Patent Publication Number: US-11038499-B2

Title: Gate drive apparatus and switching apparatus

Description:
The contents of the following Japanese patent application(s) are incorporated herein by reference: 
     No. 2018-091618 filed in JP on May 10, 2018 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a gate drive apparatus and a switching apparatus. 
     2. Related Art 
     About switching circuits that use power semiconductor elements as main switching elements, conventionally proposed drive circuits include one that can apply a positive bias and a negative bias to the gate (or base) of a main switching element using one power source (see Patent Literatures 1 and 2). 
     Patent Literature 1: Japanese Patent Application Publication No. 2009-201110 
     Patent Literature 2: WO2009/004715 
     If turning on/turning off of the main switching element is early, current flowing through the main switching element changes abruptly, and a voltage that is applied to the main switching element overshoots, and so on, which possibly results in the main switching element&#39;s shorter lifetime. If turning on/turning off of the main switching element is late, the transition period becomes long, and the switching loss increases. Because of this, it is desired to be able to flexibly configure the rate of transition for turning on/turning off of the main switching element according to current to flow through the switching circuit, a voltage to be applied to the switching circuit, and/or the state of biasing of the gate, and the like. 
     SUMMARY 
     In order to overcome the above-mentioned drawbacks, a first aspect of the present invention provides a drive apparatus that drives a control terminal of a main switching element that establishes or cuts off an electrical connection between a first main terminal and a second main terminal, the drive apparatus including: a first switching element that establishes or cuts off an electrical connection between a positive terminal of at least one power source and the control terminal; a second switching element that establishes or cuts off an electrical connection between the positive terminal of the power source and the second main terminal; a third switching element that establishes or cuts off an electrical connection between the control terminal and a negative terminal of the power source; and a fourth switching element that establishes or cuts off an electrical connection between the second main terminal and the negative terminal of the power source, wherein at least one resistance among: a first path resistance in a first path establishing a connection between the control terminal and the second main terminal via the first switching element and the second switching element; a second path resistance in a second path establishing a connection between the control terminal and the second main terminal via the first switching element and the fourth switching element; a third path resistance in a third path establishing a connection between the control terminal and the second main terminal via the second switching element and the third switching element; and a fourth path resistance in a fourth path establishing a connection between the control terminal and the second main terminal via the third switching element and the fourth switching element, is different from at least one of the other resistances. 
     A second aspect of the present invention provides a drive apparatus that drives a control terminal of a main switching element that establishes or cuts off an electrical connection between a first main terminal and a second main terminal, the drive apparatus including: a first switching element that establishes or cuts off an electrical connection between a positive terminal of a power source and the control terminal; a second switching element that establishes or cuts off an electrical connection between the positive terminal of the power source and the second main terminal; a third switching element that establishes or cuts off an electrical connection between the control terminal and a negative terminal of the power source; a fourth switching element that establishes or cuts off an electrical connection between the second main terminal and the negative terminal of the power source; and a drive control unit that controls the first switching element, the second switching element, the third switching element and the fourth switching element, wherein during at least one switching of: switching from a connection through a second path establishing a connection between the control terminal and the second main terminal via the first switching element and the fourth switching element to a connection through a third path establishing a connection between the control terminal and the second main terminal via the second switching element and the third switching element; and switching from a connection through the third path to a connection through the second path, the drive control unit provides at least one of: a period during which a connection through a first path to establish a connection between the control terminal and the second main terminal is established via the first switching element and the second switching element; and a period during which a connection through a fourth path to establish a connection between the control terminal and the second main terminal is established via the third switching element and the fourth switching element. 
     A third aspect of the present invention provides a switching apparatus including: the drive apparatus; and the main switching element. 
     The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the configuration of a switching apparatus  10  according to the present embodiment. 
         FIG. 2  illustrates connection modes of a drive apparatus  110  according to the present embodiment. 
         FIG. 3  shows a first example illustrating transitions of a bias voltage across the control terminal of a main switching element  100  according to the present embodiment. 
         FIG. 4  shows a second example illustrating transitions of a bias voltage across the control terminal of the main switching element  100  according to the present embodiment. 
         FIG. 5  shows a third example illustrating transitions of a bias voltage across the control terminal of the main switching element  100  according to the present embodiment. 
         FIG. 6  shows a fourth example illustrating transitions of a bias voltage across the control terminal of the main switching element  100  according to the present embodiment. 
         FIG. 7  illustrates a variant of connection modes of the drive apparatus  110  according to the present embodiment. 
         FIG. 8  illustrates the configuration of the switching apparatus  10  according to a first variant. 
         FIG. 9  illustrates connection modes of the drive apparatus  110  according to the first variant. 
         FIG. 10  illustrates the configuration of the switching apparatus  10  according to a second variant. 
         FIG. 11  illustrates connection modes of the drive apparatus  110  according to the second variant. 
         FIG. 12  illustrates the configuration of the switching apparatus  10  according to a third variant. 
         FIG. 13  illustrates transitions of bias voltages across the control terminals of main switching elements  1100   a - b  according to the third variant. 
         FIG. 14  illustrates the configuration of the switching apparatus  10  according to a fourth variant. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, (some) embodiment(s) of the present invention will be described. The embodiment(s) do(es) not limit the invention according to the claims, and all the combinations of the features described in the embodiment(s) are not necessarily essential to means provided by aspects of the invention. 
       FIG. 1  illustrates the configuration of a switching apparatus  10  according to the present embodiment. The switching apparatus  10  includes a main switching element  100 , a drive apparatus  110  and a power source  130 . The main switching element  100 , for example, is a power semiconductor element, has a control terminal, a first main terminal and a second main terminal, and establishes (turns on) or cuts off (turns off) an electrical connection between the first main terminal and the second main terminal according to a voltage or current input to the control terminal. In the present embodiment, the main switching element  100 , for example, is an nMOS transistor having the gate as the control terminal, the drain as the first main terminal, and the source as the second main terminal, and is turned off if a voltage (also denoted as “bias voltage”) across the control terminal relative to the second main terminal, that is, for example a gate-source voltage Vgs, is equal to or lower than a threshold voltage Vth, and is turned on if the gate-source voltage Vgs exceeds Vth. 
     Although in the present embodiment, an example in which the main switching element  100  is an nMOS transistor is explained, each embodiment explained in the present application can be applied to various types of the main switching element  100  such as a MOS transistor having a control terminal which is referred to as the gate, and two main terminals which are referred to as the drain and source, an IGBT (insulated gate bipolar transistor) having a control terminal which is referred to as the gate, and two main terminals which are referred to as the collector and emitter, or a bipolar transistor having a control electrode which is referred to as the base, and two main electrodes which are referred to as the collector and emitter. 
     Upon being supplied with a power source voltage Vin from the power source  130 , the drive apparatus  110  drives the control terminal (the gate in the figure) of the main switching element  100  according to a control signal CTL input from the outside. Thereby, the drive apparatus  110  performs switching of ON/OFF between the first main terminal and the second main terminal (between the drain and the source in the figure) of the main switching element  100 . The drive apparatus  110  may be realized by one or more integrated circuits having the control terminal and second main terminal of the main switching element  100 , the positive terminal and negative terminal of the power source  130 , and a plurality of terminals for input or output of the control signal CTL, or may be realized by a combination of a plurality of discrete components. The drive apparatus  110  has a first switching element SW 1 , a second switching element SW 2 , a third switching element SW 3 , a fourth switching element SW 4  and a drive control unit  120 . 
     The first switching element SW 1  is provided between the positive terminal of the power source  130  and the control terminal of the main switching element  100 , and establishes or cuts off an electrical connection between the positive terminal of the power source  130  and the control terminal of the main switching element  100 . The second switching element SW 2  is provided between the positive terminal of the power source  130  and the second main terminal of the main switching element  100 , and establishes or cuts off an electrical connection between the positive terminal of the power source  130  and the second main terminal of the main switching element  100 . The third switching element SW 3  is provided between the control terminal of the main switching element  100  and the negative terminal of the power source  130 , and establishes or cuts off an electrical connection between the control terminal of the main switching element  100  and the negative terminal of the power source  130 . The fourth switching element SW 4  is provided between the second main terminal of the main switching element  100  and the negative terminal of the power source  130 , and establishes or cuts off an electrical connection between the second main terminal of the main switching element  100  and the negative terminal of the power source  130 . Here, each among the first to fourth switching elements SW 1 - 4  may be a semiconductor switching element such as a MOS transistor or a bipolar transistor, and turns on or off the state of a connection between the main terminals (between the drain and the source, or between the collector and the emitter) according to a drive signal input to the control terminal (the gate or base). In the present embodiment, each among the first to fourth switching elements SW 1 - 4 , for example, is an nMOS transistor, and is turned off if the gate-source voltage becomes equal to or lower than a threshold voltage, and is turned on if the gate-source voltage exceeds the threshold voltage. 
     Note that, in the specification of the present application, phrases like “an element is provided between two terminals” refer not only to configurations in which “the element is connected directly electrically to the two terminals”, but also to configurations in which the element is electrically connected to the terminals, with another element/other elements or the like interposed therebetween unless noted otherwise. Similarly, phrases like “an element A is electrically connected to an element B” refer not only to configurations in which the elements A and B are directly electrically connected, but also to configurations in which the elements A and B are indirectly electrically connected, with another element/other elements or the like being connected therebetween. 
     In the present embodiment, a first connection resistance between the positive terminal of the power source and the control terminal via the first switching element SW 1  is defined as R 1 , a second connection resistance between the positive terminal of the power source and the second main terminal via the second switching element SW 2  is defined as R 2 , a third connection resistance between the control terminal and the negative terminal of the power source via the third switching element SW 3  is defined as R 3 , and a fourth connection resistance between the second main terminal and the negative terminal of the power source via the fourth switching element SW 4  is defined as R 4 . These connection resistances may be a combined resistance produced by a switching element and other elements connected between two points, and if the resistance of a wire is not negligible, the wire resistance may be included in the combined resistance. If the wire is the only member provided between the two points other than the switching element, and the resistance of the wire is negligible, the switching element is equivalent to the resistor in the case where the two points are in a connection state. 
     According to the control signal CTL input from the outside, the drive control unit  120  outputs a drive signal D 1  to drive the control terminal of the first switching element SW 1 , a drive signal D 2  to drive the control terminal of the second switching element SW 2 , a drive signal D 3  to drive the control terminal of the third switching element SW 3 , and a drive signal D 4  to drive the control terminal of the fourth switching element SW 4 , and controls the first to fourth switching elements SW 1 - 4 . The drive control unit  120  may be realized by a dedicated hardware circuit that may include a state machine or the like, may be realized by a programmable circuitry such as a field-programmable gate array (FPGA), or may be realized by execution of a program at a micro controller, and so on. 
     The power source  130  generates, between the positive terminal and the negative terminal, the power source voltage Vin that the drive apparatus  110  uses for applying a positive bias voltage or a negative bias voltage between the control terminal and second main terminal of the main switching element  100 . 
       FIG. 2  illustrates connection modes of the drive apparatus  110  according to the present embodiment. The drive control unit  120  controls the signal values of the drive signals D 1 -D 4 , and switches ON/OFF of the first to fourth switching elements SW 1 - 4  to thereby switch the connection path between the control terminal and second main terminal of the main switching element  100 . Thereby, the drive control unit  120  can switch the bias voltage (or bias current) to be applied to the control terminal of the main switching element  100 . 
     In a connection mode  1 , the drive control unit  120  turns on the first switching element SW 1  and second switching element SW 2 , and turns off the third switching element SW 3  and fourth switching element SW 4  so as to establish a connection between the control terminal and second main terminal of the main switching element  100  via the first switching element SW 1  and second switching element SW 2 . In the present embodiment, the drive control unit  120  supplies the control terminals of the first to second switching elements SW 1 - 2  with the drive signals D 1 - 2  having logic H, and turns them on, and supplies the control terminals of the third to fourth switching element SW 3 - 4  with the drive signals D 3 - 4  having logic L, and turns them off. Here, if the first to fourth switching elements SW 1 - 4  have parasitic diodes (or diodes connected in anti-parallel; the same applies hereinbelow) as shown in  FIG. 1 , the drive control unit  120  may make the switching elements enter the conducting state via the parasitic diodes without supplying the switching elements with drive signals having logic H. For example, if the voltage across the control terminal of the main switching element  100  is higher than the voltage across the second main terminal, the drive control unit  120  may give the drive signal D 1  logic L, and make the first switching element SW 1  enter the conducting state through the parasitic diode. In addition, if the voltage across the control terminal is lower than the voltage across the second main terminal, the drive control unit  120  may give the drive signal D 2  logic L, and make the second switching element SW 2  enter the conducting state through the parasitic diode. Note that a variant in which at least one of the first to fourth switching elements SW 1 - 4  is caused to enter the conducting state via the parasitic diode(s) is explained with reference to  FIG. 7 . 
     In the connection mode  1 , the drive apparatus  110  forms a first path that establishes a connection between the control terminal and second main terminal of the main switching element  100  via the first switching element SW 1  and second switching element SW 2 . The first path does not include the power source  130 , and the voltage (“Vgs (V)” in the figure) between the control terminal and second main terminal becomes almost 0 when in the stable state. But if any of the first to second switching elements SW 1 - 2  is in the conducting state through the parasitic diode, a potential difference is generated between the control terminal and second main terminal due to a forward voltage across the parasitic diode. The first path resistance (the “gate drive resistance” in the figure), which is the resistance between the control terminal and second main terminal in the first path, becomes R 1 +R 2 . 
     In a connection mode  2 , the drive control unit  120  turns on the first switching element SW 1  and fourth switching element SW 4 , and turns off the second switching element SW 2  and third switching element SW 3  so as to establish a connection between the control terminal and second main terminal of the main switching element  100  via the first switching element SW 1  and fourth switching element SW 4 . In the present embodiment, the drive control unit  120  supplies the control terminals of the first switching element SW 1  and fourth switching element SW 4  with the drive signals D 1  and D 4  having logic H, and turns them on, and supplies the control terminals of the second to third switching elements SW 2 - 3  with the drive signals D 2 - 3  having logic L, and turns them off. 
     In the connection mode  2 , the drive apparatus  110  forms a second path that establishes a connection between the control terminal and second main terminal of the main switching element  100  via the first switching element SW 1  and fourth switching element SW 4 . The second path includes the power source  130  in the forward direction (the direction in which the control terminal is located on the positive side), and the voltage between the control terminal and second main terminal can be regarded as being the power source voltage Vin when in the stable state. The second path resistance, which is the resistance between the control terminal and second main terminal in the second path, becomes R 1 +R 4 . 
     In a connection mode  3 , the drive control unit  120  turns on the second switching element SW 2  and third switching element SW 3 , and turns off the first switching element SW 1  and fourth switching element SW 4  so as to establish a connection between the control terminal and second main terminal of the main switching element  100  via the second switching element SW 2  and third switching element SW 3 . In the present embodiment, the drive control unit  120  supplies the control terminals of the second to third switching elements SW 2 - 3  with the drive signals D 2 - 3  having logic H, and turns them on, and supplies the control terminals of the first switching element SW 1  and fourth switching element SW 4  with the drive signals D 1  and D 4  having logic L, and turns them off. 
     In the connection mode  3 , the drive apparatus  110  forms a third path that establishes a connection between the control terminal and second main terminal of the main switching element  100  via the second switching element SW 2  and third switching element SW 3 . The third path includes the power source  130  in the reverse direction (the direction in which the control terminal is located on the negative side), and the voltage between the control terminal and the second main terminal can be regarded as being the reverse bias value of the power source voltage Vin, that is, −Vin, when in the stable state. The third path resistance, which is the resistance between the control terminal and the second main terminal in the third path, becomes R 2 +R 3 . 
     In a connection mode  4 , the drive control unit  120  turns on the third switching element SW 3  and fourth switching element SW 4 , and turns off the first switching element SW 1  and second switching element SW 2  so as to establish a connection between the control terminal and second main terminal of the main switching element  100  via the third switching element SW 3  and the fourth switching element SW 4 . In the present embodiment, the drive control unit  120  supplies the control terminals of the third to fourth switching elements SW 3 - 4  with the drive signals D 3 - 4  having logic H, and turns them on, and supplies the control terminals of the first to second switching elements SW 1 - 2  with the drive signals D 1 - 2  having logic L, and turns them off. Similar to the connection mode  1 , the drive control unit  120  may make any of the third to fourth switching elements SW 3 - 4  enter the conducting state through the parasitic diode. Note that a variant in which at least one of the first to fourth switching elements SW 1 - 4  is caused to enter the conducting state via the parasitic diode(s) is explained with reference to  FIG. 7 . 
     In the connection mode  4 , the drive apparatus  110  forms a fourth path that establishes a connection between the control terminal and second main terminal of the main switching element  100  via the third switching element SW 3  and the fourth switching element SW 4 . The fourth path does not include the power source  130 , and the voltage between the control terminal and second main terminal becomes almost 0 when in the stable state. But if any of the third to fourth switching elements SW 3 - 4  is in the conducting state through the parasitic diode, a potential difference is generated between the control terminal and second main terminal due to a forward voltage across the parasitic diode. The fourth path resistance, which is the resistance between the control terminal and second main terminal in the fourth path, becomes R 3 +R 4 . 
     In the present embodiment, the resistance of each path resistance is configured appropriately so as to configure well the rate of transition of the switching state of the main switching element  100  in each among the case where a positive bias voltage is supplied to the control terminal of the main switching element  100 , the case where a negative bias voltage is supplied to the control terminal, and the case where the bias voltage of 0 V is supplied to the control terminal. In the thus-configured drive apparatus  110 , at least one resistance among the first path resistance, the second path resistance, the third path resistance and the fourth path resistance is different from at least one of the other resistances. 
     For example, the second path resistance and the third path resistance may have different resistances. For example, if turning on of the main switching element  100  is caused to occur gradually as compare with turning off of the main switching element  100 , the second path resistance may be configured to be higher than the third path resistance. 
     In addition, for example, the first path resistance and the fourth path resistance may have different resistances. By using different resistances of the first path and the fourth path, both of which make the control terminal of the main switching element  100  zero-biased in the same way, the drive apparatus  110  can properly use zero-biasing of a higher path resistance and zero-biasing of a smaller path resistance for different purposes. 
     In order to realize such suitable path resistances, the drive apparatus  110  may be configured to have the first connection resistance R 1 , second connection resistance R 2 , third connection resistance R 3 , and fourth connection resistance R 4 , at least one resistance of which is different from at least one of the other resistances. For example, by configuring the first connection resistance R 1  and second connection resistance R 2  to be equal, and configuring the third connection resistance R 3  smaller than the fourth connection resistance R 4 , the second path resistance R 1 +R 4  becomes higher than the third path resistance R 2 +R 3 , and turning on of the main switching element  100  can be configured to occur gradually as compare with turning off of the main switching element  100 . 
     Here, the phrase “the resistances are different” means that the resistances are substantially different, and means that they are designed to be different resistances. Accordingly, it does not mean that there are variations due to manufacturing errors in the resistances between individual paths and/or individual connections that are designed to have the same resistance. Such substantial differences in resistances may be observed as significant differences in the resistances, which are equal to or larger than 1%, equal to or larger than 3%, equal to or larger than 5%, equal to or larger than 10%, equal to or larger than 20%, or the like, for example, and may be observed as differences in the resistances exceeding the range of precision of the resistances to be used. 
       FIG. 3  shows a first example illustrating transitions of a bias voltage across the control terminal of the main switching element  100  according to the present embodiment. In the present example, the drive apparatus  110  turns off the main switching element  100  by changing the bias voltage Vgs across the control terminal of the main switching element  100  from +Vin to −Vin. 
     Before a time t 1 , the drive control unit  120  is receiving an input of the control signal CTL (for example, logic H) instructing to make the main switching element  100  enter the ON-state. Upon receiving it, the drive control unit  120  makes the drive apparatus  110  enter the connection mode  2 , and is supplying the bias voltage Vgs at +Vin to the control terminal of the main switching element  100 . For example, if the threshold voltage across the main switching element  100  is 3 to 8 V, +Vin may be 10 to 20 V. Thereby, the main switching element  100  enters the ON-state. 
     At the time t 1 , the drive control unit  120  receives an input of the control signal CTL (for example, logic L) to instruct switching of the main switching element  100  to enter the OFF state. 
     Upon receiving it, the drive control unit  120  makes the drive apparatus  110  enter the connection mode  1  (or the connection mode  4 ), and lowers the bias voltage Vgs from +Vin to 0 V. Here, if the threshold voltage Vth of the main switching element  100  has a value between Vin and 0 V (for example 3 to 8 V), the main switching element  100  is turned off. 
     Here, depending on the characteristics of a system in which the switching apparatus  10  is provided, if the main switching element  100  exits the ON-state and enters the OFF state, and current flowing through the main switching element  100  decreases abruptly, the voltage between the first main terminal and the second main terminal might increase abruptly from almost 0 V to a high voltage. In such a case, the bias voltage Vgs is increased, and the main switching element  100  might be erroneously turned on even in the connection mode  1  (or  4 ). In view of this, at a time t 2 , the drive control unit  120  makes the first to fourth switching elements SW 1 - 4  enter the connection mode  3  so as to surely make the main switching element  100  enter the OFF state, and applies the negative bias voltage −Vin to the control terminal. 
     In this case, the drive control unit  120  may set the period of the connection mode  1  or connection mode  4  between the times t 1  and t 2  to a predetermined period or longer such that the period during which the bias voltage Vgs is maintained at substantially 0 V between the times t 1  and t 2  becomes a predetermined minimum maintenance period or longer. This minimum maintenance period may be longer than the transition time of a voltage or current between the first to second main terminals that accompanies turning on or turning off of the main switching element  100 , and for example may be several hundred ns or longer. Thereby, the drive apparatus  110  does not change the bias voltage across the control terminal of the main switching element  100  in one go from +Vin to −Vin, but can surely make it 0 V temporarily, and then start the transition to −Vin; therefore, occurrences of noises such as spikes due to switching can be suppressed. 
     The drive control unit  120  may also maintain the control terminal of the main switching element  100  at the negative bias voltage −Vin after turning off of the main switching element  100  ends. Instead of this, as indicated by a broken line in the figure, in switching of the main switching element  100 , the drive control unit  120  may perform switching from a connection through the second path (the connection mode  2 ) to a connection through the third path (the connection mode  3 ), then at a time t 3  perform switching to a connection through the first path (the connection mode  1 ) or a connection through the fourth path (the connection mode  4 ), and then end the switching. If the main switching element  100  is a voltage-controlled switching element, current does not flow almost at all through the path between the control terminal and the second main terminal even if the main switching element  100  is kept negatively biased, but, if the OFF-period of the main switching element  100  is long, power consumption of the power source  130  due to gate leakage current of the main switching element  100  can be suppressed by performing switching of the control terminal from the negatively-biased state to the zero-biased state. In addition, since the period during which a bias voltage is applied to the main switching element  100  can be made shorter, the service life of the main switching element  100  can also be made longer. In addition, if for example there is a finite resistance such as a pull-down resistance between the control terminal and the second main terminal, if the main switching element  100  is a current-controlled switching element, or in other cases also, the drive control unit  120  can suppress power consumption of the power source  130  by the above-mentioned operation. 
     Furthermore, the magnitude of the third path resistance is configured according to a target rate of transition at which the main switching element  100  is turned off, and in some cases is configured to have a value which is large to some extent so as to avoid occurrences of abrupt current changes. In contrast to this, in the steady state that follows after the main switching element  100  is turned off, it is in some cases desirable to make the resistance between the control terminal and second main terminal of the main switching element  100  low so as to increase noise tolerance. In view of this, in switching of the main switching element  100 , after performing switching from the connection through the second path to the connection through the third path, the drive control unit  120  may perform switching to a connection through a path with a smaller path resistance among the connection through the first path and the connection through the fourth path, and then end the switching. Here, if the path resistance of the first path or fourth path used at and after the time t 3  is configured to be smaller than the third path resistance, the drive control unit  120  can maintain, at 0 V, the bias voltage across the control terminal more stably in the steady state that follows after the main switching element  100  is turned off. 
     In the above-mentioned explanations, at least one among the path resistance of the first path or fourth path used between the times t 1  and t 2 , the path resistance of the third path used between the times t 2  and t 3 , and the path resistance of the first path or fourth path used at and after the time t 3  may be configured to have a different resistance such that they become suitable resistances in those individual periods. 
     The drive control unit  120  may have, as fixed values in advance, various types of parameters defining voltage waveforms of the control terminal like those shown above, or instead of this may have the function of making it possible to configure the parameters in a register or the like inside the drive control unit  120  upon receiving an instruction from an external apparatus, may have the function of making it possible to configure the parameters via a terminal of the drive apparatus  110 , and so on. Such parameters are, for example, a period of the connection mode  1  (the connection mode  4 ) provided between the connection mode  2  and the connection mode  3  (the period between the times t 1  and t 2 ), and a period of the connection mode  3  (a period from the times t 2  to t 3 ). 
     In addition, the drive control unit  120  may make it possible to configure which of the connection through the first path and the connection through the fourth path is to be used in the connection state where a connection is established between the control terminal and second main terminal of the main switching element  100  bypassing the power source  130 . The drive control unit  120  may make it possible to perform such configuration individually for each among multiple types of periods such as a turn-off period, a turn-on period, or an OFF-period after turning off. 
     In addition, the drive control unit  120  may use the same path or different paths for making the control terminal zero-biased between the times t 1  and t 2  during a turn-off transition of the main switching element  100 , and for making the control terminal zero-biased at and after the time t 3  after turning off of the main switching element  100 . For example, the drive control unit  120  may use one of the first path and fourth path as a path for turning off both between the times t 1  and t 2  and at and after the time t 3 . In this case, the drive control unit  120  can use the other of the first path and fourth path for another purpose. In addition, the drive control unit  120  may use different path resistances by using one of the first path and the fourth path during a turn-off transition period and the other path after turning off as explained above. 
     Note that the switching sequence of performing switching of the control terminal of the main switching element  100  temporarily to the negatively-biased state at the time of turning on or turning off of the main switching element  100 , and then making it the zero-biased state (or giving it a bias with a small absolute value) and ending the transition can be employed also for various types of drive apparatuses  110  that can output a positive bias voltage, a negative bias voltage and an intermediate bias voltage. That is, for example, such a switching sequence can be employed in a drive apparatus  110  that receives inputs from two or more power sources, or a drive apparatus  110  in which the resistances of the first to fourth connection resistances R 1 - 4  are substantially the same. 
       FIG. 4  shows a second example illustrating transitions of a bias voltage across the control terminal of the main switching element  100  according to the present embodiment. In the present example, the drive apparatus  110  turns on the main switching element  100  by changing, to +Vin, the bias voltage Vgs across the control terminal of the main switching element  100  that is turned off in the first example. 
     If the control terminal of the main switching element  100  is kept negatively biased at the end of turning off of the main switching element  100  shown in the first example, the drive apparatus  110  is in the connection mode  3 . In this case, at a time t 4 , the drive control unit  120  receives an input of the control signal CTL (for example, logic H) instructing to make the main switching element  100  enter the ON-state. Upon receiving it, the drive control unit  120  makes the drive apparatus  110  enter the connection mode  1  (or the connection mode  4 ), and increases the bias voltage Vgs across the control terminal of the main switching element  100  from −Vin to 0 V. If the threshold voltage Vth of the main switching element  100  has a value between Vin and 0 V, the main switching element  100  continues with the OFF state. 
     At a time t 5 , the drive control unit  120  makes the drive apparatus  110  enter the connection mode  2 , increases the bias voltage Vgs to +Vin, and turns on the main switching element  100 . Here, similar to the period between the times t 1  and t 2  in the first example, the drive control unit  120  may set the period of the connection mode  1  or connection mode  4  to a predetermined period or longer such that the period during which the bias voltage across the control terminal of the main switching element  100  is maintained at substantially 0 V between the times t 4  and t 5  becomes a predetermined minimum maintenance period or longer. This minimum maintenance period may be longer than the transition time of a voltage or current between the first to second main terminals that accompanies turning on or turning off of the main switching element  100 , and for example may have a value of several hundred ns or larger. 
     Note that, if the control terminal of the main switching element  100  is zero-biased at the end of turning off of the main switching element  100  shown in the first example, at the time t 5 , the drive control unit  120  may perform the above-mentioned operation by receiving an input of the control signal CTL (for example, logic H) instructing to make the main switching element  100  enter the ON-state. 
     In the above-mentioned explanations, at least one among the path resistance of the first path or fourth path used between the times t 4  and t 5 , and the path resistance of the first path or fourth path used at and after the time t 5  may be configured to have a different resistance such that they become suitable resistances in those individual periods. 
     The drive control unit  120  may have, as fixed values in advance, parameters such as the period of the connection mode  1  (connection mode  4 ) provided between the connection mode  3  and the connection mode  2  (the period between the times t 1  and t 2 ), or, instead of this, it may be made possible to configure parameters for the drive control unit  120 . 
       FIG. 5  shows a third example illustrating transitions of a bias voltage across the control terminal of the main switching element  100  according to the present embodiment. The drive control unit  120  may provide both a period for establishing the connection through the first path and a period for establishing the connection through the fourth path during at least one switching of switching from the connection through the second path to the connection through the third path (for example, switching of the bias voltage from +Vin to −Vin) and switching from the connection through the third path to the connection through the second path (for example, switching of the bias voltage from −Vin to +Vin). Thereby, the drive control unit  120  can more suitably configure the voltage waveform of the control terminal using the first path resistance and fourth path resistance that have different resistances in a transition period of turning on and/or turning off of the main switching element  100 . 
     The present example is a variant of the first example, and, in the present example, the drive control unit  120  provides both a period for establishing the connection through the fourth path and a period for establishing the connection through the first path during switching from the connection through the second path to the connection through the third path (that is, between the times t 1  and t 2 ). At the time t 1 , the drive control unit  120  receives an input of the control signal CTL (for example, logic L) to instruct switching of the main switching element  100  to enter the OFF state. Upon receiving it, the drive control unit  120  makes the first to fourth switching elements SW 1 - 4  enter the connection mode  4 , and starts lowering the bias voltage Vgs across the control terminal of the main switching element  100  from +Vin. The drive control unit  120  lowers the bias voltage Vgs to a value larger than the threshold voltage Vth. Instead of this, the drive control unit  120  may lower the bias voltage Vgs to a voltage equal to or lower than the threshold voltage Vth. 
     At a time t 1 ′, the drive control unit  120  makes the drive apparatus  110  enter the connection mode  1 , and further lowers the bias voltage Vgs from the voltage value observed at the time t 1 ′, and changes it to a zero bias. If the threshold voltage Vth of the main switching element  100  has a value between Vin and 0 V (for example 3 to 8 V), the main switching element  100  is turned off. 
     Here, during switching from the connection through the second path to the connection through the third path, the drive control unit  120  may perform switching to a connection through a path with a smaller resistance among the connection through the first path and the connection through the fourth path after switching to a connection through a path with a larger resistance among the connection through the first path and the connection through the fourth path. In the present example, by setting the fourth path resistance in the connection mode  4  to have a value larger than the first path resistance in the connection mode  1 , changes in the bias voltage between the times t 1  and t 1 ′ become gradual as compared to changes in the bias voltage between the times t 1 ′ and t 2 . Thereby, the drive apparatus  110  can lower the rate of change of the voltage across the first main terminal of the main switching element  100  immediately after starting turning off, and can lower the possibility of occurrences of malfunctions. 
     The drive control unit  120  may have, as fixed values in advance, parameters such as the period of the connection mode  4  provided between the connection mode  2  and the connection mode  3  (the period between the times t 1  and t 1 ′) or the period of the connection mode  1  (the period between the times t 1 ′ and t 2 ), or, instead of this, it may be made possible to configure the parameters. In addition, the drive apparatus  110  may have a comparator that judges whether or not the voltage across the control terminal of the main switching element  100  became equal to or lower than a predetermined voltage, and the drive control unit  120  may detect the timing of the time t 1 ′ using a result of comparison performed by the comparator. 
     In the present example shown, in the period between the times t 1  and t 2 , the drive control unit  120  performs switching of the first to fourth switching elements SW 1 - 4  in the order of the connection modes  4 ,  1 , but the drive control unit  120  may perform switching in the order of the connection modes  1 ,  4 . In addition, in the period between the times t 1  and t 2 , the drive control unit  120  may first perform switching to a connection mode with a smaller path resistance among the connection modes  1  and  4 , and then perform switching to a connection mode with a larger path resistance. Furthermore, in the period between the times t 1  and t 2 , the drive control unit  120  may perform switching between three or more connection modes in order, such as switching in the order of the connection modes  4 ,  1 ,  4 . 
       FIG. 6  shows a fourth example illustrating transitions of a bias voltage across the control terminal of the main switching element  100  according to the present embodiment. The present example is a variant of the second example, and, in the present example, the drive control unit  120  provides both a period for establishing the connection through the fourth path and a period for establishing the connection through the first path during switching from the connection through the third path to the connection through the second path (that is, between the times t 4  and t 5 ). At the time t 4 , the drive control unit  120  receives an input of the control signal CTL (for example, logic H) instructing to make the main switching element  100  enter the ON-state. Upon receiving it, the drive control unit  120  makes the drive apparatus  110  enter the connection mode  4 , and starts increasing the bias voltage Vgs across the control terminal of the main switching element  100  from −Vin. 
     At a time t 4 ′, the drive control unit  120  makes the drive apparatus  110  enter the connection mode  1 , and further increases the bias voltage Vgs from the voltage value observed at the time t 4 ′, and changes it to a zero bias. 
     Here, during switching from the connection through the third path to the connection through the second path, the drive control unit  120  may perform switching to a connection through a path with a smaller resistance among the connection through the first path and the connection through the fourth path after switching to a connection through a path with a larger resistance among the connection through the first path and the connection through the fourth path. In the present example, by setting the fourth path resistance in the connection mode  4  to have a value larger than the first path resistance in the connection mode  1 , changes in the bias voltage between the times t 4  and t 4 ′ become gradual as compared to changes in the bias voltage between the times t 4 ′ and t 5 . Thereby, the drive apparatus  110  can lower the rate of change of the voltage across the first main terminal of the main switching element  100  immediately after starting turning on, and can lower the possibility of occurrences of malfunctions. 
     The drive control unit  120  may have, as fixed values in advance, parameters such as the period of the connection mode  4  provided between the connection mode  3  and the connection mode  2  (the period between the times t 4  and t 4 ′) or the period of the connection mode  1  (the period between the times t 4 ′ and t 5 ), or, instead of this, it may be made possible to configure the parameters for the drive control unit  120 . In addition, the drive apparatus  110  may have a comparator that judges whether or not the voltage across the control terminal of the main switching element  100  became equal to or higher than a predetermined voltage, and the drive control unit  120  may detect the timing of the time t 4 ′ using a result of comparison performed by the comparator. 
     In the present example shown, in the period between the times t 4  and t 5 , the drive control unit  120  performs switching of the first to fourth switching elements SW 1 - 4  in the order of the connection modes  4 ,  1 , but the drive control unit  120  may perform switching in the order of the connection modes  1 ,  4 . In addition, in the period between the times t 4  and t 5 , the drive control unit  120  may first perform switching to a connection mode with a smaller path resistance among the connection modes  1  and  4 , and then perform switching to a connection mode with a larger path resistance. Furthermore, in the period between the times t 4  and t 5 , the drive control unit  120  may perform switching between three or more connection modes in order, such as switching in the order of the connection modes  4 ,  1 ,  4 . 
     Note that, if the main switching element  100  has a positive threshold voltage, and the inductance of the first path and second path is suppressed sufficiently, the drive apparatus  110  can suppress overshoot of the bias voltage Vgs even if a path with a smaller path resistance among the first path and the fourth path is used in the period between the times t 4  and t 5 , and can prevent erroneous turning on of the main switching element  100  in the period between the times t 4  and t 5 . In such a case, the drive control unit  120  may use the bias voltage waveform shown in  FIG. 5  when it turns off the main switching element  100 , and may use the bias voltage waveform shown in  FIG. 4  when it turn on the main switching element  100 . 
       FIG. 7  illustrates a variant of connection modes of the drive apparatus  110  according to the present embodiment. In the present variant, the drive control unit  120  uses connection codes indicating ON/OFF patterns of the first to fourth switching elements SW 1 - 4  to control the connection path between the control terminal and second main terminal of the main switching element  100 . For example, a connection code may have a one-bit field corresponding to each among the first to fourth switching elements SW 1 - 4 , and each bit may have a value indicating whether a corresponding switching element is turned on or turned off. In the present variant, in a connection code, bit  0  (most-significant bit) indicates the ON/OFF state of the first switching element SW 1 , bit  1  indicates the ON/OFF state of the second switching element SW 2 , bit  2  indicates the ON/OFF state of the third switching element SW 3 , and bit  3  (least-significant bit) indicates the ON/OFF state of the fourth switching element SW 4 . Each bit is set to “0” if a corresponding switching element is turned on, and is set to “1” if the corresponding switching element is turned off. 
     The connection modes corresponding to connection codes  3  (binary 0011),  6  (0110),  9  (1001), and  12  (1100) are the same as the connection modes  1 - 4  in  FIG. 2 . 
     In the connection modes corresponding to connection codes  7  (0111),  11  (1011),  13  (1101), and  14  (1110), one of the first to fourth switching elements SW 1 - 4  is turned on, and the rest are turned off. In these connection modes, one of the switching elements which is turned off enters the conducting state via the parasitic diode according to the potential difference between the control terminal and second main terminal of the main switching element  100 . 
     In the connection mode corresponding to the connection code  7 , the first switching element SW 1  is turned on, and the second switching element SW 2  enters the conducting state using the parasitic diode if the potential of the control terminal is lower than the potential of the second main terminal by a difference which is equal to or larger than the forward voltage drop Vd 2  of the parasitic diode. In this case, the bias voltage across the control terminal of the main switching element  100 , when in the stable state, becomes −Vd 2 , and the path resistance between the control terminal and the second main terminal becomes “resistance R 1 +parasitic diode resistance Rd 2 ”. Here, if the parasitic diode causes current to flow within the range of ampacity, the path resistance can be substantially regarded as being the resistance R 1 . 
     In the connection mode corresponding to the connection code  11 , the second switching element SW 2  is turned on, and the first switching element SW 1  enters the conducting state using the parasitic diode if the potential of the control terminal is higher than the potential of the second main terminal by a difference which is equal to or larger than the forward voltage drop Vd 1  of the parasitic diode. In this case, the bias voltage across the control terminal, when in the stable state, becomes Vd 1 , and the path resistance between the control terminal and the second main terminal becomes “parasitic diode resistance Rd 1 +resistance R 2 ”. Here, if the parasitic diode causes current to flow within the range of ampacity, the path resistance can be substantially regarded as being the resistance R 2 . 
     In the connection mode corresponding to the connection code  13 , the third switching element SW 3  is turned on, and the fourth switching element SW 4  enters the conducting state using the parasitic diode if the potential of the control terminal is higher than the potential of the second main terminal by a difference which is equal to or larger than the forward voltage drop Vd 4  of the parasitic diode. In this case, the bias voltage across the control terminal, when in the stable state, becomes Vd 4 , and the path resistance between the control terminal and the second main terminal becomes “resistance R 3 +parasitic diode resistance Rd 4 ”. Here, if the parasitic diode causes current to flow within the range of ampacity, the path resistance can be substantially regarded as being the resistance R 3 . 
     In the connection mode corresponding to the connection code  14 , the fourth switching element SW 4  is turned on, and the third switching element SW 3  enters the conducting state using the parasitic diode if the potential of the control terminal is lower than the potential of the second main terminal by a difference which is equal to or larger than the forward voltage drop Vd 3  of the parasitic diode. In this case, the bias voltage across the control terminal, when in the stable state, becomes −Vd 3 , and the path resistance between the control terminal and the second main terminal becomes “parasitic diode resistance Rd 3 +resistance R 4 ”. Here, if the parasitic diode causes current to flow within the range of ampacity, the path resistance can be substantially regarded as being the resistance R 4 . 
     In the connection modes corresponding to connection codes  0  (0000),  1  (0001),  2  (0010),  4  (0100),  5  (0101),  8  (1000), and  10  (1010), both the first switching element SW 1  and the third switching element SW 3  or both the second switching element SW 2  and the fourth switching element SW 4  are turned on, and the power source  130  short-circuits. Accordingly, the drive control unit  120  may be configured to not use those connection modes, and in the figure the cells for the gate-source voltage Vgs and gate drive resistance are filled with “−”. However, if at least one of the resistance R 1 +R 3  and the resistance R 2 +R 4  is sufficiently large, and failures of and an increase of consumed power of switching elements due to short-circuiting do not become issues, the drive control unit  120  may make use of a connection mode in which two switching elements along a path with a sufficiently large resistance are turned on, and the voltage between the control terminal and second main terminal of the main switching element  100  in such a case becomes like ones explained below. 
     In the connection mode corresponding to the connection code  0 , the voltage across the control terminal of the main switching element  100  in the stable state becomes a voltage obtained by dividing Vin by the resistances R 1  and R 3  using the potential of the negative terminal of the power source  130  as reference potential, and the voltage across the second main terminal of the main switching element  100  becomes a voltage obtained by dividing Vin by the resistances R 2  and R 4  using the potential of the negative terminal of the power source  130  as reference potential. The difference between those voltages becomes the voltage between the control terminal and the second main terminal. 
     In the connection mode corresponding to the connection code  1 , the bias voltage across the control terminal of the main switching element  100  becomes −VinxR 1 /(R 1 +R 3 ) in the stable state. In the connection mode corresponding to the connection code  2 , the bias voltage becomes VinxR 2 /(R 2 +R 4 ) in the stable state. In the connection mode corresponding to the connection code  4 , the bias voltage becomes VinxR 3 /(R 1 +R 3 ) in the stable state. In the connection mode corresponding to the connection code  8 , the bias voltage becomes −VinxR 2 /(R 2 +R 4 ) in the stable state. 
     In the connection mode corresponding to the connection code  5 , the voltage across the control terminal of the main switching element  100  becomes a voltage obtained by dividing the voltage between the positive terminal and negative terminal of the power source  130  by the resistances R 1  and R 3 . For example, by selecting this connection mode in a situation where the parasitic diode of the second switching element SW 2  enters the conducting state, the drive control unit  120  can make the bias voltage across the control terminal of the main switching element  100  equal −VinxR 1 /(R 1 +R 3 )−Vd 2 . In addition, by selecting this connection mode in a situation where the parasitic diode of the fourth switching element SW 4  enters the conducting state, the drive control unit  120  can make the bias voltage equal Vin×R 3 /(R 1 +R 3 )+Vd 4 . 
     In the connection mode corresponding to the connection code  10 , the voltage across the second main terminal of the main switching element  100  becomes a voltage obtained by dividing the voltage between the positive terminal and negative terminal of the power source  130  by the resistances R 2  and R 4 . For example, by selecting this connection mode in a situation where the parasitic diode of the first switching element SW 1  enters the conducting state, the drive control unit  120  can make the bias voltage equal Vin×R 2 /(R 2 +R 4 )+Vd 1 . In addition, by selecting this connection mode in a situation where the parasitic diode of the third switching element SW 3  enters the conducting state, the drive control unit  120  can make the bias voltage −Vin×R 4 /(R 2 +R 4 )−Vd 3 . 
     In the connection mode corresponding to the connection code  15  (binary “1111”), all the first to fourth switching elements SW 1 - 4  are turned off. If a relatively highly resistive protective resistor or the like is connected between the control terminal and second main terminal of the main switching element  100  outside the drive apparatus  110 , the drive control unit  120  may be configured to select this connection mode upon detection of a failure. 
     By using a connection mode corresponding to another connection code as necessary in addition to or instead of each connection mode of the connection code  3 ,  6 ,  9 , or  12 , it becomes possible for the drive apparatus  110  to more finely control the bias voltage and gate-source resistance. 
       FIG. 8  illustrates the configuration of the switching apparatus  10  according to a first variant. Components in the present variant that are given the same signs as those in  FIG. 1  have the same configurations and functions as those in  FIG. 1 , so explanations thereof are not repeated below unless they are necessary. 
     In the configuration employed for the drive apparatus  110 , at least one of the second connection resistance R 2  and the fourth connection resistance R 4  may be larger than the first connection resistance R 1  and third connection resistance R 3 . In the present variant, the drive apparatus  110  further includes a resistor  780  provided in the second connection path and a resistor  790  provided in the fourth connection path. 
     The resistor  780  is one example of a first resistive element, and is connected between the positive terminal of the power source  130  and the second main terminal of the main switching element  100  and in series with the second switching element SW 2 . The resistor  780  may be a resistive element that can be formed on an integrated circuit such as a polysilicon resistor. Thereby, the second connection resistance R 2  in the second connection path becomes the sum of the resistance R 2   a  of the second switching element SW 2  and the resistance R 2   b  of the resistor  780 . The resistor  790  is one example of a second resistive element, and is connected between the second main terminal of the main switching element  100  and the negative terminal of the power source  130  and in series with the fourth switching element. The resistor  790  also may be a resistive element such as a polysilicon resistor that can be formed on an integrated circuit. Thereby, the fourth connection resistance R 4  in the fourth connection path becomes the sum of the resistance R 4   a  of the fourth switching element SW 4  and the resistance R 4   b  of the resistor  790 . 
       FIG. 9  illustrates connection modes of the switching apparatus  10  according to the first variant. In this this figure, the second connection resistance R 2  in  FIG. 2  is replaced with R 2   a +R 2   b , and the fourth connection resistance R 4  in  FIG. 2  is replaced with R 4   a +R 4   b . In other respects, it is the same as  FIG. 2 . 
     By connecting a resistive element(s) in series with a corresponding one(s) among the first to fourth switching elements SW 1 - 4  in at least one of the first to fourth connection paths, a connection resistance of each connection path can be adjusted also if the same switching elements are used. Thereby, a path resistance of each among the connection modes  1 - 4  can be configured to have a suitable value. 
     Note that, instead of or in addition to connecting the resistors in series with the first to fourth switching elements SW 1 - 4 , the ON-resistances of the first to fourth switching elements SW 1 - 4  themselves may be adjusted by changing the channel widths and/or channel lengths of the first to fourth switching elements SW 1 - 4 , and so on. 
     In addition, the configuration employed for the drive apparatus  110  may include one of the resistor  780  and the resistor  790 , for example, may include the resistor  780  but not include the resistor  790  or may not include the resistor  780  but include the resistor  790 , such that a path resistance assumes preferable magnitude in each connection mode. In addition, the drive apparatus  110  may have a resistive element(s) connected in series with a corresponding one(s) of the first to second switching elements SW 1 - 2  in at least one of the first connection path and the second connection path. 
     In the switching apparatus  10  according to the first variant also, at least one of various types of connection modes like those shown in  FIG. 7  may be employed similar to the switching apparatus  10  shown in  FIGS. 1-6 . 
       FIG. 10  illustrates the configuration of the switching apparatus  10  according to a second variant. Components in the present variant that are given the same signs as those in  FIG. 1  have the same configurations and functions as those in  FIG. 1 , so explanations thereof are not repeated below unless they are necessary. 
     In the present variant, the switching apparatus  10  includes two power sources, the power source  130  and a power source  930 . The power source  130  is one example of a first power source, and generates, between the positive terminal and the negative terminal, a power source voltage Vin 1  that the drive apparatus  110  uses for applying a positive bias voltage between the control terminal and second main terminal of the main switching element  100 . The power source  930  is one example of a second power source, and generates, between the positive terminal and the negative terminal, a power source voltage Vin 2  that the drive apparatus  110  uses for applying a negative bias voltage between the control terminal and second main terminal of the main switching element  100 . Here, the power source  930  has an output voltage that is different from that of the power source  130 . 
     In the present variant, the first switching element SW 1  in the drive apparatus  110  is electrically connected between the positive terminal of the power source  130  and the control terminal of the main switching element  100 . The second switching element SW 2  is electrically connected between the positive terminal and second main terminal of the power source  930 . The third switching element SW 3  is electrically connected between the control terminal of the main switching element  100  and the negative terminals of the power source  130  and power source  930 . The fourth switching element is electrically connected between the second main terminal of the main switching element  100  and the negative terminals of the power source  130  and power source  930 . 
     In addition, in the present variant, the drive apparatus  110  further includes a resistor  990  provided in the fourth connection path. The resistor  990  is one example of a second resistive element, and is connected between the second main terminal of the main switching element  100  and the negative terminal of the power source  130  and in series with the fourth switching element. The resistor  990  may be a resistive element such as a polysilicon resistor that can be formed on an integrated circuit. Thereby, the fourth connection resistance R 4  in the fourth connection path becomes the sum of the resistance R 4   a  of the fourth switching element SW 4  and the resistance R 4   b  of the resistor  990 . 
       FIG. 11  illustrates connection modes of the switching apparatus  10  according to the second variant. In this figure, the fourth connection resistance R 4  in  FIG. 2  is replaced with R 4   a +R 4   b , the bias voltage Vgs in the connection mode  1  in  FIG. 2  is replaced with Vin 1 −Vin  2 , and the bias voltage Vgs in the connection mode  3  in  FIG. 2  is replaced with −Vin  2 . In other respects, it is the same as  FIG. 2 . 
     In the connection mode  1 , the drive apparatus  110  forms a first path that establishes a connection between the control terminal and second main terminal of the main switching element  100  via the first switching element SW 1 , power source  130 , power source  930  and fourth switching element SW 4 . In this first path, the power source  130  is included in the forward direction, the power source  930  is included in the reverse direction, and the voltage between the control terminal and the second main terminal can be regarded as being the power source voltage Vin 1 −Vin  2 , when in the stable state. 
     In the present variant, the drive apparatus  110  can provide four types of the bias voltage Vgs to the control terminal of the main switching element  100  using the two power sources, the power source  130  and power source  930 . By properly using two types of intermediate biases of the connection mode  1  and connection mode  4  in addition to a positive bias in the connection mode  2  and a negative bias in the connection mode  3  for different purposes, the drive control unit  120  can make the waveform of the bias voltage a suitable waveform. For example, if Vin  1 &gt;Vin  2 , it becomes also possible for the drive control unit  120  to use the connection mode  1  in the period between the times t 1  and t 1 ′ in  FIG. 5  to lower the bias voltage Vgs to Vin 1 −Vin  2 , and use the connection mode  4  in the period between the times t 1 ′ and t 2  to lower the bias voltage Vgs to 0 V. In addition, for example, if Vin  1 &lt;Vin  2 , it becomes also possible for the drive control unit  120  to use the connection mode  1  in the period between the times t 3  and t 3 ′ in  FIG. 5  to increase the bias voltage Vgs to Vin 1 −Vin  2 , and use the connection mode  4  in the period at and after the time t 3  to increase the bias voltage Vgs to 0 V. 
     Note that, in the switching apparatus  10  according to the second variant also, at least one of various types of connection modes like those shown in  FIG. 7  may be employed similar to the switching apparatus  10  shown in  FIGS. 1-6 . 
       FIG. 12  illustrates the configuration of the switching apparatus  10  according to a third variant. The switching apparatus  10  according to the present variant includes main switching elements  1100   a - b , drive apparatuses  1110   a - b  and power sources  1130   a - b . In the present variant, the main switching elements  1100   a - b  are the same as the main switching element  100  in  FIG. 1 , the drive apparatuses  1110   a - b  are almost the same as the drive apparatus  110  in  FIG. 1 , and the power sources  1130   a - b  are the same as the power source  130  in  FIG. 1 , so explanations thereof are not repeated below except for those related to differences therebetween. 
     The main switching elements  1100   a - b  are connected in series such that the first main terminal of the main switching element  1100   a  on the lower side and the second main terminal of the main switching element  1100   b  on the higher side are connected, and an output voltage Vout is output from the output terminal between the main switching element  1100   a  and the main switching element  1100   b . If control is performed such that the main switching element  1100   a  is turned on and the main switching element  1100   b  is turned off, the output terminal is electrically connected to the second main terminal side of the main switching element  1100   a , and if control is performed such that the main switching element  1100   a  is turned off and the main switching element  1100   b  is turned on, the output terminal is electrically connected to the first main terminal side of the main switching element  1100   b.    
     Upon being supplied with the power source voltage Vin from the power source  1130   a , the drive apparatus  1110   a  drives the control terminal of the main switching element  1100   a  according to a control signal CTL 1  input from the outside. The drive apparatus  1110   a  according to the present variant also receives an input of a control signal CTL 2  for controlling the main switching element  1100   b , which is an opposite switching element connected in series with the main switching element  1100   a  which is a control target. The drive control unit  120  in the drive apparatus  1110   a  has the function of driving the control terminal of the main switching element  1100   a  according to the control signal CTL 2 . This function is described below in explanations with reference to  FIG. 13 . 
     Upon being supplied with the power source voltage Vin from the power source  1130   b , the drive apparatus  1110   b  drives the control terminal of the main switching element  1100   b  according to the control signal CTL 2  input from the outside. The drive apparatus  1110   b  according to the present variant also receives an input of the control signal CTL 1  for controlling the main switching element  1100   a , which is an opposite switching element connected in series with the main switching element  1100   b  which is a control target. The drive control unit  120  in the drive apparatus  1110   b  has the function of driving the control terminal of the main switching element  1100   b  according to the control signal CTL 1 , and this function is described below in explanations with reference to  FIG. 13 . 
     The power sources  1130   a - b  generate the power source voltage Vin between the positive terminal and the negative terminal, and supplies it to the drive apparatuses  1110   a - b.    
       FIG. 13  illustrates transitions of bias voltages across control terminals of the main switching elements  1100   a - b  according to the third variant. In the present variant, the drive control units  120  in the drive apparatuses  1110   a - b  have the function of performing switching from the connection through the first path or the connection through the fourth path to the connection through the third path before switching of an opposite switching element (the main switching element  1100   b  or  1100   a ) connected in series with the main switching element  1100   a  or  1100   b  which is the control target in an OFF-period of the main switching element  1100   a  or  1100   b  which is the control target (an ON-period in a main switching element  1100  which is turned off if Vgs&gt;Vth). 
     The lower half of this figure illustrates temporal changes of the bias voltage Vgs applied to the control terminal of the main switching element  1100   a , the connection mode of the drive apparatus  1110   a , and the ON/OFF state of the main switching element  1100   a . The upper half of this figure illustrates temporal changes of the bias voltage Vgs applied to the control terminal of the main switching element  1100   b , the connection mode of the drive apparatus  1110   b , and the ON/OFF state of the main switching element  1100   b.    
     At and before the time t 1 , the drive control unit  120  in the drive apparatus  1110   a  makes the drive apparatus  1110   a  enter the connection mode  4 , and makes the main switching element  1100   a  enter the OFF state. The drive control unit  120  in the drive apparatus  1110   b  makes the drive apparatus  1110   b  enter the connection mode  2 , and makes the main switching element  1100   b  enter the ON-state. At the time t 1 , upon receiving an input of the control signal CTL 2  (for example, logic L) instructing to make the main switching element  1100   b  enter the OFF state, the drive control unit  120  in the drive apparatus  1110   b  accordingly turns off the main switching element  1100   b  over the time range between the times t 2  and t 5  that follows micro time after the time t 1 . This process related to turning off and the waveform of a bias voltage are the same as those in  FIG. 3 , so explanations thereof are not repeated. 
     Upon receiving, at the time t 1 , an input of the control signal CTL 2  (for example, logic L) instructing to make the main switching element  1100   b , which is the opposite switching element, enter the OFF state, the drive control unit  120  in the drive apparatus  1110   a  makes the drive apparatus  1110   a  enter the connection mode  3  before switching of the main switching element  1100   b , and performs switching from the connection through the fourth path (or the connection through the first path) to the connection through the third path. After maintaining the connection through the third path until the transition period of turning off of the main switching element  1100   b  ends, that is, for example until the time t 5 , the drive control unit  120  in the drive apparatus  1110   a  may perform switching back of the drive apparatus  1110   a  to the connection mode  1  or  4 , and continue with the OFF state of the main switching element  1100   a.    
     In addition, upon receiving, at the time t 6 , an input of the control signal CTL 1  (for example, logic H) instructing to make the main switching element  1100   a  enter the ON-state, the drive control unit  120  in the drive apparatus  1110   a  accordingly turns on the main switching element  1100   a  over the time range between the times t 7  and t 8  that follows micro time after the time t 6 . This process related to turning on and the waveform of a bias voltage are the same as those in  FIG. 4 , so explanations thereof are not repeated. 
     Upon receiving, at the time t 6 , an input of the control signal CTL 1  (for example, logic H) instructing to make the main switching element  1100   a  enter the ON-state, the drive control unit  120  in the drive apparatus  1110   b  makes the drive apparatus  1110   b  enter the connection mode  3  before switching of the main switching element  1100   a , and performs switching from the connection through the fourth path (or the connection through the first path) to the connection through the third path. After maintaining the connection through the third path until the transition period of turning on of the main switching element  1100   a  ends, that is, for example until the time t 8 , the drive control unit  120  in the drive apparatus  1110   b  may perform switching back of the drive apparatus  1110   b  to the connection mode  1  or  4 , and continue with the OFF state of the main switching element  1100   b.    
     With the drive apparatuses  1110   a - b  according to the present variant, by applying a negative bias voltage to the control terminals of the main switching elements  1100   a - b  before switching of an opposite switching element, it is possible to prevent the main switching element  1100  from being erroneously turned on due to noises or the like that result from abrupt current changes generated by switching of the opposite switching element. 
       FIG. 14  illustrates the configuration of the switching apparatus  10  according to a fourth variant. The switching apparatus  10  according to the present variant includes the main switching elements  1100   a - b , the drive apparatuses  1110   a - b , a power source  1330 , and a diode  1370 . Components in the present variant that are given the same signs as those in  FIG. 12  have the same configurations as those in  FIG. 12 , so explanations thereof are not repeated below except for those related to differences. 
     The power source  1330  generates the power source voltage Vin between the positive terminal and the negative terminal. Similar to the power source  1130   a  in  FIG. 12 , the capacitor C 1  has a positive terminal that is connected to the first main terminals of the first to second switching elements SW 1 - 2  in the drive apparatus  1110   a , and has a negative terminal that is connected to the second main terminals of the third to fourth switching element SW 3 - 4  in the drive apparatus  1110   a . The capacitor C 1  is charged by the power source voltage Vin of the power source  1330 , and functions as a power source for the drive apparatus  1110   a.    
     Similar to the power source  1130   b  in  FIG. 12 , the capacitor C 2  has a positive terminal that is connected to the first main terminals of the first to second switching elements SW 1 - 2  in the drive apparatus  1110   b , and has a negative terminal that is connected to the second main terminals of the third to fourth switching element SW 3 - 4  in the drive apparatus  1110   b . The capacitor C 2  is charged by the power source voltage Vin of the power source  1330 , and functions as a power source for the drive apparatus  1110   b.    
     The diode  1370  has an anode that is connected to the positive terminal side of the power source  1130 , has a cathode that is connected to the positive terminal side of the capacitor C 2 , causes charging current to flow from the power source  1130  to the capacitor C 2 , and prevents reverse flow of current from the drive apparatus  1110   b  to the power source  1130  side. 
     With the above-mentioned configuration, the main switching element  1000  can use the single power source  1330  to charge the capacitors C 1 -C 2 , and supply the drive apparatuses  1110   a - b  with a power source voltage. Here, the capacitor C 1  can be directly charged by the power source  1130 . However, the potential on the second main terminal side of the main switching element  1100   b  varies according to switching of the main switching elements  1100   a - b , and accordingly the potential of the negative terminal of the capacitor C 2  also varies; therefore, the period during which the capacitor C 2  can be charged is limited. In the present variant, the drive control unit  120  in the drive apparatus  1110   b  makes the drive apparatus  1110   b  enter the connection mode  4  while the drive apparatus  1110   a  is in the connection mode  2  and the main switching element  1100   a  is in the ON-state. At this time, the portion between the second positive terminal of the main switching element  1100   a , and the capacitor C 1  and the negative terminal of the power source  1330  enters the conducting state via the fourth switching element SW 4  in the ON-state, the portion between the second positive terminal of the main switching element  1100   a  and the second positive terminal of the main switching element  1100   b  enters the conducting state via the main switching element  1100   a  in the ON-state, and the portion between the second positive terminal of the main switching element  1100   b  and the negative terminal of the capacitor C 2  enters the conducting state via the fourth switching element SW 4  in the ON-state. Accordingly, the negative terminal of the capacitor C 2  is at almost the same potential as the negative terminal of the power source  1330 , the power source voltage Vin is supplied from the positive terminal of the power source  1330  to the capacitor C 2  via the diode  1370 , and the capacitor C 2  is charged to the voltage Vin. 
     In this manner, by providing the capacitors C 1 -C 2  in the drive apparatuses  1110   a - b , and appropriately selecting the connection modes in the drive apparatuses  1110   a - b , the switching apparatus  10  can use the single power source  1330  to drive the control terminals of the main switching elements  1100   a - b  connected in series. 
     Note that, in the present variant, the drive apparatus  1110   b  is caused to enter the connection mode  4  to charge the capacitor C 2  while the drive apparatus  1110   a  is in the connection mode  2 . Because of this, in the present variant, the drive control unit  120  in the drive apparatus  1110   b  may not have the function of making the drive apparatus  1110   b  enter the connection mode  3  in the period between the times t 7  and t 8  in  FIG. 13 . 
     Various embodiments of the present invention may be described with reference to flowcharts and block diagrams whose blocks may represent sections of apparatuses responsible for performing operations. Certain sections may be implemented by dedicated circuitry, programmable circuitry supplied with computer-readable instructions stored on computer-readable media, and/or processors supplied with computer-readable instructions stored on computer-readable media. Dedicated circuitry may include digital and/or analog hardware circuits and may include integrated circuits (IC) and/or discrete circuits. Programmable circuitry may include reconfigurable hardware circuits comprising logical AND, OR, XOR, NAND, NOR, and other logical operations, flip-flops, registers, memory elements, etc., such as field-programmable gate arrays (FPGA), programmable logic arrays (PLA), etc. 
     Computer-readable media may include any tangible device that can store instructions for execution by a suitable device, such that the computer-readable medium having instructions stored therein comprises an article of manufacture including instructions which can be executed to create means for performing operations specified in the flowcharts or block diagrams. Examples of computer-readable media may include an electronic storage medium, a magnetic storage medium, an optical storage medium, an electromagnetic storage medium, a semiconductor storage medium, etc. More specific examples of computer-readable media may include a floppy disk, a diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an electrically erasable programmable read-only memory (EEPROM), a static random access memory (SRAM), a compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a BLU-RAY(registered trademark) disc, a memory stick, an integrated circuit card, etc. 
     Computer-readable instructions may include assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, JAVA (registered trademark), C++, etc., and conventional procedural programming languages, such as the “C” programming language or similar programming languages. 
     Computer-readable instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus, or to programmable circuitry, locally or via a local area network (LAN), wide area network (WAN) such as the Internet, etc., to execute the computer-readable instructions to create means for performing operations specified in the flowcharts or block diagrams. Examples of processors include computer processors, processing units, microprocessors, digital signal processors, controllers, microcontrollers, etc. 
     While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention. 
     The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order.