Patent Publication Number: US-7719204-B1

Title: Method for controlling striations in a lamp powered by an electronic ballast

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application is a divisional application of co-pending U.S. patent application Ser. No. 11/003,539 filed Dec. 3, 2004, entitled “Method of Controlling Striations in a Lamp Powered by an Electronic Ballast” which claims the benefit of U.S. Provisional Application No. 60/540,187, filed Jan. 29, 2004, entitled “Methods of Striation Control for Current Fed, Parallel Resonant Inverters”. 
   A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark office patent file or records, but otherwise reserves all copyright rights whatsoever. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to controlling striations in lamps powered by electronic ballasts. 
   More particularly, this invention pertains to methods for controlling lamp striations by supplying the lamp with asymmetric lamp currents. 
   Methods of controlling lamp striations are known in the art. For example, U.S. Pat. Nos. 5,041,763 and 5,192,896 teach the use of methods that include the step of supplying DC or low frequency AC current to a lamp in order to control striations. The primary disadvantage of these methods is that they require electronic ballasts that include additional components for generating the DC or low frequency AC currents and these additional components increase the costs and losses associated with the electronic ballasts. 
   U.S. Pat. No. 6,465,972 also teaches a method of controlling striations. In this patent, striations are controlled by varying the amplitude of the current that is supplied to the lamp using an amplitude modulation circuit. This method suffers from the same disadvantage as the method taught by the &#39;763 patent. That is, it requires an electronic ballast that includes additional components and those components increase the costs and losses associated with that ballast. 
   U.S. Pat. No. 6,069,453 teaches a method of controlling striations by supplying a lamp with a current that includes a DC component, a high frequency AC component, and a low frequency component. In addition, the amplitude of the high frequency AC component must be at least 500 times higher than the amplitude of the low frequency component. As was the case with the &#39;763, &#39;896, and &#39;972 patents, this method requires the use of an electronic ballast having additional components that increase the costs and losses associated with the ballast. 
   U.S. Pat. Nos. 5,701,059, 5,369,339, and 5,001,386 all teach methods of controlling striations in lamps. All of these methods, however, require the use of additional components that are not required to perform the functions most commonly performed by an electronic ballast. For example, the method taught by the &#39;059 patent requires the use of an impedance that is connected in parallel with a lamp. The method taught by the &#39;339 patent requires a DC device that can supply DC current to a lamp. And, the method taught by the &#39;386 patent requires a back end rectifier that is used to supply a DC current to a lamp. The requirement of these additional components increases the cost and losses associated with the electronic ballasts required to implement these methods. 
   What is needed, then, is a method of controlling striations in a lamp that does not require the use of an electronic ballast that includes additional components that increase the costs and losses associated with that ballast. 
   BRIEF SUMMARY OF THE INVENTION 
   One of the objects of the present invention is to provide a method of controlling striations in a lamp powered by an electronic ballast that does not require the use of additional components specifically included in the ballast for that purpose. Or, to put it another way, one of the objects of the present invention is to provide a method of controlling striations in a lamp using a conventional electronic ballast that has been modified in some way that prevents striations from occurring in a lamp powered by that ballast. 
   The applicant of the present application has determined that this object can be achieved by forcing the inverter transistors in a conventional electronic ballast to have different conduction times. The difference in conduction time causes an asymmetric voltage to be generated by the electronic ballast and this voltage causes an asymmetric current to flow in a lamp connected to the ballast. The asymmetric current, in turn, causes striations in the lamp to move at a rate that makes them invisible to the human eye. Thus, by varying the conduction times, one can control the striations in the lamp. 
   The conduction time is essentially the “on-time” of the transistor, i.e., the time that current is flowing through the transistor. The difference in conduction time does not need to be significant; differences ranging from fractions of a microsecond to one and a half microseconds have been found to provide acceptable results. 
   The applicant has determined that the inverter transistors can be forced to have different conduction times by redesigning some of the components included in a conventional electronic ballast. For example, in an electronic ballast that includes a self-oscillating, current fed, parallel resonant push-pull inverter, the conduction times of the inverter transistors can be modified by offsetting the center-tap on the output transformer included in that type of inverter by a few turns. As was the case with the variation in the conduction times, the amount of offset does not need to be significant. An offset of no more than five percent of the total primary turns has been found to cause the conduction times of the inverter transistors to vary the appropriate amount. In an electronic ballast that includes a self-oscillating, current fed, parallel resonant half-bridge inverter, the same effect can be achieved by offsetting the number of turns in one winding of the DC choke that is included in this type of inverter by approximately five percent. Of course, an offset greater than five percent may be used in either case as well 
   By redesigning the components of a conventional electronic ballast in this manner, the striations in a lamp powered by the ballast can be controlled. Importantly, it should be noted that this control is achieved without adding any additional components to the electronic ballast. As a result, the present invention does not suffer from the increased costs and losses associated with prior art methods that require the use of additional components specifically designed to control striations. 
   The applicant has found that the present invention is particularly useful when supplying power to new fluorescent lamps that include an increased amount of Krypton in the fill gas. While the increased amount of Krypton causes these lamps to have reduced arc voltages, it also increases the probability that visible striations will be produced by the lamp. By using electronic ballasts that have been modified according to the present invention, these visible striations are eliminated. 
   In summary, then, and according to one aspect, the present invention includes a method of controlling striations in a lamp powered by an electronic ballast that includes the step of generating an asymmetric lamp current using an unbalanced circuit component in the electronic ballast and supplying that current to the lamp. According to another aspect, the present invention includes an electronic ballast having an unbalanced circuit component that causes the ballast to supply asymmetric lamp current to a lamp when the lamp is connected to the ballast. In one embodiment, the unbalanced circuit component is an unbalanced output transformer, while in another embodiment the unbalanced circuit component is an unbalanced DC choke. According to a third aspect, the present invention includes a method of modifying an electronic ballast so that it produces an asymmetric lamp current without any additional components specifically included for that purpose. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a schematic drawing of a conventional parallel resonant push pull electronic ballast. 
       FIG. 2  is an enlarged schematic drawing of the push pull inverter shown in  FIG. 1 . 
       FIG. 3  is an oscilloscope printout showing the center tap voltage, the primary winding voltage, and the output current generated by the electronic ballast shown in  FIG. 1 . 
       FIG. 4  is an oscilloscope printout showing the center tap voltage, the primary winding voltage, and the output current generated by the electronic ballast shown in  FIG. 1  after it has been modified according to the teachings of the present invention. 
       FIG. 5  is a schematic drawing of a conventional parallel resonant half bridge electronic ballast. 
       FIG. 6  is an enlarged schematic drawing of the half bridge inverter shown in  FIG. 5 . 
       FIG. 7  is an oscilloscope printout showing the voltage across C 11  in  FIG. 6 , the primary winding voltage, and the output current generated by the electronic ballast shown in  FIG. 5 . 
       FIG. 8  is an oscilloscope printout showing the voltage across C 11  in  FIG. 6 , the primary winding voltage, and the output current generated by the electronic ballast shown in  FIG. 5  after it has been modified according to the teachings of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following discussion, the present invention is discussed with reference to what is commonly referred to as a current-fed, parallel resonant push pull ballast and a current-fed, parallel resonant half bridge ballast. It should be noted that the various aspects of the present invention are not limited to these particular types of ballasts and may be applied to other ballasts as well. 
     FIGS. 1-2  are schematic drawings of a conventional parallel resonant push pull electronic ballast circuit  10 .  FIG. 1  is a drawing of the entire ballast  10  and  FIG. 2  is an enlarged view of a portion of the ballast  10  including a push pull inverter circuit  12 . The operation of this type of ballast is well known in the art and will not be discussed in detail. In general, however, the ballast  10  is operable to receive a low frequency, 60 Hz, voltage input, convert that input into a substantially constant DC voltage, and then convert that DC voltage into a high frequency (typically greater than 20 kHz, but can be lower if desired) sinusoidal output voltage that can be applied to a lamp. The specific operational features of this ballast that are relevant to the present invention are discussed below. 
     FIG. 3  is an oscilloscope printout showing various waveforms that are generated by the conventional electronic ballast  10  shown in  FIG. 1 . The waveform on Channel  1  is the voltage that appears at the center tap  14  of the primary winding  16  on the output transformer  18 . The waveform on Channel  2  is the output current that is supplied to a lamp by this ballast and the waveform on Channel  3  is the voltage across the primary winding  16  on the output transformer  18  of this ballast  10 . 
   A review of this figure clearly shows that the voltage peaks of the center tap voltage waveform, Channel  1 , are very similar. Peak  1  is approximately 350 volts and Peak  2  is approximately 350 volts as well. Peaks  1 ,  3 , and  5  represent the voltage across one half of the primary winding of the output transformer and Peaks  2 ,  4 , and  6  represent the voltage that appears across the other half of the primary winding. 
   In addition, this figure indicates that the inverter transistors,  20  and  22 , are both on, or conducting current, for approximately the same amount of time, i.e., both transistors have approximately the same conduction times. This is shown by the duty cycle values included in the figure and waveform on Channel  3 . The positive duty cycle is approximately 50.74%, while the negative duty cycle is approximately 49.26%. In other words, both transistors have approximately a 50% duty cycle. 
   The resulting output current, Channel  2 , is very symmetrical. This is caused by the approximately equal conduction times of the transistors,  20  and  22 . 
     FIG. 4  is an oscilloscope printout showing various waveforms that are generated by the conventional electronic ballast  10  shown in  FIG. 1  after it have been modified in the manner taught by the present invention. As was the case with  FIG. 3 , the waveform on Channel  1  is the voltage that appears at the center tap  14  of the primary winding  16  on the output transformer  18 , the waveform on Channel  2  is the output current that is supplied to a lamp by this ballast, and the waveform on Channel  3  is the voltage across the primary winding  16  on the output transformer  18  of this ballast  10 . 
   The waveforms shown in  FIG. 4 , however, are slightly different from the waveforms shown in  FIG. 3 . For example, the waveform showing the voltage at the center tap  14  of the primary winding  16  of the output transformer  18 , Channel  1 , now includes peaks that are unequal. Peak  1  is now approximately 300 volts and Peak  2  is now approximately 350 volts; a difference of approximately 50 volts. The difference in voltage was achieved by constructing the output transformer  18  so that one half of the primary winding  16  has 54 turns and the second half has 58 turns. The difference in the number of turns causes a different voltage to appear across each half of the primary winding  16 . In alternative embodiments, the number of turns in each half of the primary winding  16  may be different. 
   In addition, the conduction times of the inverter transistors,  20  and  22 , as shown by the waveform on Channel  3 , are now unequal. The positive duty cycle is now 52.32% and the negative duty cycle is now 47.69%. In other words, one transistor is now on for approximately 52% of the time and the other transistor is on for approximately 48% of the time. 
   The inverter transistors,  20  and  22 , are on for different periods of time because the resonant tank, which includes the output transformer  18  and the capacitor C 12 , tries to keep the volt-second product of each half of the primary winding  16  of the transformer  18  equal. In other words, the resonant tank tries to balance the energy in the primary winding  16  of the output transformer  18 . The circuit accomplishes this by causing the transistors,  20  and  22 , to conduct for different periods of time. 
   Importantly, the output current waveform shown on Channel  2  is now asymmetric. As shown in  FIG. 4 , the peak value of the positive half of the current waveform is approximately 100.8 mV (with the instrumentation used 1 mV corresponds to 10 mA) and the negative half is approximately −103.3 mV. The asymmetry in the output current is caused by the fact that different voltages appear across each half of the primary winding  16  of the output transformer  18 . This asymmetric voltage is coupled to the secondary  24  of the output transformer  18  and causes the output current to be asymmetrical as well. 
   When this type of asymmetric output current is supplied to a lamp, and, in particular, a low wattage, high Krypton, T8, fluorescent lamp, it causes the striations in that lamp to be invisible to the human eye. 
   In summary, then, by offsetting the number of turns in each half of the primary winding  16  of the output transformer  18 , the peaks of the haversine voltages measured from the center tap  14  of the transformer  18  to the emitters of the inverter transistors,  20  and  22 , are forced to different levels. This, in turn, forces the inverter transistors,  20  and  22 , to have different conduction (on) times in order to keep balanced the volt-second product of the resonant tank components, transformer  18  and C 12 . The result is an asymmetric output lamp current that can be used to control striations in a lamp. 
   In some applications it may be necessary to adjust the turn-off times of the inverter transistors,  20  and  22 , so that they are approximately equal. In the circuit shown in  FIG. 2 , the turn-off times are not equal, and, in some applications, this may cause the lamp current crest factor to increase to an undesirably high level. In such applications, the lamp current crest factor can be decreased by adjusting the base drive circuit for the inverter transistors so that the turn-off times are approximately equal. This, in turn, can be accomplished by simply adjusting the resistance values for the base resisters,  26  and  28 , shown in  FIG. 2 . 
   Turning now to  FIGS. 5-6 , the present invention is discussed in connection with a parallel resonant half bridge electronic ballast circuit  30 .  FIG. 5  is a schematic showing the entire ballast circuit  30  and  FIG. 6  is an enlarged schematic showing the half bridge inverter circuit portion  32  of the ballast circuit  30  shown in  FIG. 5 . Half bridge inverters and electronic ballasts including these types of inverters are well known in the art. As a result, a detailed discussion of their operation is not included. A discussion of the operational features that are necessary to understand the present invention, however, is included below. 
   As shown in  FIGS. 5-6 , the half bridge electronic ballast circuit  30  does not include a center tap connection on the primary winding  16  of the output transformer  18  as shown in  FIGS. 1-2 . As a result, the primary winding cannot be offset in order to generate an asymmetric output lamp current. This circuit, however, does include a DC choke  34  and that circuit element is used instead. 
   By offsetting the number of turns in each winding,  36  and  38 , of the DC choke  34 , the peaks of the haversine voltages on complimentary inverter transistors,  40  and  42 , are forced to different levels. This, in turn, forces the inverter transistors,  40  and  42 , to have different conduction (on) times in order to keep balanced the volt-second product of the resonant tank components, transformer  18 , C 11 , and C 12 . The result is an asymmetric output lamp current that can be used to control striations in a lamp. As indicated above with regard to the push-pull circuit shown in  FIG. 2 , forcing the inverter transistors to have different conduction times may cause the lamp current crest factor to increase to unacceptably high levels. If this is the case, the base drive circuit for the inverter transistors can be adjusted so that the turn off times for these transistors is approximately equal. The turn-off times can be adjusted by simply adjusting the resistance values of the base drive resistors,  44  and  46 , shown in  FIG. 5 . 
     FIG. 7  is an oscilloscope printout showing various waveforms that are generated by the conventional half bridge electronic ballast circuit  30  shown in  FIG. 5 . The waveform on Channel  1  is the voltage that appears capacitor C 11 . The waveform on Channel  2  is the output lamp current that is supplied to a lamp by this ballast and the waveform on Channel  3  is the voltage across the primary winding  16  on the output transformer  18  of this ballast. In this prior art half bridge ballast  30 , the DC choke windings,  36  and  38 , are very well balanced and the inverter transistors,  40  and  42 , have very little voltage, or conduction time, offset. In addition, the resulting output lamp current, Channel  2 , is very symmetrical. 
     FIG. 8  is an oscilloscope printout showing various waveforms that are generated by the conventional electronic ballast  30  shown in  FIG. 5  after it have been modified in the manner taught by the present invention. As was the case with  FIG. 7 , the waveform on Channel  1  is the voltage across capacitor C 11 , the waveform on Channel  2  is the output lamp current, and the waveform on Channel  3  is the voltage across the primary winding  16  on the output transformer  18  of the ballast. It should be noted that the voltage on C 11  is analogous to the center tap voltage in the push pull inverter and can be used to display the effects of the DC choke offset. 
   The waveforms shown in  FIG. 8  are different from the waveforms shown in  FIG. 7 . The waveform showing the voltage on C 11 , Channel  1 , now includes peaks that are unequal. Peak  1  is now approximately 640 volts and Peak  2  is now approximately 720 volts; a difference of approximately 80 volts. The difference in voltage is achieved by constructing the DC choke  34  so that one winding has 108 turns and the other winding has 102 turns. The difference in the number of turns causes a different voltage to appear across each winding. 
   In contrast to  FIG. 7 , the voltage across the primary winding  16  of the output transformer  18 , Channel  3 , is now asymmetric in time and magnitude. The waveform is positive for approximately 18.69 microseconds and negative for approximately 19.47 microseconds. In addition, the peak of the positive portion of the waveform is approximately 693 mV (with the instrumentation used this is actually 346.5 volts) and the peak of the negative portion is approximately −688.6 mV (with the instrumentation used this is actually −344.15 volts). As was the case with the push pull ballast discussed previously, this asymmetric voltage is coupled to the secondary winding  24  of the output transformer  18  and is used to generate an asymmetric lamp current, Channel  2 . The asymmetry can be seen in the positive and negative “halves” of the waveform shown on Channel  2 . 
   As discussed previously, one of the primary benefits of the present invention is that it can be implemented at a very low cost. No additional circuitry is required to produce the desired asymmetric current waveform. Striations are not visible when low wattage, high Krypton, T8 lamps are used with electronic ballasts that include offset transformers or DC chokes. The resulting difference in conduction time with the method of the present invention may be fractions of a microsecond to one and a half microseconds. Thus, very little asymmetry is needed to mask the visible striations in a lamp using this method. 
   In alternative embodiments, additional components can be added to slow the switch-off time of one of the inverter transistors. For example, a capacitor may be included in the base drive. Alternatively, additional components, such as a Baker clamp, can be added to decrease the switch-off time of one of the inverter transistors. While these methods can reduce visible striations, they may increase the lamp current crest factor to unacceptable levels. And, of course, these alternatives require some additional components and are not as desirable for this reason. 
   In other alternative embodiments, the inverter transistors can be forced to have different conduction times by operating the transistors at different switching frequencies. 
   Thus, although there have been described particular embodiments of the present invention of a new and useful Method For Controlling Striations In A Lamp Powered By An Electronic Ballast, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.