Design Of High Efficiency With Full Load Range Buck-Boost DC-DC Converter

Portable device now becomes the most famous topic all over the world. With the rapid development of the portable devices, it challenges the designer of power management circuits. And the demand of the power system in portable system becomes more and more critical. For example: longer battery life, higher power density, smaller volume, more reliability and etc.The Li-battery is widely used in portable devices for its high energy density, low self-discharge current and low cost. So a buck-boost topology is worthiness to enlarge the output voltage range of the battery. It will maintain the battery life.Based on widely literature reading and a great deal of studying, a four switches buck-boost topology is used in this thesis. It can achieve non-invert voltage transfer converter with small value of L/C compared with the traditional buck-boost topology. And the converter works in different modes at different supply voltages to reduce the number of working switches at the same time. It can improve the efficiency of the converter.Portable device usually works in light load condition, even standby. But the efficiency of traditional PWM modulation will fall dramatically with the decreasing of the load current. In this paper, different modulation modes according to different load conditions are used to improve the efficiency in light load condition——PWM in heavy/medium load condition and Burst modulation mode in light load condition.Several problems are resulted from using different working states: the seamless transfer between different modes, the automatic transfer between different load conditions and the circuits reusing in different states. In this paper, the seamless transfer between different modes is achieved by judging the duty cycle; the automatic transfer between different load conditions is achieved by judging the output of error amplifier; most of the blocks can be reuse in different states in the design of the circuits.Synchronous MOSFET switches are used and all the power switches are intergraded on the chip for higher efficiency and reducing the complexity of the periphery circuit.At last, all the circuit and the system are accomplished in BCD 1.5μm technology and simulated in Candence Specter S. The simulation results show all the circuits and the system works well, and almost all expected specifications were realized.