Abstract:
A method and a ballast for starting a discharge lamp capable of making a rapid start while restraining the overshoot of the light output. The ballast can separately give the initial start time period of applying a maximum power rating to the lamp and the subsequent curve along which the power decreases to a normal power rating of the lamp. A power is varied along a particular run-up curve so as to apply the maximum power rating and subsequently apply the power decreasing to the normal power rating. The run-up curve is derived from a reference curve having a power level decreasing with time. The reference curve has a maximum value above the maximum power rating, and has an inflection point near the maximum power rating to define first and second reference curves above and below the inflection point, respectively. The first reference curve has a first average slope for a first time period from a point of the maximum value to the inflection point. The second reference curve has a second average slope for a second time period which starts from the inflection point and has the same length as the first time period. The second average slope is greater than the first average slope. The run-up curve is a continuous composite curve of the maximum power rating defined by a portion thereof below the reference curve and the remainder of the reference curve defined between the maximum power rating and the normal power rating.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a method and a ballast for starting a discharge lamp, particularly a high intensity discharge lamp (HID) such as a metal halide lamp. 
     2. Description of the Prior Art 
     HID lamps are known to be slow in reaching a stable operation of emitting a destined light output when starting the lamp at a cold state. Particularly, when the lamp is used for a vehicular headlamp or a light source for an LCD projector, it is highly desired to enable a cold start with prompt rise in the light output. To this end, Japanese Patent Laid-open Publication Nos. 4-141988 and 9-82480 propose a ballast which provides, at the start of the lamp, a run-up power greater than a normal power rating required for maintaining the operation of lamp. Then, the run-up power is made to decrease with time from a maximum power rating to the normal power rating over a transition period along a particular curve. The curve of the run-up power is derived from a single charging curve of a capacitor, as shown in FIG. 15A, and is represented as a reversal of the charging curve, as shown in FIG.  15 B. Since the ballast has a fixed maximum power rating for the discharge lamp, a portion of the run-up power curve above the maximum power rating should be limited to the maximum power rating, resulting a composite curve in which the maximum power rating is maintained for an initial start time period and then decrease with time to the normal power rating. As the initial start time is required to be longer for attaining a more rapid start of the lamp as indicated by solid lines in the above figures relative to those indicated by dotted lines, both curves, i.e., the charging curve and the run-up curve are made more moderate. Thus, the run-up power decreases along a more moderate slope, applying a more amount of power to the lamp during a transition period from the start to the stable lamp operation, resulting in an overshoot of the light output, as shown in FIG.  16 . In order to avoid this problem, it is desired to separately control the initial start time and the curve shape in the transition period which cannot be made in the above prior art, thereby decreasing the run-up power along a moderate slope to exclude a possibility of the overshoot in the light output. 
     SUMMARY OF THE INVENTION 
     In view of the above problem, the present invention has been accomplished to provide a method and a ballast for starting a discharge lamp which is capable of making a rapid start while restraining the overshoot of the light output. More specifically, the present invention enables to separately give the initial start time period of applying a maximum power rating to the lamp and the subsequent curve along which the power decreases to a normal power rating of the lamp in an optimized manner. The method in accordance with the present invention utilizes a ballast having a power converter capable of varying a power being applied to the discharge lamp within a range between the maximum power rating given to the lamp and the normal power rating given to the lamp. The method comprises varying the power along a particular run-up curve so as to apply the maximum power rating and subsequently apply the power decreasing to the normal power rating. The run-up curve is derived from a reference curve having a power level decreasing with time from the energization of the ballast. The reference curve has a maximum value exceeding the maximum power rating, and has an inflection point near the maximum power rating so as to define a first reference curve above the inflection point and a second reference curve below the inflection point, respectively. The first reference curve has a first average slope for a first reference time period from a point of the maximum value to the inflection point. The second reference curve has a second average slope for a second reference time period which starts from the inflection point and lasts for the same time interval as the first reference time period. The second average slope is greater than the first average slope. The run-up curve is a continuous composite curve of a straight line of the maximum power rating defined by a portion thereof below the reference curve and the remainder of the reference curve defined between the maximum power rating and the normal power rating. Thus, the initial start time period defined by the straight line of the run-up curve can be determined by the first reference curve, while the subsequent curve along which the power decreases to the normal power rating can be determined substantially by the second reference curve below the inflection point. With this result, the initial start time and the subsequent curve can be designed separately from each other in order to give a sufficient initial start time period for rapid start of the lamp and at the same time to give an optimum configuration to the subsequent curve for assuring a stable transition from the start to the normal operation of the lamp without causing an overshoot or insufficient light output. Accordingly, it is a primary object of t e present invention to provide a method of starting the discharge lamp with an optimum power characteristic to enable a rapid start with sufficient light output. 
     Most preferably, the inflection point is set to lie on the maximum power rating so that the second reference curve can defines itself the decreasing curve of applying the decreasing power to the discharge lamp after the initial start time period. 
     The present invention also provides the ballast which is specifically designed to realize the above method. The ballast includes a power converter capable of applying a varying power to discharge lamp, and a power commander which generates the run-up curve of the power with reference to time and is connected to the power converter to vary the power along the run-up curve. 
     Preferably, the power commander includes a function generator having a capacitor, a power source and a regulator for charging the capacitor by the power source at different rates to give a charging curve. The reference curve is obtained as a reversal of the charging curve so that the inflection point is given on the reference curve where the charging rate changes critically. 
     As will be seen in the detailed description of the embodiments of the present invention, various and advantageous configurations are made for the function generator to obtain the inflection point on the reference curve. These and still other object and advantageous features of the present invention will become more apparent from the following description of the embodiments when taken in conjunction with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a ballast in accordance with a first embodiment of the present invention; 
     FIG. 2A is a graph of a capacitor charging curve obtained in the above ballast; 
     FIG. 2B is a graph illustrating a reference curve and the resulting run-up curve obtained in the above ballast; 
     FIGS. 3A to  3 C are graphs illustrating the operation of the ballast; 
     FIG. 4 is a block diagram of a ballast in accordance with a second embodiment of the present invention; 
     FIG. 5 is a block diagram of a ballast in accordance with a third embodiment of the present invention; 
     FIG. 6 is a block diagram of a power commander utilized in the ballast in accordance with the fourth embodiment of the present invention; 
     FIG. 7A is a graph of a capacitor charging curve obtained in the ballast of FIG. 6; 
     FIG. 7B is a graph illustrating a reference curve and the resulting run-up curve obtained in the ballast; 
     FIG. 8 is a block diagram of a power commander utilized in the ballast in accordance with the fifth embodiment of the present invention; 
     FIG. 9A is a graph of a capacitor charging curve obtained in the ballast of FIG. 8; 
     FIG. 9B is a graph illustrating a reference curve and the resulting run-up curve obtained in the ballast; 
     FIG. 10 is a block diagram of a power commander utilized in the ballast in accordance with the sixth embodiment of the present invention; 
     FIG. 11A is a graph of a capacitor charging curve obtained in the ballast of FIG. 10; 
     FIG. 11B is a graph illustrating a reference curve and the resulting run-up curve obtained in the ballast; 
     FIG. 12 is a block diagram of a power commander utilized in the ballast in accordance with the seventh embodiment of the present invention; 
     FIG. 13 is a block diagram of a power commander utilized in the ballast in accordance with the eighth embodiment of the present invention; 
     FIG. 14 is a block diagram of a power commander utilized in the ballast in accordance with the ninth embodiment of the present invention; 
     FIGS. 15A and 15B are graphs of a capacitor charging curve and a power curve applied to the discharge lamp for illustration of the background of the present invention; and 
     FIG. 16 is a graph of relative luminous flux for illustration of the background of the present invention in which the relative flux is represented by a percentage of the luminous flux in relation to the luminous flux attained after 3 minutes from the start of a discharge lamp. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment &lt;FIGS.  1  to  3 &gt; 
     Referring now to FIG. 1, there is shown a ballast for a discharge lamp in accordance with a first embodiment of the present invention. The discharge lamp L is high-intensity discharge lamp such as a metal halide lamp in use, for example, a headlamp of an automobile and a light source for LCD projector. The ballast is required to give a maximum power rating for starting the lamp and a normal power rating for continuously operating the lamp based upon the specification of the discharge lamp. 
     The ballast includes a power converter  10 , an output controller  20 , and a power commander  30 . The power converter  10  includes a DC-to-DC converter  12  providing an raised DC voltage from a DC source  11  such as a battery, and an inverter  14  providing a low frequency AC voltage to the discharge lamp L through an igniter  16 . The igniter  16  generates from the output of the inverter a high voltage pulse sufficient for igniting the lamp. The output controller  20  is connected to monitor a voltage and current in the power converter  10  to control a lighting operation of the lamp in a feedback manner. The output controller  20  includes a current value processor  22  which detects an output voltage of the DC-to-DC converter  12  and receives a power command from the power commander  30  designating a power of operating the lamp. Then, the current value processor  22  acts to divide the power by the detected voltage to provide a current request to an error amplifier  26  through a current limiter  24  where an excessive current request is neglected. The error amplifier  26  compares the current request with a current detected by a current sensor  28  to flow into the inverter  14 , and provides an output control signal indicative of the compared result. The output control signal is fed back to regulate the DC-to-DC converter  12  in such a manner as to assure a stable operation of the lamp. 
     The power commander  30  is responsible for providing to the current value processor  22  the power command designating the power varying from the maximum power rating down to the normal power rating. The power command is provided in the form of a combination of a run-up curve C IGN  and a straight line L NOR  indicative of the normal power rating, as indicated by solid lines in FIG.  2 B. The power commander  30  includes a function generator  40  which provides a power curve to the power processor  32  where an offset value of the normal power rating is added or superimposed to the power curve to give a reference curve C REF  as will be discussed later in detail with reference to FIG.  2 B. Thus superimposed curve or the reference curve C REF  is subsequently fed to a power limiter  34  where the maximum of the reference curve C REF  is limited to the maximum power rating W MAX  to give the power command to be supplied to the current value processor  22 . The function generator  40  has a capacitor  41  and a variable voltage source composed of a first voltage source  42 - 1  and a second voltage source  42 - 2  for charging the capacitor  41  at different rates to give a charging curve C as shown in FIG.  2 A. The charging curve C is then inverted or reversed at a reversing section  70  to provide the power curve to the power processor  32  where it is shaped into the reference curve C REF  with the addition of the offset value of the normal power rating W NOR . 
     Upon the energization of the ballast which is made by closing a power switch  13 , a line voltage monitor  15  responds to issue a lighting enable signal to the DC-to-DC converter  12  as well as to the function generator  40  when the monitored input voltage level is within a predetermined operating voltage range, activating the two components  12  and  40 . The lighting enable signal closes a switch  43  to start charging the capacitor  41  through a resistor  44 . A timer  71 , which is connected to actuate a switch  45  for selectively connecting the first and second voltage sources  42 - 1  and  42 - 2  to the capacitor  41 , is also activated by the enable signal to start counting time. At first, the timer  71  turns a switch  46  for charging the capacitor  41  by the first voltage source  42 - 1  and, after the elapse of predetermined period, turns the switch  46  for charging the capacitor  41  by the second voltage source  42 - 2 . The second voltage source  42 - 2  gives a higher voltage than the first voltage source  42 - 1 , so that the charging curve C sees an inflection point P inf  as shown in FIG. 2A at a timing corresponding to the switching of the first voltage source to the second voltage source. Therefore, a corresponding inflection point P INF  is given to the resulting reference curve C REF , as shown in FIG. 2B, to define a first reference curve C 1ST  and a second reference curve C 2ND  above and below the inflection point P INF . The inflection point P INF  is selected to lie on or near the level of the maximum power rating W MAX  so that the run-up curve C IGN  is composed of a straight line of the maximum power rating extending over a portion of the first reference curve C 1ST  above the maximum power rating and the second reference curve C 2ND . The characteristic of the run-up curve can be represented in terms of an average slope of the curves over particular time periods. That is, the first reference curve C 1ST  or the portion of the reference curve above the inflection point P INF  has a first average slope over a period T A  from the energization of the ballast (time  0 ) to the inflection point, and the second reference curve C 2ND  or the portion of the reference curve below the inflection point P INF  has a second average slope greater than the first average slope over the same time period T B  starting from the inflection point. 
     With the provision of the inflection point on the reference curve, the second reference curve of decreasing the power down to the normal power rating can be selected independently of the shape of the first reference curve which determines the period of applying the maximum power rating. Thus, the resulting igniting curve can be optimized, assuring to start the lamp successfully by applying the maximum power rating over a sufficient time period and also to decrease the power to the normal power rating successfully through a transition period from the starting of the lamp to the stable operation of the lamp. 
     When the power switch  13  is turned off, the line voltage monitor  15  issues a disable signal to inactivate the DC-to-DC converter  12  as well as to open the switch  43 , allowing the capacitor  41  to discharge through a discharge path of resistor  44  and resistor  45 . The decreasing voltage of the capacitor  41  is indicative of an elapsed time from the extinction of the lamp, i.e., a cooling extent of the lamp such that, when the switch  13  is closed, the voltage of the capacitor  41  gives an initial power setting which increases from zero with the elapsed time, as shown in FIG.  3 C. The initial power setting is given to the reversing section  70  to vary the starting point of the decreasing the power on the reference curve C REF  as a function of the elapsed time. When the lamp is started short time at time T 1  after the extinction, i.e., with some residual heat from the prior operation, the reference curve C REF  is modified, as indicated by solid lines in FIG. 3A, to start at the power level corresponding to the initial power setting W 1  at time T 1  in FIG.  3 C. When the lamp is started after a relatively long time T 2  elapsed from the lamp extinction, i.e., with less residual heat, the reference curve C REF  is modified, as indicated by solid lines in FIG. 3B, to start at the level corresponding to the initial power setting W 2  at time T 2  in FIG.  3 C. In this manner, it is possible to make a successful re-ignition of the lamp in well consideration of the residual heat of the lamp. 
     Second Embodiment &lt;FIG.  4 &gt; 
     FIG. 4 illustrates a ballast in accordance with a second embodiment of the present invention which is identical to the first embodiment except for configuration of a function generator  40 A. Like parts are designated by like numerals with a suffix letter of “A”. The function generator  40 A includes a comparator  48  which compares a voltage developed across the capacitor  41 A with a reference voltage  49 . The comparator  48  is connected to the switch  46 A for charging the capacitor from the first voltage source  42 - 1  when the voltage of capacitor  41 A is below the reference voltage  49  and otherwise for charging the capacitor  41 A from the second voltage source  42 - 2 , thereby giving the inflection point on the reference curve, as in the first embodiment. 
     Third Embodiment &lt;FIG.  5 &gt; 
     FIG. 5 illustrates a ballast in accordance with a third embodiment of the present invention which is identical to the first embodiment except for the configuration of a power commander  30 B. Like parts are designated by like numerals with a suffix letter of “B”. The power commander  30 B has a like function generator  40 B which includes a comparator  48 B is connected to receive the output of the power processor  32 B, i.e., the reference curve and to receive the maximum power rating W MAX  which is set at a reference voltage source  49  and is given to the power limiter  34 B. The comparator  48 B has its output connected to a switch  46 B so that, while the power level command from the power processor  32 B exceeds the maximum power level, the first voltage source  42 - 1 B of low voltage is responsible for charging the capacitor  41 B. When the voltage across the capacitor  41 B increases to such a level such that the power command on the resulting reference curve from the power processor  32 B goes below the maximum power rating W MAX , the comparator  48 B responds to turn the switch  46 B for charging the capacitor  41 B by the second voltage source  42 - 2 B at a greater charging rate, thereby giving the inflection point, as seen in FIG. 2B at or adjacent below the maximum power rating. In this manner, the inflection point can be easily given in a feedback manner. 
     Fourth Embodiment &lt;FIGS.  6  and  7 &gt; 
     FIG. 6 illustrates a ballast in accordance with a fourth embodiment of the present invention which is identical to the first embodiment except for the configuration of a function generator  40 C. Like parts are designated by like numerals with a suffix letter of “C”. The function generator  40 C includes a variable power source  42 C for charging a capacitor  41 C at varying rates. The power source  42 C has its output voltage regulated by a time-varying function circuit  50 . The circuit  50  includes a fixed voltage source  51  and a switch  52 , which is actuated by the lighting enable signal L ENB  from a like line voltage monitor (not shown) as in the first embodiment to charge a capacitor  53  through a resistor  54  by the voltage source  51 . It is the charged voltage across the capacitor  53  that is responsible for varying the output voltage of the variable power source  42 C in such a manner that, as shown in FIG. 7A, the output voltage of the source  42 C increases as the charged voltage of capacitor  53  increases. Thus, the circuit  50  functions as a timer which causes the output voltage of the power source  42 C to increase gradually from a first level to a second level and to fix at the second level at a predetermined period after the energization of the ballast, i.e., when the voltage across capacitor  53  reaches to a predetermined level. With this result, the reference curve can be given the inflection point at or adjacent the maximum power rating, as shown in FIG. 7B, as a consequence of that the output voltage of capacitor  41 C is fixed to the second level. When the lighting enable signal is removed, the switch  52  is opened to allow the capacitor  53  to discharge through resistors  54  and  55 , and at the same time, the switch  46 B is opened to discharge the capacitor  41 C. 
     Fifth Embodiment &lt;FIGS.  8  and  9 &gt; 
     FIG. 8 illustrates a ballast in accordance with a fifth embodiment of the present invention which is identical to the first embodiment except for the configuration of a function generator  40 D. Like parts are designated by like numerals with a suffix letter of “D”. The function generator  40 D includes a variable voltage source  42 D and a time-varying function circuit  50 D which is connected to regulate the output voltage of the source  42 D based upon the voltage detected to develop across the capacitor  41 D. Upon receiving the lighting enable signal L ENB , a switch  43 D is closed to start charging the capacitor  41 D by the voltage source  42 D, and at the same the function circuit  50 D provides a linearly increasing value as a function of the detected voltage of capacitor  41 D. 
     The function circuit  50 D provides a value (y=f(x), where x is the detected capacitor voltage) which increases from a first level (y1) and a second level (y2) as the detected voltage of capacitor  41 D increases and is fixed to the second level after the detected voltage reaches a predetermined voltage. The output of the voltage source  42 D is regulated as a function of the value such that the capacitor  41 D is charged along a charging curve of FIG.  9 A and that the inflection point is given on the reference curve, as shown in FIG. 9B, when the output is fixed to the high voltage level after increasing thereto. 
     Sixth Embodiment &lt;FIGS.  10  and  11 &gt; 
     FIG. 10 illustrates a ballast in accordance with a sixth embodiment of the present invention which is identical to the first embodiment except for the configuration of a function generator  40 E. Like parts are designated by like numerals with a suffix letter of “E”. The function generator  40 E includes a variable power source  42 - 1 E and a fixed voltage source  42 - 2 E which provides a higher output voltage than the variable power source. These voltage sources are selectively connected through a switch  45 E to charge a capacitor  41 E. The switch  45 E is normally turned to a position of connecting the variable voltage source  42 - 1 E to the capacitor  41 E, and is controlled to turn to another position of connecting the fixed voltage source  42 - 2 E to the capacitor  41 E, by a comparator  47 E which compares the voltage detected to develop across the capacitor  41 E with a reference voltage corresponding to the maximum power rating W MAX  through reversal of the charged voltage, i.e., on the reference curve. Upon receiving the lighting enable signal L ENB , a switch  43 E is closed to start charging capacitor  41 E by the variable voltage source  42 - 1 E. As the capacitor  41 E is charged up to a level corresponding to the maximum power rating, the comparator  47 E responds to turn the switch  45 E to connect the fixed voltage source  42 - 2 E for charging the capacitor  41 E. In this manner, the capacitor  41 E is continuously charged to have a charging curve, as shown in FIG. 11A, to provide the reference curve of FIG. 11B in which the inflection point is given at or near the maximum power rating. The variable power source  42 - 1 E is regulate to provide the output voltage which is expressed by a function of y=f( 1   44 ·R 44 +x), where I 44  is a current flowing through resistor  44 E, R 44  is a resistance of resistor  44 E, and x is a charged voltage of capacitor  41 E. Thus, the voltage of capacitor  41 E increases linearly with the increase in the output voltage of variable power source  42 - 1 E, as shown in FIG.  11 A. With this result, the time period of applying the maximum power rating can be easily set simply by selecting a slope of the linear function. 
     Seventh Embodiment &lt;FIG.  12 &gt; 
     FIG. 12 illustrates a ballast in accordance with a seventh embodiment of the present invention which is identical to the first embodiment except for the configuration of a function generator  40 F. Like parts are designated by like numerals with a suffix letter of “F”. The function generator  40 F includes a fixed power source  42 F, and a parallel combination of a first resistor  44 - 1  and a second resistor connected  44 - 2  in series with switches  43 F and  60  between the power source  42 F and a capacitor  41 F. The first resistor  44 - 1  is selected to have a high impedance or resistance than the second resistor  44 - 2 . The switch  60  is normally set to connect the first resistor  44 - 1  of high resistance to the capacitor  41 F, and is controlled by a timer  62  through an AND gate  61  so as to connect the second resistor  44 - 2  of low resistance after a predetermined period from the energization of the ballast. Upon receiving the lighting enable signal L ENB , the switch  43 F is closed to charge the capacitor  41 F by the power source  42 F through the first resistor  44 - 1 . At this occurrence, the timer  62  starts counting time and provide a set signal to one input of AND gate  61  after the elapse of the predetermined time period. The AND gate  61 , which has the other input end receiving the light signal, responds to give an output of turning the switch  60  for switching the first resistor  44 - 1  to second resistor  44 - 2 , thereby changing the impedance to the charging current and therefore changing the charging rate of charging capacitor  41 F. Consequently, the like charging curve and the reference curve as shown in FIGS. 2A and 2B are obtained in which the inflection point is given at a timing of switching the first to the second resistor. It is noted in this connection that the turn-over of the switch  60  may be made based upon the detected charged voltage as seen in the second embodiment or based upon the maximum power rating as seen in the sixth embodiment. 
     Eighth Embodiment &lt;FIG.  13 &gt; 
     FIG. 13 illustrates a ballast in accordance with an eighth embodiment of the present invention which is identical to the first embodiment except for the configuration of a function generator  40 G. Like parts are designated by like numerals with a suffix letter of “G”. The function generator  40 G includes a variable resistor  44 G connected in series with a switch  43 G between a fixed power source  42 G and a capacitor  41 G. The variable resistor  44 G is controlled by a time-varying function circuit  50 G to vary its resistance for varying a charging rate of charging the capacitor  41 G by the power source  42 G. The time-varying function circuit  50 G varies the resistance of the resistor  44 G in such a manner as to give an abrupt change in the charging rate at a certain time after the energization of the ballast and therefore to give the inflection point on the resulting reference curve as shown in FIG.  2 B. The switch  43 G is closed and the function circuit  50 G is activated simultaneously upon receiving the lighting enable signal L ENB . 
     Ninth Embodiment &lt;FIG.  14 &gt; 
     FIG. 14 illustrates a ballast in accordance with a ninth embodiment of the present invention which is identical to the first embodiment except for the configuration of a function generator  40 H. Like parts are designated by like numerals with a suffix letter of “H”. The function generator  40 H includes a PWM circuit  64  which provides a pulse width modulated signal to repetitively turning on and off a switch  43 H for charging a capacitor  41 H by a power source  42 H. A time-varying function circuit  50 H is connected to increase the duty cycle of the signal with time, thereby increasing a charging rate of the capacitor  41 H with time. An AND gate  66  is provided to receive the lighting enable signal LENB as well as the modulate signal from the PWM circuit  64  so as to turn on and off the switch  43 H at the presence of the lighting enable signal. The duty cycle of the signal is controlled at the function circuit  50 H such that the charging curve sees an abrupt change to thereby give the inflection point on the resulting reference curve, as shown in FIGS. 2A and 2B, after a predetermined time period from the energization of the ballast.