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The case temperature of the rectifier bridge diode is the key. Generally speaking, it is determined by the case temperature.

        The determination of the case temperature of the rectifier bridge during forced air cooling is based on the analysis and calculation of three different cooling methods in the above two cases. The ambient thermal resistance (Rja) is used to calculate the junction temperature of the rectifier bridge, so that it can be easily checked whether our design meets the temperature derating standard of the power components; the forced air cooling without a radiator is used for the rectifier bridge. It is rarely used in actual use, and will not be discussed too much here. If this situation is involved in the application, the calculation method of the natural cooling of the rectifier bridge can be used for reference; when the rectifier bridge is cooled by a radiator, we can only refer to the junction-to-case thermal resistance (Rjc) provided by the manufacturer, by measuring the rectifier bridge The case temperature of the bridge can be calculated to calculate its junction temperature to achieve the purpose of inspection. Here, we focus on the selection of measurement points for the calculation of the shell temperature and the related calculation methods, and propose a feasible and reliable measurement method in practical applications.

       From the previous analysis of the rectifier bridge with a radiator to realize its heat dissipation process, it can be seen that the main loss of the rectifier bridge is dissipated through the radiator on the back of the rectifier bridge. Therefore, when discussing how to determine the case temperature of the rectifier bridge, it is about its heat transfer through the pin. Now combined with the loss of the RS2501M rectifier bridge applied to the 110VAC power module (maximum 22.0W) to analyze. Assuming that the temperature on the outer surface of the rectifier bridge housing is the junction temperature (ie 150.0C), the surface heat transfer coefficient is 50.0W/m2C (under normal circumstances, the convection heat transfer coefficient of forced air cooling is 20~40W/m2C). Then when the ambient temperature is 55.0C, the heat dissipated to the environment through the front surface of the rectifier bridge is:

     Ignoring the heat transfer of the rectifier bridge pins, the heat transfer through the back of the rectifier bridge is:

Since the thermal resistances on the two heat transfer paths on the surface of the rectifier bridge case (front of the case, back of the case) are:

According to the definition of thermal resistance:

so:

       It can be seen from the above formula that the temperature difference between the junction temperature of the rectifier bridge and the front of the case is much smaller than the temperature difference between the junction temperature and the back of the case, that is to say, the temperature of the front surface of the case of the rectifier bridge is much larger than its temperature of the back. If we use the front side temperature of the rectifier bridge case (usually it is better to measure) as the case temperature we calculate when we measure, then we will overestimate the junction temperature of the rectifier bridge! So in that case, how should we determine the calculated shell temperature? Since the back of the rectifier bridge is interconnected with the radiator, and the heat is mainly dissipated through the radiator, there is only a contact thermal resistance between the substrate temperature of the radiator and the temperature of the back shell of the rectifier bridge. Generally speaking, the value of the contact thermal resistance is very small, so we can use the value of the substrate temperature of the heat sink to replace the case temperature of the rectifier bridge, which is not only easy to implement in measurement, but also will not bring intolerable to the final calculation. error.