Investigation of on-chip inductors for fully-integrated DC-DC converters

Abstract

With the increase in the switching frequency of DC-DC switching converters, the volume of SMD (surface mount device) inductors has drastically decreased. These minute off-chip inductors, however, still occupy a substantial PCB (printed circuit board) area that restrict their use in SoC (system-on-chip) applications. While on-chip inductors have long been used in radio-frequency integrated circuits (RFICs), their deployment for power management applications has been quite limited. It is attractive yet challenging to investigate using an inexpensive bulk technology to fabricate on-chip inductors using only available metal layers, such that the corresponding fully-integrated switching converters could be used in SoC applications. This investigation starts with a review of the past works on fully-integrated power converters. It is then followed by the modeling and analysis of on-silicon inductors using a bulk 0.18 µm CMOS process. By shunting different metal layers in parallel, both the inductance and the quality factor (Q) have been enhanced. Inductors with good performance are then employed in two open-loop buck converters for comparison: the first uses an off-chip discrete diode, and the second uses an on-chip active diode. All inductors and converters have been sent for fabrication. The area of the inductors was designed to be 4 mm² each that could carry a peak current of 80 mA and was measured to have values of around 250 nH over a wide range of frequency. The measured quality factor Q varied linearly with respect to frequency, conforming to the simulation results. The buck converters were switched at 30 MHz with a fixed duty ratio of 0.5, to generate an output voltage of approximately 1.2 V from an input voltage of 2.4 V. The peak efficiency was verified to be 69.1% for a light load current of 12.9 mA. If the same voltage conversion ratio were to be implemented by a low dropout regulator, the efficiency would be 50%, and hence, the efficiency enhancement factor (EEF) is more than 20%.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology