SOLID STATE BALLAST FOR HIGH INTENSITY DISCHARGE LAMPS HAVING CONSTANT LINE AND LAMP WATTAGE

Abstract: A high efficiency, electronic ballast is taught for high intensity discharge (HID) lamps in wattages of 50W to 1000W or greater. The ballast is powered by either 240 or 115 volts, 50-60 hertz, of alternating current. The input alternating current is rectified so that 320 - 340 VDC powers the lamp. The ballast maintains constant wattage to the lamp and constant line wattage. The ballast also has an undervoltage protection circuit, a voltage spike protector, and a radio frequency interference (RFI) filter.

Description: SOLID STATE BALLAST FOR HIGH INTENSITY DISCHARGE LAMPS HAVING CONSTANT LINE AND LAMP WATTAGE Field of the Invention; This invention relates to the field of electronic, solid state ballasts for high-intensity discharge lamps. Background of the Invention: In high" * intensity discharge lamps, light is generated when an electric current is passed through a gaseous medium. The lamps have variable resistance characteristics that require operation in conjunction with a ballast to provide appropriate voltage and current limiting means. Control of the voltage, frequency, and current supply to the lamps is necessary for proper operation and determines the efficiency of the lamps. In particular, it determines the size and weight of the required ballast. The appropriate voltage, frequenc and current for efficient running of a lamp in its normal operating state is not appropriate for the lamp during its warm-up state. A high-intensity lamp typically takes several minutes to warm up from the time it is struck or turned on to its normal operating state. Initially, the lamp is an open circuit. Short pulses of current are sufficient to strike the lamp, provided they are of adequate voltage. Subsequent to striking, the lamp's resistance drops radically. The resistance then slowly rises during warmup to its normal operating ^ level. Hence,- ' subsequent to striking and during warm-up the current of the lamp must be limited to prevent internal lamp damage. Summary of the Invention; An efficient, electronic ballast is taught for high intensity discharge (HID) lamps in wattages of 50W to 1000W or greater. The ballast is powered by either 240 or 115.volts, 50-60 hertz, of alternating current. The input alternating current is rectified so that 320 - 340 VDC powers the lamp. The ballast maintains constant wattage to the lamp by using a known, reference lamp voltage at a specified current and a current integration feedback loop. The feedback loop monitors the lamp current and causes the width of the DC pulse to the lamp to be varied accordingly. The ballast according to the present invention maintains a constant line wattage by maintaining constant wattage in the lamp and by using a circuit configuration that yields a ballast efficiency (power out/power in) of greater than 90%. That is, the power dissipated by the ballast is less than 10% of the input power. Line voltage variations of ± 10% result in lamp wattage variations of only ± 1.5%, in line wattage variations of less than ±2.5%, and only in a ± 1.0% variation in ballast power dissipation. Prior art wire iron constant wattage ballasts typically have ±5% line wattage variations when the line voltage varies by ± 10%. Therefore, it is apparent that the present invention achieves line wattage variations which are much less than such prior art devices. The present invention also maintains a constant lamp wattage by maintaining a constant, regulated current in the lamp circuit. The lamp circuit current is regulated by the current feedback loop as described herein. The voltage across the lamp is constant at the regulated current. resulting in a constant lamp wattage. The amount of lamp wattage regulation increases as the line voltage increases. The ballast also has an undervoltage protection circuit, a voltage spike protector, and a radio frequency interference (RFI) filter. • Since the ballast provides very pure direct current to the lamp, the lamp does not have the strobe effect typical of lamps powered by alternating current. This makes the ballast particularly suitable for lighting sporting events and work areas having fast moving equipment. It is a feature of the present invention to provide an AC line-powered ballast for HID lamps whereby line wattage is kept constant without use of special circuitry. It is another feature of the present invention to precisely regulate the input signal to the ballast by using a RFI filter, and undervoltage and voltage spike protection circuits. These and other features of the invention will be apparent-to those skilled in the art by reference to ' the drawings and the detailed description. Brief Description of the Drawings; Fig 1 is a block diagram illustrating the control sequences of a preferred embodiment of the present invention. Figs 2A and 2B are circuit diagrams of the embodiment that uses 240 VAC as the input current. Fig ' s 3A and 3B are circuit diagrams of the embodiment that uses 115 VAC as the input current. Fig 4 is a block diagram of pulse width control subcircuit 52. Detailed Description; As shown in Fig 1, the ballast is powered by alternating current. In one embodiment, the input current is 240 volts, 50 - 60 hertz AC. See Figs. 2A and 2B. In another embodiment, the input current is 115 volts, 50 - 60 hertz AC. See Figs 3A and 3B. A spike protector 10 prevents intermittent high voltages or spikes from reaching the ballast. Also, the ballast contains a radio frequency interference (RFI) filter 11 to prevent RFI signals generated by the ballast from being picked up on the power line. RFI filter 11 is comprised of capacitors 52, 53 and 54, and inductors 64 and 65. If the input is 240 VAC, bridge rectifier 12, in combination with capacitor 55, then rectifies the alternating current into direct current waves of 340 VDC. If the input is 115 VAC, two half-wave bridge rectifiers are connected as a voltage doubler to provide 320 VDC to the ballast. See Figs. 3A and 3B. Referring to Fig. 1, low voltage power supply 13, fed by input from rectifier 12, supplies 15 VDC to oscillator 14, dead- time controller 15, and pulse width modulator 16. Low voltage power supply 13 is comprised of resistor 28, capacitor 58, and 15 volt zener diode 48. See Fig. 2A. As more fully discussed below, undervoltage protector 27 shuts off current to lamp 23 if the input line voltage drops below a safe limit. The ballast also contains a 5 VDC reference power supply 88 (Fig. 4) that regulates the voltage output of pin 14 of subcircuit 52. Referring again to Fig. 1, oscillator 14, dead time controller 15, and pulse width modulator 16, together with switch control 18, form the means for driving hexfet switches 17A and 17B. The frequency of oscillator 14 determines the frequency of the direct current waves in the lamp circuit. The high frequency waves produced by oscillator 14 are supplied to dead time controller 15 and pulse width modulator 16. Pulse width modulator 16 is also supplied with input from a current integration feedback loop 19 and from ambient light sensor 20. Based upon the current sensed by current sensor 22A, current integration feedback loop 19 determines whethe the current to hexfets 17A and 17B exceeds a reference value. If so, feedback loop 19 sends a signal to pulse width modulator 16 causing it to vary its output signal accordingly. Ambient light sensor 20 senses the amount of light present in the surroundings and sends a signal to pulse width modulator 16, causing it to output a zero pulse if th sensed amount of ambient light is greater than th.e fixed value. This turns off lamp 23. Ambient light sensor 20 does not affect the output of pulse width modulator 16 if the ambient light is greater than the fixed value. When ambient light sensor 20 detects ambient light below the fixed value, dead time controller 15 produces a modulated output signal that corresponds to a maximum duty cycle of slightly less than 100%. Dead time controller 15 provides a dead time between the direct current waves. Switch βontrol 18 combines the output of dead time controller 15, pulse width modulator 16 and * feedback loop 19, and sends the wave forms to hexfets 17A and 17B. Switch control 18 also controls the frequency at which hexfets 17A and 17B are switched on and off. This frequency corresponds to the frequency of oscillator 14. Referring now to Fig. 2A, the ballast utilizes a pulse width control subcircuit 52. One suitable, commercially available IC chip is a Motorola TL 494. Use of the TL 494 is convenient but not necessary. Fig. .4 is a block diagram of subcircuit 52, using a TL 494 IC chip. As shown by Fig. 4, subcircuit 52 includes the following components: 1. Pulse width modulator 16; 2. On-chip oscillator 14; 3. Two user available operational amplifiers, error amplifiers 86 and 87; 4. An internal 5 VDC reference power supply 88; 5. Variable dead time controller 15. Flip-flop 89, depicted in Fig. 4, is disabled by grounding pin 13 of the TL 494. This permits the TL 494 to be used in its single-ended operating mode (i.e., as oppose to its push-pull operating mode) to achieve the higher output-drive currents required in the present invention. Referring to Fig. 2B, the grounding of pin 13 causes the output pulse train of the two output transistors 70 and 71 to operate in parallel. Referring again to Fig. 2A, the frequency of oscillator 14 is controlled by resistor 38 and capacitor 61.. Oscillator 14 develops a frequency equal to 1.1 RC which, in a preferred embodiment, equals 65 kilocycles per second. This frequency corresponds to a repetitive period of 15 microseconds. The output at pin 10 of subcircuit 52 is 15 VDC. The collectors of output transistors 70 and 71 (Fig. 2B) are connected to the 15 VDC power supply. The emitter of transistor 70 is not connected. The emitter of transistor 71 develops a 15 VDC signal at pin 10 of subcircuit 52. The period of this signal corresponds to 95% of the repetitive period of oscillator 14. Dead time controller 15 limits the maximum period of the +15 VDC signal to pin 10 to 52% of the repetitive period of oscillator 14, or 7.8 microseconds. Error amplifiers 86 and 87 (Fig. 4) are used to control the pulse width of this 7.8 microsecond signal. Error amplifier 87 operates as a Schmitt trigger and performs the function of an on/off switch. Its output voltage is a function of the input from a voltage divider containing ambient light sensor 20. Error amplifier 87 turns pulse width modulator 16 to an "off" state when ambient light sensor 20 senses that it is not dark outside. Error amplifier 87 does not affect the output of pulse width modulator 16 at all when it is dark outside. A current integration feedback loop 19 (Fig. 1) is use to control the current to the lamp. Feedback loop 19 operates in the following manner. Error amplifier 86 sense the voltage developed across resistor 46. Referring to Figs. 2A and 4 this voltage is integrated by means of resistors 29 and 30, diode 49, and capacitor 59. The junction of resistors 29 and 30 is connected to the + input of error amplifier 86. The - input of error amplifier 86 i connected to the voltage developed across resistor 39, whic is the reference voltage. This reference voltage is used t set the root-mean-square (RMS) current in the lamp circuit. Error amplifier 86 controls the period of the 7.8 microsecond pulse from zero to 7.8 microseconds, thereby controlling the current flowing in the lamp circuit. The +15 VDC signal at pin 10 of subcircuit 52 is also used to drive the gates of hexfet switches 17A and 17B, causing them to go into conduction. When the signal at pin 10 is reduced to zero, output transistor 71 conducts, thereby discharging the internal gate capacitances of hexfets 17A and 17B. This configuration generates turn-off times for hexfets 17A and 17B of 100 nanoseconds or less, resulting in minimum switching power dissipation by hexfets 17A and 17B. In another embodiment (not shown) , resistor 46 is replaced by a current transformer. This replacement may be desirable since resistor 46 places an impedance on the sources of hexfets 17A and 17B, which causes degenerative feedback. This makes hexfets 17A and 17B more difficult to drive. If resistor 46 is replaced by a current transformer, the transformer's primary winding is used to sense the current in the lamp circuit. Its secondary voltage is stepped up to 2.5 volts, resulting in more reliable current sensing and the elimination of the temperature variations that are otherwise present in diode 49. A suitable transformer is a toroidal ferrite core transformer that is 0.5 inches in diameter, having one or two turns in the primary, winding and a step up secondary winding. Refering to Fig. 2B, current to lamp 23 is provided by the lamp circuit by use of a step down converter or down switcher as discussed below. When lamp 23 is not conducting, capacitor 62 charges to 340 volts, which strike the lamp. After lamp 23 is struck it goes into conduction. Hexfets 17A and 17B are used to switch 340 VDC across inductor 80 and lamp 23 in repetitive cycles. If the input current is 115 VAC, then 320 VDC is switched across lamp 23 These repetitive cycles result in a linear ramp current in inductor 80 that reaches a known peak value which depends upon the lamp manufacturer's recommended magnitude.- Error amplifier 86 senses whether the value current has been reached. If it has been reached, error amplifier 86 causes pin 10 of subcircuit 52 to go to zero, which- drives hexfets 17A and 17B out of conduction. Thus, the regulated current to lamp 23 is kept constant, so that the bulb wattage is also kept .constant. - When hexfets 17A and 17B go out of conduction, the magnetic field that was created in inductor 80 collapses, causing the anode of diode 51 to go positive. This discharges inductor 80 through capacitor 62 and lamp 23. Since inductor 80 is now discharged, it is ready to receive more current. Capacitor 62 filters the voltage across lamp 23 and also provides inductor 80 with a return path if lamp 23 goes out of conduction. The ballast of the present invention also has an undervoltage protector 27. The purpose of undervoltage protector 27 is to prevent damage to hexfets 17A and 17B in the event that the input line voltage drops to a point wher the output of low voltage power supply 13 would also drop. At that point, the voltage to the gates of hexfets 17A and 17B would be reduced when they are under a full current load, resulting in such increased power dissipation by hexfets 17A and 17B as to possibly destroy them. The undervoltage protector 27 operates as follows. Referring to Fig. 2A, when the voltage across zener diode 4 drops to a dangerously low level, transistor 71 conducts. This brings pin 4 of subcircuit 52 to +5 volts, which, by dead time control, reduces the repetitive period of the lamp's duty cycle to zero. This cuts off all current to lamp 23. The hot lamp cannot relight at this "point because the lamp's striking voltage is then greater then the voltag that the circuit is able to supply. The ballast described herein is able to achieve efficiencies of greater than 90%. At the same time, the input line wattage remains constant to within ± 2.5%, even though the line voltage varies by ± 10%. The following example illustrates typical values that may be achieved by the present invention: EXAMPLE Ballast Rating - 175 watts Input Power (optimal) - 185 watts Lamp Power (optimal) - 175 watts Line Line Laπp Ballast Ballast % Deviation % Deviation Input Wattage Wattage Efficiency Wattage of Input of Lamp Voltage % Line Watts Wattage 115VAC 185 172 92.97 13.0 0 0 125VAC 188.83 174.58 92.45 14.25 +2.07 +1.5 105VAC 181.08 169.42 93.56 11.66 -2.12 -1.5 Several elements or features of- the present inventio contribute to its high efficiency. These include the following: A. Rectification of the AC input line voltage to a voltage of sufficient magnitude to strike the HID lamp. I the lamp is a mercury vapor, a DC voltage , of at least 300 volts is required. E. The use of a step down or buck converter circuit to apply power to the lamp. Other electronic ballasts, including those employing a push-pull circuit, use a step converter. A step up converter must draw approximately two times the lamp current from the power supply to power the lamp. For a 175 watt lamp, a step up converter circuit requires approximately 1.3 Amps. However, a step down converter, like that used in the present invention, requires much less current to power the lamp. For example, when the present invention is used to power a 175 watt, 135 volt mercury lamp, the current drawn from the power supply is only about one-half of the current through the lamp. It is thus apparent that the use of a step down converter results in a higher ballast efficiency. The particular elements of the step down converter circuit that contribute to its efficiency include the use of: 1) One magnetic element or inductor 80; 2) A single switching element, which may be wired in parallel for higher current capability; 3) Current integration feedback loop 19 to regulate the current in the lamp circuit; 4) A diode 51 connected from the output of hexfet switches 17A and 17B to the power supply; 5) A capacitor 62 across the load, lamp 23; 6) An operating frequency in the range of 65 to 75 kilocycles per second. C. The use of hexfet or mosfet switches to switch current across the lamp. Such switches require very small amounts of power to turn them on and off. Bipolar switching devices would have difficulty achieving an efficiency similar to that of a hexfet or mosfet switch. D. The use of a drum core inductor 80 having multiple strands of Litz wire for increased efficiency. E. The use of an integrated circuit for pulse width control subcircuit 52. The integrated circuit requires very little power. F. The use of a switching element whose total on time is less than 50%. The two switches used in a typical push-pull electronic ballast each have an on time of slightly less than 50%, for a total on time of nearly 100% G. The peak current in the hexfet switches 17A and 17B and in inductor 80 is about two times the average current through the lamp. In a typical push-pull electron ballast, the peak current is on the order of four times th average current through the lamp.

Claims: WHAT IS CLAIMED IS: 1. A constant line and lamp wattage ballast for high intensity discharge lamps, comprising: a direct current source; a lamp circuit containing a high intensity discharge lamp and an inductor in series with sai lamp; a current feedback means for sensing the current present in said lamp circuit, comparing said current with a reference value, and generating an output signal; a pulse- * ,width modulator which, in response to said output signal of said current feedback means, varies the width of the direct current pulse that powers said lamp; a high frequency oscillator; a switch which gates direct current to said lamp; and a switch control means, responsive to said oscillator, for controlling said switch and thereby controlling the frequency of the direct current pulses that power said lamp. 2. A constant line and lamp wattage ballast for high intensity discharge lamps, comprising: a direct current source; a lamp circuit containing a high intensity discharge lamp; an undervoltage protector means for sensing the lamp circuit voltage, comparing it with a predetermined value, and extinguishing power to said lamp if the lamp circuit voltage is less than the predetermined value; a current feedback means for sensing the current present in said lamp circuit, comparing said current with a reference value, and generating an output signal; a pulse width modulator which, in response to said output signal of said current feedback means, varies the width of the direct current pulse that powers said lamp; a high frequency oscillator; a switch which gates direct current to said lamp; and a switch control means, responsive to said oscillator, for controlling said switch and thereby controlling the frequency of the direct current pulses that power said lamp. 3. The ballast of Claims 1 or 2, further comprising: a spike protector means for preventing damage to said lamp circuit caused by intermittent high voltage. 4. The ballast of Claims 1 and 2, further comprising: a radio frequency interference (RFI) filter which removes RFI signals from the ballast circuitry. 5. The ballast of Claims 1 and 2, further comprising: a dead time controller whose output signal causes said pulse width modulator to vary the width of the direct current pulse that powers said lamp by a predetermined value. 6. The ballast of Claims 1 and 2, further comprising: a low voltage direct current source which provides power to said pulse width modulator and to said high frequency oscillator. 7. The ballast of Claims 1 and 2, further comprising: an ambient light sensor which causes said pulse width modulator to output a zero pulse when the ambient light sensed by said sensor is greater that a fixed value. 8. A constant line and lamp wattage ballast for high intensity discharge lamps, comprising: a direct current source; a lamp circuit, containing: a high intensity discharge lamp; an inductor connected in series with said lamp; a capacitor connected across said lamp; - a switch that gates direct current to said lamp; a diode interconnected with said direct current source and the output of said switch; a current feedback means for sensing the current present in said lamp circuit, comparing said current with a reference value, and generating an output signal; a pulse width modulator which, in response to said output signal of said current feedback means, varies the width of the direct current pulse that powers said lamp; a high frequency oscillator; and a switch control means, responsive to said oscillator, for controlling said switch and thereby controlling the frequency of the direct current pulses that power said lamp. 9. A constant line and lamp wattage ballast for high intensity discharge lamps, comprising: a direct current source; a high intensity discharge lamp; a step down converter, containing: an inductor connected in series with said lamp; a capacitor connected across said lamp; a switch that gates direct current to said lamp; a diode interconnected with said direct current source and the output of said switch; a current feedback means for sensing the current present in said converter, comparing said current with a reference value, and generating an output signal; a high frequency oscillator; a pulse width modulator which, in response to said output signal of said current feedback means, - varies the width of the direct current pulse that powers said lamp; and a switch control means, responsive to said oscillator, for controlling said switch and thereby controlling the frequency of the direct current pulses that power said lamp. 10. The ballast of claims 1, 2, 8 or 9 wherein said direct current source is comprised of: an alternating current source; and means for rectifying the current from said alternating current source into waves of direct current. 11. The ballast of claims 1, 8 ot 9 wherein said inductor is of the drum core type and has multiple strands of wire. 12. The ballast of claims 1, 2, 8 or 9 wherein the frequency of said high frequency oscillator is in the range of 65 to 75 kilocycles per second.

Inventors: OTTENSTEIN SIDNEY A, OTTENSTEIN, SIDNEY, A.
Applicants: INNOVATIVE CONTROLS INC, INNOVATIVE CONTROLS, INC.
Application year: 1987
Main classification: H05B41
IPC codes: H05B41