A System Perspective on Specifying Electronic Power Supplies: Efficiency

In the last installment of this series entitled “A System Perspective on Specifying Electronic Power Supplies,” we discussed the effects of source characteristics upon power supply specification. In this installment, we will learn about the importance of efficiency for your system and how to specify it.

What is Power Supply Efficiency?

Figure 1 shows the typical power flow from a source, through the power supply, and on to the load. Power supplies are not ideal, so not all of the power supply input power is transferred to the load as useful power. A portion of the input power is instead dissipated to the power supply environment as heat — as represented by the red ellipse around the power supply.

Power supply efficiency, then, is a measure of how much input power is transferred to the load as useful and desirable power, rather than dissipated in the form of heat in the power supply. Efficiency is expressed either as a percentage figure, or as a decimal figure. When expressed as a percentage, 100% is perfect efficiency and typical efficiencies for power supplies range anywhere from 35% to 92% depending on the type and application of the supply. When expressed as a decimal, 1 is perfect efficiency and typical efficiencies range anywhere from 0.35 to 0.92.

The following equation summarizes power flow:

Importance of Efficiency

The importance of efficiency has three main aspects: 1) energy conservation, 2) package size, and 3) temperature rise.

Energy Conservation

Energy conservation is an important consideration in:

1. Systems where the source is in the form of stored energy such as batteries or ultracapacitors,
2. Energy harvesting applications.
3. Green applications.

With stored energy systems, power supply efficiency has a major impact upon battery life or battery and ultracapacitor discharge time. A perfect example is how long a notebook computer will be able to run while on battery. An efficient power supply will maximize battery discharge time.

Energy harvesting is a relatively recent application for power electronics whereby energy is “captured” from the environment and converted to useful energy. Since, with present technology, the rate of capturing energy from the environment is relatively low, power supply efficiency plays a major role in converting as much of that energy as possible into useful power.

Green applications are a long term vision for transforming our culture into responsible users of energy. Efficiently transforming energy is a main effort for green design. An example is the effort to switch from inefficient incandescent light bulbs to efficient flourescent which require efficient power supplies.

Package Size and Temperature Rise

To the engineer not well versed in power electronics, a less obvious effect of efficiency is power supply package size and/or power supply temperature rise. In today’s design world, the power supply is often thought of as a necessary complement to the main function of the product in design. But since the power supply is not the main function, it often takes a back seat in the design process. One of the natural results is that the power supply must be unobtrusive with small size.

However, efficiency is one of the main determiners of power supply size. The reason for this is that the less efficient a power supply is, the more waste heat generated. For a given package surface area, the package temperature increases as the waste heat increases. Package temperature directly affects reliability and life. (An old thumb rule for reliability and life vs. temperature is that each 10 degree C increase in temperature reduces reliability and life by a factor of two.) In other words, for a given package temperature which results in a given reliability and life, greater waste heat requires greater package surface area or a more sophisticated and more expensive waste heat removal method.

As an example, for a given input power, an 80% efficient power supply requires twice the surface area to maintain the package at a given temperature compared to a 90% efficient power supply. Required surface area is proportional to the complement of efficiency (1-Efficiency) when expressed as a decimal.

How to Calculate Efficiency

Calculating efficiency in all cases can be done by dividing the power output by the power input:

Power is simply RMS voltage times RMS current multiplied by the power factor if the input is AC, or just voltage multiplied by current if the input or output is DC. Power factor is the distortion factor multiplied by the displacement factor. The distortion factor accounts for the effect of a non-linearly changing power supply input impedance over one line cycle. The displacement factor accounts for an effective phase difference between line voltage and line current due to reactance at the input of the power supply.

Linear Regulator Efficiency

In the special case of a DC input linear regulator, the efficiency calculation is simply the output voltage divided by the input voltage.

Switching Power Supply vs. Linear Power Supply Efficiency

In most applications, switching power supplies are more efficient than linear power supplies, and therefore offer smaller size due to the lesser waste heat. Switching power supply efficiencies typically range from 70% to 92%. Linear power supplies efficiencies can be 35% or even lower, requiring larger sizes to handle the greater waste heat.

One case where the linear power supply can offer comparable or even better efficiency than a switching power supply is the case of a low drop-out regulator with a relatively high output voltage. In this case, the voltage dropped across the regulating pass element is a very small part of the input voltage, and the output voltage is near the input voltage. Efficiencies can easily be over 90%.

How to Specify Efficiency

Efficiency is best specified for applications requiring energy conservation as discussed earlier. Efficiency varies depending on input voltage and load current. Optimal efficiency should be specified at the operating point(s) where the product is most likely to operate. For other operating points, a tolerable minimum should be specified.

For other applications not involving energy-limited sources such as batteries, ultra-capacitors, energy harvesting, or green applications, it is best not to specify efficiency. Instead, other requirements such as package size limits, and/or temperature rise limits should be specified. Efficiency in these cases should be left to the power supply designer as a design variable that is determined based on other requirements such as package size limits and temperature rise limits that are more typically specified by the application demands.