Abstract:
A semiconductor circuit device includes a semiconductor circuit including a switching element, a temperature monitoring unit, and a control unit. The temperature monitoring unit detects or estimates a temperature of a component connected to an inside or an outside of the semiconductor circuit. Here, the temperature of the component changes in accordance with a frequency of a current flowing through the component, and the frequency of the current flowing through the component changes in accordance with a switching frequency of the switching element. The control unit adjusts the switching frequency of the switching element such that the temperature of the component is equal to a target temperature.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor circuit device including a switching element, and particularly to a semiconductor circuit device including a direct current (DC) to direct current (DC) conversion circuit or a direct current (DC) to alternating current (AC) conversion circuit using a switching element. 
         [0003]    2. Description of the Background Art 
         [0004]    In DC-DC conversion circuits (DC-DC converters) and DC-AC conversion circuits (inverter circuits), overheating due to switching loss in a semiconductor switching element often becomes problematic. As a method of suppressing overheating of a semiconductor switching element, for example, the following methods are known. 
         [0005]    A method described in Japanese Patent Laying-Open No. 03-178565 is directed to an inverter device in which a switching element in an inverter main circuit is switched in response to a pulse width modulation signal having a given carrier frequency. The inverter device described in this document includes temperature detection means detecting a temperature of the switching element, comparison means comparing the temperature detected by the temperature detection means with a reference temperature, and correction means correcting the carrier frequency of the pulse width modulation signal based on an output signal from the comparison means. 
         [0006]    Japanese Patent Laying-Open No. 2005-198406 discloses a boost device (boost chopper) capable of reducing switching loss. In the boost device, a gate resistance value of an IGBT is optimized in accordance with a boost voltage. Thereby, the switching loss can be reduced. 
         [0007]    Similarly, in semiconductor integrated circuits, as a result of an increase in power consumption along with an improvement in the degree of integration, overheating and thermal runaway of an integrated circuit often become problematic. As a technique for reducing power consumption, for example, Japanese Patent Laying-Open No. 2008-124125 discloses a semiconductor integrated circuit device with a configuration described below. The semiconductor integrated circuit device includes a temperature sensor capable of detecting a temperature, determining, for each reference value, whether or not the detection result exceeds the reference value, and outputting a signal representing the determination result, and a control block capable of controlling an operation of a calculation block based on the output signal of the temperature sensor. The control block returns from a suspended state to an operation state by an interrupt signal based on the output signal of the temperature sensor, and determines an operation condition for the calculation block to satisfy a temperature condition for the calculation block. 
         [0008]    In addition to a semiconductor switching element, overheating of a motor connected to an inverter circuit may become problematic. Japanese Patent Laying-Open No. 08-182387 discloses a method for suppressing temporary overheating of an AC motor without stopping an operation of the AC motor. According to the method, if a Central Processing Unit (CPU) detects that a temperature detected by a temperature detection element attached to a stator winding of the AC motor exceeds an upper set temperature, the CPU decreases an inverter output frequency by Δf with an appropriate time gradient. If the detected temperature becomes not more than a lower set temperature, the CPU increases the inverter output frequency by Δf with an appropriate time gradient. 
         [0009]    In a Pulse Width Modulation (PWM) DC-DC converter, a higher switching frequency is preferable because ripples in an output are reduced and a coil (reactor) can be downsized. However, as the switching frequency is increased, high frequency loss in the coil is increased, and thus overheating due to the high frequency loss becomes problematic. Similarly, in a case where a motor is driven by a PWM inverter circuit, if a carrier frequency is increased, overheating of a stator winding due to high frequency loss becomes problematic. 
         [0010]    As a method of suppressing overheating of a coil, a stator winding, and the like, it is contemplated to reduce an output voltage of a DC-DC converter, or to reduce an output voltage (current) or an output frequency of an inverter circuit. Although such a method is effective as a method of suppressing temporary overheating, it has a problem in a case where a device connected to a DC-DC converter or an inverter circuit is steadily operated. For example, in a system in which an inverter circuit is driven by an output of a DC-DC converter and a motor is driven by an output of the inverter circuit, overheating of a coil provided to the DC-DC converter can be suppressed by reducing an output voltage of the DC-DC converter. However, since it is necessary to increase an output current of the inverter circuit to maintain an output of the motor, overheating may occur in a stator winding of the motor. 
       SUMMARY OF THE INVENTION 
       [0011]    One object of the present invention is to prevent overheating of a component connected to a semiconductor circuit including a switching element such as a DC-DC converter and an inverter circuit. 
         [0012]    A semiconductor circuit device according to one aspect of the present invention includes a semiconductor circuit including a switching element, a temperature monitoring unit, and a control unit. The temperature monitoring unit detects or estimates a temperature of a component connected to an inside or an outside of the semiconductor circuit. Here, the temperature of the component changes in accordance with a frequency of a current flowing through the component, and the frequency of the current flowing through the component changes in accordance with a switching frequency of the switching element. The control unit adjusts the switching frequency of the switching element such that the temperature of the component is equal to a first target temperature. 
         [0013]    Therefore, a main advantage of the present invention is that overheating of the component can be prevented by feedback-controlling the switching frequency of the switching element such that the temperature of the component is equal to the first target temperature. 
         [0014]    The foregoing and other objects, features, aspects and advantages of the present invention will become apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram showing a configuration of a semiconductor circuit device  1  according to Embodiment 1 of the present invention. 
           [0016]      FIG. 2  is a block diagram showing an operation of a controller  40  in  FIG. 1 . 
           [0017]      FIG. 3  is a view of a temperature versus life line of a synthetic resin used to mold a reactor  16 . 
           [0018]      FIG. 4  is a block diagram showing a configuration of a semiconductor circuit device  2  according to Embodiment 2 of the present invention. 
           [0019]      FIG. 5  is a circuit diagram showing an exemplary configuration of a variable resistance element VR in  FIG. 4 . 
           [0020]      FIG. 6  is a block diagram showing an operation of a controller  40 A in  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. It is to be noted that identical or corresponding parts will be designated by the same reference numerals, and the description thereof will not be repeated 
       Embodiment 1 
       [0022]      FIG. 1  is a block diagram showing a configuration of a semiconductor circuit device  1  according to Embodiment 1 of the present invention. 
         [0023]    Referring to  FIG. 1 , semiconductor circuit device  1  includes a boost chopper  10  (DC-DC converter), an inverter circuit  20 , a voltage detection unit  41 , a current detection unit  44 , temperature sensors RT, MT, and a controller  40 . A motor  99  is connected to an output of inverter circuit  20 . 
         [0024]    Boost chopper  10  includes Insulated Gate Bipolar Transistors (IGBTs) QA 1 , QA 2 , diodes DA 1 , DA 2 , resistance elements  13 ,  14 , gate drive circuits  11 ,  12 , a reactor  16 , and a capacitor  17 . IGBTs QA 1 , QA 2  are connected in series in this order between a positive-electrode-side node P 1  and a negative-electrode-side node N 1 . Diodes DA 1 , DA 2  are connected in parallel to IGBTs QA 1 , QA 2 , respectively, in a reverse bias direction. Gate drive circuit  11  is connected to a gate of IGBT QA 1  via resistance element  13  (gate resistor). IGBT QA 1  is controlled by a gate control signal SA 1  to be always in an OFF state. Gate drive circuit  12  is connected to a gate of IGBT QA 2  via resistance element  14 , to switch IGBT QA 2  in accordance with a gate control signal SA 2 . Reactor  16  has one end connected to a connection node  18  between IGBTs QA 1  and QA 2 , and the other end to which a DC voltage is applied by a DC power source  15 . Capacitor  17  is a smoothing capacitor connected between nodes P 1  and N 1 . 
         [0025]    An output voltage Vout of boost chopper  10  output from between nodes P 1  and N 1  is represented as follows, using a duty ratio α=Ton/Ts: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Vout 
                         = 
                           
                          
                         
                           Vin 
                           × 
                           
                             
                               ( 
                               
                                 Ton 
                                 + 
                                 Toff 
                               
                               ) 
                             
                             / 
                             Toff 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           Vin 
                           × 
                           
                             Ts 
                             / 
                             Toff 
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           = 
                             
                            
                           
                             Vin 
                             × 
                             
                               1 
                               / 
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   α 
                                 
                                 ) 
                               
                             
                           
                         
                         , 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0026]    where Vin represents an input voltage from DC power source  15 , Ton represents ON time of IGBT QA 2 , Toff represents OFF time of IGBT QA 2 , and Ts (an inverse of a switching frequency fs) represents a switching cycle of IGBT QA 2 . 
         [0027]    Inverter circuit  20  is a PWM inverter converting DC power input from boost chopper  10  into three-phase AC power by the PWM method. Inverter circuit  20  includes IGBTs QU 1 , QU 2 , QV 1 , QV 2 , QW 1 , QW 2 , diodes DU 1 , DU 2 , DV 1 , DV 2 , DW 1 , DW 2 , resistance elements  27  to  32 , and gate drive circuits  21  to  26 . IGBTs QU 1 , QU 2  (U phase arms) are connected in series in this order between a positive-electrode-side node P 2  and a negative-electrode-side node N 2 . IGBTs QV 1 , QV 2  (V phase arms) are connected in series in this order between positive-electrode-side node P 2  and negative-electrode-side node N 2 , and connected in parallel to the U phase arms. IGBTs QW 1 , QW 2  (W phase arms) are connected in series in this order between positive-electrode-side node P 2  and negative-electrode-side node N 2 , and connected in parallel to the U phase and V phase arms. Diodes DU 1 , DU 2 , DV 1 , DV 2 , DW 1 , DW 2  are connected in parallel to IGBTs QU 1 , QU 2 , QV 1 , QV 2 , QW 1 , QW 2 , respectively, in the reverse bias direction. Resistance elements  27  to  32  are gate resistors provided corresponding to IGBTs QU 1 , QU 2 , QV 1 , QV 2 , QW 1 , QW 2 , respectively. Gate drive circuits  21  to  26  are provided corresponding to IGBTs QU 1 , QU 2 , QV 1 , QV 2 , QW 1 , QW 2 , respectively, to switch the corresponding IGBTs in accordance with gate control signals SU 1 , SU 2 , SV 1 , SV 2 , SW 1 , SW 2 , respectively. Pseudo three-phase AC power generated from a connection node U between IGBTs QU 1  and QU 2 , a connection node V between IGBTs QV 1  and QV 2 , and a connection node W between IGBTs QW 1  and QW 2  by the PWM method is output to motor  99 . 
         [0028]    Voltage detection unit  41  includes resistance elements  42 ,  43  connected in series between nodes P 1  and N 1 . Voltage detection unit  41  detects the voltage obtained by dividing the output voltage of boost chopper  10  by resistance elements  42 ,  43 . 
         [0029]    Current detection unit  44  is a shunt resistor connected between negative-electrode-side nodes N 1  and N 2 , and an output current of inverter circuit  20  is monitored by detecting a voltage of the shunt resistor. Instead of the shunt resistor provided on an input side of inverter circuit  20 , an instrument current transformer may be provided to each phase on an output side of inverter circuit  20 . 
         [0030]    Temperature sensor RT is attached to an insulator made of a synthetic resin for molding reactor  16  (coil) to monitor a temperature of the insulator made of the synthetic resin. As a switching frequency of IGBT QA 2  is increased, a temperature of reactor  16  is increased by high frequency loss due to skin effect, proximity effect, eddy current loss, and the like. Deterioration of the synthetic resin for molding is suppressed by adjusting the switching frequency of IGBT QA 2  such that the temperature of reactor  16  coincides with a target temperature, as described later. 
         [0031]    Instead of a method of directly measuring the temperature with temperature sensor RT, the temperature of reactor  16  may be estimated based on a current flowing through reactor  16 . In this case, the relationship among the current flowing through reactor  16 , the switching frequency of IGBT QA 2 , and the temperature of reactor  16  is measured beforehand, and the temperature of reactor  16  is estimated based on the current flowing through reactor  16  actually measured and the switching frequency. 
         [0032]    Temperature sensor MT is attached to an insulator made of a synthetic resin for molding a stator winding of motor  99  to monitor a temperature of the insulator made of the synthetic resin. As a frequency of a carrier wave used in PWM inverter circuit  20  is increased, a temperature of the stator winding is increased by high frequency loss due to skin effect, proximity effect, eddy current loss, and the like. Deterioration of the synthetic resin for molding is suppressed by adjusting switching frequencies of IGBTs QU 1 , QU 2 , QV 1 , QV 2 , QW 1 , QW 2  such that the temperature of the stator winding of motor  99  coincides with a target temperature, as described later. 
         [0033]    Controller  40  generates gate control signal SA 2  based on output signals of voltage detection unit  41  and temperature sensor RT. Further, controller  40  generates gate control signals SU 1 , SU 2 , SV 1 , SV 2 , SW 1 , SW 2  based on output signals of current detection unit  44  and temperature sensor MT. Hereinafter, an operation of controller  40  will be described in detail. 
         [0034]      FIG. 2  is a block diagram showing an operation of controller  40  in  FIG. 1 . 
         [0035]    Referring to  FIGS. 1 and 2 , controller  40  includes a control unit  68  controlling boost chopper  10  (DC-DC converter), and a control unit  70  controlling inverter circuit  20 . Firstly, controller  40  determines a target boost voltage  61  and a target current  71  such that motor  99  generates a target torque  60 , based on a table prepared beforehand. Target boost voltage  61  is used to control boost chopper  10 , and target current  71  is used to control inverter circuit  20 . 
         [0036]    Control unit  68  controlling boost chopper  10  includes feedback controllers  62 ,  65 , and a gate control signal generation unit  67 . Feedback controller  62  determines a duty ratio of IGBT QA 2  based on an actual voltage detected by voltage detection unit  41  in  FIG. 1  and target boost voltage  61 . Specifically, feedback controller  62  functions as a proportional (P) control system that determines the duty ratio by multiplying a difference between the actual voltage and target boost voltage  61  by a constant. Instead of the proportional control, proportional-integral (PI) control, proportional-integral-derivative (PID) control, or the like may be used. 
         [0037]    Feedback controller  65  determines the switching frequency of IGBT QA 2  based on the reactor temperature detected by temperature sensor RT in  FIG. 1  and a preset target reactor temperature  64 . Specifically, feedback controller  65  functions as a proportional (P) control system that determines the switching frequency by multiplying a difference between the detected reactor temperature and target reactor temperature  64  by a constant. Instead of the proportional control, PI control, PID control, or the like may be used. 
         [0038]    Gate control signal generation unit  67  generates gate control signal SA 2  based on the duty ratio and the switching frequency determined by feedback controllers  62 ,  65 , respectively, and outputs gate control signal SA 2  to gate drive circuit  12  for IGBT QA 2 . By repeating the feedback control described above, the actual voltage coincides with target boost voltage  61 , and the detected reactor temperature coincides with target reactor temperature  64 . 
         [0039]    Control unit  70  controlling inverter circuit  20  includes feedback controllers  72 ,  75 , and a gate control signal generation unit  77 . Feedback controller  72  determines an amplitude of a modulation wave used for PWM control based on an actual current detected by current detection unit  44  and target current  71 . Specifically, feedback controller  72  functions as a proportional (P) control system that determines the amplitude of the modulation wave by multiplying a difference between the actual current and target current  71  by a constant. Instead of the P control, proportional-integral (PI) control, proportional-integral-derivative (PID) control, or the like may be used. 
         [0040]    Feedback controller  75  determines the frequency of the carrier wave (the switching frequencies of the IGBTs) used for PWM control based on the stator winding temperature detected by temperature sensor MT and a preset target winding temperature  74 . Specifically, feedback controller  75  functions as a proportional (P) control system that determines the carrier frequency by multiplying a difference between the detected winding temperature and target winding temperature  74  by a constant. Instead of the P control, PI control, PID control, or the like may be used. 
         [0041]    Gate control signal generation unit  77  generates gate control signals SU 1 , SU 2 , SV 1 , SV 2 , SW 1 , SW 2  based on the modulation wave amplitude and the carrier frequency determined by feedback controllers  72 ,  75 , respectively, and outputs the signals to gate drive circuits  21  to  26  for IGBTs QU 1 , QU 2 , QV 1 , QV 2 , QW 1 , QW 2 . By repeating the feedback control described above, the actual current coincides with target current  71 , and the detected stator winding temperature coincides with target winding temperature  74 . 
         [0042]    As described above, according to semiconductor circuit device  1  of Embodiment 1, control can be performed to cause motor  99  to rotate with a desired torque and cause the temperature of reactor  16  and the temperature of the stator winding of motor  99  to coincide with respective predetermined target temperatures. As a result, overheating of these components can be prevented. 
         [0043]    Target reactor temperature  64  and target winding temperature  74  are each set to an appropriate value beforehand through experiments and the like. Since switching loss of an IGBT increases with an increase in a switching frequency, it is necessary to set the target temperatures of reactor  16  and the stator winding of motor  99  such that the IGBTs have temperatures within an allowable range. Thereby, if the temperatures of reactor  16  and the stator winding of motor  99  are feedback-controlled to coincide with the target temperatures, the IGBTs can also have temperatures within the allowable range. 
         [0044]    Set values for target reactor temperature  64  and target winding temperature  74  may be fixed, or changed in accordance with accumulated energization time for reactor  16  and motor  99 , as described below. 
         [0045]      FIG. 3  is a view of a temperature versus life line of the synthetic resin used to mold reactor  16 . Hereinafter, a method of setting the target temperature of reactor  16  will be described with reference to  FIG. 3 . The same applies to a method of setting the target temperature of the stator winding of motor  99 . 
         [0046]    Generally, a synthetic resin gradually undergoes a chemical reaction due to ultraviolet rays, moisture, and the like, and is deteriorated. If it is assumed that the chemical reaction follows the Arrhenius reaction rate theory, the synthetic resin has a life proportional to an inverse of an absolute temperature. Specifically, in a case where the axis of ordinates represents a logarithm of time for which the synthetic resin is used, and the axis of abscissas represents an inverse of an absolute temperature at which the synthetic resin is used as shown in  FIG. 3 , the life of the synthetic resin is represented by a straight line LT 100  determined depending on a synthetic resin material.  FIG. 3  also shows a straight line LT 70  corresponding to 70% of the life, and a straight line LT 50  corresponding to 50% of the life. 
         [0047]    The target temperature is set utilizing the temperature versus life line. Specifically, controller  40  in  FIG. 1  controls boost chopper  10  using a temperature T 1  as an initial set value for the target temperature of reactor  16 . If the accumulated energization time for reactor  16  reaches 50% of the life of the synthetic resin for molding, controller  40  changes the target temperature to T 2  lower than T 1 . Then, if the accumulated energization time for reactor  16  reaches 70% of the life of the synthetic resin, controller  40  changes the target temperature to T 3  lower than T 2 . Thus, a life of reactor  16  can be extended by changing the target temperature in accordance with the accumulated energization time as described above. 
         [0048]    Although the above description has exemplified a boost chopper as a DC-DC converter, it is needless to say that the type of the DC-DC converter is not limited thereto. The present invention is applicable to non-insulation type DC-DC converters such as a buck chopper and a buck-boost chopper, and insulation type DC-DC converters such as a flyback converter. Although the above description has exemplified an IGBT as a semiconductor switching element, the type of the semiconductor switching element is not limited thereto. Other semiconductor switching elements such as a metal-oxide semiconductor field-effect transistor (MOSFET) and a bipolar transistor can be used. 
       Embodiment 2 
       [0049]    A semiconductor circuit device  2  according to Embodiment 2 controls the temperature of reactor  16  and the temperature of the stator winding of motor  99  to coincide with the respective target temperatures, and controls temperatures of semiconductor chips on which the IGBTs are formed to coincide with a target temperature. 
         [0050]      FIG. 4  is a block diagram showing a configuration of semiconductor circuit device  2  according to Embodiment 2 of the present invention. A boost chopper  10 A in  FIG. 4  is different from boost chopper  10  in  FIG. 1  in that it includes a switching time adjusting unit  52  instead of resistance element  14  connected to the gate of IGBT QA 2 . An inverter circuit  20 A in  FIG. 4  is different from inverter circuit  20  in  FIG. 1  in that it includes switching time adjusting units  53  to  58  instead of resistance elements  27  to  32  connected to gates of IGBTs QU 1 , QU 2 , QV 1 , QV 2 , QW 1 , QW 2 , respectively. Each of switching time adjusting units  52  to  58  includes a variable resistance element VR used as a gate resistor for the corresponding IGBT, and a variable resistance control circuit VRC controlling a resistance value of variable resistance element VR. 
         [0051]    Semiconductor circuit device  2  of  FIG. 4  further includes temperature sensors CT 2  to CT 8  detecting the temperatures of the semiconductor chips on which IGBTs QA 2 , QU 1 , QU 2 , QV 1 , QV 2 , QW 1 , QW 2  are formed, respectively. Instead of a method of directly measuring chip temperatures with temperature sensors CT 2  to CT 8 , the temperatures of the IGBTs may be estimated based on collector currents, collector voltages, switching frequencies, and the like of the IGBTs. 
         [0052]    Controller  40 A generates resistance control signals (instruction signals) ST 2  to ST 8  based on outputs of temperature sensors CT 2  to CT 8 , and outputs the generated resistance control signals ST 2  to ST 8  to variable resistance control circuits VRCs provided in switching time adjusting units  52  to  58 . Since other components in  FIG. 4  are identical to those in  FIG. 1 , identical or corresponding parts will be designated by the same reference numerals, and the description will not be repeated. 
         [0053]      FIG. 5  is a circuit diagram showing an exemplary configuration of variable resistance element VR in  FIG. 4 . Referring to  FIG. 5 , variable resistance element VR includes a plurality of resistance elements RE connected in cascade, and a plurality of bipolar transistors BT connected in parallel to the plurality of resistance elements RE, respectively. Variable resistance control circuit VRC controls each bipolar transistor BT to be in an ON state or an OFF state in accordance with a resistance control signal ST from controller  40 A. Thereby, the resistance value of variable resistance element VR, that is, a gate resistance value of an IGBT, can be changed. 
         [0054]    Switching time of an IGBT (time in which the IGBT changes from an ON state to an OFF state, or time in which the IGBT changes from an OFF state to an ON state) is proportional to the product of a gate resistance value and an input capacitance. Here, the input capacitance is represented by the sum of a capacitance between a gate and a collector and a capacitance between the gate and an emitter of the IGBT. Therefore, the switching time can be changed by changing the gate resistance value of the IGBT. Since the change in the switching time of the IGBT causes a change in switching loss, a temperature of an IGBT chip can be changed. 
         [0055]    Specifically, if the switching time of the IGBT is increased, the switching loss is increased, and thus the temperature of the IGBT chip is increased. If the switching time of the IGBT is decreased, the switching loss is decreased, and thus the temperature of the IGBT chip is decreased. However, since a surge voltage caused when the IGBT is turned on is increased with a decrease in the switching time, that is, a decrease in the gate resistance value, it is necessary to change the gate resistance value in a range in which the surge voltage does not exceed a withstand voltage of the IGBT. Hereinafter, a specific operation of controller  40 A will be described. 
         [0056]      FIG. 6  is a block diagram showing an operation of controller  40 A in  FIG. 4 . 
         [0057]    Referring to  FIGS. 4 and 6 , controller  40 A further includes resistance control signal generation units  80 ,  90  in addition to control units  68 ,  70  described in  FIG. 2 . Since the operations of control units  68 ,  70  are identical to those in  FIG. 2 , the description will not be repeated. 
         [0058]    Resistance control signal generation unit  80  generates resistance control signal ST 2  based on a preset target chip temperature  81  and a chip temperature of IGBT QA 2  in boost chopper  10 A detected by temperature sensor CT 2 . Specifically, resistance control signal generation unit  80  performs proportional (P) control multiplying a difference between the detected chip temperature and target chip temperature  81  by a constant. Instead of the proportional control, PI control, PID control, or the like may be used. Resistance control signal generation unit  80  outputs the resistance control signal ST 2  generated by the above proportional control to variable resistance control circuit VRC in switching time adjusting unit  52 . Variable resistance control circuit VRC changes a gate resistance value of IGBT QA 2  (a resistance value of variable resistance element VR) in accordance with resistance control signal ST 2 . By repeating the feedback control described above, the detected chip temperature substantially coincides with target chip temperature  81 . 
         [0059]    Resistance control signal generation unit  90  generates resistance control signals ST 3  to ST 8  based on a preset target chip temperature  91  and chip temperatures of IGBTs QU 1 , QU 2 , QV 1 , QV 2 , QW 1 , QW 2  in inverter circuit  20 A detected by temperature sensors CT 3  to CT 8 . Specifically, resistance control signal generation unit  90  performs proportional (P) control multiplying a difference between each of the detected chip temperatures and target chip temperature  91  by a constant. Instead of the P control, PI control, PID control, or the like may be used. Resistance control signal generation unit  90  outputs the resistance control signals ST 3  to ST 8  generated by the above proportional control to variable resistance control circuits VRCs in switching time adjusting units  53  to  58 . Each variable resistance control circuit VRC changes a gate resistance value of a corresponding IGBT (a resistance value of variable resistance element VR) in accordance with a corresponding resistance control signal of resistance control signals ST 3  to ST 8 . By repeating the feedback control described above, each of the detected chip temperatures substantially coincides with target chip temperature  91 . 
         [0060]    As described above, according to semiconductor circuit device  2  of Embodiment 2, the temperature of each IGBT chip can be controlled to coincide with a predetermined target temperature in a state where control is performed to cause motor  99  to rotate with a desired torque and cause the temperature of reactor  16  and the temperature of the stator winding of motor  99  to coincide with respective predetermined target temperatures. As a result, overheating of reactor  16 , the stator winding of motor  99 , and each IGBT chip can be prevented. 
         [0061]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the terms of the appended claims.