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 BR1, a power factor correction (PFC) circuit, an electrolytic capacitor C1, 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 C1 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 C1 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'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.