The leakage inductance of high-power high-frequency transformers is a key factor for bidirectional full-bridge DC/DC converters. The frequency-dependent characteristic curve of leakage inductance in a wide frequency band is presented using analytical method, which can be divided into three regions and verified by comparing with finite element simulation. Also, the concept of turning frequency is proposed and the changing trend of turning frequency is studied for transformers with varying-thickness copper foils. Below the turning frequency, the simulation model simplification is implemented by replacing multi-layer copper foil with single-layer copper foil with the same thickness, which is verified by comparing the three-dimensional finite element calculation result with the measurement.