Design Considerations on Voltage and Current Transfer Ratio of High-Frequency Transformers

A high frequency (HF) transformer is key equipment for dc-dc converter to achieve galvanic isolation and voltage transformation. As operating frequency increases, voltage and current transfer ratio of HF transformer may significantly deviate from the turn ratio due to parasitic effects, failing to transfer expected voltage and current from the primary side to the secondary side. In this paper, design considerations on voltage and current transfer ratio of the HF transformer are investigated to obtain expected transfer performance. By revealing parasitic effects on transfer characteristic (TC) of the HF transformer with load, deviation between voltage/current transfer ratio and turn ratio is correlated with the ratio of voltage/current TC resonant frequency and operating frequency. Accordingly, constraints for parasitic parameters to obtain expected transfer ratio are proposed, and practical measures to achieve improvements in transformer design are discussed. A 20kHz HF transformer prototype is used as an application, by which evaluations on deviation of voltage/current transfer ratio and turn ratio are verified, and voltage/current transfer performance of the prototype are effectively improved by optimal design based on the proposed principles.