Efficient Resonant Converters for Power Factor Correction in Solid State Lighting Applications

Frederik Monrad Spliid

Research output: Book/ReportPh.D. thesis

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Abstract

With the increasing demand and popularity of LED-based lighting systems, research interest in power electronics has turned towards the drivers in these systems and, in particular, how to combine high conversion efficiency with a compact design. In order to satisfy the requirements on both input and output currents, state-of-the art LED drivers are composed of two stages: An AC/DC power factor correction (PFC) stage responsible for shaping the input current and a DC/DC stage responsible for supplying the desired output voltage and current for the LED. In recent years, considerable research effort has been put into the subject of DC/DC Switch Mode Power Supplies (SMPS) and how their physical size can be reduced by increasing their switching frequencies. Through the use of resonant converter topologies, operation in the high frequency (HF) and very high frequency (VHF) ranges have been demonstrated with reduced converter sizes as a result. This thesis focuses on the PFC stage of LED drivers for European mains input voltage, and investigates several methods and techniques that can be applied in order to enable the use of resonant converter topologies in PFC applications, while maintaining high conversion efficiency. By implementing resonant converters in the PFC stage, it is expected that AC/DC conversion can undergo the same process of increasing switching frequencies and shrinking volumes as DC/DC converters. The three following groups are the main research contributions of this thesis: First, the operation of common resonant converter topologies is analysed in detail, in order to derive conditions that allow their use over a wide input voltage range, while achieving zero-voltage-switching (ZVS) and linear input characteristics. A 50 W prototype of a class DE converter capable of operating from DC input voltages in the range of 60–325 V is presented and shown to achieve ZVS over a full decade of output power variation through modulation of the switching frequency. The prototype shows conversion efficiencies of up to 94 %. Further more, the concept of power factor correction through the of use resonant-converter driven charge pump circuits is explored, and a new PF Cconverter structure is presented. This structure utilizes a resonant converter as a parallel “power factor port”, and reduces the stresses on the converter significantly compared to conventional cascaded structures. A 50 W prototype is demonstrated, achieving conversion efficiencies of up to 92.2% and providing a power factor of 0.99 for output power in the range of 20–50W. Finally, the topic of high-frequency inductor design is studied, as losses in magnetic components constitute one of the main limitations on the switching frequency in resonant converters. Stresses on the magnetic components in resonant converters are investigated, and methods to reduce the resulting losses are presented. Through a series of simulations and measurements, the benefits of implementing quasi-distributed air gaps in a standard ferrite core, by the use of custom-made ferrite slices for gap-separation, is documented. For a reference inductor design, the technique is shown to give a 25 % increase in inductor Q value.
Through the application of the research described in this work, resonant converters can be implemented as efficient PFC stages in LED drivers, replacing hard-switched converter topologies that limits the converter switching frequency.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages162
Publication statusPublished - 2020

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