Thermal sensing and control of transistor plates

Abstract

An electric motor circuit includes a motor controller having a plurality of gate drivers and an inverter configured to receive direct current (DC) power from a source and output alternating current (AC) power to an electric motor. The inverter includes a plurality of transistor elements. Each transistor element includes a plurality of transistors arranged on a transistor plate. A plurality of temperature sensors are configured to monitor a plurality of temperatures on each transistor element. The motor controller is configured to vary pulse width modulation (PWM) control of the first subset of transistors on the first transistor element in response to the first subset of temperature sensors detecting a temperature exceeding a threshold temperature and the second subset of temperature sensors not detecting a temperature exceeding a threshold.

Description

Technical Field

[0001] The present invention generally relates to circuit board design and control, and to the operational control of transistors on a circuit board. Background Art

[0002] Electric vehicles and hybrid electric vehicles include direct current (DC) power storage elements that provide operating power to motors. The motors, in turn, drive corresponding wheels to rotate. In order to provide the correct power from the DC power storage elements to the motors, an inverter is used to convert the power into alternating current (AC). The inverter is usually in the form of a network of transistors that are actively controlled to produce a signal that approximates a sine wave.

[0003] In reality, each transistor within an inverter topology or similar circuit is constructed from a transistor board that includes an array of identical transistors electrically connected in parallel, each connected to the same gate command. As used herein, a transistor element refers to a transistor shown in a circuit topology and is understood to include multiple constituent components. During operation, small variations and tolerance differences can result in temperature gradients on the transistor board. Temperature gradients, in turn, lead to hot spots and cold spots.

[0004] Existing temperature monitoring systems use a single temperature sensor located on a transistor board to monitor the temperature of one location. It is assumed that the monitored temperature represents the temperature of the entire transistor board. Systems using this form of monitoring position the sensor at or near the portion of the transistor board that is expected to be hottest during normal operation, and set the overheat threshold at the lowest temperature at which any transistor component may overheat. When the detected temperature exceeds the threshold, the transistor board is deactivated (no current is supplied to the transistor board) until a predetermined condition (such as time, detected temperature threshold, etc.) has occurred. When the transistor board is deactivated, the circuit including the transistor element needs to work without the transistor element, or needs to be completely shut down. In the case of an inverter, this usually results in the inverter being turned off until the transistor element is sufficiently cooled.

[0005] It would therefore be desirable to provide a system that is able to identify and respond to specific hot spots on a transistor board without requiring the circuits including the transistor elements to be deactivated. Summary of the invention

[0006] In an exemplary embodiment, an electric motor circuit includes: a motor controller having multiple gate drivers; an inverter configured to receive direct current (DC) power from a source and output alternating current (AC) power to an electric motor, the inverter including multiple transistor elements; each transistor element including multiple transistors arranged on a transistor board, wherein multiple temperature sensors are configured to monitor multiple temperatures of each transistor element; and wherein the motor controller is configured to change pulse width modulation (PWM) control of a first subset of transistors on a first transistor element of the multiple transistor elements in response to the first subset of temperature sensors detecting a temperature exceeding a threshold temperature and the second subset of temperature sensors not detecting a temperature exceeding the threshold.

[0007] In addition to one or more features described herein, the plurality of transistors on each transistor element are arranged in a thermally symmetric arrangement having multiple sets of thermally symmetric regions.

[0008] In addition to one or more features described herein, the thermally symmetric arrangement includes a single row linear array.

[0009] In addition to one or more features described herein, the thermally symmetric arrangement includes a multi-row linear array, and wherein each thermally symmetric region includes transistors in more than one row of the multi-row linear array.

[0010] In addition to one or more features described herein, a number of temperature sensors in the plurality of temperature sensors is equal to a total number of thermally symmetric regions.

[0011] In addition to one or more features described herein, a number of temperature sensors in the plurality of temperature sensors is equal to a total number of the sets of thermally symmetric regions.

[0012] In addition to one or more features described herein, each set of thermally symmetric regions includes a single temperature sensor.

[0013] In addition to one or more features described herein, varying the PWM control of the first subset of transistors on the first transistor element includes at least one of skipping a pulse and reducing a pulse width of the PWM control of the first subset of transistors.

[0014] In addition to one or more features described herein, changing the PWM control of the first subset of transistors on the first transistor element includes one of maintaining the change for a predetermined number of pulses and periodically changing the pulses according to a set period for the predetermined number of pulses.

[0015] In addition to one or more features described herein, changing the PWM control of the first subset of transistors on the first transistor element includes reducing a pulse width of the PWM control of the first subset of transistors, and the magnitude of the pulse width reduction depends on the magnitude by which the detected temperature exceeds the threshold temperature.

[0016] In another exemplary embodiment, a method of operating a motor drive inverter includes: receiving, at a motor controller, a plurality of temperature readings from a plurality of transistors on a transistor board, the transistor board operating as transistor elements of an inverter; and in response to one of the plurality of temperature readings exceeding an overheat threshold, changing pulse width modulation (PWM) control of a first subset of transistors on the transistor board.

[0017] In addition to one or more features described herein, varying the PWM control includes at least one of skipping one or more pulses and reducing a pulse width of one or more pulses.

[0018] In addition to one or more features described herein, reducing a pulse width of the one or more pulses includes reducing the pulse width by an amount corresponding to an amount by which one of the temperature readings exceeds an overheat threshold.

[0019] In addition to one or more features described herein, varying the PWM control includes skipping at least one pulse at a periodic rate until the temperature reading falls below a second threshold.

[0020] In addition to one or more features described herein, the number of temperature readings in the plurality of temperature readings is equal to the number of thermal symmetry zone groups on the transistor plate, and wherein the motor controller determines the temperature at each thermal symmetry zone in the group of thermal symmetry zones as a single temperature reading.

[0021] In addition to one or more features described herein, the thermally symmetric region includes non-contiguous regions.

[0022] In addition to one or more features described herein, the transistors are located on a transistor board in a single row array.

[0023] In addition to one or more features described herein, the transistors are positioned in a multi-row array on the transistor plate.

[0024] In addition to one or more features described herein, changing the PWM control of a first subset of transistors on the transistor board also includes maintaining standard PWM control of at least a second subset of transistors on the transistor board.

[0025] In addition to one or more features described herein, changing PWM control of a subset of transistors on the transistor board also includes changing a third subset of transistors on the transistor board, the PWM control of the third subset being changed differently than the PWM control of the first subset is changed.

[0026] In another exemplary embodiment, a vehicle includes an energy storage system and a propulsion system including an electric motor, the propulsion system including an electric motor circuit for the electric motor, wherein the electric motor circuit includes: a motor controller having a plurality of gate drivers; an inverter configured to receive direct current (DC) power from a source and output alternating current (AC) power to the electric motor, the inverter including a plurality of transistor elements; each transistor element including a plurality of transistors arranged on a transistor board, wherein a plurality of temperature sensors are configured to monitor a plurality of temperatures of each transistor element; and wherein the motor controller is configured to change a pulse width modulation (PWM) control of a first subset of transistors on a first transistor element of the plurality of transistor elements in response to the first subset of temperature sensors detecting a temperature exceeding a threshold temperature and the second subset of temperature sensors not detecting a temperature exceeding the threshold.

[0027] The above features and advantages and other features and advantages of the present disclosure will become apparent from the following detailed description when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Other features, advantages and details appear, by way of example only, in the following detailed description, which refers to the accompanying drawings, in which:

[0029] Figure 1 is a motor vehicle embodying features of the invention;

[0030] Figure 2 is used for Figure 1 A motor drive circuit of a vehicle;

[0031] Figure 3 According to an example, Figure 2 A transistor board for a motor drive circuit;

[0032] Figure 4 During a heat event Figure 3 Pulse width modulation operation of the transistor board;

[0033] Figure 5 yes Figure 2 A replacement transistor board for a motor drive circuit; and

[0034] Figure 6 During a heat event Figure 6 The transistor board is operated by pulse width modulation. DETAILED DESCRIPTION

[0035] The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or use. It should be understood that in all figures, corresponding reference numerals represent the same or corresponding parts and features. As used herein, the term module refers to a processing circuit, which may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated or group) and memory that executes one or more software or firmware programs, a combinational logic circuit and / or other suitable components that provide the described functions.

[0036] According to an exemplary embodiment, a circuit, such as a motor drive inverter for a vehicle, includes a plurality of transistor elements. Each transistor element is composed of a transistor board, which includes a plurality of electrically parallel transistors physically distributed on the transistor board. A plurality of temperature sensors are disposed on the transistor board, the temperature sensors being located near the transistors and configured to determine the temperature at the plurality of transistors on the transistor board. When a temperature limit is detected at one or more temperature sensors, a controller determines a correspondingly changed pulse width modulation (PWM) operation of the transistor components on the transistor board. The correspondingly changed PWM operation allows the overheated area of ​​the transistor board to cool while allowing the operation of the advanced circuit including the transistor element to continue.

[0037] Continuing to refer to the general systems and operations above, Figure 1 An embodiment of a motor vehicle 10 is shown that is configured to detect and respond to a plurality of different temperatures of transistors on a transistor board. The vehicle 10 includes a vehicle body 12 that at least partially defines an occupant compartment 14. The vehicle body 12 also supports various vehicle subsystems, including a propulsion system 16, a battery system 22, and other subsystems to support the functions of the propulsion system 16 and other vehicle components, such as a braking subsystem, a suspension system, a steering subsystem, a fuel injection subsystem, an exhaust subsystem, etc.

[0038] The vehicle 10 may be an internal combustion engine vehicle, an electric vehicle (EV), or a hybrid vehicle. In an embodiment, the vehicle 10 is a hybrid vehicle that includes an internal combustion engine system 18 and at least one electric motor assembly. For example, the propulsion system 16 includes a first electric motor 20 and a second electric motor 21. The motors 20 and 21 may be configured to drive wheels 23 on opposite sides of the vehicle 10. Any number of motors located in various additional locations around the vehicle 10 may be used to provide mechanical rotation for the corresponding systems and subsystems.

[0039] The battery system 22 may be electrically connected to the motors 20 and 21 and / or other components, such as vehicle electronics. The battery system 22 may be configured as a rechargeable energy storage system (RESS) and include a plurality of battery cells divided into subgroups.

[0040] The battery system controller 24 is included within the battery system 22 and controls the charging and discharging functions of the batteries within the battery system 22. In an alternative configuration, the battery system controller 24 may be a general vehicle controller remote from the battery system 22 and configured to control multiple systems and / or subsystems. The general vehicle controller may be located anywhere within the vehicle 10. In yet another alternative, the controller 24 may be a distributed control system including multiple coordinated controllers throughout the vehicle 10, including controllers within the battery system 22 and controllers remote from the battery system 22.

[0041] In order to provide operating power from the battery system 22 to the motors 20, 21, thereby driving the wheels 23 to rotate, one or more inverters 26 are used to convert the direct current (DC) power provided from the battery system 22 into alternating current (AC) power. The one or more inverters 26 use a network of active switching transistor elements controlled by a gate driver to convert the DC power into an approximately sinusoidal wave using a conventional inverter topology and control system.

[0042] Continue to refer Figure 1 , Figure 2 An exemplary motor drive circuit 100 is shown including a battery system 122 connected to a motor 120 / 121 via an inverter 126. The inverter 126 is controlled by a controller 110. The exemplary inverter 126 is arranged as a basic three-phase inverter, however it should be understood that the systems and processes described herein may be applied to any inverter topology utilizing multiple transistor elements and are not limited to the exemplary topology nor the exemplary inverter application to a motor drive inverter.

[0043] Within the inverter 126 are a plurality of transistor elements 130. Each transistor element 130 is schematically shown as a single transistor having a drain D and a source S. In practice, each transistor element 130 shown in the topology consists of a transistor board 240 (see FIG. Figure 3 )、440(see Figure 5 ) in which a plurality of transistors are arranged electrically in parallel and physically distributed around the transistor plates 240, 440. Similarly, the schematic controller 110 includes a single gate driver 112 for each transistor element 130. It should be understood that in practice, the number of gate drivers 112 for the transistor elements 130 can be increased to provide finer control, the actual number of gate drivers 112 corresponding to the number of control areas on the transistor plates 240, 440, and the number of gate drivers at most equals the number of transistor elements. The controller 110 uses a pulse width modulation (PWM) control signal provided by the gate driver 112, the positive voltage output by the gate driver 112 turns on (closes) the transistor element 130, and the zero voltage output by the gate driver 112 turns off (disconnects) the transistor element 130.

[0044] The transistor element 130, like any other electronic component, is not 100% efficient. A portion of the current flowing through any given portion of the electronics forming the inverter 126 is converted into heat energy. Continued operation of the inverter 126 causes heat to build up, which in turn can lead to an overheat condition. Continued operation of the transistor in an overheat condition can damage the transistor and / or affect the performance of surrounding components. To monitor heat, a temperature sensor 250, 450 (see Figure 3 , 5 ) is included on each transistor element 130, and the controller 110 changes the PWM operation of a given transistor element 130 to allow the transistor element 130 to cool when an overtemperature condition is detected.

[0045] Continue to refer Figure 2 , Figure 3 An example transistor element 130 (corresponding to a gate driver 212 ) is shown as being operated by a controller 210 having a plurality of gate drivers 212 ′, 212 ″, 212 ″′ (collectively referred to as gate drivers 212 ). Figure 2 1 . The transistor element 130 includes five transistors 231, 232, 233, 234, 235 arranged linearly on a physical transistor board 240. The physical transistor board 240 may be a different printed circuit transistor board, a portion of a larger printed circuit transistor board, an application specific integrated circuit (ASIC), a plurality of connected physical boards, or any other physical structure for mounting and connecting the transistors 231, 232, 233, 234, 235.

[0046] During operation, transistors 231, 232, 233, 234, 235 generate heat that accumulates on transistor plate 240. In some examples, due to the physical layout of transistor plate 240, heat accumulates faster in some areas than in other areas, resulting in uneven heat distribution. Transistors located in relatively cool areas of transistor plate 240 can continue to operate even when transistors in hot portions of transistor plate 240 overheat. When transistors are distributed in an area, such as the linear array shown, areas near the center (e.g., near transistor 233) tend to be the hottest, while areas near the edges (e.g., transistors 231 and 235) tend to be the coolest.

[0047] Transistors in corresponding thermal positions are referred to as being in symmetrical positions (e.g., transistors 231, 235 at the edge are symmetrical, and transistors 232, 234 between the edge and the center are symmetrical). Each transistor 231, 232, 233, 234, 235 in a symmetrical position shares a control signal with other transistors 231, 232, 233, 234, 235 that are symmetrical thereto. In addition, symmetrical positions tend to generate heat at the same rate and are referred to as thermal symmetry. Based on this principle, in some examples, a single temperature sensor 250 may be included in each symmetrical region. In addition, thermal symmetry may include any region of the transistor plate 240 that accumulates and releases heat at the same rate, and is not limited to physically symmetrical regions. The thermal symmetry of a given transistor plate layout may be determined by simulation, empirical testing, or a combination of both. Although a temperature sensor mounted on the transistor plate 240 is shown here, it should be understood that an alternative form may be implemented using a temperature sensor that detects temperature rather than a directly mounted temperature sensor. For example, one such sensor is an IR temperature sensor.

[0048] In the example shown, a separate temperature sensor 250 is located near each transistor 231, 232, 233, 234, 235. In an alternative example, the sensor 250 may be included near the transistors 231, 232, 233 on one side of the transistor plate 240, and the controller 210 assumes that the temperatures of the symmetrical transistors (transistor 231 is symmetrical with transistor 235, and transistor 232 is symmetrical with transistor 234) are approximately the same based on thermal symmetry.

[0049] Each transistor 231, 232, 233, 234, 235 is controlled via a gate current output by a connected gate driver 212. When an overheat condition does not exist, the transistors 231, 232, 233, 234, 235 are controlled by a conventional PWM signal, and all transistors 231, 232, 233, 234, 235 on the transistor board 240 receive the same PWM signal at the same time. Therefore, the transistors 231, 232, 233, 234, 235 operate as a single unit and are electrically identical to a single exemplary transistor element 130.

[0050] When an over-current condition is detected at one of the sensors 250 but not all of the sensors 250, the controller 210 changes the PWM operation of the transistors 231, 232, 233, 234, 235 corresponding to the detected over-temperature. The changed operation varies according to the varying temperature of the transistors and from the sensors 250.

[0051] Continue to refer Figure 1-3 , Figure 4A set of exemplary PWM signals output from gate drivers 212 ′, 212 ″, 212 ′″ in response to an overheat condition detected at temperature sensor 250 corresponding to transistor 233 at the center of transistor board 240 and hot but not overheat conditions detected at adjacent transistors 232 , 234 are shown.

[0052] Before an overheat condition is detected, each pulse 310 lasts for the same duration (t). When an overheat condition is detected, the controller 210 causes the gate drivers 212' corresponding to transistors that are not overheated to continue to operate normally by maintaining the same pulse width. At the same time, the transistors 233 in the overheated area skip the pulse entirely and allow the overheated area to cool for one pulse. In addition, in order to provide more time between pulses to cool the hot but not overheated areas, the controller 210 controls the gate driver 212" to output a shorter pulse by starting the pulse 312 at a later time and ending the pulse 312 at an earlier time than the standard pulse 310. In the example shown, the shortened pulse 312 remains centered, truncated by the same amount of time from the beginning and end of the standard pulse 310 to produce a pulse of reduced width.

[0053] exist Figure 4 In the example of the embodiment, the changed PWM operation 312 maintains a single pulse and resumes operation at the next pulse. In actual implementations, the PWM operation can be changed by a constant change in a set number of pulses (e.g., five pulses of reduced duration), by a periodic change of alternating regular pulses and changed pulses, by a change of one pulse every three pulses, etc., or any similar change. In some examples, the change can be constantly re-evaluated based on the current temperature reading from the sensor 250.

[0054] Due to the parallel connection of transistors 231, 232, 233, 234, 235, the current flowing through transistor element 130 during the modified operation behaves the same as during normal operation because the current is redirected through fully turned-on transistors 231, 235 and reduced cycle width transistors 232, 234. Therefore, when operating in the modified pulse form, transistor element 130 provides normal transistor operation for inverter 126 including transistor element 130. The modified operation allows inverter 126 to maintain uninterrupted standard operation while allowing the temperature on transistor plate 240 of each transistor element 130 to be equalized and maintained below the overheating threshold.

[0055] Continue to refer Figure 1-4 , Figure 5An alternative layout transistor board 440 is shown, which includes eight transistors 431, 432, 433, 434, 435, 436, 437, 438 arranged in two rows and evenly spaced across the transistor board 440. The transistors 431, 432, 433, 434, 435, 436, 437, 438 are controlled by a controller 410, which instructs a gate driver 412 to turn the transistors 431, 432, 433, 434, 435, 436, 437, 438 on and off.

[0056] The transistor plate 440 includes two thermally symmetric regions 442, 444, wherein the central thermally symmetric region 442 surrounds the central four transistors 432, 433, 436, 437, and the outer thermally symmetric region 444 surrounds the outer four transistors 431, 434, 435, 438. All transistors 431, 432, 433, 434, 435, 436, 437, 438 within a single thermally symmetric region 442, 444 are controlled by a single corresponding gate driver 412', 412".

[0057] The outer thermal symmetry region 444 is not connected, which means that the thermal symmetry region 444 includes two separate regions. During operation, the two separate regions are heated and cooled at the same rate. In each thermal symmetry region 442, 444 is a single temperature sensor 450. The temperature sensor 450 in the unconnected thermal symmetry region 444 can be located in any part of the thermal symmetry region 444, and the optional alternative position 450' is shown by the dotted line portion. In an example, the thermal sensor 450 is located at the point where the transistor plate 440 is most likely to generate heat in each thermal symmetry region 442, 444. In another example, the thermal sensor 450 is located at a position that can determine the temperature of the most likely transistors 431, 432, 433, 434, 435, 436, 437, 438 monitored by the thermal sensor 450 in the thermal symmetry region 442, 444.

[0058] During operation, when one of the thermal sensors 450 detects the onset of overheating in the corresponding thermal symmetry region 442, the PWM operation output by the corresponding gate driver 412 is changed so that the PWM signal provided to the transistors 432, 433, 436, 437 in the corresponding thermal symmetry region 442 alternates between full pulses 510 and skipped pulses 512, as shown in FIG. Figure 6 As shown. Figure 3 and Figure 4 As an example, specific changes to the PWM output may include periodic pulse skipping, single pulse skipping, skipping multiple consecutive pulses, reducing the pulse width of a pulse, a combination of pulse skipping and pulse shortening, or any similar changes that reduce the time that an overheated thermal symmetric region receives current.

[0059] In some examples, the PWM changes are predefined within the controller 210, 410, and the controller 210, 410 will always provide a consistent changed PWM signal to the corresponding transistor in response to a particular sensor detecting an overtemperature condition. In alternative examples, the controller 210, 410 may change the PWM in a manner that depends on a particular output of the thermal sensor 250, 450. For example, the PWM output may be set to reduce the pulse width based on how much the temperature sensor exceeds a defined temperature threshold, with higher temperatures corresponding to shorter pulse widths, up to a maximum value where the pulses are skipped entirely.

[0060] refer to Figure 3 and Figure 5 It will be appreciated that the positioning of transistor components on a given transistor board 240, 440 may be designed based on a particular thermal distribution. For example, a single row linear layout (e.g. Figure 3 The linear layout shown) can maximize the individual control of the transistors while exploiting thermal symmetry. Similarly, multiple rows of array transistor components (e.g. Figure 5 A dual-row linear layout) can minimize the number of temperature sensors required, and the corresponding number of gate drivers required.

[0061] Incorporating temperature sensing and control capabilities within the motor drive inverter allows for substantial improvements to the switching cycles, current, torque, and loss profiles of the electric motor by allowing the inverter 126 to continue operating when one or more transistor components within the transistor element experience high temperatures or overheating. For example, the load current can be modulated to achieve hysteretic control of temperature while allowing multiple bursts of torque that would normally result in overheating. In addition, a single torque pulse with a well-defined profile is possible within the same control system. This in turn can allow for longer acceleration times without the need for thermal shutdown due to transistor overheating.

[0062] Additionally, in some inverters or similar electrical modules, the temperature variation on a given circuit board can be as much as 25°C. In a system operating with a single temperature sensor and a single temperature threshold, temperature variations can cause the power rating of components to be reduced (derating) by as much as 15%. Implementing multiple temperature sensors and regulated PWM control as described herein equalizes the temperature on the transistor board and reduces the magnitude of the power rating reduction.

[0063] The terms "one" and "an" do not indicate a limitation of quantity, but rather indicate the presence of at least one of the referenced items. The term "or" means "and / or", unless the context clearly indicates otherwise. References to "an aspect" throughout the specification mean that a particular element (e.g., feature, structure, step, or characteristic) described in conjunction with that aspect is included in at least one aspect described herein, and may or may not be present in other aspects. In addition, it should be understood that the described elements may be combined in any suitable manner in various aspects.

[0064] When an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.

[0065] Unless otherwise indicated herein, all test standards are the most recent standards in effect as of the filing date of the present application or, if priority is claimed, the filing date of the earliest priority application in which the test standards appear.

[0066] Unless defined otherwise, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0067] Although the above disclosure has been described with reference to exemplary embodiments, it will be appreciated by those skilled in the art that various changes may be made and equivalents may be substituted for its elements without departing from the scope thereof. In addition, many modifications may be made to adapt specific situations or materials to the teachings of the disclosure without departing from the essential scope of the disclosure. Therefore, it is intended that the disclosure is not limited to the specific embodiments disclosed, but will include all embodiments falling within its scope.

Claims

1. An electric motor circuit comprising: a motor controller having multiple gate drivers; an inverter configured to receive direct current (DC) power from a source and output alternating current (AC) power to the electric motor, the inverter comprising a plurality of transistor elements; Each transistor element includes a plurality of transistors arranged on a transistor board, wherein the plurality of temperature sensors are configured to monitor a plurality of temperatures of each transistor element; and The motor controller is configured to change pulse width modulation (PWM) control of a first subset of transistors on a first transistor element of the plurality of transistor elements in response to the first subset of temperature sensors detecting a temperature exceeding a threshold temperature and the second subset of temperature sensors not detecting a temperature exceeding the threshold.

2. The electric motor circuit according to claim 1, wherein: The plurality of transistors on each transistor element are arranged in a thermally symmetric arrangement having a plurality of sets of thermally symmetric regions.

3. The electric motor circuit according to claim 2, wherein: The thermally symmetric arrangement includes one of a single-row linear array and a multi-row linear array, wherein each thermally symmetric region includes transistors in more than one row of the multi-row linear array.

4. The electric motor circuit according to claim 2, wherein: The number of temperature sensors in the plurality of temperature sensors is equal to the total number of thermally symmetric regions, or wherein the number of temperature sensors in the plurality of temperature sensors is equal to the total number of groups of thermally symmetric regions, and optionally wherein each group of thermally symmetric regions comprises a single temperature sensor.

5. The electric motor circuit according to claim 1, wherein: Changing the PWM control of the first subset of transistors on the first transistor element includes at least one of skipping pulses and reducing a pulse width of the PWM control of the first subset of transistors.

6. The electric motor circuit according to claim 5, wherein: Changing the PWM control of the first subset of transistors on the first transistor element includes one of maintaining the change for a predetermined number of pulses and periodically changing the pulses according to a set period for the predetermined number of pulses.

7. The electric motor circuit according to claim 5, wherein: Changing the PWM control of the first subgroup of transistors on the first transistor element comprises reducing a pulse width of the PWM control of the first subgroup of transistors, and the magnitude of the pulse width reduction depends on the magnitude by which the detected temperature exceeds a threshold.

8. A method for operating a motor drive inverter according to any one of claims 1 to 7, comprising: receiving, at a motor controller, a plurality of temperature readings from a plurality of transistors on a transistor board operating as transistor elements of an inverter; and In response to a temperature reading in the plurality of temperature readings exceeding an overtemperature threshold, pulse width modulation (PWM) control of a first subset of transistors on a transistor board is changed.

9. The method according to claim 8, wherein: Changing the PWM control includes at least one of skipping one or more pulses and reducing a pulse width of one or more pulses.

10. The method according to claim 9, wherein: Reducing the pulse width of one or more pulses includes reducing the pulse width by an amplitude corresponding to the amplitude by which the one temperature reading exceeds the overheat threshold, and wherein at least one pulse is skipped at a periodic rate until the temperature reading drops below a second threshold.

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