Discharge lamp ballast, lighting system and projector

1 Field of the Invention

The invention relates to discharge lamp ballasts, lighting systems and projectors that apply starting voltage across discharge lamps at a starting mode and supply the lamps with DC power for steady operating (lighting) at a steady operating mode after the starting mode.

2 Description of the Related Art

A discharge lamp ballast for a DC discharge lamp comprises a voltage step down converter in order to supply the lamp with DC power for a steady operating state at a steady operating mode. Also, in case that the lamp is a high pressure discharge lamp (HID lamp) such as a metal halide lamp or the like, the ballast is provided with an igniter that generates high voltage pulse from several kV to 10 s kV with a pulse transformer (see, e.g., Japanese Patent Publication number H10-144488).

However, when the above transformer provides the lamp with high voltage from several kV to 10 s kV, electromagnetic induction noise (flux) is radiated from the transformer and therefore there is a problem that the noise gives the ballast and peripheral circuits erroneous operation.

It is therefore an object of the present invention to reduce noise from a starting means that applies starting voltage across a discharge lamp.

A discharge lamp ballast of the present invention comprises: a voltage step down converter connected to a DC power source with a positive terminal and a negative terminal; a converter control means that controls the converter; a first capacitor that applies DC voltage across a discharge lamp having a first end and a second end through DC power from the converter; and a starting means that applies starting voltage across the lamp in case of a starting mode. The converter is constructed with a diode, a first switching element and a first inductor. The diode has a cathode and an anode, and the anode is connected to the negative terminal of the DC power source and a negative voltage side of the first capacitor. The first switching element is connected between the cathode of the diode and the positive terminal of the DC power source. The first inductor is connected between the cathode of the diode and a positive voltage side of the first capacitor. In case of a steady operating mode after the starting mode, the converter control means turns the first switching element on and off at a high frequency so as to supply DC power for steady operating to the lamp via the first capacitor. For an aspect of the present invention, the starting means comprises a second inductor, a second capacitor, a second switching element, a third switching element and a starting control means. The second inductor is connected between the first end of the lamp and the positive voltage side of the first capacitor. The second capacitor is connected in parallel with the lamp and forms a resonance circuit together with the second inductor. The second switching element is connected between the positive terminal of the DC power source and the second end of the lamp. The third switching element is connected between the second end of the lamp and the negative voltage side of the first capacitor. The starting control means controls the second switching element and the third switching element. In case of the steady operating mode, the starting control means operates so as to include an on period of the third switching element while keeping the second switching element turned off. In case of the starting mode, the starting control means alternately turns the second switching element and the third switching element on and off so as to contribute resonance voltage of the resonance circuit for starting of the lamp. Thus, by contributing the resonance voltage for starting of the lamp, noise from the starting means can be reduced.

The present invention may comprise a transformer with a primary winding and a secondary winding, and utilize the primary winding as the second inductor. In this case, the secondary winding is connected in series with the lamp, while the series combination of the secondary winding and the lamp is connected in parallel with the secondary capacitor. Thus, induction voltage responding to a resonance current passing through the primary winding is superposed onto resonance voltage across the second capacitor, so that the starting voltage applied across the lamp is increased.

The second capacitor of the present invention may have capacitance smaller than that of the first capacitor. Thus, the second capacitor has capacitance smaller than that of the first capacitor and therefore the resonance current is reduced, while the first capacitor has capacitance larger than that of the second capacitor and therefore ripple voltage across the first capacitor for the lamp is reduced.

In case of the steady operating mode, the starting control means of the present invention may turn the third switching element on and off while synchronizing the turning on and off of the third switching element with the turning on and off of the first switching element.

In case of the starting mode, the starting control means of the present invention may alternately turn the second switching element and the third switching element on and off approximately at a resonance frequency of the resonance circuit.

In case of the starting mode, the starting control means of the present invention may alternately turn the second switching element and the third switching element on and off approximately at a frequency f0×1/ODD, where f0 is a resonance frequency of the resonance circuit and ODD is an odd number. In this invention, because an odd harmonic frequency of square wave voltage applied across the LC resonance circuit becomes approximately equal to the resonance frequency of the resonance circuit, the lamp can be started with the resonance voltage of the resonance circuit.

In case of the starting mode, the starting control means of the present invention may alternately turn the second switching element and the third switching element on and off at a switching frequency of a continuous sweep frequency or a switching frequency of multistage frequency. It is also preferable that the starting control means sweeps the switching frequency from a first frequency to a second frequency, while the means repeats the sweeping operation. It is further preferable that the first frequency is higher than the second frequency.

In case of a grow-arc transition mode between the starting mode and the steady operating mode, the starting control means of the present invention may alternately turn the second switching element and the third switching element on and off at a switching frequency lower than that in the starting mode. In this invention, the lamp is able to preferably transit from grow discharge to arc discharge after breakdown.

Therefore, the present invention achieves reduction of noise from the starting means and gives benefit of the noise reduction and high reliability in equipment such as a lighting system constructed with the ballast and the lamp, a projector constructed with the ballast and the lamp, or the like.

Exemplary embodiments of the invention will now be described in further details. Other features and advantages of the present invention will become better understood with regard to the following detailed description and accompanying drawings where:

FIG. 1 is a circuit diagram of a discharge lamp ballast according to a first embodiment of the present invention;

FIG. 2 illustrates control signals to switching elements of the ballast of FIG. 1 ;

FIG. 3 is a circuit diagram of a discharge lamp ballast according to a second embodiment of the present invention;

FIG. 4 is a circuit diagram of a discharge lamp ballast according to a third embodiment of the present invention;

FIG. 5 illustrates control signals to switching elements of the ballast of FIG. 4 ;

FIG. 6 is a circuit diagram of a discharge lamp ballast according to a fourth embodiment of the present invention;

FIG. 7 illustrates control signals to switching elements of the ballast of FIG. 6 ;

FIG. 8 illustrates waveform of resonance voltage (starting voltage) through the ballast of FIG. 6 ;

FIG. 9 is a circuit diagram of a discharge lamp ballast according to a fifth embodiment of the present invention;

FIG. 10 illustrates control signals to switching elements of the ballast of FIG. 9 ;

FIG. 11 is a circuit diagram of a discharge lamp ballast according to a sixth embodiment of the present invention;

FIG. 12 illustrates control signals to switching elements of the ballast of FIG. 11 ;

FIG. 13 illustrates the signals to switching elements of the ballast of FIG. 11 and waveform of resonance voltage (starting voltage) through the ballast;

FIG. 14 illustrates resonance voltage (lamp voltage) and a lamp current in case that a discharge lamp does not reach breakdown through the ballast of FIG. 11 ;

FIG. 15 illustrates resonance voltage (lamp voltage) and a lamp current in case that the lamp reaches breakdown through the ballast of FIG. 11 ;

FIG. 16 is a circuit diagram of a discharge lamp ballast according to a seventh embodiment of the present invention;

FIG. 17 illustrates control signals to switching elements of the ballast of FIG. 16 ;

FIG. 18 illustrates resonance voltage (lamp voltage) and a lamp current in case that a discharge lamp does not reach breakdown through the ballast of FIG. 16 ;

FIG. 19 illustrates resonance voltage (lamp voltage) and a lamp current in case that the lamp reaches breakdown through the ballast of FIG. 16 ;

FIG. 20 is a circuit diagram of a discharge lamp ballast according to an eighth embodiment of the present invention;

FIG. 21( a ) illustrates another example of arrangement of a pulse transformer in the ballast of FIG. 20 ;

FIG. 21( b ) illustrates another example of arrangement of a pulse transformer in the ballast of FIG. 20 ; and

FIG. 21( c ) illustrates another example of arrangement of a pulse transformer in the ballast of FIG. 20 .

FIG. 1 illustrates a discharge lamp ballast 10 for a discharge lamp DL 1 (e.g., a DC discharge lamp such as a HID lamp or the like). This ballast 10 comprises a voltage step down converter 11 connected to a DC power source DC 1 with a positive terminal and a negative terminal, and a capacitor C 11 that applies DC voltage across the lamp DL 1 having a first end and a second end through DC power from the converter 11 , and also comprises a converter controller (converter control means) 12 and a starter (starting means) 13 .

The voltage step down converter 11 is constructed with a diode D 11 , a switching element Q 11 and an inductor L 11 . The diode D 11 has a cathode and an anode, and the anode is connected to the negative terminal of the source DC 1 and a negative voltage side of the capacitor C 11 .

The switching element Q 11 is connected between the cathode of the diode D 11 and the positive terminal of the source DC 1 . The element Q 11 is, for example, a power MOSFET with a diode (body diode) BD 11 , and its drain and source are connected to the positive terminal of the source DC 1 and the cathode of the diode D 11 , respectively. A cathode and an anode of the diode BD 11 are also connected to the drain and the source of the power MOSFET, respectively. The inductor L 11 is connected between the cathode of the diode D 11 and a positive voltage side of the capacitor C 11 .

The converter controller 12 is constructed with a low-resistance resistor R 10 (current detection means), series resistors R 11 and R 12 (voltage detection means), an operational circuit 121 and a PWM (pulse width modulation) circuit 122 , and controls the converter 11 .

The resistor R 10 is located between the negative voltage side of the capacitor C 11 and a switching element Q 13 of the starter 13 , and detects a lamp current. The resistors R 11 and R 12 are connected in parallel with the capacitor C 11 , and detects lamp voltage (voltage across the capacitor C 11 ).

In case of a steady operating mode after a starting mode, the operational circuit 121 figures out lamp power based on the lamp current detected through the resistor R 10 and the lamp voltage detected through the resistors R 11 and R 12 , and then calculates difference (voltage) between target power and the lamp power. The PWM circuit 122 controls pulse widths of a control signal to (a gate of) the switching element Q 11 so that the difference calculated through the circuit 121 becomes zero.

In short, the converter controller 12 turns the switching element Q 11 on and off at a high frequency so as to supply DC power (target power) for steady operating to the lamp DL 1 via the capacitor C 11 in case of the steady operating mode.

The starter 13 is constructed with an inductor L 12 , a capacitor C 12 having capacitance smaller than that of the capacitor C 11 , switching elements Q 12 and Q 13 , and a starting controller (starting control means) 130 that controls the elements Q 12 and Q 13 , and applies starting voltage across the lamp DL 1 in case of the starting mode.

The inductor L 12 is connected between the first end of the lamp DL 1 and the positive voltage side of the capacitor C 11 . The capacitor C 12 is connected in parallel with the lamp DL 1 and forms a resonance circuit together with the inductor L 12 . The inductor L 12 and the capacitor C 12 also constitutes a low pass filter. For example, a value of the inductor L 12 may be 600 μH and a value of the capacitor C 12 may be 3,300 pF.

The switching element Q 12 is, for example, a power MOSFET with a diode (body diode) BD 12 , and its drain and source are connected to the positive terminal of the source DC 1 and the second end of the lamp DL 1 , respectively. The switching element Q 13 is, for example, a power MOSFET with a diode (body diode) BD 13 , and its drain and source are connected to the second end of the lamp DL 1 and the negative voltage side of the capacitor C 11 , respectively. A cathode and an anode of each body diode are the drain and the source of the power MOSFET, respectively.

The starting controller 130 is constructed with a pulse generation circuit 131 and an organization circuit 132 . In case of the starting mode, the pulse generation circuit 131 alternately turns the switching elements Q 12 and Q 13 on and off so that the lamp DL 1 is started by resonance voltage of the above resonance circuit. In case of the starting mode, the circuit 131 in the first embodiment alternately turns the switching elements Q 12 and Q 13 on and off approximately at a resonance frequency (e.g., 115 KHz) of the resonance circuit in order to secure the starting voltage of the lamp DL 1 through the resonance voltage.

In case of the steady operating mode, the organization circuit 132 operates so as to include an on period of the switching element Q 13 while keeping the switching element Q 12 turned off. In the first embodiment, the circuit 132 turns the switching element Q 13 on and then holds the turn on, while keeping the switching element Q 12 turned off in case of the steady operating mode.

The operation of the discharge lamp ballast 10 is now explained with reference to FIG. 2 . In a starting mode, the switching elements Q 12 and Q 13 are alternately turned on and off approximately at the resonance frequency of the resonance circuit. When the switching element Q 12 is turned on, the DC power source DC 1 applies square wave voltage mainly across the capacitor C 12 , the inductor L 12 and the capacitor C 11 . In this case, by fundamental frequency (i.e., switching frequency of Q 12 , Q 13 ) component of the square wave voltage, a resonance current mainly passes through a closed circuit constructed of the source DC 1 , the switching element Q 12 , the capacitor C 12 , the inductor L 12 and the capacitor C 11 , or a closed circuit constructed of the inductor L 12 , the capacitor C 11 , the resistor R 10 , the switching element Q 13 (BD 13 ) and the capacitor C 12 . When the resonance current reverses its direction, the current mainly passes through a closed circuit constructed of the capacitor C 12 , the switching element Q 13 , the resistor R 10 , the capacitor C 11 and the inductor L 12 . By the resonance operation, resonance voltage across the capacitor C 12 is applied across the lamp DL 1 , and thereby the lamp DL 1 is started. After the starting of the lamp DL 1 , the operation mode is sifted to a steady operating mode.

In the steady operating mode, the switching element Q 12 is held off and also the switching element Q 13 is turned and held on, while the switching element Q 11 is turned on and off at a high frequency so as to supply DC power for steady operating to the lamp DL 1 via the capacitor C 11 . By holding the switching elements Q 12 and Q 13 off and on, respectively, the circuit of the ballast 10 is organized into a circuit for DC operating (lighting).

When the switching element Q 11 is turned on, a charging current flows from the source DC 1 to the capacitor C 11 via the switching element Q 11 and the inductor L 11 , and thereby the capacitor C 11 is charged. When the switching element Q 11 is turned off, a regenerative current by energy accumulated in the inductor L 11 flows from the inductor L 11 to the capacitor C 11 via diode D 11 . On time of the switching element Q 11 is controlled with pulse widths of a control signal from the PWM circuit 122 , and thereby DC power for steady operating is supplied to the lamp DL 1 .

According to the first embodiment of the present invention, starting of the lamp DL 1 is possible through the resonance voltage of the resonance circuit with no use of a pulse transformer, and therefore it is possible to reduce noise from the starter 13 that applies the starting voltage across the lamp DL 1 . Also, because the starting voltage is AC, electrode wear of the lamp DL 1 is reduced. Moreover, the capacitor C 12 has a capacitance smaller than that of the capacitor C 11 and therefore the resonance current can be reduced, while the capacitor C 11 has capacitance larger than that of the capacitor C 12 and therefore ripple voltage across the capacitor C 11 for the lamp DL 1 (DC discharge lamp) can be reduced.

In an alternate embodiment, the pulse generation circuit 131 alternately turns the switching elements Q 12 and Q 13 on and off approximately at a frequency (switching frequency) f0×1/ODD in case of the starting mode, where f0 is a resonance frequency of the above resonance circuit and ODD is an odd number (e.g., 3). In this embodiment, because an odd harmonic frequency of square wave voltage applied across the LC resonance circuit becomes approximately equal to the resonance frequency of the resonance circuit, it is possible to secure the starting voltage of the lamp DL 1 through the resonance voltage of the resonance circuit as well as the first embodiment. For example, when a value of the inductor L 12 is 100 μH and a value of the capacitor C 12 is 2,200 pF, the switching frequency is 115 KHz. According to this embodiment, it is possible to make the resonance circuit compact. The switching frequency can be also reduced (e.g., ⅓,⅕, 1/7,. . . ).

FIG. 3 illustrates a discharge lamp ballast 20 for a discharge lamp DL 2 (e.g., a DC discharge lamp such as a HID lamp or the like). This ballast 20 is characterized by a transformer T having a primary winding nil and a secondary winding n 2 in a starter 23 as compared with the first embodiment that is different only in that the inductor L 12 is provided with the starter 13 .

In this second embodiment, the inductor L 12 of FIG. 1 is replaced by the primary winding n 1 . The secondary winding n 2 is utilized to superpose induction voltage responding to a resonance current passing through the primary winding n 1 onto resonance voltage across a capacitor C 22 . The winding n 2 is connected in series with the lamp DL 2 , while the series combination of the winding n 2 and the lamp DL 2 is connected in parallel with the capacitor C 22 . In FIG. 3 , the winding n 2 is also directly connected in series with the winding n 1 . The level of the induction voltage can be adjusted with a turn ratio (n 1 :n 2 ) of the transformer T.

According to the second embodiment of the present invention, since the induction voltage responding to the resonance current passing through the primary winding n 1 is superposed onto the resonance voltage across the capacitor C 22 , staring voltage applied across the lamp DL 2 can be increased.

FIG. 4 illustrates a discharge lamp ballast 30 for a discharge lamp DL 3 (e.g., a DC discharge lamp such as a HID lamp or the like). This ballast 30 is characterized by an intermittent organization circuit 332 provided in a starting controller 330 of a starter 33 as compared with the first embodiment that is different only in that the organization circuit 132 is provided in the starting controller 130 of the starter 13 .

In the steady operating mode ( FIG. 5 ), the intermittent organization circuit 332 in this third embodiment holds the switching element Q 32 off and also turns the switching element Q 33 on and off, while the circuit 332 synchronizes the turning on and off of the switching element Q 33 with the turning on and off of the switching element Q 31 .

According to the third embodiment of the present invention, it is possible to reduce noise from the starter 33 that applies starting voltage across the lamp DL 3 as well as the first embodiment. The intermittent organization circuit 332 of the third embodiment is also applicable to the starting controller 230 in the second embodiment.

FIG. 6 illustrates a discharge lamp ballast 40 for a discharge lamp DL 4 (e.g., a DC discharge lamp such as a HID lamp or the like). This ballast 40 is characterized by a frequency sweep circuit 433 further provided in a starting controller 430 of a starter 43 as compared with the first embodiment that is different only in that the starting controller 130 consists of the pulse generation circuit 131 and the organization circuit 132 .

In the starting mode ( FIG. 7 ), the frequency sweep circuit 433 in this fourth embodiment alternately turns the switching elements Q 42 and Q 43 on and off at a switching frequency of a continuous sweep frequency through a pulse generation circuit 431 . The range of the continuous sweep frequency includes a resonance frequency of a resonance circuit constructed with an inductor L 42 and a capacitor C 42 , and is set to, for example, 50 KHz-160 KHZ when the resonance frequency is 115 KHz.

According to the fourth embodiment of the present invention, starting voltage is able to include the resonance voltage of the resonance circuit ( FIG. 8 ) without influence of each unevenness of the inductor L 42 and the capacitor C 42 . As a result, the lamp DL 4 can be started with the starting voltage. The frequency sweep circuit 433 of the fourth embodiment is also applicable to the starting controller 230 in the second embodiment or the starting controller 330 in the third embodiment.

In an alternate embodiment, the above range of the continuous sweep frequency (substantially) includes a frequency f0×1/ODD, where f0 is the resonance frequency of the resonance circuit and ODD is an odd number. According to this embodiment, the starting voltage is able to include the resonance voltage of the resonance circuit, and the lamp DL 4 can be started with the starting voltage as well as the fourth embodiment.

FIG. 9 illustrates a discharge lamp ballast 50 for a discharge lamp DL 5 (e.g., a DC discharge lamp such as a HID lamp or the like). This ballast 50 is characterized by a frequency step circuit 534 further provided in a starting controller 530 of a starter 53 as compared with the first embodiment that is different only in that the starting controller 130 consists of the pulse generation circuit 131 and the organization circuit 132 .

In this fifth embodiment, the frequency step circuit 534 alternately turns switching elements Q 52 and Q 53 on and off at a switching frequency of a multistep frequency through a pulse generation circuit 531 in a starting mode. As shown in FIG. 10 , the above switching frequency of the multistep frequency consists of, for example, stepped down frequencies f 51 , f 52 or f 53 (f 51 >f 52 >f 53 ). In a preferred embodiment, the frequency f 51 is set to approximately a resonance frequency of a resonance circuit constructed with an inductor L 52 and a capacitor C 52 , while the frequencies f 52 and f 53 are set so that a lamp current of the lamp DL 5 steps up after breakdown of the lamp DL 5 .

According to the fifth embodiment of the present invention, the lamp DL 5 is able to start through the starting voltage with approximately resonance voltage of the resonance circuit, and moreover the lamp DL 5 can ideally transit from grow discharge to arc discharge after breakdown. As a result, starting performance (prevention of non-lighting) of the lamp DL 1 can be improved. The frequency step circuit 534 of the fifth embodiment is also applicable to the starting controller 230 in the second embodiment or the starting controller 330 in the third embodiment.

In an alternate embodiment, the above frequency f 51 is approximately a frequency f0×1/ODD, where f0 is the resonance frequency of the resonance circuit and ODD is an odd number. According to this embodiment, the lamp is able to start through the starting voltage with approximately the resonance voltage of the resonance circuit as well as the fifth embodiment.

In another alternate embodiment, when the lamp DL 5 is started at the frequency f 52 , the frequency f 52 is set to approximately the resonance frequency of the resonance circuit or approximately the frequency f0×1/ODD, where f0 is the resonance frequency of the resonance circuit and ODD is an odd number.

FIG. 11 illustrates a discharge lamp ballast 60 for a discharge lamp DL 6 (e.g., a DC discharge lamp such as a HID lamp or the like). This ballast 60 is characterized by a repetition circuit 635 further provided in a starting controller 630 of a starter 63 as compared with the fourth embodiment that is different only in that the starting controller 430 consists of the pulse generation circuit 431 , the organization circuit 432 and the frequency sweep circuit 433 .

In this sixth embodiment, the repetition circuit 635 repeats sweep operation of a frequency sweep circuit 633 in case of a starting mode. As shown in examples of FIGS. 12 and 13 , when one cycle of the continuous sweep frequency from the frequency f 61 to the frequency f 62 (<f 61 ) is about 400 μsec and a period of a starting mode is 1 sec, the sweep operation is repeated about 2,500 times. FIG. 14 illustrates resonance voltage (lamp voltage) and a lamp current in case that the lamp DL 6 does not reach breakdown, and FIG. 15 illustrates resonance voltage (lamp voltage) and a lamp current in case that the lamp DL 6 reaches breakdown.

According to the sixth embodiment of the present invention, because starting voltage including the resonance voltage is repeatedly applied across the lamp DL 6 , more preferable starting of the lamp DL 6 is possible. The repetition circuit 635 of the sixth embodiment is also applicable to the starting controller 530 in the fifth embodiment.

FIG. 16 illustrates a discharge lamp ballast 70 for a discharge lamp DL 7 (e.g., a DC discharge lamp such as a HID lamp or the like). This ballast 70 is characterized by a transition auxiliary circuit 736 further provided in a starting controller 730 of a starter 73 as compared with the sixth embodiment that is different only in that the starting controller 630 consists of the pulse generation circuit 631 , the organization circuit 632 , the frequency sweep circuit 633 and the repetition circuit 635 .

In case of the grow-arc transition mode between the starting mode and the steady operating mode ( FIG. 17 ), the transition auxiliary circuit 736 in this seventh embodiment alternately turns switching elements Q 72 and Q 73 on and off at a switching frequency f 73 (<f 72 ) lower than a switching frequency of f 71 -f 72 (f 71 >f 72 ) in the starting mode through a pulse generation circuit 731 . A period of the grow-arc transition mode and the switching frequency f 73 is set based on time taken until breakdown of the lamp DL 7 and state leading to stable transition from grow to arc of the lamp DL 7 . For example, the switching frequency of f 71 -f 72 is set with 115 KHz and the period of the starting mode is set for 1 second, while the switching frequency f 73 is set to 52 KHz and the period of the grow-arc transition mode is set for 0.5 second. FIG. 18 illustrates resonance voltage (lamp voltage) and a lamp current in case that the lamp DL 7 does not reach breakdown, and FIG. 19 illustrates resonance voltage (lamp voltage) and a lamp current in case that the lamp DL 7 reaches breakdown.

According to the seventh embodiment of the present invention, it is possible to stably lead the lamp DL 7 to arc discharge and to stably operate the lamp DL 7 .

FIG. 20 illustrates a discharge lamp ballast 80 for a discharge lamp DL 8 (e.g., a DC discharge lamp such as a HID lamp or the like). This ballast 80 further comprises an igniter 837 in a starter 83 as compared with the first embodiment that is different only in that the starter 13 consists of the inductor L 12 , the capacitor C 12 , the switching elements Q 12 and Q 13 , and the starting controller 130 .

In this eighth embodiment, the igniter 837 is constructed with a diode D 837 , a capacitor C 837 , a pulse transformer PT with a primary winding n 831 and a secondary winding n 832 , and a gap G, and superposes pulse voltage responding to voltage applied across the primary winding n 831 onto resonance voltage across a capacitor C 82 . An anode of the diode D 837 is connected between an inductor L 82 and the lamp DL 8 . The capacitor C 837 is connected in series with the diode D 837 , while the series combination of the capacitor C 837 and the diode D 837 (hereinafter referred to as a “combination A”) is connected in parallel with the capacitor C 82 . The winding n 831 is connected in series with the gap G, while the series combination of the winding n 831 and the gap G is connected in parallel with the capacitor C 837 . The winding n 832 is connected in series with the lamp DL 8 , while the series combination of the winding n 832 and the lamp DL 8 is connected in parallel with each of the capacitor C 82 and the combination A.

During a starting mode, resonance voltage (high frequency peak voltage) across the capacitor C 82 is applied across the capacitor C 837 via the diode D 837 , and therefore voltage across the capacitor C 837 rises toward threshold voltage of the gap G. When the voltage across the capacitor C 837 reaches the threshold voltage of the gap G, the capacitor C 837 discharges against the primary winding n 831 of the pulse transformer PT. As a result, pulse voltage is induced in the secondary winding n 832 of the transformer PT. At this point, the pulse voltage generates electric field toward a negative terminal (second end) of the lamp DL 8 from its positive terminal (first end). The pulse voltage is also generated in response to a turn ratio (n 831 :n 832 ) of the transformer PT.

In case of any mode except the starting mode, resonance voltage across the capacitor C 82 is not applied across the capacitor C 837 via the diode D 837 , and therefore voltage across the capacitor C 837 does not reach the threshold voltage of the gap G.

According to the eighth embodiment of the present invention, starting voltage is created by superposing the pulse voltage onto the resonance voltage across the capacitor C 82 , it is possible to reduce by the resonance voltage from the pulse voltage, so that noise from the starter 83 can be reduced. The igniter 837 of the eighth embodiment is also applicable to a starter in the above each embodiment.

FIG. 21 illustrate various examples of arrangement of a pulse transformer PT. In arrangement of FIG. 21( a ), the pulse voltage generates electric field toward the negative terminal of the lamp DL 8 from its positive terminal in the starting mode. In arrangement of FIG. 21( b ), the pulse voltage generates electric field toward the positive terminal of the lamp DL 8 from its negative terminal in the starting mode. In arrangement of FIG. 21( c ), the pulse transformer PT has secondary windings 832 a and 832 b , and in the starting mode, the pulse voltage generates electric field toward the negative terminal of the lamp DL 8 from its positive terminal and electric field toward the positive terminal of the lamp DL 8 from its negative terminal.

Therefore, the present invention achieves reduction of noise from the starting means (starter) and gives benefit of the noise reduction and high reliability in equipment such as a lighting system constructed with the ballast and the lamp, a projector constructed with the ballast and the lamp, or the like. Especially, in a liquid crystal projector, many minute electric circuits are located around a discharge lamp ballast, and therefore reducing noise from the starting means makes it possible to improve reliability.

Although the present invention has been described with reference to certain preferred embodiments, numerous modifications and variations can be made by those skilled in the art without departing from the true spirit and scope of this invention. For example, the embodiments include switching elements, such as power MOSFETs, but such elements may be replaced with bipolar transistors and diodes. In another example, the converter controller ( 12 , 22 , 32 , 42 , 52 , 62 or 82 ) may turn the switching element (Q 11 , Q 21 , Q 31 , Q 41 , Q 51 , Q 61 , Q 71 or Q 81 ) on and off at a high frequency of a specific pulse width.