Patent Publication Number: US-2022213858-A1

Title: Internal combustion engine ignition device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is the U.S. bypass application of International Application No. PCT/JP2020/029937 filed on Aug. 5, 2020, which designated the U.S. and claims priority to Japanese Patent Application No. 2019-174921, filed on Sep. 26, 2019, the contents of both of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an internal combustion engine ignition device. 
     BACKGROUND 
     An internal combustion engine ignition device includes an ignition coil having a primary coil and a secondary coil, an ignition plug for generating a spark discharge in a combustion chamber of an internal combustion engine by the ignition device, an ignition circuit to perform energization and interruption of energization to the primary coil, and others. Also, after an induced electromotive force has been generated in the secondary coil in response to interruption of energization to the primary coil, a discharge current by the induced electromotive force is maintained to lengthen a discharge time of the spark discharge in the ignition plug. 
     SUMMARY 
     A first aspect of the present disclosure is an internal combustion engine ignition device including an ignition coil having a primary coil to be applied with a DC voltage by a DC power source and a secondary coil to generate an induced electromotive force in response to interruption of energization to the primary coil, an ignition plug for generating a spark discharge in a combustion chamber of an internal combustion engine by the induced electromotive force, a main ignition circuit having a first switching element that performs energization and interruption of energization to a first coil part, which constitutes at least a portion of the primary coil, for generating the induced electromotive force using the DC voltage, and an energy input circuit having a second switching element that performs energization and interruption of energization to a second coil part, which constitutes at least a portion of the primary coil, for keeping a discharge current in the secondary coil within an intended range directly using the DC voltage after the induced electromotive force has been generated, and a soft-off circuit to slow a turn-off speed of the second switching element. 
     The energy input circuit is configured to, when decreasing a signal voltage added to a gate of the second switching element, execute a first decreasing stage of decreasing the signal voltage until it reaches the vicinity of a gate-source threshold voltage and a second decreasing stage of gradually decreasing the signal voltage in the vicinity of the threshold voltage. 
     A second aspect of the present disclosure is an internal combustion engine ignition device including an ignition coil having a primary coil to be applied with a DC voltage by a DC power source and a secondary coil to generate an induced electromotive force in response to interruption of energization to the primary coil, an ignition plug for generating a spark discharge in a combustion chamber of an internal combustion engine by the induced electromotive force, a main ignition circuit having a first switching element that performs energization and interruption of energization to a first coil part, which constitutes at least a portion of the primary coil, for generating the induced electromotive force using the DC voltage, and an energy input circuit having a second switching element that performs energization and interruption of energization to a second coil part, which constitutes at least a portion of the primary coil, for keeping a discharge current in the secondary coil within an intended range directly using the DC voltage after the induced electromotive force has been generated, the energy input circuit being configured to make a turn-off speed of the second switching element slower than a turn-on speed of the second switching element. 
     The energy input circuit is configured to, when decreasing a signal voltage added to a gate of the second switching element, execute a first decreasing stage of decreasing the signal voltage until it reaches the vicinity of a gate-source threshold voltage and a second decreasing stage of gradually decreasing the signal voltage in the vicinity of the threshold voltage. 
     A third aspect of the present disclosure is an internal combustion engine ignition device including an ignition coil having a primary coil to be applied with a DC voltage by a DC power source and a secondary coil to generate an induced electromotive force in response to interruption of energization to the primary coil, an ignition plug for generating a spark discharge in a combustion chamber of an internal combustion engine by the induced electromotive force, a main ignition circuit having a first switching element that performs energization and interruption of energization to a first coil part, which constitutes at least a portion of the primary coil, for generating the induced electromotive force using the DC voltage, and an energy input circuit having a second switching element that controls an energization state to a second coil part, which constitutes at least a portion of the primary coil, for keeping a discharge current in the secondary coil at an intended value directly using the DC voltage after the induced electromotive force has been generated, the energy input circuit being configured to maintain a state in which a voltage applied to the second coil part by the second switching element is lower than the DC voltage. The energy input circuit is configured to gradually increase a gate-source voltage of the second switching element in the vicinity of a threshold voltage thereby to limit and gradually increase a current flowing between the drain and the source of the second switching element, such that the discharge current of the secondary coil is kept at a certain value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features of the present disclosure will be made clearer by the following detailed description, given referring to the appended drawings. In the accompanying drawings: 
         FIG. 1  is a circuit diagram illustrating an internal combustion engine ignition device according to a first embodiment; 
         FIG. 2  is a schematic diagram illustrating a peripheral components of an internal combustion engine according to the first embodiment; 
         FIG. 3  is a timing chart illustrating an action of the internal combustion engine ignition device in a combustion step of an internal combustion engine according to the first embodiment; 
         FIG. 4  is a timing chart illustrating an action of maintaining a discharge current of a secondary coil using a second switching element of an energy input circuit according to the first embodiment; 
         FIG. 5  is a graph illustrating a relationship between a gate voltage (gate-source voltage) and a drain current (drain-source current) according to the first embodiment; 
         FIG. 6  is a flowchart illustrating an action of an internal combustion engine ignition device according to the first embodiment; 
         FIG. 7  is a circuit diagram illustrating a configuration of a soft-off circuit of an energy input circuit according to a second embodiment; 
         FIG. 8  is a timing chart illustrating an action of maintaining a discharge current of a secondary coil using a second switching element of an energy input circuit according to the second embodiment; 
         FIG. 9  is a circuit diagram illustrating a configuration of a soft-off circuit of an energy input circuit according to a third embodiment; 
         FIG. 10  is a timing chart illustrating an action of maintaining a discharge current of a secondary coil using a second switching element of an energy input circuit according to the third embodiment; 
         FIG. 11  is a circuit diagram illustrating a configuration of a soft-off circuit of an energy input circuit according to a fourth embodiment; 
         FIG. 12  is a timing chart illustrating an action of an internal combustion engine ignition device in a combustion step of an internal combustion engine according to the fourth embodiment; and 
         FIG. 13  is a graph illustrating a relationship between a drain-source voltage and a drain current according to the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For example, in the internal combustion engine ignition device disclosed in WO2017/006487 A, a primary coil includes a main primary coil and a sub primary coil. The main primary coil generates an energization magnetic flux in a positive direction by energization from a DC power source and thereafter generates an interruption magnetic flux in a reverse direction by interruption of energization. The sub primary coil generates an additional magnetic flux in the same direction as that of the interruption magnetic flux by energization from the DC power source. 
     Then, the energization to the main primary coil is interrupted by a main semiconductor switch to generate a discharge spark in an ignition plug. In a discharge period after this interruption timing, the sub primary coil is energized by a sub semiconductor switch for a predetermined superimposed time to increase the discharge current generated in the secondary coil in a superimposed manner. The sub semiconductor switch repeats energization and interruption of energization to the sub primary coil such that the discharge current is within a range between a predetermined upper limit value and a predetermined lower limit value. 
     In the internal combustion engine ignition device of WO2017/006487 A, it became clear that a voltage at both ends of the sub primary coil oscillates to a large extent in response to interruption of an energization state to the sub primary coil by the sub semiconductor switch. Furthermore, while the voltage at both ends of the sub primary coil is lower than a current-inputtable voltage by the sub semiconductor switch due to the oscillation, a current cannot be input to the sub primary coil even when the sub semiconductor switch is turned on. In other words, input of a current to the sub primary coil for continuing a discharge current of the secondary coil comes to be delayed until the voltage at both ends of the sub primary coil recovers to equal to or more than a current-inputtable voltage by the sub semiconductor switch. 
     During a period in which input of a current to the sub primary coil is delayed, the discharge current continues to decrease, which may cause the discharge current to become lower than a desired control lower limit. In order to control the discharge current not to become lower than a desired control lower limit, the control upper limit value of the discharge current needs to be increased. However, increasing the control upper limit value of the discharge current may uselessly consume electric energy. 
     The present disclosure has been made in view of such a problem and achieved in an attempt to provide an internal combustion engine ignition device that appropriately inputs a current to the primary coil for continuing the discharge current of the secondary coil and suppresses consumption of electric energy. 
     A first aspect of the present disclosure is an internal combustion engine ignition device including an ignition coil having a primary coil to be applied with a DC voltage by a DC power source and a secondary coil to generate an induced electromotive force in response to interruption of energization to the primary coil, an ignition plug for generating a spark discharge in a combustion chamber of an internal combustion engine by the induced electromotive force, a main ignition circuit having a first switching element that performs energization and interruption of energization to a first coil part, which constitutes at least a portion of the primary coil, for generating the induced electromotive force using the DC voltage, and an energy input circuit having a second switching element that performs energization and interruption of energization to a second coil part, which constitutes at least a portion of the primary coil, for keeping a discharge current in the secondary coil within an intended range directly using the DC voltage after the induced electromotive force has been generated, and a soft-off circuit to slow a turn-off speed of the second switching element. 
     The energy input circuit is configured to, when decreasing a signal voltage added to a gate of the second switching element, execute a first decreasing stage of decreasing the signal voltage until it reaches the vicinity of a gate-source threshold voltage and a second decreasing stage of gradually decreasing the signal voltage in the vicinity of the threshold voltage. 
     A second aspect of the present disclosure is an internal combustion engine ignition device including an ignition coil having a primary coil to be applied with a DC voltage by a DC power source and a secondary coil to generate an induced electromotive force in response to interruption of energization to the primary coil, an ignition plug for generating a spark discharge in a combustion chamber of an internal combustion engine by the induced electromotive force, a main ignition circuit having a first switching element that performs energization and interruption of energization to a first coil part, which constitutes at least a portion of the primary coil, for generating the induced electromotive force using the DC voltage, and an energy input circuit having a second switching element that performs energization and interruption of energization to a second coil part, which constitutes at least a portion of the primary coil, for keeping a discharge current in the secondary coil within an intended range directly using the DC voltage after the induced electromotive force has been generated, the energy input circuit being configured to make a turn-off speed of the second switching element slower than a turn-on speed of the second switching element. 
     The energy input circuit is configured to, when decreasing a signal voltage added to a gate of the second switching element, execute a first decreasing stage of decreasing the signal voltage until it reaches the vicinity of a gate-source threshold voltage and a second decreasing stage of gradually decreasing the signal voltage in the vicinity of the threshold voltage. 
     A third aspect of the present disclosure is an internal combustion engine ignition device including an ignition coil having a primary coil to be applied with a DC voltage by a DC power source and a secondary coil to generate an induced electromotive force in response to interruption of energization to the primary coil, an ignition plug for generating a spark discharge in a combustion chamber of an internal combustion engine by the induced electromotive force, a main ignition circuit having a first switching element that performs energization and interruption of energization to a first coil part, which constitutes at least a portion of the primary coil, for generating the induced electromotive force using the DC voltage, and an energy input circuit having a second switching element that controls an energization state to a second coil part, which constitutes at least a portion of the primary coil, for keeping a discharge current in the secondary coil at an intended value directly using the DC voltage after the induced electromotive force has been generated, the energy input circuit being configured to maintain a state in which a voltage applied to the second coil part by the second switching element is lower than the DC voltage. 
     The energy input circuit is configured to gradually increase a gate-source voltage of the second switching element in the vicinity of a threshold voltage thereby to limit and gradually increase a current flowing between the drain and the source of the second switching element, such that the discharge current of the secondary coil is kept at a certain value. 
     Internal Combustion Engine Ignition Device of First Aspect 
     In the internal combustion engine ignition device of the first aspect, the energy input circuit has a soft-off circuit that slows a turn-off speed of the second switching element. The second switching element performs energization and interruption of energization to the second coil part of the primary coil for keeping the discharge current in the secondary coil within an intended range after the induced electromotive force has been generated in the secondary coil. 
     When the turn-off speed of the second switching element is slowed by the soft-off circuit, oscillation of a voltage at both ends of the second coil part of the primary coil can be suppressed when energization to the second switching element is interrupted, that is, when the second switching element is turned off. This can prevent a voltage at both ends of the second coil part of the primary coil from becoming lower than a current-inputtable voltage by the second switching element when energization to the second switching element is interrupted. 
     Therefore, after energization to the second coil part of the primary coil has been interrupted by the second switching element, energization to the second coil part of the primary coil is quickly resumed by the second switching element. As a result, input of a current to the second coil part of the primary coil for continuing the discharge current of the secondary coil is quickly performed. Consequently, the discharge current can be controlled such that it does not become lower than the control lower limit value without increasing the control upper limit value of the discharge current, and consumption of electric energy is prevented from increasing. 
     Also, energization to the second coil part of the primary coil by the second switching element and the soft-off circuit is performed directly using a DC voltage of the DC power source. Furthermore, a circuit to boost the DC voltage is not used for energizing the second coil part of the primary coil. This suppresses, for example, an increase in size and cost of a device for continuing the discharge current of the secondary coil. 
     Therefore, according to the internal combustion engine ignition device of the first aspect, input of a current to the primary coil for continuing the discharge current of the secondary coil is adequately performed, and consumption of electric energy is suppressed. 
     Internal Combustion Engine Ignition Device of Second Aspect 
     In the internal combustion engine ignition device of the second aspect, the energy input circuit is configured to make a turn-off speed of the second switching element slower than a turn-on speed of the second switching element. This configuration enables oscillation of a voltage at both ends of the second coil part of the primary coil to become suppressed when the second switching element is turned off, similarly to the internal combustion engine ignition device of the first aspect. The turn-off speed indicates a speed at which the second switching element is turned from on to off, and the turn-on speed indicates a speed at which the second switching element is turned from off to on. 
     Therefore, according to the internal combustion engine ignition device of the second aspect, input of a current to the primary coil for continuing the discharge current of the secondary coil is also adequately performed, and consumption of electric energy is suppressed. 
     Internal Combustion Engine Ignition Device of Third Aspect 
     In the internal combustion engine ignition device of the third aspect, the energy input circuit is configured such that a voltage applied to the second coil part of the primary coil by the second switching element is kept in a state of being lower than the DC voltage of the DC power source. This state can be formed by, for example, forming a state in which the second switching element does not become completely on. This configuration enables oscillation of a voltage at both ends of the second coil part of the primary coil to become suppressed when the energization state of the second switching element is controlled. 
     Therefore, according to the internal combustion engine ignition device of the third aspect, input of a current to the primary coil for continuing the discharge current of the secondary coil is also adequately performed, and consumption of electric energy is suppressed. 
     It is noted that although a parenthesized reference sign of each constituent illustrated in the internal combustion engine ignition device of the present disclosure represents a correspondence relation with a reference sign in the drawing in each embodiment, each constituent is not limited to only the contents of each embodiment. 
     Preferable embodiments of the above-described internal combustion engine ignition device will be described with reference to the drawings. 
     First Embodiment 
     An internal combustion engine ignition device  1  of the present embodiment (hereinafter, merely referred to as an ignition device  1  includes, as illustrated in  FIG. 1  and  FIG. 2 , an ignition coil  2 , an ignition plug  3 , a main ignition circuit  5 , and an energy input circuit  6 . The ignition coil  2  has a primary coil  21  to be applied with a DC voltage VB by a DC power source  11  and a secondary coil  22  to generate an induced electromotive force in response to interruption of energization to the primary coil  21 . The ignition plug  3  generates a spark discharge in a combustion chamber  81  of an internal combustion engine  8  by the induced electromotive force. 
     As illustrated in  FIG. 1  and  FIG. 3 , the main ignition circuit  5  has a first switching element  51  that performs energization and interruption of energization to a first coil part  211 , which constitutes a portion of the primary coil  21 , for generating an induced electromotive force using the DC voltage VB. The energy input circuit  6  has a second switching element  61  and a soft-off circuit  62 . The second switching element  61  performs energization and interruption of energization to a second coil part  212 , which constitutes another portion of the primary coil  21 , for keeping a discharge current I 2  in the secondary coil  22  within an intended range Ir directly using the DC voltage VB after the induced electromotive force has been generated. The soft-off circuit  62  is configured to slow a turn-off speed of the second switching element  61 . 
     Hereinafter, the ignition device  1  of the present embodiment will be described in detail. As illustrated in  FIG. 2 , the internal combustion engine  8  is an engine having a plurality of cylinders, and the ignition device  1  is used for igniting a fuel-gas mixture in the combustion chamber  81  of each cylinder of an engine in a vehicle. 
     Ignition Coil  2   
     As illustrated in  FIG. 1 , the primary coil  21  of the ignition coil  2  has the first coil part  211  and the second coil part  212  that is connected to the first coil part  211  and generates a magnetic flux in the same direction as a magnetic flux generated in response to interruption of energization of the first coil part  211 . One end of the first coil part  211  is connected to the DC power source  11  through a diode  12 , and the other end of the first coil part  211  is connected to the first switching element  51 . One end of the second coil part  212  is connected to the DC power source  11  through the diode  12 , and the other end of the first coil part  211  is connected to the second switching element  61 . In other words, the DC power source  11  is connected to a position between the first coil part  211  and the second coil part  212  through the diode  12 . The DC power source  11  is a power source mounted on a vehicle and constituted by a battery of 12 V, 24 V, or the like, a power source circuit, or others. 
     The secondary coil  22  of the ignition coil  2  is formed by winding a wire thinner than a wire constituting the primary coil  21  with the number of turns larger than the number of turns of the wire constituting the primary coil  21 . The secondary coil  22  is disposed concentrically to the primary coil  21 . In response to interruption of energization to the first coil part  211  of the primary coil  21 , an induced electromotive force is generated in the secondary coil  22  such that a change in magnetic flux in the first coil part  211  can be prevented by mutual induction effects. 
     Ignition Plug  3   
     As illustrated in  FIG. 1  and  FIG. 2 , the ignition plug  3  is connected to the secondary coil  22  in the ignition coil  2  and generates a spark discharge by the discharge current I 2  generated in the secondary coil  22 . The ignition plug  3  has a center electrode connected to the secondary coil  22  and an earth electrode connected to a ground GND. A discharge gap  31  between the center electrode and the earth electrode is disposed in the combustion chamber  81  of each cylinder. While the discharge current I 2  flows through the secondary coil  22 , a spark discharge is generated at the discharge gap  31  in the ignition plug  3 . 
     The ignition device  1  of the present embodiment does not have, for example, a booster circuit to boost the DC voltage VB of the DC power source  11 . Furthermore, as previously described, one end of the second coil part  212  of the primary coil  21  is directly connected to the DC voltage VB of the DC power source  11  through the diode  12 . The first coil part  211  and the second coil part  212  of the primary coil  21  are configured such that the DC voltage VB of the DC power source  11  is directly used for a current to flow. 
     Ignition Control Circuit  4 , Main Ignition Circuit  5 , Energy Input Circuit  6 , and Electronic Control Unit  7   
     As illustrated in  FIG. 1 , the main ignition circuit  5  and the energy input circuit  6  are activated by an ignition control circuit  4  that receives a control command from an electronic control unit (ECU)  7  constituted by a computer. The electronic control unit  7  is connected to the ignition control circuit  4  that performs ignition control of each cylinder of an engine, and the main ignition circuit  5  and the energy input circuit  6  are connected to the ignition control circuit  4 . An ignition signal IGt and a discharge signal IGw, which are a control command by the electronic control unit  7 , are transmitted to the ignition control circuit  4 . The ignition control circuit  4  also includes a current detection circuit part  41  that detects the discharge current I 2  flowing through the secondary coil  22 . The current detection circuit part  41  detects a voltage generated in a resistor  13  for detecting the discharge current I 2 . 
     In response to reception of the ignition signal IGt and the discharge signal IGw, which are a control command from the electronic control unit  7 , the ignition control circuit  4  outputs a gate voltage (gate-emitter voltage) to the first switching element  51  of the main ignition circuit  5  and a gate signal voltage Vg to the second switching element  61  of the energy input circuit  6 . Also, the ignition control circuit  4  compares the discharge current I 2  detected by the current detection circuit part  41  to a control upper limit value Imax and a control lower limit value Imin of discharge current maintenance control to generate the gate signal voltage Vg and outputs the generated gate signal voltage Vg to the energy input circuit  6 . 
     As illustrated in  FIG. 1 , the main ignition circuit  5  is configured to perform energization control to the first coil part  211  of the primary coil  21  and may have an element other than the first switching element  51 , an electronic component, and others. The first switching element  51  of the main ignition circuit  5  is constituted by an IGBT (insulated-gate bipolar transistor) or others. A gate G of the first switching element  51  is connected with the ignition control circuit  4 , and a collector C of the first switching element  51  is connected with one end of the first coil part  211 . Also, an emitter E of the first switching element  51  is connected to the ground GND. 
     The energy input circuit  6  is configured to perform energization control to the second coil part  212  of the primary coil  21  and may have an element other than the second switching element  61 , an electronic component, and others. The second switching element  61  of the energy input circuit  6  is constituted by a MOSFET (MOS type field effect transistor) or others. A gate G of the second switching element  61  is connected with the ignition control circuit  4  through the soft-off circuit  62 , and a drain D of the second switching element  61  is connected with one end of the second coil part  212 . Also, a source S of the second switching element  61  is connected to the ground GND. It is noted that the soft-off circuit  62  may be contained in the ignition control circuit  4 . 
     The ignition control circuit  4  controls the gate signal voltage Vg transmitted to the second switching element  61  of the energy input circuit  6 , such that the discharge current I 2  flowing through the secondary coil  22  is kept within the intended range Ir between the control upper limit value Imax and the control lower limit value Imin, after a spark discharge has been generated in the secondary coil  22 . The ignition control circuit  4  changes the gate signal voltage Vg between Hi (High) and Lo (Low), such that the discharge current I 2  detected by the current detection circuit part  41  is kept within the intended range Ir. 
     Soft-Off Circuit  62   
     As illustrated in  FIG. 1 , the soft-off circuit  62  constitutes a portion of the energy input circuit  6  and is disposed between the ignition control circuit  4  and the second switching element  61 . The soft-off circuit  62  slowly decreases a gate voltage (gate-source voltage) Vgs as a signal voltage added to the gate G, when the second switching element  61  is turned off, thereby to slow the turn-off speed when the second switching element  61  is turned from on to off. 
     As illustrated in  FIG. 4 , the soft-off circuit  62  is configured to, when decreasing the gate voltage Vgs added to the gate G of the second switching element  61 , execute a first decreasing stage T 1  of decreasing the gate voltage Vgs until it reaches the vicinity of a gate-source threshold voltage Vth and a second decreasing stage T 2  of gradually decreasing the gate voltage Vgs in the vicinity of the threshold voltage Vth. In other words, the decreasing speed of the gate voltage Vgs in the second decreasing stage T 2  is made slower than the decreasing speed of the gate voltage Vgs in the first decreasing stage T 1 . The threshold voltage Vth indicates a voltage at a boundary where the second switching element  61  is switched between on and off. The first decreasing stage T 1  allows the gate voltage Vgs to quickly decrease until it reaches the vicinity of the threshold voltage Vth to ensure the turn-off speed of the second switching element  61 . Also, the second decreasing stage T 2  allows the gate voltage Vgs to gradually decrease thereby to slowly increase a voltage Vc at both ends of the second coil part  212  of the primary coil  21  when the second switching element  61  is turned off, so that the oscillation of this voltage Vc can be suppressed. 
     It is noted that the soft-off circuit  62  may decrease the gate voltage Vgs added to the gate G of the second switching element  61  in three or more stages. Also, the soft-off circuit  62  may decrease the gate voltage Vgs added to the gate G of the second switching element  61  steplessly and curvilinearly. 
     As illustrated in  FIG. 5 , in a MOSFET constituting the second switching element  61 , a drain current (drain-source current) Ids starts flowing when the gate voltage (gate-source voltage) Vgs reaches equal to or more than the threshold voltage Vth as a predetermined voltage. In a region where the gate voltage Vgs is equal to or more than the threshold voltage Vth, enhancement properties are exhibited in which as the gate voltage Vgs increases, the drain current Ids increases. It is noted that the threshold voltage Vth is, for example, about 3 V. 
     As illustrated in  FIG. 4 , in the first decreasing stage T 1  of the present embodiment, the gate voltage Vgs added to the gate G of the second switching element  61  is decreased to a voltage that is somewhat higher than the threshold voltage Vth. Subsequently, in the second decreasing stage T 2  of the present embodiment, the gate voltage Vgs is decreased from a voltage that is somewhat higher than the threshold voltage Vth to the threshold voltage Vth, such that the voltage Vc at both ends of the second coil part  212  of the primary coil  21  gradually increases. 
     The soft-off circuit  62  is configured to change the gate voltage Vgs added to the gate G of the second switching element  61  of the energy input circuit  6  between Hi voltage and the threshold voltage Vth. While the second switching element  61  is off, the soft-off circuit  62  activates the second switching element  61  in the vicinity of the threshold voltage Vth to form a state in which a minute drain current Ids flows between the drain and the source of the second switching element  61 . This gradually increases the voltage Vc at both ends of the second coil part  212  of the primary coil  21 . 
     It is noted that the gate voltage Vgs is not necessarily decreased to the threshold voltage Vth during turn-off of the second switching element  61 . That is, the gate voltage Vgs may be slowly decreased while maintaining a voltage that is higher than the threshold voltage Vth, during turn-off of the second switching element  61 . As illustrated in  FIG. 5 , as the gate voltage Vgs decreases, the drain current Ids decreases. Therefore, the voltage Vc at both ends of the second coil part  212  of the primary coil  21  can also be made not to become lower than a current-inputtable voltage Vi by decreasing the gate voltage Vgs to a voltage higher than the threshold voltage Vth for squeezing the drain current Ids. 
     Also, since a MOSFET has a parasitic capacitance, the drain current Ids sometimes flows even when the gate voltage Vgs is decreased to a voltage that is somewhat lower than the threshold voltage Vth during turn-off of the second switching element  61 , which somewhat increases the gate voltage Vgs. Therefore, in the first decreasing stage T 1 , there is some case where the gate voltage Vgs may be decreased to a voltage that is about the same voltage as the threshold voltage Vth or to a voltage that is somewhat lower than the threshold voltage Vth. 
     The ignition device  1  of the present embodiment is configured such that the gate voltage Vgs becomes around the threshold voltage Vth when the second switching element  61  is turned off. Therefore, the voltage Vc added to the second coil part  212  by the second switching element  61 , i.e., the voltage Vc at both ends of the second coil part  212 , is kept in a state of being lower than the DC voltage VB of the DC power source  11 . 
     Action of Ignition Device  1   
     Hereinafter, an action of the ignition device  1  will be described with reference to the timing charts of  FIG. 3  and  FIG. 4  and the flowchart of  FIG. 6 . In the timing chart of  FIG. 4 , waveforms of a voltage and a current when the energy input circuit  6  has the soft-off circuit  62  are illustrated with solid lines. 
     In each cylinder of an engine, a fuel-gas mixture is ignited by the ignition device  1  in the combustion step when a combustion cycle is repeated. For generating a spark discharge in the combustion step, the first switching element  51  of the main ignition circuit  5  is turned on in response to reception of the ignition signal IGt by the electronic control unit  7  and the ignition control circuit  4 , and the first coil part  211  of the primary coil  21  is energized, as illustrated in  FIG. 3  (step S 101  in  FIG. 6 ). Then, as illustrated in  FIG. 3 , when the energization to the first coil part  211  is interrupted in response to turn-off of the first switching element  51 , mutual induction effects are exerted so that a high voltage proportional to how much the number of turns of the wire of the secondary coil  22  is relative to the number of turns of the wire of the first coil part  211  is generated in the secondary coil  22 , and the discharge current I 2  is generated (step S 102 ). At this time, a spark discharge is generated at the discharge gap  31  of the ignition plug  3 . 
       FIG. 3  illustrates a state in which the discharge current I 2  of the secondary coil  22  repeatedly increases and decreases between the control upper limit value Imax and the control lower limit value Imin, in response to energization and interruption of energization to the second coil part  212  of the primary coil  21  by the gate signal voltage Vg. 
     It is noted that the discharge control of the secondary coil  22  ends after a lapse of a discharge continuation setting time represented by a time period during which the discharge signal IGw is Hi (step S 103 ), regardless of the magnitude of the discharge current I 2 . 
     Subsequently, the discharge current I 2  generated in the secondary coil  22  is detected by the current detection circuit part  41  and the ignition control circuit  4  (step S 104 ). Then, whether the discharge current I 2  has become the control lower limit value Imin or less is detected (step S 105 ). As illustrated in  FIG. 4 , in response to the discharge current I 2  becoming the control lower limit value Imin or less, the second switching element  61  of the energy input circuit  6  is turned on in response to receipt of the gate signal voltage Vg by the ignition control circuit  4 , and energization to the second coil part  212  of the primary coil  21  starts (step S 106 ). Accordingly, a current I 1  flows through the second coil part  212 , and this current I 1  increases. Also, mutual induction effects are exerted to increase the discharge current  12  flowing through the secondary coil  22 . 
     Subsequently, the discharge current I 2  generated in the secondary coil  22  is detected again by the current detection circuit part  41  and the ignition control circuit  4  (step S 108 ). Then, whether the discharge current I 2  has become the control upper limit value Imax or more is detected (step S 109 ). In response to the discharge current I 2  becoming the control upper limit value Imax or more, the ignition control circuit  4  recognizes that the energization to the second coil part  212  of the primary coil  21  needs to stop (step S 110 ). It is noted that after a lapse of the discharge continuation setting time (step S 107 ), step S 110  is executed without executing step S 108  and S 109 . 
     When step S 110  is executed, the soft-off circuit  62  of the energy input circuit  6  decreases the gate voltage Vgs added to the gate G of the second switching element  61  to a voltage that is somewhat higher than the gate-source threshold voltage Vth, as the first decreasing stage T 1 , as illustrated in  FIG. 4  (step S 111 ). This decrease of the gate voltage Vgs in the first decreasing stage T 1  is performed rapidly. Subsequently, the soft-off circuit  62  decreases the gate voltage Vgs added to the gate G of the second switching element  61  to the gate-source threshold voltage Vth, as the second decreasing stage T 2  (step S 112 ). This decrease of the gate voltage Vgs in the second decreasing stage T 2  is performed slowly such that the voltage Vc at both ends of the second coil part  212  gradually increases. 
     Then, as illustrated in  FIG. 4 , the drain current Ids of the second switching element  61  decreases, while the discharge current I 2  of the secondary coil  22  decreases. At this time, the gate voltage Vgs added to the gate G of the second switching element  61  slowly decreases in the second decreasing stage T 2 , so that a drain-source voltage Vds of the second switching element  61  and the voltage Vc at both ends of the second coil part  212  slowly increase. Accordingly, the voltage Vc at both ends of the second coil part  212  can be prevented from oscillating. 
     Subsequently, when the discharge continuation setting time has not lapsed (step S 103 ), the discharge current I 2  generated in the secondary coil  22  is detected again by the current detection circuit part  41  and the ignition control circuit  4  (step S 104 ). Then, whether the discharge current I 2  has become the control lower limit value Imin or less is detected (step S 105 ). In response to the discharge current I 2  becoming the control lower limit value Imin or less, the second switching element  61  is turned on again in response to receipt of the gate signal voltage Vg by the ignition control circuit  4 , and energization to the second coil part  212  of the primary coil  21  starts again (step S 106 ). 
     At this time, the voltage Vc at both ends of the second coil part  212  does not become lower than the current-inputtable voltage Vi, or a time period during which the voltage Vc is lower than the current-inputtable voltage Vi is short. Therefore, energization to the second coil part  212  immediately starts, and the drain current Ids of the second switching element  61  immediately starts increasing. This can prevent the timing of inputting a current to the second coil part  212  from delaying at turn-on when energization of the second switching element  61  starts. 
     The current-inputtable voltage Vi is set based on a phenomenon in which in an attempt to allow a current to flow through the second coil part  212  by the second switching element  61 , a current does not flow through the second coil part  212  when the voltage Vc at both ends of the second coil part  212  is lower than a certain value. The current-inputtable voltage Vi is set as a voltage value which allows a current to flow through the second coil part  212 . 
     Thereafter, steps S 103  to S 112  are repeated, and discharge control of the secondary coil  22  ends when the discharge continuation setting time has lapsed (step S 103 ). Then, in response to reception of the gate signal voltage Vg by the ignition control circuit  4 , a state in which the second switching element  61  is off is continued. It is noted that steps S 101  to S 112  are repeatedly executed every time the combustion step is performed in each cylinder of an engine. 
     Timing Chart of Comparative Embodiment 
     In the timing chart of  FIG. 4 , waveforms of a voltage and a current for a comparative embodiment in which the energy input circuit  6  does not have the soft-off circuit  62  are illustrated with broken lines. In this case, in response to the discharge current I 2  of the secondary coil  22  becoming the control upper limit value Imax or more, the second switching element  61  is turned off, and the gate voltage Vgs added to the gate G of the second switching element  61  rapidly decreases until it reaches around 0 V. At this time, the drain current Ids of the second switching element  61  rapidly disappears, and the drain-source voltage Vds of the second switching element  61  and the voltage Vc at both ends of the second coil part  212  oscillate to a large extent. Especially, when the voltage Vc at both ends of the second coil part  212  decreases lower than the current-inputtable voltage Vi due to an undershoot of an oscillation of the voltage Vc, the drain current Ids of the second switching element  61  does not immediately increase in response to turn-off of the second switching element  61 , even when a voltage added to the gate G of the second switching element  61  increases again. This causes the timing of inputting the current I 1  to the second coil part  212  to be delayed. As a result, input of electric energy to the discharge current I 2  of the secondary coil  22  is delayed, and the fluctuation range of the discharge current I 2  of the secondary coil  22  increases. 
     Operation Effect 
     In the ignition device  1  of the present embodiment, the energy input circuit  6  has the soft-off circuit  62  that slows the turn-off speed of the second switching element  61 . The second switching element  61  performs energization and interruption of energization to the second coil part  212  of the primary coil  21  for keeping the discharge current I 2  in the secondary coil  22  within the intended range Ir directly using the DC voltage VB, after the induced electromotive force has been generated in the secondary coil  22 . 
     When the turn-off speed of the second switching element  61  is slowed by the soft-off circuit  62 , oscillation of the voltage Vc at both ends of the second coil part  212  of the primary coil  21  can be suppressed in response to interruption of energization to the second switching element  61 , that is, in response to turn-off of the second switching element  61 . This can prevent the voltage Vc at both ends of the second coil part  212  of the primary coil  21  from becoming lower than the current-inputtable voltage Vi by the second switching element  61 , when energization to the second switching element  61  is interrupted. 
     Therefore, after energization to the second coil part  212  of the primary coil  21  has been interrupted by the second switching element  61 , energization to the second coil part  212  of the primary coil  21  is quickly resumed by the second switching element  61 . As a result, input of the current I 1  to the second coil part  212  of the primary coil  21  for continuing the discharge current I 2  of the secondary coil  22  is quickly performed. Accordingly, the discharge current I 2  can be controlled such that it does not become lower than the control lower limit value Imin without increasing the control upper limit value Imax of the discharge current I 2 , and the increase of electric energy consumption is suppressed. In other words, the intended range (control width) Ir of the discharge current I 2  can be decreased, and consumption of electric energy is reduced. 
     Also, in the present embodiment, energization to the second coil part  212  of the primary coil  21  by the second switching element  61  and the soft-off circuit  62  is performed directly using the DC voltage VB of the DC power source  11 . A circuit to boost the DC voltage VB is not used for energizing the second coil part  212  of the primary coil  21 . Also, since the oscillation of the voltage Vc at both ends of the second coil part  212  of the primary coil  21  can be suppressed by using the soft-off circuit  62 , there is no need to use a large-sized condenser between the end of the second coil part  212  and the ground GND. Since the need for the booster circuit and the large-sized condenser is eliminated, the increase in size and cost of the ignition device  1  is suppressed. 
     It is noted that a small-sized condenser may be connected between the end of the second coil part  212  and the ground GND. The condenser in this case may be small in size, because suppression of the oscillation of the voltage Vc at both ends of the second coil part  212  is not intended. On the other hand, as illustrated in the present embodiment, when the gate voltage Vgs is decreased through a plurality of decreasing stages T 1  and T 2  at turn-off of the second switching element  61 , energization to the second coil part  212  may be performed by boosting the DC voltage VB of the DC power source  11  in some cases. 
     Therefore, according to the ignition device  1  of the present embodiment, input of a current to the primary coil  21  for continuing the discharge current I 2  of the secondary coil  22  is adequately performed, and the consumption of electric energy and the increase in size of the ignition device  1  are suppressed. 
     Second Embodiment 
     In the ignition device  1  of the present embodiment, a specific configuration of the soft-off circuit  62  of the energy input circuit  6  will be illustrated. As illustrated in  FIG. 7 , the soft-off circuit  62  is configured using a plurality of comparators  631  and  632  by an operational amplifier, a transistor  64 , a plurality of resistors  65 , and others. The soft-off circuit  62  has two types of control resistors  621  and  622  having different resistance values connected to the gate G of the second switching element  61  such that a current is allowed to flow from the gate G to the ground GND. 
     As illustrated in  FIG. 8 , the soft-off circuit  62  executes, similarly to in the first embodiment, a first decreasing stage T 1  of decreasing the gate voltage Vgs added to the gate G of the second switching element  61  until it becomes a voltage that is somewhat higher than the gate-source threshold voltage Vth and a second decreasing stage T 2  of decreasing the gate voltage Vgs until it reaches the threshold voltage Vth. In the first decreasing stage T 1  of the present embodiment, the gate voltage Vgs added to the gate G of the second switching element  61  is decreased by using the first control resistor  621  having a lower resistance value among two types of control resistors  621  and  622 . Since the resistance value of the first control resistor  621  is low, the speed of a current flowing through the first control resistor  621  can be relatively increased to form the first decreasing stage T 1 . 
     Also, in the second decreasing stage T 2  of the present embodiment, the gate voltage Vgs added to the gate G of the second switching element  61  is decreased by using the second control resistor  622  having a higher resistance value among two types of control resistors  621  and  622 . Since the resistance value of the second control resistor  622  is high, the speed of a current flowing through the second control resistor  622  can be slowed to form the second decreasing stage T 2 . 
     Also, as illustrated in  FIG. 7 , the first control resistor  621  of the present embodiment is connected between the collector C of the transistor  64  and the gate G of the second switching element  61  and is switchable between when a current flows and when it does not flow by on and off of the transistor  64 . The first control resistor  621  may be connected between the emitter E of the transistor  64  and the ground GND. On the other hand, the second control resistor  622  of the present embodiment is connected between the gate G of the second switching element  61  and the ground GND and discharges a minute current from the gate G to the ground GND, regardless of on or off of the second switching element  61 . 
     The first control resistor  621  and the second control resistor  622  are connected in parallel. In the first decreasing stage T 1 , electrical charges at the gate G of the second switching element  61  rapidly decrease by the first control resistor  621  and the second control resistor  622 . Also, in the second decreasing stage T 2 , electrical charges at the gate G of the second switching element  61  slowly decrease by the second control resistor  622 . 
     As illustrated in  FIG. 7 , the soft-off circuit  62  has, other than two types of control resistors  621  and  622 , the first comparator  631 , the second comparator  632 , the transistor  64 , and others. The first comparator  631  is configured such that when the gate voltage Vgs added from the ignition control circuit  4  to the gate G of the second switching element  61  is higher than a predetermined first setting voltage Vc 1  formed by the resistor  65 , Lo (Low) voltage is output to keep the transistor  64  OFF. Also, the first comparator  631  is configured such that in response to the gate voltage Vgs added from the ignition control circuit  4  to the gate G of the second switching element  61  becoming lower than the first setting voltage Vc 1 , Lo voltage is changed to Hi (High) voltage so that the transistor  64  is turned ON. 
     The output terminal of the first comparator  631 , the output terminal of the second comparator  632 , and a base terminal B of the transistor  64  are connected to one another, and this connection point is applied with a circuit voltage V 0  for performing an on and off action of the transistor  64  through the resistor  65 . The circuit voltage V 0  may be the same as the DC voltage VB of the DC power source  11  or may be a predetermined DC voltage that is lower than the DC voltage VB of the DC power source  11 . 
     As illustrated in  FIG. 7 , the second comparator  632  is configured such that when the gate voltage Vgs added from the ignition control circuit  4  to the gate G of the second switching element  61  is higher than a predetermined second setting voltage Vc 2  formed by the resistor  65 , Hi voltage is output. Also, the second comparator  632  is configured such that in response to the gate voltage Vgs added from the ignition control circuit  4  to the gate G of the second switching element  61  becoming lower than the second setting voltage Vc 2 , Hi voltage is changed to Lo voltage so that the transistor  64  is turned OFF. 
     A voltage value that is higher than the gate-source threshold voltage Vth of the second switching element  61  and the second setting voltage Vc 2  of the second comparator  632  is set to the first setting voltage Vc 1  of the first comparator  631 . A voltage value that is higher than the gate-source threshold voltage Vth of the second switching element  61  is set to the second setting voltage Vc 2 . A voltage value that is higher by 0.2 to 1 V than the threshold voltage Vth, for example, can be set to the second setting voltage Vc 2 . 
     Action of Ignition Device  1   
     Hereinafter, an action of the ignition device  1  will be described with reference to the timing chart of  FIG. 8 . In the timing chart of  FIG. 8 , waveforms of a voltage and a current when the energy input circuit  6  has the soft-off circuit  62  are illustrated with solid lines. 
     In the ignition device  1  of the present embodiment, the current I 1  is allowed to intermittently flow though the second coil part  212  of the primary coil  21 , such that the discharge current I 2  is kept within the intended range Ir after the discharge current I 2  has been generated in the secondary coil  22 . The timing chart of  FIG. 8  illustrates changes in voltage and current of each component of the ignition device  1  during a process in which the gate signal voltage Vg from the ignition control circuit  4  changes in the following order: Hi voltage (merely indicated as Hi), Lo voltage (merely indicated as Lo), and Hi voltage. 
     In  FIG. 7  and  FIG. 8 , when the gate signal voltage Vg of the ignition control circuit  4  is Hi, the gate voltage (gate-source voltage) Vgs of the second switching element  61  is Hi. At this time, the output voltage of the first comparator  631  is Lo, the output voltage of the second comparator  632  is Hi, and the transistor  64  is OFF. Also, at this time, the drain-source voltage Vds of the second switching element  61  and the voltage Vc at the high-voltage-side terminal of the second coil part  212  are low. Also, at this time, as illustrated in  FIG. 8 , the drain current (drain-source current, current of the second coil part  212 ) Ids of the second switching element  61  and the discharge current I 2  of the secondary coil  22  slowly increase. 
     Subsequently, as illustrated in  FIG. 7  and  FIG. 8 , in response to the discharge current I 2  of the secondary coil  22  becoming the control upper limit value Imax or more, the gate signal voltage Vg of the ignition control circuit  4  changes from Hi to Lo. When the gate signal voltage Vg becomes lower than the first setting voltage Vc 1  of the first comparator  631  during a process in which the gate signal voltage Vg changes from Hi to Lo, the output voltage of the first comparator  631  changes from Lo to Hi. Then, in response to the output voltage of the first comparator  631  becoming Hi, the transistor  64  is turned from OFF to ON, and electrical charges at the gate G of the second switching element  61  are discharged to the first control resistor  621  by the transistor  64 . Accordingly, the gate voltage Vgs of the second switching element  61  starts decreasing. 
     Subsequently, as illustrated in  FIG. 7  and  FIG. 8 , in response to the gate voltage Vgs of the second switching element  61  becoming lower than the second setting voltage Vc 2  of the second comparator  632 , the output voltage of the second comparator  632  changes from Hi to Lo, while the transistor  64  is turned from ON to OFF. At this time, electrical charges at the gate G of the second switching element  61  are not discharged to the first control resistor  621  anymore, and minor amounts of electrical charges at the gate G are discharged to the second control resistor  622 . 
     Then, as illustrated in  FIG. 8 , due to the fact that electric charges at the gate G of the second switching element  61  are slowly discharged, the drain-source voltage Vds of the second switching element  61  starts slowly increasing, while the voltage Vc at the high-voltage-side terminal of the second coil part  212  starts slowly increasing. Accordingly, the drain-source voltage Vds of the second switching element  61  and the voltage Vc at the high-voltage-side terminal of the second coil part  212  are prevented from oscillating. Also, at this time, the drain current (current of the second coil part  212 ) Ids of the second switching element  61  and the discharge current I 2  of the secondary coil  22  start slowly decreasing. 
     Subsequently, as illustrated in  FIG. 7  and  FIG. 8 , in response to the discharge current I 2  of the secondary coil  22  becoming the control lower limit value Imin or less, the gate signal voltage Vg of the ignition control circuit  4  changes from Lo to Hi. At this time, the output voltage of the first comparator  631  changes from Hi to Lo, while the output voltage of the second comparator  632  changes from Lo to Hi, and the gate voltage Vgs of the second switching element  61  changes from around the threshold voltage Vth to Hi. Also, at this time, the drain-source voltage Vds of the second switching element  61  and the voltage Vc at the high-voltage-side terminal of the second coil part  212  change from the highest state to the lowest state. 
     In  FIG. 8 , the voltage Vc at the high-voltage-side terminal of the second coil part  212  decreases to a voltage in the vicinity of the current-inputtable voltage Vi of the second coil part  212 . Even if the voltage Vc at the high-voltage-side terminal of the second coil part  212  becomes lower than the current-inputtable voltage Vi, this time period is a moment, and input of a current to the second coil part  212  is hardly delayed. Then, when the gate signal voltage Vg of the ignition control circuit  4  changes to Hi, the voltage Vc at the high-voltage-side terminal of the second coil part  212  is higher than the current-inputtable voltage Vi, and the drain current Ids of the second switching element  61  and the discharge current I 2  of the secondary coil  22  immediately start increasing. The discharge current I 2  of the secondary coil  22  is intended to be kept within the intended range Ir between the control lower limit value Imin and the control upper limit value Imax. However, the intended range Ir may be somewhat outside the range between the control lower limit value Imin and the control upper limit value Imax, depending on the switching timing of the second switching element  61 . 
     Timing Chart of Comparative Embodiment 
     In the timing chart of  FIG. 8 , waveforms of a voltage and a current for a comparative embodiment in which the energy input circuit  6  does not have the soft-off circuit  62  are illustrated with broken lines. In this case, in response to the discharge current I 2  of the secondary coil  22  becoming the control upper limit value Imax or more, the second switching element  61  changes from ON to OFF, and the gate voltage Vgs added to the gate G of the second switching element  61  rapidly decreases from Hi to Lo. At this time, the drain current Ids of the second switching element  61  rapidly disappears, and the drain-source voltage Vds of the second switching element  61  and the voltage Vc at the high-voltage-side terminal of the second coil part  212  oscillate to a large extent. 
     Especially, when the voltage Vc at the high-voltage-side terminal of the second coil part  212  decreases lower than the current-inputtable voltage Vi due to an undershoot of an oscillation of the voltage Vc, start of the increase of the drain current Ids of the second switching element  61  is delayed when the second switching element  61  changes from OFF to ON. As a result, the discharge current I 2  of the secondary coil  22  decreases to a large extent, and the discharge current I 2  of the secondary coil  22  does not start increasing until the voltage Vc at the high-voltage-side terminal of the second coil part  212  is restored to the current-inputtable voltage Vi or more. 
     Operation Effect 
     In the present embodiment, the first decreasing stage T 1  of decreasing the gate voltage Vgs of the second switching element  61  using the first control resistor  621  and the second control resistor  622  enables electric charges at the gate G of the second switching element  61  to be quickly discharged, so that a time taken for turning off the second switching element  61  is prevented from being extremely lengthened. Also, the second decreasing stage T 2  of decreasing the gate voltage 
     Vgs of the second switching element  61  using the second control resistor  622  enables electric charges at the gate G of the second switching element  61  to be slowly discharged, so that an oscillation of the voltage Vc at the high-voltage-side terminal of the second coil part  212  is suppressed, and the fluctuation range of the discharge current I 2  of the secondary coil  22  is easily kept small. 
     Also, since the resistance value of the second control resistor  622  which always discharges electric charges at the gate G of the second switching element  61  is large, leakage of electric charges from the gate G of the second switching element  61  to the second control resistor  622  is suppressed when the second switching element  61  is turned on by the gate signal voltage Vg of the ignition control circuit  4 , and a delay at turn-on of the second switching element  61  is suppressed. 
     Other configurations, operation effects, and others in the ignition device  1  of the present embodiment are the same as in the first embodiment. In the present embodiment, components assigned with identical reference signs to those assigned in the first embodiment are also the same as in the first embodiment. 
     Third Embodiment 
     In the ignition device  1  of the present embodiment, a case where a condenser  66  connected between the gate and the source of the second switching element  61  is used in a soft-off circuit  62 A of the energy input circuit  6 , as illustrated in  FIG. 9 , will be illustrated. Also, a resistor  67  for always discharging electric charges at the gate G of the second switching element  61  is disposed between the gate G of the second switching element  61  and the ground GND. The soft-off circuit  62 A of the present embodiment blunts (slows) the rate of decrease of the gate voltage Vgs at turn-off of the second switching element  61  with the time constant by the resistor  67  and the condenser  66 . 
     The soft-off circuit  62 A of the energy input circuit  6  of the present embodiment is configured such that the turn-off speed of the second switching element  61  is made slower than the turn-on speed of the second switching element  61 . 
     The ignition device  1  of the present embodiment is also configured such that the gate voltage Vgs as a signal voltage added to the gate G becomes around the threshold voltage Vth in response to turn-off of the second switching element  61 . Therefore, the voltage Vc added to the second coil part  212  by the second switching element  61 , i.e., the voltage Vc at both ends of the second coil part  212 , is kept in a state of being lower than the DC voltage VB of the DC power source  11 . 
     As illustrated in  FIG. 10 , in the action of the ignition device  1  of the present embodiment, in response to the gate signal voltage Vg of the ignition control circuit  4  changing from Hi to Lo, the gate voltage (gate-source voltage) Vgs of the second switching element  61  rapidly decreases at first and slowly decreases after reaching near the gate-source threshold voltage Vth. In other words, the gate voltage Vgs of the second switching element  61  decreases in a curved manner. Accordingly, the drain-source voltage Vds of the second switching element  61  and the voltage Vc at the high-voltage-side terminal of the second coil part  212  can be slowly increased. 
     Also, in response to the gate signal voltage Vg of the ignition control circuit  4  changing from Lo to Hi, the gate voltage Vgs of the second switching element  61  quickly increases. Then, since the voltage Vc at the high-voltage-side terminal of the second coil part  212  hardly oscillates, energization to the second coil part  212  can be quickly started. 
     In the timing chart of  FIG. 10 , waveforms of a voltage and a current for a comparative embodiment in which the energy input circuit  6  does not have the condenser  66  are also illustrated with broken lines. 
     Therefore, the ignition device  1  of the present embodiment can also achieve the same operation effect as in the first embodiment. Other configurations in the ignition device  1  of the present embodiment are the same as in the first embodiment. In the present embodiment, components assigned with identical reference signs to those assigned in the first embodiment are also the same as in the first embodiment. 
     Fourth Embodiment 
     In the ignition device  1  of the present embodiment, a case where a voltage control circuit  68 , configured such that the voltage Vc added to the second coil part  212  by the second switching element  61  maintains a state of being lower than the DC voltage VB of the DC power source  11 , is applied to the energy input circuit  6 , as illustrated in  FIG. 11 , will be illustrated. The energy input circuit  6  of the present embodiment has the second switching element  61  and the voltage control circuit  68 . The second switching element  61  of the energy input circuit  6  of the present embodiment controls an energization state to the second coil part  212  of the primary coil  21  by the voltage control circuit  68 , such that the discharge current I 2  in the secondary coil  22  is kept at an intended value directly using the DC voltage VB of the DC power source  11 , after the induced electromotive force has been generated. 
     As illustrated in  FIG. 12 , the voltage control circuit  68  is configured to gradually increase the drain current (current flowing between the drain and the source) Ids of the second switching element  61  such that the discharge current I 2  in the secondary coil  2  is kept at a certain value. In other words, the voltage control circuit  68  is configured to gradually increase the gate voltage (gate-source voltage) Vgs of the second switching element  61  around the threshold voltage Vth thereby to limit the drain current Ids of the second switching element  61  such that the discharge current I 2  in the secondary coil  22  is kept at a certain value. The voltage control circuit  68  functions as a linear regulator that dulls the gate voltage Vgs added to the gate G of the second switching element  61  for maintaining a state in which the second switching element  61  does not completely become on. 
     After the discharge current I 2  has been generated in the secondary coil  22  by interruption of energization to the first coil part  211  of the primary coil  21 , this discharge current I 2  gradually decreases unless energy is newly input to the primary coil  21 . In the first to third embodiments, a current to energize the second coil part  212  of the primary coil  21  was intermittently controlled such that the discharge current I 2  changes between the control upper limit value Imax and the control lower limit value Imin On the other hand, in the present embodiment, the current I 1  to energize the second coil part  212  of the primary coil  21  is gradually increased in association with a speed at which the discharge current I 2  gradually decreases, such that the change of the discharge current I 2  decreases. 
     The second switching element  61  is constituted by a MOSFET. As illustrated in  FIG. 13 , when the gate voltage Vgs of the MOSFET is, for example, in a range of 0.7 V to 1.3 V as the vicinity of the threshold voltage Vth, a relationship between the drain-source voltage Vds and the drain current (drain-source current) Ids in the MOSFET forms a linear region A 1  and a saturation region A 2 . The linear region A 1  indicates a region where the drain current Ids increases as the drain-source voltage Vds increases while the drain-source voltage Vds is around low. The saturation region A 2  indicates a region where the drain current Ids does not increase much even when the drain-source voltage Vds increases. 
     Also, in the saturation region A 2 , when the gate voltage Vgs increases, for example, from 0.7 V to 1.3 V, the drain current Ids increases as the gate voltage Vgs increases. Then, as illustrated in  FIG. 12 , the voltage control circuit  68  of the present embodiment forms a state in which the gate voltage Vgs added to the gate G of the second switching element  61  gradually increases in the vicinity of the threshold voltage Vth such that the drain current Ids of the second switching element  61  gradually increases, by taking advantage of the saturation region A 2  of the MOSFET. 
     In the present embodiment, a state in which the second switching element  61  becomes incompletely on around the threshold voltage Vth of the gate G is formed, without performing on or off of the second switching element  61 , i.e., without performing energization and interruption of energization of the second switching element  61 . This enables the voltage Vc at the high-voltage-side terminal of the second coil part  212  of the primary coil  21  to hardly oscillate, and thus not to become lower than the current-inputtable voltage Vi. 
     Then, the discharge current I 2  of the secondary coil  22  is kept at an intended current value in response to input of electric energy to the second coil part  212  of the primary coil  21 , so that the input amount of a current to the second coil part  212  is adequately controlled. This reduces consumption of electric energy for continuing the discharge current I 2  of the secondary coil  22 . 
     Therefore, according to the internal combustion engine ignition device  1  of the present embodiment, input of a current to the primary coil  21  for continuing the discharge current I 2  of the secondary coil  22  is also adequately performed while suppressing consumption of electric energy. Other configurations and operation effects in the ignition device  1  of the present embodiment are the same as in the first embodiment. Also, in the present embodiment, components assigned with identical reference signs to those assigned in the first embodiment are the same as in the first embodiment. 
     Other Embodiments 
     The first coil part  211  and the second coil part  212  of the primary coil  21  can also be formed as the entirety of the primary coil  21 . 
     The present disclosure is not limited to only the embodiments, and further different embodiments can be configured within the scope that does not depart from the gist thereof. Also, the present disclosure includes various variation examples, variation examples within the equivalent scope, and others. Furthermore, various combinations of constituents, embodiments, and others, which are assumed from the present disclosure, are also included in the technical idea of the present disclosure.