Method and device for setting the dead time of a half-bridge switching element and inverter

Abstract

The invention relates to a method for setting a dead time between the disconnection of a first switching element (31) of a half-bridge (2) and the closing of a second switching element (32) of the half-bridge (2), the method comprising the following steps: reducing the dead time of a switching cycle relative to the dead time of a previous switching cycle; and determining the temperature of at least one of the switching elements (31, 32); wherein the steps of reducing the dead time and determining the temperature are repeated for subsequent switching cycles until a critical dead time is reached, for which an interruption condition dependent on the determined temperature is satisfied; and wherein the dead time is set taking into account the critical dead time.

Description

Technical Field

[0001] The invention relates to a method and a device for setting a dead time between opening a first switching element of a half-bridge and closing a second switching element of the half-bridge. The invention also relates to an inverter and preferably to an inverter for an electric machine. Background Art

[0002] The motor of the drive system can be supplied with an AC voltage via an inverter. An exemplary control scheme of an inverter is known from the open document DE 10 2011 081173 A1. The inverter includes a plurality of half bridges, wherein the number of the half bridges corresponds to the number of phases of the motor. The half bridges each have two switching elements connected in series, wherein the switching elements must not be closed at the same time to prevent a short circuit. During commutation, it must be additionally noted that the second switching element is not immediately closed after the first switching element is disconnected. Otherwise, due to the delayed formation of the electromagnetic field of the half-bridge power semiconductor, a lateral influence may be generated, which may cause undesirable heating of the half-bridge power semiconductor. For this reason, during the dead time after the first switching element is disconnected, both switching elements remain disconnected.

[0003] In order to ensure efficient energy transmission by the inverter, however, it is desirable to quantify the dead time as narrowly as possible. Summary of the invention

[0004] The present invention provides a method for setting the dead time between the disconnection or opening of a first switching element of a half bridge and the connection or closing of a second switching element of the half bridge. In addition, the present invention provides an apparatus for setting the dead time between the disconnection of the first switching element of a half bridge and the closing of the second switching element of the half bridge. Finally, the present invention provides an inverter.

[0005] Therefore, according to a first aspect, the present invention relates to a method for setting a dead time between the opening of a first switching element of a half bridge and the closing of a second switching element of the half bridge. To this end, the dead time of a switching cycle is reduced relative to the dead time of the previous switching cycle. The temperature of at least one of the switching elements is measured. The reduction of the dead time and the measurement of the temperature are repeated for a long time for subsequent switching cycles until a critical dead time is reached, for which an interruption condition is met. Whether the interruption condition is met is determined with the aid of the measured temperature. The dead time is set taking into account the critical dead time. And the dead time is set by adding the critical dead time to a predefined buffer safety time.

[0006] Therefore, according to a second aspect, the present invention relates to a device for setting a dead time between the disconnection of a first switching element of a half bridge and the closing of a second switching element of the half bridge. The device comprises a control mechanism and a temperature measuring mechanism. The control mechanism is configured to control the half bridge so that the second switching element of the half bridge is closed after the dead time after the disconnection of the first switching element. The control mechanism is configured to reduce the dead time of a switching cycle relative to the dead time of the previous switching cycle, wherein the temperature measuring mechanism measures the temperature of at least one of the switching elements after reducing the dead time. The reduction of the dead time and the measurement of the temperature are repeated for such a long time for the subsequent switching cycles until a critical dead time is reached, for which an interruption condition is met. The interruption condition depends on the measured temperature. The control mechanism preferably sets the dead time dynamically while taking into account the critical dead time. And the dead time is set by adding the critical dead time to a predetermined buffer safety time.

[0007] According to a third aspect, the invention relates to an inverter having a plurality of half-bridges, each of which has two switching elements. The inverter also has the above-described device for adjusting the dead time.

[0008] The present invention also relates to other preferred embodiments.

[0009] Advantages of the present invention

[0010] The invention allows a preferably dynamic adjustment of the dead time for the operation of the switching elements of the half-bridge. For this purpose, the dead time is preferably reduced continuously, that is, it is reduced compared to the dead time of the previous switching cycle. However, the dead time can also be reduced after a predetermined number of switching cycles and thus remain constant during these switching cycles.

[0011] A switching cycle includes a plurality of different switching states or switching positions of the switching elements of the half-bridge, which are typically passed through in a fixed sequence. During the corresponding switching state, the switching elements of the half-bridge remain open or closed respectively. The transition to a new switching state within a switching cycle is carried out by opening or closing one of the switching elements. Each switching cycle preferably includes exactly one dead time between the opening of the first switching element of the half-bridge and the closing of the second switching element of the half-bridge. Another dead time is between the opening of the second switching element and the closing of the first switching element. In addition, for a plurality of half-bridges, corresponding dead times can also be assigned to the half-bridges.

[0012] For example, the first switching element is opened during a switching cycle and the second switching element is closed after the first dead time has expired. Subsequently, the second switching element is opened and the first switching element is closed. Now, in the switching cycle that follows immediately thereafter, the first switching element can be opened again until the second switching element is closed again after the second dead time has expired. The duration of the second dead time can be reduced relative to the duration of the first dead time.

[0013] With the aid of the determined temperature, it is possible to detect whether, due to the reduction of the dead time, thermally effective cross-currents, in particular short circuits, occur between the switching elements. Once this point is reached, the critical dead time is reached, and a further reduction of the dead time leads to an even greater heating of the switching elements, which is to be avoided. With the aid of accurate knowledge of the critical dead time, a value for the dead time that can be used in practice can be set so that it is as small as possible in order to achieve good efficiency of the inverter, that is, the maximum power yield, without undesirable thermal effects occurring.

[0014] The slowly developing degradation of the switching element has an effect on thermal transverse influences and accordingly on the change in temperature when the dead time is changed. Due to the dynamic adjustment of the dead time, these aging effects of the switching element can be compensated.

[0015] The switching element can preferably include a power semiconductor switching element, in particular a metal oxide field effect transistor (MOSFET) or in particular a bipolar transistor with an insulated gate terminal (IGBT).

[0016] According to a preferred development of the method, the temperature determination includes the measurement of the current and / or voltage at the output of the corresponding switching element. If the switching element is a MOSFET, for example the drain current, the drain-source voltage and the gate-source voltage can be measured during the PWM switching cycle using a fast analog-to-digital converter and used to determine the temperature. If the switching element is an IGBT, the analog current and voltage at the gate, collector and emitter are measured.

[0017] According to a preferred embodiment of the method, the temperature of the switching element is determined by means of the measured current and voltage and by means of a predetermined characteristic curve of the corresponding switching element. The voltage and current of the switching element are temperature dependent. The exact relationship between the value of the current or voltage and the corresponding temperature can be stored in a lookup table (LUT) by means of data page values.

[0018] The temperature of the corresponding switching element can thus be determined very quickly by means of a lookup table using the measured current and voltage. As an alternative, the temperature dependence of the current and voltage can be calculated using a physical model. With the aid of a suitable μC circuit, the instantaneous temperature of the switching element can be determined using a corresponding mathematical function and taking into account the characteristic curve of the switching element.

[0019] According to a preferred improvement of the method, the dead time is set by adding a critical dead time to a predetermined buffer safety time. The buffer safety time is preferably selected in such a way that the set dead time is sufficiently far from the critical dead time so that the thermally effective transverse currents between the switching elements are sufficiently eliminated. According to another embodiment, the critical dead time can be multiplied by a predetermined factor greater than 1 in order to obtain an adjusted dead time. According to a preferred improvement of the method, in order to determine the critical dead time, the measured temperature is compared with a predetermined temperature threshold value. If the measured temperature exceeds the predetermined temperature threshold value, the interruption condition is met.

[0020] According to a preferred embodiment, the interruption condition depends on a change in the temperature measured after at least one switching cycle. For example, a change in the temperature difference between two switching cycles following one another can be considered. If the change exceeds a predetermined threshold value, the interruption condition is met.

[0021] According to a preferred development of the method, the determined temperature gradient is compared with a predefined gradient threshold value in order to determine the critical dead time. The determined temperature gradient can be, for example, the change in the determined temperature as a function of time during the shutdown process or also the change in the determined temperature as a function of the dead time. If the gradient exceeds a predefined gradient threshold value, the termination condition is met.

[0022] According to a preferred embodiment, it is possible to determine whether a significant temperature increase occurs within a predetermined measuring time period after the dead time has been changed. For this purpose, the temperature increase can be compared with a predetermined threshold value. The measuring time period can be a few milliseconds to several hundred milliseconds due to the inertial thermal effects that act depending on the measures for heat dissipation of the semiconductor.

[0023] According to a preferred development of the method, the setting of the dead time is performed gradually for all half-bridges of the inverter. For each of the half-bridges, a dead time is set between the opening of the first switching element of the corresponding half-bridge and the closing of the second switching element of the corresponding half-bridge, and furthermore, a dead time is set between the opening of the second switching element of the corresponding half-bridge and the closing of the first switching element of the corresponding half-bridge. For an exemplary B6 bridge consisting of three half-bridges, six different dead times are correspondingly adjusted and set.

[0024] According to a preferred development of the method, the half-bridge is an element for supplying power to a power machine. In particular, the half-bridge can be part of an inverter for such a power machine. The method is preferably carried out while the power machine is operated at zero load, that is, no torque is output or at least only a very small torque is output. The power machine can be a drive for a motor vehicle. Operation at zero load ensures continuous switching of the half-bridge.

[0025] According to a further development of the method, the half-bridge is an element of an inverter, wherein the dead time is set during a switch-on process of the inverter and / or during a switch-off process of the inverter. As a result, the dead time can be adjusted during the operation or service life of the inverter, so that in particular aging effects can be corrected.

[0026] The temperature is preferably determined during the dead time. For example, a continuous temperature curve can be determined by generating a plurality of temperature measurement values ​​and interpolating the temperature measurement values, or only a single measurement value can be determined during the dead time. However, it is also possible to measure the temperature at a time outside the dead time, for example shortly after the dead time has expired.

[0027] According to a preferred development of the device, the temperature determination device comprises sensors for measuring the current and the voltage at the output of the respective switching element.

[0028] According to a preferred development of the device, the control device is designed to determine the temperature of the switching element using the measured current and voltage and taking into account a predefined characteristic curve of the respective switching element. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] It shows:

[0030] Figure 1 A schematic block diagram of a device for setting dead time according to an embodiment of the present invention is shown; Figure 2A schematic block diagram of an inverter according to an embodiment of the present invention is shown;

[0031] Figure 3 A schematic circuit diagram of a half bridge of the inverter is shown; and

[0032] Figure 4 A schematic flow chart of a method for setting a dead time according to an embodiment of the present invention is shown.

[0033] In all figures, identical or functionally identical elements and devices are provided with the same reference numerals. The numbering of the method steps is for the sake of clarity and should generally not imply any particular chronological order. In particular, a plurality of method steps can also be carried out simultaneously. DETAILED DESCRIPTION

[0034] Figure 1 A schematic block diagram of a device 1 for setting a dead time tTD between the opening of a corresponding first switching element 31 and the closing of a corresponding second switching element 32 of a half-bridge 2 is shown. The half-bridge 2 can be part of an inverter, which in turn can be a component of an electric drive system. For this purpose, the inverter is connected to a DC voltage source and converts the DC voltage provided on the input side into an AC voltage, which is provided to a multi-phase motor on the output side. For each phase of the motor, the inverter includes a corresponding half-bridge 2. For example, a three-phase motor can be controlled by means of a B6 bridge consisting of three half-bridges 2.

[0035] The switching elements 31, 32 are arranged in series between the positive input connection of the inverter and the negative input connection of the inverter. The connection point between the first and second switching elements of the half bridge 2 is respectively coupled to the phase connection of the motor. The switching elements 31, 32 can have MOSFETs or IGBTs, for example. However, the present invention is not limited to such elements. Rather, the switching elements can be any semiconductor switching elements.

[0036] The device 1 includes a control mechanism 11 which is configured to control the half-bridge 2 of the inverter. To this end, the device 1 itself can be a component of the inverter or can also be a unit separate from the inverter. The control mechanism 11 transmits a control signal to the corresponding half-bridge 2 so that the second switching element 32 of the half-bridge 2 is closed after the dead time tTD after the first switching element 31 is disconnected. The dead time tTD is fine-tuned by adjusting the switch-on and switch-off times of the switching elements 31 and 32, which can be performed by a software program of a microcontroller. The dead time tTD can be varied in the nanosecond range. The switching edge can be moved on the PWM output end by means of software.

[0037] The switching on and off processes of the switching elements 31, 32 are influenced by a large number of factors. These factors include, for example, the switching on and off delay times of the switching elements and other minimum switching on and off times or dead times necessary for control and blocking, measuring, protection and monitoring functions. In addition, delay times occur in the drive, the temperature dependency and aging behavior of which are unknown.

[0038] For a dead time tTD that is determined too short, undesirable thermal cross-effects may occur. Therefore, the control device 11 is designed to first determine the dead time tTD to an initial value, which is selected to be large enough so that no thermal interference effects are to be expected. Such a value can be determined empirically, for example. The control device 11 is designed to gradually reduce the dead time tTD starting from the initial value.

[0039] Furthermore, the device 1 comprises a temperature determination device 12 which determines the temperature T of the two switching elements 31, 32. For reasons of accuracy, the temperature determination device 12 is preferably designed to detect the temperature of the first switching element 31 in the same manner as for detecting the temperature of the second switching element 32. In order to set the dead time tTD between the opening of the first switching element 31 and the closing of the second switching element 32, the temperature determination device 12 can, for example, determine the temperature of the first switching element 31. However, according to other embodiments, the temperature determination device 12 can also determine the first temperature T1 of the first switching element 31 and the second temperature T2 of the second switching element 32.

[0040] The temperature determination device 12 preferably includes a current sensor and a voltage sensor, which are configured to measure the voltage applied to the output terminals of the switching elements 31, 32 or the current flowing through the output terminals. The sensor elements can in particular measure the drain current, drain-source voltage and gate-source voltage of a MOSFET or the corresponding current and voltage on the gate, collector and emitter of an IGBT. In addition, the device 1 can have a storage device in which characteristic curves of the corresponding switching elements are stored, which characteristic curves indicate the temperature dependence of the switching elements. Using these characteristic curves and the measured current and voltage, the temperature determination device 12 determines the junction temperature of the corresponding switching elements 31, 32.

[0041] The control device 11 monitors the measured temperature during the adjustment of the dead time tTD. For this purpose, the corresponding temperature value or temperature trend can be measured during each dead time tTD. If the absolute temperature or the temperature rise or the change in the measured temperature exceeds a predetermined threshold value for two consecutive dead times, the control device 11 recognizes that the temperature rise is too severe, so that the interruption criterion is met and the critical dead time tK is reached. The control device 11 can especially compare the gradient of the measured temperature with a predetermined threshold value. The threshold value can be determined according to the thermal characteristics of the corresponding switching elements 31, 32. In particular, the temperature sensitivity of the switching elements 31, 32 can be considered for determining the threshold value.

[0042] The control device 11 now determines the new dead time tTD to be predefined or set by adding the critical dead time tK to the predefined buffer safety time. Alternatively, the critical dead time tK can be multiplied by a predefined factor greater than one.

[0043] The control device 11 is designed to subsequently actuate the half-bridge in such a way that the dead time tTD between the opening of the first switching element 31 and the closing of the second switching element 32 is set to the determined value.

[0044] The described adjustment is then preferably repeated for the dead time tTD between the opening of the second switching element 32 and the closing of the first switching element 31. The corresponding dead times tTD are then determined dynamically for all other half-bridges 2.

[0045] exist Figure 2 , a schematic block diagram of an inverter 10 according to an embodiment of the present invention is shown. The inverter 10 includes a plurality of half bridges 2, for example, three half bridges 2 in the form of B6 bridges. Each of the half bridges 2 has a first switching element 31 and a second switching element 32. In addition, the inverter 10 includes the above-described device 1 for setting the dead time tTD between the disconnection of one switching element 31, 32 of the half bridge 2 and the closing of the corresponding other switching element 31, 32 of the half bridge 2.

[0046] exist Figure 3An exemplary B6 bridge circuit as it can be used in the inverter 10 according to the present invention is shown in FIG. The inverter 10 includes three half-bridges 2 connected between a positive high side 51 (T+) and a negative low side 52 (T-). Each of the half-bridges 2 includes a first (high-side) switching element 31 and a second (low-side) switching element 32, wherein the first (high-side) switching element and the second (low-side) switching element have corresponding semiconductor switches 41 and free-wheeling diodes 42 connected in parallel. The semiconductor switch 41 is, for example, a MOSFET or an IGBT. The connection nodes between the first switching element 31 and the second switching element 32 are respectively connected to the phase connectors of the motor. However, the present invention is not limited to the embodiments shown. Therefore, the inverter 10 can have any number of half-bridges 2.

[0047] exist Figure 4 2 shows a flow chart of a method for setting a dead time tTD between the opening of the first switching element 31 of the half bridge 2 and the closing of the second switching element 32 of the half bridge 2 .

[0048] The half bridge 2 can be a component of an inverter 10, which converts the electrical energy provided by a DC voltage source into an AC voltage and supplies it to an electric machine, in particular an electric machine of a motor vehicle. To this end, in a first method step S1, it is ensured that the vehicle is reliably stopped, that is, for example, rolling is prevented by activating the vehicle's handbrake. In addition, the inverter 10 is placed in an operating mode in which no torque or only an insignificant torque is generated on the drive wheels of the vehicle. In this zero-torque control, continuous switching of the switching elements 31, 32 must be ensured at the same time. Therefore, the method is preferably performed at zero load.

[0049] In method step S2 , the dead time tTD between the opening of the first switching element 31 and the closing of the second switching element 32 is set to an initial value which is non-critical for all semiconductor elements.

[0050] In method step S3 , the dead time tTD is gradually reduced while the temperature of at least one of the switching elements 31 , 32 is determined in parallel.

[0051] In method step S4, it is identified whether the change in the dead time tTD causes a significant change in temperature. For this purpose, for example, the temperature or the temperature gradient can be compared with a predetermined threshold value. If the temperature or the temperature gradient, that is, for example, the derivative of the temperature with respect to the dead time tTD does not exceed the threshold value, then the dead time tTD continues to be reduced, S3. Otherwise, in method step S5, it is identified that the critical dead time tK has been reached. The interruption criterion is met and the gradual reduction of the dead time tTD is ended. The dead time tTD is set taking into account the critical dead time tK. For this purpose, it is preferred to add a buffer safety time to the critical dead time tK in order to obtain an adjusted dead time tTD.

[0052] In method step S6, it is checked whether further dead times tTD must be adjusted. For each half-bridge 2, two dead times tTD must be adjusted, on the one hand, the dead time tTD between the opening of the first switching element 31 of the half-bridge 2 and the closing of the second switching element 32 of the half-bridge 2, and on the other hand, the dead time tTD between the opening of the second switching element 32 of the half-bridge 2 and the closing of the first switching element 31 of the half-bridge 2. Method steps S2 to S5 are performed for all half-bridges 2 and for both switching elements 31, 32, respectively. Therefore, in Figure 3 A total of six dead times tTD are set in the B6 half-bridge shown in FIG. If all dead times tTD have been adjusted, the method ends.

[0053] The determined adjusted dead time tTD can be stored in a non-volatile memory and can be taken into account when the method for setting the initial value of the dead time tTD is repeatedly performed. The method is preferably performed again after a predetermined period of time in order to compensate for aging effects and changing ambient conditions. The method can be performed, for example, in each switching process and / or switching process or in a passive phase of the inverter.

Claims

1. A method for setting a dead time between the opening of a first switching element (31) of a half-bridge (2) and the closing of a second switching element (32) of the half-bridge (2), the method comprising the following steps: reducing the dead time of one switching cycle relative to the dead time of a previous switching cycle; and measuring the temperature of at least one of the switching elements (31, 32); wherein the steps of reducing the dead time and determining the temperature are repeated for subsequent switching cycles until a critical dead time is reached, for which a termination condition dependent on the determined temperature is met; and wherein the dead time is set taking into account the critical dead time, The dead time is thereby set by adding the critical dead time to a predefined buffer safety time.

2. The method according to claim 1, wherein the determination of the temperature comprises measuring a current and / or a voltage at an output of the corresponding switching element (31, 32).

3. The method according to claim 2, wherein the temperature of the switching element (31, 32) is determined with the aid of the measured current and voltage of the respective switching element (31, 32) and a predefined characteristic curve.

4. The method according to any one of claims 1 to 3, wherein the interruption condition comprises: The determined temperature exceeds a predefined temperature threshold value. 5 . The method according to claim 1 , wherein the interruption condition is dependent on a change in temperature determined after at least one switching cycle.

6. The method according to any one of claims 1 to 3, wherein the interruption condition comprises: The determined temperature gradient exceeds a predefined threshold value.

7. A method according to any one of claims 1 to 3, wherein the setting of the dead time is gradually performed for all half-bridges (2) of the inverter (10), wherein a first dead time between the disconnection of the first switching element (31) of the corresponding half-bridge (2) and the closing of the second switching element (32) of the corresponding half-bridge (2) and a second dead time between the disconnection of the second switching element (32) of the corresponding half-bridge (2) and the closing of the first switching element (31) of the corresponding half-bridge (2) are set respectively.

8. The method according to any one of claims 1 to 3, wherein the half-bridge (2) is an element of a power machine, and wherein the power machine is operated at zero load when the dead time is set.

9. A method according to any one of claims 1 to 3, wherein the half-bridge (2) is an element of an inverter (10), wherein the dead time is set during a switch-on process of the inverter (10) and / or during a switch-off process of the inverter (10).

10. The method according to claim 1, wherein the temperature of at least one of the switching elements (31, 32) is determined during the dead time.

11. A device (1) for setting a dead time between the opening of a first switching element (31) of a half-bridge (2) and the closing of a second switching element (32) of the half-bridge (2), the device comprising a control device (11) which is designed to control the half-bridge (2) in such a way that the second switching element (31) of the half-bridge (2) is closed after a dead time after the first switching element (31) is opened; and A temperature measuring device (12) configured to: measure the temperature of at least one of the switching elements (31, 32); wherein the control device (11) is configured to reduce the dead time of a switching cycle relative to the dead time of a previous switching cycle, wherein the temperature measuring device (12) is configured to measure the temperature of at least one of the switching elements (31, 32) after reducing the dead time, wherein the control device (11) and the temperature measuring device (12) are configured to repeat the reduction of the dead time and the measurement of the temperature for subsequent switching cycles until a critical dead time is reached for which an interruption condition dependent on the measured temperature is satisfied, and wherein the control device (11) is configured to set the dead time taking into account the critical dead time, The dead time is thereby set by adding the critical dead time to a predefined buffer safety time.

12. The device (1) according to claim 11, wherein the temperature determination device (12) comprises a sensor (13) for measuring the current and / or the voltage at the output of the corresponding switching element.

13. A device (1) according to any one of claims 11 or 12, wherein the control device (11) is constructed to: determine the temperature (T) of the switching element (31, 32) with the help of the measured current and voltage of the corresponding switching element (31, 32) and a predetermined characteristic curve.

14. An inverter (10), the inverter having a plurality of half bridges (2), each of which has two switching elements (31, 32); and the inverter having a device (1) according to any one of claims 11 to 13, the device being used to set a dead time between opening and closing of the corresponding switching elements (31, 32) of the half bridge (2).

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