Shunt-inserted transformers are widely used to obtain high leakage inductance. This paper investigates this method in depth to make it applicable to integrate resonant inductor for the LLC resonant converters. The analysis and model of magnetizing inductance and leakage inductance for shunt-inserted planar transformers with and without air gaps are presented. Magnetic shunt permeability and thickness significantly affect the magnetizing inductance and leakage inductance. Air gaps in traditional transformers only have a slight influence on the leakage inductance. However, air gaps in shunt-inserted planar transformers can provide a significant difference. The way to obtain the desirable magnetizing and leakage inductance value for LLC resonant converters is simplified by the creation of air gaps together with a magnetic shunt. The calculation and relation are validated by finite element analysis (FEA) simulations and experimental measurements. AC resistances for the shunt-inserted planar transformers are discussed and three transformers with the same magnetizing inductance are selected for comparison. The results indicate that the magnetic shunt can in some extent minimize fringing effects.
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