Development and Characterization of Resonant Capacitors and Inductors for Switched Tank Converters

A new class of Switch Tank Converters (STC) have been developed to achieve more efficient step down (up) power conversion for the next-generation 48V bus conversion on data center server boards. A 98.92% efficiency STC product evaluation board (4-to-1, 650W) was developed and demonstrated. The STCs uniquely employ LC resonant tanks to partially replace the flying capacitors for energy transfer. This paper describes the key characteristics and performance of resonant capacitors and inductors developed for this type of application. Class I U2J Multi-Layer Ceramic Capacitors (MLCC) were produced with higher capacitance than more common C0G type capacitors. The U2J dielectric has a linear change of capacitance with temperature with no loss of capacitance with voltage. The resulting capacitors are very stable with little variation making them ideal for use in these tank circuits. The U2J MLCC have low Equivalent Series Resistance (ESR) resulting in very low power dissipation at 300kHz. To maximize capacitance within a given circuit space multiple MLCC were combined into surface mountable Leadless Stacks using Transient Liquid Phase Sintering (TLPS) interconnect technology. The resulting capacitors have low self-heating even at high currents of 20Arms 300kHz and remain unaffected by long term exposure to these conditions at 85°C. The resonant inductors have been developed to minimize AC losses by ferrite material selection and optimization of magnetic structure improving the full load efficiency. The properties of these components make them suitable for use in the LC pairs in the STC.