Abstract:
A DC/AC converter includes: a resonant circuit including a transformer having a primary winding and a secondary winding and at least one capacitor, in which the capacitor is connected to at least one of the primary winding and secondary winding of the transformer, and an output terminal to which the load is to be connected is provided on the secondary winding side; a switching circuit connected to both ends of a direct current power supply and having a bridge configuration composed of switching elements for flowing a current through the primary winding of the transformer and the capacitor in the resonant circuit; and a control circuit that turns on/off the switching elements by a pair of drive signals, and flows a current through the load bidirectionally, thereby performs a PWM control for the current flowing through the load, wherein the control circuit includes step drive circuits which turn on the switching elements in steps, and the step drive circuits are provided so as to individually correspond to the switching elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a DC/AC converter that converts a direct current into an alternating current and supplies alternating current power to a load. 
     2. Description of the Related Art 
     In an inverter circuit that composes a DC/AC converter from a bridge circuit such as a half-bridge circuit and a full-bridge circuit, and supplies power to a load, when switching elements such as MOSFETs which compose the bridge circuit and are connected in series in a longitudinal direction are turned on simultaneously, a pass-through current flows through the bridge circuit, and the switching elements are broken. 
     For the purpose of preventing the switching elements from being turned on simultaneously, in usual, there is known a method of providing a dead time to a gate drive signal for driving the switching elements. However, by this method, the switching elements cannot be prevented from being turned on simultaneously in the case where noise is superimposed on the gate drive signal. 
     Japanese Patent Laid-Open Publication No. 2001-258268 (Patent Publication 1) discloses a half-bridge-type inverter circuit that prevents the switching elements from being turned on simultaneously by the noise. 
     This inverter circuit includes: a half-bridge-type switching circuit including a high-side switching element and a low-side switching element; a drive circuit that outputs an output signal driving the switching circuit and having a dead time period provided therein; a high-side dead time control circuit and a low-side dead time control circuit, each of which creates the dead time period from an input signal of the drive circuit; a pulse generator that generates a set output signal and a reset output signal from an output of the high-side dead time control circuit; a level shift circuit that boosts the set output signal and the reset output signal; a pulse filter circuit that allows passage of the set output signal and the reset output signal, of which pulse width has a fixed value or more; and an output circuit that outputs a high-side output signal by the set output signal and reset output signal from the pulse filter circuit, and an output circuit that outputs a high-side output signal by an output of the low-side dead time control circuit. Then, when a pulse width of the output of the high-side dead time control circuit is narrow, and the reset output signal of the pulse generator is not outputted, the pulse filter circuit does not allow the passage of the set output signal. With such a configuration, the inverter circuit of Patent Publication 1 prevents both of the switching elements from being turned on simultaneously. 
     SUMMARY OF THE INVENTION 
     However, the half-bridge-type inverter circuit disclosed in Patent Publication 1 cannot completely prevent the breakage of the switching elements, which is caused by the following phenomenon. 
     Specifically, the MOSFET to be used as the switching element has a parasitic diode (body diode) in terms of structure thereof. In a resonant-type inverter circuit, in the case where a switching circuit thereof is driven at a drive frequency lower than a circuit resonant frequency, when the high-side switching element and the low-side switching element are turned off simultaneously, the parasitic diode of the high-side switching element is turned on. Then, a recovery (reverse) current flows through the high-side switching element. 
     At this time, when the low-side switching element through which the recovery current does not flow is turned on, the pass-through current unfavorably flows through the switching circuit. Specifically, since each of the parasitic diodes has a reverse recovery time trr, the parasitic diodes are not turned off instantaneously at the time of being reverse biased, and a reverse current will flow therethrough. It is possible that the switching elements may be broken by the reverse current flowing therethrough at this time. 
     The present invention provides a DC/AC converter capable of preventing the breakage of the switching elements, which is caused by the reverse current generated by the noise and the reverse recovery time of each of the parasitic diodes present in the switching elements. 
     In order to solve the above-described problems, a DC/AC converter according to a first aspect of the present invention is a DC/AC converter for converting a direct current into an alternating current, and supplying power to a load, the DC/AC converter including: a resonant circuit including a transformer having a primary winding and a secondary winding and at least one capacitor, in which the capacitor is connected to at least one of the primary winding and secondary winding of the transformer, and an output terminal to which the load is to be connected is provided on the secondary winding side; a switching circuit connected to both ends of a direct current power supply and having a bridge configuration composed of switching elements for flowing a current through the primary winding of the transformer and the capacitor in the resonant circuit; and a control circuit that turns on/off the switching elements by a pair of drive signals, and flows a current through the load bidirectionally, thereby performs a PWM control for the current flowing through the load, wherein the control circuit includes step drive circuits which turn on the switching elements in steps, and the step drive circuits are provided so as to correspond to at least one of the switching elements. 
     In such a configuration, it is preferable that each of the step drive circuits include: a first-step drive circuit that connects a constant current source in series to a first CMOS inverter, and outputs a first drive signal slightly larger than a threshold voltage of the switching element to the switching element; and a second-step drive circuit that connects a delay circuit to an output of a second CMOS inverter, and outputs a second drive signal, of which value becomes a predetermined one, to the switching element. 
     The switching circuit may be a half-bridge circuit composed of two switching elements. In this case, each of the step drive circuits provided so as to correspond to each of the two switching elements turns on each of the two switching elements in steps. 
     Alternatively, the switching circuit may be a full-bridge circuit composed of: a first low-side switching element and a first high-side switching element, which are connected in series to each other; and a second low-side switching element and a second high-side switching element, which are connected in series to each other. In this case, the step drive circuits provided so as to individually correspond to the first low-side switching element and the second low-side switching element turn on the first low-side switching element and the second low-side switching element, respectively in steps. 
     In accordance with the DC/AC converter according to the first aspect of the present invention, each of the step drive circuits provided so as to correspond to the switching elements turns on the switching element in steps. Accordingly, the pass-through current, which flows by the reverse recovery time of each of the parasitic diodes present in the switching elements, can be reduced. Therefore, the breakage of the switching elements, which occurs by the noise and the reverse recovery time of each of the parasitic diodes present in the switching elements, is prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a configuration of a DC/AC converter according to Embodiment 1 of the present invention. 
         FIG. 2  is a circuit diagram of a first step-driver provided in the DC/AC converter according to Embodiment 1 of the present invention. 
         FIG. 3  is a circuit diagram of a second step-driver provided in the DC/AC converter according to Embodiment 1 of the present invention. 
         FIG. 4  is a chart showing drive waveforms of an SW network provided in the DC/AC converter according to Embodiment 1 of the present invention, and operation waveforms when the SW network is driven at a resonant frequency. 
         FIG. 5  is a chart showing drive waveforms of the SW network provided in the DC/AC converter according to Embodiment 1 of the present invention, and operation waveforms when the SW network is driven in a lagging phase range of a resonance. 
         FIG. 6  is a chart showing drive waveforms of the SW network provided in the DC/AC converter according to Embodiment 1 of the present invention, and operation waveforms when the SW network is driven in a leading phase range of the resonance. 
         FIG. 7  is a chart showing drive waveforms of an SW network provided in a conventional DC/AC converter, and operation waveforms when the SW network is driven in the leading phase range of the resonance. 
         FIG. 8  is a circuit diagram showing a configuration of a DC/AC converter according to Embodiment 2 of the present invention. 
         FIG. 9  is a chart showing drive waveforms of an SW network provided in the DC/AC converter according to Embodiment 2 of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A description will be made below in detail of embodiments of a DC/AC converter of the present invention while referring to the drawings. 
     Embodiment 1 
     A DC/AC converter according to Embodiment 1 of the present invention, which is shown in  FIG. 1 , is a device for converting a direct current into an alternating current, and supplying alternating current power to a load. Here, as an example, it is illustrated that the load is a discharge lamp, and that the DC/AC converter is applied to a discharge lamp lighting device. 
     The DC/AC converter of Embodiment 1 does not prevent switching elements from being turned on simultaneously, but prevents a breakage of the switching elements, which is caused by a pass-through current, and is characterized in that step-drivers are used in a control circuit. 
     In  FIG. 1 , a first series circuit composed of a high-side P-type MOSFET Qp 1  and a low-side N-type MOSFET Qn 1  is connected between a direct current power supply Vin and the ground. The first series circuit composed of the P-type MOSFET Qp 1  and the N-type MOSFET Qn 1  compose a half-bridge-type switching circuit (SW network)  7 . A series circuit composed of a capacitor C 3  and a primary winding P of a transformer T is connected between a node of the P-type MOSFET Qp 1  and N-type MOSFET Qn 1  and the ground GND. A series circuit composed of a reactor Lr and a capacitor C 4  is connected to both ends of a secondary winding S of the transformer T. The series circuit composed of the capacitor C 3  and the transformer T, the series circuit composed of the reactor Lr and the capacitor C 4 , and the secondary winding S of the transformer T compose a resonant circuit  9 . 
     The direct current power supply Vin is supplied to a source of the P-type MOSFET Qp 1 . A gate of the P-type MOSFET Qp 1  is connected to a DRV 1  terminal of a control IC  1 . A gate of the N-type MOSFET Qn 1  is connected to a DRV 2  terminal of the control IC  1 . 
     Note that a capacitor C 10  is connected between the reactor Lr and the load (discharge lamp)  3 . In Embodiment 1, both of the capacitor C 3  and the capacitor C 10  are provided; however, for example, only either one of the capacitor C 3  and the capacitor C 10  may be provided. 
     One end of the capacitor C 10  is connected to one of electrodes of the discharge lamp  3 . The other electrode of the discharge lamp  3  is connected to a lamp current detection circuit  5 . The lamp current detection circuit  5  is composed of diodes D 1  and D 2  and resistors R 3  and R 4 . The lamp current detection circuit  5  detects a current flowing through the discharge lamp  3 , and outputs a voltage proportional to the detected current to an inverting input terminal of an error amplifier  15  through a feedback terminal FB of the control IC  1 . 
     The control IC  1  includes a start circuit  10 , a constant current decision circuit  11   a , an oscillator  12   a , the error amplifier  15 , a subtractor  19   a , first and second PWM comparators  16   a  and  16   c , a first AND gate  17   c , a second AND gate  17   d , and first and second step-drivers  18   a  and  18   b . The constant current decision circuit  11   a  is connected to one end of a constant current value decision resistor R 1  through an RF terminal. The oscillator  12   a  is connected to one end of a capacitor C 1  through a CF terminal. 
     The constant current decision circuit  11   a  flows a constant current, which is set arbitrarily by the constant current value decision resistor R 1 , trough the oscillator  12   a . The oscillator  12   a  charges and discharges the capacitor C 1  by the constant current of the constant current decision circuit  11   a , and generates a triangular wave signal CF(C 1 ) (that indicates a charge/discharge voltage of the capacitor C 1  at a terminal CF) that is as shown in  FIG. 4 . Moreover, the oscillator  12   a  creates a clock CK based on the triangular wave signal CF(C 1 ), and sends the clock CK to the first AND gate  17   c  and the second AND gate  17   d . In the triangular wave signal CF(C 1 ), a rising slope thereof and a falling slope thereof are the same. The rising slope and falling slope of the triangular wave signal CF(C 1 ) are set by values of the capacitor C 1  and the resistor R 1 . 
     The error amplifier  15  amplifies an error voltage FBOUT between the voltage from the lamp current detection circuit  5 , which is inputted to the inverting input terminal, and a reference voltage E 1  inputted to a noninverting input terminal thereof. An output terminal of the error amplifier  15  is connected to noninverting input terminals of the first and second PWM comparators  16   a  and  16   c . A resistor R 5  is connected between an inverting input terminal and output terminal of the subtractor  19   a.    
     The subtractor  19   a  converts the triangular wave signal CF(C 1 ) into an inverted voltage C 1 ′ (shown in  FIG. 4 ) obtained by inverting the triangular wave signal CF(C 1 ) at a midpoint potential between an upper limit value VH and lower limit value VL thereof, and outputs the inverted voltage C 1 ′. This inverted voltage C 1 ′ is inputted to an inverting input terminal of the second PWM comparator  16   c . A reference voltage E 2  inputted to a noninverting input terminal of the subtractor  19   a  is equal to (VL+VH)/2, and gives the midpoint potential between the upper limit value VH and lower limit value VL of the triangular wave signal CF(C 1 ). 
     The second PWM comparator  16   c  creates a pulse signal that rises to an H level when the error voltage FBOUT from the error amplifier  15 , which is inputted to the noninverting terminal, is equal to or more than the inverted voltage C 1 ′ from the subtractor  19   a , which is inputted to the inverting input terminal, and drops to an L level when the error voltage FBOUT is less than the inverted voltage C 1 ′. Then, the second PWM comparator  16   c  outputs the created pulse signal to the second AND gate  17   d . The second AND gate  17   d  takes AND of an inverted signal of the clock CK from the oscillator  12   a  and the signal from the second PWM comparator  16   c . Then, an output of the second AND gate  17   d  is inputted to the N-type MOSFET Qn 1  through the second step-driver  18   b.    
     The first PWM comparator  16   a  creates a pulse signal that rises to the H level when the error voltage FBOUT from the error amplifier  15 , which is inputted to the noninverting input terminal, is equal to or more than a voltage of the triangular wave signal from the CF terminal, which is inputted to the inverting input terminal, and drops to the L level when the error voltage FBOUT is less than the voltage of the triangular wave signal. Then, the first PWM comparator  16   a  outputs the created pulse signal to the first AND gate  17   c.    
     The first AND gate  17   c  takes NAND of the clock CK from the oscillator  12   a  and the signal from the first PWM comparator  16   a , and outputs an output of the NAND to the P-type MOSFET Qp 1  through the first step-driver  18   a.    
     The first PWM comparator  16   a , the first AND gate  17   c  and the first step-driver  18   a  generate a first drive signal that drives the P-type MOSFET Qp 1  to flow a current through the discharge lamp  3  at a pulse width corresponding to the current flowing through the discharge lamp  3  in less than a half cycle of the triangular wave signal CF (C 1 ). The subtractor  19   a , the second PWM comparator  16   c , the second AND gate  17   d  and the second step-driver  18   b  generate a second drive signal that has substantially the same pulse width as that of the first drive signal and a phase difference of approximately 180 degrees therefrom, and drives the N-type MOSFET Qn 1  to flow a current through the discharge lamp  3  in a direction reverse to that when the first drive signal is generated. 
     The first step-driver  18   a  turns on the P-type MOSFET Qp 1  in two steps, and outputs a two-step gate drive signal to the gate of the P-type MOSFET Qp 1 . The second step-driver  18   b  turns on the N-type MOSFET Qn 1  in two steps, and outputs a two-step gate drive signal to the gate of the N-type MOSFET Qn 1 . 
       FIG. 2  is a circuit diagram of the first step-driver provided in the DC/AC converter shown in  FIG. 1 . The first step-driver  18   a  includes a first-step drive circuit, and a second-step drive circuit. The first-step drive circuit connects a constant current source I 1  in series to a first CMOS inverter composed of a P-type MOSFET Q 3  and an N-type MOSFET Q 4 , and outputs a drive signal slightly larger than a threshold voltage between the gate and source of the P-type MOSFET Qp 1  to the gate of the P-type MOSFET Qp 1 . The second-step drive circuit connects a delay circuit to an output of a second CMOS inverter composed of a P-type MOSFET Q 1  and an N-type MOSFET Q 2 , and outputs a drive signal, of which value becomes a predetermined one, to the gate of the P-type MOSFET Qp 1 . 
     Next, a description will be made of a more specific connection configuration. An input terminal IN is connected to a gate of the P-type MOSFET Q 1  and a gate of the N-type MOSFET Q 2 . A source of the P-type MOSFET Q 1  is connected to a power supply REG. A drain of the P-type MOSFET Q 1  is connected to a drain of the N-type MOSFET Q 2  through a resistor R 12 . A source of the N-type MOSFET Q 2  is grounded. To a node of the P-type MOSFET Q 1  and the resistor R 12 , one end of a capacitor C 11  and an input end of an inverter  183  are connected. The other end of the capacitor C 11  is grounded. An output end of the inverter  183  is connected to a gate of an N-type MOSFET Q 5 . 
     Moreover, the input terminal IN is connected through an inverter  181  and an inverter  182  to a gate of the P-type MOSFET Q 3  and a gate of the N-type MOSFET Q 4 . A source of the P-type MOSFET Q 3  is connected to the power supply REG. A drain of the P-type MOSFET Q 3  is connected to a drain of the N-type MOSFET Q 4 . A source of the N-type MOSFET Q 4  is grounded through the current source I 1 . The power supply REG is connected to a drain of the N-type MOSFET Q 5  through a resistor R 11 . A source of the N-type MOSFET Q 5  is grounded. A node of the P-type MOSFET Q 3  and the N-type MOSFET Q 4  is connected to a node of the resistor R 11  and the N-type MOSFET Q 5 . An output OUT is taken out from the node of the resistor R 11  and the drain of the N-type MOSFET Q 5 . 
       FIG. 3  is a circuit diagram of the second step-driver provided in the DC/AC converter shown in  FIG. 1 . The second stepdriver  18   b  includes a first-step drive circuit, and a second-step drive circuit. The first-step drive circuit connects a constant current source I 2  in series to a third CMOS inverter composed of a P-type MOSFET Q 8  and an N-type MOSFET Q 9 , and outputs a drive signal slightly larger than a threshold voltage between the gate and source of the N-type MOSFET Qn 1  to the gate of the N-type MOSFET Qn 1 . The second-step drive circuit connects a delay circuit to an output of a fourth CMOS inverter composed of a P-type MOSFET Q 6  and an N-type MOSFET Q 7 , and outputs a drive signal, of which value becomes a predetermined one, to the gate of the N-type MOSFET Qn 1 . 
     Next, a description will be made of a more specific connection configuration. In  FIG. 3 , an input terminal IN is connected to a gate of the P-type MOSFET Q 6  and a gate of the N-type MOSFET Q 7 . A source of the P-type MOSFET Q 6  is connected to a power supply REG. A drain of the P-type MOSFET Q 6  is connected to a drain of the N-type MOSFET Q 7  through a resistor R 14 . A source of the N-type MOSFET Q 7  is grounded. To a node of the N-type MOSFET Q 7  and the resistor R 14 , one end of a capacitor C 12  and an input end of an inverter  187  are connected. The other end of the capacitor C 12  is grounded. An output end of the inverter  187  is connected to a gate of a P-type MOSFET Q 10 . 
     Moreover, the input terminal IN is connected through an inverter  185  and an inverter  186  to a gate of the P-type MOSFET Q 8  and a gate of the N-type MOSFET Q 9 . A source of the P-type MOSFET Q 8  is connected to the power supply REG through the current source I 2 . A drain of the P-type MOSFET Q 8  is connected to a drain of the N-type MOSFET Q 9 . A source of the N-type MOSFET Q 9  is grounded. The power supply REG is connected to a source of the P-type MOSFET Q 10 . A drain of the P-type MOSFET Q 10  is grounded through a resistor R 13 . A node of the P-type MOSFET Q 8  and the N-type MOSFET Q 9  is connected to a node of the resistor R 13  and the P-type MOSFET Q 10 . An output OUT is taken out from the node of the drain of the P-type MOSFET Q 10  and the resistor R 13 . 
     Next, a description will be made of operations of the DC/AC converter of Embodiment 1, which is configured as described above, while referring to operation waveforms of  FIG. 4 . 
     In  FIG. 4 , V_DRV 1  is the output of the first step-driver  18   a , V_DRV 2  is the output of the second step-driver  18   b , I_P is a primary-side current of the transformer T, I_Qp 1  is a drain current of the P-type MOSFET Qp 1 , and I_Qn 1  is a drain current of the N-type MOSFET Qn 1 . 
     Here, a description will be made of operations of the first step-driver  18   a  and the second step-driver  18   b  by using drive waveforms of the SW network, which are shown in  FIG. 4 , and operation waveforms when the SW network is driven at a resonant frequency. Here, the resonant frequency refers to a frequency at which the capacitors C 3  and C 4  and the reactor Lr, which compose the resonant circuit  9 , resonate with each other. 
     In the first step-driver  18   a  shown in  FIG. 2 , at a time t 1 , a PWM signal of the H level, which is inputted to the input terminal IN, turns on the N-type MOSFET Q 4  through the inverter  181  and the inverter  182 . Then, a current flows along a route of: the power supply REG→the resistor R 11 →the N-type MOSFET Q 4 →the current source I 1 →the ground. Therefore, the first step-driver  18   a  outputs, as the output OUT, a voltage obtained by multiplying the resistor R 11  and the current I 1 . 
     This voltage is slightly larger than the threshold voltage Vgs(th) between the gate and source of the P-type MOSFET Qp 1 . Therefore, the first step-driver  18   a  turns on the P-type MOSFET Qp 1  in first-step, and a slight current flows through the P-type MOSFET Qp 1 . 
     Meanwhile, at the time t 1 , the PWM signal of the H level, which is inputted to the input terminal IN, turns on the N-type MOSFET Q 2 . Then, an electric charge stored in the capacitor C 11  is discharged to the ground through the resistor R 12  and the N-type MOSFET Q 2 . At this time, when a time determined by a time constant of the resistor R 12  and the capacitor C 11  elapses to reach a time t 12 , the inverter  183  outputs the H level to the gate of the N-type MOSFET Q 5 . Therefore, the N-type MOSFET Q 5  is turned on, and a current flows along a route of: the power supply REG→the resistor R 11 →the N-type MOSFET Q 5 →the ground. Therefore, the first step-driver  18   a  outputs a voltage of a substantial ground level (L level) as the output OUT. 
     This voltage of the L level is fairly larger than the threshold voltage Vgs(th) between the gate and source of the P-type MOSFET Qp 1 . Therefore, the first step-driver  18   a  turns on the P-type MOSFET Qp 1  in second-step, and a larger current than the current at the time of turning on the first-step flows through the P-type MOSFET Qp 1 . 
     At a time t 2 , the PWM signal of the L level, which is inputted to the input terminal IN of the first step-driver  18   a , turns on the P-type MOSFET Q 3  through the inverter  181  and the inverter  182 . Then, a power supply voltage, that is, a voltage of the H level is outputted from the power supply REG through the P-type MOSFET Q 3  to the output OUT. Moreover, at the time t 2 , the PWM signal of the L level, which is inputted to the input terminal IN of the first step-driver  18   a , turns on the P-type MOSFET Q 1 . Then, the capacitor C 11  is charged by the power supply REG, and the inverter  183  outputs a voltage of the L level to the gate of the N-type MOSFET Q 5 , and accordingly, the N-type MOSFET Q 5  is off. 
     Next, in the second step-driver  18   b  shown in  FIG. 3 , at a time t 3 , a PWM signal of the H level, which is inputted to the input terminal IN, turns on the P-type MOSFET Q 8  through the inverter  184 , the inverter  185 , and the inverter  186 . Then, a current flows along a route of: the power supply REG→the current source I 2 →the P-type MOSFET Q 8 →the resistor R 13 →the ground. Therefore, the second step-driver  18   b  outputs, as the output OUT, a voltage obtained by multiplying the resistor R 13  and the current I 2 . 
     This voltage is slightly larger than the threshold voltage Vgs(th) between the gate and source of the N-type MOSFET Qn 1 . Therefore, the second step-driver  18   b  turns on the N-type MOSFET Qn 1  in first-step, and a slight current flows through the N-type MOSFET Qn 1 . 
     Meanwhile, at the time t 3 , the PWM signal of the H level, which is inputted to the input terminal IN, turns on the P-type MOSFET Q 6  through the inverter  184 . Then, a current flows along a route of: the power supply REG→the P-type MOSFET Q 6 →the resistor R 14 →the capacitor C 12 . Therefore, the capacitor C 12  is charged at a time constant determined by the resistor R 14  and the capacitor C 12 . At this time, when a time determined by the time constant of the resistor R 14  and the capacitor C 12  elapses to reach a time t 34 , the inverter  187  outputs a voltage of the L level to the gate of the P-type MOSFET Q 10 . Therefore, the P-type MOSFET Q 10  is turned on, and a current flows along a route of: the power supply REG→the P-type MOSFET Q 10 →the resistor R 13 →the ground. Therefore, the second step-driver  18   b  outputs a voltage of the H level as the output OUT. 
     This H-level voltage of the second step-driver  18   b  is fairly larger than the threshold voltage Vgs(th) between the gate and source of the N-type MOSFET Qn 1 . Therefore, the second step-driver  18   b  turns on the N-type MOSFET Qn 1  in second-step, and a larger current than the current at the time of turning on the first-step flows through the N-type MOSFET Qn 1 . 
     At a time t 3 ′, the PWM signal of the L level, which is inputted to the input terminal IN of the second step-driver  18   b , turns on the N-type MOSFET Q 9  through the inverter  184 , the inverter  185 , and the inverter  186 . Then, a voltage of the ground level (L level) is outputted to the output OUT. Moreover, at the time t 3 ′, the PWM signal of the L level, which is inputted to the input terminal IN of the second step-driver  18   b , turns on the N-type MOSFET Q 7  through the inverter  184 . Then, the capacitor C 12  is discharged, and the inverter  187  outputs a voltage of the H level to the gate of the P-type MOSFET Q 10 , and accordingly, the P-type MOSFET Q 10  is off. 
       FIG. 5  is a chart showing drive waveforms of the SW network provided in the DC/AC converter shown in  FIG. 1 , and operation waveforms when the SW network is driven in a lagging phase range of the resonance.  FIG. 6  is a chart showing drive waveforms of the SW network provided in the DC/AC converter shown in  FIG. 1 , and operation waveforms when the SW network is driven in a leading phase range of the resonance.  FIG. 7  is a chart showing drive waveforms of an SW network provided in a conventional DC/AC converter, and operation waveforms when the SW network is driven in the leading phase range of the resonance. 
     In the conventional DC/AC converter shown in  FIG. 7 , when the SW network is driven in the leading phase range of the resonance, a large pass-through current has occurred at the time when the P-type MOSFET Qp 1  is turned on and at the time when the N-type MOSFET Qn 1  is turned on. 
     As opposed to this, in the DC/AC converter of Embodiment 1, which is shown in  FIG. 6 , when the SW network is driven in the leading phase range of the resonance, the pass-through current, which occurs at the time when the P-type MOSFET Qp 1  is turned on and at the time when the N-type MOSFET Qn 1  is turned on, is reduced to a great extent. Specifically, the step-driver  18   a  turns on the P-type MOSFET Qp 1  in two steps, and the step-driver  18   b  turns on the N-type MOSFET Qn 1  in two steps. Accordingly, the pass-through current, which flows by a reverse recovery time of each of the parasitic diodes present in the P-type MOSFET Qp 1  and the N-type MOSFET Qn 1 , can be reduced. As a result, in Embodiment 1, the breakage of the P-type MOSFET Qp 1  and the N-type MOSFET Qn 1 , which is caused by the large pass-through current, is prevented. 
     Therefore, the DC/AC converter of Embodiment 1 is suitable for composing a discharge lamp lighting inverter with a high input voltage, for example, an input voltage of 100V to 400V, which uses high withstand-voltage MOSFETs in each of which the reverse recovery time of the parasitic diode is longer. 
     Embodiment 2 
     The DC/AC converter of Embodiment 1, which is shown in  FIG. 1 , uses the half-bridge-type SW network  7  composed of the first series circuit of the P-type MOSFET Qp 1  and the N-type MOSFET Qn 1 . 
     As opposed to this, a DC/AC converter of Embodiment 2 of the present invention, which is shown in  FIG. 8 , is characterized by using a full-bridge-type switching circuit (SW network)  7   b  composed of the first series circuit formed of the first P-type MOSFET Qp 1  and the first N-type MOSFET Qn 1 , and of a second series circuit formed of a second P-type MOSFET Qp 2  and a second N-type MOSFET Qn 2 . 
     The series circuit of the capacitor C 3  and the primary winding P of the transformer T is connected between the node of the first P-type MOSFET Qp 1  and the first N-type MOSFET Qn 1  and a node of the second P-type MOSFET Qp 2  and the second N-type MOSFET Qn 2 . The gate of the first P-type MOSFET Qp 1  is connected to the DRV 1  terminal. The gate of the first N-type MOSFET Qn 1  is connected to a DRV 3  terminal. A gate of the second P-type MOSFET Qp 2  is connected to the DRV 2  terminal. A gate of the second N-type MOSFET Qn 2  is connected to a DRV 4  terminal. 
     A control IC  1   b  is an IC in which the control IC  1  shown in  FIG. 1  further includes inverters  20   a  and  20   b , first and second dead time creation circuits  21   a  and  21   b , and third and fourth step-drivers  18   c  and  18   d . The third and fourth step-drivers  18   c  and  18   d  have the same configurations as that of the second step-driver  18   b  shown in  FIG. 1  and  FIG. 3 . 
     The first dead time creation circuit  21   a  directly outputs a NAND signal, which is from a NAND gate  17   e , to the first P-type MOSFET Qp 1 , and delays an inverted signal, which is obtained by inverting the NAND signal from the NAND gate  17   e  in the inverter  20   a , by a predetermined dead time DT, and outputs the inverted signal thus delayed to the third step-driver  18   c . The third step-driver  18   c  turns on the N-type MOSFET Qn 1  in two steps based on the signal from the first dead time creation circuit  21   a , and outputs a two-step gate drive signal to the gate of the first N-type MOSFET Qn 1 . 
     The second dead time creation circuit  21   b  directly outputs a logic signal, which is from a logic gate  17   f , to the second P-type MOSFET Qp 2 , and delays an inverted signal, which is obtained by inverting the logic signal from a logic gate  17   f  in the inverter  20   b , by the predetermined dead time DT, and outputs the inverted signal thus delayed to the fourth step-driver  18   d . The fourth step-driver  18   d  turns on the N-type MOSFET Qn 2  in two steps based on the signal from the second dead time creation circuit  21   b , and outputs a two-step gate drive signal to the gate of the second N-type MOSFET Qn 2 . 
       FIG. 9  is a chart showing drive waveforms of the SW network  7   b  provided in the DC/AC converter shown in  FIG. 8 . In  FIG. 9 , reference symbol DT denotes the dead time. 
     In  FIG. 8 , at a time t 1 , when the first P-type MOSFET Qp 1  and the second N-type MOSFET Qn 2  are turned on, a current flows along a route of: the power supply Vin→Qp 1 →C 3 →P→Qn 2 , the ground. Thereafter, at a time t 2 , when the first P-type MOSFET Qp 1  is turned off, a recovery current flows through the parasitic diode of the first P-type MOSFET Qp 1  along a route of: the ground→Qn 2 →P→C 3 →the parasitic diode of the first P-type MOSFET Qp 1 →the power supply Vin. If the N-type MOSFET Qn 1  is turned on at the time when this recovery current flows therethrough, then the pass-through current flows. Therefore, the N-type MOSFET Qn 1  is turned on in two steps by using the third step-driver  18   c  as a drive circuit of the N-type MOSFET Qn 1 , whereby the reduction of the pass-through current is achieved. 
     Next, at a time t 3 , when the second P-type MOSFET Qp 2  and the first N-type MOSFET Qn 1  are turned on, a current flows along a route of: the power supply Vin→Qp 2 →P→C 3 →Qn 1 →the ground. Thereafter, at a time t 3 ′, when the second P-type MOSFET Qp 2  is turned off, a recovery current flows through the parasitic diode of the second P-type MOSFET Qp 2  along a route of: the ground→Qn 1 →C 3 →P→the parasitic diode of the second P-type MOSFET Qp 2 →the power supply Vin. If the N-type MOSFET Qn 2  is turned on at the time when this recovery current flows therethrough, then the pass-through current flows through. Therefore, the N-type MOSFET Qn 2  is turned on in two steps by using the fourth step-driver  18   d  as a drive circuit of the N-type MOSFET Qn 2 , whereby the reduction of the pass-through current is achieved. As a result, in Embodiment 2, the breakage of the first and second P-type MOSFETs Qp 1  and Qp 2  and the first and second N-type MOSFETs Qn 1  and Qn 2 , which is caused by the large pass-through current, is prevented.