frequency of 0.1 rad / s or an attenuation of 0 dB, at a second electrical frequency f2 of 0.5 rad / s or a loss frequency of 1 rad / s or an attenuation of 3 dB, and a third electrical frequency f3 of 5 rad / s or a loss frequency of 10 rad / s or an attenuation of 20 dB. From a fourth electrical frequency f4 of 50 rad / s ora loss frequency of 100 rad / s or an attenuation of 40 dB or at a fifth electrical frequency f5 with a loss frequency >> 100 rad / s or an attenuation of 40 dB or more, the first thermal model M1 is applied.

[0002] List of reference symbols 1 Power electronics 2 Power semiconductor 3 Thermal control unit f Frequency f1 First electrical frequency f2 Second electrical frequency f3 Third electrical frequency f4 Fourth electrical frequency f5 Fifth electrical frequency F1 First frequency range F2 Second frequency range M1 First thermal model M2 Second thermal model

Claims

Patent claims 1. Power electronics (1) for an electrical machine, with controllable power semiconductors (2) and a thermal control unit (3) for thermally monitoring the power semiconductors (2), characterized in that the thermal control unit (3) is configured to select a thermal model (M1, M2) for calculating a power loss of the power semiconductors (2) as a function of a frequency (f, F1, F2) of an electrical output current of the power electronics (1).

2. Power electronics (1) according to claim 1, characterized in that the thermal control unit is configured to calculate the power loss of the power semiconductors (2) in a first frequency range (F1) using a first thermal model (M1) and in a second frequency range (F2) using a second thermal model (M2). 3.Power electronics (1) according to claim 2, characterized in that the first thermal model (M1) is a current rms model.

4. Power electronics (1) according to claim 2 or 3, characterized in that the second thermal model (M2) is an instantaneous current model.

5. Power electronics (1) according to one of claims 2 to 4, characterized in that the first frequency range (F1) contains higher frequencies than the second frequency range (F2).

6. Power electronics (1) according to one of claims 2 to 5, characterized in that the first thermal model (M1) requires less computing power than the second thermal model (M2).

7. Power electronics (1) according to one of claims 2 to 6, characterized in that the power electronics (1) has a safety function control unit, wherein the power electronics (1) is configured such that, in the second frequency range (F2), computing power assigned to the safety function control unit is made available to the thermal control unit (3).

8. Power electronics (1) according to one of claims 2 to 7, characterized in that the first frequency range (F1) and / or the second frequency range (F2) are / is defined as a function of a thermal time constant of the power semiconductors (2). 9.Power electronics (1) according to claim 8, characterized in that a loss frequency is twice the current frequency and a thermal cutoff frequency of the power semiconductors (2) is a reciprocal of the thermal time constant of the power semiconductors (2), wherein a boundary between the first frequency range (F1) and the second frequency range (F2) is selected such that the loss frequency is 100 times greater than the thermal cutoff frequency.

10. An electrical machine with power electronics (1) according to one of claims 1 to 9.

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