DE102024108004A1 - Power electronics and electrical machine - Google Patents

Abstract

The present disclosure relates to power electronics (1) for an electrical 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 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). Furthermore, the present disclosure relates to an electrical machine comprising such power electronics (1).

Description

The present disclosure relates to power electronics for an electric machine, in particular for an electric drive motor of a (motor) vehicle. Furthermore, the present disclosure relates to an electric machine having such power electronics.

In electrical drive technology, power electronics is used to control or regulate electrical machines or traction drives, or for energy conversion. Modulation methods based on space vector pulse width modulation (SVPWM) are used. These commutations utilize power semiconductor switches, such as metal oxide semiconductor field-effect transistors (MOSFETs) or insulated-gate bipolar transistors (IGBTs), which are switched on or off depending on the position of the space vector. These commutation processes follow a mathematically predetermined pattern defined by a control algorithm and the modulation method, thereby controlling the state of the power semiconductors—namely, conducting or non-conducting.

Depending on the mechanical speed of the electrical machine or the frequency of the electrical output current of the power electronics, the dwell time in a commutation pattern of the power semiconductors in a rotating machine changes. The dwell time is shorter the higher the speed or frequency (i.e., the faster the machine rotates) and longer the lower the speed or frequency (i.e., the slower the machine rotates). In a stationary machine, the commutation pattern remains constant.

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 more 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 occur due to slowly changing or static commutation patterns. This could lead to overheating of the power semiconductors, which in turn significantly lowers the achievable torque of the electric machine 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 moment when 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. This is assumed to be a requirement of a few seconds, such as 5 seconds, and these operating points occur relatively rarely. To remedy this condition, however, it is possible to size or oversize the power semiconductors. This, however, requires increased silicon usage (in the form of larger or multiple parallel chips), which in turn leads to increased costs and is therefore disadvantageous.

Against this background, the present disclosure is based on the object of avoiding or at least mitigating 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 simultaneously be manufactured cost-effectively.

This problem is solved by a power electronics system having the features of the independent patent claim and by an electric machine having the features of the subordinate patent claim. Advantageous further developments are the subject of the dependent claims.

Accordingly, the present disclosure relates to power electronics for an electric machine, in particular for an electric drive motor of a motor vehicle, comprising power semiconductors whose state can be controlled, in particular as a function of the position of a space vector or a 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, the dwell time, the 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 level.

According to the preferred embodiment, the first thermal model can be a current rms model. This means that in the first thermal model, the power loss is calculated based on the mean 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 model. This means that in the second thermal model, the power loss is calculated based on an instantaneous current value of the power electronics. This ensures efficient thermal monitoring of the power semiconductors in this area.

According to the preferred embodiment, the first frequency range can contain higher frequencies than the second frequency range. This means that an application range of the first thermal model is a high frequency or speed range (or a higher frequency range compared to the second frequency range), and an application range 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 computationally intensive than the first thermal model. In particular, if the application area of the second thermal model is a low frequency or speed range, higher computing power can be made available for the second thermal model.

According to a preferred embodiment, the power electronics can include 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 configured 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 can be distributed as needed or depending on the frequency.

According to a preferred embodiment, the first frequency range and/or the second frequency range can be defined as a function of 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 semiconductor can ter 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 machine of a (motor) vehicle, with 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 power semiconductors and 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. Thus, particularly at low speeds and high currents, such as when starting on a hill or at a curb, where safety functions such as ESP, collision warning, etc. require less computing power than at high speeds, computing power can be shifted to the thermal model. At high speeds, a thermal model with low computing requirements can be used to make the 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 effective value model and current instantaneous value model), the thermal behavior of which 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 occurring in the power semiconductors is directly proportional to the square of the current. In particular, for a sinusoidal current, the fundamental oscillation of the power loss is twice the frequency of the current 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 higher than the cutoff frequency, the fundamental attenuation of the power loss 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 of f _{g thermal} . The power modules act as a mean value filter for a power loss frequency of f _{g thermal} . Furthermore, the power loss is directly proportional to the square of the current, where the square of the current is: ${\widehat{\text{I}}}^2{\text{sin}}^2\left(2\pi f_{el}\cdot t\right)={\widehat{\text{I}}}^2\cdot \frac{1}{2}\cdot \left(1-\text{cos}\left(2\cdot \left(2\pi f_{el}\cdot t\right)\right)\right)\to {\text{sin}}^2\left(x\right)=\frac{1}{2}\cdot \left(1-\text{cos}\left(2x\right)\right).$ $$ Consequently, the fundamental frequency of the power loss is 2 · f _el . This means that electrical $\text{Frequenzen}>\frac{1}{2}\cdot f_{gthermisch}$ significantly thermally dampened → $\zeta >\frac{1}{\sqrt{2}},$ where at very high attenuation ζ > 40dB only the mean value ${\overline{P}}_v$ the power loss remains, namely $P_v\left(t\right)=R_{dson}\cdot {\left(\widehat{\text{I}}\text{ sin}\left(\omega t\right)\right)}^2=R_{dson}\cdot \frac{1}{2}\cdot {\widehat{\text{I}}}^2\cdot \left(1-\text{cos}\left(2\pi f_{el}t\right)\right)\text{ bzw}\text{. }{\overline{P}}_v=\frac{R_{dson}}{T}{\displaystyle \int {}_0^T{\left({\widehat{\text{I}}}^2\text{sin}\left(\omega t\right)\right)}^{2}dt={\left(\sqrt{\frac{R_{dson}}{T}{\displaystyle \int {}_0^T{\left({\widehat{\text{I}}}^2\text{sin}\left(\omega t\right)\right)}^{2}dt}}\right)}^2}=R_{dson}\cdot I_{rms}^2=R_{dson}\cdot \frac{1}{2}\cdot {\widehat{\text{I}}}^2.$ $$ $$ Thus, for high frequencies, the mean square of the current (squared effective value) can be used for calculations. For low electrical frequencies f _el ≤ 50 · f _{g thermal} → f _Pv ≤ 100 · f _{g thermal} (attenuation < 40 dB), the instantaneous power loss P _v (t) must be used for calculations, and for high electrical frequencies f _el > 50 · f _{g thermal} → f _Pv > 100 · f _{g thermal} (attenuation > 40 dB), the average power loss P _v (t) can be used.

• 1 zeigt eine schematische Darstellung einer Leistungselektronik gemäß der vorliegenden Offenbarung, und
• 2 und 3 zeigen unterschiedliche zeitliche Verläufe einer Temperatur von Leistungshalbleitern bei verschiedenen elektrischen Frequenzen der Leistungselektronik.

The disclosure is explained below with the help of drawings:

• 1 shows a schematic representation of a power electronics according to the present disclosure, and
• 2 and 3 show different temporal profiles of a temperature of power semiconductors at different electrical frequencies of the power electronics.

The figures are merely schematic in nature and serve solely to facilitate understanding of the present disclosure. Like elements are identified by like reference numerals.

1 shows a schematic representation of a power electronics system 1 according to the present disclosure. The power electronics system 1 is used in an electrical machine, in particular in an electric drive motor of a motor vehicle.

The power electronics 1 comprises 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 to calculate it 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 based on the mean square of the current of 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 based on an instantaneous current value of 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 an application range of the first thermal model M1 is a higher frequency range than the second frequency range F2, and an application range of the second thermal model M2 is a lower frequency range than the first frequency range F1.

2 and 3 represent different electrical frequencies f or their loss frequencies of a fundamental oscillation and the junction temperature T in °C of the power semiconductor over time t in seconds. 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, the second thermal model M2 is applied. From a fourth electrical frequency f4 of 50 rad/s or a 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.

List of reference symbols

1

Power electronics

2

Power semiconductors

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 (10)

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 set up 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). Power electronics (1) to Claim 1 , characterized in that the thermal control unit is arranged 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). Power electronics (1) to Claim 2 , characterized in that the first thermal model (M1) is a current rms model. Power electronics (1) to Claim 2 or 3 , characterized in that the second thermal model (M2) is an instantaneous current model. Power electronics (1) according to one of the Claims 2 until 4 , characterized in that the first frequency range (F1) contains higher frequencies than the second frequency range (F2). Power electronics (1) according to one of the Claims 2 until 5 , characterized in that the first thermal model (M1) requires less computing power than the second thermal model (M2). Power electronics (1) according to one of the Claims 2 until 6 , characterized in that the power electronics (1) has a safety functions control unit, wherein the power electronics (1) is set up such that in the second frequency range (F2) a computing power assigned to the safety functions control unit is made available to the thermal control unit (3). Power electronics (1) according to one of the Claims 2 until 7 , characterized in that the first frequency range (F1) and/or the second frequency range (F2) are/is determined as a function of a thermal time constant of the power semiconductors (2). Power electronics (1) to Claim 8 , characterized in that a loss frequency is twice the current frequency and a thermal cut-off frequency of the power semiconductors (2) is an inverse 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 cut-off frequency. Electrical machine with power electronics (1) according to one of the Claims 1 until 9 .