Patent Publication Number: US-9835670-B2

Title: Isolator, semiconductor device, and method for controlling isolator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 14/745,837, filed on Jun. 22, 2015, which is based upon and claims the benefit of priority from Japanese patent application No. 2014-136464, filed on Jul. 2, 2014, the disclosure of which is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND 
     The present invention relates to an isolator, a semiconductor device, and a method for controlling the isolator and, for example, can be suitably utilized for an isolator including first and second insulating elements, a semiconductor device, and a method for controlling the isolator. 
     In a case where a signal is transmitted between circuits having largely different power supply voltages, when the signal is directly transmitted by a wiring, breakage of the circuits and failure of signal transmission may occur due to a voltage difference generated in a direct-current voltage component of the signal to be transmitted. Therefore, utilization of an isolator as a circuit that transmits a signal while insulating circuits having different power supply voltages from each other has spread. 
     An isolator utilizing a photo coupler has been known as a related isolator, and in recent years, research on an isolator utilizing insulating elements, such as a coil (a transformer) and a capacitor, has been advanced. The isolator utilizing the insulating elements connects semiconductor chips having different power supply voltages to each other by the insulating elements through an insulating film, and transmits only an alternating-current signal by alternating-current-coupling (AC-coupling) the insulating elements to each other. 
     Note that as related technologies, Japanese Unexamined Patent Application Publications No. 2010-48746, No. 2002-222477, and No. 2010-130325, and Masayuki Hikita et al., “New Approach to Breakdown Study by Measuring Pre-Breakdown Current in Insulating Materials”, Japanese Journal of Applied Physics (JJAP), vol. 23, No. 12, December, 1984, pp. L886-L888, have been known. 
     SUMMARY 
     In recent years, isolators have begun to be utilized for various applications (application systems). Further, since utilization of isolators for applications, such as an in-vehicle system, has rapidly increased, improvement in reliability of the isolators has been strongly desired. 
     However, in the related art, reliability has not been sufficiently considered in the isolator utilizing the insulating elements. For example, when a breakdown occurs in the insulating film between the insulating elements, an excessive short-circuit current flows so as to possibly lead to malfunction or destruction of peripheral circuits. Since it is impossible to prevent a breakdown at the time of actual use beforehand in the related isolator, a system cannot be safely utilized. 
     As described above, the related isolator has a problem that it is difficult to improve reliability. 
     Other problems of the conventional art and a new feature of the present invention will be apparent from the description of the specification and accompanying drawings. 
     According to one embodiment, an isolator includes: a transmission circuit; a first insulating element; a second insulating element; a reception circuit; an impedance control unit; and a leakage current detection unit. 
     The transmission circuit generates an alternating-current transmission signal in which a first potential is set to be a reference potential based on input transmission data. The generated alternating-current transmission signal is supplied to the first insulating element. The second insulating element is alternating-current-coupled to the first insulating element through an insulating film, and thereby generates an alternating-current reception signal in which a second potential different from the first potential is set to be a reference potential according to the alternating-current transmission signal. The reception circuit reproduces reception data based on the generated alternating-current reception signal. The impedance control unit controls an impedance of the first or the second insulating element to be higher than an impedance before the control. The leakage current detection unit detects a leakage current that flows between the first and the second insulating elements through the first or the second insulating element in which the impedance has been controlled. 
     According to the one embodiment, reliability of the isolator can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, advantages and features will be more apparent from the following description of certain embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a configuration diagram showing a configuration of a semiconductor device including an isolator in accordance with an embodiment 1; 
         FIG. 2  is a configuration diagram showing a configuration of a control system including an isolator in accordance with an embodiment 2; 
         FIG. 3A  is a plan diagram schematically showing a mounting example of the isolator in accordance with the embodiment 2; 
         FIG. 3B  is a plan diagram schematically showing a mounting example of the isolator in accordance with the embodiment 2; 
         FIG. 3C  is a cross-sectional diagram schematically showing a mounting example of the isolator in accordance with the embodiment 2; 
         FIG. 4A  is a plan diagram schematically showing a mounting example of the isolator in accordance with the embodiment 2; 
         FIG. 4B  is a cross-sectional diagram schematically showing a mounting example of the isolator in accordance with the embodiment 2; 
         FIG. 5  is a flow chart showing a method for controlling an isolator in accordance with the embodiment 2; 
         FIG. 6  is a timing chart showing operation of the isolator in accordance with the embodiment 2; 
         FIG. 7  is a flow chart showing a method for controlling an isolator in accordance with the embodiment 3; 
         FIG. 8  is a waveform chart showing operation of the isolator in accordance with the embodiment 3; 
         FIG. 9  is a configuration diagram showing a configuration of a control system including an isolator in accordance with an embodiment 4; 
         FIG. 10  is a flow chart showing a method for controlling an isolator in accordance with the embodiment 4; 
         FIG. 11  is a configuration diagram showing a configuration of a control system including an isolator in accordance with an embodiment 5; 
         FIG. 12  is a configuration diagram showing a configuration of a control system including an isolator in accordance with an embodiment 6; 
         FIG. 13  is a configuration diagram showing a configuration of a control system including isolators in accordance with an embodiment 7; 
         FIG. 14  is a flow chart showing a method for controlling isolators in accordance with the embodiment 7; 
         FIG. 15  is a configuration diagram showing a configuration of a control system including isolators in accordance with an embodiment 8; 
         FIG. 16  is a configuration diagram showing a configuration of a motor control system in accordance with a premise example of the embodiments; 
         FIG. 17  is a perspective diagram schematically showing a mounting example of an isolator in accordance with the premise example of the embodiments; and 
         FIG. 18  is a graph showing a relation between a leakage current and destruction of an insulating film that is described in Masayuki Hikita et al., “New Approach to Breakdown Study by Measuring Pre-Breakdown Current in Insulating Materials”, Japanese Journal of Applied Physics (JJAP), vol. 23, No. 12, December, 1984, pp. L886-L888. 
     
    
    
     DETAILED DESCRIPTION 
     Premise Example of Embodiments 
     Before embodiments are explained, a premise example before the embodiments are applied will be explained.  FIG. 16  shows a configuration of a motor control system in accordance with the premise example of the embodiments. 
     As shown in  FIG. 16 , a motor control system  900  includes: a plurality of isolators  910   a  and  910   b  (either of them is also referred to as an isolator  910 ); an MCU  920 ; a plurality of gate drivers  930   a  and  930   b  (either of them is also referred to as a gate driver  930 ); a plurality of IGBTs  940   a  and  940   b  (either of them is also referred to as an IGBT  940 ); and a motor  950 . For example, the MCU  920 , the plurality of isolators  910   a  and  910   b , and the plurality of gate drivers  930   a  and  930   b  are mounted on the one-package semiconductor device  901 . 
     The motor  950  is a three-phase motor having U-phase, V-phase, and W-phase coils. The IGBT  940   a , which is a high-side motor driver, and the IGBT  940   b , which is a low-side motor driver, are connected for each phase of the U phase, the V phase, and the W phase. The isolators  910   a  and  910   b  are connected to the high side and the low side of each phase of the IGBTs  940   a  and  940   b , and the motor  950 , respectively through the gate drivers  930   a  and  930   b.    
     The MCU  920  is connected to the isolators  910   a  and  910   b , transmits a control signal to the IGBTs  940   a  and  940   b  through the isolators  910   a  and  910   b , and the gate drivers  930   a  and  930   b , and alternately switches the IGBTs  940   a  and  940   b . The high-side IGBT  940   a  makes a current flow to the motor  950 , the low-side IGBT  940   b  extracts the current from the motor  950 , and thereby the motor  950  is rotationally driven. 
     As shown in  FIG. 16 , a power supply voltages of the MCU  920  side is 3.3 to 5 V, and a power supply voltage of a side of the IGBT  940  and the motor  950  is 1 kV. Accordingly, since reference potentials (corresponding to GND) of the MCU  920  side and the side of the IGBT  940  and the motor  950  have a difference of approximately several hundreds to several kilovolts due to a difference in the power supply voltages, the control signal cannot be directly transmitted. Therefore, the isolator  910  that insulates circuits having different potentials from each other in a DC manner is made to intermediate in order to transmit a drive signal from the MCU  920  to the motor  950 . Since inductors or capacitors are used for insulating elements of the isolator  910 , and the insulating elements transmit and receive a signal by AC coupling through an insulating film, a difference in reference potentials between transmission and reception circuits can be absorbed. 
       FIG. 17  schematically shows a mounting structure of the isolator  910  in accordance with the premise example. As shown in  FIG. 17 , in the isolator  910  included in the semiconductor device  901 , a transmission-side chip  960  and a reception-side chip  970  are mounted on a mounting substrate  911  including an external terminal  912 . 
     In the transmission-side chip  960 , a transmission circuit  961 ; a transmission-side coil (a primary-side coil)  962   a  and a reception-side coil (a secondary-side coil)  962   b  that are included in an on-chip transformer  962 ; and pads  964  and  965  connected to the reception-side coil  962   b  are formed. In the reception-side chip  970 , a reception circuit  971 ; and pads  972  and  973  connected to the reception circuit  971  are formed. 
     The pads  964  and  972  are connected to each other through a bonding wire  981 , and the pads  965  and  973  are connected to each other through a bonding wire  982 . Namely, the reception circuit  971  and the reception-side coil  962   b  are connected to each other through the pad  972 , the bonding wire  981 , and the pad  964 , and are also connected to each other through the pad  973 , the bonding wire  982 , and the pad  965 . 
     In the on-chip transformer  962 , the transmission-side coil  962   a  and the reception-side coil  962   b  are formed in first and second wiring layers in a semiconductor chip, respectively, and an interlayer insulating film (also simply referred to as an insulating film)  963  is formed between the transmission-side coil  962   a  and the reception-side coil  962   b.    
     As described above, in the isolator  910 , the transmission-side coil  962   a  and the reception-side coil  962   b  are AC-coupled to each other through the interlayer insulating film  963 , and thereby the signal is transmitted and received. For this reason, the difference in the reference potentials of the transmission-side circuit and the reception-side circuit, which reaches several kilovolts, is applied to the interlayer insulating film  963  between the insulating elements, and thus there is a problem that the insulating film deteriorates along with use of the isolator. 
     A graph of  FIG. 18  is a result of a measurement of a leakage current that flows through an insulating film described in Masayuki Hikita et al., “New Approach to Breakdown Study by Measuring Pre-Breakdown Current in Insulating Materials”, Japanese Journal of Applied Physics (JJAP), vol. 23, No. 12, December, 1984, pp. L886-L888. In  FIG. 18 , a time elapsed until a breakdown and a leakage current at the time the breakdown are observed in the insulating film formed of polyimide having a film thickness of 25 um. 
     In  FIG. 18 , an applied voltage is monotonously increased at 30 V/s, and it can be regarded to be substantially constant in a time range of the graph. Although an absolute value of the applied voltage is unknown, it is at least not less than 3000 V since an observation time is not less than 100 s. Since a withstand voltage of polyimide having the 25 um thickness is 5000 to 6000 V, the insulating film is expected to be in a comparatively high-stress state. 
     An experimental result of  FIG. 18  shows that a phenomenon in which the leakage current increases in accordance with a time elapse under a substantially constant voltage of 10 us to 10 ns before the breakdown occurs, and suggests that the increase in the leakage current and the breakdown (progress of insulation deterioration) are related to each other. 
     As described above, the insulating property of the insulating film decreases as a result of long-term use etc. even if a rated voltage operation is performed. In that case, a short circuit occurs between the transmission-side circuit and the reception-side circuit of the isolator, to thereby cause a failure of peripheral equipment. In motor control for automobiles, securement of reliability is particularly emphasized since a serious accident might arise if it is not secured. 
     A Note that although it can also be considered to include a fuse in a motor control system in order to prevent the above-described malfunction and failure due to a short circuit, a function to safely stop the system at the time of the breakdown has not been achieved. In addition, measures against the breakdown in an isolator level are not taken. 
     Consequently, in the embodiments explained hereinafter, a state of the isolator just before it becomes a breakdown state is detected utilizing a fact that there is a relation between increase in the leakage current and the breakdown. Namely, in the embodiments, a leakage current detection function to detect the leakage current of the insulating film in the isolator and control means thereof are provided. According to the embodiments, the leakage current that increases due to deterioration of the insulating film can be detected before occurrence of chip destruction, and operation of the system can be safely stopped. For example, since current rise continues for several microseconds before the breakdown in the example of  FIG. 18 , an increase in the current can be detected during the current rise, to thereby safely control the system. 
     Embodiment 1 
     Hereinafter, an embodiment 1 will be explained with reference to drawings.  FIG. 1  shows a configuration of a semiconductor device including an isolator in accordance with the embodiment. As shown in  FIG. 1 , a semiconductor device  1  includes an isolator  100  and a controller  200 , which is an MCU etc. In addition, controlled equipment  300 , which is a controlled object of the controller  200 , is connected to the semiconductor device  1 . 
     The isolator  100  includes: a transmission-side chip  120 ; a reception-side chip  130 ; and external terminals T 1  to T 4 . The isolator  100  is connected to the controller  200  through the external terminals T 1  to T 3 , and is connected to the controlled equipment  300  through the external terminal T 4 . 
     The external terminal T 1  is the terminal for inputting an impedance control signal S 1  for controlling an impedance from the controller  200 . The external terminal T 2  is the terminal for inputting transmission data VIN from the controller  200 . The external terminal T 3  is the terminal for outputting an error signal S 2 , which is a detection result of a leakage current, to the controller  200 . The external terminal T 4  is the terminal for outputting reception data VOUT received through an insulating element to the controlled equipment  300 . 
     The transmission-side chip  120  includes: a transmission circuit  101 ; an insulating element (a transmission-side insulating element)  102 ; an insulating element (a reception-side insulating element)  103 ; pads P 1  and P 2  connected to the insulating element  103 ; an impedance control unit  105 ; and a leakage current detection unit  106 . The reception-side chip  130  includes a reception circuit  104 , and pads P 3  and P 4  connected to the reception circuit  104 . 
     The transmission data VIN is input to the transmission circuit  101  from the controller  200  through the external terminal T 2 , and the transmission circuit  101  generates an alternating current transmission signal based on the input transmission data VIN. The alternating current transmission signal is the signal on the basis of a reference potential GND 1  (a first reference potential). 
     The insulating elements  102  and  103  are, for example, a coil and a capacitor. The alternating current transmission signal is supplied to the insulating element (a first insulating element)  102  from the transmission circuit  101  through the impedance control unit  105 . An insulating film  107  is formed between the insulating elements  102  and  103 . 
     The insulating element (a second insulating element)  103  is alternating current-coupled to the insulating element  102  through the insulating film  107 , and thereby generates an alternating-current reception signal. The alternating current reception signal is the signal on the basis of a reference potential GND 2  (a second reference potential) different from the reference potential GND. 
     The insulating element  103  and the reception circuit  104  are connected to each other through the pad P 1  of the transmission-side chip  120  and the pad P 3  of the reception-side chip  130 , and are also connected to each other through the pad P 2  of the transmission-side chip  120  and the pad P 4  of the reception-side chip  130 . The alternating current reception signal is input to the reception circuit  104  through the pads P 1  and P 3  and the pads P 2  and P 4 . The reception circuit  104  reproduces the reception data VOUT based on the input alternating-current reception signal, and outputs the reception data VOUT to the controlled equipment  300  through the external terminal T 4 . 
     An impedance control signal S 1  is input to the impedance control unit  105  from the controller  200  through the external terminal T 1 , and the impedance control unit  105  controls an impedance of the insulating element  102  to be high based on the input impedance control signal S 1 . The leakage current detection unit  106  detects the leakage current that flows between the insulating elements. Namely, the leakage current detection unit  106  detects the leakage current through the insulating element  102  in which the impedance has been controlled, and outputs an error signal S 2 , which is a detection result according to the leakage current, to the controller  200  through the external terminal T 3 . 
     Note that although in the example of  FIG. 1 , the impedance of the transmission-side insulating element  102  is controlled and the leakage current is detected through the insulating element  102  in which the impedance has been controlled, an impedance of the reception-side insulating element  103  may be controlled and the leakage current may be detected through the insulating element  103  in which the impedance has been controlled. 
     In testing the leakage current between the insulating elements, the controller  200  inputs a test preset signal that maintains a high level as the transmission data VIN. As a result of this, since the reception data VOUT of the reception circuit  104  and the controlled equipment  300  are controlled to be in a constant state, and the reference potential GND 2  becomes higher than the reference potential GND 1 , the leakage current is generated. The controller  200  controls the impedance of the transmission-side insulating element  102  of the isolator  100  to be high, and detects the leakage current through the transmission-side insulating element  102  in the leakage current detection unit  106 . The leakage current detection unit  106  transmits to the controller  200  the error signal S 2  according to a leakage current amount. The controller  200  determines an insulating property (a deterioration state of the insulating film) from the received error signal S 2 , and controls operation of the isolator  100 . 
     As described above, in the embodiment, the isolator controls the impedance of the insulating element to detect the leakage current, and outputs the error signal according to the detected leakage current. As a result of this, since deterioration of the insulating film can be detected before a breakdown, and it becomes possible to safely stop the isolator, reliability of the isolator can be improved. 
     Embodiment 2 
     Hereinafter, an embodiment 2 will be explained with reference to the drawings. In the embodiment, a configuration and operation of the isolator more specific than those shown in the embodiment 1 will be explained. 
       FIG. 2  shows a configuration of a motor control system including an isolator in accordance with the embodiment. The motor control system controls rotational operation of a motor  301  as one example of a controlled equipment  300 , which is the controlled object. Note that the controlled equipment  300  may be a power supply circuit, a measuring instrument, a sensor, etc. in addition to the motor. As the motor and an IGBT, as long as the system of the embodiment is a system (an application) that can maintain a reference potential of a reception circuit side (a secondary side) higher than a reference potential of a transmission circuit side, other systems may be employed. 
     For example, similar to  FIG. 16 , the motor  301  is a three-phase motor, has a high-side IGBT and isolator and a low-side IGBT and isolator for each phase, only the high-side IGBT and isolator and the low-side IGBT of one phase of the three phases being shown in  FIG. 2 . Note that a gate driver may be included as in  FIG. 16  if needed. 
     The semiconductor device  1  is connected to the motor  301  and IGBTs  1  and  2  in order to control the rotational operation of the motor  301 . The IGBT  1  is a high-side motor driver, and the IGBT  2  is a low-side motor driver. In the IGBT  1 , a gate is connected to the external terminal T 4 , a power supply VD 3 , which is a DC power supply for the IGBT, is supplied to a collector, and an emitter is connected to the motor  301  through the reference potential GND 2 , a collector of the IGBT  2 , and a coil L 3 . 
     The semiconductor device  1  of  FIG. 2  has a configuration similar to that of the semiconductor device  1  of  FIG. 1 , and further, includes specific circuit configurations of the insulating elements  102  and  103 , the impedance control unit  105 , and the leakage current detection unit  106 . 
     Namely, similar to  FIG. 1 , the semiconductor device  1  includes the isolator  100  and the controller  200 . The isolator  100  is connected to the controller  200  through the external terminals T 1  to T 3 , and is connected to the IGBT  1  through the external terminal T 4 . The isolator  100  includes the transmission-side chip  120  and the reception-side chip  130 . 
     The transmission-side chip  120  includes: the transmission circuit  101 ; the insulating element  102 ; the insulating element  103 ; the pads P 1  and P 2 ; the impedance control unit  105 ; and the leakage current detection unit  106 . A power supply VD 1  of several volts is supplied to the transmission circuit  101 , and the reference potential GND 1  of the transmission circuit  101  side from the insulating element  102  becomes approximately 0 V. 
     The reception-side chip  130  includes the reception circuit  104  and the pads P 3  and P 4 . A power supply VD 2  of several volts is supplied to the reception circuit  104 , the power supply VD 3  of several kilovolts is supplied to the IGBT  1  connected to the reception circuit  104 , and the reference potential GND 2  of the reception circuit  104  side from the insulating element  103  becomes approximately several kilovolts. 
     In the example of  FIG. 2 , a transmission-side coil L 1  and a reception-side coil L 2  are included as the insulating elements  102  and  103 . Switches SW 1  and SW 2  are included as the impedance control unit  105 . A capacitor C 1  (parasitic capacitance) and a comparator CMP 1  are included as the leakage current detection unit  106 . 
     A connection relation of these configurations will be explained. A first output terminal to output an alternating-current transmission signal of the transmission circuit  101  is connected to one end of the coil L 1  through the switch SW 1 , and a second output terminal to output the alternating-current transmission signal of the transmission circuit  101  is connected to another end of the coil L 1  through the switch SW 2 . 
     The switch SW 1  is connected between the first output terminal of the transmission circuit  101  and the one end of the coil L 1 , and a control terminal thereof is connected to the external terminal T 1 . The switch SW 1  is turned on/off according to the impedance control signal S 1  input from the controller  200  to the control terminal, and switches connection/disconnection of the first output terminal of the transmission circuit  101  and the one end of the coil L 1 . 
     The switch SW 2  is connected between a second output terminal of the transmission circuit  101  and the other end of the coil L 1 , and a control terminal thereof is connected to the external terminal T 1 . The switch SW 2  is turned on/off according to the impedance control signal S 1  input from the controller  200  to the control terminal, and switches connection/disconnection of the second output terminal of the transmission circuit  101  and the other end of the coil L 1 . 
     One end of the capacitor C 1  is connected to the other end of the coil L 1 , and another end thereof is connected to the reference potential GND 1 . The capacitor C 1  charges a leakage current that flows through the coil L 1 , and generates a leakage voltage VL, which is a voltage according to the leakage current. 
     In the comparator CMP 1 , a positive input terminal is connected to the one end of the capacitor C 1  (the other end of the coil L 1 ), a threshold value (a threshold value voltage) Vth is input to a negative input terminal, and an output terminal is connected to the external terminal T 3 . The comparator CMP 1  compares the leakage voltage VL charged by the capacitor C 1  with the threshold value Vth, and outputs a comparison result to the controller  200  as the error signal S 2 . When the leakage voltage VL is larger than the threshold value Vth, the comparator CMP 1  outputs a high level signal to the error signal S 2 . 
       FIG. 3A  is one example of a plan view of a wiring layer in which the transmission-side coil L 1  of the transmission-side chip  120  included in the isolator  100  of  FIG. 2  is formed,  FIG. 3B  is one example of a plan view of a wiring layer in which the reception-side coil L 2  of the transmission-side chip  120  is formed, and  FIG. 3C  is one example of a line A-A′ cross-sectional diagram of the transmission-side chip  120  of  FIGS. 3A and 3B . 
     As shown in  FIGS. 3A to 3C , in the transmission-side chip  120 , agate electrode layer GL 10  and wiring layers WL 11  to WL 18  are formed on a silicon substrate SUB 1  in a stacked manner in that order. In the gate electrode layer GL 10  and the wiring layers WL 11  to WL 18 , a gate electrode and a metal wiring included in a circuit of the transmission-side chip  120  are formed in desired patterns, and an interlayer insulating film  107  is formed so as to bury the gate electrode and the metal wiring. 
     The transmission circuit  101  is formed in the gate electrode layer GL 10  and the wiring layers WL 11  to WL 18 . A wiring is formed in the wiring layer WL 15  so as to connect the external terminal T 2  to which the transmission data VIN is input and the transmission circuit  101 . 
     The switch SW 2  of the impedance control unit  105  includes an MOS transistor  105   a . The MOS transistor  105   a  has two diffusion layers formed at a surface of the silicon substrate SUB 1 , and a gate electrode formed in gate electrode layer GL 10  on the silicon substrate SUB 1 . 
     A wiring of the wiring layer WL 13 , and a contact hole penetrating from the wiring layer WL 13  to the gate electrode layer GL 10  are formed so as to connect the transmission circuit  101  and one diffusion layer of the MOS transistor  105   a . A contact hole penetrating from the wiring layer WL 14  to the gate electrode layer GL 10  is formed so as to connect a gate of the MOS transistor  105   a  and the external terminal T 1  to which the impedance control signal S 1  is input. 
     Note that although a cross-sectional diagram is omitted, the switch SW 1  of the impedance control unit  105  includes, similar to the switch SW 2 , an MOS transistor, the transmission circuit  101  and one diffusion layer of the MOS transistor are connected to each other, a gate of the MOS transistor is connected to the external terminal T 1 , and another diffusion layer of the MOS transistor is connected to the transmission-side coil L 1 . 
     The comparator CMP 1  of the leakage current detection unit  106  includes MOS transistors  106   a  and  106   b . The MOS transistors  106   a  and  106   b  respectively have two diffusion layers formed at the surface of the silicon substrate SUB 1 , and a gate electrode formed in the gate electrode layer GL 10  on the silicon substrate SUB 1 . 
     A contact hole penetrating from the wiring layer WL 16  to the gate electrode layer GL 10  is formed so as to connect one diffusion layer of the MOS transistor  106   a  and the external terminal T 3  from which the error signal S 2  which has detected the leakage current is output. A wiring of the wiring layer WL 12  and two contact holes penetrating from the wiring layer WL 12  to the gate electrode layer GL 10  are formed so as to connect a gate of the MOS transistor  106   a  and the one diffusion layer of the MOS transistor. 
     The capacitor C 1  of the leakage current detection unit  106  includes wirings formed in the wiring layer WL 11  and the wiring layer WL 13 , and the insulating film  107  between the wirings. A contact hole penetrating the gate electrode layer GL 10  is formed so as to connect the wiring of the wiring layer WL 11  side of the capacitor C 1  and the silicon substrate SUB 1 . 
     The transmission-side coil L 1  includes a wiring spirally patterned in the wiring layer WL 14 . The contact hole penetrating from the wiring layer WL 13  to the gate electrode layer GL 10 , the wiring of the wiring layer WL 13 , the contact hole penetrating from the wiring layer WL 14  to the gate electrode layer GL 10 , and a wiring of the wiring layer WL 14  are formed so as to connect the transmission-side coil L 1  and another diffusion layer of the MOS transistor  105   a , a gate of the MOS transistor  106   b , and the wiring of the wiring layer WL 13  side of the capacitor C 1 . 
     The reception-side coil L 2  includes a wiring spirally patterned in the wiring layer WL 18 . The transmission-side coil L 1  and the reception-side coil L 2  are formed so as to face each other through the interlayer insulating film  107  formed in the wiring layers WL 15  to WL 17 . The reception-side coil L 2  is connected to the pads P 1  and P 2  formed in the wiring layer WL 18 . 
       FIG. 4A  is one example of a plan diagram of the reception-side chip  130  included in the isolator  100  of  FIG. 2 , and  FIG. 4B  is one example of a line B-B′ cross-sectional diagram of the reception-side chip  130  of  FIG. 4A . 
     As shown in  FIGS. 4A and 4B , similarly to the transmission-side chip  120 , in the reception-side chip  130 , a gate electrode layer GL 20  and wiring layers WL 21  to WL 28  are formed on a silicon substrate SUB 2  in a stacked manner in that order, and an interlayer insulating film  207  is formed between the respective wirings. 
     The reception circuit  104  is formed in the gate electrode layer GL 20  and the wiring layers WL 21  to WL 28 . The pad P 3  is formed in the wiring layer WL 28 , and a wiring is formed in the wiring layer WL 28  so as to connect the pad P 3  and the reception circuit  104 . Note that although a cross-sectional diagram of the pad P 4  is omitted, similarly to the pad P 3 , the pad P 4  is formed in the wiring layer WL 28 , and is connected to the reception circuit  104  by the wiring of the wiring layer WL 28 . In addition, a wiring is formed in the wiring layer WL 27  so as to connect the external terminal T 4  from which the reception data VOUT is output and the reception circuit  104 . 
     Next, a method for controlling an isolator in accordance with the embodiment will be explained using  FIGS. 5 and 6 .  FIG. 5  is a flow chart showing a method for controlling the isolator  100  (a test to detect a leakage current or a method for testing insulating film deterioration), and  FIG. 6  is a timing chart showing an example of signal wave forms of the control method. 
     As shown in  FIG. 5 , first, the controller  200  transmits a signal so as to raise a high-side voltage (S 101 ). In detecting a leakage current between the insulating elements, the controller  200  connected outside the isolator  100  first transmits the transmission data VIN (a preset signal) keeping a high level (Hi) to the isolator  100  ( 401  of  FIG. 6 ). The transmission circuit  101  then applies a voltage to the transmission-side coil L 1  according to rise of the transmission data VIN, and a current I 1  in a positive direction temporarily flows through the transmission-side coil L 1  ( 402  of  FIG. 6 ). For this reason, in the reception-side coil L 2 , an electromotive force according to change of the current I 1  of the transmission-side coil L 1  is generated, and an induced voltage V 2  temporarily rises ( 403  of  FIG. 6 ). The reception circuit  104  raises the reception data VOUT to a high level since the induced voltage V 2  rises ( 404  of  FIG. 6 ). 
     As a result of this, the state of the IGBT  1  connected to a subsequent stage of the isolator  100  becomes an always ON state, the reference potential GND 2  is fixed to approximately a power supply voltage (to kV) of the IGBT  1  ( 405  of  FIG. 6 ), and a voltage V 21  of the reception-side coil is also fixed to approximately the power supply voltage level (to kV) after the change of the induced voltage V 2  ( 406  of  FIG. 6 ). 
     Subsequently, the controller  200  turns off the switches SW 1  and SW 2  (S 102 ). The controller  200  raises the impedance control signal S 1  to a high level ( 407  of  FIG. 6 ). As a result of this, since the switches SW 1  and SW 2  of the transmission-side chip  120  of the isolator  100  become off, and the coil L 1 , which is a transmission-side insulating element, and the transmission circuit  101  are electrically cut off, both terminals of the coil L 1 , which is the transmission-side insulating element, come to have high impedances. 
     Subsequently, the isolator  100  detects the leakage voltage VL of the coil L 1  after a certain time (S 103 ). By turning off the switches SW 1  and SW 2 , inflow and outflow of a current between the coil L 1 , which is the transmission-side insulating element, and the transmission circuit  101  are prevented, and detection sensitivity of the leakage current between the insulating elements of the coils L 1  and L 2  improves. When the switches SW 1  and SW 2  are turned off, the coil L 1 , which is the transmission-side insulating element, is in a potential state just before the switches are turned off. Since a potential of the reception circuit  104  is higher than a potential of the transmission circuit  101  (by several kilovolts), a leakage current according to a deterioration state of the insulating film  107  flows into the coil L 1 , which is the transmission-side insulating element, from the coil L 2 , which is a reception-side insulating element. The leakage current is charged in voltage detection means added to the coil L 1 , which is the transmission-side insulating element, for example, the capacitor C 1 , which is capacitance (parasitic capacitance is also included), and is converted into the voltage (the leakage voltage VL). Since the leakage voltage VL is a value obtained by integrating the leakage current, it rises along with time ( 408  of  FIG. 6 ). 
     Subsequently, the isolator  100  determines whether or not the leakage voltage VL of the coil L 1  is more than the threshold value Vth (S 104 ). A voltage of the capacitor C 1  is input to the comparator CMP 1 , a voltage value corresponding to a leakage current value used as a criterion for determining normality/abnormality of the insulating film is set to the threshold value (a reference voltage) Vth of the comparator CMP 1 , and thereby deterioration of the insulating film is detected. The comparator CMP 1  compares the leakage voltage VL obtained by integrating a leakage current that flows while the impedance control signal S 1  is in a high level, with the threshold value Vth. A comparison result of the comparator CMP 1  is output to the controller  200  as the error signal S 2 , and the controller  200  controls operation of the isolator  100  according to the error signal S 2 . 
     When the leakage voltage VL of the coil L 1  is not more than the threshold value Vth, the controller  200  starts a usual operation (S 105 ). When the leakage voltage VL is not more than the threshold value, the comparator CMP 1  keeps the error signal S 2  to be a low level ( 409  of  FIG. 6 ). Since the error signal S 2  of the low level corresponding to the leakage current not more than a prescribed value is input for a predetermined period (a period when the impedance control signal S 1  is in the high level), the controller  200  determines the isolator  100  to be normal, and starts a usual operation ( 410  of  FIG. 6 ). The controller  200  inputs the transmission data VIN for controlling the motor  301  to the isolator  100 , drives the IGBT  1 , and starts rotation of the motor  301 . 
     In addition, when the leakage voltage VL of the coil L 1  is larger than the threshold value, the controller  200  outputs an alarm (S 106 ), and performs a safety stop (S 107 ). When the leakage voltage VL exceeds the threshold value, the comparator CMP 1  raises the error signal S 2  to a high level ( 411  of  FIG. 6 ). Since the error signal S 2  of the high level corresponding to the leakage current not less than the prescribed value was input within the predetermined period, the controller  200  determines the insulating film of the isolator  100  to be abnormal, and stops the operation of the isolator  100  ( 412  of  FIG. 6 ). The controller  200  inputs the transmission data VIN of a low level to the isolator  100 , turns off the IGBT  1 , and stops the rotation of the motor  301 . As a result of this, destruction of the whole system is prevented beforehand. 
     As described above, in the embodiment, as a specific example of the embodiment 1, the insulating elements  102  and  103  include the coils L 1  and L 2 , the impedance control unit  105  includes the switches SW 1  and SW 2 , and the leakage current detection unit  106  includes the capacitor C 1  and the comparator CMP 1 . In this configuration, the impedance of the coil, which is the insulating element, is controlled to detect the leakage current, and the error signal according to the detected leakage current is transmitted. As a result of this, since deterioration of the insulating film can be detected before a breakdown between the coils, and it becomes possible to safely stop the isolator, reliability of the isolator can be improved. 
     Embodiment 3 
     Hereinafter, an embodiment 3 will be explained with reference to the drawings. The embodiment is an example of adding means for applying an initial potential to an insulating element that detects a leakage current to the isolator shown in the embodiment 2, and of determining whether or not fluctuation of the leakage current falls within a range of a first threshold value to a second threshold value. 
     A method for controlling the isolator in accordance with the embodiment will be explained using  FIG. 7  and  FIG. 8 .  FIG. 7  is a flow chart showing a method for controlling the isolator  100  (a test to detect a leakage current or a method for testing insulating film deterioration), and  FIG. 8  shows an example of signal wave forms of the voltage VL of a transmission-side insulating element in the control method. 
     As shown in  FIG. 7 , first, similarly to  FIG. 5  of the embodiment 2, the controller  200  transmits a signal so as to raise a high-side voltage (S 101 ). Similarly to  FIG. 6 , when the controller  200  transmits a preset signal of a high level to the isolator  100 , the state of the IGBT  1  becomes an on state through the coils L 1  and L 2 , and the reference potential GND 2  rises. 
     Subsequently, the controller  200  turns on the switch SW 0  (S 111 ), and after that, turns off the switches SW 0 , SW 1 , and SW 2  (S 112 ). 
     In the embodiment 2, after S 101 , the transmission-side insulating element and the transmission circuit are cut off by the switches SW 1  and SW 2 . At this time, since the transmission-side insulating element is in a potential state before the switches are turned off, the potential is unfixed. Since the initial potential of the insulating element greatly affects detection sensitivity of the leakage current, the initial potential is controlled by the switch SW 0  in the embodiment. 
     Namely, in the embodiment, before controlling an impedance of the insulating element, the controller  200  sets the bias control signal S 0  to be a high level, and turns on the switch SW 0  ( 501  of  FIG. 8 ). In that case, an arbitrary initial voltage (for example, a VDD/2) is previously applied by the transmission circuit  101  connected to the coil L 1 , which is the transmission-side insulating element, through the switch SW 0 , and a power supply Vbias. After that, the controller  200  sets the bias control signal S 0  to be a low level, sets the impedance control signal S 1  to be a high level, and turns off the switches SW 0 , SW 1 , and SW 2  ( 502  of  FIG. 8 ). 
     Subsequently, the isolator  100  detects the leakage voltage VL of the coil L 1  after the certain time (S 103 ), and determines whether or not the leakage voltage VL of the coil L 1  is larger than the first threshold value Vth 1 , or whether or not it is smaller than the second threshold value Vth 2  (S 113 ). 
     In the embodiment, in detection of the leakage current, the two comparators CMP 1  and CMP 2  connected in parallel with each other are used as voltage detectors. The threshold values (reference voltages) Vth 1  and Vth 2  are input to these comparators CMP 1  and CMP 2 , respectively. The first threshold value Vth 1  is connected to a reference side of the one comparator CMP 1 , the second threshold value Vth 2  is connected to an input side of the other comparator CMP 2 , it is determined whether or not the leakage voltage VL fluctuated according to the leakage current falls within a range of the threshold values Vth 1  to Vth 2  ( 503  of  FIG. 8 ), and a determination result is output from logic circuit OR 1  to the controller  200  as the error signal S 2 . 
     When a voltage of the coil L 1  falls within the range of the threshold values Vth 1  to Vth 2 , the controller  200  determines that the isolator  100  is normal (absence of the leakage current), and starts a usual operation (S 105 ). In the comparator CMP 1 , when the leakage voltage VL is not more than the threshold value Vth 1 , an output signal is at a low level, while in the comparator CMP 2 , when the leakage voltage VL is not less than the threshold value Vth 2 , an output signal is at a low level, and the logic circuit OR 1  keeps the error signal S 2  to be a low level. Since the error signal S 2  of the low level corresponding to the leakage current within a range of a prescribed value is input for a predetermined period (a period when the impedance control signal S 1  is at a high level), the controller  200  determines the isolator  100  to be normal, and starts the usual operation. 
     In addition, when voltage rise of the leakage voltage VL of the coil L 1  becomes larger than the threshold value Vth 1 , or when voltage drop thereof becomes lower than the threshold value Vth 2 , the controllers  200  determines the isolator  100  to be abnormal (presence of the leakage current), outputs an alarm (S 106 ), and performs a safety stop (S 107 ). Since the comparator CMP 1  sets the output signal to be a high level when the leakage voltage VL exceeds the threshold value Vth 1 , and the comparator CMP 2  sets the output signal to be the high level when the leakage voltage VL becomes lower than the threshold value Vth 2 , the logic circuit OR 1  raises the error signal S 2  to be a high level. 
     Since the error signal S 2  of the high level corresponding to the leakage current that does not fall within the range of the prescribed value was input within the predetermined period, the controller  200  determines the insulating film of the isolator  100  to be abnormal, and stops operation of the isolator  100 . As a result of this, destruction of the whole system is prevented beforehand. 
     As described above, in the embodiment, with respect to the embodiment 2, the initial potential of the insulating element is set to be constant by the switch SW 0 , and the leakage current within the range of the threshold values Vth 1  and Vth 2  is detected by the comparators CMP 1  and CMP 2 . According to such a configuration, the leakage current can be detected not only in a case where a current flows into the leakage current detection unit  106 , but also in a case where the current flows out of the leakage current detection unit  106 . Namely, there is a case where the leakage current flows from the coil L 2  to the coil L 1  and a case where the leakage current flows from the coil L 1  to the coil L 2  depending on a potential difference between the coil L 1  and the coil L 2 , and in the embodiment, the leakage current can be accurately detected in either case. 
     Embodiment 4 
     Hereinafter, an embodiment 4 will be explained with reference to the drawings. The embodiment is an example where memory means for storing a leakage voltage has been added to the isolator shown in the embodiment 2. 
       FIG. 9  shows a configuration of a motor control system including an isolator in accordance with the embodiment. In  FIG. 9 , compared with  FIG. 2  of the embodiment 2, the leakage current detection unit  106  includes a memory circuit M 1  in addition to the capacitor C 1  and the comparator CMP 1 . Other configurations are similar to those of  FIG. 2 . 
     The memory circuit M 1  is connected to a positive input terminal of the comparator CMP 1 , and is connected to a negative input terminal of the comparator CMP 1 . The leakage voltage VL compared by the comparator CMP 1  this time is input to the memory circuit M 1 , and the memory circuit M 1  stores the input voltage VL. In addition, the memory circuit M 1  outputs the leakage voltage VL+ΔV as the threshold value Vth of the comparator CMP 1  of this time, the leakage voltage VL having been used in a previous comparison of the comparator CMP 1  and having been stored. 
       FIG. 10  is a flow chart showing a method for controlling the isolator  100  (a test to detect a leakage current or a method for testing insulating film deterioration) in accordance with the embodiment. 
     As shown in  FIG. 10 , first, similarly to  FIG. 5  of the embodiment 2, the controller  200  transmits a signal so as to raise a high-side voltage (S 101 ), and turns off the switches SW 1  and SW 2  (S 102 ). 
     Subsequently, the isolator  100  detects the leakage voltage VL of the coil L 1  after a certain time (S 103 ). The isolator  100  then reads the threshold value Vth from the memory circuit M 1  (S 121 ), and determines whether or not the leakage voltage VL of the coil L 1  is not more than the read threshold value Vth (S 104 ). 
     The memory circuit M 1  which has stored the leakage voltage VL detected in a previous test of the leakage current reads the previous leakage voltage VL+ΔV as the threshold value Vth, and inputs it to the negative input terminal of the comparator CMP 1 . In that case, the comparator CMP 1  compares a current leakage voltage of the coil L 1  with the previous leakage voltage VL+ΔV. 
     When the leakage voltage VL of the coil L 1  is not more than the threshold value Vth, the memory circuit M 1  stores the leakage voltage of the coil L 1  (S 122 ), and the controller  200  starts a usual operation (S 105 ). When the leakage voltage VL of this time is not more than the previous leakage voltage VL+ΔV, the comparator CMP 1  keeps the error signal S 2  to be a low level. In that case, the memory circuit M 1  stores the leakage voltage VL compared by the comparator CMP 1  this time. In addition, since the error signal S 2  of the low level is input for a predetermined period, the controller  200  determines the isolator  100  to be normal, and starts the usual operation. 
     In addition, when the leakage voltage VL of the coil L 1  is larger than the threshold value Vth, the controller  200  outputs an alarm (S 106 ), and performs a safety stop (S 107 ). When the leakage voltage VL of this time exceeds the previous leakage voltage VL+ΔV, the comparator CMP 1  raises the error signal S 2  to be a high level. Since the error signal S 2  of the high level was input within the predetermined period, the controller  200  determines the insulating film of the isolator  100  to be abnormal, and stops operation of the isolator  100 . As a result of this, destruction of the whole system is prevented beforehand. 
     With the method for prescribing an abnormality determination criterion of the insulating film by a constant leakage current value, sufficient detection accuracy may not be obtained due to effects of various variations and noise. Consequently, in the embodiment, a state of the insulating film at the time of the previous test is set to be a determination criterion, a difference of the leakage current from this state is observed, and thereby abnormality is determined. When the leakage voltage VL of the comparator CMP 1  generated by charging the leakage current exceeds the threshold value (the reference voltage) Vth, the error signal S 2  is transmitted to the controller  200  as indicating insulating film deterioration. Since a voltage applied between the insulating elements is decided substantially by a power supply voltage of the IGBT  1 , the leakage current value becomes substantially the same in every test as long as there is no deterioration in the insulating film. 
     Consequently, a leakage current measurement result (VL) is held in the memory circuit M 1 , such as a register, at the time of the test, and the read previous leakage voltage VL+ΔV is set to the threshold value Vth at the time of a next test. Here, ΔV is an arbitrary voltage margin, and may be introduced to eliminate the effect of variations or noise. 
     As described above, in the embodiment, with respect to the embodiment 2, the memory circuit M 1  is included that stores the leakage voltage according to the leakage current, and the leakage voltage used in the previous test is used for the threshold value for determining a next leakage voltage. As a result of this, since fluctuation of the leakage current can be detected, a deterioration state of the insulating film can be accurately detected. 
     Embodiment 5 
     Hereinafter, an embodiment 5 will be explained with reference to the drawings. The embodiment is an example where means for stopping the isolator according to detection of a leakage current has been added to the isolator shown in the embodiment 2. 
       FIG. 11  shows a configuration of a motor control system including an isolator in accordance with the embodiment. In  FIG. 11 , compared with  FIG. 2  of the embodiment 2, a switch SW 4  is included in the transmission-side chip  120  of the isolator  100 . Other configurations are similar to those of  FIG. 2 . 
     The switch SW 4  is connected between the other end of the coil L 1  and the reference potential GND 1 , and a control terminal is connected to the output terminal of the comparator CMP 1 . The switch SW 4  is turned on/off according to the error signal S 2  input to the control terminal from the comparator CMP 1 , and switches connection/disconnection of the other end of the coil L 1  and the reference potential GND 1 . The switch SW 4  can also be called a reference potential supply circuit in which the coil L 1  is set to be the reference potential GN 1  according to the error signal S 2 . 
     In each of the above-described embodiments, it is important to secure accuracy of detection of insulation deterioration and a feedback time of a detection result. For example, in an actual measurement result of Masayuki Hikita et al., “New Approach to Breakdown Study by Measuring Pre-Breakdown Current in Insulating Materials”, Japanese Journal of Applied Physics (JJAP), vol. 23, No. 12, December, 1984, pp. L886-L888, a time when an effect of insulating film deterioration starts to appear is several microseconds to several nanoseconds before destruction, and the more the insulating film approaches the destruction, the more a leakage current increases. 
     As described above, there is a trade-off in which the larger a leakage current is set as an error determination criterion in order to improve the accuracy of detection of the insulation deterioration, the shorter a time to spare becomes before a detection result is fed back to the controller  200 , to thereby control the system. For example, when a scale of a circuit/system is large, etc., a system control time through the controller  200  cannot be sufficiently secured, to thereby possibly cause a problem. 
     Consequently, in the embodiment, in addition to an output signal from the comparator CMP 1  being transmitted to the controller  200 , the isolator  100  is directly stopped. 
     In an example of  FIG. 11 , the reference potential GND 1  (to 0 V) is connected to the coil L 1 , which is the transmission-side insulating element, through the switch SW 4 . The switch SW 4  is turned off only when an output of the comparator CMP 1  is a high level (Hi), i.e., an insulation deterioration error is output, and a potential level of the transmission-side insulating element is forcibly dropped to a low level (Low). As a result of this, the IGBT  1  returns to an off state, and a high voltage applied state between the insulating elements is released. 
     As described above, in the embodiment, with respect to the embodiment 2, the switch SW 4  is included that controls the potential of the coil according to detection of the leakage current. As a result of this, when the leakage current before a breakdown is detected, it is possible to stop the whole system by the controller  200  after stopping an isolator body in an emergency. Accordingly, the system can be reliably stopped without causing a short circuit between the insulating elements. 
     Embodiment 6 
     Hereinafter, an embodiment 6 will be explained with reference to the drawings. The embodiment is an example of converting a leakage current into a voltage using an amplifier instead of a capacitor with respect to the isolator shown in the embodiment 2. 
       FIG. 12  shows a configuration of a motor control system including an isolator in accordance with the embodiment. In  FIG. 12 , compared with  FIG. 2  of the embodiment 2, the leakage current detection unit  106  includes the comparator CMP 1  and an amplifier AMP 1 . Other configurations are similar to those of  FIG. 2 . 
     The amplifier AMP 1  is a current-voltage conversion amplifier circuit that converts a leakage current into a voltage and amplifies it. In the amplifier AMP 1 , a positive input terminal is connected to the reference voltage Vth 1 , a negative input terminal is connected to the other end of the coil L 1 , and an output terminal is feedback-connected to the negative input terminal through a resistor R 1 , and is connected to the negative input terminal of the comparator CMP 1 . 
     As in the above-described embodiments, a capacity may not be necessarily used for detection of the leakage current. In the embodiment, as shown in  FIG. 12 , after the leakage current is directly amplified by the amplifier AMP 1 , the amplified leakage current is input to the comparator CMP 1  for error detection. When the leakage current is set as IL, and a feedback resistor is set as R 1 , a difference voltage ΔVL between the reference voltage Vth 1 , which is output voltage of the amplifier AMP 1 , is expressed by the following (Expression 1).
 
Δ VL=IL*R 1  (Expression 1)
 
     It is preferable that the leakage current IL be large enough to obtain the leakage voltage VL according to the leakage current IL by amplification of the amplifier AMP 1 . Additionally, if a voltage corresponding to a leakage current value without insulating film deterioration is set to the threshold value Vth 2  of the comparator CMP 1 , an error can be detected when an abnormal increase of the leakage current occurs. 
     As described above, in the embodiment, instead of the capacitor, the amplifier is used as current-voltage conversion means of the leakage current detection unit with respect to the embodiment 2. Even when the amplifier is used, the leakage current, similarly to the above-described embodiments, can be detected. Accordingly, since it becomes possible to generate the error signal according to the leakage current, and to stop the isolator before destruction of the insulating film, reliability of the isolator can be improved. 
     Embodiment 7 
     Hereinafter, an embodiment 7 will be explained with reference to the drawings. The embodiment is an example of controlling an operation of all the isolators according to detection results of the plurality of isolators shown in the embodiment 2. 
       FIG. 13  shows a configuration of a motor control system including isolators in accordance with the embodiment. As shown in  FIG. 13 , the semiconductor device  1  includes the controller  200  and a plurality of isolators  100  ( 100   a  to  100   c ), and a plurality of IGBTs (IGBTs  1   a  to  1   c ) are connected thereto. Further, the semiconductor device  1  includes a logic circuit OR 2  between the controller  200  and the plurality of isolators  100 . 
     In the logic circuit OR 2 , a plurality of input terminals are connected to the external terminal T 3  from which the error signal (an individual error signal) S 2  of each isolator  100  is output, and an output terminal is connected to an input terminal of an error signal (a whole error signal) SE of the controller  200 . The logic circuit OR 2  performs an OR logic operation of inputs from the plurality of isolators, and outputs an operation result. When the error signal S 2  of any of the plurality of the isolators  100  is a high level, the logic circuit OR 2  sets the error signal SE to be a high level, and outputs it to the controller  200 . 
       FIG. 14  is a flow chart showing a method for controlling the isolator  100  (a test to detect a leakage current or a method for testing insulating film deterioration) in accordance with the embodiment. 
     As shown in  FIG. 14 , first, the controller  200  selects the isolator  100  to be tested (S 131 ). For example, the controller  200  selects the isolators  100   a  to  100   c  in an order from the isolator  100   a . Note that although the isolators are sequentially selected to then perform a test here, the plurality of isolators may be simultaneously tested. In this case, S 101  to S 104  are simultaneously performed for all the isolators. 
     Subsequently, similarly to  FIG. 5  of the embodiment 2, the controller  200  transmits a signal to the selected isolator  100  so as to raise a high-side voltage (S 101 ), and turns off the switches SW 1  and SW 2  (S 102 ). Further, the selected isolator  100  detects the leakage voltage VL of the coil L 1  after a certain time (S 103 ), and determines whether or not the leakage voltage VL of the coil L 1  is not more than the threshold value Vth (S 104 ). 
     When the leakage voltage VL of the coil L 1  of the selected isolator  100  is not more than the threshold value Vth, the controller  200  determines whether or not tests of all the isolators  100  are ended (S 132 ). When testing the remaining isolators, the controller  200  returns to S 131  and selects the next isolator  100 , and when the tests of all the isolators are ended, the controller  200  starts a usual operation (S 105 ). 
     When the leakage voltage VL is not more than the threshold value Vth, the comparator CMP 1  of the isolator  100  outputs the error signal S 2  remaining to be the low level. Since the error signal S 2  to be input is the low level, the logic circuit OR 2  outputs the error signal SE remaining to be a low level to the controller  200 . 
     Since the error signal SE of the low level is input for a predetermined period, the controller  200  determines the selected isolator  100  to be normal, repeats the test of the next isolator, and starts a usual operation when the tests of all the isolators are normally ended. 
     In addition, when the leakage voltage VL of the coil L 1  of the selected isolator  100  is larger than the threshold value Vth, the controller  200  outputs an alarm (S 106 ), and safely stops all the isolators (S 133 ). 
     When the leakage voltage VL exceeds the threshold value, the comparator CMP 1  of the isolator  100  raises the error signal S 2  to the high level. Since the input error signal S 2  is the high level, the logic circuit OR 2  raises the error signal SE. Since the error signal SE of the high level is input within a predetermined period, the controller  200  determines an insulating film of the selected isolator  100  to be abnormal. In that case, the controller  200  stops operation of all the isolators  100 . As a result of this, destruction of the whole system is prevented beforehand. 
     The isolators for three phases are needed in motor control, and if at least one of them is short-circuited, malfunction and system damage occur. Therefore, in the embodiment, when all the isolators are monitored, and insulating property of any of them deteriorates, operation of all the ICs (isolators) is stopped. As a result of this, reliability of the isolator can be further improved. 
     Embodiment 8 
     Hereinafter, an embodiment 8 will be explained with reference to the drawings. The embodiment is an example of including an isolator that detects a leakage current of a reception-side insulating element in addition to the isolator shown in the embodiment 2. 
       FIG. 15  shows a configuration of a motor control system including isolators in accordance with the embodiment. As shown in  FIG. 15 , the semiconductor device  1  includes: the controller  200 ; the isolator  100 ; an isolator  100 - 2 ; and a logic circuit OR 3 . 
     The isolator  100  transmits a signal from the controller  200  to the motor  301 . The isolator  100 - 2  transmits a signal from the motor  301  to the controller  200 . 
     The isolator  100 - 2  includes a transmission-side chip  120 - 2  and a reception-side chip  130 - 2 . The transmission-side chip  120 - 2  includes a transmission circuit  101 - 2 . The reception-side chip  130 - 2  includes: the insulating element  102 ; the insulating element  103 ; a reception circuit  104 - 2 ; the impedance control unit  105 ; and the leakage current detection unit  106 . 
     The transmission data VIN is input to the transmission circuit  101 - 2  from an error detection circuit  302  through the external terminal T 4 , and the transmission circuit  101 - 2  generates an alternating-current transmission signal based on the input transmission data VIN. This alternating-current transmission signal is the signal based on the reference potential GND 2  (the second reference potential). 
     The alternating-current transmission signal is supplied to the insulating element  103  from the transmission circuit  101 . The insulating film  107  is formed between the insulating elements  102  and  103 . The insulating element  102  is alternating current-coupled to the insulating element  103  through the insulating film  107 , and thereby generates an alternating current reception signal. This alternating current reception signal is the signal based on the reference potential GND 1  (the first reference potential) different from the reference potential GND. 
     The alternating current reception signal is input to the reception circuit  104  from the insulating element  102  through the impedance control unit  105 . The reception circuit  104  reproduces the reception data VOUT based on the input alternating current reception signal, and outputs the reception data VOUT to the controller  200  through the external terminal T 2 . 
     Similarly to the isolator  100 , the impedance control signal S 1  is input to the impedance control unit  105  from the controller  200  through the external terminal T 1 , and the impedance control unit  105  controls an impedance of the insulating element  102  to be high based on the input impedance control signal S 1 . Similarly to the isolator  100 , the leakage current detection unit  106  detects a leakage current through the insulating element  102  in which the impedance has been controlled, and outputs the error signal S 2 , which is a detection result according to the leakage current, to the controller  200  through the external terminal T 3 . 
     In the logic circuit OR 3 , a plurality of input terminals are connected to the external terminal T 3  from which the error signal S 2  of each isolator is output, and an output terminal is connected to an input terminal of the error signal SE of the controller  200 . The logic circuit OR 3  performs an OR logic operation of inputs from the plurality of isolators, and outputs an operation result. When the error signal S 2  of any of the plurality of isolators  100  is a high level, the logic circuit OR 3  sets the error signal SE to be a high level, and outputs it to the controller  200 . 
     In applications, such as control of an in-vehicle motor in which reliability is emphasized, not only a state of an isolator but states of an IGBT and the motor are monitored. For example, as in  FIG. 15 , the error detection circuit  302  is included that monitors and outputs isolator interruption detection, IGBT overcurrent detection, overheat detection, etc. In the embodiment, another isolator  100 - 2  is included in order to return a signal of a monitoring result to a primary side. Additionally, a leakage current detection unit is provided at a reception side of the feedback isolator. 
     As described above, even when the leakage current detection unit is provided at the reception-side, the leakage current, similarly to the above-described embodiments, can be detected. Accordingly, since it becomes possible to generate the error signal according to the leakage current, and to stop the isolator before destruction of the insulating film, reliability of the isolator can be improved. 
     Hereinbefore, although the invention made by the present inventor has been specifically explained based on the embodiments, the present invention is not limited to the above-described embodiments, and it is needless to say that various changes can be made without departing from the scope of the invention. 
     Note that the isolator is a circuit that can transmit a signal between circuits having different potentials in a non-contact manner. Signal transmission schemes of the isolator include schemes through a magnetic field (magnetic coupling) by a coil (L), an electric field (capacitive coupling) by a capacitor (C), light by a coupler, an electric wave by an antenna, etc., and any scheme can transmit a signal in the non-contact manner, and can control the potential of the reception circuit side (the secondary side) as in the above-described embodiments. If an isolator having at least insulating elements through an insulating film is employed, the above-described embodiments are applied, thereby a state before destruction of the insulating film is detected, and improvement in reliability can be achieved. 
     The first to eighth embodiments can be combined as desirable by one of ordinary skill in the art. 
     While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention can be practiced with various modifications within the spirit and scope of the appended claims and the invention is not limited to the examples described above. 
     Further, the scope of the claims is not limited by the embodiments described above. 
     Furthermore, it is noted that, Applicant&#39;s intent is to encompass equivalents of all claim elements, even if amended later during prosecution.