<p>This paper presents a power balance control algorithm that employs duty control for input-series-output-parallel (ISOP) LLC resonant converters. With increasing demands for power capacity and system reliability, modular power converters are being widely adopted, particularly in high-voltage, high-current applications where ISOP structures are prevalent. In LLC converters, voltage gain is governed by the resonant tank parameters. Therefore, parameter mismatches among modules may result in power imbalances. Conventional power balance control methods compensate for input voltage discrepancies by allowing independent switching frequencies per module. However, in frequency-controlled LLC converters, such approaches can cause mismatches in current cycles, leading to increased output voltages and current ripples that degrade power quality. To overcome this limitation, the proposed method synchronizes the switching frequency across all modules while individually adjusting the duty ratios to achieve power sharing. The proposed algorithm is validated through PSIM simulations and experimentally verified via hardware-in-the-loop simulation.</p>