(An) on-chip Cockcroft-Walton switched-capacitor converter using split-phase control for improved soft-charging with 9.72 conversion gain and 70.1% power efficiency

Abstract

This work propose a switched-capacitor(SC) converter for high voltage generation required for medical ultrasound imaging system. Among high voltage generation circuits, an inductor-based DC-DC converter generates a high output voltage from low input voltage by storing magnetic energy in the inductor. However, when the conversion gain is large, a high current inductor is required, and accordingly an off-chip inductor having a large area is used. Therefore, due to large area of this inductor, it is difficult to apply it to the biomedical applications. Capacitor has a greater energy density in the same area than inductor. Besides, capacitor can be integrated more easily than inductors. Unlike the inductor-based DC-DC boost converter that employs magnetic energy storage, SC converter employs only switches and capacitors. Inductor-based DC-DC boost converter requires a bulky power inductor and high voltage (HV) devices that requires a large area. Therefore, SC converter is quite advantageous for the implementation of an integrated circuit(IC) in terms of area. Besides, SC converter has relatively less conduction loss than inductive DC-DC boost converter. For these reasons, the SC converter is suitable for generating high voltages in ICs. However, the power loss of the SC converter can be analyzed by dividing it into three major losses. This work analyzes these losses and applies techniques to reduce them in order to improve the conversion gain and power efficiency. As a result, an on-chip Cockcroft-Walton SC converter using split-phase control for improved soft-charging with conversion gain 9.72 and power efficiency 70.1% suitable for the portable ultrasound imaging device is implemented.