Physically, we can say that this is the result of a poor coupling between primary and secondary windings. The method consists of considering different winding structures.
Thanks to a finite element analysis software – like Maxwell – electromagnetic simulations can be performed to investigate different winding structures for a specific leakage inductance. This will provide the required input inductance value without adding a choke.
OBC: LLC topology
An LLC topology is more suitable for OBC applications (400V to 400V) since the output voltage should be adapted according to the battery charging voltage range (270V to 470V). The resonant cell allows the level of voltage to be adjusted with the switching frequency. Efficiencies up to 98% can be achieved with LLC resonant converters when integrated magnetics are used. Resonant converters have been around for a long time. However, power electronics for automotive HEV/EV is boosting demand as there is an absolute need to improve efficiency. The typical structure of a resonant converter LLC topology is shown in figure 6.
The resonant tank consists of Lr, Cr and Lp. The magnetizing inductance of the transformer itself can be used to create the Lp function of the resonant cell. Thanks to the resonant tank filter, the voltage is naturally adapted by adjusting the switching frequency, so the total losses of the converter are limited compared to a pure PWM structure.