Change the heat dissipation method to achieve low cost
As explained at the beginning of the article, under the stimulus of Sharp, Toshiba Lighting achieved low price by changing the design of its original products. The original product was available for purchase in August 2009. In order to confirm the changes in the design, the dismantling group also dismantled the original product*8.
The difference in appearance is only the silver decorative ring above the heat sink (between the globe) (Figure 10). Because the silver decorative ring does not help to improve heat dissipation and improve luminous efficiency, the new product omits the ring. Although the mold of the radiator needs to be modified accordingly, considering the production of hundreds of thousands, it is more beneficial to reduce the cost by omitting the decorative ring.
Figure 10: Compared with the original products, the Toshiba Lighting LED bulbs have a decorative ring that is not related to heat dissipation, except that the appearance of the coating color changes from white to silver. (Click to enlarge)
Removing the spherical lampshade does not reveal the difference between the original product and the new product. However, after removing the metal cover, the difference is clear: the original product is in the resin case, and there is a filling material inside the power circuit board (Fig. 11). Only the power circuit board is inserted into the resin case of the new product.
Figure 11: The lower part of the Toshiba Illuminated LED Bulb (original product) is filled with filler material between the back of the backplane and the heat sink in order to efficiently conduct heat from the power circuit backplane to the heat sink. (Click to enlarge)
It can be seen that the resin shell of the original product is shorter than the new product by pulling out the resin case from the heat sink (Fig. 12). Moreover, the side of the resin case has an open part that is not found in the new product. The purpose is to bring the filling material in the shell into contact with the heat sink.
Figure 12: Toshiba Lighting LED Bulb (original product) Resin shell and power circuit The bottom plate of the resin case has an open part for the filling material to contact the inner surface of the heat sink. (Click to enlarge)
Why do new products do not require filling materials? In this regard, Toshiba Lighting's answer is: 1 the original product uses 6-chip parallel LED package, and the new product is changed to 3 chip series; 2 the output current of the power supply circuit is reduced, and the heat generation is also reduced; so the new product There is no need to use the filler material to conduct heat to the power supply circuit board. Amplifying the LED package can indeed see the difference in the number of chips. By saving the filling material injection process, the assembly cost can also be reduced accordingly.
Toshiba also said that changes to the power circuit board also help reduce costs. The new product uses a phenolic paper base plate, while the original product uses an epoxy glass base plate. According to the technicians familiar with electronic circuits: "The cost of the backplane is related to the output, but the floor can be reduced from 300 to 500 yen to about 50 yen."
The actual bottom plate is taken out and the original product is indeed an epoxy glass bottom plate. Compared with the phenolic paper base plate of the new product, the wiring of the phenolic paper base plate is only one side, and the epoxy glass bottom plate has two sides. Therefore, the bottom plate of the original product is relatively large, that is, the new product is expanded by using phenolic paper. The area of ​​the bottom plate and the resin case accommodating the bottom plate also have to be increased.
This design change requires modification of the heat sink mold, which is a cost increase factor. But Toshiba said: "By changing the design, the amount of material used in the radiator will be slightly reduced. In general, re-opening can still reduce costs."
Radiator using ceramic
Stanley Electric is also one of the new companies entering the LED bulb market. The company, which has a strong track record in automotive lighting, plans to begin mass production of LED bulbs*9 in September 2009. Because the mass production of the company has not yet started at the time of dismantling, the dismantling group failed to disassemble the physical object, and the improvement of the heat dissipation method that was learned in the interview was introduced.
The biggest feature of Stanley Electric's LED bulbs is that the heat sink material is ceramic (Figure 13). Because the thermal conductivity of ceramics to air is higher than that of Toshiba Lighting and Sharp's aluminum alloy, the heat sink area can be reduced.
However, from the perspective of cost, the cost of aluminum alloy castings is low, and many manufacturers have abandoned ceramics. But Stanley Electric has discovered the huge benefits of shrinking the heatsink: it can achieve all-round illumination.
Now, LED bulbs with aluminum alloy heat sinks must be placed near the top. Therefore, it is difficult for light to diverge in the lateral direction near the metal cover. It cannot be applied like a incandescent light bulb to a lighting fixture with a reflector near the lateral direction of the metal cover, and a floor lamp with a low bulb setting position and requiring omnidirectional illumination. With the ceramic heat sink, the LED can be placed close to the metal cover to meet the needs of the above applications, and the difficulty of replacing the incandescent light bulb is also reduced.
Stanley Electric's aluminum alloy LED substrate has also been improved in heat transfer to the heat sink (Figure 14). The LED substrate is supported by the top end of the flange on the inside of the ceramic heat sink. Due to the small contact area, sufficient heat conduction cannot be obtained by contact. Moreover, although the thermal conductivity of the ceramic surface to the air is high, the thermal conductivity inside the ceramic is low, which also affects the heat conduction effect.
Figure 13: Stanley Electric LED bulbs use high heat dissipation ceramics,
The radiator (outer casing) can be shrunk, and in addition to the top, it can illuminate the surroundings.
Figure 14: Cross-sectional imaginary view of the Stanley Electric LED bulb. Because the contact between the LED substrate and the heat sink does not sufficiently conduct heat, the interior is filled with a filler material that connects the two.
To this end, the company filled the interior of the heat sink with a high thermal conductivity silicon-based filler material, so that the back side of the LED substrate also made contact with the filling material. This improvement opens the way for LED substrates to efficiently conduct heat through the filler material to the ceramic heat sink. In order to bring the filling material into close contact with the back surface of the LED substrate, the assembly method is also improved accordingly.
Stanley Electric failed to put into production as scheduled due to the large amount of time spent on the production preparation process. Although the cost has reached the best balance point when it is decided to put into production, it takes time to realize the cost of low-cost LED bulbs. (To be continued)
For more details, please see the (5) published tomorrow: Improve the heat dissipation of LED packages.
*8: The price is basically the same as the new product.
*9: Stanley Electric originally planned to start production of LED bulbs in May of the same year when it was released in March 2009, but plans to postpone the supply of samples in August of the same year and mass production in September of the same year. The company does not publicly sell LED bulbs, and sales are handled by lighting fixture manufacturers and distributors.
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