Patent Publication Number: US-8120264-B2

Title: Discharge lamp lighting circuit

Description:
RELATED APPLICATION 
     This application claims the benefit of priorities of Japanese patent applications no. JP2008-054970, filed on Mar. 5, 2008 and no. JP2008-299638, filed on Nov. 25, 2008, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a discharge lamp lighting circuit for preventing flame failure of a discharge lamp. 
     BACKGROUND 
     When a discharge lamp such as a metal halide lamp or the like is turned ON in AC mode, it is desirable that, in order to avoid occurrence of a phenomenon in which the lighting frequency resonates with an air flow in a discharge tube (i.e., the so-called acoustic resonance), the discharge lamp should be lit at a frequency of several hundred Hz or less. For example, in an automobile discharge lamp, a recommended value of lighting frequency of the discharge lamp is 250 Hz to 750 Hz. Nevertheless, the existing discharge lamp lighting circuit for the vehicle has a circuit configuration such that an input DC voltage from a battery is raised to a DC voltage necessary for the discharge lamp by a DC/DC converter, and then this DC voltage is AC-converted into the lighting frequency of the discharge lamp by a full-bridge inverter provided at a later stage. Also, a starter circuit generates high-voltage pulses of about 25 kV required to start the discharge lamp. A lighting auxiliary circuit promotes a discharge growth from a glow discharge to an arc discharge by supplying an excessive current immediately after the discharge lamp is started, so that this lighting auxiliary circuit allows instantaneous lighting. 
     Japanese Patent Document JP-A-11-329777 discloses a discharge lamp lighting circuit for the vehicle equipped with the DC/DC converter and the full-bridge inverter. 
     However, the conventional discharge lamp lighting circuit has a standardized configuration and contains a large number of large-size electronic components, which prevents a size reduction and a cost reduction of the discharge lamp lighting circuit and also becomes a major cause for the fact that the automobile discharge lamp cannot become sufficiently available. 
     For example, in the circuit configuration disclosed in JP-A-11-329777, there is a large amount of heat produced by the switching element and the rectifier diode in the DC/DC converter, and the temperature of respective elements tends to rise locally. In addition, according to the same configuration, a driver circuit for driving four switching elements of the full-bridge inverter is needed, which also limits size reduction and cost reduction of the discharge lamp lighting circuit. 
     SUMMARY 
     The present invention, in some implementations, achieves a size reduction and a cost reduction of a lighting circuit by changing fundamentally the above-mentioned standardized configuration. 
     According to a first aspect of the present invention, a discharge lamp lighting circuit for supplying an AC power to a discharge lamp includes first and second converters for receiving a DC voltage and stepping up the voltage. A controlling circuit for driving the first and second converters alternately at a first frequency and stopping an operation of a side that is not driven to apply a control such that the AC power is supplied to the discharge lamp to execute a lighting operation. 
     Therefore, the circuit itself can be reduced in size by employing the first and second converters. 
     In some implementations, the first and second converters are implemented by any one of (a) a converter having an insulated-type first transformer, a first switching element connected in series between a DC power supply and a primary winding of the first transformer, and a second switching element connected in series between an output end and a secondary winding of the first transformer, whereby the first and second switching elements are driven in synchronism with each other in each half period of the first frequency at a second frequency higher than the first frequency, (b) a converter having an insulated-type second transformer, a third switching element connected in series between the DC power supply and a primary winding of the second transformer, a first rectifying element connected in series between one output end and a secondary winding of the second transformer, and a fourth switching element connected between one output end and the other output end, whereby the fourth switching element is put in an OFF state while the third switching element executes a driving operation and also the fourth switching element is put in an ON state while the third switching element executes a stopping operation, or (c) a converter having an insulated-type third transformer, a fifth switching element connected in series between a DC power supply and a primary winding of the third transformer, a sixth switching element connected in series between an output end and a secondary winding of the third transformer, and a second rectifying element connected in parallel with the sixth switching element, whereby the sixth switching element is put in an OFF state while the fifth switching element executes a driving operation and also the sixth switching element is put in an ON state while the fifth switching element executes a stopping operation. 
     Also, any one of the second, fourth, and sixth switching elements on secondary sides of the first and second converters can be implemented by an insulated gate bipolar transistor. 
     The insulated-type first to third transformers in the first and second converters can be implemented such that a part of magnetic members is shared mutually. 
     According to some implementations, the discharge lamp lighting circuit includes a starter circuit for starting the discharge lamp, and having a starter capacitor that receives a charging voltage from one of the first and second converters. The converter on a side from which the charging voltage is supplied to the starter circuit is implemented the converter set forth in (b) or (c), and the fourth or sixth switching element on the secondary side is implemented by the insulated gate bipolar transistor. 
     In the discharge lamp lighting circuit in one mode, each of the first and second converters includes an insulated second transformer, a third switching element connected in series with a primary winding of the second transformer, a first rectifying element provided between an output end and a secondary winding of the second transformer, and a fourth switching element and a current detecting resistor connected in series between the output end and a fixed voltage terminal. Also, the controlling circuit puts the fourth switching element in an OFF state while the third switching element is caused to drive/operate, and puts the fourth switching element in an ON state while the third switching element is caused to stop an operation. The controlling circuit adjusts an ON/OFF duty ratio of the third switching element on the first converter side based, at least in part, on a voltage drop produced in the current detecting resistor on the second converter side while the first converter is caused to drive/operate, and adjusts an ON/OFF duty ratio of the third switching element on the second converter side based, at least in part, on a voltage drop produced in the current detecting resistor on the first converter side while the second converter is caused to drive/operate. 
     According to this mode, the lamp current flowing through the discharge lamp in the first direction and the lamp current flowing through the discharge lamp in the second direction opposite to the first direction can be detected, for example, by using the current detecting resistors provided to the first and second converters, respectively, and the third switching elements provided to the first and second converter respectively can be driven. Also, the charges stored in the smoothing capacitors provided to the output ends of the first and second converters cannot flow in the discharge lamp in the open state of the discharge lamp. Therefore, such charges tend to flow in the fixed voltage terminal (e.g., the ground terminal) via the fourth switching element. At this time, the current flowing through the fourth switching element can be restricted by the current detecting resistor, and thus the circuit can be protected. 
     In the discharge lamp lighting circuit in another mode, each of the first and second converters includes an insulated second transformer, a third switching element connected in series with a primary winding of the second transformer, a first rectifying element provided between an output end and a secondary winding of the second transformer, and a fourth switching element whose one end is connected to the output end. Another end of the fourth switching element on the first converter side and other end of the fourth switching element provided on the second converter side are connected in common. The discharge lamp lighting circuit further includes a current detecting resistor provided between the other end of the fourth switching element connected in common and a fixed voltage terminal. The controlling circuit puts the fourth switching element in an OFF state while the third switching element is caused to drive/operate, and puts the fourth switching element in an ON state while the third switching element is caused to stop an operation, and the controlling circuit adjusts an ON/OFF duty ratio of the third switching element on the first converter side based, at least in part, on a voltage drop produced in the current detecting resistor on the second converter side while the first converter is caused to drive/operate, and adjusts an ON/OFF duty ratio of the third switching element on the second converter side based, at least in part, on a voltage drop produced in the current detecting resistor on the first converter side while the second converter is caused to drive/operate. 
     According to yet another mode, the lamp current flowing through the discharge lamp in the first direction and the lamp current flowing through the discharge lamp in the second direction opposite to the first direction can be detected, for example, by using the current detecting resistor provided in common to the first and second converters respectively. The charges stored in the smoothing capacitors provided to the output ends of the first and second converters cannot flow in the discharge lamp in the open state of the discharge lamp. Therefore, such charges tend to flow in the fixed voltage terminal (e.g., the ground terminal) via the fourth switching element. At that time, the current flowing through the fourth switching element can be restricted by the current detecting resistor, and thus the circuit can be protected. This mode is advantageous from the perspective of circuit area and a cost. 
     In some implementations, the discharge lamp lighting circuit results in a size reduction and a cost reduction. Also, since the first and second converters are alternately operated, the amount of produced heat can be reduced, and locations where the heat is produced can be scattered so that the local temperature rise can be reduced or eliminated, and reliability of the circuit can be improved. 
     According to some implementations, a reduction in the number of components, and a size reduction and a cost reduction of the circuit can be attained. 
     According to some implementations, since a part of the magnetic material of the first and second transformers is shared with each other, an installing volume can be reduced and a size reduction can be attained. 
     According to some implementations, heat generation of the circuit can be reduced. 
     According to some implementations, the IGBT is employed as the switching element of the converter on the side where a charging voltage is supplied to the starter circuit, which is effective in reducing the heat generation of the circuit. 
     As described in greater detail below, the discharge lamp lighting circuit can be used as part of car lighting equipment, such as a head lamp, for example. Also, the electronic components are made compatible by combining the DC/DC converting function in the DC/DC converter with the DC/AC converting function in the full-bridge inverter and thus a size reduction and a cost reduction of the circuit can be attained. 
     Other features and advantages will be readily apparent from the detailed description and the accompanying drawings and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [ FIG. 1 ] A conceptual view of a discharge lamp lighting circuit according to a first embodiment of the present invention. 
       [ FIG. 2 ] A conceptual view of a discharge lamp lighting circuit according to a second embodiment of the present invention. 
       [ FIG. 3 ] A timing chart showing a switching operation made by the discharge lamp lighting circuit according to a second embodiment of the present invention. 
       [ FIG. 4 ] A conceptual view of a discharge lamp lighting circuit according to a third embodiment of the present invention. 
       [ FIG. 5 ] A timing chart showing a switching operation made by the discharge lamp lighting circuit according to the third embodiment of the present invention. 
       [ FIG. 6 ] A conceptual view of a discharge lamp lighting circuit according to a fourth embodiment of the present invention. 
       [ FIG. 7 ] A timing chart showing a switching operation made by the discharge lamp lighting circuit according to the fourth embodiment of the present invention. 
       [ FIG. 8 ] A conceptual view of a discharge lamp lighting circuit according to a fifth embodiment of the present invention. 
       [ FIG. 9 ] A timing chart showing a switching operation made by the discharge lamp lighting circuit according to the fifth embodiment of the present invention. 
       [ FIG. 10 ] A conceptual view of a discharge lamp lighting circuit according to a sixth embodiment of the present invention. 
       [ FIG. 11 ] A timing chart showing a switching operation made by the discharge lamp lighting circuit according to the sixth embodiment of the present invention. 
       [ FIG. 12 ] A conceptual view of a discharge lamp lighting circuit according to a seventh embodiment of the present invention. 
       [ FIG. 13 ] A timing chart showing a switching operation made by the discharge lamp lighting circuit according to the seventh embodiment of the present invention. 
       [ FIG. 14 ] A view showing arrangements of transformers in first and second converters. 
       [ FIG. 15 ]  FIGS. 15(   a ) ( b ) are a configurative view showing the discharge lamp lighting circuit according to the embodiments respectively. 
       [ FIG. 16 ]  FIGS. 16(   a ) ( b ) are an operation waveform diagram of the discharge lamp lighting circuit in  FIG. 15(   a ) and an operation waveform diagram of the comparative art in an open state respectively. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     A configuration of a discharge lamp lighting circuit according to a first embodiment of the present invention is shown in  FIG. 1 , and is explained below. 
     As shown in  FIG. 1 , the discharge lamp lighting circuit can be part of car lighting equipment, such as a head lamp, for example. A low-frequency AC lighting system is employed, and a power supply  1  such as a battery, a switch SW 0 , two systems of first and second converters CON 1 , CON 2 , switching elements S 1 , S 2 , a starter circuit  2 , and a discharge lamp  3  are provided. The first and second converters CON 1 , CON 2  are a step-up/down converter that not only steps up a voltage but also steps down a voltage respectively, and output terminals as the converter outputs are connected electrically to electrodes of the discharge lamp  3  respectively. 
     In such configuration, when the discharge lamp  3  is lit by turning ON the switch SW 0 , a controlling circuit  4  causes the first and second converters CON 1 , CON 2  to operate alternately. Then, the controlling circuit  4  connects the output of the converter CON 1  or CON 2  whose operation is stopped to the ground GND via the switching element S 1  or S 2 . AC power is supplied to the discharge lamp  3  by repeating such operations. A frequency at which the first and second converters CON 1 , CON 2  are operated alternately is defined as a “lighting frequency”. 
     Operation of the discharge lamp lighting circuit is now explained. 
     For example, when a current IL 1  (whose flowing direction is indicated by an arrow along with the same symbol) flows through the discharge lamp  3 , respective operation and states are set under control of the controlling circuit  4  such that the first converter CON 1  has a voltage step-up operation, the second converter CON 2  is set in a stopped state, the switching element S 1  is set in an open state, and the switching element S 2  is set in a short state (GND state). 
     When the direction of the current flowing through the discharge lamp  3  is switched to a current IL 2  (whose flowing direction is indicated by an arrow along with the same symbol) from this state, following controls are made sequentially. 
     That is, to provide a time in which the operations of both the first converter CON 1  and the second converter CON 2  are stopped, initially the controlling circuit  4  stops the operation of the first converter CON 1 . Then, the controlling circuit  4  switches the switching element S 2  from a short state to an open state, and switches the switching element S 1  from an open state to a short state. Then, the controlling circuit  4  causes the second converter CON 2  to start the operation from its stop state. With the above operations, the direction of the current flowing through the discharge lamp  3  is switched from the current IL 1  to the current IL 2 . 
     In contrast, the direction of the current flowing through the discharge lamp  3  is switched from the current IL 2  to the current IL 1 , controls are made by the controlling circuit  4  in following sequences. 
     That is, to provide a time in which the operations of both the first converter CON 1  and the second converter CON 2  are stopped, initially the controlling circuit  4  stops the operation of the second converter CON 2 . Then, the controlling circuit  4  switches the switching element S 1  from a short state to an open state, and switches the switching element S 2  from an open state to a short state. Also, the controlling circuit  4  causes the first converter CON 1  to start the operation from its stop state. With the foregoing operations, the direction of the current flowing through the discharge lamp  3  is switched from the current IL 2  to the current IL 1 . 
     In this manner, the discharge lamp lighting circuit according to the first embodiment is the discharge lamp lighting circuit of the low-frequency AC lighting system and has two systems of step-up/down converters. Converter outputs of the first and second converters CON 1 , CON 2  are connected to the electrodes of the discharge lamp  3 , respectively. Operation/stop of the first and second converters CON 1 , CON 2  is repeated alternately at the “lighting frequency” of the discharge lamp  3  under control of the controlling circuit  4 . Therefore, the low-frequency AC power suitable for this discharge lamp  3  can be supplied. 
     Next, discharge lamp lighting circuits according to second to seventh embodiments are described. 
     Second Embodiment 
     A configuration of a discharge lamp lighting circuit according to a second embodiment of the present invention is shown in  FIG. 2 , and is explained below. 
     As shown in  FIG. 2 , the discharge lamp lighting circuit includes the first and second converters CON 1 , CON 2 , the starter circuit  2 , the discharge lamp  3 , and the controlling circuit  4 . 
     A power supply voltage supplied from a power supply (not shown) is supplied to the primary side (primary winding T 1   a ) of a transformer T 1  of the first converter CON 1  via an inductor L 1 . One end of a capacitor C 1  is connected to a terminal of the inductor L 1  on the power supply side, and the other end is connected to the ground. Winding starting ends of respective windings T 1   a , T 1   b  of the transformer T 1  are indicated with a black dot in  FIG. 2 . The first converter CON 1  is constructed such that a switching element Q 2  is connected to a winding terminating end of the primary winding T 1   a  of the transformer T 1 , a rectifier diode D 3  and a smoothing capacitor C 3  are arranged to a winding terminating end of the secondary winding T 1   b  of the transformer T 1 , and a terminal voltage of the smoothing capacitor C 3  is extracted as an output voltage. In this example, an N-channel MOSFET (field effect transistor) is employed as the switching element Q 2 , a control signal SW 1  from the controlling circuit  4  is supplied to a gate of the N-channel MOSFET, and the switching of the switching element Q 2  is controlled. Thus, an output voltage value is controlled variably. 
     Energy stored in the transformer T 1  is accumulated in the smoothing capacitor C 3  via the rectifier diode D 3  by ON/OFF control of the switching element Q 2 . The switching element S 1  is provided to an output end of the first converter CON 1 . 
     In this example, the N-channel MOSFET (field effect transistor) is employed as the switching element S 1 , and a control signal SW 3  is supplied from the controlling circuit  4  to a gate of the N-channel MOSFET, and thus the switching of the switching element S 1  is controlled. A drain of the switching element S 1  is connected to a cathode of the rectifier diode D 3 , and a source of the switching element S 1  is connected to a ground end GND of the smoothing capacitor C 3  via a current detecting resistor R 1 . A connection point between the resistor R 1  and the switching element S 1  is connected to the controlling circuit  4 , and the current IL 1  is detected from this connection point. 
     The power supply voltage supplied from the power supply (not shown) also is supplied to the primary side (primary winding T 2   a ) of a transformer T 2  of the second converter CON 2  via the inductor L 1 . Winding starting ends of respective windings T 2   a , T 2   b  of the transformer T 2  are indicated with a black dot in  FIG. 2 . The second converter CON 2  is constructed such that a switching element Q 4  is connected to a winding terminating end of the primary winding T 2   a  of the transformer T 2 , a rectifier diode D 4  and a smoothing capacitor C 4  are arranged to a winding terminating end of the secondary winding T 2   b  of the transformer T 2 , and a terminal voltage of the smoothing capacitor C 4  is extracted as an output voltage. In this example, the N-channel MOSFET (field effect transistor) is employed as the switching element Q 4 , a control signal SW 2  from the controlling circuit  4  is supplied to the gate of the N-channel MOSFET, and the switching of the switching element Q 4  is controlled. Thus, the output voltage value is controlled variably. 
     Energy stored in the transformer T 2  is accumulated in the smoothing capacitor C 4  via the rectifier diode D 4  by ON/OFF control of the switching element Q 4 . The switching element S 2  is provided to an output end of the second converter CON 2 . In this example, an insulated gate bipolar transistor (IGBT) is employed as the switching element S 2 , a control signal SW 4  is supplied to a gate of the IGBT from the controlling circuit  4 , and thus the switching of the switching element S 2  is controlled. A collector of the switching element S 2  is connected to a cathode of the rectifier diode D 4 , and an emitter of the switching element S 2  is connected to a ground terminal GND of the smoothing capacitor C 4  via a current detecting resistor R 2 . A connection point between the resistor R 2  and the switching element S 2  is connected to the controlling circuit  4 , and the current IL 2  is detected from this connection point. 
     The starter circuit  2  has a transformer T 3 . A capacitor C 5  and a switching element SG 1  are provided to a primary winding of the transformer T 3  of the primary side circuit that contains the primary winding of the transformer T 3 . In this example, a spark gap element or the like is employed as the switching element SG 1 . That is, one end of the capacitor C 5  is connected to one ends of the primary winding and a secondary winding of the transformer T 3 , and the other end of the capacitor C 5  is connected to the other end of the primary winding via the switching element SG 1 . More particularly, when a voltage fed to the switching element SG 1  exceeds a threshold value and the switching element SG 1  becomes conductive, the charges stored in the capacitor C 5  are discharged. As a result, a high-voltage pulse generated in the secondary winding of the transformer T 3  is superposed on the output of the first or second converter CON 1 , CON 2 , and then fed to the discharge lamp  3  as a starting pulse. 
     In this example, the high voltage required to produce a charging voltage to the starter circuit can be applied to the switching element S 2 . Therefore, when the IGBT, which has a high withstand voltage and whose ON voltage is low, is employed as this switching element S 2 , a power loss caused in the element can be suppressed and heat generation in the circuit can be reduced. 
     Operation of the foregoing configuration is explained with reference to  FIG. 3 . 
     In the above configuration, the controlling circuit  4  causes the first converter CON 1  to start an operation by turning ON the switching element Q 2 , stops an operation of the second converter CON 2  by turning OFF the switching element Q 4 , and turns OFF the switching element S 1  and turns ON the switching element S 2 . Accordingly, the current IL 1  flows through the discharge lamp  3  (period PA 1 ). 
     When the direction of the current flowing through the discharge lamp  3  is switched to the current IL 2  from this state, initially the controlling circuit  4  stops the operation of the first converter CON 1  by turning OFF the switching element Q 2  to create a time during which both the first converter CON 1  and the second converter CON 2  stop the operation. Then, the controlling circuit  4  turns OFF the switching element S 2  and turns ON the switching element S 1 . Also, the controlling circuit  4  causes the second converter CON 2  to start the step-up operation by turning ON the switching element Q 4 . With the above operations, the direction of the current flowing through the discharge lamp  3  is switched from the current IL 1  to the current IL 2  (period PA 2 ). 
     Then, when the direction of the current flowing through the discharge lamp  3  is switched again from the current IL 2  to the current IL 1 , initially the controlling circuit  4  stops the operation of the second converter CON 2  by turning OFF the switching element Q 4  to create a time during which both the first converter CON 1  and the second converter CON 2  stop the operation. Then, the controlling circuit  4  turns OFF the switching element S 1 , turns ON the switching element S 2 , and turns ON the switching element Q 2 , and thus causes the first converter CON 1  to start the operation. With the above operations, the direction of the current flowing through the discharge lamp  3  is switched from the current IL 2  to the current IL 1  (period PA 3 ). 
     The operations based upon such lighting periods are repeated at the lighting frequency of the discharge lamp  3 . Therefore, the low-frequency AC power suitable for this discharge lamp  3  can be supplied. 
     As explained above, in the discharge lamp lighting circuit according to the second embodiment, the output side of the first and second converters CON 1 , CON 2  (the secondary windings T 1   b , T 2   b  side of the transformers T 1 , T 2 ) are connected in parallel with the switching elements S 1 , S 2  (referred to as “secondary side switching elements” hereinafter). First ends, from which the current is sent out, of respective converters produce the output power by virtue of the secondary side rectification in the secondary side switching elements, which gives the current flowing through the discharge lamp  3 . The other ends, which are located on the side to take the current of respective converters, take the current flowing from the discharge lamp  3  when the secondary side switching elements become conductive. 
     Third Embodiment 
     A configuration of a discharge lamp lighting circuit according to a third embodiment of the present invention is shown in  FIG. 4 , and is explained below. 
     Here, the same reference symbols are used for features that are the same or substantially the same as those in  FIG. 2 , and detailed explanation of those features is omitted. 
     In the above second embodiment ( FIG. 2 ), the switching elements S 1 , S 2  are provided to the output ends of the first and second converters CON 1 , CON 2 . These elements are omitted in the third embodiment. Also, the rectifier diodes D 3 , D 4  are provided to the first and second converters CON 1 , CON 2 , but such configuration is omitted herein. 
     In the first converter CON 1 , a switching element Q 3  is provided to the secondary winding starting end of the transformer T 1 . In this example, the N-channel MOSFET is employed as the switching element Q 3 . A drain of the switching element Q 3  is connected to the winding starting end of the secondary winding T 1   b  of the transformer T 1 , and a source of the switching element Q 3  is connected to the ground. 
     While the first converter CON 1  is executing the step-up operation, the switching element Q 3  also functions as the rectifying switching element. 
     The smoothing capacitor C 3  is arranged to the secondary winding terminating end of the transformer T 1 . A terminal voltage of the smoothing capacitor C 3  is picked up as the output voltage. 
     At this time, a control signal RECT 1  is supplied from the controlling circuit  4  to a gate of the switching element Q 3 , and the switching of the switching element Q 3  is controlled based on the control signal RECT 1 . Therefore, a value of the output voltage is controlled variably. 
     In contrast, in the second converter CON 2 , a switching element Q 5  is provided to the secondary winding starting end of the transformer T 2 . In this example, the N-channel MOSFET is also employed as the switching element Q 5 . A drain of the switching element Q 5  is connected to the winding starting end of the secondary winding T 2   b  of the transformer T 2 , and a source of the switching element Q 5  is connected to the ground. 
     Similarly, while the first converter CON 2  is executing the step-up operation, the switching element Q 5  also functions as the rectifying switching element. 
     The smoothing capacitor C 4  is arranged to the secondary winding terminating end of the transformer T 2 . A terminal voltage of the smoothing capacitor C 4  is picked up as the output voltage. 
     At this time, a control signal RECT 2  is supplied from the controlling circuit  4  to a gate of the switching element Q 5 , and the switching of the switching element Q 5  is controlled based on the control signal RECT 2 . Therefore, a value of the output voltage is controlled variably. 
     Operation of the foregoing configuration is explained in detail below with reference to  FIG. 5 . 
     While the first converter CON 1  is operating, the switching element Q 2  acts as the step-up switching element, the switching element Q 3  acts as the rectifying switching element, and the switching element Q 3  executes a synchronous rectification. Also, the second converter CON 2  is brought into a non-operative state by turning OFF the switching element Q 4 , and the switching element Q 5  is rendered conductive, and thus the current IL 1  flows through the discharge lamp  3  (period PB 1 ). 
     A behavior of the synchronous rectification is shown by numeral  100  in  FIG. 5  in an enlarged fashion. Here, ON/OFF states of the switching elements Q 2 , Q 3  and characteristics (Q 2 ID, Q 3 ID) of the currents flowing through the switching elements Q 2 , Q 3  in response to these states are illustrated. The direction of the current flowing from the drain to the source of the switching element Q 2  is set positive in a waveform Q 2 ID, and the direction of the current flowing from the source to the drain of the switching element Q 3  is set positive in a waveform Q 3 ID. 
     Conversely, while the second converter CON 2  is operating, the switching element Q 4  acts as the step-up switching element, the switching element Q 5  acts as the rectifying switching element, and the switching element Q 5  executes a synchronous rectification. Also, the first converter CON 1  is brought into a non-operative state by turning OFF the switching element Q 2 , and the switching element Q 3  is rendered conductive, and thus the current IL 2  flows through the discharge lamp  3  (period PB 2 ). 
     The operations based upon such lighting periods are repeated at the lighting frequency of the discharge lamp  3 . Therefore, low-frequency AC power suitable for this discharge lamp  3  can be supplied. 
     As explained above, in the third embodiment, the first and second converters CON 1 , CON 2  are insulated-type switching converters. The switching elements Q 3 , Q 5  are connected in series with the secondary windings T 1   b , T 2   b  of the transformers T 1 , T 2  of respective converters. 
     First ends, from which the current flows, of respective converters produce the output power by virtue of the synchronous rectification of the switching elements Q 3 , Q 5 , which gives the current flowing through the discharge lamp  3 . The other ends, which are located on the side to take the current, of respective converters, take the current flowing from the discharge lamp  3  when the switching elements Q 3 , Q 5  becomes conductive. In this manner, the switching elements Q 3 , Q 5  can also function as the rectifying elements, and therefore the number of components can be reduced. 
     Fourth Embodiment 
     A configuration of a discharge lamp lighting circuit according to a fourth embodiment of the present invention is shown in  FIG. 6 , and is explained below. 
     Here, the same reference symbols are used for features that are the same or substantially the same as those shown in  FIG. 4 . Accordingly, detailed explanation of those features is omitted. 
     In the third embodiment ( FIG. 4 ), the N-channel MOSFET is provided to the secondary winding starting ends of the transformers T 1 , T 2  of the first and second converters CON 1 , CON 2  as the switching elements Q 3 , Q 5  respectively, and the switching elements Q 3 , Q 5  also function as the rectifying switching elements while respective converters are executing the step-up operation. In contrast, in the discharge lamp lighting circuit according to the fourth embodiment, the switching elements Q 3 , Q 5  are replaced with switching elements S 3 , S 4  constructed by the IGBT respectively, and diodes D 5 , D 6  are added. 
     That is, in the first converter CON 1 , the switching element S 3  is provided to the secondary winding starting end of the transformer T 1 . In this example, the IGBT is employed as the switching element S 3 . A collector of the switching element S 3  is connected to the winding starting end of the secondary winding T 1   b  of the transformer T 1 , and an emitter of the switching element S 3  is grounded. 
     Then, while the first converter CON 1  is executing the step-up operation, the current is rectified by the diode D 5  that is connected between the emitter-collector of the switching element S 3 . An anode of the diode D 5  is connected to an emitter of the switching element S 3 , and a cathode of the diode D 5  is connected to a collector of the switching element S 3 . 
     The smoothing capacitor C 3  is arranged to the secondary winding terminating end of the transformer T 1 . A terminal voltage of the smoothing capacitor C 3  is picked up as the output voltage. 
     At this time, a control signal RECT 3  is supplied from the controlling circuit  4  to a gate of the switching element S 3 , and the switching of the switching element S 3  is controlled based on the control signal RECT 3 . 
     Similarly, in the second converter CON 2 , the switching element S 4  is provided to the secondary winding starting end of the transformer T 2 . In this example, the IGBT is employed as the switching element S 4 . A collector of the switching element S 4  is connected to the winding starting end of the secondary winding T 2   b  of the transformer T 2 , and an emitter of the switching element S 3  is grounded. 
     Then, while the second converter CON 2  is executing the step-up operation, the current is rectified by the diode D 6  that is connected between the emitter-collector of the switching element S 4 . An anode of the diode D 6  is connected to an emitter of the switching element S 4 , and a cathode of the diode D 6  is connected to a collector of the switching element S 4 . 
     The smoothing capacitor C 4  is arranged to the secondary winding terminating end of the transformer T 1 . A terminal voltage of the smoothing capacitor C 4  is picked up as the output voltage. 
     At this time, a control signal RECT 4  is supplied from the controlling circuit  4  to a gate of the switching element S 4 , and the switching of the switching element S 4  is controlled based on the control signal RECT 4 . 
     In this example, the high voltage required to produce a charging voltage to the starter circuit can be applied to the switching element S 4 . Therefore, when the IGBT, which has a high withstand voltage and whose ON voltage is low, is employed as this switching element S 4 , a power loss caused in the element can be suppressed and heat generation in the circuit can be reduced. 
     Operation of the above configuration is explained in detail below with reference to  FIG. 7 . 
     While the first converter CON 1  is operating, the switching element Q 2  functions as the step-up switching element and the switching element Q 2  is ON/OFF-operated, and the switching element S 3  is turned OFF and the current is rectified by the diode D 5  that is connected between the emitter-collector of the switching element S 3 . In this state, the switching element S 4  is brought into a conductive state and thus the current IL 1  flows through the discharge lamp  3  (period PC 1 ). 
     In contrast, while the second converter CON 2  is operating, the switching element Q 4  functions as the step-up switching element and the switching element Q 4  is ON/OFF-operated, and the switching element S 4  is turned OFF and the current is rectified by the diode D 6  that is connected between the emitter-collector of the switching element S 4 . In this state, the switching element S 3  is brought into a conductive state and thus the current IL 2  flows through the discharge lamp  3  (period PC 2 ). 
     The operations of the first and second converters CON 1 , CON 2  are repeated at the lighting frequency of the discharge lamp  3 . Therefore, the low-frequency AC power can be applied to the discharge lamp  3 . 
     As explained above, in the discharge lamp lighting circuit according to the fourth embodiment, the switching elements S 3 , S 4  are connected in series with the secondary windings T 1   b , T 2   b  of the transformers T 1 , T 2 . 
     Then, at one ends of respective converters from which the current is sent out, the current is rectified by the diodes D 5 , D 6  being connected between the emitter-collector of the switching elements S 3 , S 4  respectively, which gives the current flowing through the discharge lamp  3 . Also, the other ends, which are located on the side to take the current of respective converters, take the current flowing out from the discharge lamp  3  when the switching elements S 3 , S 4  become conductive. 
     In this manner, the diodes D 5 , D 6  connected in parallel with the switching elements S 3 , S 4  also function as the rectifier diode respectively. Here, the number of components can be reduced by using the element in which the diodes D 5 , D 6  and the IGBT are sealed in the same package. 
     Fifth Embodiment 
     A configuration of a discharge lamp lighting circuit according to a fifth embodiment of the present invention is shown in  FIG. 8 , and is explained below. 
     The discharge lamp lighting circuit according to the fifth embodiment has a similar configuration as that in the aforementioned third embodiment. In this case, in the aforementioned third embodiment ( FIG. 4 ), when the first converter CON 1  is executing the step-up operation, the switching element Q 3  functions as the rectifying switching element, and this switching element Q 3  performs the synchronous rectification. Also, when the second converter CON 2  is executing the step-up operation, the switching element Q 5  functions as the rectifying switching element, and this switching element Q 5  performs the synchronous rectification. 
     In contrast, in the fifth embodiment, the rectification is executed by using passively parasitic diodes of the switching elements Q 5 , Q 6 , which is implemented by the N-channel MOSFET and correspond to the switching elements Q 3 , Q 5  respectively. 
     Operation of the foregoing configuration is explained in detail below with reference to  FIG. 9 . 
     While the first converter CON 1  is operating, the switching element Q 2  acts as the step-up switching element and the switching element Q 2  is ON/OFF-operated, and the switching element S 5  acts as the rectifying switching element and the current is rectified by using passively the parasitic diode of the switching element S 5 . In this state, a switching element S 6  is brought into a conductive state and thus the current IL 1  flows through the discharge lamp  3  (period PD 1 ). 
     In contrast, while the second converter CON 2  is operating, the switching element Q 4  acts as the step-up switching element and the switching element Q 4  is ON/OFF-operated, and the switching element S 6  acts as the rectifying switching element and the current is rectified by using passively the parasitic diode of the switching element S 6 . In this state, the switching element S 5  is brought into a conductive state and thus the current IL 2  flows through the discharge lamp  3  (period PD 2 ). 
     The operations of the first and second converters CON 1 , CON 2  are repeated at the lighting frequency of the discharge lamp  3 . Therefore, the low-frequency AC power can be applied to the discharge lamp  3 . 
     As explained above, in the fifth embodiment, the switching elements S 5 , S 6  are connected in series with the secondary windings T 1   b , T 2   b  of the transformers T 1 , T 2  of the first and second converters CON 1 , CON 2 . Then, at one ends of respective converters from which the current is sent out, the current is rectified by using passively the parasitic diodes of the switching elements S 5 , S 6 , which gives the current flowing through the discharge lamp  3 . Also, the other ends, which are located on the side to suck the current, of respective converters suck the current being flown out from the discharge lamp  3  when the switching elements S 5 , S 6  become conductive. 
     In this manner, the parasitic diodes of the switching elements S 5 , S 6  function as the rectifier diodes. Therefore, the number of components can be reduced. 
     Sixth Embodiment 
     A configuration of a discharge lamp lighting circuit according to a sixth embodiment of the present invention is shown in  FIG. 10 , and is explained below. 
     In the discharge lamp lighting circuit according to the sixth embodiment, the first converter CON 1  side has the similar configuration to that in the second embodiment ( FIG. 2 ), and the second converter CON 2  has the similar configuration to that in the fifth embodiment ( FIG. 8 ). The same reference symbols are used for features that are the same or substantially the same as those in  FIG. 2  and  FIG. 8 . Accordingly, detailed description of those features is omitted. 
     As shown in  FIG. 10 , a switching element S 7  is provided to the output end of the first converter CON 1 . The N-channel MOSFET (field effect transistor) is employed as the switching element S 7 , a control signal RECT 7  is supplied from the controlling circuit  4  to a gate of the N-channel MOSFET, and the switching of the switching element S 7  is controlled. 
     In contrast, in the second converter CON 2 , a switching element S 8  is provided to the secondary winding starting end of the transformer T 2 . In this example, the N-channel MOSFET is also employed as the switching element S 8 . A drain of the switching element S 8  is connected to the winding starting end of the secondary winding T 2   b  of the transformer T 2 , and a source of the switching element S 8  is grounded. 
     While the first converter CON 2  is executing the step-up operation, the switching element Q 8  also functions as the rectifying switching element. 
     The smoothing capacitor C 4  is arranged to the secondary winding terminating end of the transformer T 2 . A terminal voltage of the smoothing capacitor C 4  is picked up as the output voltage. 
     At this time, a control signal RECT 0  is supplied from the controlling circuit  4  to a gate of the switching element Q 8 , and the switching of the switching element Q 8  is controlled based on the control signal RECT 8 . Therefore, a value of the output voltage is controlled variably. 
     Operation of the foregoing configuration is explained below with reference to  FIG. 11 . 
     The controlling circuit  4  causes the first converter CON 1  to start the operation by turning ON the switching element Q 2 , stops the step-up operation of the second converter CON 2  by turning OFF the switching element Q 4 , and turns OFF the switching element S 7  and turns ON the switching element S 8 . Accordingly, the current IL 1  flows through the discharge lamp  3  (period PE 1 ). 
     When the second converter CON 2  is caused to operate, the switching element Q 4  acts as the step-up switching element and the switching element Q 4  is ON/OFF-operated, and the switching element S 8  acts as the rectifying switching element and the current is rectified by using passively the parasitic diode of the switching element S 8 . In this state, the switching element S 5  is brought into a conductive state and thus the current IL 2  flows through the discharge lamp  3  (period PE 2 ). 
     The operations of the first and second converters CON 1 , CON 2  are repeated at the lighting frequency of the discharge lamp  3 . Therefore, the low-frequency AC power can be applied to the discharge lamp  3 . 
     As explained above, in the sixth embodiment, the switching element S 7  is provided to the output end of the first converter CON 1 , and the switching element S 8  is connected in series with the secondary winding T 2   b  of the transformer T 2  in the second converter CON 2 . 
     Then, the current is rectified by using passively the parasitic diode of the switching element S 8  at one end of the second converter CON 2  from which the current is sent out, which gives the current flowing through the discharge lamp  3 . The other end, which is located on the side to take the current, of the second converter CON 2  takes the current flowing from the discharge lamp  3  when the switching element S 8  becomes conductive. 
     In this manner, the parasitic diode of the switching element S 8  functions as the rectifier diode. Therefore, the number of components can be reduced. 
     Seventh Embodiment 
     A configuration of a discharge lamp lighting circuit according to a seventh embodiment of the present invention is shown in  FIG. 12 , and is explained below. 
     In the discharge lamp lighting circuit according to the seventh embodiment, a switching element S 9  is provided to the output end of the first converter CON 1 . The IGBT whose ON voltage is lower is employed as the switching element S 9 , and a heat generation of the element can be suppressed. A control signal RECT 9  is supplied from the controlling circuit  4  to a gate of the switching element S 9 , and the switching of the switching element S 9  is controlled. 
     The configuration of the second converter CON 2  side is similar to the configuration in the third embodiment ( FIG. 4 ). That is, in the second converter CON 2 , a switching element S 10  is provided to the secondary winding starting end of the transformer T 2 . In this example, the N-channel MOSFET is employed as the switching element S 10 . 
     A drain of the switching element S 10  is connected to the winding starting end of the secondary winding T 2   b  of the transformer T 2 . While the second converter CON 2  is executing the step-up operation, the switching element S 10  also functions as the rectifying switching element. The smoothing capacitor C 4  is arranged to the secondary winding terminating end of the transformer T 2 , and a terminal voltage of the smoothing capacitor C 4  is extracted as an output voltage. 
     At this time, a control signal RECT 10  is supplied from the controlling circuit  4  to a gate of the switching element S 10 , and the switching of the switching element S 10  is controlled based on the control signal RECT 10 . Therefore, a value of the output voltage is controlled variably. 
     Operation of the foregoing configuration is explained in detail below with reference to  FIG. 13 . 
     The controlling circuit  4  causes the first converter CON 1  to start the operation by turning ON the switching element Q 2 , stops the step-up operation of the second converter CON 2  by turning OFF the switching element Q 4 . Also, the controlling circuit  4  turns OFF the switching element S 9  and turns ON the switching element S 10 . Accordingly, the current IL 1  flows through the discharge lamp  3  (period PF 1 ). 
     Then, when the second converter CON 2  is operated, the switching element Q 4  acts as the step-up switching element, and the switching element S 10  acts as the rectifying switching element, and thus the switching element S 10  performs the synchronous rectification. Then, the switching element S 9  is rendered conductive and thus the current IL 2  flows through the discharge lamp  3  (period PF 2 ). 
     The operations of the first and second converters CON 1 , CON 2  are repeated at the lighting frequency of the discharge lamp  3 . Therefore, the low-frequency AC power can be applied to the discharge lamp  3 . 
     As explained above, in the seventh embodiment, the switching element S 9  is provided to the output end of the first converter CON 1 , and the switching element S 10  is connected in series with the secondary winding T 2   b  of the transformer T 2  in the second converter CON 2 . 
     Then, the switching element S 10 , which is connected in series with to one end of the second converter CON 2  from which the current is sent out, performs the synchronous rectification, which gives the current flowing through the discharge lamp  3 . The other end, which is located on the side to take the current, of the second converter CON 2  takes the current flowing from the discharge lamp  3  when the switching element S 10  becomes conductive. In this manner, the number of components can be reduced by the synchronous rectification of the switching element S 10 . 
     Next, arrangements of the transformers, which can be employed in respective embodiments, are explained in detail. 
     Arrangements of the transformers T 1 , T 2  in the first and second converters CON 1 , CON 2  are shown in  FIG. 14 . Normally each DC/DC converter is required of the transformer on a one-to-one basis. This transformer is one of several large-size components in the discharge lamp lighting circuit. In this case, in the first to seventh embodiments, the magnetic substances that are employed separately in known transformers T 1 , T 2  are used commonly as one transformer, and thus a reduction in the number of components, a size reduction, and a cost reduction can be implemented further. 
     That is, the step-up transformers T 1 , T 2  in two systems of converter constitute two systems of switching converter transformer in which magnetic paths of the magnetic fluxes are provided by one closed magnetic member and two systems of strong magnetic coupling are formed. Also, two systems of magnetic coupling are given by the magnetically loose coupling, and the magnetically interference between two systems is very small. 
     According to the embodiments of the present invention, the discharge lamp lighting circuit in which commonality of the electronic components is achieved by combining the DC/DC converting function in a known DC/DC converter with the DC/AC converting function in the full-bridge inverter to attain a size reduction and a cost reduction can be provided. Also, the first and second converters can be operated alternately, whereby the amount of generated heat is reduced and the locations where the heat is generated are separated, the temperature is raised locally, and reliability can be improved. 
     That is, the first and second converters CON 1 , CON 2  are implemented by any one of 
     (a) a converter having an insulated-type first transformer (e.g., T 1 ), a first switching element (e.g., Q 2 , Q 4 ) connected in series between a DC power supply and a primary winding of the first transformer, and a second switching element (e.g., Q 3 , Q 5 , S 10 ) connected in series between an output end and a secondary winding of the first transformer, whereby the first and second switching elements are driven in synchronism with each other in each half period of a first frequency at a second frequency higher than the first frequency, 
     (b) a converter having an insulated-type second transformer (e.g., T 2 ), a third switching element (e.g., Q 2 , Q 4 ) connected in series between a DC power supply and a primary winding of the second transformer, a first rectifying element connected in series between one output end and a secondary winding of the second transformer, and a fourth switching element (e.g., S 1 , S 2 , S 7 , S 9 ) connected between one output end and the other output end, whereby the fourth switching element is put in an OFF state while the third switching element executes a driving operation and also the fourth switching element is put in an ON state while the third switching element executes a stopping operation, or 
     (c) a converter having an insulated-type third transformer (e.g., T 1 ), a fifth switching element (e.g., Q 2 , Q 4 ) connected in series between a DC power supply and a primary winding of the third transformer, a sixth switching element (e.g., S 3  to S 6 , S 8 ) connected in series between an output end and a secondary winding of the third transformer, and a second rectifying element connected in parallel with the sixth switching element, whereby the sixth switching element is put in an OFF state while the fifth switching element executes a driving operation and also the sixth switching element is put in an ON state while the fifth switching element executes a stopping operation. 
     In this case, the first and second converters CON 1 , CON 2  have a capacitive element at on output end on the secondary side respectively, and operate only one converter before the discharge lamp is started, to charge the capacitive element on the secondary side at a voltage of several hundred V. The first and second converters CON 1 , CON 2  flow the charges stored in the capacitive element to the discharge lamp (takeover current) immediately after the discharge lamp is started, to promote the discharge growth from the glow discharge to the arc discharge. Here, the switching element on the secondary side of the other converter is operated in the active range (i.e., exhibits behavior as a suction type current source), and thus the takeover current can be controlled. 
     In all embodiments, the lighting frequency of the discharge lamp (first frequency) preferably should be set to 250 Hz to 750 Hz, for example, as a headlamp of a car. 
     Also, it is desirable that a driving frequency of the primary switching element (Q 2 , Q 4 ) (second frequency) be set higher than the first frequency by 10 times or more, and preferably 100 times or more. For example, 50 kHz to 2 MHz is desirable. 
     The discharge lamp lighting circuit can have a configuration such that two converters are arranged on both sides of the discharge lamp  3 , respectively (called as a “double converter type” hereinafter). In such a circuit mode of the double converter type discharge lamp lighting circuit, the peculiar problem explained below can arise. 
     In the discharge lamp lighting circuit of double converter type, a first state, in which a high voltage is applied from one converter CON 1  to one end side of the discharge lamp  3  and the other end side is kept at a ground potential, and a second state, in which a high voltage is applied from the other converter CON 2  to the other end side of the discharge lamp  3  and one end side is kept at a ground potential, are repeated alternately every lighting period (called an “inverter operation” hereinafter). 
     The charges accumulated in the smoothing capacitors C 3 , C 4  provided to the output ends of the first and second converters CON 1 , CON 2  should essentially flow in the discharge lamp  3 ; nevertheless such charges have nowhere to go when the inverter operation is continued in a situation that the load of the discharge lamp lighting circuit of double converter type is opened, for example, as the result of the failure of the discharge lamp as the driven subject, the bad contact of the connector. Therefore, the smoothing capacitors C 3 , C 4  continue to discharge such charges abruptly via the switching elements that are provided in series with these smoothing capacitors. Concretely, the smoothing capacitor C 3  shown in  FIG. 2  discharges the charges via the switching element S 1 , and the smoothing capacitor C 4  discharges the charges via the switching element S 2 . Also, the smoothing capacitor C 3  shown in  FIG. 4  discharges the charges via the switching element Q 3 , and the smoothing capacitor C 4  discharges the charges via the switching element Q 5 . Also, the smoothing capacitor C 3  shown in  FIG. 6  discharges the charges via the switching element S 3 , and the smoothing capacitor C 4  discharges the charges via the switching element S 4 . Also, the smoothing capacitor C 3  shown in  FIG. 8  discharges the charges via the switching element S 5 , and the smoothing capacitor C 4  discharges the charges via the switching element S 6 . 
     When the overcurrent flows through the switching element on account of the abrupt discharge, there is a danger that reliability of the switching element decreases. This problem can be solved preferably by several of the aforementioned embodiments and other embodiments. 
     In order to solve this problem, it is desirable that the current limiting circuit element be provided to the location that is positioned in series of the switching element through which the overcurrent may flow. As this circuit element, the impedance element, preferably the resistor element, is suitable. In the assumption that the resistor is provided, this circuit element is called a “protection resistor.” 
     When the protection resistor is provided, the smoothing capacitor and the protection resistor constitute a CR circuit. Therefore, the load of the discharge lamp lighting circuit becomes open, a rate of discharge conducted via the switching element is limited by a CR time constant, and thus the discharge current can be suppressed. In turn, reliability of the switching element can be enhanced. 
     Preferably, the protection resistor should be provided on the common terminal (i.e., the emitter or the source) sides of the switching elements S 1  to S 6 . In this case, when a large current flows through the protection resistor, a voltage drop in the protection resistor is increased. Thus, a potential of the common terminals of the switching elements S 1  to S 6  rises, and the gate-emitter voltages (the gate-source voltages, the base-emitter voltages) of the switching elements S 1  to S 6  is decreased. As a result, a negative feedback is applied in the direction along which an extent of ON level of the switching elements S 1  to S 6  is weakened, and thus the switching elements S 1  to S 6  can be protected preferably. 
     The discharge lamp lighting circuit in  FIG. 2  shows an example in which the protection resistors are provided. Therefore, the discharge lamp lighting circuit in  FIG. 2  will be explained, with the resistors R 1 , R 2  acting as the protection resistors.  FIGS. 15(   a ) ( b ) are circuit diagrams showing a configuration of a discharge lamp lighting circuit having protection resistors according to the embodiment respectively. The discharge lamp lighting circuit in  FIG. 15(   a ) is equivalent to the discharge lamp lighting circuit in  FIG. 2 . Thus, the same reference symbols are used for the corresponding members, and detailed explanation is omitted. 
     In  FIG. 15(   a ) (and  FIG. 2) , the resistor R 1  is provided in series with the switching element S 1  that is provided on the discharge path of the smoothing capacitor C 3 . Similarly, the resistor R 2  is provided in series with the switching element S 2  that is provided on the discharge path of the smoothing capacitor C 4 . As described above, these resistors R 1 , R 2  are the elements that are provided essentially to detect a lamp current IL in the normal operation of the discharge lamp  3 , but these resistors R 1 , R 2  act as the protection resistors at a time of open failure of the discharge lamp  3  to suppress the discharge current. Therefore, according to the configuration shown in  FIG. 2 , even when the open failure is caused in the discharge lamp  3 , it can be suppressed that the overcurrent flows through the switching elements S 1 , S 2 , and reliability of respective circuit elements, in turn the overall discharge lamp lighting circuit, can be enhanced. 
     In the discharge lamp lighting circuit in  FIG. 15(   a ), two differential amplifiers AMP 1 , AMP 2  are provided to the controlling circuit  4 . The differential amplifiers AMP 1 , AMP 2  amplify the voltage drop in two current detecting resistors R 1 , R 2 . Diodes D 11 , D 12  constitute a diode OR circuit (maximum value circuit) whose cathodes are connected in common, and output a higher output out of output voltages of the differential amplifiers AMP 1 , AMP 2 . The later circuit (not shown) receives the signals from the cathodes of the diodes D 11 , D 12 , being connected commonly, as the signal corresponding to the lamp current IL, and controls a duty ratio of the switching elements Q 2 , Q 4  by virtue of the pulse modulation. In this case, the configuration of the controlling circuit  4  can be set arbitrarily, and various type circuits can be utilized. 
     In this case, the configuration of the controlling circuit  4  in  FIG. 15(   a ) corresponds to the case where the diodes D 11 , D 12  are assumed as an ideal rectifying element (diode whose forward voltage Vf=0 V) respectively. However, the forward voltage of the actual rectifying element has some non-zero value (e.g., 0.7 V). Therefore, in order to eliminate the influence of the voltage drop of the forward voltage Vf, it is preferable that a pair of the differential amplifier AMP 1  and the diode D 11  and a pair of the differential amplifier AMP 2  and the diode D 12 , for example, should have a circuit mode shown at the lower stage in  FIG. 15(   a ) respectively. 
     In the circuit configuration at the lower stage, a voltage of the cathode of the diode D 11  (D 12 ) is fed back to the inverting input terminal of the differential amplifier AMP 1  (AMP 2 ) via a pair of resistors R 13  and R 14 . Also, the voltage drop in the current detecting resistor R 1  (R 2 ) is divided by resistors R 11 , R 12 , and is input into the non-inverting input terminal of the differential amplifier AMP 1  (AMP 2 ). According to this configuration, the influence of the forward voltage Vf of the diode D 11  (D 12 ) (i.e., level shift of the voltage) can be substantially eliminated, and thus the signal processing with high linearity with respect to the voltage drop in the resistor R 1  (R 2 ), i.e., the lamp current can be applied. 
       FIG. 16  ( a ) is an operation waveform diagram in a state that the load of the discharge lamp lighting circuit in  FIG. 15(   a ) is opened. This waveform diagram focuses on the first converter CON 1  side. Ic denotes a discharge current flowing through the switching element S 1 , V GE  denotes a gate-emitter potential of the switching element S 1 , V EE  denotes an emitter potential of the switching element S 1 , and V GG  denotes a gate potential of the switching element S 1 . 
     When the switching element S 1  is turned ON in a state that the load is open, the charges accumulated in the smoothing capacitor C 3  cannot flow in the discharge lamp  3 , and flow in the ground terminal GND via the switching element S 1 . When the switching element S 1  is turned ON, the discharge current Ic starts to increase sharply and accordingly the voltage drop of the resistor R 1 , i.e., the emitter potential V EE  is increased. The relationship
 
 V   GE   =V   GG   −V   EE  
 
     is satisfied between the emitter potential V EE , the gate potential V GG , and the gate-emitter potential V GE . Therefore, according to an increase of the emitter potential V EE , the gate-emitter potential V GE  of the switching element S 1  becomes small, an ON level of the switching element S 1  becomes low, and a resistance component R 0   n   1  of the switching element S 1  is increased. Since a CR time constant that specifies a waveform of the discharge current Ic is defined by a synthetic resistance of the resistance component R 0   n   1  of the switching element S 1  and the resistor R 1 , a peak of the discharge current Ic can be suppressed further by increasing the resistance component R 0   n   1 . Also, the discharge current flowing through the switching element S 2  can be suppressed by the resistor R 2 , on the other end (the second converter CON 2 ) side of the discharge lamp  3 . 
     In this manner, according to the discharge lamp lighting circuit in  FIG. 15(   a ) and  FIG. 2 , the large current flowing through the switching elements S 1 , S 2  can be suppressed in a load open state, and reliability of the circuit can be enhanced. 
     The advantages achieved by the discharge lamp lighting circuit in  FIG. 15(   a ) and  FIG. 2  can be made clearer in contrast to the discharge lamp lighting circuit in which the protection resistors are not provided.  FIG. 16(   b ) is an operation waveform diagram in a state that the load of the circuit, in which the protection resistors R 1 , R 2  are removed from the circuit in  FIG. 15(   a ) according to the comparative art, is opened. 
     When the switching element S 1  repeats ON/OFF intermittently in a lighting period in a state that the load is open, the charges stored in the smoothing capacitor C 3  cannot flow in the discharge lamp  3 . Thus, such charges flow in the ground terminal GND via the switching element S 1 . For the purpose of reducing a power loss as small as possible in a state that the discharge lamp  3  is normally turned ON, the gate potential V GG  applied to the switching element S 1  is set such that the switching element S 1  should be turned ON in a full-ON state or a similar state. As a result, the smoothing capacitor C 3  is grounded via the switching element S 1 , and thus the large current Ic flows through the switching element S 1 . This is the problem peculiar to the double converter type the discharge lamp lighting circuit. 
     Referring to  FIG. 16(   a ) to see the discharge current Ic, it is apparent that a peak is suppressed in contrast to a waveform in  FIG. 16(   b ). That is, according to the discharge lamp lighting circuit in  FIG. 15(   a ) an  FIG. 2 , such a problem peculiar to the double converter type can be solved that the large current flows through the switching elements S 1 , S 2  upon opening the load. 
       FIG. 15(   b ) is a circuit diagram showing a variation of the discharge lamp lighting circuit in  FIG. 15(   a ). In  FIG. 15(   b ), the configuration that the protection resistors R 1 , R 2  in  FIG. 15(   a ) are shared with two switching elements S 1 , S 2  is shown. In  FIG. 15(   b ), the constituent elements that are common to  FIG. 15(   a ) are omitted appropriately. 
     More concretely, one end of the switching element S 1  is connected to the output terminal of the first converter CON 1 , and one end of the switching element S 2  is connected to the output terminal of the second converter CON 2 . The other ends of the switching elements S 1 , S 2  are connected in common. 
     A current detecting resistor R 4  also acting as the protection resistor is provided between the other ends of the switching elements S 1 , S 2  connected in common and the fixed voltage terminal (ground terminal). A voltage drop of the current detecting resistor R 4  is input into the controlling circuit  4  as a signal corresponding to the lamp current IL. 
     While the controlling circuit  4  causes the first converter CON 1  to drive/operate, it adjusts an ON/OFF duty ratio of the switching element Q 2  on the first converter CON 1  side, based on at least the voltage drop produced in the current detecting resistor R 4 . Similarly, while the controlling circuit  4  causes the second converter CON 2  to drive/operate, it adjusts an ON/OFF duty ratio of the switching element Q 4  on the second converter CON 2  side, based on at least the voltage drop produced in the current detecting resistor R 4 . 
     The controlling circuit  4  has a differential amplifier AMP 3  that amplifies the voltage drop in the current detecting resistor R 4 . A circuit (not shown) at the later stage receives an output of the differential amplifier AMP 3  as a signal responding to the lamp current IL, and controls duty ratios of the switching elements Q 2 , Q 4  in terms of the pulse modulation. 
     According to the discharge lamp lighting circuit in  FIG. 15(   b ), the lamp current IL that flows through the discharge lamp  3  in the first direction or the second direction as the opposite direction to the first direction can be detected by the single current detecting resistor R 4 . As in the circuit in  FIG. 15(   a ), the discharge lamp lighting circuit in  FIG. 15(   b ) can suppress preferably the current flowing through the switching elements S 1 , S 2  in the load open state. 
     In the circuit in  FIG. 15(   b ), the number of current detecting resistors (protection resistors) can be reduced by one rather than the circuit in  FIG. 15(   a ). This is advantageous from the viewpoint of circuit area and a cost. Also, the matching between two the current detecting resistors R 1 , R 2  with high precision is needed in the circuit in  FIG. 15(   a ), but is not needed in the circuit in  FIG. 15(   b ). 
     To receive two detection signals in response to two directions of the lamp current IL, pads (terminals) in two systems must be provided to the controlling circuit  4 . On the other hand, the lamp signals detected in two directions can be integrated into one system in  FIG. 15(   b ), and therefore the number of pads (terminals) provided to the controlling circuit  4  can be reduced. Also, the internal configuration of the controlling circuit  4  in  FIG. 15(   b ) can be simplified rather than the configuration in  FIG. 15(   a ). 
     In the discharge lamp lighting circuit in  FIGS. 15(   a ), ( b ), the case where the current detecting resistors R 1 , R 2 , R 4  for detecting the lamp current IL also function as the protection resistor in the open load state is explained. But the present invention is not limited to this case. That is, another protection resistor may be provided on the path, which is provided in series with the switching element to be protected, separately from the current detecting resistor. In this case, in order to attain the current suppressing effect by the aforementioned negative feedback, it is desirable that the protection resistor should be arranged on the emitter (source) side of the switching element. 
     For example, in  FIG. 4 , the protection resistor may be provided between the source of the switching element Q 3  and the ground terminal GND. In  FIG. 6 , the protection resistor may be provided between the emitter of the switching element Q 3  and the ground terminal GND. In  FIG. 8  and  FIG. 10 , the protection resistor may be provided between the sources of the switching elements S 5 , S 7  and the ground terminal GND respectively. In  FIG. 12 , the protection resistor may be provided between the emitter of the switching element Q 4  and the ground terminal GND. It is obvious for those skilled in the art that various variations may be provided in addition to these illustrations. 
     In respective embodiments, the case where the positive voltage is produced by two converters CON 1 , CON 2 , and is applied to the discharge lamp  3  (referred to as a “positive polarity lighting” herein) is explained. But the negative voltage may be produced to drive the discharge lamp  3  (referred to as a “negative polarity lighting” herein). In this case, the direction of the rectifier diodes D 3 , D 4 , the polarity of the secondary windings of the first and second transformers, the direction of the switching elements connected to the secondary winding side of respective transformers, and the direction of the rectifying elements connected in parallel with the switching elements may be reversed in Figures respectively. 
     Various modifications are within the scope of the following claims.