Abstract:
A ballast configured for multiple parallel lamp operation includes a system for detecting unbalanced loads or absent lamps in a parallel multiple lamp ballast. The ballast includes one or more starting transformers having windings, each winding connected in series with a lamp connected to the ballast. A winding of one or more of the starting transformers that is not connected in series with one of the lamps is monitored for a voltage in excess of a threshold. A voltage in excess of the threshold across the winding is indicative of an unbalanced load (e.g., an absent or broken lamp). If a voltage above the threshold is detected across the monitored load, the ballast ceases powering the lamps.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to and hereby incorporates by reference in its entirety U.S. Provisional Patent Application Ser. No. 61/856,245 entitled “A METHOD TO DETECT UNEVEN AC LOAD OR PARALLEL LOAD REMOVAL” filed on Jul. 19, 2013. 
    
    
     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 reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to parallel multiple lamp ballasts. More particularly, the invention relates to protecting parallel multiple lamp ballasts from unsafe high voltage conditions following removal of a lamp from connection with the ballast and protecting parallel multiple lamp ballasts from unbalanced loads which can affect reliability of the ballast. 
     Parallel multiple lamp ballasts are widely used due to their low cost and low load impedance, especially when operating at dimmed output levels. Sensing removal of a lamp or of unbalanced lamp currents is difficult and expensive because the lamps are connected in parallel with respect to an oscillator (e.g., inverter) and resonant tank of the ballast. This difficulty is amplified when the ballast is operating at a dimmed output level. If a lamp is removed from electrical connection with the ballast during operation of the ballast, the voltage at the contacts that the lamp was removed from will become excessive and unsafe if the ballast continues operating normally. Further, unbalanced loads can cause breakdown in various components of the ballast if the unbalanced load becomes too great (e.g., when a lamp is removed from connection with the ballast while the ballast is operating). This can reduce the reliability of parallel multiple lamp ballasts. 
     BRIEF SUMMARY OF THE INVENTION 
     Aspects of the present invention provide a system for detecting unbalanced loads or absent lamps in a parallel multiple lamp ballast. The ballast includes one or more starting transformers having windings, each winding connected in series with a lamp connected to the ballast. A winding of one or more of the starting transformers that is not connected in series with one of the lamps is monitored for a voltage in excess of a threshold. A voltage in excess of the threshold across the winding is indicative of an unbalanced load (e.g., an absent or broken lamp). If a voltage above the threshold is detected across the monitored load, the ballast ceases powering the lamps. 
     In one aspect, a ballast configured to connect to a plurality of lamps and provide an alternating current (AC) power to the plurality of lamps includes an oscillator, a starting transformer, and an unbalanced load sensing circuit. The oscillator receives direct current (DC) power from the DC power rail and provides the AC power to the plurality of lamps. The starting transformer includes a primary winding, a first secondary winding, and a second secondary winding. The primary winding of the starting transformer is configured to connect in series with a first lamp of the plurality of lamps. The first secondary winding of the starting transformer is configured to connect in series with a second lamp of the plurality of lamps. The unbalanced load sensing circuit is connected to the second secondary winding of the starting transformer. The unbalanced load sensing circuit monitors a voltage across the second secondary winding of the starting transformer and provides a shut down signal to the oscillator when the voltage across the second secondary winding of the starting transformer exceeds a threshold. The oscillator is further configured to cease providing the AC power to the plurality of lamps in response to receiving the shut down signal from the unbalanced load sensing circuit. 
     In another aspect, a ballast is configured to connect to plurality of lamps, provide AC power to the plurality of lamps, and cease providing AC power to the plurality of lamps in response to a missing lamp or uneven lamp current. The ballast includes an oscillator, a first starting transformer, a second starting transformer, a third starting transformer, and an unbalanced load sensing circuit. The first starting transformer includes a primary winding and a first secondary winding. The primary winding of the first starting transformer connects in series with a first lamp of the plurality of lamps. The first secondary winding of the first starting transformer connects in series with a second lamp of the plurality of lamps. The second starting transformer has a primary winding and a first secondary winding. The primary winding of the second starting transformer connects in series with the first secondary winding of the first starting transformer and the second lamp of the plurality of lamps. The first secondary winding of the second starting transformer connects in series with a fourth lamp of the plurality of lamps. The third starting transformer has a primary winding and a first secondary winding. The primary winding of the third starting transformer connects in series with a third lamp of the plurality of lamps. The first secondary winding of the third starting transformer connects in series with the primary winding of the first starting transformer and the first lamp. The unbalanced load sensing circuit is connected to a secondary winding of one of the starting transformers. The unbalanced load sensing circuit monitors a voltage across the connected second secondary winding of the starting transformer and provides a shut down signal to the oscillator when the voltage across the connected second secondary winding of the starting transformer exceeds a threshold. The oscillator ceases providing the AC power to the plurality of lamps in response to receiving the shut down signal from the unbalanced load sensing circuit. 
     In another aspect, a light fixture is configured to receive power from a power source and provide power to a plurality of lamps. The light fixture includes an input stage, a ballast, and a housing. The input stage is configured to connect to the power source and provide a DC power rail. The housing supports the input stage, the ballast, and the plurality of lamps. The ballast is configured to connect to the plurality of lamps and provide alternating current power to the plurality of lamps. The ballast includes an oscillator, a starting transformer, and an unbalanced load sensing circuit. The oscillator receives DC power from the DC power rail and provides the AC power to the plurality of lamps. The starting transformer includes a primary winding, a first secondary winding, and a second secondary winding. The primary winding of the starting transformer connects in series with a first lamp of the plurality of lamps. The first secondary winding of the starting transformer connects in series with a second lamp of the plurality of lamps. The unbalanced load sensing circuit is connected to the second secondary winding of the starting transformer. The unbalanced load sensing circuit monitors a voltage across the second secondary winding of the starting transformer and provides a shut down signal to the oscillator when the voltage across the second secondary winding of the starting transformer exceeds a threshold. The oscillator ceases providing the AC power to the plurality of lamps in response to receiving the shut down signal from the unbalanced load sensing circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  a block diagram and partial schematic diagram of a light fixture including a parallel multiple lamp ballast having an unbalanced load sensing circuit configured to monitor multiple starting transformers. 
         FIG. 2  a block diagram and partial schematic diagram of a light fixture including a parallel multiple lamp ballast having an unbalanced load sensing circuit configured to monitor a single starting transformer. 
     
    
    
     Reference will now be made in detail to optional embodiments of the invention, examples of which are illustrated in accompanying drawings. Whenever possible, the same reference numbers are used in the drawing and in the description referring to the same or like parts. 
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     To facilitate the understanding of the embodiments described herein, a number of terms are defined below. The terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as “a,” “an,” and “the” are not intended to refer to only a singular entity, but rather include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as set forth in the claims. 
     The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. Conditional language used herein, such as, among others, “can,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
     The terms “coupled” and “connected” mean at least either a direct electrical connection between the connected items or an indirect connection through one or more passive or active intermediary devices. 
     The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. 
     The terms “power converter” and “converter” unless otherwise defined with respect to a particular element may be used interchangeably herein and with reference to at least DC-DC, DC-AC, AC-DC, buck, buck-boost, boost, half-bridge, full-bridge, H-bridge or various other forms of power conversion or inversion as known to one of skill in the art. 
     Terms such as “providing,” “processing,” “supplying,” “determining,” “calculating” or the like may refer at least to an action of a computer system, computer program, signal processor, logic or alternative analog or digital electronic device that may be transformative of signals represented as physical quantities, whether automatically or manually initiated. 
     As used herein, “ballast” and “driver circuit” refer to any circuit for providing power (e.g., current) from a power source to a light source. Additionally, “light source” refers to one or more light emitting devices such as fluorescent lamps, high intensity discharge lamps, incandescent bulbs, and solid state light-emitting elements such as light emitting diodes (LEDs), organic light emitting diodes (OLEDs), and plasmaloids. Further, “connected between” or “connected to” means electrically connected when referring to electrical devices in circuit schematics or diagrams. 
     Referring to  FIG. 1 , a light fixture  100  receives power from a power supply  108  and provides power to a plurality of lamps (e.g., a first lamp L 1 , a second lamp L 2 , a third lamp L 3 , and a fourth lamp L 4 ). The light fixture  100  includes an input stage  106 , a ballast  104 , and a housing  102 . The housing  102  supports the input stage  106 , the ballast  104 , and the plurality of lamps. In one embodiment, the housing  102  is configured to support the lamps in electrical contact with the ballast  104 . The input stage  106  connects to the power supply  108  and provides a direct current (DC) power rail V_RAIL. In one embodiment, the power supply  108  is an alternating current (AC) power line (e.g., 120 V at 60 Hz), and the input stage  106  is a power factor correcting AC to DC converter. In one embodiment, the lamps of the plurality of lamps are fluorescent lamps, and the lamps are connected in parallel to the ballast  104 . 
     The ballast  104  connects to the plurality of lamps and provides power to the plurality of lamps. The ballast  104  includes an oscillator VAC, a starting transformer T 1 , and an unbalanced load sensing circuit  110 . The oscillator receives DC power from the DC power rail V_RAIL and provides the AC power to the plurality of lamps. The starting transformer T 1  includes a primary winding T 1 _P, a first secondary winding T 1 _S 1 , and a second secondary winding T 1 _S 2 . The primary winding T 1 _P of the starting transformer T 1  connects in series with the first lamp L 1  of the plurality of lamps when the first lamp L 1  is connected to the ballast  104  (e.g., when the first lamp L 1  is inserted into the housing  102  of the fixture  100 ). The first secondary winding T 1 _S 1  of the starting transformer T 1  connects in series with the second lamp L 2  of the plurality of lamps when the second lamp L 2  is connected to the ballast  104 . In one embodiment, the primary winding T 1 _P and the first secondary winding T 1 _S 1  of the starting transformer T 1  have approximately the same number of turns. In another embodiment, all of the windings of the first transformer T 1  (i.e., the primary winding T_P, the first secondary winding T 1 _S 1 , and the second secondary winding T 1 _S 2 ) have approximately the same number of turns. 
     The unbalanced load sensing circuit  110  is connected to the second secondary winding T 1 _S 2  of the starting transformer T 1 . The unbalanced load sensing circuit  110  monitors a voltage across the second secondary winding T 1 _S 2  of the starting transformer T 1  and provides a shut down signal SHUT_DOWN to the oscillator VAC when the voltage across the second secondary winding T 1 _S 2  of the starting transformer T 1  exceeds a threshold. The oscillator VAC is configured to cease providing the AC power to the plurality of lamps in response to receiving the shut down signal SHUT_DOWN from the unbalanced load sensing circuit  110 . 
     In one embodiment, the unbalanced load sensing circuit  110  includes a first Zener diode Z 1 , a first diode D 1 , and an output capacitor C 6 . The second secondary winding T 1 _S 2  of the starting transformer T 1  has a first terminal and a second terminal. The oscillator VAC has a first terminal and a second terminal. The second terminal of the second secondary winding T 1 _S 2  of the starting transformer T 1  is connected to the second terminal of the oscillator VAC. The first Zener diode Z 1  has a threshold voltage, an anode, and a cathode. The cathode of the first Zener diode Z 1  is connected to the first terminal of the second secondary winding T 1 _S 2  of the starting transformer T 1 . The first diode D 1  has an anode and a cathode. The anode of the first diode D 1  is connected to the anode of the first Zener diode Z 1 . The output capacitor C 6  is connected between the cathode of the first diode D 1  and the second terminal of the second secondary winding T 1 _S 2  of the starting transformer T 1 . The cathode of the first diode D 1  provides the shut down signal SHUT_DOWN to the oscillator VAC when the voltage across the second secondary winding T 1 _S 2  of the starting transformer T 1  exceeds the threshold voltage of the first Zener diode Z 1 . 
     In the embodiment shown in  FIG. 1 , the ballast  104  includes a second starting transformer T 2  and a third starting transformer T 3 . The second starting transformer T 2  has a primary winding T 2 _P, a first secondary winding T 2 _S 1 , and a second secondary winding T 2 _S 2 . The primary winding T 2 _P of the second starting transformer T 2  connects in series with the first secondary winding T 1 _S 1  of the first starting transformer T 1  and the second lamp L 2  when the second lamp L 2  is connected to the ballast  104 . The first secondary winding T 2 _S 1  of the second starting transformer T 2  connects in series with a fourth lamp L 4  of the plurality of lamps when the fourth lamp L 4  is connected to the ballast  104 . 
     The third starting transformer T 3  has a primary winding T 3 _P, a first secondary winding T 3 _S 1 , and a second secondary winding T 3 _S 2 . The primary winding T 3 _P of the third starting transformer T 3  connects in series with a third lamp L 3  of the plurality of lamps when the third lamp L 3  is connected to the ballast  104 . The first secondary winding T 3 _S 1  of the third starting transformer T 3  connects in series with the primary winding T 1 _P of the first starting transformer T 1  and the first lamp L 1  when the first lamp L 1  is connected to the ballast  104 . In one embodiment, the primary winding T 1 _P of the first starting transformer T 1 , the first secondary winding T 1 _S 1  of the first starting transformer T 1 , the primary winding T 2 _P of the second starting transformer T 2 , the first secondary winding T 2 _S 1  of the second starting transformer T 2 , the primary winding T 3 _P of the third starting transformer T 3 , and the first secondary winding T 3 _S 1  of the third starting transformer T 3  each have approximately the same number of turns. In one embodiment, the second secondary windings of the starting transformers have the same number of turns as the primary windings and first secondary windings of the starting transformers. In operation, the first, second, and third starting transformers cooperate to balance current between the lamps. Because the current is the approximately the same through the transformer windings in a balanced load condition, the second secondary windings of the first, second, and/or third starting transformers generally have little to no voltage across them. 
     In the embodiment of the unbalanced load sensing circuit  110  shown in  FIG. 1 , the unbalanced load sensing circuit  110  monitors a voltage of the second secondary winding of each of the first transformer T 1 , the second transformer T 2 , and the third transformer T 3 . The unbalanced load sensing circuit  110  is connected to the second secondary winding T 1 _S 1  of the first starting transformer T 1 , the second secondary winding T 2 _S 2  of the second starting transformer T 2 , and the second secondary winding T 3 _S 2  of the third starting transformer T 3 . The unbalanced load sensing circuit  110  provides the shut down signal SHUT_DOWN to the oscillator VAC when the voltage across any of the second secondary windings exceeds the threshold (e.g., a threshold of a Zener diode associated with each of the second secondary windings). 
     In one embodiment, the second secondary winding T 2 _S 2  of the second starting transformer T 2  has a first terminal and a second terminal, and the second secondary winding T 3 _S 2  of the third starting transformer T 3  has a first terminal and a second terminal. The second terminal of the second secondary winding T 2 _S 2  of the second starting transformer T 2  is connected to the second terminal of the oscillator VAC, and the second terminal of the second secondary winding T 3 _S 2  of the third starting transformer T 3  is connected to the second terminal of the oscillator VAC. In one embodiment, the unbalanced load sensing circuit  110  further includes a second Zener diode Z 2 , a third Zener diode Z 3 , a second diode D 2 , and a third diode D 3 . The second Zener diode Z 2  has a threshold voltage, and anode, and a cathode. The cathode of the second Zener diode Z 2  is connected to the first terminal of the second secondary winding T 2 _S 2  of the second starting transformer T 2 . The third Zener diode D 3  has a threshold voltage, and anode, and a cathode. The cathode of the third Zener diode Z 3  is connected to the first terminal of the second secondary winding T 3 _S 2  of the third starting transformer T 3 . The second diode D 2  has an anode and a cathode, and the anode of the second diode D 2  is connected to the anode of the second Zener diode D 2 . The third diode D 3  has an anode and a cathode, and the anode of the third diode D 3  is connected to the anode of the third Zener diode D 3 . The first terminal of the bleed down resistor R 1  is connected to the cathode of the first diode D 1 , the cathode of the second diode D 2 , and the cathode of the third diode. 
     In one embodiment, the threshold voltage of the first Zener diode Z 1 , the threshold voltage of the second Zener diode Z 2 , and the threshold voltage of the third Zener diode D 3  are all approximately the same threshold voltage, and the cathode of the first diode D 1  (i.e., the first terminal of the output capacitor C 6 ) provides the shut down signal SHUT_DOWN to the oscillator VAC when the voltage across the second secondary winding of any of the first second or third starting transformers exceeds the common threshold voltage of the Zener diodes. The Zener diodes thus set the sensing threshold of the unbalanced load sensing circuit  110 . The diodes (e.g., D 1 , D 2 , and D 3 ) prevent negative current at output capacitor C 6  while the bleeding resistor R 1  discharges the output capacitor C 6  during normal operating conditions. 
     When a lamp is removed or damaged, a large voltage appears at the monitored second secondary winding(s), the threshold of the corresponding Zener diode is exceeded, and a large charge develops at the output capacitor C 6 , providing the shut down signal SHUT_DOWN to the oscillator VAC. The threshold voltage of the Zener diode(s) is selected in conjunction with the turns ratio between the monitored second secondary winding and the primary winding and/or first secondary winding such that small transient voltage spikes during normal operation of the ballast  104  do not exceed the threshold of the Zener diode(s) and lead to inadvertent shut down of the ballast  104 . 
     In one embodiment, the ballast  104  further includes a resonant tank circuit  112  connected between the oscillator VAC and the plurality of lamps. The resonant tank circuit  112  includes a resonant inductor L 5  and a resonant capacitor C 5 . The resonant inductor L 5  has a first terminal connected to the first terminal of the oscillator VAC. The resonant capacitor C 5  has a first terminal connected to the second terminal of the resonant inductor L 5  and a second terminal connected to the second terminal of the oscillator VAC. 
     In one embodiment, the ballast  104  further includes a plurality of lamp capacitors connected between the resonant tank  112  and the plurality of lamps. A first lamp capacitor C 1  of the plurality of lamp capacitors is connected in series with the first lamp L 1  and the primary winding T 1 _P of the starting transformer T 1  between the first terminal and the second terminal of the resonant capacitor. For example, the first lamp capacitor C 1  connects the second terminal of the resonant inductor L 5  to the first lamp L 1  when the first lamp L 1  is connected to the ballast  108 . A second lamp capacitor C 2  of the plurality of lamp capacitors connects in series with the second lamp L 2  and the first secondary winding T 1 _S 1  of the starting transformer T 1 . For example, the second lamp capacitor C 2  connects the second terminal of the resonant inductor L 5  to the second lamp L 2  when the second lamp L 2  is connected to the ballast  108 . In one embodiment, a third lamp capacitor C 3  connects in series with the third lamp L 3  of the plurality of lamps when the third lamp L 3  is connected to the ballast  104 , and a fourth lamp capacitor C 4  connects in series with the fourth lamp L 4  of the plurality of lamps when the fourth lamp L 4  is connected to the ballast  104 . 
     Referring to  FIG. 2 , the unbalanced load sensing circuit  110  is shown monitoring exactly one second secondary winding Tn_S 2  of exactly one of the starting transformers Tn (where n is representative of any of the first, second or third transformers). This is possible because any load imbalance causes an excess voltage to appear at the second secondary windings of all of the starting transformers. 
     In one embodiment, all of the windings of all of the transformers have the same number of turns. It is contemplated within the scope of the claims that the transformer windings may be connected in configurations other than as described herein to achieve current balance among the parallel lamps. It is further contemplated within the scope of the claims that any of the second secondary windings may be monitored for excess voltage by the unbalanced load sensing circuit  110  to determine an absent or damaged lamp. Although referred to herein as a primary winding, first secondary winding, and second secondary winding, it is contemplated that these terms are interchangeable (e.g., when the windings all have the same number of turns), and that the unbalanced load sensing circuit  110  is monitoring the winding of the starting transformer not connected in series with the lamps (e.g., the second secondary winding). 
     It will be understood by those of skill in the art that information and signals may be represented using any of a variety of different technologies and techniques (e.g., data, instructions, commands, information, signals, bits, symbols, and chips may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof). Likewise, the various illustrative logical blocks, modules, circuits, and algorithm steps described herein may be implemented as electronic hardware, computer software, or combinations of both, depending on the application and functionality. Moreover, the various logical blocks, modules, and circuits described herein may be implemented or performed with a general purpose processor (e.g., microprocessor, conventional processor, controller, microcontroller, state machine or combination of computing devices), a digital signal processor (“DSP”), an application specific integrated circuit (“ASIC”), a field programmable gate array (“FPGA”) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. Similarly, steps of a method or process described herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. Although embodiments of the present invention have been described in detail, it will be understood by those skilled in the art that various modifications can be made therein without departing from the spirit and scope of the invention as set forth in the appended claims. 
     This written description uses examples to disclose the invention and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
     It will be understood that the particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention may be employed in various embodiments without departing from the scope of the invention. Those of ordinary skill in the art will recognize numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims. 
     All of the compositions and/or methods disclosed and claimed herein may be made and/or executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of the embodiments included herein, it will be apparent to those of ordinary skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit, and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope, and concept of the invention as defined by the appended claims. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful METHOD TO DETECT UNEVEN AC LOAD OR PARALLEL LOAD REMOVAL 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.