Effect of a Split Transformer Leakage Inductance in the LLC Converter with Integrated Magnetics

The effects of the transformer leakage inductance in a LLC converter that exploits the integrated magnetic concept are investigated. The vast literature on this subject always models the transformer considering its magnetizing inductance, together with a lumped leakage inductance located at the primary side. However, when the transformer windings are arranged so as to increase the leakage magnetic flux with the aim of achieving the desired value, two leakage inductances are indeed created, one associated to each winding. Using the Fundamental Harmonic Approximation (FHA), the converter behavior at steady-state is revised so as to highlight the effects of a split leakage inductance, while the dynamic behavior is investigated using the Generalized Averaging Method. A suitable design procedure is proposed that accounts for the more complex transformer model. Measurements taken on a 100 W rated LLC converter prototype confirm the theoretical forecasts.