Internal resistance of power supply: the culprit of DC-DC conversion efficiency - Power Circuit - Circuit Diagram
Abstract: DC-DC converters are commonly used in battery-powered portable and other high-efficiency systems that are more than 95% efficient when boosting, stepping down, or inverting the supply voltage. Internal resistance of the Power Supply is an important factor limiting efficiency. This paper describes the effect of internal resistance of the power supply on efficiency, describes how to calculate efficiency, matters needing attention in practical applications, design considerations, and gives a practical application example.
DC-DC converters are very commonly used in battery-powered devices or other applications that require power savings. Similar to a linear regulator, a DC-DC converter can generate a lower regulated voltage. However, unlike linear regulators, DC-DC converters can also boost the input voltage or invert it to a negative voltage. There is another benefit. The DC-DC converter is capable of giving over 95% conversion efficiency under optimized conditions. However, this efficiency is limited by the energy consuming components, and one of the main factors is the internal resistance of the power supply.
The energy consumption caused by the internal resistance of the power supply will reduce the efficiency by 10% or more, which does not include the loss of the DC-DC converter! If the converter has enough input voltage, the output will be normal and there is no clear indication that power is wasted.
Fortunately, measuring input efficiency is a simple matter (see the Power section).
Larger power supply internal resistance also produces other less obvious effects. In extreme cases, the converter input will go into bistable or the output will fall under maximum load. The bistable means that the converter exhibits two stable input states, each having its own different efficiency. The converter output is still normal, but the system efficiency may be very different (see How to avoid bistable).
Is it just a matter of simply reducing the internal resistance of the power supply to solve the problem? Otherwise, the system may require additional solutions because of limited real-world conditions and trade-offs in cost/benefit. For example, a reasonable choice of input supply voltage can significantly reduce the requirements for internal resistance of the power supply. For DC-DC converters, higher input voltages limit the input current requirements and also reduce the internal resistance of the power supply. From a general point of view, the 5V to 2.5V conversion may be much more efficient than the 3.3V to 2.5V conversion. Various options must be evaluated. The goal of this paper is to provide an analytical and intuitive approach to simplifying this evaluation task.
As shown in Figure 1, any conventional power distribution system can be divided into three basic components: a power supply, a regulator (in this case a DC-DC converter), and a load. The power supply can be a battery or a regulated or unregulated DC power supply. Unfortunately, there are a variety of energy consuming components between the DC output and the load that form part of the power supply: voltage source output impedance, wire resistance and contact resistance, PCB pads, series filters, series switches, A resistor such as a hot swap circuit. These factors can seriously affect system efficiency.
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