适配器的热设计.doc

Thermal Management Designs forNatural Convection Cooled Low Profile Adaptor 1412mm Slim Adaptor, Design Approach White PaperRev:1.00 12-Mar-2003BackgroundThis 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 Specifiers guide 19951996 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 requirementsIn 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 modelThermal 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 alone the shorter side. The enclosure rests on its largest face and spaced 1mm apart from a flat surface. It can be see air enters from the side, gradually heated up by the surface of the enclosure, and then rise vertically at a region roughly at the centre of the flat enclosure. The surface temperature is around 5566C and the temperature inside rises up to 82C as shown. Ambient temperatrure is 25C.Below (Fig. B) is a cross section perpendicular to the plan to Fig. A. The compactness of the box and the requirement of spreading the heat over the two larger surfaces to avoid hot spot and improve convection, majority of the space inside the adaptor is heated to a high temperature. All components inside the box will be exposed to this high temperature internal ambient.The compromises:- Semiconductor and magnetic components are able to operate at higher temperature.- Some components, especially electrolytic capacitors, have significant lifetime reduction at higher temperature.- Components that capable of withstanding higher operating temperature are more expensive.- A previous study on a competitors products reveals similar issues.It is therefore desirable, to divide the internal space of the enclosure into two different temperature zones. One has lower temperature such that the lifetime of certain components can be increased to match the reliability of other parts without cost penalty.To verify the usefulness of this idea, a first model was built having a lower-temperature chamber located in-between two cooling barriers where air passing through (Fig. C and Fig. D).As can be seen in the simulation cross-section of Fig.D, there is significant temperature difference between the two separated sections . A complete model as shown in Fig.E was built and simulated. Components with lower operating temperature is put in the middle, isolated by two air passages from the hotter components on the left and right hand side.Fig. E is a final analysis according to the application in the direction of views as Fig. A and C. The Actual Package ConstructionLegendDescription1Bottom housing2,5Heat Spreaders, made of Copper3Power Semiconductor Device4Primted Circuit board6Upper Housing7PCB mount heat spreader8Power Transformer9Heat flow barrier10Air channel, see Fig.2,3 for better details11Capacitor compartment12Ribs to improve thermal dissipationLegendDescription1Bottom housing3Power Semiconductor Device6Upper Housing10Air channel11Capacitor compartment14Barrier walls15Air rising throughh slots , cooling barrier walls16Texture/Dimples for Improving surface area and cosmetic purposeFig. 14 and 15 are the close up view of the chamber. When the top and bottom covers are cliped together, the electrolytic capacitors are contained in the capacitor compartment. Two sides of the capacitor compartment is separated from the rest of the enclosure by means of two ventilation channels. Heat transfer from the hotter part of the enclosure is reduced by:(1) stops conduction and convection by separating the two sections.(2) air passing thru the channels furtuer reduces temperature in the inside of the air channels.(3) a curved surface around the capacitors to increase surface area as well as serving a cosmatic feature.The remaining heat transfer path is via the leads of the capacitors. Conduction thru lead is unavoidable. A small piece of heat flow barrier (Fig.1 item 9) made of insulation material is inserted to further reduce the heat transfer to the capacitors. The effective increase of thermal resistance from the rest of the enclosure to the capacitor compartment combined with the reduced thermal resistance from the capacitor compartment to the ambient resulted in overall capacitor temperature reduction.Other Features- Low cost PCB heat SpreaderFig.4 shows the PCB assembly housed inside the plastic enclosure. With a 12mm external height, the actual headroom allowed for the PCB assembly is less than 8mm. For this reason the PCB assembly utilize surface mounted component and manufacturing process to meet the 12mm low profile requirement. Considering cost effectiveness and circuit complexity a double sided PCB is selected.The heat generated by surface mounted power components need to be brought to the outer surface of the enclosure. Common thermal management techniques on a PCB are:1. Employ large Copper Pad for heat spreading.A large PCB real estate is required, reduces power density.2. Thermal vias connecting the top and bottom Copper surfaces.Interfere with soldering quality.3. Uses multi-layer PCB to enhance thermal conductivity.Expensive, unless the density of products requires multi-layer PCB.4. Employ Insulated Metal Substrate.Many times more expensive than PCB, additional weight constrain.5. Uses PCB with internal metal-core.Expensive, Limited source.6. Make use of other components on PCB for thermal conduction.Intellectual property protected.7. Application of ceramic substrate, Alumina, Aluminium Nitride etc.Expensive material and processing cost.8. Conformable Silicone gap filler.A convenient heat spreader technique is proposed here, it has the following advantages:1. Low cost, a single piece standard part stamped Copper, combined with low cost double sided PCB.2. Thicker and all-metal part has better thermal conductivity, PCB area consumption is smaller.3. Thermal management requirement compatible with manufacturing process, conducts heat away from the non SMD side of PCB. Exactly what SMD power packages are designed for.4. Compatible with SMD mounting process.Power Device Fig.5 is the assembly diagram to show how the power device, PCB and heat spreader come together.The PCB has a cutout and three holes (in this specific design) in place to accept the heat spreader. The bigger cutout is approx the same size as the larger back surface of the power device. The heat spreader is press-fitted onto the bottom side PCB before the SMD pick and place process. The tub-shaped protrusion fits into the cutout on the PCB. The flat metal surface of the heat spreader will emerge from the top side of the PCB and is flush with the top surface. The power device goes throught the same pick-n-place process as usual. The power device is in contact with the heat spreader after pick and place process. The assembly is the reflow soldered.Fig.6 is the cross-sectional view of the assembly. The back of the power device (3) is now in contact with the heat spreader (7), creating a good conduction path from the top side of the PCB to bottom side without the the use of thermal vias. The thicker metal will work much better than thermal vias through FR4 material. The heat spreader is then allow to make contact with a conformable insulating material and distribute the heat to the heat spreader located at the inside surface of the enclosure.Other Features - Clip-on Heat dissipatorFig.7 Shows the PCB assembly with the clip on heat dissipators. Indentations (24)