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Cooling Device for Power Adapter

Principles, procedures and related parameters of thermal design of power adapters in China. Power adapters have been widely used in various current electronic equipment, and their unit power density is also increasing. The definition of high power density has reached hundreds of watts per cubic inch from 25W/in3 in 1991, 36W/in3 in 1994, 52W/in3 in 1999, and 96W/in3 in 2001.


Since a large number of high-power semiconductor devices are used in the power adapter, such as rectifier bridge stacks, high-current rectifiers, high-power triodes or field effect transistors, they will generate a lot of heat when they work. If the heat cannot be discharged in time Keeping it at a reasonable level will affect the normal operation of the power adapter, and even damage the power adapter in severe cases.


In order to improve the reliability of the power adapter work, thermal design is an essential and important part in the power adapter design.


Ⅰ. The design points of the power adapter radiator


1. Under the premise of ensuring the heat dissipation requirements, try to choose a radiator with small size and light weight.


2. When installing, try to increase the contact area and pressure between the device and the heat sink, and apply silicone grease to the contact surface. Pay attention to the way and direction of installation.


3. The surface of the heat sink should be rough, but the contact surface with the device should have a good finish and be painted black.


4. When the tube is insulated from the casing, the radiator should be insulated from the casing, not only electrical insulating sheets.


5. In the case of sealing, pay attention to no convection.


6. Air duct design during air cooling.


While reducing the heat generated by the power switch as much as possible by optimizing the design, it is also necessary to use the heat transfer principles of conduction, convection and radiation through the radiator to quickly release the heat generated by the components to the surrounding environment to reduce the internal heat accumulation, so that the operating temperature of the components is reduced, which is thermal design.


Ⅱ. The basic principles of thermal design of the power adapter


1. It is necessary to clearly understand the working environment of the power adapter, including the variation range of ambient temperature, the radiation of the sun or other surrounding objects, etc. The heat dissipation scheme of the power adapter should meet the requirements of the application environment.


2. For components with class II derating, the maximum allowable temperature can be determined according to the data of the components.


3. The thermal design of the power adapter should be considered at the same time as the industrial modeling design, electrical design, structural design, reliability design and electromagnetic compatibility design. On the premise of ensuring electrical performance and reliability requirements, trade-off analysis and compromise solutions.


For example, when designing ventilation mesh panels, it is generally desirable to have a high opening rate, which involves hole spacing and hole size. From the size of the hole, of course, it is hoped that the bigger the better, but on the one hand, considering the requirements of electromagnetic compatibility, the diameter of the opening generally does not exceed 1/20 of the interference wavelength; on the other hand, according to the requirements of safety regulations, the opening of the system Any of the following requirements must be met.


①Measured in any direction, the size of the hole should not exceed 5mm.


②If the width of the hole is within 1mm, the length is not limited.


③The size of the hole is not limited, but it must be ensured that foreign objects will not fall directly into the hole and touch the parts with dangerous voltage.


4. It is necessary to ensure that the cooling system has a simple structure, reliable operation and low cost. An important principle is to strive to use mature technology and mature technology to design high-reliability products.


5. Consider the vibration and noise of cooling equipment.


6. If forced air cooling is used, the inlet temperature of the cooling air should be limited. Even if the inlet of the cooling air is far away from the outlet of the hot air of other equipment, the cooling air cannot be used for secondary cooling.


7. To improve the maintainability of the cooling system. When repairing the cooling system, it should have good safety and reliability. The dustproof device and fan should be easy to disassemble, clean and replace quickly, and the dustproof device must have a sign reminding the user to clean.


8. The thermal design of the power device in the power adapter should be considered together with the thermal design of the power adapter. For example, the thermal design of the power adapter rectifier should take into account the thermal design of the power adapter, the thermal design of the transmission rack should take into account the thermal design of the entire cabinet, and vice versa.


The heat dissipation of forced air cooling is more than 10 times larger than that of natural cooling, but the addition of fans, fan power supplies, interlocking devices, etc., not only increases the cost and complexity of the power adapter, but also reduces the reliability of the power adapter. Increased noise and vibration. Therefore, in general, natural cooling should be used as much as possible instead of air cooling.


When laying out components, heat-generating components should be placed downwind or on the upper part of the PCB. The radiator is treated with an oxidation blackening process to improve the emissivity, and it is not allowed to be coated with black paint. After the radiator is sprayed with conformal paint, it will affect the heat dissipation effect, and it is necessary to appropriately increase the margin. The plane where the radiator is installed is required to be smooth and flat, and the contact surface is generally coated with silicone grease to improve thermal conductivity. Thicker wires should be used for transformers and inductance coils to suppress temperature rise.

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