Effect of cooling method on power supply operating temperature
The heat dissipation of the power supply generally adopts the two ways of direct conduction and convection conduction, direct heat conduction is the heat energy along the object from the high temperature end to the low temperature end, the ability of heat conduction is stable. Convective conduction is a process in which a liquid or gas tends to homogenise its temperature by rotary motion. Because convection conduction involves a kinetic process, cooling is more smooth and rapid.
Mounting the hair element on a metal heat sink enables the transfer of energy by squeezing the hot surface to achieve a high and low energy body, and not much energy can be radiated out by relying on the large area of the heat sink. This type of heat transfer is called natural cooling, which has a long delay time for heat dissipation. Heat transfer Q = KA △ t (K heat transfer coefficient, A heat transfer area, △ t temperature difference), if the indoor ambient temperature is high, the absolute value of △ t is small, when the heat dissipation performance of this heat transfer method will be greatly reduced.
Natural cooling
Natural cooling is the traditional cooling method of switching power supply in the early days, this method mainly relies on a large metal heat sink to carry out direct heat conduction type heat dissipation. Heat transfer Q = KA△t (K heat transfer coefficient, A heat transfer area, △t temperature difference). When the rectifier output power increases, the temperature of its power components will rise, △ t temperature difference also increases, so when the rectifier A heat transfer area is enough, there is no time lag in its heat dissipation, the power components of the temperature difference is small, its thermal stress and thermal shock is small. However, the main disadvantage of this approach is the heat sink volume and weight. Transformer winding for the lowest possible temperature rise, to prevent the rise in temperature affects its performance, so its material selection margin is larger, the volume and weight of the transformer is also large. Rectifiers have high material costs and are inconvenient to maintain and replace. Because of its cleanliness requirements for the environment is not high, at present for small-capacity communication power supply, in some small professional communication network and some applications, such as electric power, petroleum, radio and television, military, water conservancy, national security, public security and so on.
Fan cooling
With the development of fan manufacturing technology, fan stability and service life has been a big step forward, the average failure-free time is 50,000 hours. The use of fan cooling can be reduced after the bulky radiator, so that the volume and weight of the rectifier greatly improved, the cost of raw materials is also greatly reduced. With the intensification of competition in the market and the decline in market prices, this technology has become the main current trend.
The main disadvantage of this approach is that the average failure-free time of the fan is shorter than the rectifier 100,000 hours, if the fan fails on the power supply failure rate. So in order to ensure the service life of the fan, the speed of the fan is changed with the temperature inside the equipment. Its heat dissipation Q = Km △ t (K heat transfer coefficient, m heat transfer air quality, △ t temperature difference). m heat transfer air quality is related to the speed of the fan, when the rectifier output power increases, the temperature of its power components will rise, and the change in the temperature of the power components to the rectifier to be able to detect this change, and then to increase the speed of the fan to strengthen the heat dissipation, there is a large lag in time. If the load is often sudden changes, or utility input fluctuations, it will cause the power components to appear rapid hot and cold changes, this sudden change in the semiconductor temperature difference generated by the thermal stress and thermal shock, will lead to components of different materials part of the stress cracks. Make it premature failure.
