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Forced air cooling
1. When the loss of power components such as rectifier bridges is high (>4.0W), the natural cooling method can not meet the heat dissipation requirements. At this time, forced air cooling must be used to ensure the normal operation of components. . When forced air cooling is used, it can be considered in two cases:
a) the rectifier bridge does not have a radiator;
b) The rectifier bridge has its own radiator.
1. The rectifier bridge does not have a radiator. For the case where the rectifier bridge does not have a radiator and uses forced air cooling, the analysis process is the same as the natural cooling, except that the thermal resistance and PCB board for calculating the heat dissipation between the rectifier bridge housing and the environment are calculated. When the heat transfer resistance between the environment and the environment is used, the heat transfer coefficient should be selected according to the forced air cooling condition, and the value is usually 20~30W/m2C.
From the forced convection cooling analysis of the above rectifier bridge without radiator, it can be seen that the heat dissipation path through the surface of the rectifier bridge shell is equivalent to the heat dissipation through the pin. On the one hand, we can increase the cooling wind speed. The size changes the heat transfer condition of the rectifier bridge. On the other hand, we can increase the copper coverage on the PCB to improve the heat transfer between the PCB and the environment to improve the heat dissipation capability of the rectifier bridge.
2, the rectifier bridge comes with a radiator. When the rectifier bridge comes with a radiator to carry out forced air cooling to achieve its heat dissipation purpose, the heat dissipation path in this case
Comparing the two cooling methods of natural cooling of the rectifier bridge and forced air cooling with the radiator, the fundamental difference is that the function of the radiator greatly improves the heat dissipation resistance between the rectifier bridge housing and the environment. If the contact thermal resistance between the heat sink and the rectifier bridge is neglected, combined with the heat transfer analysis of the rectifier bridge without the heat sink, we can obtain the thermal resistance of each heat dissipation path of the rectifier bridge with the heat sink for cooling as follows:
1), heat dissipation thermal resistance of the surface of the rectifier bridge housing:
a) Thermal resistance of the front housing of the rectifier bridge: same as forced air cooling without a radiator:
b) Thermal resistance of the rear housing of the rectifier bridge:
2), the thermal resistance of the flow bridge through the pin heat dissipation: the thermal resistance at this time is the same as when the rectifier bridge is forced air cooled without a radiator:
1 In the above three heat transfer paths (heat transfer on the front side of the rectifier bridge, heat transfer through the heat sink on the back side of the rectifier bridge, and heat transfer through the pins of the rectifier bridge), the heat transfer resistance of the back side of the rectifier bridge through the heat sink is minimal. The heat transfer resistance through the front of the housing is the largest, and the thermal resistance through the pin is centered;
2 Comparing the total thermal resistance of the rectifier bridge heat dissipation and the heat resistance value of heat transfer through the back surface heat sink, it can be found that the heat transfer resistance of the heat sink through the back of the housing is very similar to the total thermal resistance of the rectifier bridge. In fact, the conclusion also shows that in this case, the main heat transfer path of the rectifier bridge is carried out through the radiator on the back of the casing, that is, most of the loss on the rectifier bridge is discharged through the radiator. The amount of heat dissipated through other means (the front side of the pin and the housing) is small.
3 Because the heat dissipation condition of the rectifier bridge is closely related to the thermal resistance of the heat sink, the heat resistance of the heat sink will directly affect the temperature of the rectifier bridge. It can be seen that when the thermal resistance of the rectifier bridge radiator is provided in the rectifier bridge parameter table provided by the manufacturer, it may only be the crust (Rjc) on the back of the rectifier bridge or the total junction on the rectifier bridge housing. Shell thermal resistance (parallel connection of front and back thermal resistance); the thermal resistance of the junction environment at this time has no reference value because it changes significantly with the thermal resistance of the heat sink.