Harry Arbetter. Robert Erickson. and Dragan Maksimovid
Power Electronics Group Department of Electrical and Computer Engineering University of Colorado, Boulder, CO 80309-0425 USA
Abstrurt - This paper describes performance, design and optimization of de-dc converters for energy limited, battery operated systems. Variable-frequency operation is used to achieve voltage regulation and high efficiency for an extremely wide range of load currents. An experimental 15W, 3.3V buck converter has been constructed to demonstrate design and optimization techniques. The converter employs synchronous rectification to reduce conduction losses, and discontinuous, variable-frequency, current-mode control with optimum peak current to maximize efficiency for a wide range o f loads. Applications include portable computers, hand-held instruments, and telecommunications.
Energy limited, battery powered systems require the utmost in efficiency in order to provide full system capability and maximum battery life. The use of low voltage power supplies (3.3 volts or less) can reduce power consumption at the expense of lower noise margins and a requirement for better voltage regulation. Elements which require high instantaneous power. such as transmitters. microprocessors, backlit displays, and flash memory, can be switched to a low power standby mode when not needed. Applications include portable computers, hand-held instruments. and wireless telecommunications. To fully realize such systems, dc-dc converters are needed which can: (a) regulate the load voltage with (ideally) zero load current; (b) operate at high efficiency with many orders of magnitude variation in the load current; (c) operate efficiently at low output voltages (3.3 volts or less). Losses in a switch-mode converter can be classified as: loud dependent conduction losses (due to transistor onresistance, diode forward voltage drop, inductor winding resistance, capacitor equivalent series resistance); freyuencj, dependent switchrng losses (due to transistor and diode output capacitance charge and discharge, gatedrive losses, voltage/current overlap at switching transitions, inductor core losses, and controller frequencydependent power consumption); $xed losses (due to controller standby current, and leakage currents of transistors, diodes, etc.). The loss budget for a conventional fixed-frequency dc-dc converter is shown in Fig. 1 It is clear that the converter cannot meet the requirements in battery-operated systems be0-7803-2730-6195 $4.00 0 1995 IEEE
cause the switching losses do not scale with load, and the light-load efficiency is poor. Also, it is difficult to maintain output voltage regulation when the converter-is unloaded. An approach to the above problems is to allow variable frequency operation, at least at light loads. This approach has been shown to yield significantly improved efficiency in a wide load range, and is currently supported by dedicated controllers [1,2]. Fig. I shows how the variable-frequency approach offers reduced losses at light loads by reducing both conduction and switching losses in proportion to the load. The performance at zero load is ultimately limited by fixed losses. The purpose of this paper is to describe analysis, design, and optimization of a variable frequency converter for battery powered applications, and to suggest techniques to further improve efficiency and noise performance. The paper describes a variable-frequency, current-controlled buck converter which utilizes synchronous rectification and is operating the discontinuous mode at light loads. The input voltage range is 4 to 15 volts and the output voltage is 3.3 volts. Fixed Frequency Converter
Variable Frequency Converter
Loaa u r r e n t
Fig 1 Losses in fixed-frequency and variablefrequency switch-mode corwertets