High voltage insulation design of coreless, planar PCB transformers for multi-MHz power supplies

Abstract

This paper investigates the insulation design for printed, planar, coreless, and high-frequency transformers with high isolation-voltage. By using finite element analysis on 2D axial-symmetry, the transformer circuit parameters and electric field distribution are modelled and estimated. Several transformers are designed for an operating frequency of 6.78 MHz. The high frequency, coreless design allows for using thicker insulation material while ensuring a high transformer efficiency. The inclusion of the coupling capacitance in the design optimisation results in several design solutions with the same figure of merit, but with different footprint and isolation voltages. Moreover, high electric fields are identified around the sharp edges of the PCB windings. Finally, the electrical and isolation performance is verified experimentally. The measured electrical properties are close to the simulated values, validating the chosen model. Breakdown tests demonstrate the feasibility of isolation voltage levels up to several tens of kilovolts. The majority of breakdowns occurs at the outer edge of the PCB winding that was identified as a high-field area. Additionally, a concept for grading the electric field of PCB windings is also proposed. Based on the results, the design aspects are discussed in detail for planar, high-frequency isolation transformers with medium-voltage isolation level.