Patent Document

BACKGROUND OF THE INVENTION 
     The present invention is directed to improving the visual appearance of linear fluorescent lamps, and more particularly, to the elimination of visual striations which may occur in gas discharge lamps. Generally, a gas discharge lamp will have an elongated gas-filled tube having electrodes at each end. A voltage between the electrode accelerates the movement of electrons. This causes the electrons to collide with gas atoms producing positive ions and additional electrons forming a gas plasma of positive and negative charge carriers. Electrons continue to stream toward the lamp&#39;s anode and the positive ions toward its cathode sustaining an electric discharge in the tube and further heating the electrodes. The electric discharge causes an emission of radiation having a wavelength dependent on the particular fill gas and the electrical parameters of the discharge. 
     A fluorescent lamp is a gas discharge lamp in which the inner surface of the tube is coated with a fluorescent phosphor. The phosphor is excited by the ultraviolet radiation from the electric discharge and fluoresces, providing visible light. 
     During operation of a gas discharge lamp, such as a fluorescent lamp, a phenomenon known as striations can occur. Striations are zones of light intensity, appearing as dark bands. This phenomenon can give a lamp an undesirable strobing effect. An example of the striation phenomenon is shown in FIG. 1, which depicts a linear fluorescent lamp  10  employing Krypton added as a buffer gas to improve the efficacy of the lamp. In FIG. 1, lamp  10  has striation zones  12  which appear as the dark bands moving along the length of the lamp. Striations in gas discharge lamps are known to occur in cold applications and in other contexts such as Krypton content lamps. 
     A variety of theories as to why striations occur have been set forth. For example, in U.S. Pat. No. 5,001,386 to Sullivan, it is stated that striations are believed to occur as a result of high-frequency currents re-enforcing a standing wave of varying charge distribution between the lamp electrodes. 
     Sullivan attempts to solve the striation problem by injecting a dc component superimposed on top of a driving ac current. A disadvantage to this technique is that, by adding the dc bias, it is possible to cause damage to the lamp by moving mercury in the lamp to one end, creating an unbalanced light output. It has also been suggested that increasing the crest factor in a lamp lighting system will eliminate the usual striations. However, increasing the crest factor may also increase the stress on a lamp, which will lead to a shorter lamp life. 
     Therefore, it would be beneficial to provide a ballast that solves the above-described problems without adding a dc bias and without substantially increasing the crest factor. 
     SUMMARY OF INVENTION 
     The present invention provides a ballast circuit powered by a system power source. The ballast is in operative connection with the system power source wherein the ballast is designed to convert the AC system power source to a DC voltage on a DC bus included within the ballast circuit. An inverter circuit is included in the ballast circuit in operative connection with the DC bus to generate an asymmetric alternating current on a lamp input line. Further, a gas discharge lamp is in operative connection to the lamp input line, configured to receive the asymmetric alternating current, thereby eliminating visual striations otherwise occurring in the lamp. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates a typical fluorescent lamp having striation zone creating a strobing effect to an end user; 
     FIG. 2 illustrates a standing pressure wave in a closed organ pipe; 
     FIG. 3 depicts a high-level view of a system implementing the concepts of the present invention; 
     FIG. 4 illustrates a preferred embodiment of the present invention; 
     FIG. 5 a  shows a standard forcing function which may be obtain by a prior art system; 
     FIG. 5 b  depicts an input forcing function obtained by use of the concepts of the present invention; 
     FIG. 6 a  shows a standard lamp input current; 
     FIG. 6 b  depicts a lamp input current obtained by use of the concepts of the present invention; and 
     FIG. 7 illustrates an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     As depicted in FIG. 1, the striation zones  12  generate an undesirable visual effect to an end user. In addressing this problem, the inventors applied a null hypothesis to describe the striation phenomenon, and propose the physics behind striations can be modeled as a standing pressure wave  14  in an enclosed organ pipe  16 , such as shown in FIG.  2 . The frequency of resonance for a closed pipe is given by:          f   n     =       n     4      l                    C   P       C   v            P   0         ρ   0                                  
     where l is length unit, n is harmonic, c p  is molar capacity as constant volume, c v  is molar capacity at constant pressure, P 0  is undisturbed gas pressure and D 0  is density of gas outside compression zone. 
     Using this hypothesis, it has been determined that striations in a lamp can be reduced or eliminated by operating a ballast having an inverter at other than a 50% duty ratio. That is, in a two switch inverter, for example, one switch is configured to operate longer than the remaining switch. As long as this offset in the duty ratio is blocked, such as by capacitor, no DC current will flow through the lamp&#39;s arc. Rather, for example, the positive portion of the lamp current cycle will have a shorter duration but a higher amplitude than the succeeding negative portion of the cycle, or vice versa. Consequently, a ballast circuit has been developed which provides an asymmetric input current to the lamp. By altering the symmetry of the current in this manner, the repetitive resonance frequencies which are believed to create the striations are interfered with thereby eliminating the visual appearance of striations. 
     FIG. 3 sets forth an exemplary lamp lighting system  20  which incorporates the concepts of the present invention. An input power source  22  supplies power to a ballast  24 . Ballast  24  includes an AC-to-DC converter  26  which provides a DC voltage on DC bus  28  which, in turn, provides power to a lamp input current generating circuit  30 . The lamp input current generating circuit  30  is configured to generate an asymmetric alternating current on lamp input line  32  that provides power to gas discharge lamp  34 . In one embodiment, the lamp input current generating circuit  30  can be an inverter circuit or portions of the investor circuit, and will be described primarily with this focus. However, it is to be appreciated that other components and circuits capable of generating or supplying an a symmetric alternating current to lamp  34  may also be used. These additional circuits, which may be represented by block  30  of FIG. 3, may or may not be part of the inverting circuit. For example, a sub-circuit subsequent to the inverting mechanism can be used to alter asymmetric generated signal into an asymmetric form. 
     Set forth in FIG. 4 is one embodiment of inverter circuit  30  suitable for incorporating concepts of the present invention. Inverting circuits of this type are well known in the industry and, therefore, the circuit will not be described in great detail except where concepts of the present invention are implemented. The circuit comprises complementary switches  40  and  42 , bipolar junction transistors in this exemplary embodiment. The emitters of switches  40  and  42  are connected in common to a series configured resonant circuit  44  including capacitor  46  and inductor  48 . A blocking capacitor  50  is connected to the remaining end of resonant circuit  44  and is series connected to lamp  34  at node  52  with the remaining end of lamp  34  connected to the junction of capacitor  46  and inductor  48  at node  54 . A feedback inductor  56 , a tap from inductor  48 , is connected to the common emitters of switches  40  and  42  at node  58  with the remaining end of inductor  56  series connected to driving inductor  60  which is connected, in turn to feedback capacitor  62 . The remaining end of feedback capacitor  62  is connected to the base terminals of switches  40  and  42 . A first resistor  64  is connected from the base terminals of switches  40  and  42  to the collector terminal of switch  40  which is also connected to the positive lead of DC bus  28  at node  66 . The collector terminal of switch  42  is connected to ground  68  which is connected to the negative lead of DC bus  28  at node  70 . Driving inductor  60  is bridged by output clamping circuit  72  comprising back-to-back, series connected zener diodes  74  and  76 . Capacitor  78  bridges resonant circuit, and resistor  80  is connected between node  58  and ground  68 . Reverse-conducting diode  82  bridges the emitter and collector terminals of switch  40 , with the cathode of diode  82  connected to the collector terminal of switch  40 . Reverse-conducting diode  84  bridges the emitter and collector terminals of switch  42 , with the anode of diode  84  connected to the collector terminal of switch  42 . A preferred method of producing asymmetry in the lamp input current for the circuit illustrated in FIG. 4 is to configure switches  40  and  42  with mismatched h FE  (commonly called beta). This causes the transistor with a lower h FE  to conduct for a shorter period of time, thereby causing the on time of switches  40  and  42  to be asymmetrical. That is, one BJT will conduct for a shorter period of time than the other will. 
     FIG. 5 b  shows an asymmetrical forcing function  86  of the present invention compared to a typical symmetrical forcing function  88  of FIG. 5 a  of prior art ballast inverters. The forcing function is a voltage as measured from node  58  with respect to node  52  in FIG.  4 . The particular forcing function shown is configured to have a short positive duration and a long negative duration. The positive and negative durations can be reversed with equal efficacy. 
     FIG. 6 b  illustrates the effect of asymmetrical forcing function  86 . Asymmetrical load current  90 , measured as the current flowing from node  54  to node  52 , and can be compared to a symmetrical load current  92  shown in FIG. 6 a . The positive portion of the asymmetrical current cycle is of shorter duration than the negative portion of the cycle, however, the positive portion is of a higher amplitude than the negative portion. Symmetrical load current  92 , however, shows equal positive and negative durations, and equal positive and negative amplitudes. There is no DC component to asymmetrical load current  90  because DC current is blocked by blocking capacitor  50 . 
     An alternate embodiment of the present invention is shown in FIG. 7 incorporating MOSFET switches  94  and  96 . With continuing reference to FIG. 4, like numbered numerals in FIG. 7 designate similar components. Omitted in FIG. 7 are reverse-conducting diodes  82  and  84  since MOSFET switches  94  and  96  have intrinsic reverse-conducting diodes. Added in FIG. 7 are gate voltage limiting zener diodes  98  and  100 . The BJT switches of FIG. 4 did not require voltage limiting diodes because the base-emitter junction of a BJT inherently limits the input voltage. 
     In a prior art inverter incorporating complementary MOSFET switches, voltage-limiting zeners  98  and  100  would be configured with equal component voltage ratings. However, in this alternate embodiment of the present invention, zener diodes  98  and  100  are configured with unequal voltage ratings. The unequal voltage ratings cause one of switches  94  and  96  to be in an on state longer than the opposite switch. The effect of unequal on times of switches  94  and  96  will be the same as illustrated in FIGS. 5 a - 5   b  and  6   a - 6   b  for BJT switches  40  and  42 . 
     The beneficial aspect of the asymmetric input line current generated by asynchronous switching of inverter circuits begins to be noticed when even small on/off time imbalances are generated. It is to be noted however, that as the on/off times between, for example, the two switches in the described circuits are increased, a circuit&#39;s crest factor will also increase, diminishing the circuit&#39;s efficiencies. Therefore, in practical applications users will determine the benefits versus tradeoffs obtainable to provide the most efficient circuit having striations eliminated. 
     The embodiment shown in FIG.  4  and the embodiment shown in FIG. 7 are for exemplary purposes only. It is to be appreciated that other configurations can be imagined that fall within the scope of the present invention. 
     As previously noted, while the present invention may be implemented in numerous forms. In the forgoing embodiments, component designations and/or values for the circuits of FIGS. 4 and 7 would include: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Transformer Inductor 48 (56 is a tap from 48) 
                 3.5 mH 
               
               
                 Transformer Inductor 60 
                 150 μH 
               
               
                 Capacitor 46 
                 1 nF, 1 kV 
               
               
                 Capacitor 62 
                 100 nF, 50 V 
               
               
                 Capacitor 50 
                 100 nF, 500 V 
               
               
                 Capacitor 78 
                 120 pF, 1 kV 
               
               
                 Diodes 82, 84 each 
                 1N4937 
               
               
                 Zener diode 98 
                 9 V 
               
               
                 Zener diode 100 
                 11 V 
               
               
                 Zener diodes 74, 76 each 
                 24 V 
               
               
                 Resistor 64 
                 1 Meg 
               
               
                 Resistor 80 
                 1 Meg 
               
               
                 Transistor 40 
                 General Electric 13003 
               
               
                 Transistor 42 
                 General Electric 93003 
               
               
                 Transistor 94 
                 IRF310 
               
               
                 Transistor 96 
                 IRF9310 
               
               
                   
               
             
          
         
       
     
     It is to be appreciated that, while a variety of lamps may be used, for the values presented, the present lamps would operate on a power supply of line  120 / 277  Vac at 60 Hertz cycle where the lamps may be a gas discharge lamp such as rare gas filled T 8  linear fluorescent. The components listed as STM components are from STMicroelectronics of Catania, Italy. Although the present invention is described primarily in connection with fluorescent lamps, the circuit herein described may be used to control any type of gas discharge lamp. Since certain changes may be made in the above-described circuit without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not a limiting sense.

Technology Category: 5