DE102020124387A1 - Power electronic device, method for its operation and motor vehicle - Google Patents

Abstract

The invention relates to a method for operating a power electronic device with at least two series-connected power transistors, in particular a converter, such a power electronic device and a motor vehicle equipped with it. In the method, the series-connected power transistors of the power electronic device are controlled, the drain-source capacitances of the two power transistors being operated as a voltage divider by recharging the drain-source capacitance of at least one of the power transistors depending on the respective drain-source voltages. As a result, a maximum voltage to which the two power transistors are exposed can be reduced.

Description

The present invention relates to a method for operating a power electronic device, such a power electronic device and a motor vehicle equipped therewith.

Power electronic components and applications are becoming increasingly widespread in many technical areas. An example of this is the field of vehicle technology, in which increasing electrification is being sought. With increasing spread in safety-relevant areas and applications, reliability is of particular importance, in addition to the always sought-after lower costs and lower installation space requirements and weight. Influencing factors that may have been less relevant in the past can arise or gain in importance, both as a result of the spread in new areas of application and in the course of technical further development to meet ever-increasing requirements.

For example, in the DE 10 2018 221 022 A1 the phenomenon of so-called range anxiety, which can be observed in connection with electric vehicles, is addressed. In order to cope with this problem, there is proposed a power conversion device for charging a vehicle battery and a method of controlling the same. The device includes, in addition to the battery, a motor for receiving a battery power source from the battery. Further, the device includes a power controller for controlling the motor to change the battery power source to charging power, and a charging controller for selectively performing charging power supply control for supplying the charging power to the outside and charging power receiving control for receiving external power. This aims to efficiently implement a DC charging service for sharing battery power between electric vehicles.

However, there are also effects that occur within power electronic components and can affect their reliability. These include, for example, overvoltages or single event upsets that can occur as a result of the effects of cosmic radiation, and the like. A solution for this can consist, for example, in the use of power electronic components or components with a higher voltage blocking capability, ie a higher breakdown voltage. However, this can have the disadvantage of higher losses during operation due to an increased contact resistance or higher costs and a higher installation space requirement.

The object of the present invention is to enable particularly reliable operation of power electronics.

According to the invention, this object is achieved by the subject matter of the independent patent claims. Possible refinements and developments of the present invention are specified in the dependent patent claims, in the description and in the drawing.

A method according to the invention is used to operate a power electronic device, ie a device or component that deals with the conversion of electrical energy using switching electronic components, in particular for supplying power to a connected load. The electronic power device includes at least one half-bridge with two series-connected power transistors. An electrical phase, ie a supply line for the connected consumer, ie for supplying an electrical load, can for example be connected to a connecting point or node between these two power transistors. In a charging mode for charging a battery, the power transistors can be at least partially exposed to a battery voltage of the battery to be charged. The power transistors can therefore be exposed to the battery voltage at least temporarily and/or at least partially. This can be problematic since the failure rate of the power transistors available today can increase exponentially as the applied voltage increases. The failure rate, also known as FIT (Failures in Time), can indicate, for example, the number of failures per 1,000,000,000 hours of operating or load time.

According to the method according to the invention, the power transistors are controlled in the charging mode, ie operated or switched. In this case, a respective drain-source voltage of the two power transistors is monitored, that is to say the respective drain-source voltage which is present at the power transistors or which drops across them is measured.

In the method according to the invention, the drain-source capacitances of the power transistors are operated by recharging the drain-source capacitance of at least one of the power transistors as a function of the measured drain-source voltages as a voltage divider for the voltage present at the electronic power device, i.e. approximately the battery voltage , so that the two power transistors are or will be exposed to at most a predetermined proportion of the battery voltage.

In other words, the drain-source capacitances are adjusted by the charge reversal in such a way that neither of the two power transistors is exposed to at least almost the full battery voltage. In this case, for example, a voltage divider ratio of approximately 50% or, for example, in the range from 25% to 75% can be sought or set. It can thus be achieved that during the charging operation the voltages applied to the two power transistors are not too different or remain in a predetermined range or ratio, in particular do not become greater than a predetermined threshold value. The charge reversal of the at least one drain-source capacitance or a degree of this charge reversal, ie charging and discharging, of at least one of the drain-source capacitances can be carried out or adjusted automatically by a corresponding circuit or controller.

Even with power transistors that are nominally the same, i.e. with the same specifications, they can still differ in their electrical or electronic properties. For example, the power transistors can utilize predetermined tolerances to different extents and/or have different temperatures during operation and thus have different resistances or different leakage currents, for example. This can mean that even with slightly different properties, in particular leakage currents, of the two power transistors of the half-bridge, only one of these two power transistors would be exposed to at least almost the full battery voltage. As already mentioned, this can lead to a problematic increase in the failure rate, for example due to the effects of cosmic radiation during charging, especially with the higher battery or charging voltages that are aimed for nowadays.

This problem arises because during the charging operation the electronic power device, for example a converter, often remains electrically connected to the battery to be charged and is therefore subjected to a corresponding voltage load. In principle, to avoid this problem, a mechanical switch could be placed between the battery to be charged and the electronic power device and opened or disconnected during the charging operation. However, this disadvantageously involves additional costs and an increased safety risk elsewhere, for example since such a mechanical switch could open during power operation in which a consumer or an electrical load is supplied with power by the battery via the electronic power device.

The failure rate behind the power transistors and thus the electronic power device can be reduced by the present invention without introducing an additional safety risk.

In one possible embodiment of the present invention, at least one of the power transistors, in particular both power transistors, is operated during the charging operation in the active range--also referred to as the linear range or normal operation--that is to say it is driven. A variable gate voltage is used, which is automatically varied by a feedback loop in order to operate the power transistors as a voltage divider, as described, or to limit the voltage acting on an individual voltage divider. Here the gates of the power transistors are switched in such a way that the power transistors are or remain in the linear range or amplification range (forwardactive region). In the case of a bipolar transistor, for example, this means that the source or emitter diode is operated in the forward direction and the drain or collector diode is operated in the reverse direction. In the active range, ie in normal operation, the collector current is at least approximately given by the base current multiplied by the current amplification factor. The same applies to MOSFETs, for example. In the active range, that is to say in normal operation, the respective power transistor is usually only operated in the range in which the amplification is approximately linear. In the active range, i.e. in normal or boost mode, switching can be carried out particularly quickly in order to avoid delays caused by saturation effects. As a result, it is possible to react particularly quickly to differences or changes, and effective operation, that is to say for example a movement, of a load connected to the electronic power device can also be avoided by correspondingly rapid switching. The gate voltage can vary within a predetermined range, ie it can be controlled or regulated, in particular continuously. As a result, fluctuations or changed conditions can be compensated for dynamically and the load on the power transistors can be limited in a particularly flexible and reliable manner. For example, 0 V to 2 V can be specified as the voltage range for the gate voltage. Depending on the application or design, however, other gate voltages can also be possible or used. For example, an analog amplifier circuit can be used to vary or regulate the gate voltage, which can then be part of the electronic power device.

In another possible embodiment of the present invention, at least one of the power transistors, in particular both power transistors or the electronic power device, is operated in a pulsed manner with a predetermined gate voltage and with a predetermined pulse or switching frequency. In this case, the switching frequency can in particular be specified so large that in each individual half period the charge-reversed drain-source capacitance is only partially, ie not fully, charged. Here, the same fixed predetermined gate voltages can be used during operation of the power transistors or for or in each switching operation. For example, the gate voltage can be switched on at +15 V and switched off at -5 V, or the power transistors can be switched on or kept switched off at these voltage values. The power transistors can be operated with a time-average output power of at least almost zero, in particular with a square-wave pulse signal that varies symmetrically around zero. The power transistors can thus be switched on and off here in such a way that an average or integral of an output signal from the power transistors is minimal, in particular at least almost zero. The power transistors can thus be switched without a load connected on the output side being supplied with a net power on average over time. If, for example, an electrical machine is connected as a load to the electronic power device, then no torque is generated in or by this machine. In other words, the power transistors are switched here with a modulation index of zero. In this way, a separation, ie an additional switch, between the electronic power device and the load can be dispensed with and the method according to the invention can also be used when the respectively connected load is at a standstill or in an idle state. Likewise, other voltage values and/or signal forms can be used, for example depending on the application or design.

In one possible development of the present invention, a pulse duration and/or pulse or switching frequency used to drive the power transistors are selected as a function of a load connected to the electronic power device such that a starting inertia of the load is not overcome. In other words, the pulses given to the power transistors for pulsed operation are set so short or the corresponding pulse frequency is set so high that the connected load remains at least essentially in its current state. If, for example, an electrical machine is connected to the electronic power device as a load, then there is no movement or no torque of the electrical machine. The pulse or switching frequency can also be adjusted to minimize current ripple. In this way, the method according to the invention can be used in a particularly simple manner, that is to say with minimal additional effort, without undesirable effects ultimately occurring.

In one possible embodiment of the present invention, the recharging of the at least one drain-source capacitance is only started, that is to say only automatically activated, when the battery voltage is above a predetermined threshold value. In other words, when the battery voltage is below the predefined threshold value, the charge reversal of the at least one drain-source capacitance can be dispensed with. As a result, energy can be saved overall and a circuit load on the electronic power device can be reduced. The threshold value can be specified here, in particular as a function of a breakdown voltage of the power transistors. If the power transistors have a higher breakdown voltage, that is to say a higher dielectric strength or voltage blocking capability, a correspondingly higher threshold value can be specified. As a result, the corresponding outlay on circuitry can be saved in a voltage range—that is to say at a battery voltage—where ultimately the failure rate is not significantly increased or influenced by cosmic radiation or similar effects.

In a possible development of the present invention, the threshold value is more than 50% and less than 95% of the breakdown voltage of the power transistors, preferably at least 65%, particularly preferably at least 80%. A standard or system voltage currently used in the vehicle sector is, for example, 800 V. In corresponding 800 V systems, however, the battery voltage is not always exactly 800 V, but can vary more depending on the design, state of charge, age of the battery and the like, for example between 450 V and 850 V or about 900 V. If, for example, power transistors with a breakdown voltage of about 1200 V are then used, the method according to the invention, i.e. the charge reversal of the drain-source capacitances, is only used when the battery voltage reaches a predetermined threshold value of, for example, 750 V or 780 V.

A further aspect of the present invention is a power electronic device, in particular for a motor vehicle. The power electronic device according to the invention has at least one half-bridge of two power transistors connected in series and a control circuit for measuring the drain-source voltage of the power transistors and for recharging at least one drain-source capacitance of the power transistors. The power electronic device according to the invention is set up for automatic operation according to at least one embodiment of the method according to the invention. For example, the control circuit can be programmed accordingly. The power electronic device according to the invention can in particular be the power electronic device mentioned in connection with the method according to the invention and accordingly have some or all of the properties and/or features mentioned in this connection.

In one possible embodiment of the present invention, the control circuit is integrated in a gate driver of the electronic power device or in a respective gate driver of the power transistors. A common control circuit can therefore be provided for the two or all of the power transistors. Likewise, an individual control circuit can be provided for each power transistor whose drain-source capacitance is to be recharged by means of the control circuit. The integration of the control circuit in the gate driver or drivers represents a particularly effective and space-saving implementation option.

In one possible embodiment of the present invention, the electronic power device comprises a converter with at least three half-bridges with or consisting of two power transistors each connected or arranged in series. The control circuit or control circuits is or are set up for charge reversal of the drain-source capacitances of the power transistors or at least one power transistor of all three half-bridges. In the present case, therefore, an application or use of the power electronic device according to the invention or of the present invention as a whole can be provided or possible, for example in a machine or drive train. In particular, the converter can be connected between a battery of an electric motor vehicle and its electric drive or traction machine. The failure rate of the power transistors can then be reduced by the present invention without a mechanical separation of the converter from the battery being necessary during the charging operation. The converter or its power transistors can therefore be exposed to the battery voltage during the charging operation, as a result of which the corresponding outlay on hardware or circuitry can be avoided. Since in this case the converter or its power transistors can be exposed to the battery voltage during charging of the battery for significantly longer than when it is not charging - at least considered over the service life of the respective motor vehicle - the present invention can achieve a particularly large and effective reduction in the failure rate can be achieved.

Another aspect of the present invention is a motor vehicle that has a power electronic device according to the invention, a battery connected thereto—in particular as a traction battery—and an electric machine that can be supplied by the power electronic device—in particular as a traction motor or traction drive. Accordingly, the motor vehicle according to the invention is also set up to carry out the method according to the invention. The motor vehicle according to the invention can in particular be the motor vehicle mentioned in connection with the method according to the invention and/or in connection with the electronic power device according to the invention. Accordingly, the motor vehicle according to the invention can have some or all of the properties and/or features mentioned there.

Further features of the invention can result from the claims, the drawing and the description of the figures. The features and feature combinations mentioned above in the description and the features and feature combinations shown below in the description of the figures and/or in the figures alone can be used not only in the combination specified in each case, but also in other combinations or on their own, without going beyond the scope of the invention to leave.

The drawing shows in 1 a schematic representation of an electric motor vehicle with a particularly reliable electronic power device.

For example, in the field of vehicle technology, the use of 800 V systems, ie electrical devices that are set up or designed for a charging, battery and/or operating voltage of approximately 800 V, is currently being sought. However, an actual battery voltage can vary, for example as a function of a number of battery cells actually connected in series, a current state of charge and the like. In 1 an example of an electric motor vehicle 10 is shown schematically and in part or as a detail. The motor vehicle 10 here has a battery 12, a converter 14 which is electrically connected thereto and an electric one which can be supplied with electric power via this converter machine 16 on. The battery 12 can be a traction battery and the electric machine 16 can be a traction or drive machine of the motor vehicle 10 here. Ultimately, wheels 18 of motor vehicle 10 can be driven by electric machine 16--as indicated schematically here.

The converter 14 is a power electronic device or part of a power electronic device of the motor vehicle 10 and comprises, indicated here schematically, three half-bridges 20 each consisting of two power transistors 22 connected in series. The power transistors 22 can, for example, be SiC MOSFETs or IGBTs, preferably with a breakdown voltage of about 1200 V be trained. For the sake of clarity, only some of the power transistors 22 and components of one of the power transistors 22 or components assigned to one of the power transistors 22 are identified here. In principle, however, all the power transistors 22 of the converter 14 or of the half-bridges 20 can be constructed in the same way.

The power transistors 22 have a drain 24, a gate 26 and a source 28 here. In this case, a drain-source capacitance 30 is provided or arranged between the drain 24 and the source 28 in each case. The power transistors 22 are powered by a respective gate driver 32 .

The battery 12, the converter 14 and the electric machine 16 can be designed here as an 800 V system, for example. The battery 12 can therefore have a nominal battery voltage of 800 V, for example, or 850 V, for example. Despite the breakdown voltage of the power transistors 22, which is nominally well above this battery voltage at 1200 V, exposing them to the full battery voltage can have a major influence on the failure rate of the power transistors 22. On the one hand, switching the power transistors 22 when the full battery voltage is present, in particular with relatively large current increases dl/dt, can lead to a relatively high voltage stress load on the power transistors 22 . This stress can be caused by overvoltage peaks, which are a result of a relatively high inductance of a comment loop and an intermediate circuit capacitor—not shown here for the sake of clarity. On the other hand, the susceptibility of the power transistors 22 to errors caused by cosmic radiation, such as single event upsets (SEU), increases exponentially with the applied voltage, for example the battery or intermediate circuit voltage. Thus, with the full battery voltage applied to the power transistors 22, the failure rate due to cosmic rays can reach a critical level.

In general, the converter 14 can be exposed to the current battery voltage in the charging operation, in which the battery 12 is charged, even if the power transistors 22 do not switch or are not switched. In the ideal case, this can be unproblematic if the two power transistors 22 in each case within a half-bridge 20 were electrically identical. In this case, the voltage distribution across the two power transistors 22 would be the same within each half-bridge 20, so that each of the power transistors 22 would effectively only be exposed to half the battery or intermediate circuit voltage. In practice, however, this is not the case. Realistically, the power transistors 22 will differ, for example have different leakage currents, have different temperatures and the like. This can lead to one of the two power transistors 22 being exposed at least almost to the full battery or intermediate circuit voltage within a half-bridge 20 . As described, this can lead to a correspondingly high failure rate.

In order to counteract this problem, a respective control circuit is integrated into the gate driver 32 in the present case. The voltages present at the power transistors 22 are thus measured and the drain-source capacitances 30 are charged or recharged in such a way that differences between the two power transistors 22 within a half-bridge 20 are compensated for. The drain-source capacitances 30 then function as a voltage divider. This voltage division does not have to be exact in order to achieve a significant reduction in the maximum voltage applied to the power transistors 22 and thus in the failure rate, so that these effects can be achieved with particularly little effort.

The reduction in the voltage achieved in this way, to which the power transistors 22 are actually exposed, during charging operation can be particularly useful because over the entire lifetime of the converter 14, the charging operation accounts for the largest portion of a total load time during which the power transistors 22 are exposed to high voltages , can make out. For example, there can be relatively long periods of time during which the battery 12 is at least almost fully charged and is permanently kept at its maximum voltage level by a trickle charge.

Likewise, during the charging operation, the power transistors 22 can be switched with a modulation index of 0 and a duty cycle of 0.5 or 50%, for example using a rectangular pulse signal that varies symmetrically around 0. Since the power or the current on the output side of the converter 14 is 0 on average over time, no torque is generated in the electric machine 16 . This switching method can therefore also be used when the motor vehicle 10 is stationary while the battery 12 is being charged. Such a switching operation of the power transistors 22 can be implemented, for example, by a corresponding control device, for example also the control circuit integrated in the gate driver 32, which can generate a PWM signal for switching the power transistors 22 or the converter 14 with modulation index 0. A switching frequency used in this case, ie, for example, a frequency of a corresponding pulse width modulation, can be so high that, due to the short duration of a respective current flow in one direction, complete charging of the drain-source capacitances is avoided. A feedback circuit can be implemented in order to only start or activate this switching of the power transistors 22 or the converter 14 above a predetermined threshold value of the battery or intermediate circuit voltage, for example only above 780 V. To minimize losses and a Current or output signal ripple, a switching frequency can be adjusted automatically. Likewise, the power transistors 22 can be operated in the active range—that is to say continuously instead of in a pulsed manner—in which case a gate voltage can then be regulated in a predetermined voltage range via a feedback loop.

Overall, the examples described show how an error rate or failure rate due to cosmic radiation can be reduced during charging operation in a power electronic device.

Reference List

10

motor vehicle

12

battery

14

converter

16

electric machine

18

wheels

20

half bridges

22

power transistors

24

drainage

26

Gate

28

Source

30

drain-source capacitance

32

gate driver

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102018221022 A1 [0003]

Claims (10)

Method for operating a power electronic device (14), which comprises at least one half-bridge (20) with two series-connected power transistors (22), wherein in a charging operation for charging a battery (12) in which the power transistors (22) at least partially one Battery voltage of the battery to be charged (12) are exposed, a respective drain-source voltage of the two power transistors (22) is monitored and the drain-source capacitance (30) of the two power transistors (22) by recharging the drain-source capacitance (30) at least one of the power transistors (22) is operated as a voltage divider for the battery voltage depending on the drain-source voltages, so that the two power transistors (22) are each exposed to at most a predetermined portion of the battery voltage. procedure after claim 1 , characterized in that at least one of the power transistors (22) is operated during charging in the active region with a variable gate voltage, the gate voltage being varied by a feedback loop in a predetermined voltage range in order to use the power transistors (22) as a voltage divider operate. procedure after claim 1 , characterized in that at least one of the power transistors (22) is operated in a pulsed manner with a predetermined gate voltage and a predetermined switching frequency, in particular for a complete charging of the recharged drain-source capacitance in each period which is too high, in particular with a time-averaged output power of at least close to zero. procedure after claim 3 , characterized in that a pulse duration and/or pulse frequency used to drive the power transistors (22) are selected as a function of a load connected to the electronic power device (14) in such a way that a starting inertia of the load is not overcome. Method according to one of the preceding claims, characterized in that the charge reversal of the at least one drain-source capacitance (30) is only started when the battery voltage is above a predetermined threshold value. procedure after claim 5 , characterized in that the threshold is more than 50% and less than 95% of the breakdown voltage of the power transistors (22). Electronic power device (14), in particular for a motor vehicle (10), having at least one half-bridge (20) made up of two series-connected power transistors (22) and a control circuit (32) for measuring the drain-source voltage of the power transistors (22) and for recharging at least one drain-source capacitance (30) of the power transistors (22), the electronic power device (14) being set up for operation in accordance with a method according to one of the preceding claims. Power electronic device (14) after claim 7 , characterized in that the control circuit (32) is integrated in a gate driver (32) of the electronic power device (14). Power electronic device (14) after claim 7 or 8th , characterized in that the electronic power device (14) comprises a converter (14) with at least three half-bridges (20) each with two series-connected power transistors (22) and the control circuit (32) for recharging the drain-source capacitances (30) the power transistors (22) of all three half-bridges (20) are set up. Motor vehicle (10), having a power electronic device (14) according to one of Claims 7 until 9 , a battery (12) connected thereto and an electrical machine (16) that can be supplied by the electronic power device (14).

Images