How to calculate the conversion efficiency of the power adapter?

May 13, 2020

How to calculate the conversion efficiency of the power adapter?

The power adapter is actually a comprehensive transformer composed of a transformer, an AC/DC converter and a corresponding voltage regulator circuit. Simply put, this comprehensive transformer contains two main components, a transformer and a current converter, and these two components themselves There is a consumption of electric energy, and the voltage stabilizing circuit attached to them is no exception, so the power supply itself is a power consumer.

The energy of the input power supply cannot be 100% converted into effective energy used by the various components in the host, which is the problem of conversion efficiency we are talking about today.


Conversion efficiency is an important indicator of power adapters. High efficiency means that the power adapter itself has less loss and saves more energy. The conversion efficiency of a power adapter is defined as the efficiency divided by the total output power divided by the total input power: power efficiency η=Po/Pi .In the formula: Po is the output power, Pi is the input power.

The relationship between the conversion efficiency and temperature rise of the power adapter has to be discussed here. Because the corresponding power is lost inside the power adapter, the conversion efficiency of the power adapter cannot be 100%. The power consumed by the power adapter is expressed in the form of heat. The heating level of the power adapter itself mainly depends on the conversion efficiency of the power adapter and the size of the power adapter.

Under certain heat dissipation conditions, the power adapter has a certain temperature rise, that is, the difference between the shell temperature and the ambient temperature. The heat dissipation table of the power adapter shell The size of the product directly affects the temperature rise. For a rough estimate of the temperature rise, you can use this formula: temperature rise = thermal resistance coefficient × power consumption of the calibration block. For places with high temperature, the power adapter needs to be derated to reduce the power supply.

The power consumption of the adapter, thereby reducing the temperature rise, ensures that the temperature of the components of the power adapter does not exceed the limit value. In addition to meeting the working requirements of electronic appliances, when the output power of the power adapter is constant, the working temperature of the power adapter has a great influence on its mean time between failures (MTBF) The impact is great, high efficiency, low temperature rise makes the product have a long life, smaller volume, and smaller quality. When it comes to volume, we need to talk about power density.

Most power adapter manufacturers use the power density of the product as a standard to measure the effectiveness of the product. The power density is usually represented by watts/cubic inch: W/in3, if the power adapter cannot be used within the specified maximum ambient temperature range , it may not reach the maximum output power of the parameter. The average output power available for the power adapter is the available power density.

Available power density depends on the following factors.

■A. Required output power. The required output power is the maximum average power required by the application.

■B. Thermal impedance. The definition of thermal impedance is the temperature rise caused by power consumption, usually measured in °C/W.

C. Maximum operating temperature of the shell. All power devices specify the maximum operating temperature of the shell, which refers to the maximum temperature that the components inside the power device can withstand when working. In order to maintain the reliability of the power device, it should work at the highest temperature the following.

D. Working environment temperature. It refers to the worst ambient temperature when the power device is working. If the power device generates too much heat during operation, and it is too late to dissipate to the surrounding medium, the power device will exceed its normal working temperature. Guarantee the temperature and fail. Therefore, selecting a suitable heat sink is one of the important conditions for the reliable operation of components.


The main parameters required in the thermal design of power devices are as follows.

■1. The working junction temperature Tj of the power device: that is, the maximum working temperature limit allowed by the device. This parameter is provided by the manufacturer, or is mandated by the product standard.
■2. The power loss Pz of the power device: the average steady-state power consumption generated by the device itself during operation, defined as the product of the average RMS output current and the average RMS voltage drop.

■3. Dissipated power Q of power devices: refers to the heat dissipation capacity of a specific heat dissipation structure.

■4. Thermal resistance R of power devices: refers to the temperature rise generated by unit power consumption when heat is transferred between media.

The efficiency of power adapters used in household appliances is now more and more concerned. If the efficiency of the converter is low, it means more power consumption. The power adapter efficiency test should be measured under rated input voltage and full load conditions. Many power adapters often It works under half load, so we also test the light load efficiency when we test, and the power adapter designer should pay special attention when choosing the topology.

Efficiency is related to power. This standard is the standard of Energy Star in the United States, and it is also an industry standard for switching power supply management. As long as the power adapter is to be exported to developed countries in Europe and the United States, it must meet this standard. Our qinxpower adapters have all reached the highest standard of energy efficiency level VI.


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