Bidirectional DC voltage conversion for low power applications

Battery-powered mobile equipment is an important pillar of the electronic consumer market, especially since cellular phones and digital cameras have been introduced. However, all these mobile equipment have the same major weakness: their battery provides a limited operating time, which can only be increased in two ways. First, the energy density of the battery can be increased by developing new battery chemistries. Second, the battery energy can be used more efficiently by improving the energy management. This thesis focuses on the latter, and especially on the voltage conversion, which is used in mobile equipment. The novel concept exposed in this thesis consists in combining the voltage conversion unit with the battery management unit, thus building an intelligent power converter (IPC), that is integrated into the battery. This intelligent battery is able to provide a regulated and adjustable voltage directly to the mobile equipment, thus making it adaptable to every mobile equipment. Because the battery must also be recharged, the IPC must allow a bidirectional energy flow. The IPC has been designed, simulated, laid-out and manufactured in a 0.18 μm mixed-signal CMOS technology from UMC. A full-custom design-flow using Cadence software was elaborated. In addition to the models provided by UMC, Monte-Carlo models were developed for simulating the impact of abrication process variations. For the power part, electromigration design rule checks have been developed to ensure that metal overstress due to high current flows is avoided. The characteristics of the IPC a re an operating voltage range between 1.2 V-3.6 V, an average load current up to 2000 mA, and an operating frequency in the range of 100 kHz-10 MHz. Several novel solutions were developed for the IPC. First, since the direction of the energy flow is defined by the presence of a battery charger, a method was developed for detecting automatically the connection of a battery charger in parallel to the load. Second, a continuous regulation loop was developed, which enables highly efficient step-up and step-down conversion in both directions and at high switching frequencies. Third, dynamic MOSFET sizing was developed, to maximize the conversion efficiency when a light load is supplied. At switching frequencies above 1 MHz, this method provides more than 25% of absolute improvement in effi ciency. Fourth, a current sensing method has been developed for estimating the average inductor current at switching frequencies up to 10 MHz. Fifth, an I2C interface was implemented, to enable digital programming of the battery management. Since the intelligent battery contains a battery management and provides an adjustable voltage, it can be easily replaced. This enables battery upgrading (e.g., different chemistry, higher energy density), so that the operating time of the mobile equipment is extended. The integration of electronics provides protection functions against shortcuts, overcharging, or also counterfeit.