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Power adapter thermal design method

The power adapter will generate some heat during operation due to internal power consumption, and it is necessary to limit this "self heating" in every application so that the power adapter housing temperature does not exceed a specified maximum. If the power adapter generates too much heat during operation and does not dissipate to the surrounding media, the power adapter will fail due to exceeding the normal operating guaranteed temperature. Therefore, the selection of suitable radiator is one of the important conditions for reliable operation of power adapter. The main parameters required in the thermal design of the power adapter are as follows.
(1) the operating junction temperature Tj of the switching tube in the power adapter, namely the maximum allowable operating temperature limit of the switching tube. This parameter is provided by the manufacturer or mandated by the product standard.
(2) the power loss P of the power adapter is the average steady-state power consumption generated by the power adapter itself during operation, which is defined as the product of the average RMS output current and the average RMS voltage drop.
(3) the dissipated power Q of the power adapter, that is, the heat dissipation capacity of a specific heat dissipation structure.
(4) thermal resistance R between the power adapter and the working environment, that is, the temperature rise generated by unit power consumption when heat is transferred between the media.
The heat dissipation design of the power adapter depends on the maximum junction temperature (Tj) allowed by the switching tube in the power adapter. At this temperature, the loss generated by the power adapter should be calculated first, and the switch junction temperature should be raised to the allowable value according to the loss to select the heat sink. In the case of insufficient heat dissipation design, the actual operation at medium level may exceed the allowable temperature of the switch tube and lead to the performance degradation or damage of the power adapter.
In order to select the best radiator for the switch tube in the power adapter, the above parameters need to cooperate with each other. The principle of radiator configuration is to ensure that the radiator can effectively conduct the heat loss of the switch tube to the surrounding environment and that the junction temperature of the switch tube does not exceed Tj. Let the ambient temperature be Ta, expressed as:

Where, P is the loss power of the power adapter, is the average steady-state power consumption generated by the power adapter itself when it is working, and is defined as the product of the average RMS output current and the average RMS voltage drop. Q is dissipated power, namely the heat dissipation capacity of a specific heat dissipation structure; Tj is the working junction temperature of the switch tube, namely the maximum allowable working temperature limit of the switch tube. Ta is the ambient temperature; R is the thermal resistance, that is, the temperature rise generated by unit power consumption when heat is transferred between media.

Thermal resistance R is mainly composed of three parts:

Where, Rjc is the thermal resistance between the switch tube chip and the shell; Rcs is the thermal resistance from the shell to the radiator. Rsa is the heat resistance of radiator to air.
(1) Rjc has a great relationship with the technological level and structure of the switch tube and is given by the manufacturer.
(2) Rcs is closely related to the filler medium (usually air) between the switch housing and the radiator, the roughness and planeness of the contact surface, and the mounting pressure. The better the thermal conductivity of the medium, or the closer the contact, the smaller the Rcs.
(3) Rsa is an important parameter for radiator selection. It is related to material, material shape and surface area, volume and air velocity.
By integrating equations (2-4) and (2-6), the following equation can be obtained:

Formula (2-4) the basic principle that radiator chooses to match, general radiator manufacturer offers the shape parameter of specific radiator material and thermal resistance characteristic curve, design personnel can calculate the surface area that needs radiator according to this, length, weight, and obtain the thermal resistance Rsa of radiator further.
In actual design should put aside enough margin, because the accuracy that provides data, by the installation condition that switches tube to radiator, the air convection condition of radiator surface, the unsteady state distribution of quantity of heat, be not ideal factor, should consider these factors in the design.
In addition, the radiator surface to the air heat radiation is also a kind of heat dissipation. Anodic oxidation blackening and dehasting processes, which are widely used in self-cooling design, are effective methods to increase thermal radiation. However, this method is obviously not suitable for forced air cooling thermal design with convection and conduction as the main mode, because the brighter the surface of the radiator, the lower the thermal resistance, which should be paid special attention to in the design.

| release time：2019.09.12 Source：

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Power adapter thermal design method