Earlier generation Electric Vehicle (EV) onboard chargers (OBC’s) have utilized multiple LLC transformers to reach power levels up to 6.6KW and above, as covered by a previous white paper on this topic, where the primary side concentrated leakage inductance transformer eliminate the need for an external resonant inductor. However, even with this first level of magnetic integration, 2 large components are still required to reach just 6.6kW of throughput power. Along with their associated cooling requirements, these existing solutions occupy a significant volume.
EV powertrain engineers have looked to three phase systems to reduce the size of their OBC solutions when dealing with power ranges above 7kW. However, with current solutions for magnetic integration, two limitations have become evident when scaling for higher power, the thermal management of the hot spot temperature on the centre leg winding and asymmetrical current waveforms that arise from the unbalanced flux paths between the transformer phases. Besides the obvious impact on the dissipated losses within the transformer and in other circuit components due to imbalanced currents, this also presents EMC challenges due to higher ripple currents.
Addressing this challenge, Pulse Electronics was proud to present the paper ‘A Novel Design for a 22kW Transformer for a 3 Phase Full Bridge LLC EV Onboard Charger for Smaller Size, Lower Cost and Improved Performance’ at the recent PCIM conference.
In this paper, Finite Element Analysis of the traditional core shapes for integration and various alternate constructions led to the concept of a triangular core to equalizes coupling between each phase. Additional Spice modelling of the OBC circuit using a transformer based on this construction reveals that the issue of current imbalance can been eliminated. With the losses equally distributed between the three phases and an improved thermal interface, the hotpot issue suffered with the traditional construction has been avoided. The benefits of this approach are demonstrated with the realization of a CLLC transformer for a 22kW three phase OBC
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