Accurate Efficiency Measurement

The efficiency of a power supply has a direct influence on reliability. It is essential – especially in the case of convection-cooled devices – on the one hand for a small and on the other hand a reliable power supply. Without additional ventilation, heat can only be dissipated to a limited degree and the designer of a power supply must be very careful to achieve the lowest possible generation of heat. Even the other essential system elements benefit from low power losses and therefore also minimal heat generation. An increasing number of power supply manufacturers thus strive for high efficiency. This article should help users determine the accurate efficiency and thus the power losses of a power supply so that they don’t have to rely completely on data sheets with often optimistic and scarce information.

Minimum efficiency changes produce plenty of influence on no-load losses

Efficiency is a figure that helps one carry out a proper comparison of different power supplies. However, system designers and users of power supplies are even more interested in the heat which remains in the power supply – i.e. the power losses. Since these power losses cannot be electrically measured in a direct manner, one can only consider the difference between input and output power.

It is important to keep in mind that seemingly small differences in efficiency mean a big difference in power losses. The values for modern power supplies lie between 92 and 95%. The user might have the impression that one or two percent of difference wouldn’t play a big role. However, this is a misconception. Because it is not the absolute value of efficiency but rather the difference from the ideal value of 100% what matters most.

At high efficiencies, even small increases produce a significant reduction in no-load losses. With increasing efficiencies, measurement accuracy keeps gaining importance.

Multimeter, wattmeter or power analyser - which is the (measurement) tool of choice?

There are a number of measuring instruments which are used for the determination of efficiency. Nevertheless, the measurement tolerances and the capabilities of measuring instruments in measuring various signals (AC or DC) varies considerably.

Accurate multimeters do a great job measuring the voltage and current of purely DC inputs and outputs. The voltage can be measured with high precision directly at the input and output of the power supply.
Data loggers
Data loggers are even better for DC measurements. They consist of a single, usually highly accurate meter, which is used several times by multiplexing.
Watt meters are used for measuring AC signals and follow the right principle. The instantaneous values of current and voltage are multiplied and a mean value is calculated from these products. Generally, only high-precision wattmeters should be used when measuring efficiency.
Power analyser
PULS uses power analysers to measure the efficiency of its power supplies. The advantages of this are the high basic accuracy of 0.02%, the correct measuring of active power, the simultaneous and thus synchronous measuring of input and output, and the direct display of power losses and efficiency. The downside of this method of measuring is the high purchase price involved. Nevertheless, the power analyser is the tool of choice for the accurate determination of efficiency.

Taking environmental conditions into consideration

In respect to environmental conditions, temperature plays a decisive role, because the power losses from a power supply are temperature dependent. The temperature of the components in a power supply is a crucial factor. The component temperature is the sum of ambient temperature and self-heating.

Temperature: The various components in the power supply react differently to temperature. When taking any efficiency measurement, the running time and the ambient temperature should be documented so that the results remain traceable.

Altitude and air pressure: Since cooling is done by air, the air pressure has an influence on self-heating. Each component has tolerances and therefore not every device is the same.

Requesting accurate efficiency specifications

Rather than having to carry out your own measurements, it is obviously easier to rely on the manufacturer’s specifications – that is, if accurate measurements were taken. Unfortunately, the datasheets for this very important characteristic often contain general statements, such as “Up to x% efficiency”. This is a best-case statement and actually means that this value is not exceeded. For instance, power losses at different mains voltages or loads are not mentioned. Therefore, users who want to know in more detail, cannot avoid taking their own measurements. Or else they find another manufacturer that gives them accurate specifications. But even then, they might feel more reassured if they take one last measurement themselves.