适配器的热设计.doc

Thermal Management Designs for
Natural Convection Cooled Low Profile Adaptor
14
12mm Slim Adaptor,
Design Approach White Paper
Rev:1.00 12-Mar-2003

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 doesn't 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 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 55~66°C and the temperature inside rises up to 82°C as shown. Ambient temperatrure is 25°C.

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 competitor's 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 Construction

Legend
Description
1
Bottom housing
2,5
Heat Spreaders, made of Copper
3
Power Semiconductor Device
4
Primted Circuit board
6
Upper Housing
7
PCB mount heat spreader
8
Power Transformer
9
Heat flow barrier
10
Air channel, see Fig.2,3 for better details
11
Capacitor compartment
12
Ribs to improve thermal dissipation

Legend
Description
1
Bottom housing
3
Power Semiconductor Device
6
Upper Housing
10
Air channel
11
Capacitor compartment
14
Barrier walls
15
Air rising throughh slots , cooling barrier walls
16
Texture/Dimples for Improving surface area and cosmetic purpose

Fig. 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 Spreader

Fig.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 dissipator

Fig.7 Shows the PCB assembly with the clip on heat dissipators. Indentations (24) are specially punched to allow the metal surface to have better contact with heat generating components.

Fig.8 and 9 Show how the dissipators clip together. On Fig.8, the part 21 clips on to the notches cut onto the PCB by the catches (28). A locking tab (26) holds the PCB at its designed position. After part 21 is installed, Part 20 goes on from the other side, the clip 22 wraps around the previous position occupied by part 21. Utilizing the same fixation mechanism. PCB position can be controlled to tight tolerance with a simple assembly procedure.
Other Features - Ribs and Knurling

Ribs and Knurling are distributed over the surface for improvement of effective surface area. With the unit sits horizontally, air enters from the sides in a horizontal fashion, gradually changing to a vertical direction when it reach the centre portion of the box (see Fig.20). Addition of ribs at the edge of the unit takes advantage of this horizontal air flow, improving heat transfer.

End of Document