When debugging or repairing the circuit, we often mention the word "××burned". This XX is sometimes a resistor, sometimes a fuse, sometimes a chip. There may be very few people who will pursue the use of the word. Why not use it? "Bad" but "burning"? The reason is that in mechanical and electrical products, thermal failure is the most common failure mode, current overload, and a large current in a short space in a local space will be converted into heat and heat. The set is not easy to dissipate, causing the local temperature to rise rapidly. Excessive temperatures can burn the conductive copper, wires and the device itself. So a large part of the electrical failure is thermal failure.
Then ask a question, if you assume that the current overload is serious, but the heat dissipation in this part is very good, you can control the temperature rise to a very low range. Is the device not going to fail? The answer is "yes".
It can be seen that if you want to improve the reliability of the product, on the one hand, the high temperature resistance of the equipment and components can be improved, and it can withstand large thermal stress (because of environmental temperature or overload, etc.); The heat dissipation is enhanced, so that the heat caused by the ambient temperature and the overload is completely dissipated, and the reliability of the product can be improved. The general method of the thermal design is described below.
The common heat dissipation method of our electromechanical equipment is the heat dissipation method of the heat sink and the fan. Sometimes the heat dissipation is not enough, and sometimes the heat is excessively dissipated. Then, when should the heat be dissipated, which way is the most suitable for heat dissipation? This can be evaluated based on heat flux density, heat flux = heat / hot aisle area.
According to the GJB/Z27-92 Electronic Equipment Reliability Thermal Design Manual (Figure 1), an acceptable heat dissipation method is obtained based on the acceptable temperature rise requirements and the calculated heat flux density. For example, if the temperature rises to 40 °C (vertical axis) and the heat flux density is 0.04 W/cm2 (horizontal axis), the intersection is found as shown below, and falls in the natural cooling zone to obtain natural convection and radiation to meet the design requirements.
Figure 1 cooling method
Most of the thermal design applies to the above chart because essentially the heat is dissipated through the face. However, for sealed equipment, it should be estimated by volume power density, thermal power density = heat / volume. The figure below (Fig. 2) is the heat dissipation method corresponding to the power density of different volumes when the temperature rise requirement does not exceed 40 °C. For example, a power supply adjustment chip, the heat consumption is 0.01W, the volume is 0.125cm3, the volume power density is 0.1/0.125=0.08W/cm3, and the following figure shows that the metal conduction cooling can meet the requirements.
Figure 2: The heat dissipation method corresponding to different volume power densities when the temperature rise requirement does not exceed 40 °C
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