Abstract:
A ballast circuit adapted for converting an input alternating current (AC) mains power received at input terminals to an output alternating current (AC) to supply a load, e.g. a gas discharge lamp. The output terminals of the ballast connect to the load in parallel with another ballast circuit. The ballast circuit is configured to supply the output AC current to the load in parallel with an AC current output of the other ballast circuit. A synchronization module attached at the output is adapted for synchronizing the output alternating current (AC) of the ballast circuit with the AC current output of the other ballast circuit. The synchronization module is configured to synchronize phase of the output alternating current with phase of the AC current output of the other ballast circuit.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to ballast circuits for gas-discharge lamps and in particular a system and method for load balancing between ballasts and improving reliability of lighting systems. 
         [0003]    2. Description of Related Art 
         [0004]    Gas-discharge lamps include fluorescent lamps, low and high pressure sodium lamps, metal halide lamps and high-intensity discharge (HID) lamps. A high-intensity discharge (HID) lamp produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. The tube is filled with both gas and metal salts. The gas facilitates the arc&#39;s initial strike. Once the arc is started, the arc heats and evaporates the metal salts forming a plasma, which greatly increases the intensity of light produced by the arc and reduces its power consumption. 
         [0005]    Ballasts are used where an electrical load, e.g. gas-discharge lamp cannot effectively regulate current use such as when a gas discharge lamp presents a negative (differential) resistance to the supply. If a gas-charge lamp were connected to a constant-voltage power supply, the lamp would draw an increasing amount of current until it is destroyed. To prevent this, a ballast provides a positive resistance or reactance that provides current to the gas discharge lamp at an appropriate level. 
         [0006]    International patent application publication WO2006109313 of the same applicant discloses a system and method for configuring a single ballast for use with different power ratings and/or different lamp types. The electronic ballast includes hardware  25  typically including a microprocessor to support a large range of output powers, e.g. 20-1000 W, and programmable parameters (or software versions) which support different types of lamps and optional features including a dimming option, and dimming delay. The manufacturer may supply hardware and/or software to a local supplier, reseller, customer or distributor for configuring the ballast. Typically, the distributor requires for configuring the ballast a computer with a connection to a communications port of the ballast. The ballast can be monitored individually or as as part of a group through its communication interface by master control software installed on a computer. Combined with a computer and optional hub, thousands of lamps can be controlled and monitored by authorized site engineers using wired or wireless connection. With the proper security authorization, a site manager can control, dim, and monitor individual and group of lamps from virtually any place on the globe. 
         [0007]    The term “synchronization” or “synchronous” as used herein refers to maintaining identical or nearly identical phase and frequency between two or more sinusoidal alternating currents. 
       BRIEF SUMMARY 
       [0008]    According to embodiments of the present invention there is provided a ballast circuit adapted for converting an input alternating current (AC) mains power received at input terminals to an output alternating current (AC) to supply a load, e.g. a gas discharge lamp. The output terminals of the ballast connect to the load in parallel with another ballast circuit. The ballast circuit is configured to supply the output AC current to the load in parallel with an AC current output of the other ballast circuit. A synchronization module attached at the output is adapted for synchronizing the output alternating current (AC) of the ballast circuit with the AC current output of the other ballast circuit. The synchronization module is typically configured to synchronize ignition outputs of the ballast circuit and said other ballast circuit. The synchronization module is configured to synchronize phase of the output alternating current with phase of the AC current output of the other ballast circuit. The synchronization module may be configured as either a master synchronization module which provides a synchronization signal to the other ballast circuit or a slave synchronization module which receives a synchronization signal from the other ballast circuit. The ballast may include a microprocessor and a communication interface attached thereto. The microprocessor may be configured to be re-programmed through the communications interface to supply to the load a higher AC current than the output AC current previously programmed. 
         [0009]    According to an embodiment of the present invention there is provided multiple ballast circuits with respective AC outputs configured for interconnection in parallel and for supplying alternating current synchronously and in parallel to a single load. The single load is a high-intensity discharge (HID) lamp. The ballast circuits may share a synchronization signal from a single clock. Upon a failure of one (or more) of the ballast circuits, the single load continues to operate by receiving current from the remaining ballast circuits. A modular ballast system may include a chassis on which the ballast circuits are mountable. A communications junction box may be connected to the ballast circuits. The communications junction box may be adapted for communicating monitor signals from the ballast circuits and/or for communicating control signals to re-program AC output power of the ballast circuits. The communications junction box may be a single communications junction box which connects to all the ballast circuits. 
         [0010]    According to an embodiment of the present invention there is provided a method for operating a gas discharge lamp using a modular ballast system including multiple ballast circuits with respective AC output terminals. The AC output terminals are connected in parallel to a load and output currents of the ballast circuits are synchronized. The ignition outputs of the ballast circuits are typically synchronized. The ballasts are typically previously programmed to provide a previously determined AC output power and upon receiving an alarm indicating a failure of one or more of the ballast circuits, the remaining ballast circuits are reprogrammed while still operating to provide higher output currents to compensate for the failed ballast circuit(s). Alternatively, upon failure of one of the ballast circuits the gas discharge lamp continues to operate from the output current of the other remaining ballast circuits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0012]      FIG. 1  shows a ballast circuit according to an embodiment of the present invention. 
           [0013]      FIG. 2  shows a modular ballast system according to an embodiment of the present invention. 
           [0014]      FIG. 3  which shows a method according to an embodiment of the present invention 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
         [0016]    Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0017]    By way of introduction, embodiments of the present invention are directed to allow for fail-safe operation of gas discharge lamps in a ballast lighting system so that lamps in the modular ballast lighting system continue to operate despite failure of one or more of the parallel connected ballasts providing current to a single load, e.g. one or more HID lamps. 
         [0018]    In embodiments of the present invention, when multiple ballasts are used to supply current to a single load, e.g. a gas discharge lamp, one of the ballasts may be replaced for instance during scheduled maintenance. During replacement, the gas discharge lamp still operates, albeit at a lower power. Alternatively, the remaining operating ballasts are re-programmed on replacement of the failed ballast to supply higher currents so that the lamp continues to operate at the original power. According to other embodiments of the present invention, it may be more cost effective and/or reliable to use multiple ballast circuits at lower power than a single ballast circuit at higher power. 
         [0019]    Reference is now made to  FIG. 1  which illustrates schematically a ballast circuit  10  according to an embodiment of the present invention. Ballast circuit  10  typically includes a rectifier circuit  100 , a power factor control circuit  102 , an inverter circuit  104 , output terminals  106 , and lamp  114  all interconnected in the usual way. Monitor and control of power factor correction circuit  102  and/or inverter circuit  104  may be provided by a microprocessor  110 . A communications interface  112  may connect to microprocessor  110  to enable programming and/or reprogramming of ballast operation parameters and/or to send an alarm in case one of the sensors (e.g. temperature, output current, not shown) measures a parameter to be outside an expected tolerance. 
         [0020]    Rectifier  100  has a mains electricity input  118 . Input  118  is typically a 120/240 root mean square (RMS) alternating current (AC) voltage with a frequency of 60/50 Hz. Rectifier  100  rectifies mains electricity input  118  to produce a direct current (DC) output which is input into power factor correction (PFC) circuit  102 . PFC  102  is connected and controlled by microprocessor  110 . The DC output of PFC  102  is connected to the input of inverter circuit  104 , inverter  104  may be a “half bridge” or a “full bridge” inverter circuit. The AC output of inverter  104  is sinusoidal with a frequency typically of 100 kHz and typically provides a constant AC current output. The AC output of inverter  104  is then connected to output terminals  106 . Output terminals  106  are also connected across lamp  114 . Lamp  114  is typically a high intensity discharge (HID) lamp where the physical properties of lamp  114  determine the voltage across lamp  114 . Lamp  114  may also be a fluorescent lamp, low or high pressure sodium lamps and metal halide lamps. 
         [0021]    Inverter  104  is controlled by microprocessor  110  by control lines  122 . Microprocessor  110  also includes a synchronization input and/or output to synchronization module  108 . A synchronization module  108 , according to a feature of the present invention connects to microprocessor  110 . Synchronization module  108  has at least one of a synchronization output  124   b  and a synchronization input  124   a  which are connectible to synchronization modules  108  of other ballasts  10 . Alternatively, microprocessor  110  may input/output a synchronization or clock signal directly as input/output  124   a / 124   b  to microprocessors  100  of other parallel connected ballasts  10 . Output  124   b  and/or input  124   a  of multiple ballast circuits  10  are typically connected together to provide synchronization between the combined AC currents Iout supplying lamp  114 . Communication interface  112  may be used to externally monitor and/or control ballast  10  via a network  126  to a computer  128  through data interface  116 . 
         [0022]    Reference is now made  FIG. 2  which shows a modular ballast system  20  according to an embodiment of the present invention. Modular ballast system  20  includes multiple ballast circuits  10  physically and/or electrically plugged into chassis or motherboard  202 . Motherboard  202  typically provides the electrical connections for multiple ballast circuits  10  to be connected to mains  118 , communication junction box  200 , power junction box  204  and synchronization junction box  206 . Motherboard  202  typically allows multiple ballast circuits  10  to be removed or added during operation of modular ballast system  20   
         [0023]    Power junction box  204  connects output terminals  106  of each ballast  10  in parallel and provides a connection to one HID lamp  114  or multiple lamps. Synchronization junction box  206  enables synchronization output  124   b  and/or a synchronization input  124   a  of each ballast circuit  10  to be connected together so as to allow synchronization between multiple ballast circuits  10 . Connection  116  connects communication junction box  200  to communication interface  112  in each ballast circuit  10 . Communication junction box  200  enables multiple connections  116  to be connected to a computer  128  and/or network  126 . 
         [0024]    Reference is now made  FIG. 3  which shows a method  30  according to an embodiment of the present invention. Multiple ballast circuits  10  are each programmed (step  303 ) to provide a constant current output /out on terminals  106 . The constant output current may be first programmed (step  303 ) during a final test/configuration procedure in production prior to labeling and shipping. Alternatively or in addition, programming (step  303 ) may be performed by a distributor or installer prior to installation or operation by connecting computer  128  and/or network  126  to communication junction box  200  through communications interface  112  of ballast circuit  10 . 
         [0025]    Output terminals  106  of multiple ballast circuits  10  are connected (step  305 ) in parallel to lamp  114  optionally with use of power junction box  204 . Typically, synchronization of outputs is performed during ignition (step  306 ). After ignition (step  306 ), normal operation (step  307 ) of lamp occurs while synchronizing output currents. When output terminals  106  of multiple ballast circuits  10  are connected in parallel, the total current supplied to lamp  114  is the sum of the constant current outputs Iout of each ballast circuit  10 . For example, if lamp  114  is to be operated at 1200 Watts, three ballast circuits  10  with a maximum rated output of 600 watts may be programmed (step  303 ) to each produce current at 400 Watts to provide in unison the correct output AC current Iout to supply the 1200 Watts required. One or more synchronization signals  124  enable synchronization (step  307 ) of the AC output currents Iout during operation. Typically, one synchronization module  108  of one of parallel connected ballast circuits  10  may perform the role of a master synchronization module  108  which initiates and controls the transmission of synchronization signals to the other “slave” synchronization modules  108  via output  124   b  of the master to input  124   a  of the slave. Synchronization (step  307 ) may according to other embodiments of the present invention be performed with use of master microprocessor  110  by outputting a clock signal to slave microprocessors  110  of other ballast circuits  10  being synchronized. Typically, each ballast circuit  10  has sensors, e.g. temperature, current, voltage which monitor (step  308 ) ballast operation. If an alarm is received in decision box  309  because of a failure or out of specification operation in one of ballast circuits  10 , then the alarm may be communicated back through communications port  112 . Computer  128  through network  126  optionally may re-program (step  311 ) ballast circuits  10  to increase output current Iout but typically within rated specification to compensate for the failed ballast circuit  10 . Operation continues in step  307  with lamp power still at 1200 Watts being supplied by two operating ballasts  10  at 600 Watts each. Failed ballast  10  may be shut down completely (step  313 ). Replacement (step  315 )of failed ballast may be scheduled at a later date because lamp  114  is presently operating at full rated power of 1200 Watts. Alternatively, after failure of one ballast, continued operation may be at less than the fully rated power, for instance at 800 Watts without re-programming (step  311 ). 
         [0026]    The definite articles “a”, “an” is used herein, such as “a ballast”, “a lamp” have the meaning of “one or more” that is “one or more ballasts” or “one or more lamps”. 
         [0027]    Although selected embodiments of the present invention have been shown and described, it is to be understood the present invention is not limited to the described embodiments. Instead, it is to be appreciated that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and the equivalents thereof.