Abstract:
A new ballast circuit for automotive high intensity discharge (HID) applications is disclosed. The ballast utilizes two DC/DC converters and two low frequency inverter switches. The ballast also includes an integrated high voltage ignition circuit. The positive DC/DC converter builds up a high ignition voltage in addition to raising the DC positive bus. When the lamp breaks down, the DC bus voltage decreases and the ignition circuit falls inactive. The DC/DC stages are then alternately conductive to supply power to the lamp via the low frequency inverter switches. The disclosed ballast reduces the number of switches used to four from a typical six, it utilizes independently controlled voltage sources, and provides a more efficient run-up voltage waveform during pre-steady state running of the lamp.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present application relates to the electronic lighting arts. More specifically, it relates to lamp ballast circuits and, in particular, to high intensity discharge (HID) lamp electronic ballasts. One particular application is to use such a ballast in an automobile headlamp assembly, and the present application will be directed with particular attention thereto. 
         [0002]    HID lamps are considered to be one of the most effective light sources. These lamps have high electrical to lumen efficiency, long life, good color rendition and good focusing capability when the arc is made short. These favorable characteristics, and in particular the very high brightness and color temperature of commercial HID lamps, make them good candidates for sophisticated applications such as automotive headlights. Application of HID lamps in such demanding environments, however, is far form straightforward due to the many peculiarities of HID light sources. An issue with HID lamps is the need for special ballasts to drive them. 
         [0003]    Many lamps have a relatively narrow band of power in which they can operate, and require ballast circuits to rectify, filter, and convert power from a source. Thus, ballast circuits require heat generating components such as transistors, transformers, and the like. The more complex a ballast circuit is, generally the more heat it will produce, and the more likely it is for one of the components to fail. Additionally, the more complex a ballast is, generally the more it will cost. Ballast designers struggle to find the simplest designs to produce a ballast that supports particular lighting applications. Less complexity, and fewer parts lead to a less expensive, more robust and commercially viable ballast circuit. 
         [0004]    The reliability of a ballast circuit is of increased importance in the particular application of automobile headlamps, for obvious reasons. It would be undesirable to have frequent drop outs when a motorist is relying on their headlamps to drive at night. Also, with space being an issue, it is desirable to make the ballast circuit as compact as possible, and fewer components help achieve that goal. 
         [0005]    Another drawback of typical ballasts is that they use a single voltage source. Since lamp applications that require a ballast are driven by an alternating current (AC) signal, these ballasts utilize extra circuit components to construct a full bridge inverter for providing the power to drive the lamp. These additional components that make up the bridge inverter add to the problems noted above, such as size, cost, heat, and complexity. 
         [0006]    The content described in the present application contemplates a new and improved method and apparatus for a ballast circuit that overcomes the above referenced problems and others. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    According to one aspect of the present application, a high intensity discharge lamp ballast powered by an external DC voltage source is provided. The ballast includes a positive DC to DC converter that acts as a positive voltage source to a lamp. The ballast also includes a negative DC to DC converter that acts as a negative voltage source to the lamp. A first, positive low frequency switch and a second, negative low frequency switch oscillate periods of conductivity to provide power to the lamp. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a circuit diagram of an exemplary ballast incorporating concepts of the present application; 
           [0009]      FIG. 2  is a voltage/time comparison of voltage applied to a startup portion of the ballast during lamp startup. 
           [0010]      FIG. 3  depicts voltage, current, and power supplied to the ballast during pre-steady-state operation of the lamp. 
           [0011]      FIG. 4  depicts an exemplary waveform applied to the lamp during warm-up and run-up phases. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    With reference to  FIG. 1 , an exemplary HID ballast  10  connected to an HID lamp  12  is shown. High intensity discharge lamps require a high striking voltage, on the order of 18 kV to 30 kV. During steady state running, however, the voltage demands to keep the lamp  12  running after it has been ignited are significantly less. Therefore, the ballast circuit  10  includes a startup portion  14  and a steady state portion  16 . 
         [0013]    When power is first applied to the ballast  10 , the startup portion  14  is open. A spark gap  18  has a threshold voltage that must be overcome before the startup portion  14  provides power to the lamp. Resultantly, power is stored in capacitor  20  until the voltage across the spark gap  18  reaches the threshold voltage. The threshold voltage of the spark gap  18  in this embodiment is approximately 800 V, and the capacitor  20  may be a 70 nF 1000 V capacitor. 
         [0014]    As power is not yet needed in the steady state portion  16 , because the lamp  12  is not yet lit, the transformer  22  boosts the voltage applied to the steady state portion  16  to break down the spark gap  18 . Diode  23  is a rectifier that converts the AC signal of the transformer output into a DC signal and helps prevent stored energy from undesirably bleeding back across the transformer. Diode  23  is preferably a 1000 V 1 A diode. Preferably, the voltage applied to the steady state portion  16  that is, the voltage seen across the capacitor  24  is essentially doubled by transformer  22 . Capacitor  24  is preferably a 0.22 μF 450 V capacitor. 
         [0015]    With reference to  FIG. 2 , and continuing reference to  FIG. 1 , the dashed line represents the voltage seen across capacitor  24  and the solid line represents the voltage seen across the capacitor  20 . As can be seen, the voltage on capacitor  20  reaches about 800 V, making the spark gap  18  conductive and transferring the energy stored in capacitor  20  to the primary side of transformer  26  around 3 milliseconds after power is applied to the ballast  10 . 
         [0016]    Before application to the lamp  12 , however, the voltage is boosted once again by transformer  26 . Transformer  26  boosts the voltage seen across its primary winding up to the lamp ignition voltage, that is, from about 18 kV to 30 kV depending on the lamp. In the present embodiment, transformer  26  boosts the voltage to about 25 kV. As shown in  FIG. 2 , the steady state portion  16  still only sees less than about 300 V (e.g. the dashed line). So the startup portion  14  enables the lamp  12  to be struck with a relatively high ignition voltage while insulating the more sensitive components of the steady state portion  16 . 
         [0017]    After the lamp  12  is ignited, the steady state portion takes over operation of the lamp  12 . After the capacitor  20  discharges, the voltage seen across the spark gap  18  drops to below the threshold voltage and stays there as long as the steady state portion  16  is operating. Thus, the startup portion  14  is able to provide the sufficiently large voltage to ignite the lamp  12  without it being applied to the rest of the ballast circuit  10  and then cuts out during normal operation of the ballast  10 . 
         [0018]    Put another way, when the driver of an automobile activates their headlamps, what they actually are doing is providing 12 volts DC to the ballast  10 . Then a first high frequency switch  40  starts oscillating, building up voltage on capacitor  24  while also putting energy across the transformer  22 , building up voltage on capacitor  20 . Once this voltage reaches the spark gap  18  threshold voltage, it arcs across the spark gap  18  applying the energy to transformer  26 , which boosts it to the ignition voltage. At this point, the steady state portion  16  knows that the lamp has been lit and takes over control of the lamp  12  starting oscillation of the ballast circuit  10 . 
         [0019]    After ignition of the lamp  12 , the steady state portion takes over operation of the circuit. With reference to  FIG. 3 , there are three phases of operation that the ballast  10  facilitates. Right after the steady state portion  16  takes over control of the lamp  12  at time=0, there is an electrode warm up phase  30 . Following the electrode warm up phase  30  is a run up phase  32 . Finally, after the run up phase  32  the lamp ballast  10  enters steady state operation  34 . As can be seen in  FIG. 3 , from top to bottom, relative voltage, current, and power waveforms are depicted as they are applied by the ballast  10 . The warm up phase lasts for approximately 20 milliseconds, followed by the run up phase, which can last as long as 30 seconds. Then the ballast  10  enters steady state operation. Again, the lamp is already lit during all of these phases after time=0 in  FIG. 3 . By applying the depicted waveforms to the ballast  10 , no external sensor is needed to check the lamp temperature and light output. The ballast  10  will continuously monitor and control the lamp voltage and current. This is so it can keep the power applied to the lamp  12  constant over time as it is depicted in  FIG. 3 . 
         [0020]    With more particularity to the steady state portion  16  of the ballast  10 , reference is again made to  FIG. 1 , which employs a four switch design. The four switches are the first high frequency switch  40 , a second high frequency switch  42 , a first low frequency switch  44  and a second low frequency switch  46 . The high frequency switches  40 ,  42  may operate at about 100 kHz, but could operate between about 75 kHz to 125 kHz, and the low frequency switches preferably operate at about 400 Hz, but could operate anywhere in the range of 250 Hz to 500 Hz. The high frequency switches may be 500 V 20 A MOSFETs. The low frequency switches  44 ,  46  provide the inverter function to the lamp by alternately switching the positive and negative supplies during normal run operation, and may be 600 V 12 A MOSFETs. Other transistors, of course, can be used. 
         [0021]    The first high frequency switch  40  is part of a positive DC to DC converter  50  and the second high frequency switch  42  is part of a negative DC to DC converter  52 . A DC to DC converter generally includes a switch, an inductor, a diode and a capacitor. The positive DC to DC converter includes switch  40 , the primary winding of transformer  22 , capacitor  24 , and diode  54 . Diode  54  may be a 600 V 5 A ultra fast diode. The negative DC to DC converter  52  includes switch  42 , capacitor  56 , diode  58 , and inductor  60 . The capacitor  56  may be a 0.22 μF 200 V capacitor. The diode  58  may be a 600 V 5 A ultra fast diode. The value of inductor  60  varies depending on the design parameters of the ballast. It is to be understood that while component values are provided for the illustrated embodiment, component values are selected based on several factors, including, but not limited to, what type of DC to DC converters are being used, the type of lamp  12  (which would affect starting and operating specifications, etc.) the application (industrial, all-weather, indoor residential, etc.) and the like. 
         [0022]    Generally, an HID lamp must operate between positive and negative voltages. Existing systems typically utilize a single power source and a full bridge rectifier to produce the needed voltage variance. As seen from the point of view of the lamp  12 , in the present system, the positive DC to DC converter  50  is a positive power supply, and the negative DC to DC converter  52  is a negative power supply. Resultantly, the ballast  10  can produce both positive and negative drive voltages for the lamp that are independently controllable. There are several reasons for having independent positive and negative voltage sources. First, for the previously stated reason that an HID lamp operates between alternating positive and negative voltages. Another reason is to simplify the ballast by reducing the number of power switches and increasing its efficiency. 
         [0023]    Another advantage of having separate positive and negative voltage sources, as mentioned above, is that they can be independently controlled. This is not the case with a single voltage source. Generally, when the positive DC to DC converter  50  is providing power, the first low frequency switch  44  is conductive. Thus, the positive voltage is applied to the lamp  12 . Likewise, when the negative DC to DC converter  52  is providing power, then the second low frequency switch  46  is conductive, applying the negative voltage to the lamp  12 . This operation alone would produce a normal square wave to drive the lamp  12  (e.g. the steady state portion  34  in  FIG. 3 ). 
         [0024]    With independent voltage sources, however, the typical operation of single source ballasts does not have to occur. When the first low frequency switch  44  is conducting, for instance, the negative voltage source input  72  can still be providing power to the ballast  10 , and vice versa. If the negative source input  72  provides power to the ballast  10  when the first low frequency switch  44  is conducting, the lamp  12  does not see that source input  72 , but rather power is being stored in the converter  52 . The reverse is also true: when the second low frequency switch  46  is conducting and the positive source  70  is providing power, the lamp  12  does not see the positive source input  70  but power is being stored in the converter  50 . 
         [0025]    This becomes relevant when the low frequency switch  44 ,  46  that is not currently conducting becomes conductive. At this point, when the low frequency switches  44 ,  46  switch, the lamp  12  sees the current provided by the source input ( 70  or  72 , depending on which low frequency switch ( 44  or  46 ) is conductive) and the power that was stored in the DC to DC converter ( 50  or  52 ) during the last half-cycle. This provides the lamp with an in-rush current that modifies the typical square wave provided to operate the lamp  12 . As shown in  FIG. 4 , the in-rush currents manifest in the form of leading edge voltage spikes on the square wave  80 . When the first low frequency switch  44  turns conductive, and the lamp  12  sees both the voltage source input  70  and the power stored in converter  50 , a positive leading edge spike  82  occurs. Similarly, when the second low frequency switch  46  turns conductive, the lamp  12  sees both the voltage source input  72  and the power stored in the converter  52  from the last half-cycle, and a negative leading edge spike  84  occurs. The in-rush currents are provided over the course of the warm-up and run-up phases. The ballast  10  decays them over time so that no in rush current is supplied during steady state running of the lamp  10 , producing an actual square wave drive signal. The in-rush currents decay over time, but initially they are sufficient to produce leading edge voltage spikes that are about 80-100% of the running voltage of the lamp  10 . 
         [0026]    Existing devices that utilize full bridge inverters do not apply this in-rush current at the beginning of each half cycle, and thus produce a more standard square wave. Having the in-rush current is beneficial during the warm up period of the lamp ( 32  in  FIG. 3 ). It increases the lamp life, warms the lamp faster, and cures the foremost source of lamp drop-out. The amount of in-rush current provided to the lamp  12  is decreased as the lamp  12  warms. Eventually, no in-rush current is provided, so that a square wave is provided to the lamp  12  at its operating voltage for steady state operation. As shown in  FIG. 3 , the current provided to the lamp is decreased from its peak from about 2 seconds after takeover to steady state running, which can take up to about 30 seconds.  FIG. 4  shows the first 5 milliseconds, but the in-rush current is preferably provided for as long as the lamp  12  is in its run up phase  32 . As depicted in  FIG. 4 , the preferred waveform is a square wave with leading edge spikes, but with independently controlled voltage sources, many other wave shapes can be produced that cannot be produced using a single power supply. 
         [0027]    While it is to be understood the described circuit may be implemented using a variety of components with different component values, provided below is a listing for one particular embodiment when the components have the following values: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Reference Character 
                 Component 
               
               
                   
                   
               
             
             
               
                   
                 Capacitor 20 
                 70 nF 1000 V 
               
               
                   
                 Transformer 22 
                 300 V in, 800 V out 
               
               
                   
                 Diode 23 
                 1000 V 1 A 
               
               
                   
                 Capacitor 24 
                 0.22 μF 450 V 
               
               
                   
                 Transformer 26 
                 800 V in, 25,000 V out 
               
               
                   
                 Switch 40, 42 
                 500 V 20 A MOSFET 
               
               
                   
                 Switch 44, 46 
                 600 V 12 A MOSFET 
               
               
                   
                 Ultra fast diode 54 
                 600 V 5 A 
               
               
                   
                 Capacitor 56 
                 0.22 μF 200 V 
               
               
                   
                 Ultra fast diode 58 
                 600 V 5 A 
               
               
                   
                   
               
             
          
         
       
     
         [0028]    The concepts have been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the claims be construed as including all such modifications and alterations.