Discharge lamp lighting circuit

A discharge lamp lighting circuit 1 includes an electric power supply part and a control part. The control part generates a control signal (Sc) for controlling the magnitude of electric power based on a lamp voltage (VL) of a discharge lamp. The electric power supply part supplies the electric power based on the control signal (Sc) from the control part to the discharge lamp. The control part has a differential computation part for differentiating a lamp voltage corresponding signal (VS) with respect to time and generating a first differential signal Sd1 (=dVS/dt), and an integral computation part for integrating a second differential signal (Sd2) which monotonously increases and decreases as the first differential signal (Sd1) increases and decreases with respect to time and generating a first integral signal (Si1), and generates the control signal (Sc) so that the electric power decreases with an increase in the first integral signal (Si1).

TECHNICAL FIELD

The present disclosure relates to a discharge lamp lighting circuit.

BACKGROUND

A discharge lamp such as a metal halide lamp used in a vehicle headlight is lit in the following manner. A high-voltage pulse (e.g., several tens kV) for prompting a dielectric breakdown between electrodes is first applied, and a discharge arc is struck between the electrodes and the portion between the electrodes is brought into conduction. Next, relatively large electric power is supplied to increase light emission intensity quickly. Thereafter, a voltage (lamp voltage) between the electrodes of the discharge lamp increases as the light emission intensity by vaporization of metal sealed inside a tube increases, so that the supplied electric power gradually is decreased according to an increase in this lamp voltage. In this manner, the light emission intensity of the discharge lamp quickly converges to a predetermined intensity while preventing an overshoot.

At present, a trace of mercury is sealed in the discharge lamp such as the metal halide lamp. However, a (mercury-free) discharge lamp without containing mercury is being developed to prevent environmental contamination at the time of disposal.FIG. 14(a) is a graph showing a typical example of changes (from a start of lighting) in luminous flux (graph G10), lamp voltage (graph G11) and supply electric power (graph G12) in a conventional discharge lamp in which mercury is sealed. Also,FIG. 14(b) is a graph showing a typical example of changes (from a start of lighting) in luminous flux (graph G13), lamp voltage (graph G14) and supply electric power (graph G15) in a mercury-free discharge lamp.

In the conventional discharge lamp in which mercury is sealed, a lamp voltage immediately after starting lighting is about 27 V and gradually increases to about 85 V with an increase in light emission intensity as shown inFIG. 14(a). A lighting circuit decreases the supply electric power from about 70 W to about 35 W according to a change (amount of change 58 V) in this lamp voltage. On the other hand, in the mercury-free discharge lamp, a lamp voltage immediately after starting lighting is equal to that of the discharge lamp with mercury (about 27 V) and the lamp voltage increases to about 45 V with an increase in light emission intensity, but the amount of change (18 V) is smaller than that of the discharge lamp with mercury as shown inFIG. 14(b). Also, a lamp voltage value immediately after starting lighting or the amount of change in the lamp voltage with an increase in light emission intensity has variations depending on secular change or individual difference. When the amount of change in the lamp voltage is small, an influence of the variations by secular change or individual difference becomes relatively great, so that it becomes difficult to speedily converge the light emission intensity while preventing an overshoot in a method for controlling the supply electric power according to the lamp voltage value.

To address the problem of electric power supply to the mercury-free discharge lamp as described above, a discharge lamp apparatus described, for example, in Japanese Patent Reference JP-A-2003-338390, is intended to reduce an influence on electric power control by variations in lamp voltage of individual discharge lamps by storing a lamp voltage (lamp initial voltage) immediately after a start of lighting and controlling supply electric power based on the amount of change in the lamp voltage from this lamp initial voltage.

However, the discharge lamp apparatus described in JP-A-2003-338390 can present the following problem. As described above, a high-voltage pulse for prompting a dielectric breakdown between electrodes is first applied in the case of lighting a discharge lamp. A lamp voltage immediately after a start of lighting is influenced by this high-voltage pulse and becomes unstable, so that in a method using the lamp voltage immediately after the start of lighting as a lamp initial voltage, a value of the stored lamp initial voltage varies every operation and the amount of change in the calculated lamp voltage also varies every operation. Therefore, in the discharge lamp apparatus described in JP-A-2003-338390, it is difficult to control the supply of electric power with good reproducibility.

SUMMARY

The invention has been implemented in view of the problem described above. In some implementations, the discharge lamp lighting circuit disclosed below is capable of controlling supply electric power with good reproducibility while suppressing an influence of variations in a voltage between electrodes by secular change or individual difference in a discharge lamp.

Among other things, in order to address the problem, a discharge lamp lighting circuit is disclosed to supply electric power for lighting a discharge lamp to the discharge lamp. The circuit comprises a control part for generating a control signal for controlling magnitude of the electric power based on a voltage between electrodes of the discharge lamp, and an electric power supply part for supplying the electric power based on the control signal from the control part to the discharge lamp. The control part has a differential computation part for differentiating a signal according to the voltage between electrodes with respect to time and generating a first differential signal and a first integral computation part for integrating a second differential signal which monotonously increases and decreases as the first differential signal increases and decreases with respect to time and generating a first integral signal, and generates the control signal so that the electric power decreases with an increase in the first integral signal.

The present inventors found that there is a strong correlation, which has an extremely small influence of change with time or individual difference in a discharge lamp, between change in light emission intensity and a differential value and an integral value of a voltage between electrodes even when the amount of change in the voltage between electrodes of the discharge lamp with an increase in the light emission intensity is small and there are variations in magnitude of the voltage between electrodes. In the discharge lamp lighting circuit described above, a control part differentiates a signal according to the voltage between electrodes with respect to time and generates a first differential signal, and integrates a second differential signal which monotonously increases and decreases as this first differential signal increases and decreases with respect to time and generates a first integral signal, and generates a control signal so that electric power decreases with an increase in this first integral signal. Consequently, supply of electric power can be controlled while suppressing an influence of variations in the voltage between electrodes by secular change or individual difference in the discharge lamp.

Also, in some implementations of the discharge lamp lighting circuit described above, the supply electric power is controlled based on the first integral signal in which the second differential signal is integrated, so that even when a voltage between electrodes immediately after a start of lighting is influenced by a high-voltage pulse and varies, an influence on electric power control can be reduced by action of averaging the variations. Therefore, the supply electric power can be controlled during each operation with good reproducibility.

In some implementations, the first integral computation part integrates the first differential signal with respect to time and further generates a second integral signal and the control part offers the control signal based on the first integral signal to the electric power supply part after the second integral signal reaches a first predetermined value. Consequently, the electric power control described above can be started under a certain condition that an integral value of the first differential signal reaches the first predetermined value, so that even when individual difference in a voltage between electrodes immediately after a start of lighting is large, an influence of the individual difference can be suppressed more effectively.

Also, the first integral computation part can include a first conversion part for converting the second differential signal into a second current signal, a second conversion part for converting the first differential signal into a first current signal, a first capacitive element for charging the first current signal and outputting a voltage across the first capacitive element as the second integral signal and also charging the second current signal and outputting a voltage across the first capacitive element as the first integral signal and a first current control part for controlling supply of the first and second current signals to the first capacitive element based on the voltage across the first capacitive element, and the first current control part controls the first and second current signals so that the first current signal is first supplied to the first capacitive element and the second current signal is supplied to the first capacitive element after the voltage across the first capacitive element reaches the first predetermined value or the corresponding value.

In some implementations, the first current signal is first supplied to the first capacitive element and thereby, integral computation of the first differential signal is performed and the second integral signal can be generated. Then, after the voltage across the first capacitive element (indicating the second integral signal in this case) reaches the first predetermined value or the corresponding value, the second current signal instead of the first current signal is supplied to the first capacitive element and thereby, integral computation of the second differential signal is performed and the first integral signal can be generated. Also, one capacitive element (first capacitive element) combines a capacitive element for integrating the first differential signal and generating the second integral signal with a capacitive element for integrating the second differential signal and generating the first integral signal, so that a circuit size can be reduced further.

Also, in some implementations, the control part has a second integral computation part for integrating the first differential signal with respect to time and generating a second integral signal, and offers the control signal based on the first integral signal to the electric power supply part after the second integral signal reaches a first predetermined value. Consequently, the electric power control described above can be started under a certain condition that an integral value of the first differential signal reaches the first predetermined value, so that even when individual difference in a voltage between electrodes immediately after a start of lighting is large, an influence of the individual difference can be suppressed more effectively.

Also, the first integral computation part can include a first conversion part for converting the second differential signal into a second current signal and a first capacitive element for charging the second current signal and outputting a voltage across the first capacitive element as the first integral signal, and the second integral computation part includes a second conversion part for converting the first differential signal into a first current signal and a second capacitive element for charging the first current signal and outputting a voltage across the second capacitive element as the second integral signal, and the control part further has a first current control part for controlling supply of the second current signal to the first capacitive element so that the second current signal is supplied to the first capacitive element after the voltage across the second capacitive element reaches the first predetermined value or the corresponding value.

The first differential signal can be integrated by the second capacitive element and the second integral signal can be generated. Then, after the voltage across the second capacitive element (that is, the second integral signal) reaches the first predetermined value or the corresponding value, the second current signal is controlled so as to supply the second current signal to the first capacitive element and thereby, integral computation of the second differential signal is performed and the first integral signal can be generated.

In some implementations, the first integral computation part has a resistance element connected between a constant-voltage source and the first capacitive element, and a second current control part for supplying a current from the constant-voltage source to the first capacitive element when a voltage across the first capacitive element is larger than a second predetermined value. In this discharge lamp lighting circuit, when the voltage across the first capacitive element (first integral signal) reaches the second predetermined value, a current from the constant-voltage source is superposed on the second current signal. That is, a signal which monotonously increases depending on only elapsed time is superposed on the first integral signal.

When some time has elapsed since a start of lighting, a change in a state of the inside of a tube of a discharge lamp becomes small, so that it is preferable to control the supply electric power based on the elapsed time rather than to control the supply electric power based on an integral value and a time differential value of a voltage between electrodes. According to this discharge lamp lighting circuit, the signal which monotonously increases depending on only the elapsed time is superposed on the first integral signal and thereby, the discharge lamp can be shifted to a steady state while the supply electric power is gradually converged on target electric power and light emission intensity close to target intensity is maintained. Further, start timing of electric power control based on the elapsed time is defined based on the first integral signal and thereby, a gradual change in light emission intensity in the case of shifting to the electric power control based on the elapsed time can be obtained.

In some implementations, the first current control part stops supply of the second current signal to the first capacitive element after a voltage across the first capacitive element reaches a third predetermined value larger than the second predetermined value, and the third predetermined value is less than or equal to a value of the voltage across the first capacitive element at a point in time when the first differential signal becomes maximum. A discharge lamp includes means which exhibits characteristics in which the first differential signal suddenly decreases after the first differential signal becomes maximum and means which does not exhibit the characteristics. In this discharge lamp lighting circuit, before the first differential signal becomes maximum, supply of the second current signal to the first capacitive element is stopped and subsequently, only a current from a constant-voltage source is integrated by the first capacitive element. Therefore, supply electric power is controlled based on only a signal which monotonously increases depending on only elapsed time, and an influence on a control signal by variations in the first differential signal after the first differential signal becomes maximum can be avoided.

The first integral computation part can include a function computation part for receiving the first differential signal and generating the second differential signal, and the function computation part converts the first differential signal into the second differential signal according to a function having a positive first slope when magnitude of the first differential signal is smaller than a fourth predetermined value, and converts the first differential signal into the second differential signal according to a function having a positive second slope smaller than the first slope when magnitude of the first differential signal is larger than the fourth predetermined value. According to this discharge lamp lighting circuit, even in a time region in which a voltage between electrodes suddenly increases by vaporization of metal of the inside of a tube (that is, the first differential signal increases), a sudden decrease in supply electric power can be prevented and a more speedup in convergence of light emission intensity can be achieved.

Various advantages can be obtained in some implementations. For example, the supply of electric power can be controlled with good reproducibility while suppressing an influence of variations in a voltage between electrodes by secular change or individual difference in a discharge lamp.

Other features and advantages will be apparent from the following detailed description, the accompanying drawings and the claims.

DETAILED DESCRIPTION

Preferred embodiments of a discharge lamp lighting circuit according to the invention are described below in detail with reference to the drawings. In addition, in the description of the drawings, the same numerals are assigned to the same or corresponding parts.

First Embodiment

FIG. 1is a block diagram showing an example of a configuration of a first embodiment of a discharge lamp lighting circuit according to the invention. A discharge lamp lighting circuit1shown inFIG. 1is a circuit for supplying electric power for lighting a discharge lamp L to the discharge lamp L, and a DC voltage from a DC power source B is converted into an AC voltage and is supplied to the discharge lamp L. The discharge lamp lighting circuit1is mainly used in lamp fittings such as, particularly, a headlight for a vehicle. In addition, as the discharge lamp L, for example, a mercury-free metal halide lamp is suitably used, but discharge lamps with other structures may be used.

The discharge lamp lighting circuit1comprises an electric power supply part2for receiving power source supply from the DC power source B and supplying AC electric power to the discharge lamp L, and a control part10afor controlling magnitude of supply electric power to the discharge lamp L based on a voltage (hereinafter called a lamp voltage) between electrodes of the discharge lamp L.

The electric power supply part2supplies electric power of magnitude based on a control signal Sc from the control part10adescribed below to the discharge lamp L. The electric power supply part2is connected to the DC power source B (e.g., a battery) through a switch20for lighting operation, and receives a DC voltage VB from the DC power source B and makes AC conversion and a step-up. The electric power supply part2of the present embodiment has a starting circuit3for applying a high-voltage pulse to the discharge lamp L at the time of a start of lighting, two transistors5aand5b, and a bridge driver6for driving the transistors5aand5b. As the transistors5aand5b, for example, an N-channel MOSFET can be used, but other FETs or bipolar transistors may be used. In the embodiment, a drain terminal of the transistor5ais connected to a plus side terminal of the DC power source B and a source terminal of the transistor5ais connected to a drain terminal of the transistor5band a gate terminal of the transistor5ais connected to the bridge driver6. Also, a source terminal of the transistor5bis connected to a ground potential line GND (that is, a minus side terminal of the DC power source B) and a gate terminal of the transistor5bis connected to the bridge driver6. The bridge driver6alternately brings the transistors5aand5binto conduction.

The electric power supply part2of the embodiment further has a transformer7, a capacitor8and an inductor9. The transformer7is disposed in order to apply a high-voltage pulse to the discharge lamp L and transmit electric power and also step up the electric power. Also, a series resonance circuit is constructed of the transformer7, the capacitor8and the inductor9. That is, a primary winding7aof the transformer7, the inductor9and the capacitor8are mutually connected in series. Then, one end of its series circuit is connected to the source terminal of the transistor5aand the drain terminal of the transistor5band the other end is connected to the ground potential line GND. In this configuration, a resonance frequency is determined by capacitance of the capacitor8and combined reactance made of inductance of the inductor9and leakage inductance of the primary winding7aof the transformer7. In addition, the series resonance circuit is constructed by only the primary winding7aand the capacitor8and the inductor9may be omitted. Also, it may be constructed so that inductance of the primary winding7ais set extremely smaller than that of the inductor9and a resonance frequency is substantially determined by capacitance of the capacitor8and inductance of the primary winding7a.

In the electric power supply part2, using a series resonance phenomenon by an inductive element (an inductance component or an inductor) and the capacitor8, a drive frequency of the transistors5aand5bis defined at a value of this series resonance frequency or higher and the transistors5aand5bare alternately turned on and off and AC electric power is produced in the primary winding7aof the transformer7. This AC electric power is stepped up and transmitted to a secondary winding7bof the transformer7and is supplied to the discharge lamp L connected to the secondary winding7b. In addition, the bridge driver6for driving the transistors5aand5bdrives each of the transistors5aand5breciprocally so that both the transistors5aand5bdo not become a connection state.

Also, impedance of this series resonance circuit varies depending on the drive frequency of the transistors5aand5bby the bridge driver6. Therefore, magnitude of AC electric power supplied to the discharge lamp L can be controlled by changing the drive frequency. Here,FIG. 2is a graph conceptually showing a relation between magnitude of supply electric power and the drive frequency of the transistors5aand5b. As shown inFIG. 2, magnitude of electric power supplied to the discharge lamp L becomes a maximum value Pmax when the drive frequency is equal to a series resonance frequency fo, and decreases as the drive frequency becomes higher (or becomes lower) than the series resonance frequency fo. However, when the drive frequency is lower than the series resonance frequency fo, switching loss becomes large and electric power efficiency decreases. Therefore, magnitude of a drive frequency of the bridge driver6is controlled in a region (region A inFIG. 2) higher than the series resonance frequency fo. In the embodiment, the drive frequency of the bridge driver6is controlled according to a pulse frequency of the control signal Sc (signal including a frequency-modulated pulse train) from the control part10aconnected to the bridge driver6.

The starting circuit3is a circuit for applying a high-voltage pulse for starting to the discharge lamp L, and when trigger voltage and current are applied from the starting circuit3to the transformer7, the high-voltage pulse is superposed on an AC voltage generated in the secondary winding7bof the transformer7. In the starting circuit3of the embodiment, one of an output terminal is connected to the middle of the primary winding7aof the transformer7and the other of the output terminal is connected to a ground potential side terminal of the primary winding7a. An input voltage to the starting circuit3may be obtained from, for example, an auxiliary winding (not shown) for starting or the secondary winding7bof the transformer7or maybe obtained from an auxiliary winding by disposing the auxiliary winding constructing the transformer together with the inductor9.

The control part10acontrols magnitude of supply electric power to the discharge lamp L based on a lamp voltage of the discharge lamp L. The control part10aof the embodiment has an electric power computation part11for computing magnitude of electric power to be supplied to the discharge lamp L, an error amplifier12for amplifying and outputting a difference between a predetermined reference voltage and an output voltage Sp1from the electric power computation part11, and a V-F conversion part13for making voltage-frequency conversion (V-F conversion) of a signal Sp2which is an analog signal output from the error amplifier12and generating the control signal Sc.

The electric power computation part11has input ends11aand11band an output end11c. The input end11ais connected to an intermediate tap of the secondary winding7bthrough a peak hold circuit21in order to input a signal (hereinafter called a lamp voltage corresponding signal) VS indicating magnitude of a lamp voltage VL of the discharge lamp L. The lamp voltage corresponding signal VS is set at, for example, 0.35 time the peak value of the lamp voltage VL. The input end11bis connected to one end of a resistance element4disposed for detecting a lamp current of the discharge lamp L through a peak hold circuit22and a buffer23. One end of the resistance element4is further connected to one electrode of the discharge lamp L through an output terminal of the discharge lamp lighting circuit1, and the other end of the resistance element4is connected to the, ground potential line GND. Then, a lamp current corresponding signal IS indicating magnitude of the lamp current is output from the buffer23. Also, the output end11cis connected to the error amplifier12.

Here,FIG. 3is a block diagram showing a configuration of the inside and periphery of the electric power computation part11of the embodiment. Referring toFIG. 3, the electric power computation part11has a differential computation part15, an integral computation part (first integral computation part)16, a V/I conversion part17, and current sources18and19.

The differential computation part15is a circuit part for computing a time differential value (dVS/dt) of the lamp voltage corresponding signal VS and generating a first differential signal Sd1. An input end15aof the differential computation part15is connected to the input end11aof the electric power computation part11. An output end15bof the differential computation part15is connected to the integral computation part16. In addition, such a differential computation part15is suitably constructed by, for example, a differentiation circuit using the lamp voltage corresponding signal VS as input.

The integral computation part16is a circuit part for integrating a second differential signal Sd2which monotonously increases and decreases as the first differential signal Sd1increases and decreases with respect to time and generating a first integral signal Si1. An input end16aof the integral computation part16is connected to the output end15bof the differential computation part15. An output end16bof the integral computation part16is connected to the V/I conversion part17.

The V/I conversion part17is a circuit part for subtracting a first predetermined value E0(described below) from the first integral signal Si1and also converting the subtracted value into a current signal I1. An input end17aof the V/I conversion part17is connected to the output end16bof the integral computation part16. An output end17bof the V/I conversion part17is connected to the input end11bof the electric power computation part11through a resistance element24. In addition, such a V/I conversion part17is suitably constructed by, for example, a voltage-current converter and a differential amplifier using the first integral signal Si1and the predetermined value E0as input.

The V/I conversion part17outputs a current I1according to a function shown in, for example,FIG. 4. That is, the V/I conversion part17sets the current signal I1at zero when the first integral signal Si1is the first predetermined value E0or less, and outputs the current signal I1of the magnitude proportional to a value obtained by subtracting E0from the first integral signal Si1when the first integral signal Si1is the first predetermined value E0or more.

The current sources18and19are a circuit part for controlling steady electric power (for example, 35 [W]) and supply electric power (for example, 75 [W]) immediately after a start of lighting. Input ends18a,19aof the current sources18,19are connected to the input end11aof the electric power computation part11. Output ends18b,19bof the current sources18,19are connected to one input end12aof the error amplifier12through the output end11cof the electric power computation part11. In addition, the other input end12bof the error amplifier12is connected to a predetermined voltage source14for generating a predetermined reference voltage.

The current source18outputs a current I2according to a function shown, for example, inFIG. 5. That is, the current source18sets the current signal I2at zero when the lamp voltage corresponding signal VS is a certain predetermined value V1or less, and sets the current signal I2at a constant value when the lamp voltage corresponding signal VS is a certain predetermined value V2(>V1) or more, and outputs the current signal I2of the magnitude proportional to the lamp voltage corresponding signal VS when the lamp voltage corresponding signal VS is V1or more and V2or less. The current source19outputs a current I3according to a function also shown, for example, inFIG. 5. That is, the current source19outputs the current signal I3of the magnitude proportional to the lamp voltage corresponding signal VS, and its proportional coefficient is set so as to become small as the lamp voltage corresponding signal VS becomes high.

The integral computation part16is now described in further detail. The integral computation part16of the embodiment includes a function computation part161, V/I conversion parts162and163, a current control part165, and a capacitive element (first capacitive element)166.

The function computation part161is a circuit part for generating the second differential signal Sd2which monotonously increases and decreases as the first differential signal Sd1increases and decreases. An input end161aof the function computation part161is connected to the output end15bof the differential computation part15through the input end16aof the integral computation part16. An output end161bof the function computation part161is connected to the V/I conversion part162.

The V/I conversion part162is a first conversion part in the embodiment, and converts the second differential signal Sd2which is a voltage signal into a second current signal Id2. An input end162aof the V/I conversion part162is connected to the output end161bof the function computation part161. An output end162bof the V/I conversion part162is connected to one end of the capacitive element166through a switch164a. The other end of the capacitive element166is connected to the ground potential line GND.

The V/I conversion part163is a second conversion part in the embodiment, and converts the first differential signal Sd1which is a voltage signal into a first current signal Id1. An input end163aof the V/I conversion part163is connected to the output end15bof the differential computation part15through the input end16aof the integral computation part16. An output end163bof the V/I conversion part163is connected to one end of the capacitive element166through a switch164b.

The current control part165is a first current control part in the embodiment, and controls the first current signal Id1and the second current signal Id2based on a voltage V across the capacitive element166. The current control part165is constructed by including, for example, a window comparator165aand a comparator165b. An input end of the window comparator165ais connected to one end of the capacitive element166and an output end is connected to a control terminal of the switch164a. The window comparator165aoutputs a voltage corresponding to logic 0 when an input voltage (that is, the voltage V across the capacitive element166) is smaller than a predetermined value E0(first predetermined value) or the input voltage is larger than a predetermined value E2(third predetermined value), and outputs a voltage corresponding to logic 1 when the input voltage is larger than the predetermined value E0and is smaller than the predetermined value E2. Also, an input end of the comparator165bis connected to one end of the capacitive element166and an output end is connected to a control terminal of the switch164b. The comparator165boutputs a voltage corresponding to logic 1 when an input voltage (that is, the voltage V across the capacitive element166) is smaller than the predetermined value E0, and outputs a voltage corresponding to logic 0 when the input voltage is larger than the predetermined value E0. In addition, the switches164aand164bshall become a connection state when the voltage corresponding to logic1is inputted to the control terminal, and become a non-connection state when the voltage corresponding to logic 0 is inputted to the control terminal.

In addition, the current control part165of the embodiment controls supply of the first current signal Id1and the second current signal Id2to the capacitive element166by the switches164aand164b, but the current control part165may control the second current signal Id2by directly controlling the function computation part161or the V/I conversion part162and also may control the first current signal Id1by directly controlling the V/I conversion part163. Also, the current control part165of the embodiment includes the window comparator165ain order to control the second current signal Id2, but the second current signal Id2may be controlled using two comparators independent mutually. Also, the switches164aand164bdescribed above are suitably implemented by a transistor such as an FET.

The integral computation part16further includes a switch167, a resistance element168and a comparator169in addition to the above configuration. The switch167and the resistance element168are connected in series between a constant-voltage source Vcc and one end of the capacitive element166. The switch167is suitably implemented by a transistor such as an FET. Also, the comparator169is a second current control part in the embodiment, and supplies a current from the constant-voltage source Vcc to the capacitive element166when the voltage V across the capacitive element166is larger than a predetermined value E1(second predetermined value). Concretely, an input end of the comparator169is connected to one end of the capacitive element166and an output end is connected to a control terminal of the switch167. The comparator169outputs a voltage corresponding to logic 0 when an input voltage (that is, the voltage V across the capacitive element166) is smaller than the predetermined value E1, and outputs a voltage corresponding to logic 1 when the input voltage is larger than the predetermined value E1. In addition, the switch167becomes a connection state when the voltage corresponding to logic 1 is inputted to the control terminal, and becomes a non-connection state when the voltage corresponding to logic 0 is inputted to the control terminal.

An operation of the discharge lamp lighting circuit1comprising the foregoing configuration is now described.FIGS. 6(a) to6(c), respectively, show situations of changes in the lamp voltage VL (FIG. 6(a)), the first differential signal Sd1(=dVS/dt) (FIG. 6(b)) and the voltage V across the capacitive element166(FIG. 6(c)) with a lapse of time since immediately after a start of lighting. Also,FIGS. 7(a) and7(b), respectively, show situations of changes in supply electric power (FIG. 7(a)) to the discharge lamp L and light emission intensity (FIG. 7(b)) of the discharge lamp L with a lapse of time since immediately after a start of lighting.

First, while the bridge driver6shown inFIG. 1drives the transistors5aand5bat a predetermined drive frequency, a high-voltage pulse of several tens kV is applied between electrodes of the discharge lamp L and prompts a dielectric breakdown by the starting circuit3. Immediately after that, the drive frequency of the bridge driver6is controlled to a drive frequency to which the predetermined maximum electric power (75 [W] the time of a cold start) is obtained according to a control signal Sc from the control part10a. In the control part10a, an output voltage Sp1, to the error amplifier12is controlled by current signals I2, I3output from the current sources18,19(seeFIG. 3) of the electric power computation part11. Then, V-F conversion of an output voltage SP2, which is a difference between this output voltage Sp1, and a predetermined reference voltage, from the error amplifier12is made in the V-F conversion part13and the output voltage SP2is offered to the bridge driver6as the control signal Sc.

In addition, a voltage V across the capacitive element166of the integral computation part16becomes substantially a ground potential immediately after a start of lighting, so that the window comparator165aof the current control part165controls the switch164ain a non-connection state and the comparator165bcontrols the switch164bin a connection state. Also, the comparator169controls the switch167in a non-connection state.

Subsequently, when an output signal from the differential computation part15of the electric power computation part11becomes stable (time t0ofFIG. 6(c)), a first differential signal Sd1(=dVS/dt) output from the differential computation part15is converted into a current signal Id1in the V/I conversion part163of the integral computation part16and is charged into the capacitive element166through the switch164b. Consequently, the first differential signal Sd1is integrated with respect to time in the capacitive element166. At this time, the voltage V across the capacitive element166is expressed by the following mathematical formula (1) and indicates magnitude of a second integral signal.

Subsequently, when the voltage V across the capacitive element166(a second integral signal in this case) reaches a predetermined value E0(time t1ofFIGS. 6 and 7), the switch164bis controlled in a non-connection state by the comparator165band supply of the first current signal Id1to the capacitive element166is stopped and at the same time, the switch164ais controlled in a connection state by the window comparator165aand supply of a second current signal Id2to the capacitive element166is started. That is, the first differential signal Sd1output from the differential computation part15is converted into a second differential signal Sd2by the function computation part161and the second differential signal Sd2is converted into the second current signal Id2in the V/I conversion part162and is charged into the capacitive element166through the switch164a. Consequently, the second differential signal Sd2is integrated with respect to time in the capacitive element166. At this time, the voltage V across the capacitive element166is expressed by the following mathematical formula (2) and indicates magnitude of a first integral signal Si1. In addition, in the mathematical formula (2), f(x) represents a function computed in the function computation part161.

Here,FIG. 8is a graph showing one example of the function f(x) of the first differential signal Sd1and the second differential signal Sd2computed in the function computation part161of the embodiment. As shown inFIG. 8, the function computation part161converts the first differential signal Sd1into the second differential signal Sd2according to a function f1having a positive certain slope (a first slope which is 1 in the example ofFIG. 8) when magnitude of the first differential signal Sd1is smaller than a predetermined value (a fourth predetermined value which is 0.3 [V/s] in the example ofFIG. 8), and converts the first differential signal Sd1into the second differential signal Sd2according to a function f2having a positive slope (a second slope which is 0.2 in the example ofFIG. 8) smaller than the first slope when magnitude of the first differential signal Sd1is larger than the predetermined value. In addition,FIG. 8shows a proportional function as one example of the functions f1, f2, but the functions f1, f2may be a function whose slope varies according to the first differential signal Sd1.

The voltage V across the capacitive element166is output from the integral computation part16as the first integral signal Si1and is inputted to the V/I conversion part17. Then, the predetermined value E0(that is, the second term of the right side of the mathematical formula (2)) is subtracted from the first integral signal Si1and a voltage value after the subtraction is converted into a current signal I1. In the electric power computation part11of the embodiment, a current signal I4formed by joining the current signal I1from the V/I conversion part17and the current signals I2, I3from the current sources18,19flows to an input end of the buffer23through the resistance element24as shown inFIG. 3. On the other hand, a lamp current ILflows in the resistance element4, so that a voltage drop in this resistance element4occurs in an output end of the buffer23as a lamp current corresponding signal IS. That is, the output voltage Sp1from the electric power computation part11is determined by the current signal I4and the lamp current IL. When the first integral signal Si1increases gradually (FIG. 6(c)), the current signal I1increases, so that a voltage drop in the resistance element24increases and a frequency of the control signal Sc output from the V-F conversion part13becomes high gradually. Consequently, supply electric power to the discharge lamp L is reduced gradually (FIG. 7(a)).

Subsequently, when the voltage V across the capacitive element166(first integral signal Si1) reaches a predetermined value E1(time t2ofFIGS. 6 and 7), the switch167is controlled in a connection state by the comparator169. Consequently, a current from a constant-voltage source Vcc is superposed on the second current signal Id2and is integrated by the capacitive element166(FIG. 6(c)). That is, a signal (hereinafter called g(t)) which monotonously increases depending on only elapsed time is superposed on an integral value of the second differential signal Sd2and the voltage V across the capacitive element166becomes a value shown in the following mathematical formula (3). This voltage V across the capacitive element166is output as the first integral signal Si1, and electric power according to this first integral signal Si1is reduced from the supply electric power to the discharge lamp L (FIG. 7(a)).

Subsequently, when the voltage V across the capacitive element166(first integral signal Si1) reaches a predetermined value E2(>E1) (time t3ofFIGS. 6 and 7), the switch164ais controlled in a non-connection state by the window comparator165aand supply of the second current signal Id2to the capacitive element166is stopped. Consequently, only the current from the constant-voltage source Vcc is supplied to the capacitive element166. That is, the supply electric power to the discharge lamp L is reduced according to only a time function g(t) and gradually converges on target electric power (for example, 35 [W]) (FIG. 7(a)). In addition, it is preferable that the predetermined value E2be less than or equal to the voltage V across the capacitive element166(a value E3shown inFIG. 6(c)) at a point in time when the first differential signal Sd1becomes maximum.

Effects obtained by the discharge lamp lighting circuit1of the embodiment described above are as follows. As described in the Background section, in a mercury-free discharge lamp, the amount of change in a lamp voltage since immediately after a start of lighting is as small as about 18 [V] and an influence of variations by secular change or individual difference becomes relatively large. The present inventors found that there is a strong correlation, which has an extremely small influence of change with time or individual difference, between change in light emission intensity and a differential value and an integral value of a lamp voltage even when the amount of change in the lamp voltage is small and there are variations in magnitude of the lamp voltage. In the discharge lamp lighting circuit1of the embodiment, the control part10adifferentiates the lamp voltage corresponding signal VS with respect to time and generates the first differential signal Sd1, and integrates the second differential signal Sd2which monotonously increases and decreases as this first differential signal Sd1increases and decreases with respect to time and generates the first integral signal Si1, and generates the control signal Sc so that a drive frequency becomes high (that is, supply electric power decreases) with an increase in this first integral signal Si1. Consequently, the supply electric power can be controlled suitably while suppressing an influence of variations in the lamp voltage VL by secular change or individual difference in the discharge lamp L.

Also, the control part10aof the embodiment controls the supply electric power based on the first integral signal Si1in which the second differential signal Sd2is integrated, so that even when the lamp voltage VL immediately after a start of lighting is influenced by a high-voltage pulse from the starting circuit3and varies, an influence on electric power control can be reduced by action of averaging the variations. Therefore, according to the discharge lamp lighting circuit1of the embodiment, the supply electric power can be controlled every operation with good reproducibility.

Also, as shown in the embodiment, the integral computation part16preferably integrates the first differential signal Sd1with respect to time and generates a second integral signal and the control part10aoffers the control signal Sc based on the first integral signal Si1to the electric power supply part2after the second integral signal reaches the predetermined value E0. Consequently, electric power control based on the first integral signal Si1can be started under a certain condition that an integral value (second integral signal) of the first differential signal Sd1reaches the predetermined value E0, so that even when individual difference in the lamp voltage VL immediately after a start of lighting is large, an influence of the individual difference can be suppressed more effectively.

Also, as shown in the embodiment, the integral computation part16is preferably constructed by including the V/I conversion parts162and163, the current control part165, and the capacitive element166. Then, the current control part165preferably controls the first and second current signals Id1and Id2so that the first current signal Id1is first supplied to the capacitive element166and the second current signal Id2is supplied to the capacitive element166after the voltage V across the capacitive element166reaches the predetermined value E0.

Thus, the first current signal Id1is first supplied to the capacitive element166and thereby, integral computation of the first differential signal Sd1is performed and the second integral signal can be generated suitably. Then, after the voltage V across the capacitive element166(second integral signal) reaches the predetermined value E0, the second current signal Id2instead of the first current signal Id1is supplied to the capacitive element166and thereby, integral computation of the second differential signal Sd2is performed and the first integral signal Si1can be generated suitably. According to the integral computation part16thus, one capacitive element166combines a capacitive element for integrating the first differential signal Sd1and generating the second integral signal with a capacitive element for integrating the second differential signal Sd2and generating the first integral signal Si1, so that a circuit size can be reduced further.

Also, as shown in the embodiment, the integral computation part16preferably has the resistance element168connected between the constant-voltage source Vcc and the capacitive element166, and the second current control part (comparator169) for supplying a current from the constant-voltage source Vcc to the capacitive element166when the voltage V across the capacitive element166(first integral signal Si1) is larger than the predetermined value E1. Then, when the voltage V across the capacitive element166(first integral signal Si1) reaches the predetermined value E1, the signal g(t) which monotonously increases depending on only elapsed time is preferably superposed on the first integral signal Si1.

At an initial stage of a start of lighting, a change in a state of the inside of a tube of the discharge lamp L is great, so that supply electric power is controlled based on an integral value and a time differential value of the lamp voltage VL (an integral value and a time differential value (dVS/dt) of the lamp voltage corresponding signal VS in the embodiment) with a high correlation to light emission intensity and thereby, variations in the lamp voltage VL are accommodated and the supply electric power can be controlled suitably. However, when some time has elapsed since a start of lighting, the change in the state of the inside of the tube of the discharge lamp L becomes small, so that it is preferable to control the supply electric power based on elapsed time rather than to control the supply electric power based on the integral value and the time differential value of the lamp voltage VL. According to the discharge lamp lighting circuit1of the embodiment, the signal g(t) which monotonously increases depending on only the elapsed time is superposed on the first integral signal Si1and thereby, the discharge lamp L can be shifted to a steady state while the supply electric power is gradually converged on target electric power and light emission intensity close to target intensity is maintained. Further, start timing of electric power control based on the elapsed time is defined (predetermined value E1) based on the first integral signal Si1and thereby, a gradual change in light emission intensity in the case of shifting to the electric power control based on the elapsed time can be obtained.

Also, when the integral computation part16has the resistance element168and the comparator169, the current control part165preferably stops supply of the second current signal Id2to the capacitive element166after the voltage V across the capacitive element166reaches the predetermined value E2larger than the predetermined value E1as shown in the embodiment. Then, the predetermined value E2is preferably less than or equal to the voltage V across the capacitive element166(first integral signal Si1) at a point in time when the first differential signal Sd1becomes maximum.

Here,FIG. 9is a graph conceptually showing a situation of a change in the lamp voltage VL and a change in its time differential value (dVL/dt) in two discharge lamps with different characteristics. In addition, inFIG. 9, the axis of ordinate shows the lamp voltage VL or its time differential value and the axis of abscissa shows elapsed time since a start of lighting. Also, graphs G1and G2respectively show a lamp voltage VL of a certain discharge lamp and its time differential value, and graphs G3and G4respectively show a lamp voltage VL of another discharge lamp and its time differential value. As shown in the graphs, the discharge lamp includes means (graph G4) which exhibits characteristics in which a time differential value of the lamp voltage VL suddenly decreases after the time differential value becomes maximum, and means (graph G2) which exhibits characteristics in which the time differential value decreases relatively gradually. If electric power control based on the second differential signal Sd2is continued, light emission intensity of the discharge lamp having the characteristics as shown in graphs G3and G4may overshoot when the control part10ais adjusted using the discharge lamp having the characteristics as shown in graphs G1and G2. In reverse, when the control part10ais adjusted using the discharge lamp having the characteristics as shown in graphs G3and G4, light emission intensity of the discharge lamp having the characteristics as shown in graphs G1and G2may undershoot.

On the other hand, in the discharge lamp lighting circuit1of the embodiment, before the first differential signal Sd1becomes maximum, supply of the second current signal Id2to the capacitive element166is stopped and subsequently, only a current from the constant-voltage source Vcc is integrated by the capacitive element166. Therefore, supply electric power is controlled based on only the signal g(t) which monotonously increases depending on only elapsed time, and an influence on the control signal Sc by variations in the first differential signal Sd1after the first differential signal Sd1becomes maximum can be avoided.

Also, as shown inFIG. 8, the function computation part161of the integral computation part16preferably converts the first differential signal Sd1into the second differential signal Sd2according to the function f1having a positive certain slope when magnitude of the first differential signal Sd1is smaller than a certain predetermined value, and converts the first differential signal Sd1into the second differential signal Sd2according to the function f2having a positive smaller slope when magnitude of the first differential signal Sd1is larger than the predetermined value. If supply electric power is controlled based on the first differential signal Sd1without making conversion by the function computation part161, as shown inFIG. 10, in a time zone in which light emission intensity suddenly increases by vaporization of metal of the inside of a tube, that is, in the time zone (a region C inFIG. 10) in which the lamp voltage VL suddenly increases, the supply electric power is reduced more than necessary, with the result that a rise in light emission intensity is delayed. On the other hand, when the first differential signal Sd1(=dVS/dt) exceeds a certain predetermined value, the functions f1, f2in which an increase in a current signal to the capacitive element166is suppressed are applied to the first differential signal Sd1and thereby, even in a time region in which the lamp voltage VL suddenly increases (that is, the first differential signal Sd1increases), the supply electric power can be prevented from being reduced more than necessary and a more speedup in convergence of light emission intensity can be achieved.

A concrete example of the function computation part161according to the first embodiment is now described. In addition, the following example is one example of a concrete circuit configuration for implementing the function computation part161according to the embodiment, and the function computation part161can also be implemented by circuit configurations other than the following circuit configuration.

FIG. 11is a circuit diagram showing a configuration example of the function computation part161. Referring toFIG. 11, this function computation part161has an amplification circuit201, output control circuits202and203, and a suction buffer circuit204. The amplification circuit201includes an amplifier211. A non-inverting input end211aof the amplifier211is connected to an input end161aof the function computation part161. An inverting input end211bof the amplifier211is connected to an output end211cof the amplifier211through a resistance element212, and is grounded through a resistance element213. Also, the inverting input end211bis connected to an output end201aof the amplification circuit201.

The output control circuit202has a NOR circuit216and a transistor214such as an FET. A drain terminal of the transistor214is connected to the output end201aof the amplification circuit201. A source terminal of the transistor214is grounded and a gate terminal is connected to an output end of the NOR circuit216through a resistance element215. One input end of the NOR circuit216is connected to an output end of a comparator165c. In addition, the comparator165cis one comparator in the case of dividing the window comparator165aof the first embodiment into two independent comparators, and outputs a voltage corresponding to logic 1 when a voltage V across the capacitive element166(seeFIG. 3) is larger than a predetermined value E0. A signal SVLwhich becomes logic 1 when a lamp voltage VL exceeds a certain reference value is inputted to the other input end of the NOR circuit216.

The output control circuit203has a transistor221such as an FET. A drain terminal of the transistor221is connected to the output end201aof the amplification circuit201. A source terminal of the transistor221is grounded and a gate terminal is connected to an output end of a comparator165dthrough a resistance element222. In addition, the comparator165dis the other comparator in the case of dividing the window comparator165aof the first embodiment into two independent comparators, and outputs a voltage corresponding to logic 1 when a voltage V across the capacitive element166(seeFIG. 3) is larger than a predetermined value E2.

The suction buffer circuit204has an amplifier231and a diode232. A predetermined voltage E4(corresponding to a fourth predetermined value) in which resistance voltage division is made is inputted to a non-inverting input end231aof the amplifier231. An inverting input end231bof the amplifier231is connected to an anode of the diode232, and an output end231cof the amplifier231is connected to a cathode of the diode232. Also, the anode of the diode232is connected to the output end201aof the amplification circuit201through a resistance element233and a resistance element218. In addition, a point of connection between the resistance element233and the resistance element218is connected to an output end161bof the function computation part161.

When the voltage V across the capacitive element166exceeds the predetermined value E0(corresponding to time t1ofFIG. 6(c)) in this function computation part161, the transistor214becomes a non-connection state and a potential according to a first differential signal Sd1develops in the output end201aof the amplification circuit201. At this time, while the potential of the output end201aof the amplification circuit201is the predetermined voltage E4or less, the amplifier231attempts to pass a current through the resistance elements233and218, but the current is blocked by the diode232. Therefore, a potential (second differential signal Sd2) of the output end161bbecomes almost equal to the first differential signal Sd1(corresponding to the function f1shown inFIG. 8). Thereafter, when the potential of the output end201aof the amplification circuit201exceeds the predetermined voltage E4, the suction buffer circuit204sucks a current through the resistance elements218and233, so that a value of the second differential signal Sd2becomes a value shown in the following mathematical formula (4) (corresponding to the function f2shown inFIG. 8).

Sd2=⁢E4+(Sd1-E4)·R233/⁢(R233+R218)=⁢Sd1·R233/(R233+R218)+⁢E4·R218/(R233+R218)[Mathematical⁢⁢formula⁢⁢4]
In addition, in the mathematical formula (4), R218and R233respectively represent resistance values of the resistance elements218and233. Thereafter, when the voltage V across the capacitive element166exceeds the predetermined value E2, the transistor221becomes a connection state and the output end201aof the amplification circuit201is grounded and a signal output from the output end161bis stopped.

Second Embodiment

Next, another example of a control part will be described as a second embodiment of a discharge lamp lighting circuit according to the invention.FIG. 12is a block diagram showing a configuration of a control part10bof the present embodiment. The control part10bof the embodiment has an electric power computation part31instead of the electric power computation part11of the first embodiment. The electric power computation part31has input ends31aand31band an output end31c. The input end31ais connected to an intermediate tap of a secondary winding7b(seeFIG. 1) through a peak hold circuit21. The input end31bis connected to one end of a resistance element4(seeFIG. 1) disposed for detecting a lamp current IS of a discharge lamp L through a peak hold circuit22and a buffer23. The output end31cis connected to an input end12aof an error amplifier12.

The electric power computation part31has a differential computation part15, a first integral computation part32, a second integral computation part33, a current control part34, a V/I conversion part35, and current sources18and19. The differential computation part15and the current sources18and19among them are similar to those of the first embodiment, so that detailed description is omitted.

The first integral computation part32is a circuit part for integrating a second differential signal Sd2based on a first differential signal Sd1inputted from the differential computation part15with respect to time and generating a first integral signal Si1. An input end32aof the integral computation part32is connected to an output end15bof the differential computation part15. An output end32bof the integral computation part32is connected to the V/I conversion part35.

The first integral computation part32includes a function computation part161, a V/I conversion part162(first conversion part), a switch164a, a capacitive element (first capacitive element)166, a switch167, a resistance element168, and a comparator169(second current control part). These configurations are similar to those of the first embodiment.

The second integral computation part33is a circuit part for integrating a first differential signal Sd1with respect to time and generating a second integral signal Si2. The second integral computation part33includes a V/I conversion part331(second conversion part) for converting the first differential signal Sd1which is a voltage signal into a first current signal Id1, and a capacitive element332(second capacitive element) for charging the first current signal Id1. An input end331aof the V/I conversion part331is connected to the output end15bof the differential computation part15. An output end331bof the V/I conversion part331is connected to one end of the capacitive element332. In addition, the other end of the capacitive element332is grounded.

The current control part34is a first current control part in the embodiment, and controls supply of a second current signal Id2to the capacitive element166based on a voltage V across the capacitive element166(first integral signal Si1) and a voltage across the capacitive element332(second integral signal Si2). The current control part34is constructed by including, for example, comparators341and342and an AND circuit343. An input end of the comparator341is connected to one end of the capacitive element332of the second integral computation part33and an output end is connected to one input end of the AND circuit343. The comparator341outputs a voltage corresponding to logic 0 when an input voltage (that is, the voltage across the capacitive element332) is smaller than a predetermined value E0(first predetermined value), and outputs a voltage corresponding to logic 1 when the input voltage is larger than the predetermined value E0. Also, an input end of the comparator342is connected to one end of the capacitive element166and an output end is connected to the other input end of the AND circuit343. The comparator342outputs a voltage corresponding to logic1when an input voltage (that is, the voltage V across the capacitive element166) is smaller than a predetermined value E2, and outputs a voltage corresponding to logic 0 when the input voltage is larger than the predetermined value E0. In addition, an output end of the AND circuit343is connected to a control terminal of the switch164a. The switch164abecomes a connection state when the voltage corresponding to logic 1 is inputted to the control terminal, and becomes a non-connection state when the voltage corresponding to logic 0 is inputted to the control terminal.

In addition, the current control part34of the embodiment controls supply of the second current signal Id2to the capacitive element166by the switch164a, but the current control part34may control the second current signal Id2by directly controlling the function computation part161or the V/I conversion part162.

The V/I conversion part35is a circuit part for converting the first integral signal Si1into a current signal I1. An input end35aof the V/I conversion part35is connected to the output end32bof the first integral computation part32. An output end35bof the V/I conversion part35is connected to the input end31bof the electric power computation part31through a resistance element24. The V/I conversion part35outputs the current I1according to, for example, a function shown inFIG. 13. That is, the V/I conversion part35outputs the current signal I1of magnitude proportional to the first integral signal Si1.

An operation of the electric power computation part31comprising the above configuration will be described again with reference toFIGS. 6 and 7. When an output signal from the differential computation part15becomes stable after a start of lighting (time to ofFIG. 6(c)), a first differential signal Sd1(=dVS/dt) output from the differential computation part15is converted into a current signal Id1in the V/I conversion part331of the second integral computation part33and is charged into the capacitive element332. Consequently, the first differential signal Sd1is integrated with respect to time in the capacitive element332and a second integral signal Si2is generated.

Subsequently, when a voltage across the capacitive element332(that is, the second integral signal Si2) reaches a predetermined value E0(time t1ofFIGS. 6 and 7), an output of the comparator341becomes logic 1 and the switch164ais controlled in a connection state and supply of a second current signal Id2to the capacitive element166is started. That is, the first differential signal Sd1output from the differential computation part15is converted into a second differential signal Sd2by the function computation part161and the second differential signal Sd2is converted into a second current signal Id2in the V/I conversion part162and is charged into the capacitive element166through the switch164a. Consequently, the second differential signal Sd2is integrated with respect to time in the capacitive element166and a first integral signal Si1is generated.

The first integral signal Si1is output from the first integral computation part32and is inputted to the V/I conversion part35. Then, the first integral signal Si1is converted into the current signal I1in the V/I conversion part35. When the first integral signal Si1increases gradually (FIG. 6(c)), the current signal I1increases, so that a voltage drop in the resistance element24increases and a frequency of a control signal Sc output from the V-F conversion part13(seeFIG. 1)becomes high gradually. Consequently, supply electric power to the discharge lamp L is reduced gradually (FIG. 7(a)).

Subsequently, when a voltage V across the capacitive element166(first integral signal Si1) reaches a predetermined value E1(time t2ofFIGS. 6 and 7), the switch167is controlled in a connection state by the comparator169. Consequently, a current from a constant-voltage source Vcc is superposed on the second current signal Id2and the voltage V across the capacitive element166becomes a value in which a time function g(t) is superposed on an integral value of the second differential signal Sd2. This voltage V across the capacitive element166is output as the first integral signal Si1, and electric power according to this first integral signal Si1is reduced from the supply electric power to the discharge lamp L (FIG. 7(a)).

Subsequently, when the voltage V across the capacitive element166(first integral signal Si1) reaches a predetermined value E2(>E1) (time t3ofFIGS. 6 and 7), an output of the comparator342becomes logic 0 and the switch164ais controlled in a non-connection state and supply of the second current signal Id2to the capacitive element166is stopped. Consequently, only the current from the constant-voltage source Vcc is supplied to the capacitive element166, and the supply electric power to the discharge lamp L is reduced according to only the time function g(t) and gradually converges on target electric power (for example, 35 [W]) (FIG. 7(a)).

Effects that can be obtained by some implementations of the discharge lamp lighting circuit (control part10b) of the embodiment described above are as follows. The supply electric power can be controlled while suppressing an influence of variations in the lamp voltage VL by secular change or individual difference in the discharge lamp L in a manner similar to the first embodiment. Also, even when the lamp voltage VL immediately after a start of lighting is influenced by a high-voltage pulse from the starting circuit3and varies, an influence on electric power control can be reduced by action of averaging the variations and the supply electric power can be controlled every operation with good reproducibility.

Also, as shown in the embodiment, the control part10bmay have the second integral computation part33for integrating the first differential signal Sd1with respect to time and generating the second integral signal Si2, and may offer the control signal Sc based on the first integral signal Si1to the electric power supply part2(seeFIG. 1) after the second integral signal Si2reaches the predetermined value E0. Consequently, electric power control based on the first integral signal Si1can be started under a certain condition that an integral value (second integral signal Si2) of the first differential signal Sd1reaches the predetermined value E0, so that even when individual difference in the lamp voltage VL immediately after a start of lighting is large, an influence of the individual difference can be suppressed more effectively.

Also, as shown in the embodiment, the first integral computation part32may include the V/I conversion part162and the capacitive element166, and the second integral computation part33may include the V/I conversion part331and the capacitive element332, and the current control part34may control the second current signal Id2so that the second current signal Id2is supplied to the capacitive element166after the voltage across the capacitive element332(that is, the second integral signal Si2) reaches the predetermined value E0.

By this configuration, the first differential signal Sd1is integrated by the capacitive element332and the second integral signal Si2can be generated. Then, the second current signal Id2is controlled so that the second current signal Id2is supplied to the capacitive element166after the voltage across the capacitive element332(that is, the second integral signal Si2) reaches the predetermined value E0and thereby, integral computation of the second differential signal Sd2is performed and the first integral signal Si1can be generated.

The discharge lamp lighting circuit according to the invention is not limited to the specific embodiments described above, and various modifications can be made. For example, in the each of the embodiments described above, the control part (particularly, the electric power computation part) has been constructed by an analog circuit, but the control part (particularly, the electric power computation part) according to the invention may be implemented by executing predetermined software in a computer having a CPU and memory.

Other implementations are within the scope of the claims.