Effective Load Resistance

Posted on Feb 7, 2014

This article offers an alternative to efficiency measurements as an evaluation tool of power supplies. The proposed method works well with little dependency on the output voltage and temperature. Power supplies have always been evaluated and compared based on the how efficient the conversion process is. Equations 1 and 2 are two of the most used f

Effective Load Resistance
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ormulae to calculate the power conversion efficiency ·. The use of efficiency as a yardstick to evaluate DC-DC converters represents a very simple and in most cases effective way of comparison. Efficiency figure combined with the details of the converter parameters also allows the thermal engineer to calculate the thermal load of the converter and whether heatsink and/or cooling airflow is required. The efficiency tool works very well when we have established standard input and output voltages that are more or less fixed in value. We will show that the efficiency alone is not the right tool to compare converters operating at different output conditions even with a fixed input voltage and switching frequency. We will explain why that is and propose an alternative methods that measures the performance almost  independent of the output voltage that gives a much more accurate and unambiguous idea about the converter`s performance so we introduce the concept of the Effective Loss Resistance, Rol. Figure 1 depicts a simplified block diagram of a synchronous buck converter used in the evaluation of this paper. It is worthwhile mentioning that this work may be adapted to almost all DC-DC converters. Figure 2 shows the efficiency of a given converter at different output voltages. Although the power loss is mainly dependant on the load current at a fixed switching frequency and input voltage and is very lightly dependant on the...

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