<p>LLC resonant converters typically achieve high efficiency only within specific operating conditions. When working in high voltage conversion ratio scenarios, the switching frequency range increases, causing the efficiency to decline. In this paper, a novel adaptive switched capacitor control strategy is proposed to improve the efficiency over the whole load conditions. A mathematical model that can directly relate power loss to the resonant capacitance and the load current is established. The switching frequency as a dependent variable is also considered. Using this model, the optimal resonant capacitance for different load conditions to maximize efficiency can be calculated on-the-fly. By dynamically adjusting the resonant capacitance through the selective activation of a bank of capacitors, the proposed method mitigates frequency broadening and achieves significant improvement in power conversion efficiency. To validate the effectiveness of the proposed method, a 130 to 90&#xa0;V, 450 W LLC converter demonstrates a 3.6% increase in efficiency under light load (<i>I</i>_<i>out</i> = 2 A) and a 3.9% increase in efficiency under heavy load (<i>I</i>_<i>out</i> = 5 A).</p>