Patent Publication Number: US-6906474-B2

Title: Three-phase electronic ballast

Description:
FIELD OF THE INVENTION 
     The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a three-phase electronic ballast. 
     BACKGROUND OF THE INVENTION 
     In recent years, electronic ballasts have begun to displace traditional “core and coil” magnetic ballasts. In comparison with magnetic ballasts, electronic ballasts provide a host of benefits, including dramatically higher energy efficiency and better quality of illumination (e.g., little or no visible flicker in the light emitted by the lamp). On the other hand, magnetic ballasts are usually less expensive and more reliable than electronic ballasts. 
     A typical prior art single-phase electronic ballast is described in FIG.  1 . The ballast includes a 1-phase electromagnetic interference (EMI) filter, a fullwave diode bridge BR 1 , a power factor correction (PFC) circuit, an electrolytic capacitor C 1 , and a high frequency inverter. The ballast receives operating power from a single-phase alternating current (AC) voltage source. The DC bus voltage, Vbus, across capacitor C 1  is described in FIG.  2 . 
     In the prior art ballast of  FIG. 1 , the PFC circuit, which is typically realized by a controlled DC-to-DC converter such as a boost converter, is required in order to ensure that the power factor (PF) is high enough, and that the total harmonic distortion (THD) in the current drawn from the AC voltage source is low enough, to meet applicable standards for power quality. Without a PFC circuit, the PF would be much too low (e.g., about 0.5) and the THD would be much too high (e.g., about 150%). Unfortunately, a dedicated PFC circuit is materially expensive, requires a considerable amount of physical space, and has power losses that detract from the energy efficiency of the ballast. 
     In the prior art ballast of  FIG. 1 , the large electrolytic bulk capacitor C 1  is necessary in order to ensure that the amount of ripple (ΔV in  FIG. 2 ) in Vbus is sufficiently small so as to prevent excessive low frequency (e.g., 120 hertz) flicker in the illumination provided by the lamp(s). Typically, the electrolytic capacitor has a high capacitance (e.g., 47 microfarads or higher) and a high voltage rating (e.g., 250 volts or higher), and is therefore quite large. Additionally, a high value bulk capacitor causes correspondingly high levels of inrush current. Perhaps the greatest disadvantage of using electrolytic bulk capacitors is encountered in those ballasts that operate in high ambient temperature environments, in which case the ballast&#39;s operating life is largely determined by the useful operating life of the electrolytic capacitor (which decreases by a factor of two for every 10° C. increase in operating temperature). Thus, significant impetus exists for developing ballast circuits that do not require electrolytic bulk capacitors. 
       FIG. 3  describes a typical grouping scheme that is desirable in industrial/office buildings having lighting fixtures that employ single-phase electronic ballasts like the ballast of FIG.  1 . In order to equalize the loading on each phase of the 3-phase AC voltage source, it is necessary that the fixtures be divided into groups wherein each group draws about the same amount of power from the AC voltage source. As such a grouping scheme requires that the building be wired so that each of the three phases are distributed accordingly, it greatly complicates the building wiring. 
     What is needed, therefore, is an electronic ballast that does not require a dedicated PFC circuit or an electrolytic bulk capacitor in order to provide acceptable power quality and illumination without noticeable flicker. A need also exists for a ballast that does not require grouping of lighting fixtures within a building so as to equalize the loading on each phase of the AC voltage source. Such a ballast would offer a number of benefits over existing electronic ballasts, including lower material cost, reduced physical size, higher energy efficiency, enhanced life, lower inrush current, and simplified building wiring, and would thus represent a significant advance over the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  describes a single-phase electronic ballast, in accordance with the prior art. 
         FIG. 2  describes the DC bus voltage provided by the single-phase electronic ballast of FIG.  1 . 
         FIG. 3  describes a typical grouping scheme for lighting fixtures that employ the single-phase electronic ballast of FIG.  1 . 
         FIG. 4  describes a three-phase electronic ballast, in accordance with a preferred embodiment of the present invention. 
         FIG. 5  describes the DC bus voltage provided by the three-phase electronic ballast described in  FIG. 4 , in accordance with a preferred embodiment of the present invention. 
         FIG. 6  describes a group of lighting fixtures that employ the threephase electronic ballast described in  FIG. 4 , in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A ballast  10  for powering at least one gas discharge lamp  52  from a three-phase alternating current (AC) voltage source  30  is described in FIG.  4 . Ballast  10  comprises a three-phase rectifier circuit  200 , a high frequency filter capacitor  300 , and a high frequency inverter  400 . Three-phase AC voltage source  30  is a conventional 60 hertz voltage source that is provided by the electrical utility company. 
     In a preferred embodiment of ballast  10 , three-phase rectifier circuit  200  comprises a first input terminal  202 , a second input terminal  204 , a third input terminal  206 , a first output terminal  212 , a second output terminal  214 , a first diode  220 , a second diode  230 , a third diode  240 , a fourth diode  250 , a fifth diode  260 , and a sixth diode  270 . First input terminal  202  is adapted to receive a first phase  32  of three-phase AC voltage source  30 . Second input terminal  204  is adapted to receive a second phase  34  of source  30 . Third input terminal is adapted to receive a third phase  36  of AC source  30 . First diode  220  has an anode  222  coupled to first input terminal  202  and a cathode  224  coupled to first output terminal  212 . Second diode  230  has an anode  232  coupled to second output terminal  214  and a cathode  243  coupled to first input terminal  202 . Third diode  240  has an anode  242  coupled to second input terminal  204  and a cathode  244  coupled to first output terminal  212 . Fourth diode  250  has an anode  252  coupled to second output terminal  214  and a cathode  254  coupled to second input terminal  204 . Fifth diode  260  has an anode  262  coupled to third input terminal  206  and a cathode  264  coupled to first output terminal  212 . Sixth diode  270  has an anode  272  coupled to second output terminal  214  and a cathode  274  coupled to third input terminal  206 . 
     During operation, rectifier circuit  200  receives the three-phase alternating current (AC) voltage source  30  and provides a rectified output voltage. As described in  FIG. 5 , the rectified output voltage, Vbus, has a maximum value, a minimum value, an average value (Vavg), and a ripple value (ΔV). In ballast  10 , the ripple value (ΔV), which is defined as the difference between the maximum value and the minimum value, has a root-mean-square (RMS) value that is no greater than about 5% of the average value Vavg. Advantageously, rectifier circuit  200  utilizes all three phase  32 , 34 , 36  of the AC source  30  to provide a Vbus that naturally has a small amount of 360 hertz ripple and that therefore requires no-capacitive filtering in order to provide an acceptably low level of visible flicker in the illumination of the lamp(s). This is in contrast with the prior art ballast of  FIG. 1  where, in the absence of a large electrolytic capacitor C 1 , the 120 hertz ripple would be extremely high, with consequent excessive flicker in the illumination of the lamp(s). 
     Referring to  FIG. 4 , high frequency filter capacitor  300  is coupled between the first and second output terminals  212 , 214  of rectifier circuit  200 . The sole function of capacitor  300  is to provide an AC path for high frequency current drawn by inverter  400 . Capacitor  300  can thus be realized by a capacitor with a relatively low capacitance value (e.g., 0.1 microfarads when ballast  10  is designed to power four 32 watt lamps). Consequently, capacitor  300  can be implemented by a film capacitor or a ceramic capacitor. Advantageously, because ballast  10  does not require an electrolytic bulk capacitor, its operating life will be substantially greater than the prior art ballast described in  FIG. 1 , if the ballast is operated in a high ambient temperature environment. Moreover, because of the relatively low capacitance of capacitor  300 , the amount of inrush current that occurs in ballast  10  will be dramatically less than what occurs in the prior art ballast described in FIG.  1 . 
     High frequency inverter  400  is coupled to rectifier circuit  200  and high frequency filter capacitor  300 . During operation, inverter  400  powers at least one gas discharge lamp  52  and has an operating frequency that is greater than about 20,000 hertz. In general, inverter  400  includes a plurality of output terminals  40 , 42 , 44 , . . . , 48  for connection to a plurality of discharge lamps  52 , 54 , . . . , 58 . Inverter  400  may be realized by any of a number of circuit arrangements (e.g., a half-bridge inverter followed by a series resonant output circuit) that are well known to those skilled in the art of electronic ballasts. 
     During operation of ballast  10 , the line current that is drawn from each phase  32 , 34 , 36  of AC source  30  has a total harmonic distortion (THD) that is no greater than about 33%. Additionally, as the line current drawn from each phase is only moderately out of phase with the voltage between each phase and ground  38 , ballast  10  provides a power factor (PF) that is no less than about 0.9. Thus, ballast  10  is capable of approaching or meeting applicable standards for power quality without requiring an active power factor correction (PFC) circuit such as a boost converter. Consequently, in comparison with the prior art electronic ballast described in  FIG. 1 , ballast  10  provides the added benefits of lower material cost, smaller physical size, and enhanced energy efficiency (e.g., 94% versus about 88% for the ballast of FIG.  1 ). 
     Preferably, as described in  FIG. 4 , ballast  10  further comprises a three-phase electromagnetic interference (EMI) filter  100  that is interposed between rectifier circuit  200  and three-phase AC voltage source  30 . During operation, three-phase EMI filter  100  attenuates any line-conducted EMI that tends to arise due to the high frequency operation of inverter  400 . In a preferred embodiment of ballast  10 , three-phase EMI filter comprises first, second, third, and fourth input connections  22 , 24 , 26 , 28 , first, second, and third inductors  102 , 104 , 106 , and first, second, and third capacitors  112 , 114 , 120 . First input connection  22  is adapted to receive a first phase  32  of three-phase AC voltage source  30 . Second input connection is adapted to receive a second phase of source  30 . Third input connection is adapted to receive a third phase of source  30 . Fourth input connection is adapted to receive a ground  38  of source  30 . A neutral  37  of source  30  has no corresponding connection to ballast  10 . First inductor  102  is coupled between first input connection  22  and the first input terminal  202  of rectifier circuit  200 . Second inductor  104  is coupled between second input connection  24  and the second input terminal  204  of rectifier circuit  200 . Third inductor  106  is coupled between third input connection  26  and the third input terminal  206  of rectifier circuit  200 . First capacitor  112  is coupled between the first and second input terminals  202 , 204  of rectifier circuit  200 . Second capacitor  114  is coupled between the second and third input terminals  204 , 206  of rectifier circuit  200 . Third capacitor  120  is coupled between fourth input connection  28  and the second input terminal  204  of rectifier circuit  200 . 
     Turning now to  FIG. 6 , it can be seen that ballast  10  allows for installations in which all of the fixtures in a building are wired to the AC source  30  in an identical manner. This is in contrast to the arrangement described in  FIG. 3 , where the fixtures must be segregated into three groups in order to equalize the loading on each phase of the AC source. Thus, ballast  10  provides the added benefit of simplifying the electrical wiring that is routed to the lighting fixtures within a building. 
     Although the present invention has been described with reference to certain preferred embodiments, numerous modifications and variations can be made by those skilled in the art without departing from the novel spirit and scope of this invention.