Frequency-Doubler Modulation for Reduced Junction Temperatures for LLC Resonant Converters Operated in Half-Bridge Configuration

LLC resonant converters are typically unsuitable to be applied for wide voltage-transfer ratio applications. With a full-bridge inverter, however, they can be operated in a variety of different modulations. Most notably, by permanently turning on one MOSFET and turning off the other MOSFET of the same bridge leg, the LLC can be operated in half-bridge configuration reducing the gain by a factor of two. The resonant capacitor is hereby charged to an average voltage of half the input voltage. In this modulation, however, the switch that is permanently turned on is stressed by the complete resonant current while exhibiting no switching losses. This paper proves that the frequency-doubler modulation can better balance the losses among all MOSFETs and should be the preferred mode of operation favored over the conventional half-bridge modulation. This paper analyzes the beneficial loss distribution, proposes an on-the-fly morphing modulation and discusses potential operating strategies to further reduce the junction temperature. Furthermore, it is shown that this modulation can also be altered to achieve the asymmetrical LLC operation. Experimental measurement results show that the modulation results in a substantial decrease of the maximum MOSFET temperature and shows that the converter can be smoothly transitioned during operation from full-bridge modulation to the frequency-doubler half-bridge operation and back.