Lighting fixtures using light-emitting diodes (LEDs) as a light source have gradually entered the market due to their low power consumption and long life. Among them, bulb-type LED lighting (hereinafter, referred to as LED bulbs ) intended to replace conventional bulbs such as incandescent bulbs and bulb-type fluorescent lamps has recently attracted much attention. Since the unit time price of a bulb calculated according to the life of the LED has been comparable to that of a conventional bulb, it is expected to be popularized in an ordinary household.
The first to introduce low-cost LED bulbs is Sharp. Sharp entered the LED bulb market with a low price of less than 4,000 yen at the actual price. This price setting is very impactful, about half of the price of LED bulbs at the time. Toshiba Lighting responded quickly and released a new low-cost LED bulb product*1,*2 with Sharp, 11 days after the release of Sharp products.
Figure 1: Low-cost LED bulbs Toshiba Lighting and Sharp have been listed in the past half the price, that is, ordinary bulb-type LED lighting (LED bulbs) with a retail price of less than 4,000 yen. In order to prevent the decrease in luminous efficiency and shorten the life, the heat dissipation of the LED is very important. Therefore, the lower half of the LED bulb is a radiator made of aluminum alloy castings.
The lower half of the bulb is a heat sink
Low price does not mean that LED bulbs can abandon the unique advantages of low power consumption and long life. Moreover, if the product is to be based on the market, it needs to have a higher heat dissipation capability.
The light emitted by the LED bulb has less infrared components. Therefore, compared with the incandescent bulb and the bulb type fluorescent lamp, the light irradiation portion is heated slowly by *3. However, the LED itself will generate heat, so the heat dissipation countermeasure is indispensable. Once the allowable temperature of the LED chip is exceeded, the luminous efficiency of the LED will decrease, which will adversely affect the life of the bulb.
From the outside, the characteristics of the LED bulb can be said to be the result of improved heat dissipation. Looking at the LED bulb from the side, more than half of the overall lower side is the heat sink (Figure 1). Toshiba Lighting and Sharp have used radiators made of aluminum alloy castings.
Comparing the heat sinks of the two, in addition to the color, the shape difference is also very obvious. Although the Sharp product is slightly more in terms of height, the channel area of ​​the radiator is larger than that of Toshiba. The channel depth of Toshiba's lighting products gradually increases from bottom to top, while Sharp's is basically the same height.
The larger the surface area of ​​the heat sink, the higher the heat dissipation performance. In the case of a limited external dimension, increasing the channel depth is one of the methods of increasing the surface area, but as the channel depth increases, the internal mounting space of the power circuit board, the resin case, and the like is reduced by *4.
The internal space of the Toshiba lighting radiator is cylindrical, and the Sharp product is a conical shape close to the shape (Fig. 2). The resin case is mounted on the bulb housing while maintaining insulation.
Figure 2: The main structure of the LED bulb The Toshiba Illumination LED bulb radiator (housing) has 16 right-angled triangular channels on the side of the cylinder, which are covered with a circular plate. The LED substrate is directly fixed on the top. The power circuit bottom plate is fixed in the cup-shaped resin case and inserted from below the heat sink. On the other hand, the Sharp LED bulb has a tapered cylindrical shape with 60 blades with a height of less than a few mm. The LED substrate is fixed to a circular plate-shaped metal plate covered above the heat sink. The base plate of the power supply circuit is fixed in a conical (but mostly hollowed out) resin case A inserted above the heat sink. (Click to enlarge)
The LED chip is the largest heat source for the LED bulb. In the bulb, a plurality of LED chips are packaged together and then mounted on a substrate made of an aluminum alloy. This aluminum alloy LED substrate is fixed to the upper portion of the heat sink. In Sharp's products, there is a metal plate between the LED substrate and the heat sink.
This article will combine the Toshiba lighting products and Sharp's LED bulb disassembly diagram to detail its internal structure.
Radiator connection construction is different
The hemispherical portion of the diffused LED light is referred to as a "spherical shade." The Toshiba Illuminated Spherical Shade is made of polycarbonate and is fixed to four locations above the heat sink with an adhesive. The spherical lampshade of a conventional light bulb is generally made of glass. This is because the LED light is not easily heated, so resin can be used. Moreover, after the resin is used, it is not easily broken when the bulb is dropped, and the safety is thereby improved.
The LED substrate is arranged under the spherical lampshade. Among Toshiba Lighting's products, 6.9W (white, total luminous flux 565lm) rated power consumption, the number of LED packages on the LED substrate is 7 (Figure 3) * 5.
Figure 3: The back side of the upper LED substrate of the Toshiba Illumination LED Bulb is in contact with the upper surface of the heat sink (housing) and is directly fixed with 2 screws.
Toshiba's LED substrate is mounted with a connector that connects to the power circuit. The connector is a product that does not need to be soldered, and it is estimated that the ease of assembly is prioritized. Incidentally, Toshiba's LED bulbs are assembled at factories in Japan.
The LED substrate is fixed by 2 screws, and the upper surface of the heat sink can be seen after the substrate is removed. This part is planarized by machining, and the LED substrate can be screwed to the surface of the substrate to obtain thermal conductivity.
The spherical lampshade of Sharp's LED bulbs is made of glass*6. 7.5W (daylight, total luminous flux 560lm) The LED substrate of the rated power consumption is equipped with 6 LED packages (Fig. 4). The wiring between the power circuit backplanes is soldered.
Figure 4: The upper LED substrate of the Sharp LED bulb is attached to the metal plate with 3 screws, coated with heat transfer oil. In addition, the wiring of the power supply circuit board is soldered.
The LED substrate is fixed to the metal plate by 3 screws, and a thermal grease (Grease) is applied between the two. The fixed LED substrate is not a heat sink made of aluminum alloy casting, but an additional metal plate. Although the material is aluminum alloy, there are no mechanical traces on the surface. Fixing the LED substrate with such a metal plate may not be sufficient for the bonding performance of the two, so it is necessary to use a thermal grease.
The metal plate is fixed to the heat sink with three screws (not the screws that fix the LED substrate). It can be seen that the metal plate is removed and the inside of the heat sink is filled with black resin (Fig. 5)*7. It is estimated that these resins are filler materials that promote heat conduction, but these resins are not in contact with the back surface of the metal plate, so it is presumed that the main purpose is to conduct heat to the heat sink circuit board instead of the heat generated by the LED package.
Figure 5: The internal metal plate of the Sharp LED bulb is attached to the heat sink (outer casing) with 3 screws. The inside of the heat sink is filled with filler material, but it is not in contact with the back surface of the metal plate, and heat can only be conducted through the contact portion of the metal plate and the heat sink. In addition, an O-ring is arranged between the two to ensure airtightness.
The contact portion between the heat sink and the metal plate is annular, and the area is not large. The contact portion on the back side of the metal plate has a full-circle flange, which is not only easy to position when assembled, but also slightly enlarges the contact area. In addition, the outer edge of the metal plate is exposed to the outside of the bulb, which becomes an embellishment on the design of the bulb.
The use of an O-ring disposed around the contact portion of the metal plate and the heat sink is not known. If it is to maintain airtightness, the effect of the ring should be to prevent leakage of the material after filling the liquid resin and to prevent water from entering from the outside. It is also conceivable to promote the transfer of heat from the metal plate to the heat sink.
Power circuit size is significantly different
A power circuit board and a resin case are mounted inside the heat sink. Figure 6 shows the situation after removing the metal cover under the Toshiba Illuminated LED bulb. The power circuit board is inserted into a resin case that is fixed to the heat sink with one screw.
Figure 6: The lower power circuit board of the Toshiba Illumination LED bulb is inserted into the resin case. The resin case is fixed to the heat sink with one screw.
The bottom plate of the power circuit is a rectangular phenolic paper base plate, and the resin case is substantially close to the cylindrical shape (Fig. 7). A resin case is disposed between the bottom plate and the heat sink to ensure insulation between the two.
Figure 7: Resin shell of the Toshiba Illuminated LED Bulb and the power circuit bottom plate Resin shell has an open part only on the metal cover side (the right side in the figure), which ensures the insulation of the power circuit bottom plate and the heat sink. The power circuit bottom plate has a rectangular shape and is connected to the LED bottom plate through a connector.
Figure 8 is a section of Sharp's LED bulb heatsink. Because the filling material is hard, it takes a lot of time to remove it from above, so the radiator is cut directly.
Figure 8: The section of the Sharp LED bulb is filled with a dense filling material inside the heat sink, and the power circuit board and resin case are completely covered.
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