Design, Control, and Implementation of High
Frequency LLC Resonant Converter
Hwa-Pyeong Park
Department of Electrical and Computer Engineering
Abstract
A high switching frequency operation has been introduced with much interest in research and
industrial areas to improve the power density of power converters. However, its implementation is
difficult for an elaborate switch mode power supply which has high efficiency and stable operation. In
this paper, a power stage and a feedback controller design will be considered for proper operation,
stability, and high power conversion efficiency of the high frequency LLC resonant converter. The
power density can be improved by adopting high switching frequency which allows small sized passive
components. At the high switching frequency, the size reduction of the passive components such as
transformer, and output capacitor will be estimated to obtain the high power density design. In addition,
the design method of the magnetizing inductance design method will be derived to achieve the zero
voltage switching (ZVS) at the high switching frequency operation.
In aspect of frequency domain, the smaller output capacitor which has small capacitance and low
effective series resistance (ESR) changes the small-signal behavior of the converter’s power stage. It
can make the converter unstable by increasing the crossover frequency in the loop gain of the smallsignal
model. The effect of the smaller output capacitance should be analyzed for stability analysis using
a proper small-signal model of the LLC resonant converter. Therefore, the proper design methods of
the feedback compensator are derived to obtain sufficient phase margin in the bode plot of the
converter’s loop gain for its stable operation. The design considerations of the power stage and the
feedback loop will be verified with the performance comparison of 100 kHz and 500 kHz switching
frequency LLC resonant converters.
Since the switching performance of state-of-art power switches has been improved, the power
converter can operate over a 1 MHz switching frequency. In this paper, GaN E-HEMTs are used to
achieve the high switching frequency operation due to its small channel resistance and small output
capacitance. However, the GaN E-HEMTs also have different switching operation characteristics to
other conventional silicon-based MOSFETs. Therefore, the high speed switching characteristics of the
GaN E-HEMT should be analyzed to obtain proper operation for a half-bridge type LLC resonant
converter using a boostrap gate drive circuit. Moreover, a soft start algorithm for the high switching
frequency is analyzed to suppress inrush currents at the cold start operation of the converter. All the
design considerations using the GaN E-HEMT are verified with a 240 W prototype LLC resonant
converter operating at 1 MHz switching frequency.
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