Abstract:
A drive circuit is provided for driving a voltage-driven semiconductor element by producing a drive signal depending upon an input signal. The drive circuit comprises an output stage and a current-suppressing circuit. The output stage includes two semiconductor elements connected in series. The voltage-driven semiconductor element is connected to a common connection point of the two semiconductor elements. The current-suppressing circuit controls one of the two semiconductor elements to provide an output current flowing through either one of the two semiconductor elements if a voltage applied to the voltage-driven semiconductor element reaches a limit level, which is in excess of a level for conducting the voltage-driven semiconductor element by a predetermined voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and incorporates herein by reference Japanese Patent Applications No. 2006-80774 filed on Mar. 23, 2006 and No. 2006-288289 filed on Oct. 24, 2006. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a drive circuit which outputs a drive signal to a voltage-driven semiconductor element depending upon an input signal. 
       BACKGROUND OF THE INVENTION 
       [0003]    JP 7-111446A discloses a conventional drive circuit. This drive circuit may be exemplified as shown in  FIG. 5  or  FIG. 6 . 
         [0004]    In a drive circuit for driving a large power MOSFET, as shown in  FIG. 5 , two N-channel MOSFETs  1  and  2 , the sources of which are grounded, have their gates connected in common to receive a drive control signal VIN from an external unit. The drain of the FET  1  is grounded via an emitter of an NPN transistor  3  and a resistor  4 , and is further connected to the base of an NPN transistor  5 . On the other hand, the drain of the FET  2  is connected to a power source VB via a resistor  6  and is further connected to the base of the transistor  3 . 
         [0005]    The collector of the transistor  3  is connected to the power source VB via a resistor  7  and is further connected to the base of a transistor  8 . The transistor  8  has its collector connected to the power source VB and has its emitter connected to the collector of the transistor  5  via a resistor  9 . The emitter of the transistor  8  serves as an output terminal  10  which is connected to the power source via a Zener diode  11  and is further connected to the gate of a P-channel power MOSFET  12 . The source of FET  12  is connected to the power source VB, and the drain thereof is grounded via a load  13 . The FET  12  thus drives the load at the high potential side. The load  13  may be, for example, a DC motor, a solenoid or a lamp. 
         [0006]    In this drive circuit  14 , if the drive control signal VIN is of a low level, the FETS  1  and  2  are turned off, and the transistors  3  and  5  are turned on. The transistor  8  is turned off by the transistor  3 , the gate of FET  12  assumes the low level, and the FET  12  is turned on to supply a current to the load  13 . If the drive control signal VIN assumes a high level, on the other hand, these elements are turned on and off in a reversed manner, and the FET  12  is turned off to interrupt the supply of current to the load  13 . 
         [0007]    In another drive circuit  16  for driving a N-channel power MOSFET  15  instead of driving the FET  12 , as shown in  FIG. 6 , a series circuit of the load  13  and a FET  15  is connected between the power source VB and the ground. The gate of FET  15  is connected to the collector side of the transistor  5  (low side drive). Further, the Zener diode  11  is connected between the gate of FET  15  and the ground. In the drive circuit  16 , the FET  15  is turned off if the drive control signal VIN is at the low level. It is turned on if the control signal VIN is at the high level. 
         [0008]    Here, the Zener diode  11  arranged in the output stage of the drive circuits  14 ,  16  is for preventing an excess of voltage from being applied to the gates of FETs  12 ,  15 . Therefore, the resistor  9 , too, is provided in the output stage to limit the current, i.e., to limit the current that flows through the Zener diode  11 . Therefore, as a switching frequency for driving the FETs  12 ,  15  increases, the voltage waveforms across the gate and the source of FETs  12 ,  15  become less sharp due to the resistor  9 , and the FETs cannot be driven at high speeds. 
         [0009]    An output current (collector current) I 5  in the transistor  5  and voltage Vout change as shown in  FIGS. 7 and 8 , respectively, when the resistance R 9  of the resistor  9  is 180 Ω in the drive circuit  14  shown in  FIG. 5 . The current I 5  in the transistor  5  and voltage Vout change as shown in  FIGS. 9 and 10 , respectively, when the resistance R 5  of the resistor  9  is 2 kΩ in the drive circuit  14  shown in  FIG. 5 . When R 5  is 180 Ω the current I 5  that flows through the transistor  5  is about 30 mA as shown in  FIG. 7 . When R 5  is 2 kΩ, the current can be provided to be about 4 mA as shown in  FIG. 9 . In this case, however, it will be learned that the breakdown of the gate voltage Vout of FET  12  becomes slower as shown in  FIG. 10  than in the case of  FIG. 8 . 
         [0010]    According to JP 7-111446A, further, a diode or a capacitor is provided between the gate of an IGBT (insulated gate bipolar transistor) and the power source of a drive device in a device for driving the IGBT in order to bypass the charging/discharging current of a feedback capacitor to the power source side when the IGBT is to be switched. 
         [0011]    This, however, is to prevent erroneous operation (oscillation) of the IGBT relying upon the bypass diode and to provided an increase in the current by clamping the gate voltage. Therefore, the problem cannot be solved even if the above technique is applied to the problem of suppressing the output current while accomplishing a high-speed switching. 
         [0012]    U.S. Pat. No. 5,552,746 (JP 8-293774A) also discloses another conventional drive circuit for outputting a drive signal to a voltage-driven semiconductor element. As shown in  FIG. 13 , this drive circuit is provided with an active voltage clamp circuit for protecting the gate of a power transistor  144  from an excessive electric stress The transistor  144  is connected to transistors  140 ,  142  and a load. 
         [0013]    The active voltage clamp circuit has a current mirror formed by Zener diodes  132 ,  134  and transistors  136 ,  138 . The diodes  132 ,  134  are connected to a transistor  130 , which is connected to a resistor  129 . This resistor is connected in parallel to Zener diodes  127 ,  128 . If a power source voltage VB is lower than a threshold voltage Vth of the following equation, 
         [0000]        Vth=Vth 132 +Vth 134 +VBE 138 
         [0000]    the active voltage clamp is in the passive state, and the circuit operation is not affected. Conversely, if the current flows through the diodes  132 ,  134  and the transistor  138 , and the voltage VB becomes greater than the Vth voltage, the active voltage clamp is activated. A current flowing through the transistor  138  mirror-operates through the transistor  136  connected to the gate of a transistor  125 . As the voltage VB becomes higher than the threshold voltages of diodes  132  and  134 , the current starts flowing through the diodes  132  and  134 , and mirror-operates through the transistor  136 , causing the gate of transistor  125  to assume the low level. 
         [0014]    Therefore, the gate potential of a transistor  144  is clamped to a voltage which is the sum of threshold voltages of diodes  132  and  134  and a voltage drop through the transistor  138 . The electric current is limited by a feedback loop through a current mirror formed by diodes  132 ,  134  and transistors  136 ,  138 . 
         [0015]    According to the above construction, the gate potential of the transistor  125  is determined by the power source voltage of an inverter  122  connected to an input inverter  120  and a voltage drop through a gate resistor  124 . Therefore, when the power source voltage of the inverter  122  varies, the clamp current varies and the consumption of electric power fluctuates. 
       SUMMARY OF THE INVENTION 
       [0016]    It is therefore a first object of the present invention to provide a drive circuit capable of suppressing an increase in an output current and of switching a semiconductor element for large power use at higher speeds. 
         [0017]    It is a second object of the present invention to provide a drive circuit capable of suppressing fluctuation in a clamping voltage. 
         [0018]    According to a first aspect of the present invention for attaining the first object, a drive circuit is provided for driving a voltage-driven semiconductor element by producing a drive signal depending upon an input signal. The drive circuit comprises an output stage and a current-suppressing circuit. The output stage includes two semiconductor elements connected in series. The semiconductor element that is to be driven is connected to a common connection point of the two semiconductor elements. The current-suppressing circuit controls one of the two semiconductor elements to provide an output current flowing through either one of the two semiconductor elements if a voltage applied to the voltage-driven semiconductor element reaches a limit level, which is in excess of a level for conducting the voltage-driven semiconductor element by a predetermined voltage. 
         [0019]    According to a second aspect of the present invention for attaining the second object, a drive circuit is provided for driving a voltage-driven semiconductor element to be conductive depending upon an input signal. The drive circuit has a function for clamping a voltage applied to a conduction control terminal of the semiconductor element when the semiconductor element is rendered conductive. The drive circuit comprises a constant current source, a resistor, a mirror pair and a constant voltage source. The constant current source supplies a constant current. The resistor is supplied with the constant current depending upon the input signal. The mirror pair on an output side determines a current flowing through the conduction control terminal of the semiconductor element. The constant voltage element connected between the conduction control terminal and a potential point that becomes equal to the terminal voltage of the resistor on a current path on a side of a main transistor that forms the mirror pair on the output side. The constant voltage element forms a negative feedback path for a current that flows through the conduction control terminal. The constant current source includes a circuit that is operable independently of temperature and voltage, and the resistor is operable independently of temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
           [0021]      FIG. 1  is a circuit diagram illustrating a drive circuit according to a first embodiment of the present invention; 
           [0022]      FIGS. 2A and 2B  are waveform diagrams illustrating an output current of the drive circuit and gate signal developed in the first embodiment; 
           [0023]      FIG. 3  is a circuit diagram illustrating a drive circuit according to a second embodiment of the present invention; 
           [0024]      FIG. 4  is a circuit diagram illustrating a drive circuit according to a third embodiment of the present invention; 
           [0025]      FIG. 5  is a circuit diagram illustrating a conventional drive circuit; 
           [0026]      FIG. 6  is a circuit diagram illustrating another conventional drive circuit; 
           [0027]      FIGS. 7 and 8  are waveform diagrams illustrating an output current and a gate signal developed in the conventional drive circuit when R 5  is 180 Ω; and 
           [0028]      FIGS. 9 and 10  are waveform diagrams illustrating an output current and a gate signal developed in the conventional drive circuit when R 5  is 2 kΩ; 
           [0029]      FIG. 11  is a circuit diagram illustrating a drive circuit according to a fourth embodiment of the present invention; 
           [0030]      FIG. 12  is a circuit diagram illustrating a drive circuit according to a fifth embodiment of the present invention; and 
           [0031]      FIG. 13  is a circuit diagram illustrating a further conventional drive circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0032]    Referring first to  FIGS. 1 and 2 , a first embodiment of a drive circuit for driving a semiconductor element is applied to a P-channel MOSFET, which is an element to be driven. The first embodiment is an improvement of the conventional drive circuit shown in  FIG. 5 . Therefore, the same portions as those of the conventional circuit shown in  FIG. 5  are denoted by the same reference numerals  1  to  13 . It is to be noted however that the resistor  9  in  FIG. 6  is not provided in the first embodiment. 
         [0033]    In a drive circuit  21  of this embodiment, a series circuit of a resistor  22 , a diode  23  and a Zener diode  24  is connected between a power source VB and an output terminal  10 , and a Zener diode  25  is connected in parallel with a resistor  22 . Further, a series circuit of a P-channel MOSFET  26  (trigger transistor) and a N-channel MOSFET  27  is connected between the power source VB and the ground, and the gate of FET  26  is connected to the anode of diode  23 . A N-channel MOSFET  28  (transistor for suppressing current) forms a mirror pair with the FET  27 , i.e., forms a current mirror circuit  29 . Their gates are connected in common to the drain of FET  27 . The drain of FET  28  is connected to the base of transistor  5 . Further, a Zener diode  30  is connected between the gates of FETs  27 ,  28  and the ground. The Zener diodes  25  and  30  are arranged for protecting the FETs  26 ,  27  and  28 . 
         [0034]    Further, the above construction added up with the resistor  22 , diode  23 , Zener diode  24  and FET  26 , forms a clamp circuit  45 , and the clamp circuit  45  added up with the FET  27  forms a bias circuit  46 . Further, the bias circuit  46  added up with the FET  28  forms a current-suppressing circuit  47 . 
         [0035]    Next, the operation of the embodiment will be described. First, considered below are the conditions of the output voltage Vout of the drive circuit  21 , where the FET  26  is turned on at the time of turning the FET  12  (element to be driven) on with the drive control signal VIN of the low level. The resistance of the resistor  22  is R 22 , the forward voltage of the diode  23  is V 23 , the Zener voltage of the Zener diode  24  is V 24 , the threshold voltage of the FET  26  is V 26 , and the current flowing through the resistor  22  is I 22 . 
         [0036]    Here, the condition for turning on the FET  26  is R 22 ·I 22 &gt;V 26 , and there holds a relationship, 
         [0000]        VB−R 22· I 22= V out+ V 24+ V 23   (1) 
         [0000]      Since 
         [0000]        R 22· I 22= VB− ( V out+ V 24+ V 23)   (2) 
         [0000]    an output voltage Vout that satisfies, 
         [0000]        VB− ( V out+ V 24+ V 23)&gt; V 26   (3) 
         [0000]      becomes 
         [0000]        V out&lt; VB− ( V 24+ V 23+ V 26)   (4) 
         [0037]    If specific values are given, i.e., VB=15 V, V 23 =0.7 V, V 24 =5 V, V 26 =1.8 V, then, 
         [0000]        V out&lt;15−(5+0.7+1.8)= 7 . 5   V    
         [0038]    Therefore, if the output voltage Vout becomes lower than 7.5 V (limit level), the FET  26  turns on. Here, the diode  23  is arranged to adjust the gate-source voltage Vgs of the FET  12 . 
         [0039]    If the FET  26  is turned on, a base current is fed to the current mirror circuit  29 , and the FET  28  decreases the base current of the transistor  5 . That is, if the Zener voltage VZ 2  of the Zener diode  11  is set to be about 8 V the output current of the drive circuit  21  can be provided from increasing in a state where the gate-source voltage Vgs of the FET  12  is clamped to 7.5 V. The Zener voltage of the Zener diode  25  may be set to be, for example, about 2.5 V which is slightly greater than the threshold voltage V 26  of the FET  26 . 
         [0040]    An output current of the drive circuit  21  (collector current I 5  of the transistor  5 ) is shown in  FIG. 2A , and an output voltage Vout (gate signal of the FET  12 ) is shown in  FIG. 2B . The output current I 5  is provided to be about 3 mA, and the gate signal waveform is not becoming dull but sharp as shown in  FIG. 2B  when it breaks down, contrary to the conventional case shown in  FIG. 8B . It will therefore be obvious that the FET  12  can be switched at a high speed while suppressing the consumption of electric current. 
         [0041]    In the drive circuit  21  of this embodiment, if the voltage applied to the FET  12  reaches a limit level in excess of the conduction level of the element, the current suppressing circuit  47  operates to decrease the base current of the transistor  5  (semiconductor element of the ground side) that forms the output stage making it possible to provided the output current (sink current) that flows through the transistor  5 . Specifically, if the gate voltage applied to the FET  12  reaches the limit level, the bias circuit  46  renders the FET  28  to be conductive so that the base current of the transistor  5  decreases. 
         [0042]    Therefore, the output voltage of the drive circuit  21  can be limited without using the resistor  9  for limiting the current in the output stage. The current suppressing circuit  47  does not operate to provided the current from when the gate voltage applied to the FET  12  has exceeded the conduction level until when it reaches the limit level. Within this range of application voltages, therefore, the FET  12  can be switched at a high speed. 
         [0043]    The current suppressing circuit  47  further operates as the clamp circuit  45  for clamping the gate voltage applied to the FET  12  to the limit level. When it is necessary to clamp the applied voltage for suppressing the drive current that flows through the FET  12 , therefore, the circuit construction can be further simplified. The clamp circuit  45  increases the amount of current flowing through the resistor  22  as the level of the output voltage (gate voltage) decreases, renders the FET  26  conductive in a step where the terminal voltage thereof increases, and executes the clamping operation by utilizing a constant voltage V 26  that generates across the gate and the source (across the input terminal and the output terminal) of the element, making it possible to more efficiently form a circuit that exhibits both the current suppressing function and the voltage clamping function. 
         [0044]    According to this embodiment, further, the Zener diodes  25 ,  30  are connected across the source and the gate of the FET  26 , and across the gate and the source of the FET  27  to protect the FETs  26  and  27  from overvoltages. 
       Second Embodiment 
       [0045]    A second embodiment shown in  FIG. 3  is an improvement of the conventional drive circuit shown in  FIG. 6 . In a drive circuit  31  of the second embodiment, a series circuit of a diode  32 , Zener diode  33  and resistor  34  is connected between the output terminal  10  and the ground, and a Zener diode  35  is connected in parallel with the resistor  34 . Further, the gate of a N-channel MOSFET  36  for suppressing current is connected to the anode of the Zener diode  33 , and the drain of FET  36  is connected to the base of transistor  8 , and the source thereof is grounded. 
         [0046]    In the above construction, the diode  32 , Zener diode  33  and resistor  34  form a bias circuit  37 . The bias circuit  37  to which the FET  36  is added forms a clamp circuit (current-suppressing circuit)  38 . 
         [0047]    Next, the operation of the second embodiment will be described. Considered below are the conditions of the output voltage Vout of the drive circuit  31  where the FET  36  is turned on. The forward voltage of the diode  32  is V 32 , the Zener voltage of the Zener diode  33  is V 33 , the resistance of the resistor  34  is R 34 , the threshold voltage of the FET  36  is V 36 , and the current flowing through the resistor  34  is I 34 . 
         [0048]    Here, the condition for turning the FET  36  on is R 34 ·I 34 &gt;V 36  and, besides, 
         [0000]        R 34· I 34= V out− V 32− V 33   (5) 
         [0049]    Therefore, an output voltage Vout that satisfies, 
         [0000]        V out− V 32− V 33&gt; V 36   (6) 
         [0000]      becomes 
         [0000]        V out&gt; V 32+ V 33+ V 36   (7) 
         [0050]    If specific values are given, i.e., V 32 =0.7 V, V 33 =5 V, V 36 =1.8 V, then, 
         [0000]        V out&gt;0.7+5+1.8=7.5 V    (8) 
         [0051]    In this case, too, therefore, if the Zener voltage V 11  of the Zener diode  11  is set to be about 8 V, the FET  36  is turned on as the output voltage Vout exceeds 7.5 V. If the FET  36  is turned on, a decreased base current flows into the transistor  8  (semiconductor element on the power source side) suppressing an increase in the output current (source current) of the drive circuit  31 . 
         [0052]    In the drive circuit  31  of this embodiment, if the element to be driven is the P-channel MOSFET  15 , the clamp circuit  38  which is the current-suppressing circuit operates to decrease the base current of the transistor  8  that forms the output stage if the voltage applied to the FET  15  reaches the limit level in excess of the conduction level of the element, making it possible to provided the output current that flows through the transistor  8 . Specifically, if the gate voltage applied to the FET  15  reaches the limit level, the bias circuit  37  renders the FET  36  to be conductive so that the base current of the transistor  8  decreases. Therefore, the same effect as that of the first embodiment is provided. 
         [0053]    In this case, the clamp circuit  38  increases the amount of current flowing through the resistor  34  as the level of the output voltage (gate voltage) increases, renders the FET  36  conductive in a step where the terminal voltage thereof increases, and executes the clamping operation by utilizing a constant voltage VT 2  that generates across the gate and the source (across the input terminal and the output terminal) of the element, making it possible to more efficiently form a circuit that exhibits both the current suppressing function and the voltage clamping function. 
       Third Embodiment 
       [0054]    A third embodiment shown in  FIG. 4  is similar to the first embodiment, and differentiated in that a drive circuit  41  has a N-channel MOSFET  42  (semiconductor element on the ground side) in place of the transistor  5  in the drive circuit  21  of the first embodiment. 
         [0055]    In operation, the FET  26  is turned on if the output voltage Vout satisfies the condition of the equation (4) at the time when the drive control signal VIN is at the low level causing the FET  12  to be turned on. Then, the current mirror circuit  29  operates and the FET  28  draws part of the current that flows into the resistor  4 , causing the gate potential of FET  42  to decrease. Therefore, the output current of the drive circuit  21  decreases. The above construction of the third embodiment, too, exhibits the same operation and effect as those of the first embodiment. 
         [0056]    In the first to third embodiments, the diode  23  or  32  may be provided as required and two or more diodes may be provided to suitably adjust the voltage applied to the input terminal of the element to be driven. Further, the Zener voltage of the Zener diode  24  or  33 , too, may be suitably varied. The Zener diodes  11 ,  25 ,  30 ,  35  for protection may be connected as required. The bias circuit and the clamp circuit may be formed independently of each other. The element to be driven may be an IGBT. Moreover, the elements that form the drive circuit may be suitably replaced by MOSFETs or bipolar transistors. 
       Fourth Embodiment 
       [0057]    In a fourth embodiment shown in  FIG. 11 , two current sources  401  and  402  are connected to the positive terminal of the power source VB, and the drains of N-channel MOSFETs  403  and  404  are connected to the side of the current source  402 . The source of FET  403  (main transistor) is grounded via a resistor  405  and the source of FET  404  (sub-transistor) is directly grounded. Further, the drain of FET  403  is connected to its gate and is further connected to the gate of a N-channel MOSFET  406 . 
         [0058]    The current source  401  is grounded via the drain and source of N-channel MOSFET  407 , and the gate of FET  407  (main transistor) is connected to its drain and to the gate of N-channel MOSFET  408 . The drain of FET  408  (sub-transistor) is connected to the source of FET  406 , and the source of FET  408  is grounded. The sources of two P-channel MOSFETs  409  and  410  are connected to the positive terminal of the power source VB, and the gates thereof are connected to the drain of FET  409  (main transistor) and to the drain of FET  406 . Further, the gates of FETs  409  and  410  are connected to the positive terminal of the power source VB through a resistor  450 . 
         [0059]    The drain of FET  410  (sub-transistor) is connected to the drain of N-channel MOSFET  411 , the source of FET  411  is grounded and the gate thereof is connected to the gate of FET  404 . A series circuit of a load (e.g., resistor and inductance)  412  and a N-channel MOSFET  413 , which is an element to be driven, is connected between the positive terminal of power source VB and the ground. The gate of FET- 413  is connected to the drains of FETs  410  and  411 . 
         [0060]    The FET  413  is, for example, a power MOSFET A series circuit of a diode  414  and a Zener diode  415  is connected between the gate of FET  413  and the ground. The diode  414  and the Zener diode  415  are connected in reverse in polarity to each other. The gate of FET  413  is connected to the cathode of Zener diode  416  (constant voltage element), and the anode of Zener diode  416  is connected to the drain of FET  408 . 
         [0061]    The drive control signal VIN is input to the gates of FETs  404  and  411 . A pair of FETs  403  and  406  form a mirror pair  417  on the input side. A pair of FETs  407  and  408  form a mirror pair  418  for determining current. A pair of FETs  409  and  410  form a mirror pair  419  on the output side. The drive circuit  420  is connected to drive the load  412  through the FET  413 . 
         [0062]    In operation, if the drive control signal VIN is at a high level, the FETs  404  and  411  are both turned on, and the FETs  403  and  406  are both turned off. Accordingly, the FETs  409  and  410  are turned off, the gate potential Vout of FET  413  assumes the low level, the FET  413  is turned off, and no current is fed to the load  412 . 
         [0063]    If the drive control signal VIN is at a low level, on the other hand, the FETs  404  and  411  are both turned off, and the FETs  403  and  406  are both turned on. Therefore, the FETs  409  and  410  are turned on. As a result, the output signal Vout (gate potential of FET  413 ) assumes the high level, and the FET  413  is turned on to supply a current to the load  412 . Here, if it is presumed that the Zener diode  416  is not provided, a current flows into the diodes  414  and  415  through the FET  410  provided the voltage VB is, 
         [0000]        VB&gt;V 414+ V 415   (9) 
         [0064]    where V 414  is a forward voltage of the diode  414 , and 
         [0065]    V 415  is a Zener voltage of the Zener diode  415 . 
         [0066]    If the frequency of the drive control signals VIN is on the order of several hundred kHz, a current flowing through the FET  410  becomes about 100 mA provided the input capacity of FET  413  is about 100 pF, and nearly all of the current flows as a useless current into the ground through the diodes  414  and  415 . 
         [0067]    This embodiment employs the Zener diode  416 . If the voltage VB becomes, 
         [0000]        VB&gt;V 408+ V 415   (10) 
         [0000]    due to the provision of Zener diode  416 , then the current flows into the FET  408  through the Zener diode  416 . Here, V 408  is a drain potential of FET  408  and V 416  is a Zener voltage of Zener diode  416 . Here, if the current flowing into the FET  408  is denoted by I 408 , then, 
         [0000]        I 408= I 416+ I 401   (11) 
         [0068]    where I 416  is a current flowing into the Zener diode  416  and I 401  is a constant current supplied by the current source  401 . Namely, as represented by the equation (11), a current drawn by the FET  408  from the FET  409  decreases by the amount of current I 416  flowing into the drain thereof. Accordingly, the output current that flows via the FET  410  decreases correspondingly. Namely, a negative feedback acts on the output current. 
         [0069]    Further, the output voltage Vout is clamped at, 
         [0000]        V out= V 408+ V 416   (12) 
         [0070]    Here, the drain potential V 408  of FET  408  is expressed by the equation (13), 
         [0000]        V 408= VGS 2+ R 405× I 402− VGS 1   (13) 
         [0071]    where VGS 2  and VGS 1  are gate-source voltages of the FETs  403  and  406  as expressed below, R 405  is a resistance of the resistor  405 , and I 402  is a constant current supplied by the current source  402 . 
         [0000]        VGS 1=(2× I 1/β) 1/2    (14) 
         [0000]        VGS 2=(2× I 2/β) 1/2    (15) 
         [0000]      where β=(μ× Cx×W )/ L    (16) 
         [0072]    μ is a mobility of electrons of the FET, Cx is a capacity of the gate oxide film, W is a channel width, an L is a channel length. 
         [0073]    Therefore, if the constant currents I 401  and I 402  of the current sources  401  and  402  are set to be equal to each other, then 
         [0000]        V 408= R 405× I 402   (17) 
         [0074]    That is, if the resistor  405  is formed by a thin-film resistor having no temperature dependency and if the current source  402 , too, is formed having neither the temperature dependency nor the voltage dependency, the clamp voltage Vout can be liberated from the effect caused by variation in the temperature and fluctuation in the power source VB. A constant current circuit having the above characteristics are known. 
         [0075]    To turn the FET  413  on, a constant current I 402  is supplied from the constant current source  402  to the resistor  405  depending upon the input signal VIN, and a current that flows through the gate of FET  413  is determined by the mirror pair  419  on the output side. Further, the Zener diode  416  is connected between the above gate and a potential point (drain of FET  408 ) which becomes equal to the terminal voltage of the resistor  405  in the current path on the side of FET  409  of the mirror pair  419  on the output side, to thereby form a negative feedback path for flowing a current into the above potential point through the gate of FET  413 . 
         [0076]    Therefore, the gate potential of FET  413  can be clamped to the terminal voltage (=V 408 ) of the resistor  405  to which a constant voltage VZD 416  generated by the Zener diode  416  is added. The constant current source  402  is formed by a circuit having neither the temperature dependency nor the voltage dependency, and the resistor  405  is formed by an element having no temperature dependency, liberating the clamped voltage from the effect caused by variation in the power-source voltage and in the temperature. Thus, the load current that flows through the FET  413  is maintained nearly constant. 
         [0077]    Further, the output stage of the drive circuit  420  is formed by connecting two FETs  410  and  411  in series, and the gate of FET  413  is connected to a common connection point thereof, and the FET  410  is rendered to operate as a sub-transistor of the mirror pair  419  on the output side. Therefore, a current that flows through the gate when the FET  413  is turned on becomes a mirror current of the current that flows through the FET  409  of the mirror pair  419  on the output side, and a current that flows through the gate of FET  413  and the Zener diode  416  meets the current that flows through the FET  409 . Therefore, the drive current that flows during the clamping operation is provided by a current that is fed back to the side of FET  409  through the Zener diode  416 . 
         [0078]    Further, since the above potential point is disposed between the FET  409  and the FET  408  in the mirror pair  418  for determining the current, a current that flows through the FET  409  in the mirror pair  419  on the output side can be determined by the constant current I 1  from which a current flowing through the Zener diode  416  is subtracted. Moreover, the resistor  405  is connected in series with the source of FET  403  that forms the mirror pair  417  on the input side, and the FET  406  that forms the mirror pair  17  on the input side is arranged between the FET  409  and the FET  408 , making it possible to determine the current flowing into the mirror pair  419  on the output side depending upon the two constant currents I 1  and I 402 . 
         [0079]    The terminal voltage of resistor  405  becomes equal to the source potential of FET  403  that forms the mirror pair  417  on the input side, and the source potential of FET  406  becomes equal to a threshold voltage of FETs  403 ,  406  to which VGS 2 , VGS 1  are added or from which VGS 2 , VGS 1  are subtracted. By setting the two constant currents I 401  and I 402  to be equal to each other, therefore, the source of FET  406  becomes a potential point that is equal to the terminal voltage of the resistor  405 . 
       Fifth Embodiment 
       [0080]    In a fifth embodiment shown in  FIG. 12 , the semiconductor element to be driven is a P-channel MOSFET  435 . 
         [0081]    A series circuit of resistors  421 ,  422  and N-channel MOSFET  423  is connected between the positive terminal of the power source VB and the ground. A Zener diode  424  for clamping is connected in parallel with the resistor  421 . The source of P-channel MOSFET  425  is connected to the power source VB, and the drain of FET  425  is grounded via the constant current source  402 . Further, the gate of FET  425  is connected to a common connection point of the resistors  421  and  422 . 
         [0082]    A series circuit of resistor  405  and P-channel MOSFET  426  (main transistor) is connected in parallel with the FET  425 , and the gate of FET  426  is connected to its drain together with the gate of P-channel MOSFET  427  (sub-transistor). The sources of P-channel MOSFTs  428  and  429  are connected to the positive terminal of the power source VB, and the gates thereof are connected in common to the drain on the side of FET  428  (main transistor). 
         [0083]    The drain of FET  428  is grounded via the constant current source  401 . On the other hand, the drain of FET  429  (sub-transistor) is connected to the source of FET  427 , and the drain of FET  427  is connected to the drain of N-channel MOSFET  430  (main transistor) which forms a mirror pair together with the FET  411  (sub-transistor) and is, further, connected to the gates of FETs  411  and  430 . A resistor  451  is connected between the gates of FETs  411 ,  430  and the ground. 
         [0084]    Further, a series circuit of resistors  431 ,  432  and N-channel MOSFET  433  is connected between the positive terminal of the power source VB and the ground, and drive control signals VIN are supplied to the gate of FET  433  as well as to the gate of FET  423 . The gate of FET  410  is connected to a common connection point of the resistors  431  and  432 , and a Zener diode  434  is connected in parallel with the resistor  431  to clamp the gate potential of FET  410 . 
         [0085]    A series circuit of the P-channel MOSFET  435  and the load  412  is connected between the positive terminal of the power source VB and the ground. The gate of FET  435  is connected to the drains of FETs  410  and  411 . A series circuit of the diode  414  and the Zener diode  415  is connected between the positive terminal of the power source VB and the gate of FET  435 , and a Zener diode  413  is connected between the source of FET  427  and the gate of FET  435 . A negative feedback path for the drive current is thus formed. 
         [0086]    In the above construction, a pair of FETs  426  and  427  form a mirror pair  436  on the input side, a pair of FETs  428  and  429  form a mirror pair  437  for determining the current, and a pair of FETs  430  and  411  form a mirror pair  438  on the output side. The drive circuit  439  is thus connected to the load  412  through the FET  435 . 
         [0087]    In operation, if the drive control signal VIN is at the high level, FETs  423  and  433  are turned on. Then, the gate potential of FET  425  assumes the low level and the FET  425  is turned on causing the FET  426  to be turned off. Therefore, the FET  427 , too, is turned off, and the gate potential to the FETs  430  and  411  assumes the low level; i.e., the FETs  430  and  411  are turned off. Here, since the FET  433  is turned on, the FET  410  is turned on. As a result, the gate potential Vout to the FET  435  assumes the high level; i.e., the FET  435  is turned off and no current is supplied to the load  412 . 
         [0088]    On the other hand, if the drive control signal VIN is at the low level, the FETs  423  and  433  are turned off. Then, the gate potential of FET  425  assumes the high level and the FET  425  is turned off causing the FET  426  to be turned on. Therefore, the FET  427 , too, is turned on, and the gate potential to the FETs  430  and  411  assumes the high level; i.e., the FETs  430  and  411  are turned on. Here, since the FET  433  is turned off, the FET  410 , too, is turned off. 
         [0089]    As a result, the output voltage Vout (gate potential to the FET  435 ) assumes the low level; i.e., the FET  435  is turned on and a current is supplied to the load  412 . Here, if the source potential (potential point) of the FET  427  is denoted by V 427  and the Zener voltage of the Zener diode  413  is denoted by V 413 , the gate voltage Vout of FET  435  is given by, 
         [0000]        V out= V 427− V 413   (18) 
         [0090]    Due to the same principle as that of the first embodiment, the source potential V 427  of FET  427  is equal to the source potential of FET  426 , i.e., 
         [0000]        V 427= VB−R 405· I 402   (19) 
         [0091]    Therefore, the gate potential Vout of FET  435  is clamped to be, 
         [0000]        V out= VB−R 405· I 402− V 413   (20) 
         [0092]    Further, part of the current flowing through the FET  429  is branched as the current I 413  that flows through the Zener diode  413 . Therefore, the current flowing into the mirror pair  438  on the output side decreases by the current I 413 . Accordingly, the output current that flows through the FET  411  decreases, and the negative feedback acts on the output current. 
         [0093]    As described above, the fifth embodiment exhibits the same effect as that of the fourth embodiment even when the element to be driven is the p-channel MOSFET  435 . 
         [0094]    In the fourth and the fifth embodiments, the FETs may be suitably replaced by bipolar transistors and the element to be driven may be an IGBT.