Power adapter thermal design principles, procedures and related parameters Power adapters have been widely used in the current various types of electronic equipment, its unit power density is also increasing. The definition of high power density has gone from 25W/in3 in 1991， 36W/in3 in 1994, 52W/in3 in 1999，96W/in3 in 2001, to hundreds of watts per cubic inch. Due to the use of a large number of high-power semiconductor devices in the power adapter, such as rectifier bridge pile, high-current rectifier, high-power transistor or field effect tube and other devices, they will produce a lot of heat when working, if you can not discharge these heat in time and make it at a reasonable level, it will affect the normal operation of the power adapter, serious will also damage the power adapter. In order to improve the reliability of the power adapter, thermal design is an essential part in the design of the power adapter.

Power adapter radiator design points

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

2. During installation, increase the contact area and pressure between the device and the heat sink as much as possible, and apply silicone grease to the contact surface. Note the installation method and direction.

3. The surface of the radiator should be rough, but the contact surface with the device should have a good finish and be blackened.

4. When the pipe is insulated from the casing, the radiator should be insulated from the casing, and not only electrical insulation sheets should be used.

5. In sealed cases, note that there is no convection.

6. Air duct design for air cooling.

While reducing the heat generated by the power switch by means of optimal design as far as possible, it is also necessary to use the heat transfer principle 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 and reduce the operating temperature of the components, which is thermal design.

The basic principles of thermal design are as follows：

(1) To clearly understand the working environment of the power adapter, including the change range of ambient temperature, the radiation of the sun or other objects around, etc., the heat dissipation scheme of the power adapter should meet the requirements of the application environment.

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

(3) The thermal design of the power adapter should be considered at the same time as industrial modeling design, electrical design, structural design, reliability design and electromagnetic compatibility design, and weigh analysis and compromise solution under the premise of ensuring electrical performance and reliability requirements. For example, when designing ventilation panels, it is generally hoped that the opening rate is high, which involves the hole spacing and the size of the holes. From the size of the hole, of course, it is hoped that the larger the better, but on the one hand, considering the requirements of electromagnetic compatibility, the diameter of the opening hole is generally not more than 1/20 of the interference wavelength;

On the other hand, according to the requirements of safety regulations, the opening of the system must meet any of the following requirements：

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

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

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

(4) To ensure that the cooling system structure is simple, reliable and low cost. An important principle is to strive to use mature technology and mature technology to design high-reliability products.

(5) To consider the vibration and noise of the cooling equipment.

(6) If forced air cooling is used, the inlet temperature of the cooled air should be limited, and even if the inlet of the cooled air is far from the outlet of the hot air of other equipment, the cooled air can not be used for cooling twice.

(7) To improve the maintainability of the cooling system. When the cooling system is repaired, it should have good safety and accessibility, the dustproof device and fan should be easy to quickly disassemble, clean and replace, and there must be a sign on the dustproof device 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 take into account the thermal design of the power adapter, and the thermal design of the transmission sub-frame 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 it is necessary to increase fans, fan power supplies, interlocking devices, etc., which not only increases the cost and complexity of the power adapter, but also decreases the reliability of the power adapter, and also increases noise and vibration.

Therefore, in general, natural cooling should be used as far as possible, rather than wind cooling. I

n component layout, the heating component should be placed in the downwind position or on the upper part of the PCB. The heat sink is treated by oxidation blackening process to improve the radiation rate and is not allowed to be coated with black paint.

Radiator spraying three anti-paint will affect the heat dissipation effect, the need to appropriately increase the margin. The surface of the radiator installation must be smooth and flat.

Generally, the contact surface is coated with silicone grease to improve the thermal conductivity.

Transformers and inductors should use thicker wires to inhibit temperature rise.