Source Resistance: The Efficiency Killer in DC-DC Converter Circuits - AN3166

DC-DC converters, common in battery-driven, portable, and other high-efficiency systems, can deliver efficiencies greater than 95% while boosting, reducing, or inverting supply voltages. Resistance in the power source is one of the most important factors that can limit efficiency. This application note describes the effects of source resistance, how to calculate efficiency, real-world considerations, design considerations, and shows a real-world example. DC-DC converters are commonly used in battery-operated equipment and other power-conserving applications. Like a linear regulator, the DC-DC converter can regulate to a lower voltage. Unlike linear regulators, however, the DC-DC converter can boost an input voltage or invert it to create a negative voltage. As an added bonus, the DC-DC converter boasts efficiencies greater than 95% under optimum conditions. However, this efficiency is limited by dissipative components. The main cause is resistance in the power source. Losses due to source resistance can lower the efficiency by 10% or more, exclusive of loss in the DC-DC converter! If the converter has adequate input voltage, its output will be normal and there may be no obvious indication that power is being wasted. Fortunately, testing the input efficiency is a simple matter (see the Source section). A large source resistance can cause other, less obvious effects. In extreme cases, the converter's input can become bistable, or its output can decrease under maximum load conditions. Bistability means that the converter exhibits two stable input conditions, each with its own efficiency. The converter output is normal, but system efficiency may be drastically affected (see How to Avoid Bistability). Should this problem be solved simply by minimizing the source resistance? No, because the practical limits and cost/benefit trade-offs posed by the system may suggest other solutions. A prudent selection of power-supply input voltage, for example, can considerably minimize the need for low source resistance. Higher input voltage for a DC-DC converter limits the input current requirement, which in turn lessens the need for a low source resistance. From a systems standpoint, the conversion of 5V to 2.5V may be far more efficient than the conversion of 3.3V to 2.5V. Each option must be evaluated. The goal of this article is to provide analytic and intuitive tools for simplifying the evaluation task.