I. Negative incremental impedance of fluorescent lamps. II. Simple high power factor lamp ballasts

摘要

Two important subjects in high frequency fluorescent lighting are investigated. Part I. presents a fundamental understanding of the negative incremental impedance of flourescent lamps. Search for simple, high power factor, lamp ballast topologies, a more practical aspect of lighting electronics, is presented in Part II.Part I.Flourescent lamps have a special v-i characteristic. Their incremental or small-signal impedance is defined and studied in the frequency domain. It is found that the negative incremental impedance of a fluorescent lamp can be characterized by a right half plane zero. The existence of such a result is explained by the modified Francis equation. Two approaches are proposed in order to set up a stable operating point for the lamp. First, a resonant matching network, which provides impedance transformation, is placed between a voltage source and the lamp. Second, a feedback loop is closed from the lamp current to control the magnitude of the voltage source. Stability criteria based on the Nyquist criterion are developed for the impedance of the resonant matching network and for the loop gain in the above two cases, respectively.It is found that the input impedance of a switching dc-to-dc regulator, which is another case with the negative incremental impedance, and the fluorescent lamp impedance, are two dual cases and thus can be analyzed in a unified manner based on the stability of one-port circuit.The small-signal lamp impedance is measured in the most practical case of a sinusoidal carrier, and it is fit by a real rational function with a right half plane zero. The small signal models describing low frequency variations of sinusoidal magnitude are developed and are experimentally verified for the three major resonant matching networks. Stability analysis for resonant matching networks at parallel resonance are performed. It shows that the LCC network is the simplest resonant matching network that a fluorescent lamp needs. It is also predicted, and experimentally verified, that the resonant matching networks at series resonance are feasible as ballasts when the lamp current feedback loop is closed.Part II.Unlike conventional high power factor ballasts requiring two cascaded power conversion stages, the proposed new lamp ballasts achieve near unity input power factor and high frequency sinusoidal lamp current in a single power conversion stage.A single-stage, high power factor lamp ballast is proposed, which is derived from the Cuk converter. A new discontinuous inductor current mode of its input inductor makes this possible by separating the input current-shaping from the output high frequency inversion. The desirable mode of operation is presented with major waveforms. The high efficiency is further enhanced by the soft-switching improvement, which is provided naturally through the lagging current of the resonant matching network. Design equations are given. The performances of the proposed single-stage, high power factor lamp ballast are verified by the experimental results.Single-switch, unity power factor ac-to-ac converter topologies based on proper integrations of automatic current shapers and single-switch inverters, such as the Class E converter, are presented, which are suitable for lamp ballast applications. Descriptions of the desirable mode of operations and a first order analysis are given. The ballasts have the compact structure of single active switch, high input power factor and low crest factor lamp current as well as soft-switching. Experimental results confirm all the performance advantages.