Zero-Loss Switching in $LLC$ Resonant Converters Under Discontinuous Conduction Mode: Analysis and Design Methodology
David Elizondo, Ernesto L. Barrios, Iñaki Larequi, Alfredo Ursúa, Pablo Sanchis
Abstract
Many thriving applications where isolation is required, such as LED drivers, traction and EV fast charging, implement <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$LLC$</tex-math></inline-formula> resonant converters, particularly when voltage regulation is not required or an additional conversion stage is in charge of it. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$LLC$</tex-math></inline-formula> converter can be operated under discontinuous conduction mode (DCM), due to its advantages such as unregulated and sensorless operation, fixed switching frequency and voltage gain, and zero-current switching (ZCS). However, ZCS results in EMI and switching losses in the primary converter, particularly for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\geq$</tex-math></inline-formula> 1200-V devices. Alternatively, zero-loss switching (ZLS) can be accomplished by means of a proper design of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$LLC$</tex-math></inline-formula> converter, overcoming the drawbacks of ZCS. The focus of this paper is to perform an exhaustive research on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$LLC$</tex-math></inline-formula> converter under DCM-ZLS: discontinuous conduction mode with lossless switching in the primary and secondary sides. As a result of this analysis, a set of design boundaries are deduced for parameters such as the magnetizing inductance, the leakage inductance, and the gate resistance. A comprehensive, step-by-step design methodology is proposed and applied to a 18-kW, 200-kHz test bench. The designed parameters are implemented in the converter and several experiments are conducted, including a test at rated input voltage and rated power (600 V, 18 kW). The conduction states studied theoretically in the analysis of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$LLC$</tex-math></inline-formula> converter are identified in the experimental results, and the operation of the test bench under DCM-ZLS is verified.