Insulated gate semiconductor device

Abstract: In order to prevent a malfunction caused by an electrical noise and limit an excessive main current at a high speed while cutting off the same to a value close to zero, the main current is regulated by an IGBT (1) which is connected with a load. A part of this main current is shunted to another IGBT (2). The as-shunted current flows through a resistor (3), to be converted to a voltage across the resistor (3). When the main current is excessively increased by shorting of the load or the like, this voltage exceeds a prescribed value so that a transistor (5) and a thyristor (7) enter conducting states. Consequently, a voltage across a gate (G) and an emitter (E) of the IGBT (1) is so reduced as to cut off the main current. The transistor (5) prevents the main current from excessive increase since the same has a high speed of response, while the thyristor (7) cuts off the main current to zero since the same has lower resistance in conduction.

Description: The present invention relates to an insulated gate semiconductor device according to the preamble of claim 1 which is employed as a switching element in a power converter such as an inverter, and more particularly, it relates to an improvement in a function for preventing an insulated gate element from breakdown in shorting of a load. An insulated gate element such as a MOS field effect transistor element (MOSFET) or an insulated gate bipolar transistor element (IGBT) has two current electrodes and a control electrode which is insulated from these electrodes, so that the value of a current flowing across the two current electrodes is adjusted by the value of a voltage which is applied across the control electrode and one of the current electrodes. The current is increased as the applied voltage is increased, while the same is cut off when the voltage is zero. An insulated gate semiconductor device comprising such an insulated gate element is employed as a switching element for a power converter such as an inverter for switching a current (main current) flowing to a load, for example. When the load is shorted in such a power converter, an excessive main current (short-circuit current) flows to the insulated gate element, which is finally broken if this state is left intact. In the insulated gate semiconductor device, therefore, a driving circuit for the insulated gate element, which is a circuit part for driving the insulated gate element, is provided with a short-circuit current cutoff function for preventing breakdown caused by such a short-circuit current. Fig. 15 is a block diagram showing an exemplary conventional insulated gate semiconductor device having a short-circuit current cutoff function. Such a device, corresponding to the features of the preamble of claim 1, is known from US-A-4 750 079. A load (not shown) is connected to a collector C of an IGBT 1 serving as an insulated gate element, so that a collector current I C flowing from the collector C to an emitter E is supplied to the load as a main current. This collector current I C is controlled by the value of a voltage (gate voltage) across a gate G and the emitter E. A larger collector current I C flows as the gate voltage is increased. The gate voltage is regulated and supplied by a gate driving circuit 42. This insulated gate semiconductor device is provided with a current transformer 43, which detects the collector current I C . The as-detected value of the collector current I C is compared with a prescribed reference value in a compare circuit 44. The compare circuit 44 transmits a prescribed signal to the gate driving circuit 42 when the collector current I C exceeds the reference value. In response to this signal, the gate driving circuit 42 outputs a prescribed gate voltage to the gate G, in order to cut off the IGBT 1. Thus, the excessive collector current I C resulting from shorting of the load is cut off to protect the IGBT 1 against breakdown. Other exemplary conventional insulated gate semiconductor devices having short-circuit current cutoff functions are disclosed in JP-A-63-318781 (1988), JP-A-64-68005 (1989) and JP-A-2-309714 (1990). In each of the former two devices, a second MOSFET is provided in parallel with a first MOSFET for controlling a main current to shunt the main current to the second MOSFET, while a transistor which enters an ON state when the shunt current exceeds a prescribed value is connected between gate and source electrodes of the first and second MOSFETs. When the main current flows in an amount exceeding the prescribed value due to shorting of a load etc., therefore, the transistor is turned on to reduce gate voltages of these MOSFETs, thereby limiting the main current below the prescribed value. In the last one of the aforementioned conventional devices, a thyristor is provided in place of the transistor provided in each of the former techniques. When a current which is shunted to a second MOSFET temporarily exceeds a prescribed value and a voltage exceeding a prescribed value is applied across a gate and a cathode of the thyristor, the thyristor so continuously conducts that gate voltages of two MOSFETs are reduced to values close to zero, whereby a main current is continuously cut off. However, these conventional devices have the following problems: It is particularly necessary to quickly cut off the short-circuit current when a high voltage is supplied to the load, and hence the gate driving circuit 42 and the compare circuit 44 must be driven at high speeds in the conventional device shown in Fig. 15. When speeds of these circuits are increased, however, the circuits are liable to cause malfunctions by electrical noises and hence no stable operations can be obtained. Further, circuit loss is increased following such speed increase. In such a prior art system which is limiting the gate voltages of the MOSFETs with a transistor, it is difficult to sufficiently reduce the gate voltages to values close to zero with the transistor, and hence the short-circuit current cannot be sufficiently cut off in shorting of the load. In the prior art device employing a thyristor, on the other hand, the speed of response of the thyristor is so slow as compared with the transistor that a longer time is required for conduction of the thyristor after detection of an excessive main current as compared with the transistor. Therefore, an excessive short-circuit current flows in a constant period when the load is shorted, to break the MOSFETs during this period. Further, an excessive surge voltage is generated by inductance of the load since the current is cut off after a flow of such an excessive short-circuit current, also leading to breakdown of the MOSFETs. In the prior art devices, further, the value of an ON-state voltage of the transistor, i.e., the value of a voltage signal which is supplied to the transistor for turning on the transistor, is varied with the temperature of the transistor. Thus, the value of the limited main current is disadvantageously fluctuated depending on the temperature. Further on, the transistor is erroneously turned on by an electrical noise which is superposed on the aforementioned voltage signal. An influence by such an electrical noise is increased as the speed of a switching operation for the MOSFETs is increased. As can be seen from the a.m. discussion, the present invention starts out from an insulated gate semiconductor device which comprises two insulated gate elements. These two insulated gate elements are arranged in parallel, the first one being a "power device" which conducts the main current, whereas the second one is an "auxiliary" element which is used for sensing the current flowing through the semiconductor device by means of a shunt resistor. The gate electrodes of these two insulated gate elements are directly coupled to each other so that the control voltage applied thereto controls the switching behaviour of both elements in the same way. In contrast thereto, US-A-4 924 344 discloses a circuit having two insulated gate elements which, although being coupled basically in parallel, comprise a resistor between the two gate electrodes. Due to this additional resistor the operation of the auxiliary transistor is somewhat different from that of the main transistor, i.e. the MOSFET transistor is turned on only in the case of an overcurrent, whereas the auxiliary transistor of the present invention is turned on whenever the main transistor is turned on. It follows that the basic switching behaviour of this known circuit cannot be compared to that of the present invention. It is therefore the object of the present invention to improve an insulated gate semiconductor device according to the preamble of claim 1 in such a way that an excessive main current can be limited at a high speed and cut off to a level close zero. This object, according to the present invention, is solved by the advantageous measures indicated in the characterizing part of claim 1, 8 and 19, respectively. Due to these features it is possible to limit an excessive main current at a high speed and to cut same off to a level close zero. A further advantage of the invention is that the possibility of a malfunction due to an electrical noise can be remarkably reduced; furtheron, there are no fluctuations in the device characteristics caused by the temperature. Further advantageous developments of the invention are the subject matter of the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram showing a semiconductor device according to a first embodiment of the present invention; Fig. 2 is a graph showing results of observation in relation to the semiconductor device according to the first embodiment; Fig. 3 is a graph showing results of observation in relation to a circuit to be compared with the semiconductor device according to the first embodiment; Fig. 4 is a circuit diagram showing the structure of a semiconductor device according to a second embodiment of the present invention; Fig. 5 is a circuit diagram showing the structure of a semiconductor device according to a third embodiment of the present invention; Fig. 6 is a circuit diagram showing the structure of a semiconductor device according to a fourth embodiment of the present invention; Fig. 7 is a circuit diagram showing the structure of a semiconductor device according to a sixth embodiment of the present invention; Fig. 8 is a circuit diagram showing the structure of a semiconductor device according to an eighth embodiment of the present invention; Fig. 9 is a circuit diagram showing the structure of a semiconductor device according to a ninth embodiment of the present invention; Fig. 10 is a circuit diagram showing the structure of a semiconductor device according to a tenth embodiment of the present invention; Fig. 11 is a circuit diagram showing the structure of a semiconductor device according to an eleventh embodiment of the present invention; Fig. 12 is a circuit diagram showing the structure of a semiconductor device according to a twelfth embodiment of the present invention; Fig. 13 is a circuit diagram showing the structure of a semiconductor device according to a thirteenth embodiment of the present invention; Fig. 14 is a circuit diagram showing the structure of a semiconductor device according to a fourteenth embodiment of the present invention; and Fig. 15 is a block diagram showing the structure of a conventional insulated gate semiconductor device. DESCRIPTION OF THE PREFERRED EMBODIMENTS <1. Embodiment 1> <1-1. Structure of Device> Fig. 1 is a circuit diagram showing a first embodiment of the present invention. A load (not shown) is connected to a collector C (first current electrode) of an IGBT 1 (first insulated gate element), so that a collector current I C mainly flowing from the collector C to an emitter E (second current electrode) is supplied to the load as a main current. This collector current I C is controlled by the value of a gate voltage across a gate G (control electrode) and the emitter E. A higher collector current I C flows as the gate voltage is increased. This gate voltage is regulated and supplied by a gate driving circuit 9 (gate driving means). A resistor 4 is interposed between an output terminal G O of the gate driving circuit 9 and the gate G. A power source 10 which is connected to the gate driving circuit 9 supplies a source voltage to the gate driving circuit 9. Another IGBT 2 (second insulated gate element) having lower current capacity than the IGBT 1 is provided in parallel with the IGBT 1. The collectors C and the gates G of the IGBTs 1 and 2 are connected with each other. A part of the main current which is supplied to the load is shunted to the IGBT 2 as a collector current I C for the IGBT 2. A resistor 3 is connected between the emitters E of the IGBTs 2 and 1. The current which is shunted to the IGBT 2 passes through the resistor 3. Therefore, a voltage which is proportionate to the shunted current is developed across the resistor 3. Therefore, a higher voltage is developed across the resistor 3 as the main current is increased. A transistor 5 and a thyristor 7 are interposed between the gate G and the emitter E of the IGBT 1. A collector C of the transistor 5 and an anode of the thyristor 7 are connected to the gate G, while a base B of the transistor 5 is connected between the emitter E of the IGBT 2 and the resistor 3. An emitter E of the transistor 5 is connected to the emitter E of the IGBT 1 through a Schottky-barrier diode 6 (first diode). The Schottky-barrier diode 6 is connected in such a direction that a current flowing from the collector C to the emitter E of the transistor 5 forms a forward current. A cathode of the thyristor 7 is connected to the emitter E of the IGBT 1, while its gate is connected between the emitter E of the IGBT 2 and the resistor 3 through a resistor 8. <1-2. Operation of Device> While the main current is at a low value within a range of a normal operation, the voltage developed across the resistor 3 is so sufficiently low that a voltage (V BE ) across the base B and the emitter E of the transistor 5 is insufficient for bringing the transistor 5 into an ON (conducting) state and that across the gate and the cathode of the thyristor 7 is also insufficient for bringing the thyristor 7 into an ON state. Therefore, both of the transistor 5 and the thyristor 7 are in OFF (cutoff) states. At this time, the potential of the gate G coincides with that outputted from the gate driving circuit 9. In other words, the IGBTs 1 and 2 operate in response to the output potential of the gate driving circuit 9. When the main current is increased beyond the range of a normal operation by shorting of the load or the like, on the other hand, the voltage across the resistor 3 is increased in response. As the result, the base-to-emitter voltage V BE of the transistor 5 is so increased as to turn on the transistor 5, while the gate-to-cathode voltage of thyristor 7 is also so increased as to turn on the thyristor 7. Thus, the transistor 5 immediately enters an ON state, to reduce the potential of the gate G to some extent. Then the thyristor 7 enters an ON state in a short delay to reduce the potential of the gate G to a level substantially equal to that of the emitter E of the IGBT 1, whereby the IGBTs 1 and 2 enter cutoff states so that the main current finally reaches zero. Thus, the IGBT 1 is protected against breakdown. The resistance value of the resistor 3 is properly selected in correspondence to setting of an upper limit of the normal operation range of the main current. The Schottky-barrier diode 6 is provided in order to prevent the semiconductor device from oscillation. Since this Schottky-barrier diode 6 has a shorter back recovery time than that in junction between the base B and the emitter E of the transistor 5, it is possible to prevent the circuit from oscillation by providing this Schottky-barrier diode 6 as shown in Fig. 1. <1-3. Observation Data> Fig. 2 is a graph showing observation results in relation to the insulated gate semiconductor device according to this embodiment. Referring to Fig. 2, the axis of ordinates corresponds to a voltage V GO across the output terminal G O of the gate driving circuit 9 and the emitter E of the IGBT 1, and the main current (substantially corresponding to the collector current I C for the IGBT 1), while the axis of abscissas corresponds to time. A rated value of the voltage V CE across the collector C and the emitter E of the IGBT 1 is 600 V in this semiconductor device, and a half voltage of 300 V is applied in the observation. When the output terminal voltage V GO is abruptly raised from zero to about 10 V, which is a sufficient value for allowing conduction of the IGBT 1, the collector current I C is first increased in response thereto. However, the collector current I C is not unlimitedly increased but restricted to a value around 100 A since the transistor 5 is turned on. Thereafter the thyristor 7 is turned on in a slight delay, whereby the voltage V G across the gate G and the emitter E of the IGBT 1 is reduced substantially to zero. The output terminal voltage V GO is reduced to 8 V at around 9 µsec. for this reason. The width of reduction of the output terminal voltage V GO reflects a ratio of the resistor 4 to output resistance of the gate driving circuit 9. The IGBTs 1 and 2 are cut off when the thyristor 7 is turned on, whereby the main current reaches zero as understood from Fig. 2. When the time reaches 14 µsec., the output terminal voltage V GO is returned to zero to complete observation. Fig. 3 is a graph showing results of observation similar to that in Fig. 2, in relation to a circuit having such a structure that the thyristor 7 is removed from the semiconductor device according to this embodiment. In this case, the main current is limited to a value of about 100 A after rise of the output terminal voltage V GO by action of the transistor 5, similarly to the result shown in Fig. 2. However, the main current maintains a value of about 100 A up to a time of 7 µsec. when the output terminal voltage V GO is returned to zero. Namely, the main current continuously maintains a considerably high value even if abnormality is caused in the load at such a degree that the main current is increased beyond a range of a normal operation. In this circuit structure, therefore, the IGBT 1 may be broken in abnormality such as shorting of the load. In another circuit (not shown) having such a structure that the thyristor 7 is left but the transistor 5 is removed from the semiconductor device according to this embodiment shown in Fig. 1, the thyristor 7 is turned on to reduce the main current to zero after a lapse of a constant time from rise of the output terminal voltage V GO , as readily predicted from the observation results shown in Fig. 2. However, the main current runs away to a value extremely higher than that of about 100 A shown in Fig. 2, since no mechanism is provided for limiting the main current in a period before the turn-on time of the thyristor 7. The IGBT 1 may be broken such an abnormally high runaway main current. After a lapse of the constant time, the main current is abruptly reduced to zero from the abnormally high runaway current level since the thyristor 7 is turned on. Consequently, a high surge voltage is developed across the collectors and the emitters of the IGBTs 1 and 2 by inductance provided in the load or that parasitically generated in a line of the load. This surge voltage further breaks the IGBTs 1 and 2. However, the semiconductor device of this embodiment shown in Fig. 1 comprises both of the transistor 5 and the thyristor 7, whereby increase of the main current is quickly suppressed to a certain limit when the main current is abnormally increased beyond the normal operation range in shorting of the load or the like. Further, the main current is reduced to zero after the constant period, whereby the IGBT 1 is prevented from breakdown caused by an excessive collector current I C . Since the main current is inhibited from unlimited increase, it is possible to suppress a surge voltage which is developed when the thyristor 7 is turned on. Thus, the IGBT 1 is prevented from breakdown which is caused by application of an excessively high collector-to-emitter voltage V CE across the IGBTs 1 and 2. <2. Embodiment 2> Fig. 4 is a circuit diagram showing the structure of a semiconductor device according to a second embodiment of the present invention. According to this embodiment, a Zener diode 13 is connected to an emitter E of the transistor 5 in series with a Schottky-barrier diode 6. The Zener diode 13 is set in such a direction that a current flowing from a collector C to the emitter E of the transistor 5 forms a reverse current. A base-to-emitter voltage V BE of the transistor 5 is varied with a temperature change. Therefore, the voltage across the resistor 3 for turning on the transistor 5 is fluctuated in the semiconductor device according to the embodiment 1, whereby the value of the main current for turning on the transistor 5 is fluctuated. A reverse voltage (Zener voltage) of the Zener diode 13 is increased following a temperature increase, contrarily to the base-to-emitter voltage V BE . When a proper Zener diode 13 is selected and set as shown in Fig. 4, therefore, it is possible to maintain the voltage across the resistor 3 for turning on the transistor 5 at a constant value with no dependence on a temperature change. <3. Embodiment 3> Fig. 5 is a circuit diagram showing the structure of a semiconductor device according to a third embodiment of the present invention. This embodiment is so structured that a reverse voltage is applied across a gate G and an emitter E of an IGBT 1, in order to increase a speed of response for turning off the IGBT 1. Thus, a reverse bias power source 11 is connected to a gate driving circuit 9, in addition to a power source 10. Further, a diode 12 (second diode) is so provided that a potential at the emitter E of the IGBT 1 will not increase that of the gate G through a resistor 3 and a transistor 5, to prevent a gate-to-emitter voltage V G from reaching a value around zero. The diode 12 is interposed between the gate G and the collector C of the transistor 5 and an anode of a thyristor 7, to prevent a reverse current which flows from the emitter E of the IGBT 1 toward the gate G through the resistor 3 and the transistor 5. <4. Embodiment 4> Fig. 6 is a circuit diagram showing the structure of a semiconductor device according to a fourth embodiment of the present invention. According to this embodiment, a light emitting diode 14 (information means) is interposed between emitters E of a transistor 5 and an IGBT 1. The light emitting diode 14 is set in such a direction that a current flowing from a collector C to the emitter E of the transistor 5 forms a forward current of the light emitting diode 14. The light emitting diode 14 emits light when the transistor 5 is turned on. Thus, it is possible to recognize an operating state of the transistor 5 caused by abnormal increase of a main current resulting from shorting of a load or the like by such light emission. In other words, it is possible to readily recognize upon stopping of the main current whether the same is normally stopped or stopped by occurrence of abnormality. <5. Embodiment 5> In each of the aforementioned embodiments, the transistor 5, the thyristor 7 and the resistor 3 may be formed in a single semiconductor chip. Further, the resistor 8, the Schottky-barrier diode 6, the Zener diode 13 and the like may additionally formed in the single semiconductor chip. <6. Embodiment 6> <6-1. Structure of Device> Fig. 7 is a circuit diagram showing the structure of a semiconductor device according to a sixth embodiment of the present invention. A load (not shown) is connected to a collector C (first current electrode) of an IGBT 101 (first insulated gate element), so that a collector current I C mainly flowing from the collector C to an emitter E (second current electrode) is supplied to the load as a main current. This collector current I C is controlled by a value of a gate-to-emitter voltage V G , which is a voltage across a gate G (control electrode) and the emitter E. A higher collector current I C flows as the gate-to-emitter voltage V G is increased. The gate-to-emitter voltage V G is regulated and supplied by a gate driving circuit 210 (gate driving means). A resistor 109 is interposed between an output terminal G O of the gate driving circuit 210 and the gate G. A power source 103 which is connected to the gate driving circuit 210 supplies a source voltage to the gate driving circuit 210. Another IGBT 2 (second insulated gate element) having lower current capacity than the IGBT 101 is provided in parallel with the IGBT 101. The collectors C and the gates G of the IGBTs 1 and 2 are connected with each other. A small part of the main current which is supplied to the load is shunted to the IGBT 102 as a collector current I C for the IGBT 102. A resistor 104 is connected between the emitters E of the IGBTs 102 and 101. The current which is shunted to the IGBT 102 passes through the resistor 104. Therefore, a voltage V R which is proportionate to the shunted current is developed across the resistor 104. Thus, a higher voltage V R is developed across the resistor 104 as the main current is increased. A series circuit of a MOS field effect transistor element (hereinafter referred to as MOSFET) 105 and a Zener diode 106 is interposed in parallel between the gate G and the emitter E of the IGBT 101. A drain D of the MOSFET 105 is connected to the gate G through a resistor 110. This drain D is also connected to an end of the resistor 109. A cathode of the Zener diode 106 is connected to a source S of the MOSFET 105. An anode of the Zener diode 106 is connected to the emitter E of the IGBT 101. Namely, the Zener diode 106 is set in such a direction that a current flowing from the drain D to the source S of the MOSFET 105 forms a reverse current of the Zener diode 106. A gate G1 of the MOSFET 105 is coupled to the emitter E of the IGBT 102 through resistors 111 and 112 and a diode 113. The resistors 111 and 112 are connected in series with each other, while the diode 113 is connected in parallel with the resistor 112. The diode 113 is interposed in such a direction that its anode is connected to the emitter E of the IGBT 102. The IGBT 101 is connected in parallel with a freewheel diode 301 for preventing breakdown caused by a reverse voltage which is developed across the collector C and the emitter E when this element is converted from a conducting state to a cutoff state. Similarly, the MOSFET 105 is connected in parallel with a freewheel diode 302 which is adapted to protect the MOSFET 105. <6-2. Schematic Operation of Device> The MOSFET 105 enters an OFF state when a gate-to-source voltage V G1 across the gate G1 and the source S is lower than a gate threshold voltage (a gate-source threshold voltage) V GS(th) which is specific to the MOSFET 105, while the same enters an ON state when the former is higher than the latter. Therefore, the MOSFET 105 enters an OFF state when a voltage V R developed across the resistor 104 is lower than the sum of a Zener voltage Vz which is specific to the Zener diode 106 and the gate threshold voltage V GS(th) of the MOSFET 105 while the same enters an ON state in other case. While the main current is at a low value within a range of a normal operation, the voltage V R is sufficiently low as compared with the sum of the Zener voltage Vz and the gate threshold voltage V GS(th) . Therefore, the MOSFET 105 is in an OFF state. At this time, the potential of the gate G coincides with a potential which is outputted from the gate driving circuit 210. In other words, the IGBTs 101 and 102 operate in response to the output potential of the gate driving circuit 210. When the main current is increased beyond the range of the normal operation by shorting of the load or the like, on the other hand, the voltage V R is also increased in response to the same. When the main current is so increased that the voltage V R exceeds the sum of the Zener voltage Vz and the gate threshold voltage V GS(th) , the MOSFET 105 enters an ON state. Consequently, the potential of the gate G is reduced. Thus, the IGBTs 101 and 102 enter states close to cutoff states, whereby increase of the main current is prevented. Namely, this semiconductor device prevents the main current from being increased beyond an upper limit corresponding to coincidence of the voltage V R to the sum of the Zener voltage Vz and the gate threshold voltage V GS(th) . As the result, the IGBT 101 is prevented from breakdown caused by an overcurrent. The resistance value of the resistor 104 is properly selected in correspondence to the upper limit of the main current which is set within the range of the normal operation. <6-3. Characteristic Operation of Device> The resistor 104 may be replaced by another current detection circuit which detects the collector current I C of the IGBT 102 and outputs a voltage corresponding to the collector current I C . According to this embodiment forming a current detection circuit by the resistor 104, however, it is possible to most simply form a semiconductor device at the lowest cost. The resistor 104 converts the collector current I C to the voltage V R at a high speed. Further, it is possible to readily set accuracy of conversion from the collector current I C to the voltage V R at a high level by selecting a resistor having high accuracy for the resistor 104. Namely, this embodiment is excellent in accuracy of the current detection circuit and high speed responsibility, with a simple structure. According to this semiconductor device, the Zener diode 106 is provided in series to the MOSFET 105. In this semiconductor device, therefore, the voltage V R which is required for turning on the MOSFET 105 is higher by the Zener voltage Vz as compared with a conventional semiconductor device having no Zener diode 106. Thus, it is possible to suppress such a malfunction that the MOSFET 105 is erroneously turned on by an electrical noise superposed on the voltage signal which is inputted in the gate G1. Namely, this semiconductor device has a higher noise margin as compared with the conventional device. According to this embodiment, the resistors 112 and 111 are interposed in a line for transmitting the voltage V R to the gate G1. Thus, the semiconductor device is prevented from oscillation. According to this embodiment, further, the diode 113 is provided in parallel with the resistor 112. This diode 113 is set in such a direction that the voltage V R is transmitted to the gate G1 along the forward direction of the diode 113. Therefore, the voltage V R is transmitted to the gate G1 in a short time when the resistor 104 transmits the voltage V R in order to convert the operation of the MOSFET 105 from an OFF state to an ON state. Namely, the diode 113 has a function of accelerating the conversion of the MOSFET 105 from an OFF state to an ON state. Thus, it is possible to reduce a delay time required for restricting the main current to a value below the upper limit value after shorting of the load. <Embodiment 7> In the semiconductor device according to the embodiment 6, the gate threshold voltage V GS(th) of the MOSFET 105 is varied with a temperature change. This means that the upper limit value of the main current can be varied with the temperature. On the other hand, it is possible to obtain a commercially available Zener diode whose Zener voltage Vz has various temperature characteristics. Therefore, it is possible to select such a Zener diode that temperature dependence of the Zener voltage Vz and that of the gate threshold voltage V GS(th) compensate for each other for applying the same to the Zener diode 106. In the semiconductor device having such a Zener diode 106, the value of the voltage V R required for turning on the MOSFET 105 is constant with no dependence on the temperature. Namely, the upper limit of the main current is constant with no dependence on the temperature according to this semiconductor device. <8. Embodiment 8> Fig. 8 is a circuit diagram showing the structure of a semiconductor device according to an eighth embodiment of the present invention. This embodiment is so structured that a reverse voltage is applied across a gate G and an emitter E of an IGBT 101 for converting the IGBT 101 into an OFF state. Thus, a reverse bias power source 107 is connected to a gate driving circuit 210, in addition to a power source 103. Therefore, a speed of response for converting the IGBT 101 from an ON state into an OFF state is higher as compared with the device according to the embodiment 6 or 7. Further, it is possible to sufficiently stably implement an OFF state. This embodiment is so structured that a reverse voltage is applied across the gate G and the emitter E, whereby a potential of the emitter E of the IGBT 101 may increase that of the gate G through a Zener diode 106 and a MOSFET 105, to reduce a gate-to-emitter voltage V G to a value close to zero. A diode 108 (first diode) is provided in order to prevent this. This diode 108 is interposed between the gate G and a drain D of the MOSFET 105. Further, the diode 108 is interposed in such a direction that a current flowing in the MOSFET 105 forms a forward current of the diode 108. The diode 108 blocks a current flowing from the emitter E of the IGBT 101 toward the gate G through the Zener diode 106 and the MOSFET 105, i.e., a reverse current of the MOSFET 105. Thus, a negative potential which is outputted by the gate driving circuit 210 is correctly transmitted to the gate G. The diode 108 also serves to prevent the MOSFET 105 from breakdown caused by the reverse current. The diode 108 will not inhibit the current from forwardly flowing in the MOSFET 105 upon conduction of the MOSFET 105. Therefore, the diode 108 will not inhibit the function of the semiconductor device for preventing an excessive main current. <9. Embodiment 9> Fig. 9 is a circuit diagram showing the structure of a semiconductor device according to a ninth embodiment of the present invention. According to this embodiment, the diode 108 in the semiconductor device according to the embodiment 8 is replaced by a light emitting diode 303. When a MOSFET 105 is turned on, a current flowing in this MOSFET 105 also flows in the light emitting diode 303, whereby the light emitting diode 303 emits light. Therefore, it is possible to recognize that a function for limiting a main current acts due to abnormal increase of the main current caused by shorting of a load or the like by light emission of the light emitting diode 303. Namely, it is possible to readily recognize whether the semiconductor device is in a normally operating state or an abnormally operating state. The light emitting diode 303 also serves the function of the diode 108 in the embodiment 8. Namely, the semiconductor device according to this embodiment implements both of a function of preventing a reverse current of the MOSFET 105 and that of informing abnormality in an operating state of the device, by employing the light emitting diode 303. <10. Embodiment 10> Fig. 10 is a circuit diagram showing the structure of a semiconductor device according to a tenth embodiment of the present invention. According to this embodiment, the MOSFET 105 in the semiconductor device according to the embodiment 9 is formed by two MOSFETs 105a and 105b which are connected in parallel with each other. The MOSFETs 105a and 105b are independently connected with freewheel diodes 302a and 302b respectively, in place of the freewheel diode 302. Further, resistors 111a and 111b are connected to respective gates of the MOSFETs 105a and 105b, in place of the resistor 111. Functions of the freewheel diodes 302a and 302b are similar to that of the freewheel diode 302, while those of the resistors 111a and 111b are similar to that of the resistor 111. The semiconductor device according to this embodiment is provided with the two MOSFETs 105a and 105b in parallel with each other, whereby ON resistance in ON states of these elements is lower and current capacity is larger as compared with the semiconductor device provided with only one MOSFET 105. In an insulated gate semiconductor device which is applied to a switching element such as an inverter, output resistance of the gate driving circuit 210 and the resistance value of the resistor 109 are set at low values since it is necessary to turn the IGBT 101 on and off at a high speed. Particularly in a large insulated gate semiconductor device for supplying a high main current, these resistance values are set at lower levels. In a large insulated gate semiconductor device which switches at a high speed, therefore, the ON resistance of the MOSFET 105 must be sufficiently low in relation to the resistance value of the resistor 109 and the like. If the ON resistance is insufficiently low as compared with the resistance value of the resistor 109 and the like, it is impossible to sufficiently reduce the gate-to-emitter voltage V G to limit the main current below a prescribed upper limit value when the MOSFET 105 is turned on. In the large insulated gate semiconductor device, further, current capacity of the MOSFET 105 must be set at a high level since a high current flows in the MOSFET 105 upon conduction thereof. The semiconductor device according to this embodiment satisfies such requirements by forming the MOSFET 105 by the two MOSFETs 105a and 105b which are connected in parallel with each other. The number of the MOSFETs 105a and 105b is not restricted to two, but the MOSFET 105 can be formed by two or more MOSFETs which are connected in parallel with each other at need. When the same MOSFETs are employed, ON resistance of the MOSFET 105 is lowered and the current capacity is increased as the number of the MOSFETs which are connected in parallel with each other is increased, as a matter of course. <11. Embodiment 11> Fig. 11 is a circuit diagram showing the structure of a semiconductor device according to an eleventh embodiment of the present invention. According to this embodiment, a light emitting diode 304 is connected in series to the MOSFET 105 in the semiconductor device 105 according to the embodiment 6. The light emitting diode 304 is connected in such a direction that a current forwardly flowing in the MOSFET 105, i.e., that flowing from the drain D toward the source S of the MOSFET 105, forms a forward current of the light emitting diode 304. Upon conduction of the MOSFET 105, the current simultaneously flows also to the light emitting diode 304. Namely, the light emitting diode 304 emits light upon conduction of the MOSFET 105. Therefore, it is possible to recognize that a function for limiting the main current serves due to abnormal increase of the main current resulting from shorting of a load or the like by light emission of the light emitting diode 304. Namely, it is possible to readily recognize whether the semiconductor device is in a normally operating state or an abnormally operating state by a simple structure. <12. Embodiment 12> In each of the aforementioned embodiments, a circuit part including the MOSFET 105 and the Zener diode 106 can be integrated on a single semiconductor chip. Since a part of the circuit is integrated, the semiconductor device can be readily assembled. Since the MOSFET 105 and the Zener diode 106 are integrated on a single semiconductor chip, further, it is possible to form the device with excellent reproducibility so that temperature characteristics of these elements are complementary to each other. Further, the temperatures of these elements are further homogenized since the same are formed on the same semiconductor substrate. Thus, it is possible to maintain the voltage V R , which is required for turning on the MOSFET 105, in a further unchangeable state with respect to a temperature change. Fig. 12 is a circuit diagram showing a device which is obtained by integrating a part of the device shown in Fig. 7. According to this device, a circuit part 401 including a freewheel diode 302, resistors 111 and 112 and a diode 113 is integrated. On the other hand, resistors 104, 109 and 110 are not integrated but set around the integrated circuit part 401. The respective circuit components included in the integrated circuit part 401 are capable of relatively widely coping with various rated values of the semiconductor device. On the other hand, it is necessary to select resistance values, heat resistance etc. of the resistors 104, 109 and 110, which are not integrated, in response to the rated value of a main current to be controlled, for example. According to this embodiment, these resistors are independently arranged in the exterior of the integrated circuit part 401 while the circuit components relatively non-dependent on the rated values of the device are integrated, whereby the integrated circuit part 401 can be applied to semiconductor devices of various rated values in common. Namely, the device according to this embodiment can advantageously reduce a manufacturing cost. <13. Embodiment 13> Fig. 13 is a circuit diagram showing a device which is obtained by integrating a part of the device shown in Fig. 10. According to this device, MOSFETs 105a and 105b, a Zener diode 106, resistors 111a, 111b and 112 and a diode 113 are integrated to form an integrated circuit part 402. On the other hand, resistors 104, 109 and 110 are not integrated but set around the integrated circuit part 402. Also in the device according to this embodiment, the circuit components dependent on the rated values of the device are independently arranged in the exterior of the integrated circuit part 402 while the circuit components relatively non-dependent on the rated values of the device are integrated, whereby the manufacturing cost can be advantageously reduced. <14. Embodiment 14> <14-1. Structure of Device> Fig. 14 is a circuit diagram showing the structure of a semiconductor device according to a fourteenth embodiment of the present invention. This embodiment employs a current mirror circuit having two junction field effect transistors (JFETs). A series circuit of a JFET 114 (first junction field effect transistor element) and a resistor 116 (first resistor) is interposed between emitters E of IGBTs 102 and 101. A drain D of the JFET 114 is connected to the emitter E of the IGBT 102, while its source S is connected to an end of the resistor 116. Another end of the resistor 116 is connected to the emitter E of the IGBT 101. A current I1 flowing through the JFET 114 and the resistor 116 coincides with a collector current I C of the IGBT 102. A gate G of the JFET 114 is shorted with the drain D. On the other hand, another JFET 115 (second junction field effect transistor element) forms a series circuit of a light emitting diode 303 and a resistor 117 (second resistor). An anode of the light emitting diode 303 is coupled to gates G of the IGBTs 101 and 102 through a resistor 110. A cathode of the light emitting diode 303 is connected to a drain D of the JFET 115. A source S of the JFET 115 is connected to an end of a resistor 117. Another end of the resistor 117 is connected to the emitter E of the IGBT 101. Gates G of the JFETs 115 and 114 are connected with each other. According to this embodiment, a reverse bias power source 107 is connected to a gate driving circuit 210 in addition to a power source 103, similarly to the embodiment 9. <14-2. Characteristic Operation of Device> As hereinabove described, the JFETs 114 and 115 form a current mirror circuit. Further, the resistors 116 and 117 apply negative feedbacks to these JFETs 114 and 115. Therefore, the value of a current I2 flowing in the JFET 115 is mostly determined by a ratio of the resistor 116 to the resistor 117 and the current I1, with no remarkable dependence on the characteristics of the JFETs 114 and 115. Namely, the value of the current I2 regularly coincides with a product of the ratio of the resistor 116 to the resistor 117 and the current I1. Therefore, a current which is proportionate to a main current which is supplied to a load regularly flows to the JFET 115. High accuracy which is responsive to accuracy of resistance values of the two resistors 116 and 117 is guaranteed for the value of such a current I2. Further, fluctuation of the resistance values of the resistors 116 and 117 with a temperature change is so small that the relation between the main current and the current I2 is not much dependent on the temperature. In the semiconductor device according to this embodiment, therefore, a function of suppressing an overcurrent is implemented in high accuracy and this function is not much dependent on the temperature. Similarly to the semiconductor device according to the embodiment 9, further, a reverse bias power source 107 is connected to a gate driving circuit 210 in addition to a power source 103, whereby the IGBT 101 can be converted from an ON state to an OFF state at a high speed of response and the OFF state can be sufficiently stably implemented. Further, the light emitting diode 303 implements both of a function for preventing a reverse current of the JFET 115, and that for informing abnormality in an operating state of the device. <Other Modifications> (1) Although the IGBTs 101 and 102 are prepared from n-channel IGBTs in the aforementioned embodiments, it is possible to alternatively employ p-channel IGBTs according to the present invention. (2) The IGBTs 101 and 102 are employed as elements for controlling and detecting the main currents in the semiconductor devices according to the aforementioned embodiments. However, the present invention is not restricted to IGBTs but is also applicable to a semiconductor device generally employing an insulated gate element such as a MOS field effect transistor element, for example.

Claims: Insulated gate semiconductor device for switchably conducting a load current, comprising: [a] a first and second insulated gate element (1,2;101, 102), each having a first and second current electrode (C,E) and a control electrode (G), the conducting state between the two current electrodes (C, E) being controllable by the voltage applied between said control electrode (G) and the respective second current electrode (E), wherein the electrodes of said first element (1;101) are connected to corresponding electrodes of said second element (2;102); [b] a current detection means (3;104) being interposed between the second current electrodes (E) of said two elements (1,2; 101; 102) and having a sensing terminal which is connected to said second element (2; 102), developing a voltage which corresponds to said load current flowing in said semiconductor device; [c] a gate driving means (9,4;210,109) the output terminal of which is connected to both control electrodes (G) and applies a control voltage thereto; characterized by [d] a transistor element (5), the collector electrode (C) being connected to said control electrodes (G), the emitter electrode (E) being connected to said second current electrodes (E) and the base electrode (B) being connected to said sensing terminal; and [e] a thyristor element (7), the anode electrode being connected to said control electrodes (G), the cathode electrode being connected to said second current electrodes (E) and the gate electrode being connected to said sensing terminal, said thyristor element (7) conducting current upon temporary increase of the voltage applied to the gate electrode in excess of a prescribed value. Insulated gate semiconductor device according to claim 1, characterized by [f] a diode (6) being connected between said emitter electrode of said transistor element (5) and said second current electrode (E) in such a direction that a current flowing from said second collector electrode (E) to said emitter electrode forms a forward current, said diode (6) having a shorter back recovery time than that between said emitter and base electrodes of said transistor element (5). Insulated gate semiconductor device according to claim 1 or 2, characterized by a resistor (8) which is connected between said gate electrode of said thyristor element (7) and the second current electrode (E) of said second insulated gate element (2;102). Insulated gate semiconductor device according to one of claims 1 through 3, characterized by a resistor (4) which is connected between said output terminal of said gate driving means (9,4;210,109) and said control electrodes (G). Insulated gate semiconductor device according to one of claims 1 through 4, characterized by a Zener diode (13) which is connected between said emitter electrode of said transistor element (5) and said second current electrode (E) in such a direction that a current flowing from said second collector electrode (E) to said emitter electrode forms a reverse current. Insulated gate semiconductor device according to one of claims 1 through 5, characterized in that said gate driving means comprises a gate driving part (9) capable of outputting a potential being lower than that of said second current electrode (E) as said control voltage, wherein a diode (12) is connected between the control electrodes (G) of said insulated gate elements (1,2;101,102) and the collector electrode (C) of said transistor element (5) in a direction which is inhibiting a current flow from said collector electrode toward said control electrodes. Insulated gate semiconductor device according to one of claims 1 through 6, characterized by an information means (14), preferably in the form of a light emitting diode, which is connected to said transistor element (5) in such a way that the conducting state thereof is visually perceptible. Insulated gate semiconductor device according to the preamble of claim 1, characterized by [d] at least one MOS field effect transistor (105; 105a, 105b) having two current electrodes (D, S), the first being connected to said control electrodes (G) and the second being connected to said second current electrodes (E) of said insulated gate elements (101, 102), and a gate control electrode (G1) being connected to said sensing terminal; and [e] a Zener diode (106) being interposed between the second current electrode (S) of said field effect transistor (105;105a,105b) and the second current electrodes (E) of said insulated gate elements (101,102) in such a direction that a current flowing through said field effect transistor (302; 302a,302b) in a forward direction forms a reverse current. Insulated gate semiconductor device according to claim 8, characterized in that a Zener voltage in said Zener (106) diode has complementary temperature characteristics with respect to channel temperature characteristics of a gate-source threshold voltage in said MOS field effect transistor (105; 105a, 105b). Insulated gate semiconductor device according to claim 8 or 9, characterized in that said MOS field effect transistor is formed by a plurality of unit MOS field effect transistor elments (105a, 105b) which are connected in parallel. Insulated gate semiconductor device according to one of claims 8 through 10, characterized by a light emitting diode (303) being coupled to said MOS field effect transistor (105; 105a, 105b) in such a direction that a current passing through said MOS field effect transistor (105; 105a, 105b) in a forward direction forms a forward current. Insulated gate semiconductor device according to one of claims 8 through 11, characterized in that said gate driving means comprises a gate driving part (9) capable of outputting a potential being lower than that of said second current electrode (E) as said control voltage, wherein a diode (108;303) is connected between the control electrodes (G) of said insulated gate elements (1,2;101,102) and the corresponding electrode (D) of said MOS field effect transistor (105; 105a, 105b) in such a direction that a current passing through said MOS field effect transistor (105; 105a, 105b) in a forward direction forms a forward current. Insulated gate semiconductor device according to claim 12, characterized in that said diode is a light emitting diode (303). Insulated gate semiconductor device according to one of claims 8 through 12, characterized by a resistor (112) which is disposed between said gate control electrode (G1) and said sensing terminal. Insulated gate semiconductor device according to claim 14, characterized by a diode (113) being connected in parallel with said resistor (112) in a direction capable of transmitting a voltage - which is being outputted from said sensing terminal to shift said MOS field effect transistor (105;105a,105b) from a cutoff state to a conducting state - to said gate control electrode (G1) in a shorted state of said diode (113). Insulated gate semiconductor device according to claim 15, characterized in that at least said MOS field effect transistor (105; 105a, 105b), said resistor (112) and said diode (113) are integrated on a single semiconductor chip. Insulated gate semiconductor device according to one of claims 8 through 16, characterized by a resistor (109) which is connected between the output terminal of said gate driving means (9,4;210,109) and the control electrodes (G) of said insulated gate elements (101,102) and the corresponding current electrode (D) of said MOS field effect transistor (105; 105a, 105b). Insulated gate semiconductor device according to claim 17, characterized by a further resistor (110), the first end thereof being connected to said first mentioned resistor (109) and the corresponding current electrode (D) of said MOS field effect transistor (105; 105a, 105b) and the second end thereof being connected to the control electrodes (G) of said insulated gate elements (101,102). Insulated gate semiconductor device according to the preamble of claim 1, characterized by [d] a first junction field effect transistor (114) having two current electrodes (D, S), the first, together with its control electrode (G), being connected to the second current electrode (E) of said second insulated gate element (102) and the second being connected to said sensing terminal; and [e] a second junction field effect transistor (115) having two current electrodes (D,S), the first being connected to said control electrodes (G) and the second being connected to said second current electrodes (E) of said insulated gate elements (101, 102) via a resistor (117), and a gate control electrode (G1) being connected to the control electrode of said first junction field effect transistor (114). Insulated gate semiconductor device according to claim 19, characterized by a resistor (109) which is connected between the output terminal of said gate driving means (9, 4; 210, 109) and the control electrodes (G) of said insulated gate elements (101, 102) and the corresponding current electrode (D) of said second junction field effect transistor (115). Insulated gate semiconductor device according to claim 20, characterized by a further resistor (110), the first end thereof being connected to said first mentioned resistor (109) and the corresponding current electrode (D) of said second junction field effect transistor (115) and the second end thereof being connected to the control electrodes (G) of said insulated gate elements (101,102). Insulated gate semiconductor device according to one of claims 1 through 21, characterized in that said current detection means (3;104) is a resistor. Insulated gate semiconductor device according to one of claims 1 through 22, characterized in that said insulated gate elements (1,2;101,102) each comprise an insulated gate bipolar transistor element. Insulated gate semiconductor device according to claim 23, characterized in that the current capacity of the insulated gate bipolar transistor element of said second element (2;102) is smaller than that of the insulated gate bipolar transistor element of said first element (1;101).

Inventors: HATAE SHINJI, MAJUMDAR GOURAB, MARUMO TAKASHI, TABATA MITSUHARU, HATAE, SHINJI, MAJUMDAR, GOURAB, MARUMO, TAKASHI, TABATA, MITSUHARU, HATAE, SHINJI, C/O MITSUBISHI DENKI K.K., MAJUMDAR, GOURAB, C/O MITSUBISHI DENKI K.K., Marumo, Takashi, c/o Fukuryo Semiconduct.Eng.Corp., Tabata, Mitsuharu, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H02H7
IPC codes: H03K17