LLC converters: Beyond datasheets for MOSFET power loss estimation

In the past 10 years, LLC resonant converters have become a mainstream topology and multiple design tools have been developed, including LLC controllers, LLC tank regulation techniques, etc. While many design tools are available, methods of estimating power losses in LLC MOSFETs do not exist. In particular, accurate methods of estimating conduction losses, which are dominant in LLC converters, are lacking in the literature. This paper develops a method of accurately estimating MOSFET power losses in LLC converters. This is a fundamental tool for designing LLC converters, and allows the thermal behaviour of the MOSFETs to be predicted before the converter is built. The proposed method overcomes major datasheet limitations by replacing the poor, unrealistic datasheet information with a realistic MOSFET characterization applicable to LLC converters. The method focuses on the detailed characterization of the on-state resistance (R
_DS
(
_on
)), including the effects of the gate-source voltage (Vg
_S
), junction temperature (T
_j
) and drain current (I
_D
). The characterization covers the actual operating points of the LLC converter, and provides an estimating equation to calculate MOSFET losses. As a result, LLC losses can be estimated with improved accuracy, including different operating points and I
_D
polarities. As verified by simulation and experimental results, the proposed design tool provides an improvement in accuracy of over 100% compared to the results obtained using the simplistic datasheet information when the circuit is operating near resonant frequency.