Abstract:
A portable lighting ballast includes first and second transistors  20, 22  for converting direct current from a voltage source  16  into alternating current to operate a lamp  10 . The lamp has an ignition voltage that is significantly higher than the voltage that the source  16  produces. The battery is a typical 6 volt cell or a combined source of 4 “D” cells, also producing six volts. The ignition voltage of the lamp  10  is approximately 600 V. A transformer  34  boosts the alternating current signal from the transistors  20, 22  to an amplitude sufficient to ignite the lamp  10 . The transformer  34  boosts the signal to 1.2 kV. After lamp ignition, the transformer settles the voltage to a steady state value.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the artificial illumination arts. It finds particular application in providing high ignition voltages for portable lamp ballasts and will be described with particular reference thereto. It is to be appreciated, however, that the present invention is also applicable to boosting voltages in fixed ballasts and other circuits, and is not limited to the aforementioned application.  
           [0002]    Typical portable lamp ballasts utilize relatively low-cost, low-voltage sources to operate the lamp. For instance, certain types of popular fluorescent camping lanterns utilize four “D” cells. In other words, the lantern has a six volt source. Typically, much larger voltages are needed to ignite and sustain a lighted fluorescent lamp. Inexpensive fluorescent lamps, as are commonly found in such lanterns, require on the order of about 200 Volts to ignite. Consequently, when these systems initiate start-up, extremely high circulating currents are present in resonant tanks of the ballast, and relatively high-valued circuit components are required to meet the voltage demands for lamp ignition.  
           [0003]    In addition to having high startup currents, typical portable ballasts are inefficient. As a result of limited voltage available from direct current sources, typical portable lamps utilize light sources that require less voltage to ignite, but are more inefficient, lessening light output and battery life.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0004]    In accordance with one aspect of the present invention, a lighting ballast is provided. A voltage source provides current that is converted by a switching portion, the switching portion including first and second transistors. A drive portion is included. A resonant load portion receives a lamp, and a transformer boosts the voltage from the switching portion to the resonant load portion.  
           [0005]    In accordance with another aspect of the present invention, a method of igniting a lamp is included. A threshold voltage is supplied by boosting a signal significantly higher than its direct current source. The direct current is converted into alternating current by a switching portion, the switching portion including first and second transistors.  
           [0006]    In accordance with another aspect of the present invention, a portable lamp ballast is provided. A direct current battery provides power to the ballast. A complementary pair of MOSFETs convert the direct current signal from the battery into an alternating current signal. A drive inductor taps power from a resonant inductor. A transformer including primary and secondary windings boosts the alternating current signal to the lamp. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.  
         [0008]    [0008]FIG. 1 is a circuit diagram of a ballast circuit, in accordance with the present invention;  
         [0009]    [0009]FIG. 2 is a time-voltage graph showing start-up and steady-state voltages;  
         [0010]    [0010]FIG. 3 is a graph showing circuit activity in select components in time. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    With reference to FIG. 1, a lamp  10  is operably connected between contacts  12 ,  14  of a ballast housing circuit. In the preferred embodiment, the circuit is powered by a direct current (DC) battery  16  of 5 to 7 V. Preferably the bus voltage  16  is between 5.5 and 6 volts with 6 volts being ideal. The circuit is referenced at point  18  to ground. The lamp  10  is preferably a compact fluorescent lamp that operates at a particular frequency or range of frequencies. The ballast circuit provides AC power at the operational frequency of the lamp  10 .  
         [0012]    In order to convert a DC signal into an AC signal, a first transistor  20  and a second transistor  22  alternate between periods of conductivity and periods of non-conductivity, out of phase with each other. That is, when the first transistor  20  is conductive, the second transistor  22  is non-conductive, and vice-versa. The action of alternating periods of conduction of the transistors provides an AC signal across the contacts  12  and  14 . In the preferred embodiment, the transistors are MOSFETs, but it is to be understood that bipolar junction transistors or other field effect transistors are also possible.  
         [0013]    Each transistor  20 ,  22  has a respective gate and source. The voltage from gate to source on either transistor defines the conduction state of that transistor. That is, the gate-to-source voltage of transistor  20  defines the conductivity of transistor  20  and the gate-to-source voltage of transistor  22  defines the conductivity of transistor  22 . As shown, the sources of the two transistors  20 ,  22  are connected at a common node  24 . The gates of the transistors  20 ,  22  are connected at a control node  26 . The single voltage between the control node  26  and the common node  24  determines the conductivity of both transistors  20 ,  22 . The drains of the transistors  20 ,  22  are connected to the bus voltage  16  and ground  18 , respectively.  
         [0014]    A gate drive circuit, connected between the common node  24  and the control node  26  controls the conduction states of the transistors  20 ,  22 . The gate drive circuit includes a serial capacitor  27 , and a drive inductor  28  that is connected to a resonant inductor  30  at the common node  24 . The other end of the drive inductor  28  is coupled to a phase inductor  32 . The phase inductor  32  is used to adjust the phase angle of the base-emitter voltage appearing between nodes  24  and  26 . The drive inductor  28  provides a driving energy for the operation of the drive circuit. The resonant inductor  30  along with a resonant capacitor  33  connected between nodes  12  and  14  determine the operating frequency of the lamp  10 . The serial capacitor  27  charges to provide sufficient voltage to turn the first transistor  20  conductive. During steady state operation of the ballast, the serial capacitor  27  aids in switching between the two transistors  20 ,  22 .  
         [0015]    As stated previously, the voltage source is preferably a 6 V battery, or its equivalent. The lamp  10  of the preferred embodiment has a threshold ignition voltage of 500 to 700 V, more preferably in the range of 550 to 650 V with 600 V being most preferred. A transformer  34  boosts the bus voltage of 6 V to a magnitude adequate to ignite the lamp  10 . Preferably, the transformer  34  boosts the voltage to between 1.0 and 1.4 kV during a period of time between when the ballast is activated and the lamp  10  ignites. The preferred ignition voltage is between 1.1 and 1.3 kV with 1.2 kV being ideal. After the lamp ignites, the ballast circuit settles to a steady state operation mode in which the transformer  34  boosts the bus voltage to a relatively small steady state value, relative to the ignition voltage. The steady state value of the preferred embodiment is on the order of 50 V.  
         [0016]    The transformer  34  includes a primary winding  36  and a secondary winding  38 . Current that passes through the primary winding  36  induces a current in the secondary winding  38 . The secondary winding  38  is on operative connection with the lamp  10 . The number of coil turns of the secondary winding  38  exposed to current passing through the primary winding  36  is controllable. Thus, the magnitude of the voltage transform is controllable. In the preferred operation of the transformer, during lamp ignition, all of the windings of the secondary coil  38  are exposed to the primary coil  36 . This boosts the voltage to 1.2 kV, as discussed previously. Subsequent to lamp ignition, the number of secondary coil  38  windings exposed to the primary coil  36  is reduced, and the voltage across the lamp  10  drops to its steady state operating value. In an alternate embodiment, the transformer  34  is an auto transformer.  
         [0017]    In the preferred embodiment, during periods of time when the lamp  10  is lit, a user can manipulate the windings ratio between the secondary and primary coils  38 ,  36  to adjust an intensity of the lamp  10 . The user can select high medium and low settings, for instance, thereby changing the windings ratio, the voltage across the lamp  10  and ultimately the brightness of the lamp  10 . Possible windings ratios are, for high intensity, 24:1, for medium intensity, 12:1, and for low intensity, 6:1. Alternately, an analog dial may be used to select and de-select windings, giving the user a dimming control of the intensity of the lamp. Regardless of the method used to give the user intensity control, the lowest setting that the user may select still provides the lamp  10  with sufficient voltage to stay lit, unless, of course, if the user selects an off position, in which power is cut from the ballast circuit.  
         [0018]    Additionally, the ballast circuit includes smoothing capacitors  40 ,  42  between the bus voltage  16  and ground  18  to smooth abnormalities and noise in the bus voltage signal. Starting resistors  44 ,  46  prevent current in the ballast circuit from exceeding tolerable levels during startup, before the capacitors and inductors are charged. Back to back Zener diodes  48 ,  50  clamp the voltage across the transistors  20 ,  22 .  
         [0019]    During lamp ignition, and with reference to FIG. 2, the ballast circuit boosts the voltage across the lamp  52  to a temporary ignition voltage  54 . With a lamp having a steady state resistance of 400 Ω, the ballast achieves 1.2 kV with a battery voltage of 5.5 volts. This ensures sufficient voltage as the battery discharges. From the time the lamp is switched on (0 s) to lamp ignition at about 2 ms, the starting voltage of 1.2 kV is applied. After the lamp ignites, the voltage settles to a steady state voltage  56  between 40 and 60 volts, with 50 volts being preferred. The steady state voltage  56  is maintained while the lamp is in normal operation.  
         [0020]    With reference to FIG. 3, waveforms across select circuit components are provided over a period of 20 μs. The curve  58  depicts a gate-source voltage of the first and second transistors  20 ,  22 . Only one is shown, but the other transistor has a gate-source voltage preferably identical, but 180° out of phase. As is shown, the gate source voltage resembles a square wave, having transition periods of less than 2 μs, ranging from approximately 5V to −5V. The source drain voltage of the second transistor  60  is provided. This square wave function ranges from about 5.5 to 6 V (bus voltage) down to zero volts. The current across the phase inductor  62  is provided for comparison. The current  62  preferably alternates between approximately 5 A and −5 A. The curve  64  is the resultant voltage across the lamp  10 , which is an AC signal.  
         [0021]    Exemplary component values for the circuit of FIG. 1 are as follows:  
                                                                         Part Description   Part Number   Nominal Value                                        Lamp   10   23 watts           DC Bus Voltage   16   6 Volts           Circuit Reference   18   0 Volts           Serial Capacitor   27   47 nanofarads           First Transistor   20   IRLML2502           Second Transistor   22   IRLML6401           Drive Inductor   28   5.6 microhenries           Resonant Inductor   30   560 microhenries           Phase Inductor   32   220 microhenries           Resonant Capacitor   33   2.2 nanofarads           Primary Winding   36   13.9 microhenries           Secondary Winding   38   8 millihenries           Smoothing Capacitor   40   10 microfarads           Smoothing Capacitor   42   10 microfarads           Starting Resistor   44   100 k Ohms           Starting Resistor   46   3 k Ohms           Zener Diode   48   1N5227           Zener Diode   50   1N5227                      
 
         [0022]    The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.