Katsuhiko Onitsuka

New substrate for micro DC-DC converter

Recently, mobile phones and PDAs (personal digital assistants) have advanced high performances and multi functions. A wide variety of devices and electrical parts are used to enhance their performance. As the consumption of electrical energy of the equipment has increased, several small-sized DC-DC converters have been used as power sources to supply suitable voltage to the device efficiently. And, further miniaturization of the DC-DC converter is demanded. The current DC-DC converter consists of an IC and several surface mounted components on a PCB, and they are modularized. The main factor in miniaturization and low profile of the DC-DC converter is the size of chip inductors. It has been difficult for chip inductor suppliers to satisfy both low profile and high inductance. We have developed the LTCC (low temperature co-fired ceramic) substrate with embedded high value inductor to achieve further miniaturization and low profile of the DC-DC converter. The substrate size is from 3 mm SQ. to 5 mm SQ. A high value inductor from 1 muH to 10 muH is embedded with the low temperature co-fired ferrite material using multilayer technology. Use of Ag conductor makes it possible to achieve low resistance of the inductor pattern. The DC-DC converter can be miniaturized, because the devices and the chip components can be mounted on both surfaces of the substrate. This paper describes the structure of the inductor embedded substrate, the ferrite material, the inductor design, and the electrical characteristics, and the results of reliability tests