Experience in Thermal Design of Electronic Devices
The experience in and notes on the thermal solution design of electronic devices are summarized on the basis of years of design experience and experimental analysis summary, specifically as follows:
1: Enough natural convection space should be ensured. There should be a certain distance at least 13mm, between components and structures to facilitate air flow and enhance the heat dissipation of natural convection.
2: The gap between the components on a vertically mounted board and surrounding PCB should be at least 19mm.
3: In terms of the heat conduction path, the metal or heat conduction material with high heat conduction coefficient can be used to connect components with the shell or the cold plate for heat dissipation via larger surface area.
4: The emissivity can be as high as 1 by radiant heat transfer and powder coat. In the same sealed shell, the temperature rise of painted components is 10% lower than that of unpainted ones on average.
5: If large radiator cannot be installed due to high heat flux components and limited assembly space, heat pipe or vapor chamber may be used; or the heat may be led to another location where there is enough space to install a radiator.
6: Within the design life of the product, the failure rate of cooling system, such as the fan, should be lower than that of components.
7: The safety margin must be considered upon thermal design. At least a safe temperature of 20-30 ℃ should be reserved; and a safe design margin should also be reserved in consideration of an increase in heat consumption and flow resistance caused by the failure of electronic devices during use.
8: Natural convection, low speed fan and other highly reliable cooling modes should be used as far as possible.
9: When using the fan, attention should be paid to noise parameters, which must meet the design requirements for noise. Meanwhile,dust prevention and the protection device in front of the fan should be considered.
10: Product cost should be considered upon thermal design to ensure optimized cooling solution, simple product structure, high reliability, small volume and cost.
11: Cooling system, such as the fan, should be easy to monitor and maintain.
12: Each air duct should have clear air inlet and air outlet. Different air ducts should be isolated. Air flow short circuit between different air ducts should be avoided, for example, the air outlet of air duct A should not be the air inlet of air duct B.
13: In case of any vacancy of standard cabinet, a guide plate should be installed to prevent air flow short circuit for the air flow short circuit in the cabinet has great impact on the heat dissipation of components.
14: The cooling by natural convection is applicable to sealed electronic devices with small volume and dense components.
15: The air is a poor conductor of heat. When the distance between heated surfaces is very short, the heat conduction by surrounding air actually exists. In such case, The air is a poor conductor. A large number of our experiments proved that if the distance between
two surfaces < 6.35mm, the heat transfer between the two surfaces is mainly realized through the air; and the heat transfer by natural convection can be neglected. If the distance between two surfaces > 12.7mm, heat dissipation is mainly realized by convection and radiation.
空气是散热的不良导体，当受热表面之间的距离很小，周围的空气产生热传导是真实存在的，此时空气为不良导体，我们大量的实验证明，如果两个面之间距离 <6.35mm ,则两面之间主要是通过空气进行换热，自然对流 换热可以忽略不计，如果两个面之间的距离>12.7mm，散热主要是通过对流 、辐射。
16: The black anodize surface can ensure excellent heat absorption and high emissivity, while polished surface has low emissivity and can be used to shield heat radiation, for example, the emissivity of polished copper plate is 0.023 and that of black anodize can be as high as 0.95.
17: The reflectivity of metal surface is associated with roughness: the reflectivity of shinny surface is higher than that of rough surface If a polished metal surface is powder coat black, its reflectivity will be improved.
18: In order to realize the maximum radiation heat transfer, it is suggested that the surface of the fin be black. It certainly should not be understood as all surfaces should be black, for example, at 100 ℃, the emissivity of glass is equivalent to the reflectivity of black powder coating. With the increase of temperature, the emissivity of glass may be 5-10% higher than that of black coating.
19: Do not choose high-density fins for natural convection
20: At least a 30mm clearance should be reserved for the air inlet of the fan. If the air inlet is blocked, the noise will be 2-3 times higher. Besides, at least a 10mm clearance should be reserved for the air outlet for exhaust
21: Fans with different interfaces and different flows should not operate in parallel.
22: If the fan adopts a filter, the loss of pressure drop should be considered.
23: For transformer, attention must be paid to the maximum temperature value of winding group. If the maximum temperature is exceeded, the transformer will be damaged. Once the transformer is damaged, the whole device cannot work. If the transformer has a shielding case, it should be connected to the base as far as possible. The shell, magnetic core and base should be wrapped with copper strips to enhance heat conduction.
24: Experiments showed that the heat consumption of hollow coil is low, thus no serious problem of heat dissipation will occur for natural convection may be adopted. Upon thermal simulation, hollow coil can be taken as a cylinder. If only the natural convection
of the external surface can be calculated when the coil is horizontally installed, when it is vertically installed, both the internal surface and the external surface will cool down naturally.
25: During circuit design, long components should be distributed vertically (along the direction of gravity). Vertically distributed components should be alternatively arranged in horizontal direction.
26: The heat emitted from the human body is about 100W, thus such heat should be considered during the thermal scheme calculation for outer space capsule.
27: For electronic device to be used outdoors in harsh environment, protector must be considered and installed in the protective shell as far as possible.
用于户外恶劣环境的电子设备必须要考虑保护装置，尽量安装在保护机壳内面，通道宽度在6.35 – 12.7mm 之间.
28: For a natural convection electronic device with an opening, the design of the shell should ensure that the air can flow in from the bottom and flow out from the top and that the width of the air opening is between 6.35 – 12.7mm.
对于自然冷却的电子设备，有开口的，外壳在设计时应该重点考虑使空气从底部流入，从顶部流出，空气的通道宽度在6.35 – 12.7mm 之间
29: Sometimes, radiation must be controlled. Since the requirements for temperature rise are low, components with poor
temperature tolerance should be protected to avoid damage when heated. Generally, the use of smooth metal material as a shielding case can effectively protect such components from thermal radiation, but there should be a permanent connection between the thermal radiation and the PCB or the shell to form a feat transfer path.
30: Thermal design should consider ambient temperature, if a natural convection electronic product is installed in a cabinet, with the increase of cabinet temperature, the design of natural convection probably cannot meet the ambient temperature in the cabinet. If two forced convection devices are place in parallel, the distance between them should be considered, otherwise the hot air discharged from one device may enter the air inlet of the other device, causing temperature rise.
31: For electronic devices, ambient humidity should be considered. In addition, waterproof and dustproof designs should be completed, or components will be damaged. Of course, attention should also be paid to the min temperature. If the min temperature is lower than the min operating temperature of components, a heating unit should be equipped.
32: During natural convection, the temperature boundary is thick; if the fin spacing is too small, thermal boundary crossing will occur, affecting surface convection. Therefore, it is suggested that the fin spacing > 12mm during natural cooling. If the fin height is below 10 mm, the fin spacing can be calculated from the fin spacing > 1.2 times of the height.
自然冷却时温度边界比较厚，如果散热片间距太小，会产生热的边界交叉，影响表面的对流，所以一般在做自然冷却时建议散热片的Fin 之间的间距>12mm, 如果散热片Fin的高度低于10mm,可以按Fin的间距>1.2倍的高度来计算Fin之间的距离
33: During natural convection, the surface heat transfer ability of the fin is weak, the addition of corrugation on the fin has no impact on natural heat dissipation, thus the addition of corrugation on the fin is not recommended. Generally, during natural convection, black anodizing is recognized for it can increase the emissivity and radiation heat transfer on the heat-transfer surface.
34: Since the thermal balance time of natural convection is long, the thickness of fin bottom plate and the fin adopting natural convection should be considered to resist the impact of instantaneous thermal load. The thickness of fin bottom plate has great impact on thermal capacity and thermal resistance. If the bottom plate is too thin, the thermal capacity is too small. If it is too thick, thermal resistance will increase instead. According to experimental recommendation, the bottom plate should be 3-6mm thick and the fin 2-5mm thick.
由于自然对流的热平衡时间比较长，所以采用自然对流的散热片的底板厚度和Fin的厚度需要考虑， 以抗击瞬时热负荷的冲击，散热片的底板厚度对于热容量及热阻有很大的影响，如果厚度太薄热容量小，如果过厚造成热阻反而增大，根据实验建议，底板厚度3-6mm, Fin的厚度 2-5mm
35: When forced cooling is adopted, it is suggested to add corrugation on the fin. The addition of corrugation on the surface can
increase 10-20% of heat dissipation capability. If the corrugation is less than 0.5mm in height and is 0.5-1.0mm wide, the
effect of heat convection can be enhanced.
36: when the wind speed is higher than 1m/s, the natural cooling of the fin can be neglected.
37: When fluid cooling solution is adopted, the pipe must be sealed. Pipe bending should be avoided as much as possible to
reduce the loss of resistance; and the sharp expansion or contraction of the pipe should be avoided. Rectangular or square
cooling plate should be used as much as possible, so as to polish the surface and reduce the thermal resistance.
38: The material of air duct should be as smooth as possible on the surface to reduce air friction and minimize the airflow resistance.
39: For electronic device installed on walls, the air inlet and outlet should not be designed on the back of the device. If the air inlet
and outlet must be designed outside, the clearance between the device and the wall should be greater than 100mm.
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