Multilayer Ferrite Inductor Substrate for Ceramic-Based High Current Point-of-Load (POL) Converter

Abstract High power-density and high efficiency are the two driving forces for point-of-load (POL) converters used in portable electronics and other applications where system miniaturization is required. Discrete passive components, especially bulky inductors, have become the bottleneck for downsizing POL converters. Low-temperature sintered Ni-Cu-Zn ferrite tapes for multilayer chip inductors have been widely studied and used in high-frequency power electronics applications. In our previous study, a low-profile, planar inductor substrate with lateral flux pattern was fabricated using mixed commercial low-fire Ni-Cu-Zn ferrite tapes and compatible low temperature co-fired ceramic (LTCC) processing. However, thermal interface material was used between active circuit board and passive layer (ferrite substrate), which increases the total volume of the converter and becomes a potential threat for reliability due to the mismatch of coefficient of thermal expansion among different layers. Additionally, this hybrid integration method requires labor-intensive manual steps which are not compatible with cost-sensitive power electronics market. A fully ceramic-based POL module with integrated multilayer ferrite inductor has been proposed. The circuit and other components are designed to be directly built on top of the multilayer ferrite inductor substrate. This presented work focuses on the development of the multilayer ceramic substrate with embedded planar, lateral-flux inductor by co-firing of ferrite and dielectric tapes with conductor paste. Commercial dielectric LTCC and ferrite tapes were chosen for the fabrication of multilayer ferrite inductor substrate. Different silver pastes were co-fired with ceramic tapes to form the inductor winding. The sintering behavior and compatibility of dielectric, magnetic, and conductive components in one co-firing process was studied in order to realize a cohesive multilayer ceramic substrate. The embedded inductors present lower inductance than pure ferrite inductors sintered alone using the same profile when the output current is smaller than 10 A. The inductance of both types of inductors are very similar when output current is above 15 A. The inductor embedded in dielectric tapes exhibits higher core loss density than its counterpart. Future work will focus on the integration of high current POL module using this developed multilayer ferrite inductor substrate.