The principles, procedures and related parameters of thermal design of power adapter power adapter has been widely used in various electronic devices, and its 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. Because a large number of high-power semiconductor devices are used in the power adapter, such as rectifier bridge stack, high-current rectifier, high-power triode or field effect transistor, they will generate a lot of heat when they work. If these heat can not be discharged in time and kept at a reasonable level, the normal operation of the power adapter will be affected and the power adapter will be damaged in serious cases. In order to improve the reliability of power adapter, thermal design is an essential part in the design of power adapter.
Design points of power adapter radiator
1. on the premise of ensuring the heat dissipation requirements, the radiator with small volume and light weight shall be selected as far as possible. 2. during installation, try to increase the contact area and pressure between the device and the radiator, and apply silicone grease on the contact surface. Pay attention to the installation mode and direction. 3. the surface of the radiator shall be rough, but the contact surface with the device shall have a good finish and be painted black. 4. when the pipe is insulated from the casing, the radiator shall be insulated from the casing, not only the electrical insulating sheet shall be used. 5. in case of sealing, pay attention to no convection. 6. air duct design during air cooling.
While reducing the heat generation of power switch through optimization design, it is also necessary to quickly release the heat generated by components to the surrounding environment through the heat sink using the heat transfer principle of conduction, convection and radiation, so as to reduce the internal heat accumulation and reduce the working temperature of components. This is called thermal design. The basic principles of thermal design are as follows. (1) 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) Class II derating is adopted for components, and the maximum allowable temperature can be determined according to the data of components. (3) The thermal design of power adapter shall be considered at the same time with 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 shall be conducted. For example, when designing the ventilation net plate, it is generally expected that the opening rate is high, which involves the hole spacing and hole size. From the perspective of hole size, it is expected that the larger the hole, the better. However, on the one hand, considering the requirements of electromagnetic compatibility, the hole diameter generally does not exceed 1/20 of the interference wavelength; On the other hand, according to the requirements of the safety regulations, the opening of the system must meet any of the following requirements. ① The hole size shall not exceed 5mm when measured in any direction. ② If the width of the hole is within 1mm, the length is unlimited. ③ The size of the hole is unlimited, but it must be ensured that foreign objects will not directly fall into the hole and touch parts with dangerous voltage. (4) Ensure that the cooling system has simple structure, reliable operation and low cost. An important principle is to strive to design high reliability products with mature technology and technology. (5) Vibration and noise of cooling equipment shall be considered. (6) If forced air cooling is adopted, the inlet temperature of the cooling air shall be limited. Even if the inlet of the cooling air is far away from the outlet of hot air of other equipment, the cooling air cannot be reused for cooling. (7) To improve the maintainability of the cooling system. When maintaining the cooling system, it shall have good safety and accessibility. The dust-proof device and fan shall be easy to disassemble, clean and replace quickly, and the dust-proof device must have a sign to remind 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 consider the thermal design of the power adapter, and the thermal design of the transmission subrack should consider the thermal design of the entire cabinet, and vice versa. The heat dissipation capacity of forced air cooling is more than 10 times greater than that of natural cooling, but the fan, fan power supply, interlocking device, etc. should be added, which not only increases the cost and complexity of the power adapter, but also reduces the reliability of the power adapter, and also increases the noise and vibration. Therefore, in general, natural cooling should be used instead of air cooling. In the layout of components, the heating components shall be placed in the downwind position or on the upper part of PCB. The radiator is treated by oxidation blackening process to improve the emissivity. It is not allowed to use black paint for coating. After the radiator is sprayed with three proofing paint, the heat dissipation effect will be affected, and the allowance needs to be appropriately increased. The installation plane of the radiator shall be smooth and flat. Generally, the contact surface shall be coated with silicone grease to improve the thermal conductivity. The transformer and inductor coil shall use thicker wires to suppress the temperature rise.
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