Folded Flex-PCB Winding Planar Transformer for High-Frequency Isolated DC-DC Converters

In modern high-frequency DC-DC converters such as electric vehicle (EV) chargers, transformers can commonly contribute to ~45% of the losses and a third of the box volume. Meanwhile, the magnetics can strain such converters’ thermal management and electromagnetic interference (EMI), especially when targeting high power densities. This underlines the need for innovative solutions in magnetic design. One such innovation arises from recent advancements in flexible printed circuit board (PCB) manufacturing, allowing highly interleaved windings with sufficient copper thickness while eliminating termination losses between PCB boards. Therefore, this paper assesses flex-PCB designs for high-frequency transformers in EV chargers, addressing challenges and opportunities to conventional PCB-winding designs. The key modelling and trade-offs are detailed, considering the impact of parasitic capacitance, core loss, winding loss, leakage inductance, and thermal management for different configurations. From the analytical and simulation results, a design candidate with low transformer loss and parasitic effect is selected and experimentally characterised for its implementation in a 500 kHz CLLC converter.