Design and Comparison of FR4 and Flex PCB Transformers for High Frequency High Current, Low Profile Applications

PCB transformers are desirable for future high density power converter applications due to their well established manufacturing process which reduces transformer variation, labor, and system cost. While 1 ~ 3 MHz PCB transformers have been demonstrated commercially, the power and consequently current rating is low, typically below 300 W. In the literature, > 500 kHz PCB transformers have been shown but with little information about their thermal performance, which remains the bottleneck for high density magnetics. This paper analyzes flexible PCB technology with its naturally thin dielectric as a way to enable low leakage inductance and high density PCB transformers with acceptable thermal margins. 10-layer, 1 MHz, 69 A rms flex and FR4 PCB transformers were constructed and flex PCB showed 15 - 20 °C peak temperature reduction. In order to carry current among 5 parallel secondary layers in the 69 A rms transformer, 14 vias are used. This paper proposes via current sharing strategies for high frequency, high amperage PCB transformers which fully utilizes all 14 vias, as opposed to only half with a simple line or rectangle formation.