Abstract:
A snubber circuit as a conventional protection circuit for inverters, which is composed of an anode reactor, a Si diode and a resistance, needs to keep junction temperature of the Si diode at 125° C. or lower during operation and therefore requires mounting of a large-size heat sink, which causes a large number of component parts and difficulty in downsizing. As a diode of a snubber circuit for circulating electomagnetic energy of the anode reactor, a wide gap semiconductor (SiC) diode, which does not need a heat sink or requires only a small-size heat sink, is used. When the current density of SiC diode is made 20 to 30 times larger than the current density during normal temperature operation, ON resistance increases, and therefore it becomes possible to substitute the ON resistance for the resistance of the snubber circuit. The increase in current density increases the temperature of the SiC diode. However, since the SiC diode can operate at temperature near 300° C. without any problems, a large-size heat sink is not necessary.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a snubber circuit and a power semiconductor device using a self-arc-extinguishing type semiconductor element having the snubber circuit.  
       BACKGROUND ART  
       [0002]     Self-arc-extinguishing type semiconductor elements such as gate turnoff thyristors (hereinbelow abbreviated to GTOs) are provided with a protection circuit called snubber circuit for controlling a voltage applied to between a cathode and an anode as well as steep rise of current passing the elements when the semiconductor elements are turned on and off. The snubber circuit has a function to prevent the semiconductor elements from suffering failure due to voltage, current with steep rise or over voltage applied to the semiconductor elements. The snubber circuit includes a “parallel snubber circuit” which is connected to the semiconductor elements in parallel and a “series snubber circuit” which is connected to the semiconductor elements in series. Either one of or both the parallel snubber circuit and the series snubber circuit are used according to need.  
         [0003]      FIG. 7  is a circuit diagram showing a typical and conventional three-phase inverter device using a GTO as a switching device. In the drawing, a series connection composed of a series snubber circuit  14   a , a first switching circuit  15   a  and a second switching circuit  16   a  is connected to between a positive terminal  20   a  and a negative terminal  20   b  of a direct current DC. Similarly, a series connection composed of a series snubber circuit  14   b , and first and second switching circuits  15   b ,  16   b  as well as a series connection composed of a series snubber circuit  14   c , and first and second switching circuits  15   c ,  16   c  are respectively connected to between the positive terminal  20   a  and the negative terminal  20   b , by which a three-phase inverter device is structured. It is to be noted that since the first switching circuits  15   b ,  15   c  are identical to the first switching circuit  15   a , and the second switching circuits  16   b ,  16   c  are identical to the second switching circuit  16   a , each circuit is shown as a block to omit detailed circuit diagrams. A three-phase alternate current (AC) is outputted from a connection point  17   a  between the first and second switching circuits  15   a  and  16   a , a connection point  17   b  between the first and second switching circuits  15   b  and  16   b , and a connection point  17   c  between the first and second switching circuits  15   c  and  16   c.  Since the series snubber circuits  14   a ,  14   b  and  14   c  share the same structure and the same operation, detailed description will be given of the series snubber circuit  14   a  on behalf of the circuits. Since the first and second switching circuits  15   a ,  15   b ,  15   c ,  16   a ,  16   b ,  16   c  share the same structure and the same operation, detailed description will be given of the first switching circuit  15   a  (hereinbelow referred to as a switching circuit  15   a ) on behalf of the circuits.  
         [0004]     The series snubber circuit  14   a  has a structure in which a series connection composed of a silicon semiconductor (Si) diode  2   a  and a resistance  3   a  is connected in parallel to an anode reactor la that is an inductor.  
         [0005]     The switching circuit  15   a  has a Si GTO  4  as a switching element and a free wheeling diode  5  connected in antiparallel to between the anode and the cathode of the GTO  4 . A parallel snubber circuit formed by connecting a parallel connection composed of a diode  8  and a resistance  9  in series to a capacitor  10  is further connected to between the anode and the cathode of the GTO  4 . A control signal from a known control circuit which is not shown in the drawing is applied to the gate terminal of the GTO  4 . With the control signal, the switching circuits  15   a  to  15   c  and  16   a  to  16   c  are turned on and off with specified timing, so that a direct current DC is converted to an alternate current and so the alternate current AC is outputted.  
         [0006]     When the GTO  4  is turned on, a current from the direct current DC flows through the anode reactor la and the GTO  4 . In the series snubber circuit  14   a , the anode reactor la slows the rise of current (di/dt) generated when the GTO  4  is turned on.  
         [0007]     When the GTO  4  is turned off, current by electromagnetic energy stored in the anode reactor la flows through the diode  2   a  and the resistance  3   a  and is consumed by the resistance  3   a.    
         [0008]     In the parallel snubber circuit, when the GTO  4  is turned on, the capacitor  10  discharges electric charges through the resistance  9 . When the GTO  4  is turned off, the capacitor  10  is charged with electric charges by the current flowing through the diode  8 . More specifically, the current, which tends to keep on flowing by the inductance of the anode reactor la and by the inductance of the circuit interconnections, flows into the capacitor  10  through the diode  8 , so that the over voltage generated in between the anode and the cathode of the GTO  4  is suppressed. Electric charges in the capacitor  10  are gradually discharged through the resistance  9 . 
    (Patent Document 1) JP 2000-166248 A     (Patent Document 2) JP H08-196083 A    
 
         [0011]     In the conventional series snubber circuit  14   a  for the inverter device, the anode reactor la requires the diode  2   a  and the resistance  3   a  for circulation of electromagnetic energy, and this causes a problem of a relatively large number of component parts. Moreover, since the diodes  2   a ,  8  are silicon diodes, the junction temperature during operation needs to be kept at 125° C. or lower. Therefore, the diodes  2   a ,  8  are equipped with a large-size air-cooling or water-cooling heat sink for cooling with a fan and the like. Further, the diodes  2   a ,  8  are designed to be away from the GTO  4  having a large heat generation rate so as to prevent the temperature of the diodes  2   a ,  8  from increasing. However, providing the diodes  2   a ,  8  away from the GTO  4  decreases space efficiency and increases the size of the inverter device, which hinders the downsizing demanded in the field of the invention. Moreover, the length of interconnections connecting each element is prolonged, which makes the increase of inductance by the interconnections unavoidable. When a time constant of the snubber circuit is increased by the increase of the inductance and thereby operation speed is lowered, functions of the snubber circuit are degraded.  
       DISCLOSURE OF THE INVENTION  
       [0012]     A power semiconductor device of the present invention includes at least one switching section for switching current from a power supply on and off, an inductor connected to between the switching section and the power supply, and a wide-gap semiconductor diode element using a wide-gap semiconductor connected to the inductor in parallel so as to be opposed to the direction of a current flow.  
         [0013]     According to the invention, a parallel connection which is composed of the inductor and the wide-gap semiconductor diode element and which is connected to between the switching section and the power supply functions as a series snubber circuit. When the switching section is turned on, the current rise is slowed down by the inductor of the series snubber circuit, which brings about an effect of protecting the switching section.  
         [0014]     When the switching section is turned off, current by electromagnetic energy stored in the inductor flows to the wide-gap semiconductor diode element for circulation, where the current changes to heat and disappears. Although the temperature of the wide-gap semiconductor diode element rises, the wide-gap semiconductor diode element can operate at temperature considerably higher than normal temperature, and therefore a cooling means is simplified or made unnecessary. The wide-gap semiconductor diode element can be used with higher ON resistance than that of, for example, silicon diodes. For example, in a snubber circuit using a silicon diode, a resistance needs to be connected to the silicon diode in series, whereas in the present invention, the function of the resistance is fulfilled by the wide-gap semiconductor diode element.  
         [0015]     A power semiconductor device in another aspect of the present invention includes at least one switching section for switching current from a power supply on and off, an inductor connected to between the switching section and the power supply, and a series connection which is composed of a wide-gap semiconductor diode element using a wide-gap semiconductor and a resistance and which is connected to the inductor in parallel so as to be opposed to the direction of a current flow.  
         [0016]     According to the invention, the inductor connected to between the switching section and the power supply and the series connection which is composed of the resistance and the wide-gap semiconductor diode element and which is connected to the inductor in parallel functions as a series snubber circuit. When the switching section is turned on, the current rise is slowed down by the inductor of the series snubber circuit, which brings about an effect of protecting the switching section.  
         [0017]     When the switching section is turned off, current by electromagnetic energy stored in the inductor flows into the resistance through the wide-gap semiconductor diode element, so that the current changes to heat in the wide-gap semiconductor diode element and in the resistance and disappears. Although the temperature of the wide-gap semiconductor diode element rises, the wide-gap semiconductor diode element can operate at temperature considerably higher than normal temperature, and therefore a cooling means is simplified or made unnecessary. The wide-gap semiconductor diode element during high temperature operation has higher ON resistance than that of, for example, silicon diodes. With the high ON resistance, electricity derived from the electromagnetic energy stored in the inductor changes to heat. In the present invention, the function of the resistance is fulfilled by the wide-gap semiconductor diode element, and therefore a resistance value can be decreased.  
         [0018]     When the switching section is turned off, a voltage is generated between terminals of the anode reactor, and so displacement current flows to a diode for circulation of electromagnetic energy in the anode reactor. The current is superimposed on the current flowing through the anode reactor, so that a rate of rise of current in the switching section increases. However, by using a wide-gap semiconductor diode as the diode, a chip area can be made smaller than that in the case of Si diodes and thereby electrostatic capacity is decreased, so that the displacement current can be decreased and the rate of rise of current in the switching section can be decreased. Consequently, it becomes possible to delay the reverse recovery time of a free wheeling diode in the other switching section, so that the rate of rise of voltage in the switching section can also be decreased.  
         [0019]     A power semiconductor device in still another aspect of the present invention includes an inductor with one end connected to one terminal of a power supply, and a series connection which is composed of a first wide-gap semiconductor diode in backward direction of an output current from the power supply and a resistance and which is connected in parallel with the inductor. One end of a first switching circuit is connected to the other end of the inductor, while one end of a second switching circuit is connected to the other end of the first switching circuit. The other end of the second switching circuit is connected to the other terminal of the power supply. A clamp capacitor is connected to between a connection point between the resistance and the first wide-gap semiconductor diode element and the other end of the power supply. A first snubber capacitor is connected to between the connection point between the resistance and the first wide-gap semiconductor diode element and a connection point between the first and second switching circuits. A series connection composed of a second snubber capacitor and a second wide-gap semiconductor diode in forward direction is connected to between the connection point between the first and second switching circuits and the other terminal of the power supply.  
         [0020]     According to the invention, in addition to the above functions and effects, the first snubber capacitor suppresses over voltage applied to the first switching circuit when the first switching circuit is turned off. The second snubber capacitor suppresses over voltage applied to the second switching circuit when the second switching circuit is turned off.  
         [0021]     A power semiconductor device in yet another aspect of the present invention includes at least one inductor connected to one terminal of a power supply, and a series connection which is composed of a first wide-gap semiconductor diode in backward direction of an output current from the power supply and a resistance and which is connected to each inductor in parallel. A first switching circuit is connected to each inductor in series. A second switching circuit is connected to each first switching circuit in series. A second inductor is connected to between each second switching circuit and the other terminal of the power supply. A series connection composed of a second wide-gap semiconductor diode in backward direction and a second resistance is connected to the second inductor in parallel. Moreover, a capacitor is connected to between a connection point between the first resistance and the first wide-gap semiconductor diode and a connection point between the second resistance and the second wide-gap semiconductor diode.  
         [0022]     According to the invention, a series snubber circuit is each provided in between the first switching circuit and the power supply and in between the second switching circuit and the power supply. As a result, the first and second switching circuits are less likely to suffer unbalanced operations. This brings about an effect to stabilize the operation of the circuit.  
         [0023]     According to the present invention, a diode made of a wide-gap semiconductor is used as a reflux diode for circulating current by electromagnetic energy from an anode reactor in a series snubber circuit. Since the wide-gap semiconductor diode can be used at temperature considerably higher than normal temperature, it becomes possible to downsize a heat sink for cooling. In some cases, the heat sink is made unnecessary and thereby the snubber circuit is downsized, which in turns makes it possible to downsize the power semiconductor device.  
         [0024]     Structuring the reflux diode so as to function also as a diode of a parallel snubber circuit provided in the switching circuit reduces the number of component parts of the switching circuit, and thereby leads to downsizing and increased reliability of the power semiconductor device.  
         [0025]     Using a wide-gap semiconductor diode as a reflux diode makes it possible to decrease a chip area and to thereby decrease its electrostatic capacity. Consequently, it becomes possible for the reflux diode to decrease displacement current when the switching section is turned on and to decrease a rate of rise of current in the switching section. Further, a rate of rise of voltage in the other switching section can be decreased.  
         [0026]     By structuring a switching element and a free wheeling diode from a wide-gap semiconductor, the switching element and the free wheeling diode are incorporated in a single package and operated at temperature higher than normal temperature. Consequently, the cooling means such as heat sinks is made unnecessary or downsized, which makes it possible to simplify the structure of the power semiconductor device and allows downsizing, as well as to achieve the power semiconductor device operable in the environment of high temperature. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a circuit diagram showing a three-phase inverter device as a first embodiment of a power semiconductor device in the present invention;  
         [0028]      FIG. 2  is a circuit diagram showing a switching circuit of only one phase in the three-phase inverter device as a second embodiment of the power semiconductor device in the present invention;  
         [0029]      FIG. 3  is a circuit diagram showing a switching circuit of only one phase in the three-phase inverter device as a third embodiment of the power semiconductor device in the present invention;  
         [0030]      FIG. 4  is a circuit diagram showing a switching circuit of only one phase in the three-phase inverter device as a fourth embodiment of the power semiconductor device in the present invention;  
         [0031]      FIG. 5  is a circuit diagram showing a switching circuit of only one phase in the three-phase inverter device as a fifth embodiment of the power semiconductor device in the present invention;  
         [0032]      FIG. 6  is a cross sectional view showing a SiC Schottky diode for use in the three-phase inverter device as a sixth embodiment of the power semiconductor device in the present invention; and  
         [0033]      FIG. 7  is a circuit diagram showing a conventional inverter device. 
     
    
     REFERENCE NUMERALS  
       [0000]    
       
           20   a  positive terminal  
           20   b  negative terminal  
           31   a ,  31   b ,  31   c  anode reactor  
           32   a ,  32   b ,  32   c  SiC-pn diode  
           33   a ,  33   b ,  33   c  resistance  
           34   a ,  34   b ,  34   c  GTO  
           35   a ,  35   b ,  35   c  free wheeling diode  
           36   a ,  36   b ,  36   c  SiC-GTO  
           37   a ,  37   b ,  37   c  free wheeling diode  
           38   a  clamp capacitor  
           40   a ,  41   a  snubber capacitor  
           45   a  resistance  
           46   a  SiC diode  
           54   a ,  54   b ,  54   c  series snubber circuit  
           55   a ,  55   b ,  55   c  first switching circuit  
           56   a ,  56   b ,  56   c  second switching circuit  
           57   a ,  57   b ,  57   c  connection point  
           64   a  series snubber circuit  
           65   a  first switching circuit  
           66   a  second switching circuit  
           74   a  snubber circuit  
           76   a  second switching circuit  
           80  SiC Schottky diode  
           84  cathode electrode  
           85  anode electrode  
       
     
       DETAILED DESCRIPTION OF THE INVENTION  
       [0059]     Preferred embodiments of a power semiconductor device in the invention will now be described with reference to  FIG. 1  to  FIG. 6 .  
       First Embodiment  
       [0060]     A three-phase inverter device, that is a power semiconductor device in the first embodiment of the present invention, will be described with reference to  FIG. 1 . In the drawing, a series connection formed by connecting a series snubber circuit  54   a , a first switching circuit  55   a  and a second switching circuit  56   a  in this order is connected to between a positive terminal  20   a  and a negative terminal  20   b  of a direct current input DC (direct current power supply). Similarly, a series connection composed of a series snubber circuit  54   b , a first switching circuit  55   b  and a second switching circuit  56   b  is connected to between the positive terminal  20   a  and the negative terminal  20   b.  A series connection composed of a series snubber circuit  54   c , a first switching circuit  55   c  and a second switching circuit  56   c  is connected to between the positive terminal  20   a  and the negative terminal  20   b.  The series snubber circuit  54   a , and the first and second switching circuits  55   a ,  56   a  should not necessarily be connected to between the positive terminal  20   a  and the negative terminal  20   b  in this order but may be connected in arbitrary order. This also applies to other circuits including the series snubber circuits  54   b ,  54   c , the first switching circuits  55   b ,  55   c , and the second switching circuits  56   b ,  56   c.    
         [0061]     The first switching circuit  55   a  is structured from an antiparallel connection composed of a GTO  34   a  made of a Si or SiC semiconductor serving as a switching element and a free wheeling diode  35   a  made of a Si or SiC semiconductor. The first and second switching circuits  55   b ,  55   c ,  56   a  to  56   c  share the same structure with the first switching circuit  55   a.  Each of the first switching circuits  55   a  to  55   c  and the second switching circuits  56   a  to  56   c  is controlled by a control signal applied to each GTO gate from a known control circuit which is not shown in the drawing. An alternate current output AC can be obtained from a connection point  57   a  between the first switching circuit  55   a  and the second switching circuit  56   a , a connection point  57   b  between the first switching circuit  55   b  and the second switching circuit  56   b , and a connection point  57   c  between the first switching circuit  55   c  and the second switching circuit  56   c.  The circuit shown in  FIG. 1  can also operate as a converter by inputting an alternate current into the terminal of the alternate current output AC. In this case, the positive terminal  20   a  and the negative terminal  20   b  of the direct current input DC can obtain a direct current output. This operation is achieved in the same manner in second embodiment to fifth embodiment.  
         [0062]     Since the series snubber circuits  54   a ,  54   b ,  54   c  have the same structure and the same operation, detailed description will now be given of the series snubber circuit  54   a  on behalf of these circuits. Also, since the first switching circuits  55   a  to  55   c  and the second switching circuits  56   a  to  56   c  have the same structure and the same operation, detailed description will now be given of the first switching circuit  55   a  (hereinbelow simply referred to as a switching circuit  55   a ) on behalf of these circuits.  
         [0063]     The series snubber circuit  54   a  has an anode reactor  31   a  including an inductor such as coils and a series connection which is composed of a SiC-pn diode  32   a  and a resistance (reflux resistance)  33   a  and which is connected to the anode reactor  31   a  in parallel. The SiC-pn diode  32   a  is a wide-gap semiconductor pn diode (hereinbelow abbreviated to SiC diode  32   a ) with use of silicon carbide (SiC) that is a wide-gap semiconductor. Examples of the wide-gap semiconductor include gallium nitride (GaN) and diamond, though in this embodiment, description will be given of an example using SiC. One terminal of the anode reactor  31   a  is connected to the positive terminal  20   a , while the other terminal is connected to the switching circuit  55   a.    
         [0064]     When the GTO  34   a  in the switching circuit  55   a  is turned on, current from the direct current input DC flows to the GTO  34   a  through the anode reactor  31   a.  The anode reactor  31   a  slackens the rise characteristics of the current flowing into the GTO  34   a.    
         [0065]     When the GTO  34   a  is turned off, current by electromagnetic energy stored in the anode reactor  31   a  flows through the SiC diode  32   a  and the resistance  33   a , where the current is turned into heat and consumed.  
         [0066]     The SiC diode  32   a  for use in the series snubber circuit  54   a  in the present embodiment has characteristics described below.  
         [0067]     The SiC diode  32   a  operates normally in the junction temperature from 150° C. to 500° C. due to high-temperature resistance characteristics of SiC. In the present embodiment, in the state in which the inverter device shown in  FIG. 1  operates while outputting a desired steady alternate current (hereinbelow referred to as the steady operation state), the density of current flowing through the SiC diode  32   a  in the series snubber circuit  54   a  is set higher than that in the case of rated operation. More specifically, the current density of the SiC diode  32   a  is set to be 5 times or 30 times larger than the current density when the junction temperature is normal temperature (100° C. or below). The higher the current density is set, the more the junction temperature of the SiC diode  32   a  rises. In concrete, the SiC diode to be employed should have such a junction area that the temperature of the junction becomes approx. 150° C. during steady operation. In other words, the SiC diode  32   a  having much smaller capacity and smaller junction area, i.e., smaller rated capacity, is sufficient for use than those operated at normal temperature, i.e., under usual rated operation.  
         [0068]     An experiment by an inventor of the present invention proved that the junction temperature became approx. 250° C. if the current density was set approx. 20 times to 30 times larger than the current density at the rated operation. This rate changes depending on the heat radiation characteristics and the heat resistance of a heat sink mounted on the SiC diode  32   a.    
         [0069]     As a result of increasing the current density of the SiC diode  32   a , its junction temperature rises and its ON resistance increases. The increase in the ON resistance in turn increases ON voltage, which is, for example, in the range of 3 V to 20 V. With the increase in the ON resistance, electricity due to the electromagnetic energy stored in the anode reactor  31   a  is also consumed by the ON resistance of the SiC diode  32   a.  Therefore, the electricity which should be consumed by the resistance  33   a  is partially consumed by the SiC diode  32   a.  As a result, it becomes possible to use, as the resistance  33   a , a resistance having a smaller resistance value and a rated capacity than those of the resistance  3   a  in the series snubber circuit  14   a  of the conventional inverter device shown in  FIG. 7 . Further, with the presence of the SiC diode, electrostatic capacity is decreased and thereby displacement current of the SiC diode can be decreased, so that the rate of rise of current when the switching section is turned on can be reduced.  
         [0070]     The SiC diode  32   a  has a shift time from the off state to on state (on time) shorter than that of Si diodes and the like. This makes it possible to pass quickly the current from the anode reactor  31   a  to the resistance  33   a  when the GTO  34   a  is turned off. Consequently, voltage generated between both the terminals of the anode reactor  31   a  during the turned-off time is low. An experiment confirmed that the voltage was approx. 40% of the voltage of the series snubber circuit  14   a  using the Si diode  2   a  shown in  FIG. 7 .  
         [0071]     The SiC diode  32   a  of the series snubber circuit  54   a  may be replaced with a SiC Schottky diode with high withstand voltage. The ON resistance of the SiC Schottky diode is higher than that of the SiC-pn diode at high temperature. Because of this, the ON voltage of the SiC Schottky diode is also high; the ON voltage of the SiC Schottky diode for use in the present embodiment is approx. 30 V at approx. 250° C. Replacing the SiC diode  32   a  with such a SiC Schottky diode with high ON resistance makes it possible to omit the resistance  33   a.  Since the resistance  33   a  is generally equipped with a heat sink, omitting the resistance  33   a  makes the heat sink unnecessary and have a profound effect on reduction in component parts and space, thereby contributing to downsizing of the inverter device.  
         [0072]     In a concrete example in the present embodiment, in the condition that the voltage of the direct current input DC was 1000 V and that the current passing the GTO  34   a  was 50 A, the over voltage generated in the GTO  34   a  when the GTO  34   a  blocked the current of 50 A was approx 40 V. In the structure in which the Si diode  2   a  is used in the series snubber circuit  14   a  as seen in the conventional example shown in  FIG. 7   c , over voltage when a current of 50 A with a direct current voltage of 1000 V is blocked in the same condition is approx. 380 V. Therefore, the over voltage in the present embodiment is about 1/10 of the over voltage in the conventional example.  
         [0073]     In the switching circuit  55   a  of the present embodiment shown in  FIG. 1 , the GTO  34   a  and the free wheeling diode  35   a  are incorporated in a single package shown by a chain line so as to shorten connection lead wires between the GTO  34   a  and the free wheeling diode  35   a . The shortened lead wires make it possible to reduce inductance and enable the inverter device to support high frequencies. Moreover, incorporating the GTO  34   a  and the free wheeling diode  35   a  in a single package reduces the number of component parts compared to the case of assembling separate component parts, and this increases reliability and reduces the total cost as well as allows downsizing.  
         [0074]     It is also possible to structure so that the SiC diode  32   a  in the series snubber circuit  54   a  is also incorporated in the package (unshown). In this structure, a SiC-GTO is used as the GTO  34   a  serving as a switching element and a SiC diode is used as the free wheeling diode  35   a.  More specifically, forming all the semiconductor elements by SiC semiconductors and incorporating them in a single package enable all the SiC semiconductor elements in the package to operate at temperature as high as 250° C. This makes it possible to simplify a cooling device such as heat sinks in the package for achieving simple structure as well as to implement considerable downsizing.  
         [0075]     Although in the series snubber circuit  54   a  in the present embodiment, a series connection composed of the SiC diode  32   a  and the resistance  33   a  is connected to the anode reactor  31   a  in parallel, the same effects as those of the present embodiment can also be achieved by another structure, in which a series connection composed of the SiC diode  32   a  and a power regenerative circuit is connected to the anode reactor  31   a  in parallel. The power regenerative circuit is to collect and make efficient use of electric energy which is changed to heat and consumed in the resistance  33   a.    
         [0076]     In the present embodiment shown in  FIG. 1 , the same effects can also be achieved if the series snubber circuit  54   a  is connected to between the first switching circuit  55   a  and the second switching circuit  56   a.    
       Second Embodiment  
       [0077]     Description is now given of a three-phase inverter device that is a power semiconductor device in a second embodiment of the present invention with reference to  FIG. 2 .  FIG. 2  is a circuit diagram showing a switching circuit of one phase in the three-phase inverter device in the second embodiment of the present invention. In the drawing, a series connection composed of a snubber circuit  64   a , a first switching circuit  65   a  and a second switching circuit  66   a  is connected to between a positive terminal  20   a  and a negative terminal  20   b  of a direct input DC (direct current power supply). While the actual three-phase inverter device is composed of three series connections of three phases which share the same structure and the same operation and which are connected to between the positive terminal  20   a  and the negative terminal  20   b , only one phase is shown in the drawing and illustration and description of other two phases are omitted for simplification. While the three-phase inverter device in the present embodiment is controlled by a known control circuit whose illustration is omitted, the first switching circuit  65   a  and the second switching circuit  66   a  are turned on with different timing and are not turned on at the same time.  
         [0078]     In  FIG. 2 , a series snubber circuit  64   a  has an anode reactor  31   a  and a series connection which is composed of a SiC diode  32   a  serving as a reflux diode and a resistance  33   a  and which is connected to the anode reactor  31   a  in parallel. A clamp capacitor  38   a  is connected to between a connection point between the SiC diode  32   a  and the resistance  33   a  and the negative terminal  20   b.  In the present embodiment, the SiC diode  32   a  in the series snubber circuit  64   a  is incorporated in the package of the first switching circuit  65   a.  The switching elements constituting the first and second switching circuits  65   a  and  66   a  are a SIC-GTO  34   a  and a SIC-GTO  36   a , which are GTOs with use of SiC semiconductors. By forming the switching elements from SiC-GTOs  34   a ,  36   a  with use of wide gap semiconductor SiCs, the switching speed becomes not less than  10  times higher than that of the Si-GTO. A free wheeling diode  35   a  and a free wheeling diode  37   a  serve as free wheeling diodes. In the second switching circuit  66   a , the SIC-GTO  36   a  and the free wheeling diode  37   a  are also incorporated in a single package.  
         [0079]     In the present embodiment, the clamp capacitor  38   a  is connected to between the connection point between the SiC diode  32   a  and the resistance  33   a  and the negative terminal  20   b , so that the SiC diode  32   a  functions also as a clamp diode when an over voltage higher than the voltage of the direct current input DC is applied to a series body composed of the first and second switching circuits  65   a  and  66   a.  The SiC diode  32   a  functioning as a clamp diode makes it unnecessary to separately provide a clamp diode. This simplifies a circuit including the first switching circuit  65   a  and the series snubber circuit  64   a  and thereby makes it possible to shorten interconnections. As a result, the inductance of the interconnections is decreased, and therefore when the SIC-GTO  34   a  is turned off, over voltage applied to the anode and the cathode can be lowered. Therefore, it becomes possible to prevent the SiC-GTO  34   a  from being damaged by the over voltage.  
         [0080]     During operation of the inverter device shown in  FIG. 2 , when the SiC-GTO  34   a  as a switching element in the first switching circuit  65   a  is turned off, over voltage is generated by electomagnetic energy stored in the anode reactor  31   a  and the inductance of the interconnections. In order to suppress the over voltage, current by the over voltage is sent to the clamp capacitor  38   a , by which the clamp capacitor  38   a  is charged. This makes it possible to prevent the over voltage from being applied to the SiC-GTO  34   a.  Excess electricity charged to the clamp capacitor  38   a  is passed to and consumed by the resistance  33   a.  Since the SiC diode  32   a  is used as a reflux diode of the series snubber circuit  64   a , it becomes possible to pass the current flowing the anode reactor  31   a  to the reflux resistance  33   a  through the SiC diode  32   a  at high speed when the SiC-GTO  34   a  is turned off. Consequently, it becomes possible to prevent the over voltage by electomagnetic energy in the anode reactor  31   a  from being applied to the SiC-GTO  34   a.  Moreover, since the electricity by electomagnetic energy in the anode reactor  31   a  is consumed by large ON resistance of the SiC diode  32   a , the electricity can be damped at higher speed than that in the case where the electricity is consumed only by the resistance  33   a.  The high-speed operation of the SiC diode  32   a  can increase the drive frequency of the inverter device.  
         [0081]     Moreover, since the low electrostatic capacity of the SiC diode can suppress a rate of rise of current in the switching section, a rate of rise of the voltage applied to the other switching section can also be suppressed.  
         [0082]     In the present embodiment as with the first embodiment, the SiC diode  32   a  in the series snubber circuit  64   a  can operate at temperature as high as approx. 250° C. The SIC-GTO  34   a  and the free wheeling diode  35   a  incorporated in the same package as the SiC diode  32   a  are also operated with high current density and therefore reach generally the same temperature as the SiC diode  32   a . Operating the SiC diode  32   a  with high current density makes it possible to achieve the functions and effects identical to those described in the first embodiment. Since the second switching circuit  66   a  can also be operated at high temperature like the first switching circuit  65   a , heat sinks and the like of the first and second switching circuits  65   a  and  66   a  are made unnecessary or downsized, by which downsizing of the entire inverter device can be achieved. Incorporating the first and second switching circuits  65   a  and  66   a  in respective packages can reduce the number of component parts and enhance reliability, which in turn allows reduction in size and cost of the inverter device.  
         [0083]     Increasing the current density increases the ON resistance of the SiC diode  32   a  in the series snubber circuit  64   a , and therefore it also becomes possible to remove the resistance  33   a  by substituting the ON resistance of the SiC diode  32   a  for the resistance  33   a.  In this case, the clamp capacitor  38   a  is connected to between the connection point between the SiC diode  32   a  and the free wheeling diode  35   a  and the negative terminal  20   b.  This allows further downsizing.  
         [0084]     Since incorporating the first and second switching circuits  65   a  and  66   a  in respective packages shortens the length of connecting conductors for connecting each element, the inductance of the connecting conductors decreases. The decrease in the inductance can suppress the generation of over voltage attributed to the inductance and can increase the drive frequency of the inverter device.  
       Third Embodiment  
       [0085]     Description is now given of a three-phase inverter device that is a power semiconductor device in a third embodiment of the present invention with reference to  FIG. 3 .  FIG. 3  is a circuit diagram showing a switching circuit of one phase in the three-phase inverter device, and the circuits of other two phases are omitted. In  FIG. 3 , a series snubber circuit  64   a  composed of an anode reactor  31   a , a SiC diode  32   a  and a resistance  33   a  is similar to that in the second embodiment shown in  FIG. 2 .  
         [0086]     A first switching circuit  65   a  has a SiC-GTO  34   a  and a SiC free wheeling diode  35   a  connected in antiparallel, and the SiC-GTO  34   a  and the SiC diodes  32   a ,  35   a  are incorporated in a single package. A clamp capacitor  38   a  is connected to between the cathode of the SiC diode  32   a  and the negative terminal  20   b , while a snubber capacitor  40   a  is connected to between the cathode of the SiC diode  32   a  and the cathode of the SiC-GTO  34   a . The SiC diode  32   a  and the snubber capacitor  40   a  constitute a parallel snubber circuit in the first switching circuit  65   a.  Therefore, the SiC diode  32   a  functions as an element for both the series snubber circuit  64   a  and the parallel snubber circuit.  
         [0087]     The second switching circuit  76   a  has a SiC-GTO  36   a , a free wheeling diode  37   a  connected to the SiC-GTO  36   a  in antiparallel, and a series connection which is composed of a snubber capacitor  41   a  and a SiC diode  39   a  and which is connected to the anode and the cathode of the SIC-GTO  36   a . The SiC diode  39   a  and the snubber capacitor  41   a  constitute a parallel snubber circuit in the second snubber circuit  76   a.    
         [0088]     In the present embodiment as with the second embodiment, the SiC diodes  32   a ,  35   a  and the SIC-GTO  34   a  can be operated at temperature as high as approx. 250° C. The SIC-GTO  36   a  and the SiC diodes  37   a ,  39   a  in the second snubber circuit  76   a  can also be operated at normal temperature or be operated at temperature as high as approx. 250° C.  
         [0089]     In the three-phase inverter device in the present embodiment, the SiC diode  32   a  and the snubber capacitor  40   a  make it possible to hold down the over voltage applied to the SIC-GTO  34   a  when the SIC-GTO  34   a  is turned off. Also, the SiC diode  39   a  and the snubber capacitor  41   a  make it possible to hold down the over voltage applied to the SiC-GTO  36   a  when the SIC-GTO  36   a  is turned off. The energy stored in the snubber capacitor  40   a  by absorbing the over voltage is partially consumed by the resistance  33   a  in the series snubber circuit  64   a , while the remaining energy returns to the power supply.  
         [0090]     In the present embodiment, the SiC diode  32   a  for circulating the electomagnetic energy of the anode reactor  31   a  in the snubber circuit  64   a  also serves as a component of a parallel snubber circuit structured in combination with the snubber capacitor  40   a.  This decreases the number of component parts. Since each of the first and second switching circuit  65   a  and  76   a  can also be operated at temperature as high as 250° C. or higher in the present embodiment, the heat sinks can be downsized. This allows downsizing of the inverter device. Moreover, the presence of the parallel snubber circuit having the snubber capacitors  40   a ,  41   a  allows effective suppression of large over voltage generated when the first and second switching circuits  65   a  and  76   a  using the SiC-GTOs  34   a ,  36   a , which have far shorter switching time than Si-GTOs as switching elements, are turned off.  
       Fourth Embodiment  
       [0091]      FIG. 4  is a circuit diagram showing one phase of a three-phase inverter device that is a power semiconductor device in a fourth embodiment of the present invention. As the circuits of other two phases are identical to the circuit of the one phase, their illustration is omitted.  
         [0092]     In  FIG. 4 , a series snubber circuit  64   a , and first and second switching circuits  65   a  and  76   a  are identical to those in the third embodiment shown in  FIG. 3 .  
         [0093]     In the present embodiment, another anode reactor  43   a  is connected to between the cathode of a SiC-GTO  36   a  in the second snubber circuit  76   a  and the negative terminal  20   b  of the direct current input DC. Also, another resistance  45   a  is connected to between the anode of a SiC diode  46   a  and the negative terminal  20   b.  A clamp capacitor  38   a  is connected to between the cathode of a SiC diode  32   a  in the series snubber circuit  64   a  and the anode of the SiC diode  46   a.    
         [0094]     In the present embodiment, the SIC-GTO  34   a , a free wheeling diode  35   a  and the SiC diode  32   a  are incorporated in the package of the first switching circuit  65   a.  The second switching circuit  76   a  is also incorporated in another package. The SiC-GTOs  34   a ,  36   a  and the SiC diodes  32   a ,  35   a ,  37   a ,  46   a  are operated at temperature higher than normal temperature, e.g., at approx. 250° C. This allows downsizing of the heat sinks and contributes to downsizing of the inverter device.  
         [0095]     During operation of the inverter device in the present embodiment, the over voltage, which is applied to the SIC-GTO  34   a  and the free wheeling diode  35   a  at the time of recovery of the SiC diode  32   a  and the free wheeling diode  35   a  when the SIC-GTO  34   a  is turned off, is suppressed by the clamp capacitor  38   a.  The over voltage, which is applied to the SIC-GTO  36   a  and the free wheeling diode  37   a  at the time of recovery of the free wheeling diode  37   a  and the SiC diode  46   a  when the SIC-GTO  36   a  is turned off, is suppressed by the clamp capacitor  38   a.  If the inductance of the anode reactors  31   a ,  43   a  is set at a specified value, a rate of rise of the voltage applied to the SIC-GTO  34   a  or  36   a  at the time of recovery of the free wheeling diode  35   a  or  37   a  can be controlled so as to be lower than a critical rate of rise of off-state voltage of the SiC-GTOs  34   a ,  36   a . This can prevent false firing of the SiC-GTO  34   a  or  36   a  and can thereby prevent the SiC-GTOs  34   a ,  36   a  from suffering failures by the false firing.  
         [0096]     In the inverter device in the present embodiment, the anode reactor  31   a  is provided in between the first switching circuit  65   a  and the positive terminal  20   a , and the anode reactor  43   a  is provided in between the second snubber circuit  76   a  and the negative terminal  20   b , so that unbalance in over voltage and difference in rate of rise of voltage between upper and lower arms of the inverter circuit can be decreased.  
       Fifth Embodiment  
       [0097]      FIG. 5  is a circuit diagram showing one phase of a three-phase inverter device that is a power semiconductor device in a fifth embodiment of the present invention, and the circuits of other two phases are omitted.  
         [0098]     The circuit shown in  FIG. 5  is structured based on the circuit of the fourth embodiment shown in  FIG. 4 , in which the snubber capacitor  40   a  is connected to between the cathode of the SiC diode  32   a  and the cathode of the SiC-GTO  34   a  and the snubber capacitor  41   a  is connected in between the anode of the SiC diode  46   a  and the anode of the SiC-GTO  36   a.  Other structural aspects are identical to those shown in  FIG. 4 .  
         [0099]     During operation of the inverter device in the present embodiment, the over voltage generated when the SiC-GTO  34   a  or the SIC-GTO  36   a  is turned off is reduced by charging of the snubber capacitor  40   a  or  41   a.  The steep voltage rise generated at the time of recovery of the free wheeling diode  35   a  or  37   a  is also suppressed by charging of the snubber capacitor  40   a  or  41   a.  As a result, it becomes possible to prevent false firing of the SiC-GTOs  34   a ,  36   a  and to prevent the SiC-GTO  34   a  or  36   a  from being damaged or suffering failures due to the false firing.  
         [0100]     In the present embodiment, the SiC-GTO  34   a , the SiC diode  32   a  and the free wheeling diode  35   a  are also incorporated in a single package. The SiC-GTO  36   a , the SiC diode  46   a  and the free wheeling diode  37   a  are also incorporated in a single package. Since all the semiconductor elements in each package are made of wide-gap semiconductor SiCs, they can be used at temperature higher than normal temperature, e.g., at approx. 250° C. Therefore, the cooling means such as heat sinks can be simplified or can be omitted in some cases. Since the SiC-GTO  34   a , the SiC diode  32   a  and the free wheeling diode  35   a  are incorporated in a single package, and the SiC-GTO  36   a , the SiC diode  46   a  and the free wheeling diode  37   a  are also incorporated in a single package, the connecting conductors between the elements in each package can be shortened. This can enhance the frequency characteristics in the first and second switching circuits  65   a  and  76   a  and achieve downsizing.  
         [0101]     Moreover, incorporating the SIC-GTO  34   a , the SiC diode  32   a  and the free wheeling diode  35   a  in a single package makes the temperature of these three elements almost identical, which brings about an effect of making their characteristics almost identical during operation. This effect is also true for the elements in the second switching circuit  76   a.    
       Sixth Embodiment  
       [0102]      FIG. 6  is a cross sectional view showing a SiC Schottky diode for use in a series snubber circuit for a three-phase inverter device that is a power semiconductor device in a sixth embodiment of the present invention. The SiC Schottky diode in the present embodiment can be used in place of the SiC diodes  32   a ,  39   a  and  46   a  in the three-phase inverter devices in the first to the fifth embodiments shown in  FIG. 1  to  FIG. 5 .  
         [0103]     In  FIG. 6 , a drift layer  82  made of a lightly-doped (n−) n-SiC semiconductor containing nitrogen as dopant and having a thickness of 50 to 100 μm is formed on a substrate  81  made of a heavily-doped (n+) n-type 4H-SiC semiconductor having a thickness of 300 μm. The dopant concentration of the drift layer  82  should preferably be in the range of 1×10 13  to 5×10 14 cm −3 . A cathode electrode  84  made of nickel (Ni), gold (Au) or the like is provided on the lower surface of the substrate  81 , while an anode electrode  85  made of nickel is provided on the upper surface of the drift layer  82 .  
         [0104]     Generally, SiC semiconductor diodes gain higher ON resistance if the thickness of their drift layer is increased and the dopant concentration is lowered. In the present embodiment, the ON resistance is increased by increasing the thickness of the drift layer  82  of a SiC Schottky diode  80 . The ON resistance is also increased by lowering the dopant concentration. In the Sic Schottky diode  80  in the present embodiment, the thickness of the drift layer  82  is approx. 100 μm. The drift layer  82  is set to have a thickness 1.5 times larger than the thickness necessary for withstanding the backward voltage applied to the SiC Schottky diode  80 . Consequently, the ON resistance becomes 1.5 times larger than that of SiC Schottky diodes manufactured by the standard design criteria. The ON resistance of SiC-pn diodes is also increased by increasing the thickness of their drift layer and lowering the dopant concentration. Higher temperature increases the ON resistance of SiC-pn diodes but not to the level of the SiC Schottky diodes.  
         [0105]     The SiC Schottky diode  80  in the present embodiment is used in place of, for example, the SiC diode  32   a  in  FIG. 1 . Since the ON resistance of the SiC Schottky diode  80  increases electricity consumption in the element, the SiC Schottky diode  80  generates heat and raises temperature. Since the SiC Schottky diode  80  has higher ON resistance than the SiC-pn diode at high temperature, the SiC Schottky diode  80  can take over the role of the resistance  33   a.  In some cases, the resistance  33   a  can be omitted.  
         [0106]     Although SiC-GTOs are used as switching elements in the first to the fifth embodiments, other wide-gap semiconductor GTOs such as gallium nitride (GaN) and diamond other than SiC can also be used, and in such a case, the same functions and effects can be achieved. Moreover, although the GTOs used in each embodiment are structured to have a gate on the cathode side, GTOs having a gate on the anode side (unshown) can also be used while the same functions and effects can be achieved.  
       INDUSTRIAL APPLICABILITY  
       [0107]     The present invention is applicable to small-size power semiconductor devices with high withstand voltage.