Applying asymmetrical half-bridge LCC resonant inverter to EEFL lighting ballast

Abstract

In this paper, an asymmetric half-bridge LCC resonant inverter is presented, which is the kernel of the proposed lighting ballast that is used to drive an external electronic fluorescent lamp (EEFL). This lighting ballast contains three power stages if dimming is necessary; otherwise it contains two power stages only. The first power stage is constructed by a traditional boost converter with power factor correction (PFC), and such a converter is operated in the transient mode (TM) with the DC output voltage 390V. The second power stage is built up by a buck converter is used to control the duty cycle such that the corresponding output voltage can be changed and hence dimming of the EEFL can be achieved. The third power stage is established by an asymmetric half-bridge LCC resonant inverter operating under a fixed switching frequency with a fixed duty cycle of about 50%. Via LCC resonance, the power switches of this inverter are operated in zero voltage switching (ZVS) so as to reduce the switching loss, and at the same time, the inherent high voltage conversion characteristics make the voltage conversion gain is larger than one such that the turns ratio of transformer can be reduced. Most importantly, as the EEFL is operated on the rated conditions, i.e., without dimming, the second power stage is to be bypassed, thereby causing the corresponding efficiency to be upgraded. The basic operating principles and corresponding mathematical deductions of the proposed inverter are described, and applied to the constructed EEFL lighting ballast that is verified by some simulated and experimental results.