In the development of power adapter, there will be a variety of parameter performance test evaluation, the temperature is one item.   

      Normally, the surface temperature of the power adapter will be controlled within 30-35℃. As we all know, the power adapter is used indoor, so generally the temperature will not more than 40℃, normally the tempreture is around 25℃. Even if it's high as 40℃, the power adapter case temperature is within 75℃. The temperature resistance of the shell is about 100℃, and the temperature of the nameplate is about 80℃. So the 75℃ is roughly the limit value. In fact, as long as the temperature of the power adapter is within the range of design, normally there's no danger.

      It's normal that the Power adapter heating. The conversion efficiency of the power adapter to the power only can reach about 75-85％. During the voltage conversion, a part of the energy is lost, except for a small part of the loss in the form of waves, most of it comes out as heat. The greater the power of the power adapter, the more energy is lost, the more heat the power source generates.
At present, the power adapters are all fireproof and high temperature resistant plastic sealed packages on the market，the inside heat is mainly dissipated by conduction through the plastic case. Therefore, the surface temperature of the power adapter is still quite high, and the highest temperature will even reach about 70℃.