Technical Terms

Efficiency is the ratio of effective output to input. Efficiency is used as one of the indicators of specifications for a variety of equipment besides transformers, such as motors, lighting, and generators. There are two types of efficiency in electrical equipment. One is the measure efficiency through input and output power measurements. The other is commercial efficiency, which is determined by a formula established to determine efficiency.

Since actual measurement is difficult for large transformers and equipment, commercial efficiency is used. For power supply devices such as switching power supplies, efficiency is determined by actual measurement from the input power at rated output.

Causes of power supply inefficiency include ON resistance of switching devices, switching losses, diodes, transformer winding resistance, iron core hysteresis, capacitors, and control circuits. Most power supplies are at maximum efficiency at rated output.

Reducing output voltage and current also reduces input power, but with lower efficiency.

Example of power supply electrical efficiency calculation For a power supply with a rated output of 12V, 10A (120W), and real power of 140W, the formula 120/140 = 0.8571... yields an efficiency of 85.7%.