Discharge lamp lighting apparatus

The discharge lamp lighting apparatus includes a high-voltage generating coil connected in series to a discharge lamp for applying a high voltage to the discharge lamp to turn on the lamp, an inverter for inverting a dc voltage into an ac voltage in order to supply a lamp current to the discharge lamp on an alternating basis through the high voltage generating coil, a lamp voltage detecting circuit for detecting a voltage across the discharge lamp as a lamp voltage, and a lamp power control circuit for controlling an ac power supplied to the discharge lamp from the inverter on the basis of the lamp voltage detected by the lamp voltage detecting circuit. The lamp voltage detecting circuit detects the lamp voltage by subtracting a voltage in proportion to a sum of voltage drops across devices lying on a current path over which the lamp current flows other than the discharge lamp from a voltage in proportion to the dc voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a discharge lamp lighting apparatus for controlling lighting of a discharge lamp, particularly a high-pressure discharge lamp for use in a vehicle headlight.

2. Description of the Background Art

There have been proposed various types of discharge lamp lighting apparatuses configured to step up an output voltage of a vehicle-mounted battery into a high voltage by use of a transformer, invert the polarities of the high voltage by use of an inverter in order to light a high-pressure discharge lamp mounted on a vehicle as a headlight on an alternating basis. For example, refer to Japanese Patent Application Laid-Open No. 8-321389.

In such control apparatuses, the electric power supplied to the lamp is adjusted by performing PWM (Pulse Width Modulation) control on a switch device lying on a current path over which the primary current of the transformer flows in accordance with a predetermined control curve defining a relationship between a lamp voltage and a lamp current.

To sum up, an output voltage of a DC-DC converter, which is applied to an H-bridge constituting the inverter, makes the lamp voltage. This lamp voltage is used as the basis of calculation of the electric power supplied to the lamp.

Lamps having a rated power of 35 W, a rated lamp voltage of 85V, and a rated lamp current 0.41A have been conventionally used. To use such a lamp as a vehicle headlight, it is necessary to boot up the light beam, or to make the lamp bright promptly after turning on a lighting switch, and so the lamp is supplied with electric power greater than the rated power in the initial lighting stage.

To give an actual example for a conventional 35 W-lamp (D2S bulb or D2R bulb), the lamp power is controlled such that it is about 75 W in the initial lighting stage, and is decreased gradually to the rated power of 35 W in the stable lighting stage. This lamp power control is performed in accordance with a prescribed control curve defining a relationship between the lamp voltage and the lamp current. For example, by setting the lamp voltage at about 27V in the initial lighting stage and at 85 V in the stable lighting stage, that is, by raising the lamp voltage by 85−27=58 (V), the lamp power can be changed from 75 W to 35 W.

Incidentally, in consideration of environmental pollution, it is desirable to use mercury-less or mercury-free lamps instead of conventional lamps containing a trace quantity of mercury.

In the case of using the mercury-less lamp as a vehicle headlight, it is also necessary to boot up the light beam, or to make the lamp bright promptly after turning on the lighting switch. Accordingly the mercury-less lamp has to be supplied with electric power greater than its rated power in the initial lighting stage. Generally, when using a 35 W-lamp of the mercury-less type, electric power of about 90 W is supplied to the lamp in the initial lighting stage, and is decreased gradually to 35 W in the stable lighting stage. The lamp voltage of the mercury-less lamp in the stable lighting stage is approximately half the voltage lamp of the conventional lamp in the stable lighting stage, whereas the lamp voltage of the mercury-less lamp in the initial lighting stage is approximately equal to the lamp voltage (27V) of the conventional lamp in the initial lighting stage.

When the lamp power control is performed for the mercury-less lamp by use of the above mentioned control curve as in prior art, the lamp power (electric power supplied to the lamp) is set at 90 W in the initial lighting stage and is decreased gradually to be 35 W in the stable lighting stage by changing the lamp voltage from 27V to 42V. In the conventional lamp, the voltage variation required for changing the lamp power by 75 W−35 W=35 W is 85V−27V=58V, whereas in the mercury-less lamp, the voltage variation required for changing the lamp power by 90 W−35 W=55 W is 42V−27V=15V. The ratio of the lamp power variation to the lamp voltage variation in the mercury-less lamp's case is greater than that in the conventional lamp's case.

The above-described lamp voltage variation and the lamp power variation will be explained below in more detail by way of an example.

The lamp voltage used as the basis of calculating the lamp power is a voltage outputted from the DC-DC converter and applied to the H-ridge constituting the inverter as disclosed in the Japanese Patent Application Laid-Open No. 8-321389. To be more precise, a voltage drop across the lamp itself (referred to as “true lamp voltage” hereinafter) added by other voltage drops across other devices such as switch devices, a high-voltage generating coil, etc makes the lamp voltage which is used as the basis of calculation of the lamp power.

The following shows the value of the lamp voltage for each of the case of using the conventional lamp and the case of using the mercury-less lamp, assuming that the inverter (H bridge) is constituted by MOS transistors having on resistance of 0.7 ohms and the coil resistance of the high-voltage generating coil is 1.5 ohms.

In the conventional lamp's case, the lamp current is 2.6A in the initial lighting stage where the electric power supplied to the lamp is 70 W and the true lamp voltage is 27V, while it is 0.41A in the stable lighting state where the electric power supplied to the lamp is 35 W and the true lamp voltage is 85V. The voltage applied to the inverter in the initial lighting stage is calculated according to the following equation 1.
27+(0.7×2×2.6)+(1.5×2.6)=34.54(V)  Equation 1

The voltage applied to the inverter in the initial lighting stage is calculated as follows.
85+(0.7×2×0.41)+(1.5×0.41)=86.2(V)  Equation 2

Thus, the variation of the voltage applied to the inverter is 86.2−34.54=51.7V.

In the mercury-less lamp's case, the lamp current is 3.3A in the initial lighting stage where the electric power supplied to the lamp is 90 W and the true lamp voltage is 27V, while it is 0.83A in the stable lighting stage where the electric power supplied to the lamp is 35 W and the true lamp voltage is 42V. The voltage applied to the inverter in the initial lighting stage is calculated as follows.
27+(0.7×2×9.3)+(1.5×3.3)=36.57(V)  Equation 3

The voltage applied to the inverter in the stable lighting stage is calculated as follows.
42+(0.7×2×0.83)+(1.5×0.83)=44.4(V)  Equation 4

Thus, the variation of the voltage applied to the inverter is 44.4−36.57=7.83V

As described above, in the conventional lamp's case, the variation of the voltage applied to the inverter is 51.7V which is smaller by about 6% than the variation of the true lamp voltage which is 58V, since the voltage applied to the inverter includes not only the voltage drop across the lamp, but also the voltage drops across the devices other than the lamp. However, since the variation of the voltage applied to the inverter, which is 51.7V, is relatively large, the contribution ratio of the voltage drops across the devices other than the lamp inn the variation of the lamp voltage are relatively small. Accordingly, it is possible to control the lamp power accurately without difficulty by use of a lamp power calculating circuit designed with consideration given to the effects of the voltage drops across the devices other than the lamp and device-to-device variation.

On the other hand, in the mercury-less lamp's case, the variation of the voltage applied to the inverter is 7.83V which is smaller by about 48% than the variation of the true lamp voltage which is 15V. Since the variation of the voltage applied to the inverter, which is 7.83V, is relatively small, the contribution ratio of the voltage drops across the devices other than the lamp in the variation of the lamp voltage are relatively large. As explained above, to control the electric power supplied to the mercury-less lamp by controlling the voltage applied to the inverter as in prior art, it is necessary to vary the lamp power by 55 W by varying the voltage supplied to the inverter by the value as small as 7.83V. Accordingly, it is difficult to control the lamp power accurately by use of the lamp power calculating circuit even if it is designed with consideration given to the effects of the voltage drops across the devices other than the lamp and device-to-device variation.

As explained above, if the variation of the voltage supplied to the inverter is large as in the conventional lamp's case, the effects of the voltage drops across the devices other than the lamp on the lamp power control are small, since the contribution ratio of the voltage drops across the devices other than the lamp in the lamp voltage variation is small.

However, if the variation of the voltage supplied to the inverter is small as in the mercury-less lamp's case, the effects of the voltage drops across the devices other than the lamp on the lamp power control is large, since the contribution ratio of the voltage drops across the devices other than the lamp in the lamp voltage variation is large.

As a result, there arises a problem in that the build up characteristic of the vehicle headlight's beam defined in the regulation cannot be satisfied by controlling the voltage applied to the inverter as in prior art in the mercury-less lamp's case.

SUMMARY OF THE INVENTION

The present invention has been made in light of the above-described problem with an object of providing a discharge lamp lighting apparatus capable of controlling accurately the electric power supplied to the discharge lamp even if the lamp voltage variation between the initial lighting stage and the stable lighting stage is small.

The object can be achieved by a discharge lamp lighting apparatus having a structure including:

a high-voltage generating coil connected in series to a discharge lamp for applying a high voltage to the discharge lamp to turn on the lamp;

an inverter for inverting a dc voltage into an ac voltage in order to supply a lamp current to the discharge lamp on an alternating basis through the high voltage generating coil;

a lamp voltage detecting circuit for detecting a voltage across the discharge lamp as a lamp voltage; and

a lamp power control circuit for controlling an ac power supplied to the discharge lamp from the inverter on the basis of the lamp voltage detected by the lamp voltage detecting circuit;

wherein the lamp voltage detecting circuit detects the lamp voltage by subtracting a voltage in proportion to a sum of voltage drops across devices lying on a current path over which the lamp current flows from the inverter to the discharge lamp other than the discharge lamp from a voltage in proportion to the dc voltage.

With this structure, it is possible to determine the lamp power solely on the basis of the true lamp voltage without being affected by the voltage drops across the devices other than the discharge lamp such as the high voltage generating coil, the lamp current detecting resistor, and semiconductor switch device. Accordingly it becomes possible to control accurately the electric power supplied to the discharge lamp even if the lamp voltage variation between the initial lighting stage and the stable lighting stage is small as in the case of using a mercury-less lamp as the discharge lamp.

The lamp voltage detecting circuit may subtract at least a voltage drop across the high voltage generating coil from the voltage in proportion to the dc voltage.

The lamp voltage detecting circuit may subtract at least a voltage drop across a semiconductor switch device included in the inverter circuit from the voltage in proportion to the dc voltage.

The discharge lamp lighting apparatus may further include a lamp current detecting resistor through which the lamp current flows, the lamp voltage detecting circuit determining the sum of the voltage drops across the devices other than the discharge lamp on the basis of the voltage drop across the lamp current detecting resistor.

The lamp voltage detecting circuit may have a first voltage detecting circuit for detecting a first sum of voltage drops across the devices and a voltage drop across the discharge lamp on the basis of the dc voltage, a second voltage detecting circuit for detecting a second sum of the voltage drops across the devices on the basis of the lamp current, and a subtraction circuit for subtracting the second sum from the first sum.

The lamp voltage detecting circuit may have a first voltage detecting circuit for detecting a first sum of voltage drops across the devices including a semiconductor switch device within the inverter and a voltage drop across the discharge lamp on the basis of the dc voltage, a second voltage detecting circuit for detecting a second sum of voltage drops across the devices other than the semiconductor switch device on the basis of the lamp current, a voltage generating circuit for generating a voltage equivalent to a voltage drop across the semiconductor switch device, and a subtraction circuit for subtracting the second sum and the voltage generated by the voltage generating circuit from the first sum. In this case, the semiconductor switch may be made of a MOS transistor, and the voltage generating circuit may generate the voltage equivalent to the voltage drop across the semiconductor switch device by dividing down a constant voltage by a predetermined dividing ratio.

The lamp voltage detecting circuit may have a sample-and-hold circuit configured to sample the dc voltage within a time frame which is after a lapse of 1/30 a duration of a half polarity-changing cycle of the inverter from a start of the half polarity-changing cycle and is within ⅓ the duration of the start.

The lamp power control circuit may control the ac power supplied to the discharge lamp by gradually increasing the lamp voltage from a predetermined initial voltage to a predetermined saturation voltage, a difference between the initial voltage and the saturation voltage being equal to or smaller than 50V, 40V, 30V, or 20V.

The present invention is particularly advantageous when the difference between the initial voltage and the saturation voltage is small.

PREFERRED EMBODIMENTS OF THE INVENTION

First Embodiment

FIG. 1shows an overall structure of a discharge lamp lighting apparatus of the invention used for controlling the lighting of a vehicle headlight.

The discharge lamp lighting apparatus is connected, through a lighting switch3, to a vehicle-mounted battery1serving as a dc power supply, and operates to put on and out a high-pressure discharge lamp (vehicle headlight)2in response to on/off operation of the lighting switch3.

The discharge lamp lighting apparatus includes a DC power supply circuit (DC-DC converter)4, a takeover circuit5, an inverter circuit6, a starter circuit7, and a lamp current detecting resistor8.

The DC-DC converter4includes a flyback transformer41having a primary coil41aon the battery1side and a secondary coil41bon the lamp2side, a MOS transistor42used as a semiconductor switch connected to the primary coil41a, a rectifying diode43connected to the secondary coil41b, and a smoothing capacitor44. The DC-DC converter4generates a high voltage by stepping up the battery voltage VB.

To be more specific, when the MOS transistor42is turned on and a current flows through the primary coil41a, energy is stored in the primary coil41a, and when the MOS transistor42is turned off, the energy stored in the primary coil41ais supplied to the secondary coil41b. Through repetition of such an operation, the high voltage appears on the node of the diode43and the capacitor44.

The takeover circuit5including a capacitor51and a resistor52is for shifting the dielectric breakdown between the electrodes of the lamp2into the arc discharge between the electrodes by the action of the charged capacitor51promptly after the lighting switch3is turned on.

The inverter circuit6including an H-bridge circuit61and bridge driving circuits62and63is for lighting the lamp2on an alternating basis. The H-bridge circuit61includes semiconductor switch devices61ato61darranged in an H-bridge. The bridge driving circuits62and63turn on and off a combination of the semiconductor switch devices61aand61dand another combination of the semiconductor switch devices61band61calternately in accordance with a signal from an H-bridge control circuit400(to be hereinafter described). As a result, the polarity of the voltage applied to the lamp2and the direction of the discharge current within the lamp2alternate. Thus the lamp2is lit on an alternating basis.

The starter circuit7, which is connected to a neutral-potential node of the H-bridge circuit61and a negative terminal of the battery1, includes a transformer71having a primary coil71aand a secondary coil71b, diodes72,73, a resistor74, a capacitor75, and a thyristor76.

The starter circuit7triggers lighting of the lamp2. That is, when the lighting switch3is turned on, the capacitor75is charged, and subsequently the thyristor76is turned on. In consequence, the capacitor75starts discharging so that the lamp2is applied with the high voltage through the transformer71. As a result, a dielectric breakdown occurs between the electrodes of the lamp2, and so the lamp2begins to light up.

The lamp current detecting resistor8is for detecting a current flowing through the lamp2. The lamp current flowing through the lamp2can be determined on the basis of the voltage drop across the lamp current detecting resistor8. More particularly, the voltage drop IL across the lamp current detecting resistor8is detected as the lamp current IL indicative of the value of the current flowing through the lamp2.

The MOS transistor42, bridge driving circuits62,63, and thyristor76are controlled by a control circuit10which receives the voltage outputted from the DC-DC converter4and applied to the inverter circuit6), the lamp current IL indicative of the value of the current flowing from the inverter circuit6to the negative terminal of the battery1, etc.

FIG. 2is a block diagram showing a structure of the control circuit10. As shown in this figure, the control circuit10includes a PWM control circuit100, a lamp voltage detecting circuit200, a lamp power control circuit300, an H-bridge control circuit400, and a high-voltage generation control circuit500.

The PWM control circuit100is for turning on and off the MOS transistor42by outputting a PWM signal. The lamp voltage detecting circuit200is for converting the voltage applied to the inverter circuit6into a lamp voltage VL. The lamp power control circuit300is for controlling the electric power supplied to the lamp2(lamp power) to a desired value on the basis of the lamp voltage VL and the lamp current IL.

The H-bridge control circuit400, which is for controlling the H-bridge circuit61, turns on and off the semiconductor switch devices61ato61dby outputting a control signal to the bridge driving circuits62,63. The high-voltage generation control circuit500is for generating the high voltage to be applied to the lamp2by turning on the thyristor76.

The operation of the discharge lamp lighting apparatus having the above-described structure will now be explained.

When the lighting switch3is turned on, the electric power is supplied to each section of the apparatus shown inFIG. 1, and the MOS transistor42is PWM-controlled by the PWM control circuit100. In consequence, the high voltage resulting from stepping up the battery voltage VB by the action of the flyback transformer41is outputted from the DC-DC converter4.

The H-bridge control circuit400turns on and off the combination of the semiconductor switches61aand61dand the other combination of the semiconductor switches61band61calternately, so that the high voltage outputted form the DC-DC converter4is supplied to the capacitor75of the starter circuit7through the H-bridge circuit61, thereby charging the capacitor75.

Subsequently, the high-voltage generation control circuit500outputs a gate drive signal to the thyristor76to turn on the thyristor76in accordance with a signal which the H-bridge control circuit400produces to indicate timing of selection between the combination of the semiconductor switches61a,61dand the combination of the semiconductor switches61b,61c. When the thyristor76is turned on, the capacitor75is discharged, and accordingly the lamp2is applied with the high voltage through the transformer71. In consequence, a dielectric breakdown between the electrodes of the lamp2occurs, and the lamp2begins to light.

After that, the polarity of the voltage applied to the lamp2(the direction of the discharge current) is alternated by the operation of the H-bridge circuit61, so that the lamp2continues to light on an alternating basis. The lamp power control circuit300controls the lamp power at a desired value. The lamp voltage detecting circuit200receives the voltage VLa applied to the inverter circuit6and converts it into the lamp voltage VL. The lamp power control circuit300controls the lamp power on the basis of the lamp voltage VL received from the lamp voltage detecting circuit200and the lamp current IL equivalent to the voltage drop across the lamp current detecting resistor8.

The structure of the lamp power control circuit300will be explained below in detail with reference toFIG. 3.

The lamp power control circuit300includes an initial lighting voltage storing circuit320, a ΔVL detecting circuit350and an error amplifying circuit301.

The initial lighting voltage storing circuit320is for storing the lamp voltage VL immediately after the lamp2is turned on (lit up) and outputting it as an initial lighting voltage VLs.

The ΔVL detecting circuit350is for subtracting the initial lighting voltage VLs from the current lamp voltage VL and outputting a lamp voltage variation ΔVL indicative of difference between them.

The error amplifying circuit301is for producing a voltage representing the lighting state of the lamp2depending on the lamp voltage VL, the lamp current IL, etc. This voltage produced by the error amplifying circuit301is supplied to the PWM control circuit100. The PWM control circuit100is configured to increase the lamp power by increasing the duty ratio of the signal applied to the gate of the MOS transistor42as the voltage supplied from the error amplifying circuit301increases.

The error amplifying circuit301receives a reference voltage Vr1at its noninverting input terminal and a voltage V1at its inverting terminal as a parameter used for controlling the lamp power, and outputs a voltage depending on the difference between the reference voltage Vr1and the voltage V1.

The voltage V1depends on the lamp current IL, a constant current i1, a current i2set by a first current setting circuit302, a current i3set by a second current setting circuit303, a current i4set by a third current setting circuit304, and a current i5set by a fourth current setting circuit305.

As shown inFIG. 3, the first current setting circuit302is configured to increase the setting of the current i2with the increasing lamp voltage VL. The second current setting circuit303is configured to set the current i3at zero when the lamp voltage VL is equal to or lower than a first predetermined value, set the current i3at a constant value when the lamp voltage VL is equal to or higher than a second predetermined value, and increase the setting of the current i3with the increasing lamp voltage VL as long as the lamp voltage VL is higher than the first predetermined value and lower than the second predetermined value. The third current setting circuit304is configured to set the current i4at a constant value when the lamp voltage variation ΔVL is equal to or lower than a first predetermined value, sets the current i4at another constant value when the lamp voltage variation ΔVL is equal to or higher than a second predetermined value, and increase the setting of the current i4with the increasing ΔVL as long as the ΔVL is higher than the first predetermined value and lower than the second predetermined value. The fourth current setting circuit305is configured to increase the setting of the current i5with time T elapsed after the lamp2is turned on. For example, the fourth current setting circuit305sets the current i5at zero over a predetermined period of time after the lamp2is turned on, increases the current i4with the passage of time T, and sets the current i5at a predetermined value several tens of seconds after the lamp2is turned on.

Alternatively, it is possible to configure the fourth current setting circuit305so as to set the current i5at zero before the ΔVL reaches a predetermined voltage after the lamp2is turned on, increases the current i5with the passage of time after the ΔVL reaches the predetermined voltage, and sets the current i5at a predetermined value several tens of seconds after the lamp2is turned on.

The sum of the currents i1, i2, i3, i4, and i5is small enough compared to the lamp current IL.

The lamp power control circuit300having the above-described structure controls the lamp power by outputting, to the PWM control circuit100, the voltage depending on the time T elapsed after the lamp2is turned on, lamp voltage VL, and lamp voltage variation ΔVL. More specifically, the lamp power is set at a large value (90 W, for example) to boot up the light beam (to make the lamp2bright) promptly in the initial lighting stage, decreased gradually with the increasing light beam, and set at a predetermined value (35 W, for example) when the lamp2has reached the stable lighting stage.

Next, the structure of the lamp voltage detecting circuit200will be explained with reference toFIG. 4.

InFIG. 4, a part surrounded by a chain line represents the lamp voltage detecting circuit200. As shown in this figure, the lamp voltage detecting circuit200receives the voltage VLa outputted from the DC-DC converter4at its input terminal231(node B). This voltage VLa, which is equivalent to the sum of voltage drops across the devices lying on the current path over which the lamp current flows, is given by the equation 5.
VLa=V1+V2+V3+V4+V5  Equation 5

where V1is a voltage drop across the semiconductor switch device (MOS transistor)61a(or61c) constituting the H-bridge circuit61, V2is a voltage drop across the secondary coil of the high-voltage generating transformer71, V3is a voltage drop across the lamp2(true lamp voltage), V4is a voltage drop across the semiconductor switch device (MOS transistor)61d(or61b) constituting the H-bridge circuit61, and V5is a voltage drop across the lamp current detecting resistor8. V1, V2, V4, and V5are given by the following equations 6 to 9, respectively.
V1=r61a(orr61c)×ILEquation 6

where r61a(or r61c) is an on resistance of the semiconductor switch device61a(or61c) made of a MOS transistor.
V2=r71×ILEquation 7

where r71is a resistance of the secondary coil of the high-voltage generating transformer71.
V4=r61d(r61b)×ILEquation 8

where r61d(r61b) is an on resistance of the semiconductor switch device61d(or61b) made of a MOS transistor.
V5=R8×ILEquation 9

where R8is a resistance of the lamp current detecting resistor8.

Substituting the equations 6 to 9 into the equation 5 yields the following equation 10.
VLa=V3+(r61a+r71+r61d+R8)×ILEquation 10

The voltage VLa inputted into the H-bridge circuit61is divided down to a voltage Va by resistors201and202included in a first voltage detecting circuit200a, and supplied to an operational amplifier204serving as a voltage follower circuit for impedance conversion. A capacitor203is for reducing voltage ripples caused by the switching operation of the DC-DC converter4.

An output voltage of the operational amplifier204is stored in a sample-and-hold circuit200bincluding a switch205and a capacitor207in order to eliminate the effects of a transient voltage which the high-voltage generating transformer71produces each time the polarity (the direction of the current flowing through the transformer71) is changed in the H-bridge circuit61. FromFIG. 5showing waveforms of the transient voltage and the output voltage together with temporal change of the polarity in the H-bridge circuit61, it can be understood that if the sample-and-hold circuit200bis not provided, a large error occurs in the output voltage.

The operation of the sample-and-hold circuit200bwill be explained below in more detail. The switch205is on-off controlled by a pulse signal inputted into an input terminal232of the lamp voltage detecting circuit200. This pulse signal, which is in synchronization with the timing of the polarity change in the H-bridge circuit61, is sent from the H-bridge control circuit400. Accordingly, the capacitor207is charged to the voltage Va resulting from dividing down the voltage VLa by the resistors201,202. With this structure, it becomes possible to keep the switch205in the off state for a predetermined period of time after the polarity change is executed in the H-bridge circuit61, thereby masking the transient voltage which the secondary coil of the high-voltage generating transformer produces when the polarity change is executed in the H-bridge circuit61.

In each of half polarity-changing cycles, if the sample-and-hold operation is performed within 1/30 the duration of the half polarity-changing cycle of the start of the half polarity-changing cycle, it is difficult to obtain a correct sample. On the other hand, if the sample-and-hold operation is performed after a lapse of ⅓ the duration of the half polarity-changing cycle from the start of the half polarity-changing cycle, it is difficult to output a correct sample. Accordingly, it is preferable to perform the sample-and-hold operation after a lapse of 1/30 the duration from the start of the half polarity-changing cycle and within ⅓ the duration of the start of the half polarity-changing cycle.

The sample-and-hold circuit200boutputs a voltage Vb by way of a voltage follower circuit200cincluding an amplifier208for impedance conversion. The voltage Vb is given by the equation 11.
Vb=Va=VLa×(R2/(R+R2))=VLa×k1  Equation 11

where R1is a resistance of the resistor201, and R2is a resistance of the resistor202. k1is given by the equation 12.
k1=R2/(R1+R2)  Equation 12

Since the transient voltage appearing each time the polarity change is performed in the H-bridge circuit61is masked or eliminated, the voltage Vb just after the polarity change is performed is the same as the voltage Vb just before the polarity change is performed.

As explained above, the provision of the sample-and-hold circuit enables detecting the voltage Vb while eliminating the effects of the transient voltage, and therefore improving control accuracy.

Substituting the equation 11 into the equation 10 yields the following equation 13.
Vb=V3×k1+(r61a+r71+r61d+R8)×IL×k1  Equation 13

The lamp voltage detecting circuit200receives the voltage V5across the lamp current detecting resistor8at its input terminal233(node D). This voltage V5is divided down into a voltage Vc by a resistor224and a resistor225constituting a second voltage detecting circuit200d, and outputted through an operational amplifier223serving as a voltage follower circuit for impedance conversion. An output voltage Vd of the operational amplifier223is given by equation 14.
Vd=Vc=V5×(R11/(R10+R11))=V5×k2=R8×IL×k2  Equation 14

where R10is a resistance of the resistor224, and R11is a resistance of the resistor225. k2is given by the following equation 15.
k2=R11/(R10+R11)  Equation 15

where R8is a resistance of the current detecting resistor8, and IL is the lamp current flowing through the lamp2.

The voltages Vb and Vd given by the equations 13 and 14 are inputted to a subtraction circuit200eincluding resistors209,210,212,213, and an operational amplifier211. The resistors209,210,212,213have the same resistance (R3=R4=R5=R6) in order to output the voltage VL equal to the difference between the voltage Vb and voltage Vd from an output terminal234of the lamp voltage detecting circuit200.

The voltage VL (=Vb−Vd) is given by the following equation 16 derived by substituting the equations 13, 14 into the equation of VL=Vb−Vd.
VL=(V3×k1+(r61a+r71+r61d+R8)×IL×k1)−(R8×IL×k2)  Equation 16

If k2is set at a value satisfying the equation 17, that is, if the equation 18 holds, the equation 19 is derived.
(r61a+r71+r61d+R8)×IL×k1=(R8×IL×k2)  Equation 17
k2<(r61a+r71+r61d+R8)×k1/R8  Equation 18
VL−V3×k1  Equation 19

If the equation 19 holds, it means that the lamp voltage detecting circuit200outputs the voltage VL which is in proportion solely to the true lamp voltage V3and not to the sum of the true lamp voltage V3, the voltage drops V1, V4across the switch devices61a,61d, the voltage drop V2across the secondary coil of the high-voltage generating transformer71, and the voltage drop V5across the lamp current detecting resistor8.

The voltage Va produced by dividing down the voltage VLa outputted from the DC-DC converter4and applied to the input terminal231(node B) of the lamp voltage detecting circuit200is in proportion to the voltage inputted into the H-bridge circuit6. Accordingly, the voltage Va includes not oly the true lamp voltage V3but the voltage drops V1, V4of the switch devices61a,61d, the voltage drop V2across the secondary coil of the high-voltage generating transformer71, and the voltage drop V5across the lamp current detecting circuit8.

The voltage drops V1, V4of the switch devices61a,61d, the voltage drop V2across the secondary coil of the high-voltage generating transformer71, and the voltage drop V5across the lamp current detecting circuit8are substantially in proportion to the lamp current IL, respectively. Accordingly, by determining the resistances of the resistors R10and R11provided for dividing down the voltage applied to the input terminal233(node D), which is equivalent to the voltage drop across the lamp voltage detecting resistor8, while taking account of all of the voltage drops V1, V2, V4, V5, it becomes possible to obtain the voltage Vc which is in proportion to the sum of the voltage drops V1V2, V4, V5.

Thus, by subtracting the voltage Vd obtained by performing the impedance conversion on the voltage Vc from the voltage Vb obtained by performing the above-described processings including the impedance conversion on the voltage Va, that is to say, by subtracting the voltage Vd proportionate to the sum of the voltage drops V1, V2, V4, V5from the voltage Vb proportionate to the sum of the true lamp voltage V3and the voltage drops V1, V2, V4, V5, the voltage VL proportionate solely to the true lamp voltage V3can be picked up.

As explained above, in this embodiment, the voltage VL in proportion solely to the true lamp voltage V3is picked up, and this voltage VL is supplied to the initial lighting voltage storing circuit320and the ΔVL detecting circuit350to calculate the lamp power.

Accordingly, with this embodiment, it is possible to determine the lamp power solely on the basis of the true lamp voltage V3without being affected by the voltage drops V1, V4across the switch devices61a,61d, the voltage drop V2across the secondary coil of the high-voltage generating transformer71, and the voltage drop V5across the lamp current detecting circuit8. This embodiment enables controlling accurately the electric power supplied to the discharge lamp even if the lamp voltage variation between the initial lighting stage and the stable lighting stage is small as in the mercury-less lamp's case.

Second Embodiment

In the first embodiment, MOS transistors are used as the semiconductor switch devices61ato61dconstituting the H-bridge circuit61, whereas in the second embodiment, IGBTs (Insulated Gate Bipolar Transistors) are used instead of the MOS transistors.

When MOS transistors are used as the semiconductor switch devices61ato61das is the case with the first embodiment, it is possible to determine the voltage drops of the semiconductor switch devices61ato61don the basis of the voltage drop across the lamp current detecting resistor8through which the lamp current IL flows, because the voltage drop across the MOS transistor used as the semiconductor switch is in proportion to the drain current (equivalent to the lamp current) thereof.

However, when IGBTs are used as the semiconductor switch devices61ato61d, the voltage drops across the semiconductor switch devices61ato61dare substantially constant and independent of their collector currents (equivalent to the lamp current). Accordingly, in this embodiment, the voltage drops across the semiconductor switch devices61ato61dare determined without referring to the lamp current IL.

Since the structure of the discharge lamp lighting apparatus according to the second embodiment is the same as that according to the first embodiment except for the lamp voltage detecting circuit200, the explanation of the second embodiment set forth below focuses on the lamp voltage detecting circuit200.

FIG. 6is a circuit diagram of the lamp voltage detecting circuit200of the discharge lamp lighting apparatus according to the second embodiment.

InFIG. 6, a part surrounded by a chain line represents the lamp voltage detecting circuit200. As shown in this figure, the lamp voltage detecting circuit200receives the voltage VLa outputted from the DC-DC converter4at its input terminal231(node B). This voltage VLa, which is equivalent to the sum of voltage drops V1to V5across the devices lying on the current path over which the lamp current flows, is given by the equation 20.
VLa=V1+V2+V3+V4+V5  Equation 20

where V1is a voltage drop across the semiconductor switch61a(or61c) made of an IGBT of the H-bridge circuit61, V2is a voltage drop across the secondary coil of the high-voltage generating transformer71, V3denotes a voltage drop across the lamp2(true lamp voltage), V4is a voltage drop across the semiconductor switch61d(or61b) made of an IGBT of the H-bridge circuit61, and V5is a voltage drop across the lamp current detecting resistor8. V2and V5are given by the equations 21, 22, respectively.
V2=r71×ILEquation 21

where r71is the resistance of the secondary coil of the high-voltage generating transformer71.
V5=R8×ILEquation 22

where R8is the resistance of the lamp current detecting resistor8.

Substituting the equations 21 and 22 into the equation 20 yields the following equation 23.
VLa=V3+V1+V4+(r71+R8)×ILEquation 23

The voltage VLa inputted into the H-bridge circuit61is divided down to the voltage Va by the resistors201and202included in the first voltage detecting circuit200a, and supplied to the operational amplifier204serving as a voltage follower circuit for impedance conversion. The capacitor203is for reducing voltage ripples caused by the switching operation of the DC-DC converter4.

The output voltage of the operational amplifier204is stored in the sample-and-hold circuit200bincluding the switch205and the capacitor207to eliminate the effects of the transient voltage which the high-voltage generating transformer71produces each time the polarity (the direction of the current flowing through the transformer71) is changed in the H-bridge circuit61. The operation of the sample-and-hold circuit200bis the same as that in the first embodiment.

The sample-and-hold circuit200boutputs the voltge Vb by way of the voltage follower circuit200cincluding the amplifier208for impedance conversion. The voltage Vb is given by the equation 24.
Vb=Va=VLa×(R2/(R1+R2))=VLa×k1  Equation 24

where R1is the resistance of the resistor201, and R2is the resistance of the resistor202. k1is given by the following equation 25.
k1=R2/(R1+R2)  Equation 25

Substituting the equation 24 into the equation 23 yields the following equation 26.
Vb=V3×k1+(r61a+r71+r61d+R8)×IL×k1  Equation 26

The lamp voltage detecting circuit200receives the voltage across the lamp current detecting resistor8shown inFIG. 1at its input terminal233(node D) This voltage is divided down into the voltage Vc by the resistors224and225constituting the second voltage detecting circuit200dand outputted through the operational amplifier223serving as a voltage follower circuit for impedance conversion The output voltage Vd of the operational amplifier223is given by the equation 27.
Vd=Vc=V5×(r11/(r10+r11))=V5×k2=r8×IL×k2  Equation

where R10is the resistance of the resistor224, and R11is the resistance of the resistor225. k2is given by the equation 28.
k2−r11/(r10+R11)  Equation 28

where R8is the resistance of the current detecting resistor8, and IL is the lamp current flowing through the lamp2.

The discharge lamp lighting apparatus according to this embodiment is further provided with a voltage generating circuit as a third voltage detecting circuit200f. This third voltage detecting circuit200fis for generating a voltage equivalent to the voltage drops across the semiconductor switch devices61ato61d. A terminal221of the third voltage detecting circuit200fis connected to a constant voltage source. The constant voltage produced by the constant voltage source and applied to the terminal221is divided down into a voltage Ve by a resistor219and a resistor220, and outputted as a voltage Vf (=Ve) through an operational amplifier218serving as a voltage follower circuit for impedance conversion.

The voltages Vd and Vf are inputted to an adding circuit including resistors217,222,215,216and an operational amplifier214. The resistors217,222,215,216have the same resistance (R12=R13=R14=R15) in order to output a voltage Vg equal to the sum of the Voltage Vf and the voltage Vd from an output terminal of the operational amplifier214. The voltage Vg is given by the equation 29.
Vg=Vf+Vd=Ve+R8×IL×k2  Equation 29

The voltages Vb and Vg given by the equations 26 and 29 are inputted to the subtraction circuit200eincluding the resistors209,210,212,213, and the operational amplifier211. The resistors209,210,212,213have the same resistance (R3=R4=R5=R6) in order to output the voltage VL equal to the difference between the voltage Vb and the voltage Vg from the output terminal234of the lamp voltage detecting circuit200.

The voltage VL (=Vb−Vd) is given by the following equation 30 derived by substituting the equations 26 and 29 into the equation of VL=Vb−Vd.
VL=V3×k1+(V1+V4)×k1+(r71+R8)×IL×k1−(Ve+R IL×k2)  Equation 30

If Ve is set at a value satisfying the equation 31 and k2is set at a value satisfying the equation 32, that is, if the equation 33 holds, the equation 34 is derived.
Ve(V1+V4)×k1  Equation 31
(r71+R8)×IL×k1=R8×IL×k2  Equation 32
k2=(r71+R8)×k1/R8  Equation 33
VL=V3×k1  Equation 34

If the equation 34 holds, it means that the lamp voltage detecting circuit200outputs the voltage VL which is in proportion solely to the true lamp voltage V3, and not to the sum of the true lamp voltage V3, the voltage drops V1, V4across the switch devices61a,61d, the voltage drop V2across the secondary coil of the high-voltage generating coil71, and the voltage drop V5across the lamp detecting resistor8.

The voltage Va produced by dividing down the voltage VLa outputted from the DC-DC converter4and applied to the input terminal231(node B) of the lamp voltage detecting circuit200is in proportion to the voltage inputted into the H-bridge circuit6. Accordingly, the voltage Va includes not only the true lamp voltage V3but the voltage drops V8, V4of the switch devices61a,61d, the voltage drop V2across the secondary coil of the high-voltage generating transformer71, and the voltage drop V5across the lamp current detecting circuit B.

The voltage drop V2across the secondary coil of the high-voltage generating transformer71, and the voltage drop V5across the lamp current detecting circuit8are substantially in proportion to the lamp current IL, respectively. Accordingly, by determining the resistances of the resistors R10and R11provided for dividing down the voltage applied to the input terminal233(node D), which is equivalent to the voltage drop across the lamp voltage detecting resistor8, while taking account of the voltage drops V2and V5, it becomes possible to obtain the voltage Vc which is in proportion to the sum of the voltage drops V2and V5.

Since the voltage drops across the switch devices61aand61dare substantially constant, it is possible to obtain the voltage Ve proportionate to the sum of the voltage drops V1and V4across the switch devices61aand61dby dividing down, by use of the resistors219,220, the voltage produced by the constant voltage source.

Thus, by subtracting the voltage Vd obtained by performing the impedance conversion processing on the voltage Vc and the voltage Vf obtained by performing the impedance conversion processing on the voltage Ve from the voltage Vb obtained by performing the above-described processings including the impedance conversion on the voltage Va, that is to say, by subtracting the voltage Vd proportionate to the sum of the voltage drops V2, V5and the voltage Vf proportionate to the sum of the voltage drops V1, V4from the the voltage Vb proportionate to the sum of the true lamp voltage V3and the voltage drops V1, V2, V4, V5, the voltage VL proportionate solely to the true lamp voltage V3can be picked up.

As explained above, in the case of using IGBTs as the semiconductor switch devices61ato61d, it is possible to pick up the voltage VL which is in proportion solely to the true lamp voltage V3. By supplying this voltage VL to the initial lighting voltage storing circuit320and the ΔVL detecting circuit350to calculate the lamp power, the same advantages as the first embodiment can be obtained.

Other Embodiments.

Each of the above-described embodiments is configured to pick up the voltage VL which is in proportion solely to the true lamp voltage by eliminating all of the voltage drops V1, V2, V4, V5.

However, by eliminating at least one (preferably at least two) of the voltage drops V1, V2, V4, V5, it is also possible to control the lamp power more accurately than the prior art, since the voltage used for calculating the lamp power when at least one of the voltage drops V1, V2, V4, V5is removed is closer to the true lamp voltage than that used in the prior art where none of the voltage drops V1, V2, V4, V5is eliminated.

Although the above-described embodiments are for controlling the lighting of the mercury-less lamp, it is needless to say that the present invention is applicable to the case of controlling the light of the conventional mercury lamp.

The present invention is effective on a case where the lamp voltage variation of the mercury lamp or the mercury-less lamp between the initial lighting stage and the stable lighting stage is within 50V.

The present invention is more effective on a case where the lamp voltage variation of the mercury lamp or the mercury-less lamp between the initial lighting stage and the stable lighting stage is within 40V.

The present invention is even more effective on a case where the lamp voltage variation of the mercury lamp or the mercury-less lamp between the initial lighting stage and the stable lighting stage is within 30V.

The present invention is still more effective on a case where the lamp voltage variation of the mercury lamp or the mercury-less lamp between the initial lighting stage and the stable lighting stage is within 20V.

FIG. 7is a graph showing examples of the lamp voltage variation curve after turning on the discharge lamp. As shown in this figure, the lamp voltage VL falls rapidly after turning on the lamp to a minimal value, and then rises gradually until it reaches a saturation value. In the above-described embodiments, the lamp voltage VL in the initial lighting stage refers to the minimal value, and the lamp voltage in the stable lighting stage refers to the saturation value. The variation curve ranging from the initial lighting stage to the stable lighting stage varies depending on the lamp used. InFIG. 7, ΔVLa, ΔVLb, and ΔVLc represent the lamp voltage variations of three different lamps2a,2band2c. The present invention is particularly advantageous for controlling the lighting of the lamp whose lamp voltage variation is small.