Alternatives for high performance LED driving in off-line applications

Abstract

This doctoral thesis proposal is focused on the area of high-performance off-line LED drivers, in other words, converters that feature high efficiency, low cost, high reliability, high power density, and high power factor. The work presents alternatives for achieving these desirable characteristics. In order to obtain topological alternatives with high efficiency, this research uses resonant conversion in the second stage to reduce switching losses. In this sense, a high-accuracy methodology for designing a dc-dc LLC resonant converter has been proposed, so that the operation point of the converter and the low-frequency (LF) current ripple transmission can be defined with errors significantly lower than the ones obtained by other strategies, such as based on First Harmonic Approximation (FHA). In order to reduce the total cost of the topology, the work studies converters that integrate the power factor correction (PFC) stage with the power control (PC) stage, thus reducing the number of components that yields simplicity and low cost. The work also addresses alternatives for increasing the reliability and power density of LED drivers. These two characteristics are strongly linked to the sizing of the capacitors used to filter the low-frequency ripple of the output current, which are often large and directly related to the circuit reliability. In this regard, an alternative for Active Ripple Compensation (ARC) techniques is proposed to reduce the converter bulk capacitance. The proposed technique performs the compensation by deliberate modulation of the converter’s switching frequency, which allows for a huge capacitance reduction with a negligible increase in the THD of the input current. Experimental results gathered from a 96-W laboratory prototype supplied from a 127-V 60-Hz grid attested to the superior performance of the frequency-based active ripple compensation since a capacitance reduction of 66.6% has been obtained with an increase of only 0.9% in the THD. In addition, a systematic design methodology based on a constrained multi-objective optimization problem is proposed for designing high-performance drivers. The proposed strategy enables the simultaneous design of the power circuit elements and the controller with frequency-based ARC, while optimizing various performance parameters. Experimental results from the optimized converter demonstrated the significant improvements in performance parameters, such as a 38.25% reduction in failure rate and a 111.69% increase in power density of passives elements. The proposed converter also achieved an efficiency of 90.96%, meeting the desirable characteristics for a high-performance driver.