适配器的热设计.docVIP

Thermal Management Designs for Natural Convection Cooled Low Profile Adaptor  Background This is a report about the experience gained from the thermal management design of a slim (12mm thickness) AC/DC adaptor used in notebook computer or portable devices. It is well known that the heat dissipation capacity of an object is governed by its surface area. Package manufacturers (e.g. Hoffman Specifier’s guide 1995~1996 P 594) provide specified chart curve for selected materials as shown below; by plotting the temperature rise against the power dissipation per unit area. This provides an idea about the average temperature rise of a certain shape. However it does not show temperature distribution inside the shape and therefore cannot be used as a tool for identifying possible reliability issues. The graphical method also doesnt work very well for geometries with extreme aspect ratio. A general description of the requirements In this design the enclosure is a box measures 50x80x12mm. The box is moulded from plastic material for safety reason. Copper plates are located at the inner surface of the box that spreads the heat generated internally and equalize the temperature gradient along the inner surface of the box. The metal plates also doubles as electrical shield for minimizing electromagnetic radiation. Electronic components are mounted on a 1mm FR-4 glass epoxy double sided printed circuit board. The heat generating components are summarized as follows: - Input EMI filter. - Input rectifier. - Main primary power switch. - Primary snubber. - Primary control circuits. - Power transformer. - Secondary Rectifier. - Secondary snubber. - Secondary control circuit. - Output capacitors. The simulation model Thermal simulation is used to investigate the air flow pattern when the enclosure is sitting in a horizontal orientation. Fig.A and B are the 2 dimensional cross sectional views along the centre line. Fig. A is along the longer side and Fig. B is alon