Patent Publication Number: US-9903765-B2

Title: Apparatus for detecting temperature of semiconductor elements for power conversion

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to JP Patent Application Nos. 2013-255769 filed Dec. 11, 2013 and 2014-220603 filed Oct. 29, 2014, the entire contents of each of which are hereby incorporated by reference. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an apparatus for detecting temperatures of semiconductor elements for power conversion. 
     Related Art 
     Conventionally, a known power converter, as disclosed in Japanese Patent Application Laid-Open Publication No. 2008-206345, includes a plurality of switching elements, such as insulated gate bipolar transistors (IGBTs), is configured such that temperatures of some of the plurality of switching elements can be detected by temperature sensing elements. More specifically, in such a power converter, a prespecified one of the switching elements is assumed to be hottest among the switching elements during use. In such a power converter, the plurality of switching elements are de-energized, when the detected temperature of the prespecified switching element has reached a predetermined threshold temperature. Such a power converter is thus intended to protect the switching elements from overheating. 
     Such a configuration that temperatures of some of the plurality of switching elements are detected may give rise to the following disadvantages. 
     Even though the prespecified one of the switching elements is assumed at the time of designing to be hottest during use, another switching element may actually be hottest among the switching elements during use due to aging or the like of the power converter. There is a disadvantage that, if the power converter is not configured to detect a temperature of such another switching element, all the switching elements couldn&#39;t be reliably protected from overheating based on the detected temperature of the prespecified one of the switching elements assumed at the time of designing to be hottest during use. 
     In addition, the temperatures of the respective switching elements may vary during flowing of collector currents through the respective switching elements due to their aging or individual difference. Therefore, when the power converter is configured to detect temperatures of some of the switching elements, the threshold temperature that is used to protect the switching elements from overheating needs to be set lower than an upper limit by a large margin for safety, below which upper limit the reliability of the switching elements can be retained. However, in such a configuration, there is a disadvantage that the overheat protecting process may be performed even though the actual temperatures of the switching elements have not yet reached the upper-limit temperature, and a temperature range where use of the switching elements is restricted may enlarge. 
     Power-conversion semiconductor elements, whether switching elements or not, may suffer from such disadvantages. 
     In consideration of the foregoing, exemplary embodiments of the present invention are directed to providing an apparatus for detecting temperatures of a plurality of semiconductor elements for power conversion, capable of preventing the semiconductor elements from overheating and preventing enlargement of a temperature range where use is restricted. 
     SUMMARY 
     In accordance with an exemplary embodiment of the present invention, there is provided an apparatus for detecting temperatures of power-conversion semiconductor elements, the apparatus being applicable to a system including a plurality of power-conversion semiconductor elements that produce heat when energized, a plurality of temperature signal outputs associated with the respective power-conversion semiconductor elements, the temperature signal outputs being configured to output temperature signals correlated to temperatures of the respective power-conversion semiconductor elements, the apparatus including: a microcomputer configured to detect the temperatures of the respective power-conversion semiconductor elements based on the temperature signals outputted from the respective temperature signal outputs, at least one input-output interface having a plurality of input ports and an output port selectively connected to one of the plurality of input ports, wherein the temperature signal outputted from at least one of the plurality of temperature signal outputs is input directly to the microcomputer without passing through the at least one input-output interface, and the temperature signals outputted from the other two or more temperature signal outputs are input to the respective input ports of the at least one input-output interface, the microcomputer is configured to detect the temperatures of the respective power-conversion semiconductor elements based on the temperature signals received from the output port of the at least one input-output interface and the temperature signal received directly from the at least one of the plurality of temperature signal outputs. 
     In the above embodiment, the temperature signal outputs are provided for the respective power-conversion semiconductor elements. This allows the microcomputer to detect the temperature of each of the plurality of power-conversion semiconductor elements. Hence, even though a prespecified one of the switching elements is initially assumed to be hottest during use of the system, another switching element may actually be hottest among the switching elements during use due to aging or the like of the system. Even in such a case, the microcomputer is able to determine which semiconductor element is hottest among the plurality of switching elements. It is thus possible to prevent the semiconductor elements from overheating and prevent enlargement of a temperature range where use of the system is restricted. 
     In addition, in the above embodiment, the temperature signal outputted from at least one of the plurality of temperature signal outputs is input directly to the microcomputer without passing through the at least one input-output interface. Therefore, even if the at least one input-output interface fails, paths can be ensured between the microcomputer and the temperature signal outputs other than the at least one of the temperature signal outputs that is not connected to the at least one input-output interface. Even if the at least one input-output interface fails, this can achieve fail-safe, such as power saving, and can thus prevent substantial degradation of the reliability of the system. 
     In addition, in the above embodiment, signal transferring paths between the microcomputer and the other two or more temperature signal outputs can be established by using the input-output interface. The input-output interface is configured such that the number of output ports is less than the number of input ports. Therefore, the number of input ports of the microcomputer required to detect the temperatures of the respective power-conversion semiconductor elements can be reduced less than the total number of power-conversion semiconductor elements. This allows the apparatus to be downsized and costs of the apparatus to be reduced. 
     In another embodiment, the apparatus is applied to a system including a plurality of power-conversion semiconductor elements that produce heat when energized, and a plurality of temperature signal outputs associated with the respective power-conversion semiconductor elements, where the temperature signal outputs are configured to output temperature signals correlated to temperatures of the respective power-conversion semiconductor elements. The apparatus includes a microcomputer having a plurality of input ports for receiving the temperature signals from the respective temperature signal outputs, each for a respective one of the temperature signals, and is configured to detect the temperatures of the respective power-conversion semiconductor elements based on the temperature signals received at the respective input ports. 
     In the above other embodiment, the temperature signal outputs are provided for the plurality of power-conversion semiconductor elements, one for each power-conversion semiconductor element. The microcomputer has the plurality of input ports for respectively receiving the temperature signals from the respective temperature signal outputs. That is, in the microcomputer is configured such that the number of the temperature signal outputs is equal to the number of the input ports. Thus, the microcomputer is able to detect the temperatures of the respective whole power-conversion semiconductor elements. Even though a prespecified one of the switching elements is initially assumed to be hottest during use of the system, another switching element may actually be hottest among the switching elements during use due to aging or the like of the system. Even in such a case, the microcomputer is able to determine which semiconductor element is hottest among the plurality of switching elements. It is thus possible to prevent the semiconductor elements from overheating and prevent enlargement of a temperature range where use of the system is restricted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a three-phase inverter in accordance with a first embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the three-phase inverter taken along line  2 - 2  of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a motor control system in accordance with the first embodiment; 
         FIG. 4  is a schematic circuit diagram for transferring temperature signals in accordance with the first embodiment; 
         FIG. 5  is a schematic of temperature estimation errors for IGBTs; 
         FIG. 6  is a schematic circuit diagram for transferring temperature signals in accordance with a second embodiment of the present invention; 
         FIG. 7  is a schematic diagram of a motor control system in accordance with a third embodiment of the present invention; 
         FIG. 8  is a schematic circuit diagram for transferring temperature signals in accordance with the third embodiment; 
         FIG. 9  is a schematic circuit diagram for transferring temperature signals in accordance with a fourth embodiment of the present invention; 
         FIG. 10  is a schematic circuit diagram for transferring temperature signals in accordance with a fifth embodiment of the present invention; 
         FIG. 11  is a schematic diagram of a buck-boost converter in accordance with a sixth embodiment of the present invention; 
         FIG. 12  is a schematic diagram of a motor control system in accordance with a seventh embodiment of the present invention; 
         FIG. 13  is a schematic circuit diagram for transferring temperature signals in accordance with an eighth embodiment; and 
         FIG. 14  is an example of time-multiplexed frame signal in accordance with the eighth embodiment. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Embodiments of the present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     First Embodiment 
     There will now be explained a temperature detection apparatus applied to a vehicle-mounted power converter (a three-phase inverter) in accordance with a first embodiment of the present invention with reference to the accompanying drawings. 
     The three-phase inverter of the first embodiment will be explained with reference to  FIGS. 1 and 2 . 
     As shown in  FIGS. 1 and 2 , the three-phase inverter  10  includes a circuit board  20 , a cooler  30 , a frame  40 , U-, V-, W-phase upper-arm semiconductor modules  50 Up,  50 Vp,  50 Wp, and U-, V-, W-phase lower-arm semiconductor modules  50 Un,  50 Vn,  50 Wn. 
     Each semiconductor module  50 αβ (α=U, V, W: β=p, n) is a member, into which a switching element, a freewheel diode electrically connected in anti-parallel with the switching element, and a temperature-sensitive diode for detecting a temperature of the switching element are modularized. In the present embodiment, an insulated gate bipolar transistors (IGBT) is used as the switching element. The switching element and the temperature-sensitive diode are formed on the same chip. 
     Each semiconductor module  50 αβ has a body including therein the switching element, the freewheel diode, and the temperature-sensitive diode. The body is a flattened cuboid. A plurality of control terminals protrude from and perpendicularly to one of surfaces of the body. The plurality of control terminals include a gate terminal and an emitter terminal (referred to as a Kelvin emitter terminal) of the switching element. 
     Each semiconductor module  50 αβ is secured to the circuit board  20  via the control terminals. More specifically, each semiconductor module  50 αβ is secured to the circuit board  20  by soldering the control terminals to the circuit board  20 . 
     The frame  40  surrounds the six semiconductor modules  50 αβ. The circuit board  20  is secured on top of the frame  40 . For example, the frame  40  is stiff enough to compressively hold the cooler  30  and the respective semiconductor modules  50 αβ, and made of a material capable of blocking switching noise (e.g., metal or conductive resin). 
     The circuit board  20  is rectangular in shape as viewed from a perpendicular direction to a front surface of the circuit board  20 . The circuit board  20  is functional to turn on and off the switching elements of the respective semiconductor modules  50 αβ. This function of the circuit board  20  is realized by an electronic component, such as a microcomputer or the like, mounted on the circuit board  20 . 
     The cooler  30  includes an inlet line  30   a , an outlet line  30   b , and a plurality of (e.g., four) cooling members  30   c , and is secured to the frame  40 . The inlet line  30   a  is a member for introducing a cooling fluid for cooling the respective semiconductor modules  50 αβ into the three-phase inverter  10 . The outlet line  30   b  is a member for outputting the cooling fluid from the three-phase inverter  10 . Each of the cooling members  30   c  connects the inlet line  30   a  and the outlet line  30   b  to direct the cooling fluid from the inlet line  30   a  to the outlet line  30   b.    
     The cooling members  30   c  are each a flattened cuboid in shape and arranged in line. For each pair of upper-arm semiconductor module  50 αp and lower-arm semiconductor module  50 αn (α=U, V, W), the upper-arm semiconductor module  50 αp and the lower-arm semiconductor module  50 αn of the same phase are arranged in line between a pair of adjacent cooling members  30   c . More specifically, the semiconductor modules  50 αp,  50 αn of the same phase are arranged in line along the longitudinal direction of the cooling members  30   c  (a direction from the inlet line  30   a  to the outlet line). The upper-arm semiconductor module  50 αp is arranged upstream of the lower-arm semiconductor module  50 αn along a flow path of the cooling fluid including the inlet line  30   a , the outlet line  30   b , and the cooling members  30   c . The pairs of upper-arm semiconductor module  50 αp and lower-arm semiconductor module  50 αn (α=U, V, W) and the plurality of cooling members  30   c  are stacked alternately, thereby forming a stack. 
     A wall member  42  is provided on an inner circumferential wall of the frame  40 . On a side of the wall member  42  facing the stack is formed a wall surface. In the present embodiment, the wall member  42  is made of the same material as the frame  40 . 
     A pressing member  60  is provided on a portion of the inner circumferential wall of the frame  40  facing the wall member  42  with mounting pins  62  between the pressing member  60  and the portion of the inner circumferential wall. The pressing member  60  is a member for pushing the stack of the cooling members  30   c  and the semiconductor modules  50 αβ toward the wall member  42 . More specifically, one of opposite sides of the stack is pushed by the pressing member  60  toward the wall member  42  with the other of opposite sides of the stack (a side of the leftmost cooling member  30   c  facing the wall member  42 ) kept in contact with the wall surface of the wall member  42 . This can enhance the fit between the cooling members  30   c  and their respective adjacent semiconductor modules, thereby enhancing the heat dissipation of the semiconductor modules. In the present embodiment, a plate spring is used as the pressing member  60 . In the present embodiment, the stack is pushed by the pressing member  60  via the plate member  64 . An arc-like portion of the pressing member  60  is pressed directly onto the plate member  64 , thus indirectly onto the stack, which can prevent the cooling members  30   c  from being considerably deformed. 
     In the cooler  30  configured as above, when the cooling fluid is introduced form the inlet line  30   a , the cooling fluid flows through the respective cooling members  30   c . The cooling fluid is output from the outlet line  30   b  after flowing through the respective cooling members  30   c , thereby cooling the respective semiconductor modules  50 αβ. 
     In the present embodiment, it is assumed that there is the hottest one of the six semiconductor modules  50 αβ (α=U, V, W: β=p, n) during use (that is, while the switching elements Sαβ of the three-phase inverter  10  are turned on and off). In the present embodiment, it is assumed that the hottest one of the six semiconductor modules is the V-phase lower-arm semiconductor module  50 Vn that is on the downstream side of the cooling fluid and between the middle pair of the adjacent cooling members  30   c . In addition, it is further assumed that the second hottest one of the six semiconductor modules is the W-phase lower-arm semiconductor module  50 Wp. 
     That is, during use of the three-phase inverter  10 , a temperature of the cooling fluid on the downstream side of the flow path of the cooling fluid is greater than a temperature of the cooling fluid on the upstream side of the flow path of the cooling fluid. Therefore, during use of the three-phase inverter  10 , a temperature of the semiconductor module  50 αn on the downstream side of the flow path is greater than a temperature of the semiconductor module  50 αp on the upstream side of the flow path. Each cooling member  30   c  other than the cooling members  30   c  adjacent the V-phase semiconductor modules  50 Vβ ( 50 Vp,  50 Vn) are in contact with either the wall member  42  or the plate member  64 . Therefore, an amount of heat dissipated from each cooling member  30   c  (along the heat dissipation path) other than the cooling members  30   c  adjacent the V-phase semiconductor modules  50 Vβ is greater than an amount of heat dissipated from each cooling member  30   c  (along the heat dissipation path) adjacent the V-phase semiconductor modules  50 Vβ. Therefore, the center V-phase semiconductor modules  50 Vβ are prone to be hotter. Besides this reason, since collector currents of the switching elements of the respective semiconductor module  50 αβ have the substantially same average value, the V-phase lower-arm semiconductor module  50 Vn is hottest, and the W-phase lower-arm semiconductor module  50 Wp is second hottest. 
     An overall configuration of a motor control system of the present embodiment will now be explained with reference to  FIG. 3 . 
     As shown in  FIG. 3 , the motor control system includes the three-phase inverter  10  and a motor generator  70 . The motor generator  70  is connected to a driven wheel (not shown) and acts as a vehicle prime mover or the like. In the present embodiment, a one-motor control system including one motor generator  70  is used as the motor control system. A permanent magnet synchronous electrical motor may be used as the motor generator  70 . 
     The three-phase inverter  10  is electrically connected to a high-voltage battery  72 , such as a lithium-ion secondary battery, a nickel-metal-hydride secondary battery or the like, as a direct-current (DC) power source. 
     Collector terminals of the U-, V-, W-phase upper-arm switching elements SUp, SVp, SWp of the respective U-, V-, W-phase upper-arm semiconductor modules  50 Up,  50 Vp  50 Wp are electrically connected to each other. Emitter terminals of the U-, V-, W-phase lower-arm switching elements SUn, SVn, SWn of the respective U-, V-, W-phase lower-arm semiconductor modules  50 Un,  50 Vn,  50 Wn are electrically connected to each other. The collector terminals are electrically connected to a positive terminal of the high-voltage battery  72 . The emitter terminals are electrically connected to a negative terminal of the high-voltage battery  72 . 
     An emitter terminal of the α-phase upper-arm switching element Sαp (α=U, V, W) is electrically connected to a collector terminal of the α-phase lower-arm switching element Sαn. A junction between the emitter terminal of the α-phase upper-arm switching element Sαp and the collector terminal of the α-phase lower-arm switching element Sαn is electrically connected to an α-phase terminal of the motor generator  70 . 
     The three-phase inverter  10  includes a microcomputer  80  and drive circuits Drαβ (α=U, V, W: β=p, n). The microcomputer  80  includes a central processing unit (CPU) and others to generate operation signals gαβ and output the operation signals gαβ to the respective drive circuits Drαβ, thereby controlling a controlled variable (e.g., torque) of the motor generator  70  to a command value. Each drive circuit Drαβ is a gate drive circuit for charging and discharging a gate of the switching element Sαβ in response to the operation signal gαβ. The switching element Sαβ is turned on and off by the drive circuit Drαβ. The operation signals gαβ are signals for passing sinusoidal currents that are 120 degrees out of phase. For example, the operation signals gαβ may be generated in the well-known field-oriented control loop. 
     In the present embodiment, the vehicle includes a high-voltage domain and a low-voltage domain. The high-voltage domain includes the motor generator  70 , the high-voltage battery  72 , the semiconductor modules  50 αβ and the drive circuits Drαβ (α=U, V, W: β=p, n). The low-voltage domain includes the microcomputer  80 . A reference potential VstL of the low-voltage domain and a reference potential VstH of the high-voltage domain are different from each other. In the present embodiment, the reference potential VstH of the high-voltage domain is set to a negative-electrode potential of the high-voltage battery  72 , and the reference potential VstL of the low-voltage domain is set to a potential of the vehicle body that is a median of the positive-electrode and negative-electrode potentials of the high-voltage battery  72 . 
     Temperature detection for the respective switching elements Sαβ (α=U, V, W: β=p, n) will now be explained with reference to  FIG. 4 .  FIG. 4  shows various electronic components mounted on the respective semiconductor modules  50 αβ and the circuit board  20 . 
     As shown in  FIG. 4 , the drive circuits Drαβ (α=U, V, W: β=p, n), photocouplers Cαβ (α=U, V, W: β=p, n), first and second multiplexers  82   a ,  82   b , and the microcomputer  80  are mounted on the circuit board  20 . Each of the first and second multiplexers  82   a ,  82   b  has two input ports (first input port Tin 1 , second input port Tin 2 ), and one output port Tout. In the present embodiment, each of the first and second multiplexers  82   a ,  82   b  corresponds to an input-output interface. 
     The drive circuits Drαβ are configured to receive temperature signals from the respective temperature-sensitive diodes Tαβ. In the present embodiment, the temperature-sensitive diodes Tαβ are supplied with current from a constant current source (not shown). Therefore, the temperature-sensitive diodes Tαβ are configured to output voltage signals negatively correlated to temperatures of the respective switching elements Sαβ. In the present embodiment, each temperature-sensitive diode Tαβ corresponds to a temperature signal output. 
     The drive circuits Drαβ are configured to receive the temperature signals from the respective temperature-sensitive diodes Tαβ. The drive circuit Drαβ are configured to output the received temperature signals to the respective photocouplers Cαβ. In the present embodiment, each drive circuit Drαβ is configured to, based on comparison of the received temperature signal from the temperature-sensitive diode Tαβ with a carrier signal (e.g., a triangular waveform signal) in magnitude, convert the temperature signal into a duty ratio signal (referred to as a duty signal) and output the duty ratio signal to the photocoupler Cαβ. 
     Each photocoupler Cαβ is an optical insulating transfer element configured to transmit a signal from the high voltage domain to the low voltage domain while electrically isolating the high and low voltage domains from each other. At an input (photodiode-side) of the photocoupler Cαβ is received the temperature signal (i.e., the duty ratio signal) of the temperature-sensitive diode Tαβ from the drive circuit Drαβ. 
     From an output (phototransistor-side) of each of the U-, V-phase upper-arm photocouplers CUp, CVp, the temperature signal is input to the microcomputer  80  via the first multiplexer  82   a . More specifically, the output of the U-phase upper-arm photocoupler CUp is connected directly to the first input port Tin 1  of the first multiplexer  82   a  via a first input channel Lin 1 . The output of the V-phase upper-arm photocoupler CVp is connected directly to the second input port Tin 2  of the first multiplexer  82   a  via a second input channel Lin 2 . In the present embodiment, the input channels Lin 1 , Lin 2  use a wiring pattern formed on the circuit board  20 . 
     An output port Tout of the first multiplexer  82   a  is connected directly to a first input port T 1  of the microcomputer  80  via an output channel Lout. In the present embodiment, the output channel Lout uses the wiring pattern formed on the circuit board  20 . The first multiplexer  82   a  is configured to sequentially select one of the first and second input ports Tin 1 , Tin 2  to be connected to the output port Tout at predetermined time intervals based on a switching signal (e.g., a bit signal) received from the microcomputer  80 . This allows the temperature signal received at the output port Tout to be alternately switched between the temperature signal received at the first input port Tin 1  and the temperature signal received at the second input port Tin 2  at the predetermined time intervals. 
     From an output (phototransistor-side) of each of the W-phase upper-arm photocoupler CWp and the U-phase lower-arm photocoupler CUn, the temperature signal is input to a second input port T 2  of the microcomputer  80  via the second multiplexer  82   b . A transferring manner of the temperature signal via the second multiplexer  82   b  is similar to that of the temperature signal via the first multiplexer  82   a . Therefore, a detailed description about the transferring manner of the temperature signal via the second multiplexer  82   b  will not be given here. 
     An output of the V-phase lower-arm photocoupler CVn is connected directly to a third input port Tin 3  of the microcomputer  80  via a V-phase lower-arm electrical path LVn. An output of the W-phase lower-arm photocoupler CWn is connected directly to a fourth input port Tin 4  of the microcomputer  80  via a V-phase lower-arm electrical path LWn. In the present embodiment, the electrical paths LVn, LWn use the wiring pattern formed on the circuit board  20 . 
     The microcomputer  80  is configured to detect temperatures of the six switching elements Sαβ (α=U, V, W: β=p, n) based on the temperature signals received at the input ports T 1  through T 4 . The microcomputer  80  is configured to perform a power saving process when determining that the highest one ST of the temperatures of the six switching elements exceeds a threshold temperature Tγ. In the power saving process, the microcomputer  80  de-energizes the switching elements Sαβ or reduces power supplied to the switching elements Sαβ, thereby the respective switching elements Sαβ from overheating. 
     The temperature sensing configuration as described above is employed to prevent the respective switching elements Sαβ from overheating and prevent enlargement of a temperature range where use of the respective switching elements Sαβ is restricted. 
     The V-phase lower-arm switching element SVn is a switching element that is assumed at the time of designing to be hottest during use among the six switching elements Sαβ. However, another switching element may be hottest during use among the six switching elements Sαβ due to aging of the three-phase inverter  10  or the like. Therefore, in such a case, even if the power saving process is performed based on the temperature signal of the V-phase lower-arm switching element SVn from the temperature-sensitive diode, the overheat condition of the other switching elements could not be avoided reliably. 
     In addition, the temperatures of the switching elements Sαβ may vary during flowing of the collector currents through the respective switching elements Sαβ due to aging or individual difference of the switching elements. Therefore, when temperatures of some of the six switching elements Sαβ are detected, the threshold temperature Tγ that is used to performed the power saving process needs to be set lower than an upper limit by a large margin for safety, below which upper limit the reliability of the switching elements Sαβ can retained. Temperature variations of the switching elements will now be explained with reference to  FIG. 5 . In  FIG. 5 , a “detection element” refers to a switching element, a temperature of which is detected, and a “non-detection element” refers to a switching element, a temperature of which is not detected. 
       FIG. 5  shows an example case that a temperature error of each of the detection element and the non-detection element is a maximum with the same collector current flowing through the detection element and the non-detection element. In  FIG. 5 , an “element loss” refers to an average temperature over a plurality of mass-produced switching elements. An “element-loss variation” refers to a variation from the average temperature. A “thermal resistance variation” refers to a variation in temperature among the switching elements caused by a variation in thermal resistance among new semiconductor modules. A “thermal resistance degradation” refers to an amount of temperature increase of the switching element caused by thermal resistance variation due to aging of the semiconductor module. 
     For the detection element, the element-loss minus (the element-loss variation plus the thermal resistance variation) gives a temperature detected by the temperature-sensitive diode. In the example of  FIG. 5 , although the same collector current flows though the detection element and the non-detection element, the temperature of the non-detection element is greater than the temperature of the detection element. In this case, a temperature of the non-detection element is a sum of the element loss, the element-loss variation, the thermal resistance variation, and the thermal resistance degradation. A difference in temperature between the detection element and the non-detection element gives an actual temperature variation. 
     For example, when the actual temperature variation is 20° C. and the upper limit of temperatures of the switching elements is 150° C., the threshold temperature Tγ that is used to performed the power saving process needs to be set to 130° C. to protect the non-detection element from overheating. Thus, the threshold temperature Tγ is set lower than the upper limit by a large margin for safety, which may cause the power saving process to be performed even though the temperature of the non-detection element has not reached the upper-limit temperature. In such a case, there is a concern that the collector current can be no more increased even if the collector current may be further increased. That is, there is a disadvantage that a temperature range where use of the switching elements is restricted may enlarge. 
     To overcome such a problem, in the present embodiment, temperatures of the six switching elements Sαβ are detected. 
     There are some benefits associated with the present embodiment set forth above. 
     (1) The temperatures of all the six switching elements Sαβ are detected. Therefore, the hottest switching element among the six switching elements Sαβ during use can be determined by the microcomputer  80  even if the hottest switching element among the six switching elements Sαβ during use is changed from the switching element that is assumed at the time of designing to be hottest among the six switching elements Sαβ during use to another one of the six switching elements Sαβ. This can prevent the six switching elements Sαβ from overheating and prevent enlargement of a temperature range where the maximum value of the available collector current is constrained, even when it is difficult to determine the hottest switching element among the six switching elements Sαβ only from information about the positions of the respective semiconductor modules within the cooler. 
     (2) The temperature signals outputted from two of the six temperature-sensitive diodes Tαβ, that is, the two temperature-sensitive diodes TVn, TWn, are transferred from the outputs of the photocouplers CVn, CWn to the microcomputer  80  via the electrical paths LVn, LWn without passing through the multiplexers  82   a ,  82   b . Each of the electrical paths LVn, LWn directly connecting one of the outputs of the photocouplers CVn, CWn and the microcomputer  80  can be a path for transferring the temperature signals in the event that the multiplexers  82   a ,  82   b  fail. Thus, in the present embodiment, even if the multiplexers  82   a ,  82   b  fail, fail-safe, such as power saving, can be achieved based on the temperatures of the switching elements SVn, SWn. This can avoid substantial degradation of the reliability of the three-phase inverter  10 . 
     Particularly, in the present embodiment, only the V-phase lower-arm temperature-sensitive diode TVn and the W-phase lower-arm temperature-sensitive diode TWn output the temperature signals that are input directly to the microcomputer  80 . The V-phase lower-arm switching element SVn is assumed at the time of designing to be hottest among the six switching elements Sαβ during use. The W-phase lower-arm switching element SWn is assumed to be second hottest among the six switching elements Sαβ during use. Therefore, even if another switching element is actually hottest among the six switching elements Sαβ during use due to aging of the three-phase inverter  10  or the like, the temperatures of The V-phase lower-arm switching element SVn and the W-phase lower-arm switching element SWn are likely to remain at higher levels than the average temperature over the six switching elements Sαβ. Therefore, in the present embodiment, even if the multiplexers  82   a ,  82   b  fail, the power saving can properly protect the switching elements Sαβ from overheating. 
     (3) Paths of the temperature signals from four of the six temperature-sensitive diodes Tαβ, that is, the temperature-sensitive diodes Tαp (α=U, V, W), TUn, to the microcomputer  80  are realized by the two multiplexers  82   a ,  82   b . This can reduce the number of input ports of the microcomputer  80  required to detect temperatures of the six switching elements Sαβ to less than the total number of switching elements Sαβ. This allows the microcomputer  80  to detect the temperatures of all the six switching elements Sαβ even in the presence of a limited number of input ports of the microcomputer  80 , which allows the three-phase inverter  10  to be downsized and costs of the three-phase inverter  10  to be reduced. 
     (4) The multiplexers  82   a ,  82   b  are used, each of which has two inputs and one output. In each of the multiplexers  82   a ,  82   b , the input to be connected to the output port Tout is alternately switched between the two input ports Tin 1 , Tin 2 . This causes a delay between when the temperature signal is received at one of the two input ports and when the temperature signal is output from the output port Tout to the microcomputer  80 . The delay increases with an increasing number of input ports. A difference in delay between the multiplexers  82   a ,  82   b  increases with an increasing difference in number of input ports between the multiplexers  82   a ,  82   b . Time intervals between detection of temperatures of each of the switching elements associated with one of the multiplexers  82   a ,  82   b  having a larger number of input ports are greater than time intervals associated with the other of the multiplexers  82   a ,  82   b . Therefore, there is a concern that overheat protection for the switching elements by the power saving may be delayed. In the present embodiment, the multiplexers  82   a ,  82   b  have an equal number of input ports, which can reduce variations in time required for the temperature signals to be transferred from the respective temperature-sensitive diodes Tαp, TUn to the microcomputer  80  via signal transferring paths passing though the respective multiplexers  82   a ,  82   b . This can prevent the time intervals between detection of temperatures of each of the switching elements Sαp, SUn from increasing, thereby preventing overheat protection from being delayed. 
     Particularly, in the present embodiment, the number of input ports of each of the multiplexers  82   a ,  82   b  is set to two. This can minimize the time intervals between detection of temperatures of each of the switching elements Sαp, SUn are detected. 
     Second Embodiment 
     There will now be explained a second embodiment of the present invention. Only differences of the second embodiment from the first embodiment will be explained with reference to the accompanying drawings. 
     As shown in  FIG. 6 , the microcomputer  80  of the present embodiment includes first to sixth input ports t 1 -t 6 . The first to third input ports T 1 , T 2 , T 3  are electrically connected directly to outputs of the U-, V-, W-phase upper-arm photocouplers CUp, CVp, CWp via U-, V-, W-phase upper-arm electrical paths LUp, LVp, LWp. Fourth to sixth input ports T 4 , T 5 , T 6  of the microcomputer  80  are electrically connected directly to outputs of the U-, V-, W-phase lower-arm photocouplers CUn, CVn, CWn via U-, V-, W-phase lower-arm electrical paths LUn, LVn, LWn. In the present embodiment, the α-phase upper- and lower-arm electrical paths Lαp, Lαn (α=U, V, W) may use the wiring pattern of the circuit board  20 . 
     The present embodiment described above can also prevent each of the six switching elements Sαβ from overheating, and can also prevent enlargement of a temperature range where the available flow of collector current is constrained. 
     Third Embodiment 
     There will now be explained a third embodiment of the present invention. Only differences of the third embodiment from the first embodiment will be explained with reference to the accompanying drawings. In the present embodiment, a two-motor control system including two motor generators (rotating electric machines) is used as a control system. More specifically, as shown in  FIG. 7 , the control system includes an buck-boost converter  90 , a first inverter  100 , a first motor generator  101 , a second inverter  102 , a second motor generator  103 , and a microcomputer  80 . Each of the motor generators  101 ,  103  may be a permanent magnet synchronous electrical motor. Elements having similar functions as in the first embodiment (see  FIG. 1 ) are assigned the same numbers. 
     The buck-boost converter  90  includes an input capacitor  91 , a reactor  92 , voltage-transformation switching elements SCpa, SCpb, SCna, SCnb, freewheel diodes DCpa, DCpb, DCna, DCnb electrically connected in anti-parallel with the respective voltage-transformation switching elements, and a smoothing capacitor  93 . In the present embodiment, an insulated gate bipolar transistors (IGBT) is used for each of the voltage-transformation switching elements SCpa, SCpb, SCna, SCnb. 
     The buck-boost converter  90  includes a plurality of (e.g., two) upper-arm switching elements and a plurality of (e.g., two) lower-arm switching elements electrically connected in parallel with the plurality of upper-arm switching elements. More specifically, collectors of the respective upper-arm voltage-transformation switching elements SCpa, SCpb are electrically connected to each other, and emitters of the respective upper-arm voltage-transformation switching elements SCpa, SCpb are electrically connected to each other. In addition, collectors of the respective lower-arm voltage-transformation switching elements SCna, SCnb are electrically connected to each other, and emitters of the lower-arm voltage-transformation switching elements SCna, SCnb are electrically connected to each other. A series connection of the upper-arm voltage-transformation switching elements SCpa, SCpb and the lower-arm voltage-transformation switching elements SCna, SCnb is electrically connected in parallel with the smoothing capacitor  93 . As in the first embodiment, the voltage-transformation switching elements SCpa, SCpb, SCna, SCnb, together with the respective freewheel diodes DCpa, DCpb, DCna, DCnb and respective temperature-sensitive diodes (not shown), form semiconductor modules  110 Cpa,  110 Cpb,  110 Cna,  110 Cnb, respectively. 
     A first end of the reactor  92  is electrically connected to a junction between the upper-arm transformer switching elements SCpa, SCpb and the lower-arm voltage-transformation switching elements SCna, SCnb. A second end of the reactor  92  is electrically connected to a positive terminal of a high-voltage battery  72 . A negative terminal of the high-voltage battery  72  is electrically connected to the emitters of the lower-arm voltage-transformation switching elements SCna, SCnb. An input capacitor  91  is electrically connected to the high-voltage battery  72 . 
     The first inverter  100  includes U-, V-, W-phase upper-arm switching elements S 1 Up, S 1 Vp, S 1 Wp, and U-, V-, W-phase lower-arm switching elements S 1 Un, S 1 Vn, S 1 Wn. The switching elements S 1 Up, S 1 Vp, S 1 Wp, S 1 Un, S 1 Vn, S 1 Wn are electrically connected in anti-parallel with respective freewheel diodes D 1 Up, D 1 Vp, D 1 Wp, D 1 Un, D 1 Vn, D 1 Wn. As in the first embodiment, the switching elements S 1 Up, S 1 Vp, S 1 Wp, S 1 Un, S 1 Vn, S 1 Wn, together with the respective freewheel diodes D 1 Up, D 1 Vp, D 1 Wp, D 1 Un, D 1 Vn, D 1 Wn and respective temperature-sensitive diodes (not shown), form semiconductor modules  120 Up,  120 Vp,  120 Wp,  120 Un,  120 Vn,  120 Wn, respectively. 
     The second an inverter  102  includes U-, V-, W-phase upper-arm switching elements S 2 Up, S 2 Vp, S 2 Wp, and U-, V-, W-phase lower-arm switching elements S 2 Un, S 2 Vn, S 2 Wn. The switching elements S 2 Up, S 2 Vp, S 2 Wp, S 2 Un, S 2 Vn, S 2 Wn are electrically connected in anti-parallel with respective freewheel diodes D 2 Up, D 2 Vp, D 2 Wp, D 2 Un, D 2 Vn, D 2 Wn. As in the first embodiment, the switching elements S 2 Up, S 2 Vp, S 2 Wp, S 2 Un, S 2 Vn, S 2 Wn, together with the respective freewheel diodes D 2 Up, D 2 Vp, D 2 Wp, D 2 Un, D 2 Vn, D 2 Wn and respective temperature-sensitive diodes (not shown), form semiconductor modules  130 Up,  130 Vp,  130 Wp,  130 Un,  130 Vn,  130 Wn, respectively. 
     Each of the first and second inverter  100 ,  102  has a similar configuration to that of the three-phase inverter  10  of the first embodiment. Therefore, in the present embodiment, a detailed description about the first and second inverter  100 ,  102  will not be given here. 
     The first inverter  100  is electrically connected to the first motor generator  101 . The first motor generator  101  acts as an alternator and as a starter for applying an initial torque to a crankshaft of a vehicle engine. The second inverter  102  is electrically connected to the second motor generator  103 . Like the motor generator  70  of the first embodiment, the second motor generator  103  acts as a prime mover or the like. 
     The microcomputer  80  is configured to output operation signals to drive circuits (not shown) of the respective switching elements S 1 Up, S 1 Vp, S 1 Wp, S 1 Un, S 1 Vn, S 1 Wn to drive the first motor generator  101  as an alternator. This allows an alternating voltage input from the first motor generator  101  to the first inverter  100  to be converted into a direct voltage, which in turn is applied to the buck-boost converter  90 . In the present embodiment, the operation signals are generated through a PWM process based on comparison of U-, V-, W-phase output voltage command values for the first inverter  100  with a carrier signal (e.g., a triangular waveform signal) in magnitude. The operation signals thus generated for the respective three phases of the first motor generator  101  are, for example, signals for providing U-, V-, W-phase sinusoidal currents for the first motor generator  101  that are 120 degrees out of phase in electric angle. 
     The microcomputer  80  is configured to output operation signals to drive circuits (not shown) of the respective switching elements S 2 Up, S 2 Vp, S 2 Wp, S 2 Un, S 2 Vn, S 2 Wn to motoring-drive the second motor generator  103  as an electrical motor or regeneration-drive the second motor generator  103  as an alternator. In motoring drive, a direct voltage input from the buck-boost converter  90  to the second inverter  102  is converted into an alternating voltage, which in turn is applied to the second motor generator  103 . In regeneration drive, an alternating voltage input from the second motor generator  103  to the second inverter  102  is converted into a direct voltage, which in turn is applied to the buck-boost converter  90 . In the present embodiment, the operation signals are generated through the PWM process. 
     The microcomputer  80  is configured to output operation signals to drive circuits (not shown) of the respective voltage-transformation switching elements SCpa, SCpb, SCna, SCnb to drive the buck-boost converter  90  as a boost converter or drive the buck-boost converter  90  as a buck converter. More specifically, in voltage step-up operation to motoring-drive the second motor generator  103 , both the lower-arm voltage-transformation switching elements SCna, SCnb are turned on and off in synchronization with each other with both the upper-arm voltage-transformation switching elements SCpa, SCpb kept in an off-state. In voltage step-down operation to regeneration-drive the second motor generator  103 , the upper-arm voltage-transformation switching elements SCpa, SCpb are turned on and off in synchronization with each other with both the lower-arm voltage-transformation switching elements SCna, SCnb kept in an off-state. 
     A frequency of the carrier signal (e.g., 5 kHz) used to generate the operation signals for each of the inverters  100 ,  102  is set lower than a frequency of the carrier signal (e.g., 10 kHz) used to generate the operation signals for the buck-boost converter  90 . 
     A configuration for detecting temperatures of the respective switching elements will now be explained with reference to  FIG. 8 .  FIG. 8  corresponds to  FIG. 4  of the first embodiment. In  FIG. 8 , the circuit board  20  is not shown. 
     As shown in  FIG. 8 , the drive circuits D 2 Up, D 2 Vp, D 2 Wp, D 2 Un, D 2 Vn, D 2 Wn configured to drive the respective switching elements S 2 Up, S 2 Vp, S 2 Wp, S 2 Un, S 2 Vn, S 2 Wn forming the second an inverter  102 , photocouplers C 2 Up, C 2 Vp, C 2 Wp, C 2 Un, C 2 Vn, C 2 Wn, first and second multiplexers  140   a ,  140   b , and the microcomputer  80  are mounted on the circuit board. In the present embodiment, the first multiplexer  140   a  has two input ports and one output port, and the second multiplexer  140   b  has three input ports and one output port. The drive circuits D 2 Up, D 2 Vp, D 2 Wp, D 2 Un, D 2 Vn, D 2 Wn are configured to receive temperature signals from temperature-sensitive diodes T 2 Up, T 2 Vp, T 2 Wp, T 2 Un, T 2 Vn, T 2 Wn for detecting temperatures of the respective switching elements S 2 Up, S 2 Vp, S 2 Wp, S 2 Un, S 2 Vn, S 2 Wn. The drive circuits D 2 Up, D 2 Vp, D 2 Wp, D 2 Un, D 2 Vn, D 2 Wn are configured to convert the received temperature signals into duty signals, which in turn are input to the respective photocouplers C 2 Up, C 2 Vp, C 2 Wp, C 2 Un, C 2 Vn, C 2 Wn. 
     The drive circuits D 1 Up, D 1 Vp, D 1 Wp, D 1 Un, D 1 Vn, D 1 Wn are configured to drive the respective switching elements S 1 Up, S 1 Vp, S 1 Wp, S 1 Un, S 1 Vn, S 1 Wn forming the first inverter  100 , photocouplers C 1 Up, C 1 Vp, C 1 Wp, C 1 Un, C 1 Vn, C 1 Wn, and third and fourth multiplexers  140   c ,  140   d  are mounted on the circuit board. In the present embodiment, each of the third and fourth multiplexers  140   c ,  140   d  has three input ports and one output port. The drive circuits D 1 Up, D 1 Vp, D 1 Wp, D 1 Un, D 1 Vn, D 1 Wn are configured to receive temperature signals from temperature-sensitive diodes T 1 Up, T 1 Vp, T 1 Wp, T 1 Un, T 1 Vn, T 1 Wn for detecting temperatures of the respective switching elements S 1 Up, S 1 Vp, S 1 Wp, S 1 Un, S 1 Vn, S 1 Wn. The drive circuits D 1 Up, D 1 Vp, D 1 Wp, D 1 Un, D 1 Vn, D 1 Wn are configured to convert the received temperature signals into duty signals, which in turn are input to the respective photocouplers C 1 Up, C 1 Vp, C 1 Wp, C 1 Un, C 1 Vn, C 1 Wn. 
     The drive circuits DCpa, DCpb, DCna, DCnb are configured to drive the respective switching elements SCpa, SCpb, SCna, SCnb forming the buck-boost converter  90 , photocouplers CCpa, CCpb, CCna, CCnb, and fifth and sixth multiplexers  140   e ,  140   f  are mounted on the circuit board. In the present embodiment, each of the fifth and sixth multiplexers  140   e ,  140   f  has two input ports and one output port. The drive circuits DCpa, DCpb, DCna, DCnb are configured to receive temperature signals from temperature-sensitive diodes TCpa, TCpb, TCna, TCnb for detecting temperatures of the respective switching elements SCpa, SCpb, SCna, SCnb. The drive circuits DCpa, DCpb, DCna, DCnb are configured to convert the received temperature signals into duty signals, which in turn are input to the respective photocouplers CCpa, CCpb, CCna, CCnb. 
     Temperature signals outputted from outputs of the respective U-, V-phase upper-arm photocouplers C 2 Up, C 2 Vp are input to the respective input ports of the first multiplexer  140   a . The output port of the first multiplexer  140   a  is electrically connected to a first input port T 1  of the microcomputer  80  via a first electrical path L 1 . Outputs of the respective W-phase upper-arm photocoupler C 2 Wp and U-, V-phase lower-arm photocouplers C 2 Un, C 2 Vn are electrically connected to a second input port T 2  of the microcomputer  80  via the second multiplexer  140   b  and a second electrical path L 2 . An output of the W-phase lower-arm photocoupler C 2 Wn is electrically connected to a third input port T 3  of the microcomputer  80  via a third electrical path L 3 . 
     Outputs of the respective U-, V-, W-phase upper-arm photocouplers C 1 Up, C 1 Vp, C 1 Wp are electrically connected to a fourth input port T 4  of the microcomputer  80  via a third multiplexer  140   c  and a fourth electrical path L 4 . Outputs of the respective U-, V-, W-phase lower-arm photocouplers C 1 Un, C 1 Vn, C 1 Wn are electrically connected to a fifth input port T 5  of the microcomputer  80  via a fourth multiplexer  140   d  and a fifth electrical path L 5 . 
     Outputs of the respective upper-arm voltage-transformation photocouplers CCpa, CCpb are electrically connected to a sixth input port T 6  of the microcomputer  80  via a fifth multiplexer  140   e  and a sixth electrical path L 6 . Outputs of the respective lower-arm voltage-transformation photocouplers CCna, CCnb are electrically connected to a seventh input port T 7  of the microcomputer  80  via a sixth multiplexer  140   f  and a seventh electrical path L 7 . 
     The microcomputer  80  is configured to detect temperatures of the respective whole switching elements forming the first inverter  100 , the second inverter  102 , and the buck-boost converter  90  based on the temperature signals received at the respective input port T 1 -T 7  of the microcomputer  80 . The microcomputer  80  is configured to, when determining that the highest one ST of the detected temperatures exceeds a threshold temperature Tγ, conduct the power saving process. 
     In the present embodiment, a temperature signal from the temperature-sensitive diode T 2 Wn associated with the W-phase lower-arm switching element S 2 Wn forming the second inverter  102  is input to the microcomputer  80  without passing through any multiplexer. This is because, among the buck-boost converter  90 , the first inverter  100 , and the second inverter  102 , the switching elements forming the second inverter  102  may be assumed to be hotter than the switching elements forming the buck-boost converter  90  and the first inverter  100  during use. Particularly, in the present embodiment, among the switching elements forming the second inverter  102 , the W-phase lower-arm switching element S 2 Wn is assumed to be hottest during use. 
     The two-motor control system in a buck and boost configuration of the present embodiment can provide similar advantages to those of the first embodiment. 
     Further, in the present embodiment, only the temperature signals of the respective switching elements forming the second inverter  102  are input to the first and second multiplexers  140   a ,  140   b . Only the temperature signals of the respective switching elements forming the first inverter  100  are input to the third and fourth multiplexers  140   c ,  140   d . Only the temperature signals of the respective switching elements forming the buck-boost converter  90  are input to the fifth and sixth multiplexers  140   e ,  140   f . In such a configuration, the microcomputer  80  can readily determine each of the temperature signals received at the respective input ports of the microcomputer  80  corresponds to which of the buck-boost converter  90 , the first inverter  100 , and the second inverter  102 . For example, when a signal received at one of the input ports of the microcomputer  80  indicating the presence of an abnormality, the microcomputer  80  can determine the signal has come from which of the buck-boost converter  90 , the first inverter  100 , and the second inverter  102 . 
     Fourth Embodiment 
     There will now be explained a fourth embodiment of the present invention. Only differences of the fourth embodiment from the third embodiment will be explained with reference to the accompanying drawings. In the present embodiment, as shown in  FIG. 9 , for each of the buck-boost converter  90 , the first inverter  100 , and the second inverter  102 , one temperature signal is input directly to the microcomputer  80  without passing through any multiplexer. In  FIG. 9 , members same as the members shown in  FIG. 8  are assigned the same numbers. 
     As shown in  FIG. 9 , outputs of the respective U-, V-phase lower-arm photocouplers C 1 Un, C 1 Vn for the first inverter  100  are electrically connected to a fifth input port T 5  of the microcomputer  80  via a fourth multiplexer  140   g  and a fifth electrical path L 5 . The output of the W-phase lower-arm photocoupler C 1 Wn is electrically connected directly to a sixth input port T 6  of the microcomputer  80  via a sixth electrical path L 6  without any multiplexer. This is because, in the present embodiment, the W-phase lower-arm switching element S 1 Wn may be assumed to be hottest among the switching elements forming the first inverter  100  during use. 
     Outputs of the respective voltage-transformation photocouplers CCpa, CCpb, CCna are electrically connected to a seventh input port T 7  of the microcomputer  80  via a fifth multiplexer  140   h  and a seventh electrical path L 7 . In addition, the output of the lower-arm voltage-transformation photocoupler CCnb is are electrically connected to an eighth input port T 8  of the microcomputer  80  via an eighth electrical path L 8  without any multiplexer. This is because, in the present embodiment, the lower-arm voltage-transformation switching element SCnb may be assumed to be hottest among the switching elements forming the buck-boost converter  90  during use. 
     Advantages of the present embodiment will now be explained. The operational aspect or the value of flowing current may be different between the buck-boost converter  90 , the first inverter  100 , and the second inverter  102 . Thus, heat generation amounts of the respective switching elements may be different between the buck-boost converter  90 , the first inverter  100 , and the second inverter  102 , which leads to different temperatures of the respective switching elements between the buck-boost converter  90 , the first inverter  100 , and the second inverter  102 . In the present embodiment, for each of the buck-boost converter  90 , the first inverter  100 , and the second inverter  102 , one temperature signal is input directly to the microcomputer  80  without passing through any multiplexer. This can enhance the accuracy of detecting the temperatures of the whole switching elements. 
     Fifth Embodiment 
     There will now be explained a fifth embodiment of the present invention. Only differences of the fifth embodiment from the fourth embodiment will be explained with reference to the accompanying drawings. In the present embodiment, as shown in  FIG. 10 , for the buck-boost converter  90 , the temperature signal of one of the upper-arm switching elements and the temperature signal of one of the lower-arm switching elements are input directly to the microcomputer  80  without passing through any multiplexer. In  FIG. 10 , members same as the members shown in  FIG. 9  are assigned the same numbers. 
     As shown in  10 , the output of the upper-arm voltage-transformation photocoupler CCpa is electrically connected to the seventh input port T 7  of the microcomputer  80  via the seventh electrical path L 7  without any multiplexer. This is because, in the present embodiment, the switching element SCpa may be assumed to be hottest among the upper-arm voltage-transformation switching elements SCpa, SCpb during use. The outputs of the respective voltage-transformation photocouplers CCpb, CCna are electrically connected to the eighth input port T 8  of the microcomputer  80  via a fifth multiplexer  140   i  and the eighth electrical path L 8 . The output of the lower-arm voltage-transformation photocoupler CCnb is electrically connected to a ninth input port T 9  of the microcomputer  80  via a ninth electrical path L 9  without any multiplexer. This is because, in the present embodiment, the switching element SCpb may be assumed to be hottest among the lower-arm voltage-transformation switching elements SCna, SCnb during use. 
     Advantages of the present embodiment will now be explained. In the buck-boost converter  90 , the switching elements that are turned on and off during the voltage step-up operation are different from the switching elements that are turned on and off during the voltage step-down operation. Therefore, during the voltage step-up operation, the temperatures of the respective lower-arm voltage-transformation switching elements SCna, SCnb are higher than the temperatures of the respective upper-arm voltage-transformation switching elements SCpa, SCpb. During the voltage step-down operation, the temperatures of the respective upper-arm voltage-transformation switching elements SCpa, SCpb are higher than the temperatures of the respective lower-arm voltage-transformation a switching elements SCna, SCnb. Therefore, in the present embodiment, the temperature signal of one of the upper-arm switching elements and the temperature signal of one of the lower-arm switching elements are input directly to the microcomputer  80  without passing through any multiplexer. This can enhance the accuracy of detecting the temperatures of the whole switching elements. 
     Sixth Embodiment 
     There will now be explained a sixth embodiment of the present invention. Only differences of the sixth embodiment from the third to fifth embodiments will be explained with reference to the accompanying drawings. In the present embodiment, as shown in  FIG. 11 , the buck-boost converter  90   a  is modified in configuration. In  FIG. 11 , members same as the members shown in  FIG. 7  are assigned the same numbers. 
     In the present embodiment, as shown in  FIG. 11 , the buck-boost converter  90   a  includes a lower-arm series connection of first and second switching elements S 1 , S 2  and an upper-arm series connection of third and fourth switching elements S 3 , S 4 . In the present embodiment, an IGBT is used for each of the switching elements S 1 , S 2 , S 3 , S 4 . The switching elements S 1 , S 2 , S 3 , S 4  are electrically connected in anti-parallel with respective freewheel diodes D 1 , D 2 , D 3 , D 4 . 
     A junction between the second switching element S 2  and the third switching element S 3  is electrically connected to the first end of the reactor  92 . A junction between the first switching element S 1  and the second switching element S 2  is electrically connected to a junction between the third switching element S 3  and the fourth switching element S 4  through an auxiliary capacitor  94 . The emitter of the first switching element S 1  is the negative terminal of the high-voltage battery  72 . 
     The voltage step-up operation and the voltage step-down operation of the buck-boost converter  90   a  will now be explained. In the present embodiment, the voltage step-up operation is implemented by combinations of first to fourth modes. In the first mode, the first and second switching elements S 1 , S 2  are turned on, and the third and fourth switching elements S 3 , S 4  are turned off. In the second mode, the first and third switching elements S 1 , S 3  are turned on, and the second and fourth switching elements S 2 , S 4  are turned off. In the third mode, the second and fourth switching elements S 2 , S 4  are turned on, and the first and third switching elements S 1 , S 3  are turned off. In the fourth mode, the third and fourth switching elements S 3 , S 4  are turned on, and the first and second switching elements S 1 , S 2  are turned off. The voltage step-up operation may be implemented by a cyclic sequence of modes, for example, the first mode→the second mode→the first mode→the third mode or the second mode→the fourth mode→the third mode→the fourth mode. 
     In the present embodiment, the voltage step-up operation is implemented by combinations of fifth to eighth modes. In the fifth mode, the first to fourth switching elements S 1 -S 4  are all turned off. In the sixth mode, the third switching element S 3  is turned on, and the first, second and fourth switching elements S 1 , S 2 , S 4  are turned off. In the seventh mode, the fourth switching element S 4  is turned on, and the first to third switching elements S 1 -S 3  are turned off. In the eighth mode, the third and fourth switching elements S 3 , S 4  are turned on, and the first and second switching elements S 1 , S 2  are turned off. The voltage step-down operation may be implemented by a cyclic sequence of modes, for example, the fifth mode→the sixth mode→the fifth mode→the seventh mode or the eighth mode→the sixth mode→the eighth mode→the seventh mode. 
     As described above, as in the third to fifth embodiments, during the voltage step-up operation, only the first and second lower-arm switching elements S 1 , S 2  are turned on and off. During the voltage step-down operation, only the third and fourth upper-arm switching elements S 3 , S 4  are turned on and off. Therefore, as in the third to fifth embodiments shown in  FIGS. 8-10 , a similar configuration for detecting temperatures of the switching elements forming the buck-boost converter  90   a  is applicable in the present embodiment. More specifically, in the present embodiment, for example, the lower-arm voltage-transformation switching elements SCna, SCnb in each of the third to fifth embodiments may be replaced with the first and second switching elements S 1 , S 2 , and the upper-arm voltage-transformation switching elements SCpa, SCpb may be replaced with the third and fourth switching elements S 3 , S 4 . 
     Seventh Embodiment 
     There will now be explained a seventh embodiment of the present invention. Only differences of the seventh embodiment from the third embodiment will be explained with reference to the accompanying drawings. In the present embodiment, as shown in  FIG. 12 , the buck-boost converter  90  is absent in the motor control system. In  FIG. 12 , members same as the members shown in  FIG. 7  are assigned the same numbers. 
     The present embodiment can thus provide similar advantages to those of the third embodiment. 
     Eighth Embodiment 
     There will now be explained an eighth embodiment of the present invention. Only differences of the eighth embodiment from the fifth embodiment will be explained with reference to the accompanying drawings. In the present embodiment, as in the buck-boost converter  90 , each of the first inverter  100  and the second inverter  102  includes upper-arm parallel connections (for the respective phases) of two switching elements and lower-arm parallel connections (for the respective phases) of two switching elements. Accordingly, as shown in  FIG. 13 , in each of the first inverter  100  and the second inverter  102 , each pair of two switching elements connected in parallel with each other are turned on and off by their shared drive circuit. Each drive circuit is configured to receive temperature signals from the temperature-sensitive diodes respectively associated with the two switching elements connected in parallel with each other, and output a time-multiplexed signal for the received temperature signals to the photocoupler. This is because the temperatures of the respective two switching elements connected in parallel with each other may be different from each other. In  FIG. 13 , for each of the first inverter  100  and the second inverter  102 , each pair of semiconductor modules including the respective two switching elements connected in parallel with each other are assigned the same numbers. 
     As shown in  FIG. 13 , the temperature signals outputted from the temperature-sensitive diodes TCpa, TCpb associated with the upper-arm switching elements are input to the shaped drive circuit DCpa. The drive circuit DCpa is configured to output the time-multiplexed signal to the input of the upper-arm voltage-transformation photocoupler CCpa. The output of the upper-arm voltage-transformation photocoupler CCpa is connected directly to a seventh input port T 7  of the microcomputer  80  via a seventh electrical path L 7  without any multiplexer. The temperature signals outputted from the temperature-sensitive diodes TCna, TCnb associated with the lower-arm switching elements are input to the shaped drive circuit DCna. The drive circuit DCna is configured to output the time-multiplexed signal to the input of the lower-arm voltage-transformation photocoupler CCna. The output of the lower-arm voltage-transformation photocoupler CCna is connected directly to an eighth input port T 8  of the microcomputer  80  via an eighth electrical path L 8  without any multiplexer. In the present embodiment, the control system does not include any multiplexer associated with the buck-boost converter  90 . The temperature detection configuration for each of the first inverter  100  and the second inverter  102  based on the time multiplexing is similar to that of the buck-boost converter  90 . 
     The time multiplexing performed by each drive circuit will now be explained with reference to  FIG. 14 . In time multiplexing, the drive circuit generates and outputs a frame signal including a header, a first temperature signal Ta, and a second temperature signal Tb, in this order. In the present embodiment, the temperature signals Ta, Tb are duty signals correlated to the temperatures. More specifically, as an example, the upper-arm drive circuit DCpa of the buck-boost converter  90  operates as follows. The first temperature signal Ta is a temperature signal of the switching element SCpa based on the output signal of the temperature-sensitive diode TCpa, and the second temperature signal Tb is a temperature signal of the switching element SCpb based on the output signal of the temperature-sensitive diode TCpb. Upon receipt of the time-multiplexed frame signal, the microcomputer  80  calculates temperatures of the respective two switching elements based on the received frame signal. 
     In the present embodiment, even though the upper-arm switching elements are electrically connected in parallel with each other and the lower-arm switching elements are electrically connected in parallel with each other, the temperature detection accuracy can be enhanced while suppressing the number of components. 
     Other Embodiments 
     There will now be explained modifications to the respective above embodiments. 
     In the first embodiment, there are the plurality of multiplexers. Alternatively, the number of multiplexers may be one. 
     Power converters other than the three-phase inverter are applicable to the present invention. Electronic devices having switching elements, other than power converters, may also be applicable. The number of switching elements in each of such power converters or electronic devices is not limited to six, and may be equal to or greater than three, but not six. For each of such power converters or electronic devices, at least three switching elements allow at least one temperature signal to be input directly to the microcomputer without any multiplexer and allows at least two temperature signals to be input to the microcomputer via the multiplexer. 
     More specifically, such a system as above may include, for example, the two-motor control system having two motor generators. Since such a system includes two three-phase inverters as power converters or electronic devices, the number of switching elements is equal to or greater than twelve, where the number of multiplexers is not limited to two, and may be equal to or greater than three. For example, when the number of switching elements is twelve and the number of multiplexers is three, first to three multiplexers may have three, four, and four input ports, respectively. Even though a difference in number of input ports between these multiplexers is up to one, variations in the lag time of transfer of the temperature signal from the temperature-sensitive diode to the microcomputer via the multiplexer can be minimized. This can thus minimize the time intervals between detection of temperatures of a specific switching element. 
     In the first embodiment, only one temperature signal may be input directly to the microcomputer without any multiplexer, where, as an example, the first multiplexer may have three input ports. 
     Some specifications of the three-phase inverter  10  may cause the W-phase lower-arm semiconductor module  50 Wn that is most far away from the upstream portion of the inlet line  30   a  and adjacent the pressing member  60  to be hottest during use. 
     In the first embodiment, for each of the multiplexers, when the number of output ports is less than the number of input ports, the multiplexer may have two or more output ports. 
     Each switching element is not limited to the single IGBT, and may be a parallel connection of plural IGBTs. More specifically, collector terminals of the plural IGBTs are electrically connected to each other, and emitter terminals of the plural IGBTs are electrically connected to each other. In such a configuration, the actual temperature variation as shown in  FIG. 5  will be increased due to a “parallel-connection variation” that is a variation in temperature during use between the plural IGBTs caused by a variation in flowing collector current between the plural IGBTs. The parallel-connection variation exists in each of the detection element and the non-detection element shown in  FIG. 5 . Thus, the actual temperature variation may be doubled as compared with the actual temperature variation of  FIG. 5 . Accordingly, the threshold temperature Tγ needs to be set lower for safety. A temperature range where use is restricted is thus prone to be enlarged. Therefore, in such a configuration, it is more advantageous to detect temperatures of the respective whole switching elements. 
     The six switching elements are individually modularized as the six semiconductor modules. Alternatively, for example, the six switching elements are connected in series with each other, and lower-arm switching elements may be modularized as a single semiconductor module. 
     Each insulating transfer element for transferring a signal from the high voltage domain to the low voltage domain while electrically isolating the high and low voltage domains from each other is not limited to the optical insulating transfer element. Alternatively, the insulating transfer element may be a magnetic insulating transfer element, such as a pulse transformer. 
     Instead of the IGBT, for example, a metal-oxide semiconductor field-effect transistor (MOSFET) may be used as each of the switching elements that produce heat when energized. 
     Each temperature signal output for outputting the temperature signal correlated to the temperature of one of the switching elements is not limited to the temperature-sensitive diode. Alternatively, each temperature signal output for outputting the temperature signal correlated to the temperature of one of the switching elements may be a thermistor. A copper pattern is formed on a first side (e.g., a top surface) of an insulated substrate, and a heat sink is mounted on a second side (e.g., a bottom surface) of the substrate. The switching element and the freewheel diode are mounted on the substrate by soldering or the like, thereby forming a module. The switching element and the freewheel diode are electrically connected via the pattern, and the thermistor may be mounted on the pattern of the module. Heat produced in the switching element or the freewheel diode dissipated to the thermistor via the pattern. Therefore, the temperature of at least one of the switching element and the freewheel diode can be detected. 
     Each power-conversion semiconductor element that produces heat when energized is not limited to the IGBT. Alternatively, each power-conversion semiconductor element that produces heat when energized may include a MOSFET or an RC-IGBT. The RC-IGBT is a diode-embedded IGBT such that the IGBT and the freewheel diode are mounted on the same chip. In addition, each power-conversion semiconductor element that produces heat when energized is not limited to the switching element as a power semiconductor. Alternatively, the power-conversion semiconductor elements that produce heat when energized may include the freewheel diode. For example, in the buck-boost converter, a large amount of heat may be generated in the upper-arm freewheel diode during voltage step-up operation. Therefore, it is advantageous to detect a temperature of the freewheel diode. 
     In each of the third to sixth, and eighth embodiments, the buck-boost converter is used. Alternatively, either a boost converter or a buck converter may be used. 
     In the third embodiment, for example, one of the switching elements forming the buck-boost converter  90  or the first inverter  100  may be hottest among the whole switching elements forming the buck-boost converter  90 , the first inverter  100 , and the second an inverter  102 . For example, when one of the switching elements forming the buck-boost converter  90  is hottest during use, only the switching elements forming the buck-boost converter  90  may be connected directly to the microcomputer  80  without any multiplexer. 
     In each of the third to seventh embodiments, for each of the buck-boost converter  90 , the first inverter  100 , and the second inverter  102 , at least one multiplexer may have four or more input ports. For example, each of the buck-boost converter  90 , the first inverter  100 , and the second inverter  102  may each have a single multiplexer. In the third embodiment, for example, a single multiplexer for the second inverter  102  may have five input ports, a single multiplexer for the first inverter  100  may have six input ports, and a single multiplexer for the buck-boost converter  90  may have four input ports. 
     In each of the third to seventh embodiments, the multiplexers may each receive temperature signals from at least two of the buck-boost converter  90 , the first inverter  100  and the second inverter  102 . In the eighth embodiment, the multiplexers may each receive temperature signals from both the first inverter  100  and the second inverter  102 . 
     In each of the third to eighth embodiments, the number of motor generators may be greater than two. 
     In the third embodiment, the high-voltage battery  72  may be replaced with a low-voltage battery having a lower output voltage (e.g., 12V) than the high-voltage battery  72 . Such a configuration may be applied to a system where, for example, an engine is only one vehicle prime mover and the motor generator is used as an integrated starter generator (ISG) that integrates starter and alternator functionalities. 
     In the seventh embodiment, the high-voltage battery  72  electrically connected to the first inverter  100  may be replaced with a low-voltage battery having a lower output voltage (e.g., 12V) than the high-voltage battery  72 . Such a configuration may be applied to a system where, for example, an engine and a motor are mounted in the vehicle as vehicle prime movers and the first motor generator  101  is used as an integrated starter generator (ISG) that integrates starter and alternator functionalities. 
     In the eighth embodiment, the motor control system may not have the buck-boost converter  90 . In addition, in the eighth embodiment, the motor control system may include a single motor generator. 
     In each of the third to seventh embodiments, the buck-boost converter  90  may be formed by parallel connections of three or more switching elements, and each of the first and second inverters  100 ,  102  may be formed by parallel connections of two or more switching elements. In the eighth embodiment, each of the buck-boost converter  90 , the first inverter  100 , and the second inverter  102  may be formed by parallel connections of three or more switching elements, and each of the first and second inverters  100 ,  102  may be formed by parallel connections of three or more switching elements. Further, at least one of the first inverter  100 , the second inverter  102 , and the buck-boost converter  90  may be formed by parallel connections of plural switching elements. 
     In the eighth embodiment, each drive circuit may not perform any time multiplexing process. In such a case, each temperature-sensitive diode individually includes the drive circuit and the photocoupler. 
     In the eighth embodiment, the number of input ports for each of the multiplexers respectively corresponding to the first inverter  100  and the second inverter  102  may be equal to or greater than four. More specifically, the number of input ports for the multiplexer corresponding to the second an inverter  102  may be five, and the number of input ports for the multiplexer corresponding to the first an inverter  100  may be five. This allows the number of multiplexers for each of the first inverter  100  and the second inverter  102  to be one. Given a multiplexer having ten input ports, the first inverter  100  and the second inverter  102  may share such a multiplexer, which allows the number of multiplexers for the first inverter  100  and the second inverter  102  to be one. 
     In each of the third to eighth embodiments, the outputs of the respective whole photocouplers electrically connected to the respective drive circuits may be connected directly to the input ports of the microcomputer  80  without any multiplexer. 
     In each of the first and second embodiments, the inverter may be formed by parallel connections of plural switching elements, where for each parallel connection of plural switching elements, the temperature for the parallel connection of plural switching elements may be transferred from a drive circuit shared by the plural switching elements to the microcomputer via the photocoupler by using the time multiplexing process.