This paper investigates a cycle-skipping modulation technique based on sigma-delta modulation to improve the performance of series resonant converters, particularly those operating as a dc transformer. Compared to burst mode control, sigma-delta modulation provides a higher equivalent frequency at the output, which significantly reduces output voltage and current ripples. While dc transformer converters typically operate at a constant power with limited power regulation capabilities, the application of sigma-delta modulation enables effective power regulation. The proposed approach is experimentally validated on a 1 kW converter prototype operating at a switching frequency of 100 kHz. A peak efficiency of 98.4% and maintaining above 96% efficiency under light-load conditions has been achieved, with a 4 -point average efficiency of 97.47%, demonstrating its practicality and effectiveness.