A novel magnetic structure for use in LLC resonant converters is proposed in this paper. It is known as a Path-core type Resonant Inductance Adjustable (PRIA) transformer, which is a type of transformer with adjustable resonant inductance. In the proposed transformer, the resonant inductance and magnetizing inductance can be designed separately; therefore, it becomes less challenging to realize the required resonant frequency. Moreover, the magnetizing inductance is not affected by any variation in the resonant inductance. The proposed PRIA transformer is found to improve the efficiency of the LLC converters. In this paper, the design of a resonant inductance of a PRIA transformer is presented. In addition, the application of PRIA transformers to a 1-MHz LLC resonant converter and 1 kW 1.4-MHz LLC resonant converter is presented.