LLC Resonant Converter Modulations for Reduced Junction Temperatures in Half-Bridge Mode and Transformer Flux in the On-the-Fly Morphing Thereto

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 modulation modes to extend the operating range. Most notably, by permanently turning on one switch and turning off the other switch of the same bridge leg, the LLC can be operated in half-bridge mode reducing the gain by a factor of two. In this modulation, however, the switch that is permanently turned on is stressed by the complete resonant current while exhibiting no switching losses. This article proves that the frequency-doubler modulation can better balance the losses among all switches and should be the preferred mode of operation favored over the conventional half-bridge modulation. This article analyzes the beneficial loss distribution and proposes an on-the-fly morphing modulation to the frequency-doubler modulation and reverse. Furthermore, the morphing magnetizing current offset of the morphing is analyzed to show that the offset can be influenced by the design of the magnetizing inductance and through the morphing modulation itself. Finally, an improved morphing modulation is presented showing that the morphing flux offset can be reduced substantially. Experimental measurement results on a 1.8 kW prototype show that by employing the frequency-doubler modulation, the maximum inverter temperature can be reduced by about 20 K and that the maximum transformer flux deviation during morphing can be reduced significantly on average by 78%. Both results enable the use of switches with a lower on-state resistance and the application of transformers with a smaller core cross-section.