Abstract:
A method and ballast for driving a high intensity discharge (HID) lamp include generating a very high frequency driving signal for the HID lamp, generating a low frequency modulating signal, amplitude modulating the driving signal with the modulating signal at a predetermined low initial modulation level, measuring a lamp voltage across the HID lamp, determining a standard deviation of the lamp voltage, comparing the standard deviation with a predetermined minimum level, if the standard deviation is above the predetermined minimum level, incrementally increasing the modulation level and repeating the amplitude modulating step, the measuring step, the determining step and the comparing step, and if the standard deviation is below the predetermined minimum level, maintaining the amplitude modulation at the determined level.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. patent application Ser. No. 10/043,586, filed Jan. 10, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates to controlling the operation of various types of gas discharge lamps, and in particular, an improvement in the operational performance of electronic ballasts within a high frequency range of a gas discharge lamp. 
     2. Description of the Related Art 
     High intensity discharge (HID) gas discharge lamps as known in the art suffer from acoustic resonances when such lamps are operated at high frequencies, i.e., between a few kHz and hundreds of kHz, depending on the type of lamp. However, the acoustic resonances significantly weaken in such gas discharge lamps in which the acoustic resonances do not have a negative affect on the performance of these gas discharge lamps when the lamps are operated at very high frequencies, i.e., above the highest acoustic resonance (e.g., 150 kHz for a 400 W metal halide lamp). However, a consequence of operating the gas discharge lamp in the VHF range is the generation of electro-magnetic interference. Additionally, when a gas discharge lamp is operated at VJF lamp currents, the electrode temperature modulation (i.e., the difference in anode and cathode temperatures) vanishes. This results in a different electrode operating condition, which could cause changes in the arc attachment on the electrode. Arc instabilities related with arc-electrode attachment have been found when 400 W metal halide lamps are operated at high frequencies, even up to as high as 500 kHz. 
     Back-arcing of a gas discharge lamp involves an arc attachment of the arc on the back of the electrode coil of the lamp, as opposed to an ideal arc attachment of the arc at the tip of the electrode. This can affect thermal balance of the end of the arc tube, which, in turn, can affect the vapor pressures. Consequently, the color properties of the lamp are affected. 
     There are a number of known methods for operating HID lamps stably at high frequencies. A first method is to operate at a current frequency that is below the frequency of the lowest acoustic resonance. This method is limited to very low power lamps because acoustic resonance frequencies scale as one over an inner dimension of the lamp envelope. For higher wattage (larger) lamps, the lowest acoustic resonance frequencies are below 40 kHz power frequency (20 kHz current frequency) and the circuit can produce audible noise. A second method is to find a “resonance free window” that lies between the acoustic resonance frequencies. This method depends critically on the dimensions of the lamp. Small variations in manufacturing tolerances or changes in lamp parameters over the life of the lamp can make this “window” disappear. A variation on this method is to frequency sweep through a range of weak resonances. Again, the frequency range is very dependent on lamp dimensions. A third method for operating an HID lamp stably, is to increase the frequency sufficiently such that the acoustic resonances are damped. In this case, it is hard to guarantee that very weak resonances will not occur. The frequencies of these weak resonances vary unpredictably from lamp to lamp and can even vary from one operating period to another. Frequency sweeping at VHF has not proven totally successful in eliminating these instabilities. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to be able to drive an HID lamp at very high frequencies while eliminating arc instabilities. This object is achieved in a method of driving a high intensity discharge (HID) lamp, comprising the steps generating a very high frequency driving signal for said HID lamp; generating a low frequency modulating signal; amplitude modulating said driving signal with said modulating signal at a level of 10% to 30%; and applying said amplitude modulated driving signal to said HID lamp. 
     Applicants have found that when the modulating signal has a frequency of substantially 100 Hz, and the driving signal has a frequency in the range of 100 kHz to 500 kHz, stabilization of the arc of the HID lamp is attainable. 
     Since the properties of each lamp have a direct bearing on the stability, in a preferred embodiment of the invention, the method of driving a high intensity discharge (HID) lamp, comprises the steps generating a very high frequency driving signal for said HID lamp; generating a low frequency modulating signal; amplitude modulating said driving signal with said modulating signal at a predetermined low initial modulation level; measuring a lamp voltage across said HID lamp; determining a standard deviation of said lamp voltage; comparing said standard deviation with a predetermined minimum level; if said standard deviation is above said predetermined minimum level, incrementally increasing said modulation level and repeating said amplitude modulating step, said measuring step, said determining step and said comparing step; and if said standard deviation is below said predetermined minimum level, maintaining said amplitude modulation at said determined level. This method may be modified by first trying the amplitude modulation when the driving frequency is at an initial value. Then, if the standard deviation does not drop below the predetermined minimum level when the amount of amplitude modulation reaches a predetermined amount, the driving frequency may be incrementally increased (or decreased) and the procedure repeated until the appropriate combination of driving frequency and amount of amplitude modulation is achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     With the above and additional objects and advantages in mind as will hereinafter appear, the subject invention will be described with reference to the accompanying drawings, in which: 
     FIG. 1 shows a block circuit diagram of a ballast driving a lamp in accordance with the subject invention; 
     FIG. 2 shows a block circuit diagram of a half-bridge circuit for use in the ballast of FIG. 1; 
     FIG. 3 shows a graph of the lamp voltage in one embodiment of the subject invention; 
     FIG. 4 shows a graph of the lamp voltage in another embodiment of the subject invention; 
     FIG. 5 shows a graph of the lamp voltage when the amplitude modulation is incrementally increased; 
     FIG. 6 shows a graph of the standard deviation of one embodiment of a lamp with the lamp voltage at differing driving frequencies; 
     FIG. 7 shows a graph of the standard deviation of another embodiment of a lamp with the lamp voltage at differing driving frequencies; and 
     FIG. 8 shows a modification for the embodiment of FIG. 2, in which the standard deviation of the lamp voltage is monitored. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a block circuit diagram of a ballast  1  incorporating the subject invention for providing a lamp current IL to a conventional lamp  3 . The ballast  1  includes an electro-magnetic interference filter  10  for filtering line voltage applied thereto. A line voltage rectifier  12  rectifies the line voltage from the filter  10  and provides a DC voltage V D  to a boost converter  14 . An energy buffer  16  is connected across the output from the boost converter  14 , the output therefrom being also applied to a half-bridge circuit  18 . An output from the half-bridge circuit  18  forms the output of the ballast  1  and is applied to the lamp  3 . 
     FIG. 2 shows an embodiment of the half-bridge circuit  18 . The half-bridge circuit  18  includes the series arrangement of a first switch T 1  and a second switch T 2 , shown as MOSFETs, and an impedance Z connected between the input terminals  20  and  22  of the half-bridge circuit  18  receiving the voltage V D . A series arrangement of a primary winding PW of a transformer TF, an inductor L, a first capacitor C 1  and a second capacitor C 2  is connected between the junction between the first and second switches T 1  and T 2 , and the second input terminal  22 . The output terminals  24  and  26  of the half-bridge circuit  18  are arranged across the second capacitor C 2  and are connectable to the lamp  3 . 
     A micro-controller  28  receives the voltage V D  from the first input terminal  20  and a current I D  from the junction between the second switch T 2  and the impedance Z. In addition, the secondary winding SW of the transformer TF, having one end connected to ground, supplies the current I F  to the micro-controller  28 . In response to the voltage V D  and the currents I D  and I F , the micro-controller  28  generates a control voltage V FM  for controlling the oscillating frequency of voltage-controlled oscillator  30  at the desired operating frequency of the lamp. The voltage-controlled oscillator  30  generates a control voltage V C  at the operating frequency to a half-bridge driver circuit  32 . In response to the control voltage V C , the half-bridge driver circuit  32  generates the drive signals for the gates of the first and second switches T 1  and T 2 . Amplitude modulation of the signal to the lamp can be accomplished by amplitude modulating the bus voltage V C . To that end, amplitude modulator  34  is included between the input  20  and the first switch T 1 . The amplitude modulator  34  has a control input coupled to an output from the micro-controller  28  for receiving a control signal V AM  indicative of the desired amount of amplitude modulation. It should be understood that there are other arrangements for amplitude modulating the signal to the lamp, which may be substituted for the above-described embodiment. 
     Applicants have found that it is not sufficient to merely amplitude modulate the VHF drive voltage for the lamp in order to achieve stable operation of the HID lamp. While the amplitude modulating waveform may be a sine wave, a square wave, a ramp or a triangle wave, it is also necessary for the amplitude modulation to be significant. In one example, a 150 W HID lamp with a ceramic envelope was operated at 500 kHz current frequency. The voltage waveform to the lamp was then modulated with a 100 mV square wave signal at 100 Hz, corresponding to a 10% modulation. As shown in FIG. 3, the waveform A represents the lamp voltage V L  over an approximately 20 second time period without amplitude modulation, while waveform B represents the lamp voltage V L  over the same time with the amplitude modulation. As should be apparent, the large excursions of the lamp voltage, as shown in waveform A, are indicative of arc instabilities. With the appropriate amount of amplitude modulation, as shown in waveform B, the large excursions of the lamp voltage have been eliminated and the lamp operation is stable. In a second example, as shown in FIG. 4, the lamp was operated at 400 kHz current frequency. In FIG. 4, waveform C represents the lamp voltage V L  over an approximately 20 second time period without amplitude modulation, while waveform D represents the lamp voltage V L  over the same time with the amplitude modulation. 
     As the operating conditions of each lamp are different, and the operating parameters of a lamp may change over time, it may be necessary to change the amount of amplitude modulation. FIG. 5 shows the effects of incrementally increasing the amplitude modulation. In particular, a 150 W HID lamp with a ceramic envelope was operated at 500 kHz current frequency. The lamp voltage V L  periodically varied with time and is shown for 4 second intervals. Amplitude modulation of the VHF signal was then incrementally increased until, at 250 mV (approx. 25% modulation), the lamp stabilized. In particular, waveform E shows the lamp voltage V L  without amplitude modulation, waveform F shows the lamp voltage V L  in which the modulation level was at 100 mV, waveform G shows the lamp voltage V L  in which the modulation level was at 150 mV, waveform H shows the lamp voltage V L  in which the modulation level was at 200 mV, and waveform I shows the lamp voltage V L  in which the modulation level was at 250 mV. It should be appreciated that the lamp voltage V L  shows significantly smaller variations in waveform I as opposed to in waveforms E-H, thereby signifying stable operation. When the amplitude modulation was removed, the lamp resorted to its unstable operation (waveform J). 
     With the above in mind, Applicants have determined that the amount of needed amplitude modulation may be determined by examining the standard deviation of the lamp voltage V L . When the arc of the lamp becomes unstable, it deviates from its normal length and this produces a distribution of voltages. In an exemplary study, the lamp voltage waveform was digitized over a 10 ms period (corresponding to one period of the amplitude modulation signal) and the rms voltage was calculated. This measurement was repeated 500 times and the standard deviation of these 500 measurements was calculated. The total time for each standard deviation measurement was approximately 10 s. A 70 W cylindrical discharge lamp was operated at integer VHF current frequencies from 250 to 300 kHz without amplitude modulation. Of these 51 discrete frequencies, only 3 were stable (instabilities persisted above 400 kHz). With the addition of 30% amplitude modulation with a 100 Hz square wave, 34 of the frequencies were stable. This is illustrated in FIG. 6 which plots the standard deviation of 500 voltage measurements without amplitude modulation (circles  50 ) and with amplitude modulation (triangles  52 ) at current frequencies from 250 to 300 kHz. The horizontal line at a standard deviation of 0.1 is the approximate dividing line between arc stability (&lt;0.1) and arc instability (&gt;0.1). The effect of percentage amplitude modulation required to stabilize this 70 W lamp was investigated for a 100 Hz square wave and a 100 Hz sine wave. The VHF frequency was 285 kHz, which was unstable without amplitude modulation. In the case of the square wave modulation, the arc was stable with 20% to 30% modulation, and in the case of sine wave modulation, the arc was stable from 15% to 30% modulation. 
     The modulation frequency was investigated with 30% amplitude modulation with a square wave and a sine wave. With square wave modulation, the lamp was stable at 100 and 400 Hz, but at 200 Hz, there was a periodic movement of the discharge. By 500 Hz, there was a rapid flicker at the bottom electrode and the lamp was unstable at 1000 Hz. With sine wave modulation, the arc was stable at 100 Hz and 200 Hz, but at 400 Hz, there were intermittent instabilities. By 500 Hz, the lamp was unstable. The lower limit of the modulation frequency is determined by the perception of flicker caused by the strong modulation of the lamp power and light output. 
     In a second example, a 100 W non-cylindrical HID lamp with a quartz envelope was stabilized using amplitude modulation. The lamp was unstable in a vertical orientation at all 51 VHF frequencies from 150 to 200 kHz. With the addition of 30% square wave amplitude modulation, the lamp stability increased dramatically at many of these frequencies. This is illustrated in FIG. 7 which plots the standard deviation of 500 voltage measurements without amplitude modulation (circles  54 ) and with amplitude modulation (triangles  56 ). 
     In view of the above, Applicants have devised a modification of the circuit of FIG.  2 . As shown in FIG. 8, the lamp voltage VL is applied to an analog-to-digital (A/D) converter  40 . The digitized lamp voltage is then applied to a standard deviation circuit  42  which calculates the standard deviation of the lamp voltage over a predetermined period of time. This standard deviation is then applied to a threshold detector  44  which determines when the standard deviation is below a predetermined level indicative of stable operation of the lamp. An output of the threshold detector  44  is applied to the micro-controller  28 . 
     In operation, the micro-controller  28  initially does not generate an output modulation signal for the amplitude modulator  34 . Based on the output of the threshold detector  44 , the micro-controller  28  begins generating an output modulation signal at a predetermined minimal amount, and incrementally increases the amount of amplitude modulation, while the results are monitored by the A/D converter  40 , the standard deviation circuit  42  and the threshold detector  44 . Once the standard deviation of the lamp voltage drops below the predetermined threshold in the threshold detector  44 , the micro-controller  28  stops increasing the amount of amplitude modulation, which then remains at the optimum level. 
     It may be that after the above procedure, the standard deviation of the lamp voltage is still above the predetermined threshold. As such, it will be necessary for the micro-controller  28  to change the frequency of operation of the lamp and then repeat the incremental increasing of the amount of amplitude modulation. To that end, the above operation is modified in that the micro-controller  28  initially supplies a control signal to the VCO  30  causing the VCO  30  to operate at a predetermined initial frequency. Based on the output of the threshold detector  44 , the micro-controller  28  begins generating an output modulation signal at a predetermined minimal amount, and incrementally increases the amount of amplitude modulation, while the results are monitored by the A/D converter  40 , the standard deviation circuit  42  and the threshold detector  44 . Once the standard deviation of the lamp voltage drops below the predetermined threshold in the threshold detector  44 , the micro-controller  28  stops increasing the amount of amplitude modulation, which then remains at the optimum level. If the standard deviation of the lamp voltage does not drop below the predetermined threshold once the amount of amplitude modulation reaches, for example, 30%, the micro-controller  28  incrementally increases the frequency of the VCO  30  and then repeats the incremental increasing of the amount of amplitude modulation. This is continued until the appropriate combination of frequency and amount of amplitude modulation is achieved. 
     Numerous alterations and modifications of the structure herein disclosed will present themselves to those skilled in the art. However, it is to be understood that the above described embodiment is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.