Abstract:
In a discharge lamp lighting apparatus, in a transient period from the time a discharge lamp is lit in a cold state to the time the discharge lamp reaches a static lighting state, the power applied to the discharge lamp is reduced over time after an initial maximum power that exceeds the rated power. A maximum power regulator circuit is provided to regulate the applied power in the transient period such that the applied power does not exceed an upper limit power line M1 or M2 that is reduced over time after the application of the initial maximum power. In this way, the application of excessive transient power is prevented from continuing longer than necessary in the event of a failure in a load, thus limiting the amount of generated heat and preventing thermal destruction.

Description:
[0001]     This application claims foreign priority based on Japanese application no. JP2004-227908, filed on Aug. 4, 2004, the contents of which is incorporated herein in its entirety. This priority claim is being made concurrently with the filing of this application.  
       BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to techniques for preventing a discharge lamp from being continuously applied with excessive power more than necessary due to failure in a load when the discharge lamp is turned on from a cold state.  
         [0004]     2. Background Art  
         [0005]     When a discharge lamp is used for car illumination, light flux must be rapidly increased after the discharge lamp is turned on, so that a transient power control is conducted to supply larger power immediately after the discharge lamp is lit than the power applied for a normal lighting situation, and further, to reduce the applied power over time.  
         [0006]     For example, for a metal halide lamp with rated power of 35 W, when lighting is started from a state in which its light emitting tube is cold (so-called “cold start”), the power is controlled such that approximately 60 to 80 W of power is transiently applied to the lamp. Then, the applied power is gradually reduced based on a control value calculated in accordance with the state of the lamp (mainly, a lamp voltage) and an elapsed time from the power-on time to eventually converge the power to the rated value.  
         [0007]     Power loss of a lighting circuit increases as larger power is outputted. When large power is applied to a lamp, as with the cold start, a large loss occurs, which increases the amount of generated heat.  
         [0008]     In a related art lamp lighting process, even if increased power is temporarily applied in a transient period, the transient period lasts for several seconds, so that the lighting circuit has sufficiently durable specifications with respect to heat-resisting designs of the lighting circuit. Even so, measures should be taken for a possible failure in a load.  
         [0009]     For example, in the event of a leak or submergence of a bulb, troubles caused by manufacturing-related faults and aging deterioration (an excessively small amount of mercury, a reduction in inner pressure of an arc tube, and the like), a failure in a lamp, and the like, or when an accident brings about an equivalence to a parallel circuit of a lamp and a low resistor in a lamp connector, a lamp voltage detected by a detector circuit included in a lighting circuit indefinitely remains low, possibly resulting in continuous application of excessive power to the lamp.  
         [0010]     In the related art (for example, see Laid-open Japanese Utility Model Registration Application No. 7-8997), a discharge lamp is monitored for power supplied thereto, such that the power supply to the discharge lamp is shut off when the discharge lamp is supplied with power that exceeds the power that should be supplied after the discharge lamp has been lit. Alternatively, when there is a function of determining whether a lamp voltage falls within a normal range in a static light condition determined from information on a detected lamp voltage, the lamp voltage equal to or lower than a predefined reference value is regarded as a failure in lighting, and the power supplied to the discharge lamp can be shut off to protect the circuit.  
         [0011]     However, the related art techniques have problems of insufficient measures for protecting circuits from heat generated by a failed load, an increased cost therefor, and the like.  
         [0012]     For example, when a high-frequency switching scheme is employed to reduce the size of a lighting apparatus having a fly-back type DC-DC converter circuit, continued application of excessive power, in the event of a failure, can directly lead to thermal runaway, thermal destruction, and the like, due to a reduced thermal capacitance of the overall apparatus.  
       SUMMARY  
       [0013]     It is therefore an object of the invention to take measures to circuit protection by reducing maximum applied power (allowable upper limit value) over time in a discharge lamp lighting apparatus for preventing continued application of excessive transient power to the lighting apparatus in the event of a failure in a load. However, the present invention need not achieve this object or any other objects, and may also achieve objects other than this object.  
         [0014]     The invention provides a lighting apparatus for a discharge lamp which is configured to reduce power applied to the discharge lamp over time after it is applied with initial maximum power exceeding the rated power in a transient period from the time the discharge lamp is lit from a cold state to the time the discharge lamp reaches a static lighting state. The lighting apparatus includes a maximum power regulator circuit for regulating the applied power in the transient period such that the applied power does not exceed an upper limit power line which is reduced over time after the application of the initial maximum power.  
         [0015]     Additionally, the invention provides an apparatus for preventing initial maximum from exceeding a maximum value during a transient period when a discharge lamp is started from a cold start until a stable operation period. The apparatus includes a power conversion circuit that converts a received input into a desired output, a control circuit that generates a power control signal in response to a voltage level signal and a current level signal measured at the desired output, the control circuit comprising a power processing unit that generates at least one first output current based on at least one of the voltage level signal, the current level signal and a first reference voltage, and a maximum power regulator circuit that generates a second output current based on a timing signal and one of a second reference voltage and a power supply signal. The apparatus also includes a starting circuit coupled between the power conversion circuit and the lamp, that outputs power to the lamp during the transient period.  
         [0016]     Thus, in the transient power control, the invention can prevent the application of excessive transient power from continuing for a time more than necessary in the event of a failure in a load, thus limiting the amount of generated heat and preventing thermal destruction and the like.  
         [0017]     The invention can take sufficient circuit protection measures by preventing thermal detrimental effects resulting from the continuous application of transient power to the discharge lamp.  
         [0018]     The lighting apparatus may further comprise a DC-DC converter circuit for converting a received DC input voltage to a desired DC voltage, and a control circuit for controlling the power applied to the discharge lamp. The control circuit may include an error processing unit, and a control signal generator responsive to a signal from the error processing unit for generating a control signal which is sent to the DC-DC converter circuit. The error processing unit receives a reference signal at one input, and an output signal of the maximum power regulator circuit multiplexed on a power control signal, which is calculated based on information on a detected voltage or current associated with the discharge lamp, at the other input. With the foregoing configuration, the lighting apparatus of the invention can reduce the allowable upper limit value for the power applied to the discharge lamp over time without involving a complicated control scheme, a significant increase in cost, and the like.  
         [0019]     For example, when the power value regulated by the upper limit power line is reduced over time in accordance with an exponential function or a linear function, the circuit configuration will be effectively simplified. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  illustrates an exemplary, non-limiting basic configuration.  
         [0021]      FIG. 2  illlustrates a change in transient power applied to a lamp over time.  
         [0022]      FIG. 3  illustrates an exemplary, non-limiting circuit configuration of a main portion.  
         [0023]      FIG. 4  illustrates an exemplary, non-limiting configuration of a maximum power regulator circuit.  
         [0024]      FIG. 5  illustrates another exemplary, non-limiting configuration of a maximum power regulator circuit. DETAILED 
     
    
     DESCRIPTION  
       [0025]      FIG. 1  illustrates an exemplary, non-limiting basic configuration of a discharge lamp lighting apparatus  1 .  
         [0026]     DC voltage from a DC power supply  2  is supplied to a DC-DC converter circuit  4  through a noise filter circuit, not shown, by turning on a lighting switch  3 .  
         [0027]     The DC-DC converter circuit  4  receives a DC input voltage from the DC power supply  2 , and converts the received DC input voltage to a desired DC voltage. For example but not by way of limitation, a fly-back type DC-DC converter can be used for the DC-DC converter circuit  4 . In a circuit configuration having a transformer T and a switching element SW on the primary side of the transformer T, the switching element SW is driven by a control signal So from a control circuit  8 , later described.  
         [0028]     A DC-AC converter circuit  5  is provided for converting an output voltage of the DC-DC converter circuit  4  to an AC voltage that is supplied to a discharge lamp  6 . For example but not by way of limitation, in a circuit configuration of H-bridge (or full bridge), four semiconductor switches sw 1 -sw 4  are used to make two arms, and driving circuits are included for driving the switching elements on the respective arms independently of each other. The AC voltage is outputted by complementarily controlling on/off the two pairs of switching elements.  
         [0029]     A starting circuit  7  is provided for generating a high voltage pulse signal (starting pulse) to start the discharge lamp  6 . This signal is multiplexed on the AC voltage outputted from the DC-AC converter circuit  5 , and the resulting multiplexed signal is applied to the discharge lamp  6 . In this example, the starting circuit  7  is implemented by a trigger transformer, a cylister, a capacitor, or the like.  
         [0030]     The control circuit  8  has a power control unit for controlling the power supplied to the discharge lamp  6 . For example but not by way of limitation, in a transient period from lighting of the discharge lamp  6  started in a cold state to the static lighting state, the power control unit controls the power such that the power applied to the discharge lamp  6  is reduced over time after application of initial maximum power exceeding the rated power. As a result, the discharge lamp  6  transitions to a static lighting state.  
         [0031]     A detector unit  9  is disposed after the DC-DC converter circuit  4  for acquiring detected signals of a lamp voltage and a lamp current or a voltage and a current corresponding thereto. As the detector unit  9  sends a lamp state detection signal (see a voltage detection signal “VL” and a current detection signal “IL”) to the control circuit  8 , the control circuit  8  sends a control signal (labeled “So”) to the DC-DC converter circuit  4  to control an output voltage of the DC-DC converter circuit  4 . More specifically, the generated control signal So is sent to the switching element SW of the DC-DC converter circuit  4  for driving control. Switching control schemes used in this embodiment include, for example, a PWM (pulse width modulation) scheme, and PFM (pulse frequency modulation) scheme, but are not limited therero.  
         [0032]     The control circuit  8  is provided with a maximum power regulator circuit  8   a  for regulating the power applied to the discharge lamp  6  in a transient period until the discharge lamp  6  reaches the stable static lighting state so as not to exceed an upper limit power line, which is reduced over time after the initial maximum power has been applied. Thus, the maximum power regulator circuit  8   a  regulates the maximum power value (allowed upper limit value) in accordance with the lapse of time, to prevent the continued application of excessive power to the discharge lamp  6  longer than necessary when a load fails.  
         [0033]      FIG. 2  illustrates a change in the applied power Pw over time from power-on at a cold start, where the horizontal axis represents the time “t” and the vertical axis represents the applied power “Pw.” “Po” on the vertical axis indicates the initial maximum power supplied to the discharge lamp for a period “0&lt;t&lt;To”. While To is assumed to be a fixed value, To can be made longer if a longer unlit time is present before the discharge lamp is lit. “Pc” indicates the rated power.  
         [0034]     Curves Ga, Gb, Gc, Gd in the graph represent a difference in changes in the applied power over time due to the differences among respective discharge lamps. Variations are recognized in the change in the applied power from a difference in a lamp state and the like related to individual discharge lamps.  
         [0035]     However, the maximum allowable value (or an upper limit value), which decreases in accordance with an elapsed time, can be defined with respect to the change in the power applied to each discharge lamp. Curves M 1 , M 2  in the graph indicate upper limit power lines (or allowable maximum power lines), which should not be exceeded by the varying power in a lighting state of the discharge lamp.  
         [0036]     The curve M 1  represented by a dotted line shows an upper limit power line, which exponentially decreases as the time elapses after To. The curve M 2  represented by a one-dot chain line shows an upper limit power line, which levels off after it linearly decreases as the time elapses after To.  
         [0037]     Such an upper limit power line can be defined by statistically examining changes in the power applied to discharge lamps. The transient power applied to the lamp is regulated by the maximum power regulator circuit  8   a  so as not to exceed the upper limit power line (e.g., M 1  or M 2 ).  
         [0038]     In the transient power control at a cold start, complete measures can be taken for heat generation by reducing the allowable upper limit value for the applied power over time, and by regulating the transient power applied to the lamp so as not to exceed the allowable upper limit value indicated by the upper limit power line, even if a discharge lamp is connected to the lighting circuit.  
         [0039]      FIG. 3  is a diagram for describing an exemplary configuration of main circuits including the DC-DC converter circuit  4  and control circuit  8 . “Vin” shown in  FIG. 3  indicates a DC input voltage to the DC-DC converter circuit  4 , and “Vout” indicates a DC output voltage of the DC-DC converter circuit  4 .  
         [0040]     A capacitor  11  is disposed on the primary side of a transformer  10 . A leading end of a primary winding  10   p  is coupled to an end of the capacitor  11 , while a trailing end of the primary winding  10   p  is connected to a switching element  12  (N-channel FET in this example).  
         [0041]     A rectifying diode  13  and a smoothing capacitor  14  are disposed on the secondary side of the transformer  10 . The leading end of a secondary winding  10   s  is coupled to a connection point of the primary winding  10   p  with the switching element  12 , and the trailing end of the secondary winding  10   s  is connected to an anode of the diode  13 . One end of the capacitor  14  is connected to a cathode of the diode  13 , and its terminal voltage is outputted to a subsequent circuit (DC-AC converter circuit) as Vout.  
         [0042]     In this exemplary, non-limiting embodiment, the control circuit  8  comprises a power processing unit  15 , an error processing unit  17 , and a control signal generator unit  18 . The power processing unit  15  comprises a first processor  15   a , a second processor  15   b , and an offsetting circuit  15   c.    
         [0043]     The first processor  15   a  generates an output current (labeled “i 1 ”) in accordance with the voltage detection signal VL acquired, for example but not by way of limitation, from the output of the DC-DC converter circuit  4 , and comprises a function generator circuit that receives VL (the type of function may be arbitrary). The output of the first processor  15   a  is sent to the error processing unit  17  through a resistor R 1 .  
         [0044]     The second processor  15   b  generates an output current (labeled “i 2 ”) in accordance with the current detection signal IL acquired, for example but not by way of limitation, by a lamp current detecting resistor disposed subsequent to the DC-DC converter circuit  4 , and comprises a function generator circuit which receives IL (the type of function may be arbitrary). The output of the second processor  15   b  is sent to the error processing unit  17  through a resistor R 2 .  
         [0045]     As represented by the symbol of a regulated voltage source in  FIG. 3 , the offsetting circuit  15   c  sends a reference voltage “Eref” to the error processing unit  17  through a resistor R 3  (see an output current “i 3 ”).  
         [0046]     The maximum power regulator circuit  8   a  sends its output to the error processing unit  17  through a resistor R 4  (see an output current “i 4 ”) in order to prevent detrimental effects caused by an increased power loss and generated heat when excessive transient power is continuously applied by the output of the power processing unit  15 . However, the maximum power regulator circuit  8   a  does not affect the relationship with the output of the power processing unit  15  in normal power control.  
         [0047]     Accordingly, the error processing unit  17  is supplied, at one input thereof, with an output signal (i 4 ) of the maximum power regulator circuit  8   a  multiplexed on the power control signals (i 1 -i 3 ) that are calculated based on information on the detected voltage or current associated with the discharge lamp. Specifically, the first processing unit  15   a , second processing unit  15   b , offsetting circuit  15   c , and maximum power regulator circuit  8   a  are arranged in parallel, and weighted additions are performed in accordance with weighting coefficients determined by the respective resistances of the resistors R 1 -R 4 , to send control signals of the respective components (the sum total of respective output currents) to the error processing unit  17 .  
         [0048]     In this exemplary, non-limiting embodiment, the control signal is input to a negative input terminal of an error amplifier that forms part of the error processing unit  17 , and a positive input terminal of the error amplifier is supplied with the reference voltage “Vref” indicated by the symbol of a regulated voltage source (control is conducted to reduce the power supplied to the discharge lamp as the control signal has a higher level).  
         [0049]     An output signal of the error processing unit  17  is sent to the control signal generator  18 , which generates the aforementioned control signal So. For example but not by way of limitation, in the PWM scheme, the control signal generator  18  includes a PWM comparator and the like, and an error signal from the error processing unit  17  is supplied to the comparator. The comparator is also supplied with a ramp wave at a frequency, and generates an output signal at a duty ratio that varies in accordance with the result of a comparison between the levels of the inputs. The output signal is supplied to the switching element  12 .  
         [0050]     In the PFM scheme, the error processing unit  17  generates an output signal, the frequency of which varies in accordance with an error signal from the error processing unit  17 , and supplies this output signal to the switching element  12 .  
         [0051]      FIG. 4  illustrates an exemplary, non-limiting configuration of the maximum power regulator circuit  8   a . An operational amplifier  19  is supplied with the reference voltage “Vref” at its non-inverting input terminal. The operational amplifier  19  has an output terminal connected to an anode of a diode  20 .  
         [0052]     The diode  20  has a cathode coupled to an inverting input terminal of the operational amplifier  19  and also coupled to a capacitor  22  through a resistor  21 .  
         [0053]     An emitter-grounded NPN transistor  23  is supplied at its base with a signal (hereinafter labeled “STo”) from a circuit (timer circuit or the like), not shown, through a resistor  24 . The transistor  23  has a collector coupled to the output terminal of the operational amplifier  19 . Until a time (the aforementioned “To”) elapses after the power is turned on, the signal STo is set to H (high) level, causing the transistor  23  to transit to the on position to forcedly bring the output signal of the operational amplifier  19  to L (low) level.  
         [0054]     After the lapse of the time To, as the signal STo changes to L level to cause the transistor  23  to transit off, a charging operation is started on the capacitor  22  through the resistor  21 .  
         [0055]     A subsequent operational amplifier  25  has a non-inverting input terminal coupled to one end of the capacitor  22 . The operational amplifier  25  has an output terminal coupled to an anode of a diode  26 , which has a cathode connected to an inverting input terminal of the operational amplifier  25 , and to the resistor R 4 .  
         [0056]     In the foregoing configuration, the transistor  23  remains on while the signal STo is at H level, so that the capacitor  22  is prohibited from being charged. However, as the signal STo goes to L level after the lapse of the time To, the transistor  23  transits to the off position to charge the capacitor  22 . In other words, the voltage on the capacitor  22  exponentially increases over time (since the upper limit power line is in an opposite-phase relationship with the change, the upper limit power line exponentially decreases over time).  
         [0057]     A time constant circuit including the resistor  21  and capacitor  22  (CR integrator circuit) may be used to reduce the circuit scale.  
         [0058]      FIG. 5  shows an exemplary, non-limiting configuration of the maximum power regulator circuit  8 A. In this exemplary, non-limiting embodiment, the operational amplifier  25  is coupled to a circuit using PNP transistors  27 ,  28 ,  29  and transistors  30 ,  31  which make up a current mirror circuit.  
         [0059]     The PNP transistor  27  has its emitter connected to a power supply line  32  at a voltage (Vcc), and its collector grounded through a resistor  33 .  
         [0060]     The collector-grounded PNP transistor  28  has its base connected to the collector of the transistor  27 , and its emitter connected to bases of the transistors  27 ,  29 .  
         [0061]     The PNP transistor  29  has its base connected to the base of the transistor  27 , and its emitter connected to the power supply line  32 . Then, the transistor  29  has its collector grounded through a capacitor  34 .  
         [0062]     The emitter-grounded NPN transistor  30  is supplied with the signal STo at its base through a resistor  35 , and the transistor  30  has a collector connected to a base of the transistor  31  through a resistor  36 .  
         [0063]     The PNP transistor  31  has its emitter connected to the power supply line  32 , and its collector connected to the bases of the transistors  27 ,  29 .  
         [0064]     The operational amplifier  25  has its non-inverting input terminal connected (or coupled, as in the present application, “connected” and “coupled” are used interchangeably to refer to either the direct or indirect electrical connection of elements) to a capacitor  34  and to the collector of the transistor  29 . Then, the operational amplifier  25  has an output terminal connected to the anode of the diode  26  which has the cathode connected to an inverting input terminal of the operational amplifier  25  and to the resistor R 4 .  
         [0065]     In the foregoing configuration, the transistors  30 ,  31  remain on while the signal STo is at H level, so that the capacitor  34  is prohibited from being charged. However, as the signal STo goes to L level after the lapse of the time To, the transistors  30 ,  31  turn off, thus charging the capacitor  34  with a corrector current of the transistor  29 . In other words, a charging operation is performed with a constant current, so that the voltage on the capacitor  34  linearly increases over time (since the upper limit power line is in an opposite-phase relationship with the change, the upper limit power line linearly decreases over time).  
         [0066]     According to the configuration described above, the upper limit power line is set at a level slightly higher than a time varying maximum power value in the transient power control in consideration of variations in change over time of the applied power due to differences among individual discharge lamps, thus avoiding a power supply exceeding the upper limit power line for any load condition. In this way, sufficient measures can be taken to heating of the lighting apparatus to support a reduction in size of the lighting apparatus.  
         [0067]     It will be apparent to those skilled in the art that various modifications and variations can be made to the described preferred embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover all modifications and variations of this invention consistent with the scope of the appended claims and their equivalents.