Patent Publication Number: US-2004051472-A1

Title: Electronic ballast having valley frequency modulation for a gas discharge lamp

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The invention relates generally to electronic ballasts for gas discharge lamps and more particularly to an electronic ballast having frequency modulation for compensating for cyclic low voltage in an AC power line cycle.  
       [0003] 2. Description of the Prior Art  
       [0004] An ionized high intensity discharge (HID) light, such as a high power Sodium or Halide lamp, uses an electronic ballast for converting the frequency of a public utility AC line power to a higher frequency in order to drive the lamp. The ballast must first start the lamp at a very high voltage and then run the lamp at a much lower voltage.  
       [0005] Ballasts are commonly evaluated on the basis power efficiency, power factor, lamp lifetime, and cost. Common existing ballasts have power efficiencies of about 80% and power factors of about 0.9. Several attempts have been made to improve upon these figures. Unfortunately, these attempts have not been entirely successful and they have sometimes resulted in decreased lamp life. In some cases high voltage and high power FET switches have been used. However, these switches add significantly to the cost of the ballast. Moreover, existing ballasts commonly use ferromagnetic devices that are so heavy, for example 30 pounds, that they are costly to ship and difficult install.  
       [0006] There is a continuing need for an improved ballast for HID applications.  
       SUMMARY OF THE INVENTION  
       [0007] It is therefore an object of the present invention to provide a ballast providing high power efficiency, high power factor and long lamp life with a low weight at a low cost.  
       [0008] Briefly, in a preferred embodiment, a ballast of the present invention includes a variable frequency power generator, a lamp driver network, and a valley fill correction system. The power generator switches a rectified AC power line signal for providing a high frequency generator signal at a first or starting lamp frequency to start an HID lamp and a second or operating lamp frequency to operate the lamp. The driver network uses first and second resonant frequencies of an inductor and several capacitors for boosting the generator signal to start and then operate the lamp. The valley fill correction system fills the low voltages (valleys) in the rectified AC power cycle with voltage pedestals and further boosts the operating current to the lamp during the valley time periods by frequency modulating the generator signal.  
       [0009] An advantage of the present invention is that a single inductor having a moderate weight is used for providing the drive signals for both starting and operating an HID lamp.  
       [0010] Another advantage of the present invention is that the cyclic low voltages of the AC power line voltage cycle are compensated by adjusting a generator signal closer to a resonant frequency for boosting lamp current without significantly decreasing power factor.  
       [0011] Another advantage of the present invention is that a power generator has a low cost and a high power efficiency by operating at the relatively low voltage of a rectified AC power line signal and driving a resonant circuit for providing a high voltage generator signal to a lamp.  
       [0012] These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures.  
     
    
    
     IN THE DRAWINGS  
     [0013]FIG. 1 is an electrical diagram of a ballast of the present invention for powering a lamp;  
     [0014]FIGS. 2A and 2B are circuit diagrams of a lamp driver network of the ballast of FIG. 1 for starting and operating the lamp, respectively;  
     [0015]FIG. 3 is a rectified voltage diagram of the prior art without valley fill correction;  
     [0016]FIG. 4 is a rectified voltage diagram having valley fill correction of the ballast of FIG. 1;  
     [0017]FIG. 5 is a diagram of AC line current without valley fill correction;  
     [0018]FIG. 6 is a diagram of lamp current having valley fill correction of the ballast of FIG. 1;  
     [0019]FIG. 7 is a graph of power factor versus valley fill correction of the ballast of FIG. 1;  
     [0020]FIG. 8 is a circuit diagram of a variable frequency power generator of the ballast of FIG. 1;  
     [0021]FIG. 9 is a block diagram of a valley correction modulation system of the ballast of FIG. 1;  
     [0022]FIG. 10 is an electrical diagram of an alternative lamp driver network for the ballast of FIG. 1; and  
     [0023]FIGS. 11A and 11B are circuit diagrams of the lamp driver network of FIG. 10 for starting and operating the lamp, respectively.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0024]FIG. 1 illustrates a ballast of the present invention referred to by a general reference number  10 . The ballast  10  includes a line rectifier  12 , a variable frequency power generator  14 , a valley fill circuit  16 , a controller  18 , and a tuned lamp driver network  20 . The driver network  20  includes an inductor L, a first capacitor C 1 , a second capacitor C 2 , and a third capacitor C 3 . The object of the ballast  10  is to start and operate a gas discharge lamp  30 .  
     [0025] The line rectifier  12  receives alternating current (AC) voltage typically in a range of 200 to 304 rms or 300 to 450 peak volts from an AC power line  32  and provides a rectified voltage on a line  34  with respect to a circuit common  36 . Without a valley fill correction as described below, the rectified voltage has cyclic low voltages  37  (FIG. 3) corresponding to the portions of the AC power signal cycle that are close to the zero crossings of the AC power line cycle.  
     [0026] The rectified voltage on the line  34  is received by the power generator  14 . The power generator  14  switches the rectified voltage from the line  34  on and off for providing a generator signal across circuit nodes N 1  and N 2 . The node N 2  also connects to the circuit common  36 . The inductor L connects between the node N 1  and a resonating circuit node N 3 . The first capacitor C 1  connects between the node N 3  and one side of the lamp  30 . The other side of the lamp  30  connects to the node N 2 . The lamp  30  has a high impedance when it is off and a low impedance when it is on. The second capacitor C 2  connects between the node N 3  and the valley fill circuit  16 . The third capacitor C 3  connects between the node N 3  and the node N 2 .  
     [0027] The generator signal from the variable frequency power generator  14  has a lamp power frequency that is controlled by a frequency control signal, denoted by a reference number  38 . The frequency control signal  38  is provided by the controller  18 . In order to reduce the physical sizes of the inductor L and capacitors C 1 - 3 , the lamp frequency is much higher than the frequency of the AC power line  32 .  
     [0028] Referring to FIGS. 2A and 2B, before ignition when the lamp  30  is off, a starting serial circuit  42  is formed between the nodes N 1  and N 2  by an inductance of the inductor L in series with an effective capacitance  44  between the nodes N 3  and N 2 . The starting serial circuit  42  has a first resonant frequency termed a “starting resonant frequency”. The effective capacitance  44  is approximately the parallel combination of the second and third capacitors C 2  and C 3 . After ignition when the lamp  30  is conducting, the first capacitor C 1  is added in parallel to the second and third capacitors C 2  and C 3  to form an effective capacitance  46  between the nodes N 3  and N 2 . An operating serial circuit  48  between the nodes N 1  and N 2  is formed by the inductance of the inductor L and the effective capacitance  46 . The operating serial circuit  48  has a second resonant frequency termed an “operating resonant frequency”.  
     [0029] In order to start or ignite the lamp  30 , the controller  18  sets the frequency control signal  38  so that the power generator  14  provides a first or starting lamp frequency that matches the starting resonant frequency within a range of ±10%. Preferably, the starting resonant frequency is in a range of 50 to 500 kHz. The resonance of the inductor L and the effective capacitance  44  provides a very high alternating start voltage between the nodes N 3  and N 2 . The alternating start voltage is passed by the first capacitor C 1  to the lamp  30 .  
     [0030] When the start voltage becomes high enough, typically about 1000 to 3000 volts peak, gas in the lamp  30  breaks down (ignites) and the lamp  30  begins to conduct. When the lamp  30  is conducting, the starting serial circuit  42  is replaced by the operating serial circuit  46 . The operating resonant frequency is lower because the first capacitor C 1  now effectively parallels the second and third capacitors C 2  and C 3 .  
     [0031] The change from the serial circuit  42  having the starting resonant frequency to the serial circuit  46  having the operating resonant frequency causes the alternating voltage across the nodes N 3  and N 2  and across the lamp  30  to drop very rapidly. This reduction in voltage results in a gradual turn on of the lamp  30 , thereby avoiding long term damage to the lamp  30  that would result from a more rapid turn on. Preferably, the operating resonant frequency is in a range of 30 to 100 KHz or a range of two to five times lower than the starting resonant frequency. At this lower resonant frequency an alternating operating voltage between the nodes N 3  and N 2  is much lower than the alternating start voltage. In order to continue to run the lamp  30 , the controller  18  gradually adjusts the frequency control signal  38  until the lamp frequency is a second or operating lamp frequency that is slightly away, preferably above, the operating resonant frequency. The operating lamp frequency should be within a range of ±25% of the second or operating resonant frequency.  
     [0032] The valley fill circuit  16  includes a charge pump  54 , a valley storage capacitor C VS , a rectifier D 1 , and a valley mode detector  56  for providing a valley fill correction. The second capacitor C 2  acts as series capacitor for passing an input or feedback current from the node N 3  to the charge pump  54 . The charge pump  54  includes diodes D 2  and D 3  for pumping charge onto the valley storage capacitor C VS  for providing a valley fill correction voltage. In operation, the valley fill correction voltage on the capacitor C VS  is typically one-fourth to one-half the peak rectified voltage on the line  34 . When the rectified voltage on the line  34  is less than the voltage on the valley storage capacitor C VS , the rectifier D 1  passes a valley fill current to the line  34 . The valley fill current results in voltage pedestals  57  (FIG. 4) that cover the cyclic low voltages  37  (FIG. 3) in the rectified voltage on the line  34  during low voltage time period  58  (FIGS. 4, 5,  6 ).  
     [0033] Referring again briefly to FIGS.  2 A-B, the effective in-circuit capacitance of the second capacitor C 2  is slightly lower than the capacitance of the second capacitor C 2  measured by itself due to a small range of voltages when neither of the diodes D 2 - 3  is conducting. However, because sum of the diode voltage drops is small compared to the voltages of 600 to 3000 volts between the third circuit node and the second circuit node, the discrepancy is small.  
     [0034] Returning to FIG. 1, the valley fill detector  56  connects to the rectifier D 1  to sense the time periods  58  (FIGS. 4, 5,  6 ) when the valley fill current is flowing, and passes a fill detect signal to the controller  18 . Responsive to the fill detect signal, the controller  18  provides valley fill frequency modulation.  
     [0035] The controller  18  provides the valley fill frequency modulation to adjust the frequency control signal  38  to cause the power generator  14  to adjust the lamp frequency of the generator signal toward the operating resonant frequency. The lamp frequency nearer to the operating resonant frequency results in a boost current, denoted by  59  (FIG. 6), to the lamp  30  during the valley fill time periods  58  (FIGS. 4, 5,  6 ). The boost current  59  (FIG. 6) compensates for the lower voltage level of the voltage pedestals  57  (FIG. 4) in the rectified voltage on the line  34 . A further effect of the boost current  59  (FIG. 3) is that the driver network  20  acts as a current source (high impedance) to the lamp  30 , thereby providing more consistent drive power than would be provided with a voltage source (low impedance). The more consistent drive power increases the lifetime of the lamp  30  and reduces flicker. The more consistent drive also prevents the lamp  30  from cooling during the low voltages. The cooling of the lamp  30  might otherwise cause the lamp  30  to turn off spontaneously.  
     [0036] The ballast  10  also includes a voltage detector  62  and an external input  64 . The voltage detector  62  detects the voltage of the pedestal voltages  57  (FIG. 4) and the average of the high voltages, denoted by  66  (FIG. 4), of the rectified voltage on the line  34  and provides a detected voltage signal to the controller  18 . The external input  64  receives information from a user or an external controller. The external controller may include an occupancy detector using motion, sound, heat radiation, or the like, for determining whether there are any human occupants in the vicinity of the lamp  30 . The power generator  14  includes a current sensor  68 . The current sensor  68  provides a detected current signal, denoted by  72 , to the controller  18  for detecting a current, denoted by  74  (FIG. 6), flowing through the nodes N 1  and N 2 . The detected current signal  72  is indicative of the current flowing through the lamp  30 . The current  74  has high currents, denoted by  75  (FIG. 6), corresponding to the high voltages  66  (FIG. 4) and boost currents  59  (FIG. 6) during the low voltage times  58  (FIGS. 4, 5,  6 ).  
     [0037] Programming in the controller  18  monitors the external input  64 , the fill detect signal, the detected voltage signal, and the detected current signal  72  for adjusting the frequency control signal  38 . The power generator  14  uses the frequency control signal  38  for adjusting the lamp frequency of the generator signal. As the lamp frequency is adjusted toward the operating resonant frequency, the alternating operating voltage between the nodes N 3  and N 2  increases, thereby increasing the brightness of the lamp  30 ; and when the lamp frequency is adjusted away from the operating resonant frequency, the alternating operating voltage between the nodes N 3  and N 2  decreases, thereby dimming the lamp  30 .  
     [0038] The controller  18  uses the detected voltage signal to compensate for high and low line levels on the AC power line  32  to provide a constant brightness from the lamp  30 . The controller  18  uses the external input  64  to dim or increase the brightness of the lamp  30  in response to a user request and/or an indication of whether the vicinity of the lamp  30  is occupied. The controller  18  uses the average current indicated by the detected current signal  72  for estimating the brightness of the lamp  30  and adjusting the lamp frequency toward or away from the operating resonant frequency in order to adjust the lamp current to set the brightness of the lamp  30  to a desired level. The controller  18  also uses the indication of the boost currents  59  from the detected current signal  72  for adjusting the generator frequency away from the operating resonant frequency if necessary for maintaining a sufficient power factor. Typically, a power factor greater than 0.97 is considered to be sufficient.  
     [0039] The inductor L has an inductance in a range of 50 to 1000 uH (microHenrys). The first capacitor C 1  has a capacitance in a range of 0.005 to 0.056 uF (microfarads). The second capacitor C 2  has a capacitance in a range of 0.005 to 0.056 uF. The third capacitor C 3  has a capacitance in a range of 0.005 to 0.056 uF. In a preferred embodiment, the inductor L is about 270 uH (microHenrys) made with 42T on C—C ED3 core 56/256 wound with Litz wire and weighting about 6 ounces. In a preferred embodiment, the first capacitor C 1  is about 0.033 uF (microfarads). The second capacitor C 2  is about 0.01 uF. The third capacitor C 3  is about 0.01 uF. The valley storage capacitor C VS  is an electrolytic type of about 330 uF. A small low power conventional power supply converts AC line power to DC power for powering the circuitry in the ballast  10 . A housing for the ballast  10  may be constructed of plastic. The entire weight of the ballast  10  described herein using easily available components is in a range of 2 to 5 pounds.  
     [0040]FIG. 3 shows a rectified voltage, denoted by  82 , on the line  34  without the valley fill correction of the present invention. The rectified voltage  82  includes nulls for the cyclic low voltages  37 . The rectified voltage  82  is essentially the absolute level of a sine wave passing through zero at the nulls.  
     [0041]FIG. 4 shows the rectified voltage, denoted by  84 , on the line  34  with the valley fill correction. The rectified voltage  84  has the cyclic high voltages  66  and the voltage pedestals  57 . The voltage pedestals  57  cover the cyclic low voltages  37  during the low voltage time periods  58  of the rectified voltage on the line  34  with the valley fill correction voltage from the valley fill circuit  16 . Preferably, the ratio of the voltage pedestals  57  to the peaks of the high voltage  66  of the rectified voltage  84  is about one-half.  
     [0042]FIG. 5 shows current, denoted by  87 , pulled from the AC power line  32 . No line current is pulled during the low voltage time periods  58 .  
     [0043]FIG. 6 shows the lamp current  74  through the lamp  30 . The current  74  has the high currents  75  and the boost currents  59 . The boost currents  59  result from the valley fill frequency modulation during the low voltage time periods  58 .  
     [0044]FIG. 7 shows power factor placed on the AC power line  32  at mid line level versus a ratio of the level of the voltage pedestals  57  to the level of the peaks of the rectified voltage  84 . The power factors are shown on a vertical axis as 1.000 to 0.850 for pedestal/peak ratios on the horizontal axis of 0.0 to 0.8. A ratio of zero causes no degradation of the power factor so the power factor is 1.0. A ratio less than 0.5, for example a 200 volt pedestal and a 400 volt peak, results in a power factor that is better than about 0.97.  
     [0045]FIG. 8 is a simplified diagram of the power generator  14 . The power generator  14  includes a voltage controlled oscillator (VCO)  95 , a series switch S 1 , a shunt switch S 2 , and a resistor R 1 . Preferably, the switches S 1  and S 2  are power MOSFETs, for example models IRFPS37N50A. The VCO  95  issues x and x bar square wave drive voltages, where bar indicates the opposite phase, to the series switch S 1  to open and close between the rectified voltage on the line  34  and the circuit node N 1 , thereby chopping or switching or the rectified voltage  84  on and off for providing the generator signal.  
     [0046] The VCO  95  also issues y and y bar square wave drive voltages to the shunt switch S 2  to open and close between the circuit node N 1  and the resistor R 1 . The resistor R 1  connects the shunt switch S 2  to the circuit node N 2 . The x and y square wave voltages have the opposite phase. The effect of the phasing of the square wave drive voltages and the opening and closing of the switches S 1  and S 2  is to switch the rectified voltage on the line  34  on and off across the nodes N 1  and N 2  for providing the generator signal. The current detector  68  (FIG. 1) detects the voltage across the resistor R 1  to provide the detected current signal  72 . Preferably the resistor R 1  has a very low resistance, for example 0.05 Ohms.  
     [0047]FIG. 9 is a block diagram showing a valley frequency modulation system  100  of the present invention using frequency modulation for compensating for low voltage of the voltage pedestals  57  (FIG. 4) in the rectified voltage on the line  34 . The valley frequency modulation system  100  includes the power generator  14 , the valley fill circuit  16 , and the controller  18 . The power generator  14  receives the rectified signal on the line  34  and the frequency control signal  38  and provides the generator signal having the modulated lamp frequency to the network driver  20 . The network driver  20  drives the lamp  30  and provides the input or feedback current to the valley fill circuit  16 . The valley fill circuit  16  provides the valley fill current for the voltage pedestals  57  (FIG. 4) and a valley detect signal to the controller  18  during the low voltage time periods  58  when the valley fill current is flowing. The controller  18  uses the valley detect signal for providing the frequency control signal  38 . The frequency control signal  38  adjusts the operating lamp frequency closer to the operating resonant frequency in order to compensate for the lower voltages of the voltage pedestals  57  during the low voltage time periods  58 .  
     [0048]FIG. 10 is an electrical diagram of a second embodiment of a tuned lamp driver network of the present invention referred to by a reference number  120 . The driver network  120  receives the generator signal from the variable frequency power generator  14  for driving the lamp  30  and providing the feedback current to the valley fill circuit  16  as described above for the driver network  20 .  
     [0049] The driver network  120  includes the first, second and third capacitors C 1 - 3 . The second and third capacitors C 2  and C 3  form the effective capacitance  44  (FIG. 2A). The inductor L described above for the driver network  20  is replaced for the driver network  120  by a mutually coupled inductor L ab  having a first mutually coupled inductor section L a  and a second mutually coupled inductor section L b . The first mutually coupled inductor section L a  connects between the first circuit node N 1  and the third circuit node N 3 . The second mutually coupled inductor section L b  connects between the third circuit node N 3  and the second circuit node N 2  in series with the first capacitor C 1  and the lamp  30 . In a preferred embodiment the coupling ratio is 1:1.  
     [0050] Referring to FIGS. 11A and 11B, before ignition when the lamp  30  is off, a starting serial circuit  122  is formed between the nodes N 1  and N 2  by the first mutually coupled inductor section L a  in series with the effective capacitance  44  between the nodes N 3  and N 2  for the first resonant frequency or “starting resonant frequency”. After ignition when the lamp  30  is conducting, an effective capacitance  124  that is approximately the capacitance of the first capacitor C 1  times a factor that depends upon the turns ratio of the second to first mutually coupled inductor sections L a  and L b  is added in parallel to the second and third capacitors C 2  and C 3  to form an effective capacitance  126  between the nodes N 3  and N 2 . The first mutually coupled inductor section L a  in series with the effective capacitance  126  forms an operating serial circuit  128  between the nodes N 1  and N 2 . The operating serial circuit  128  has the second resonant frequency or “operating resonant frequency”.  
     [0051] The first or starting resonant frequency of the driver network  120  is lower than the second or operating resonant frequency. As an exemplary case, the first or starting resonant frequency is about 50 kHz and the second or operating resonant frequency is about 80 kHz. The starting lamp frequency should be within about ±10 of the first or starting resonant frequency and the operating lamp frequency should be within about ±25 of the second or operating resonant frequency. An effect of the mutually coupled inductor L ab  is to increase the starting and operating voltages at the third node N 3  to higher levels for driving the lamp  30 . The higher levels may be required for utility power of 120 VAC rms (as opposed to the 200 to 304 VAC rms given above) on the AC power line  32 .  
     [0052] Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.