Zero-Loss Switching in $LLC$ Resonant Converters Under Discontinuous Conduction Mode: Analysis and Design Methodology

Many thriving applications where isolation is required, such as LED drivers, traction and EV fast charging, implement
$LLC$
resonant converters, particularly when voltage regulation is not required or an additional conversion stage is in charge of it. The
$LLC$
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
$\geq$
1200-V devices. Alternatively, zero-loss switching (ZLS) can be accomplished by means of a proper design of the
$LLC$
converter, overcoming the drawbacks of ZCS. The focus of this paper is to perform an exhaustive research on the
$LLC$
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
$LLC$
converter are identified in the experimental results, and the operation of the test bench under DCM-ZLS is verified.