The price of LED bulbs is falling. A year ago, it cost about $50 to buy a Philips 60W dimming LED bulb that would replace the old-fashioned bulb. Today, at Best Buy, you can buy 8W, 800 lumens Insignia for your home brand for just $17. The bulb can replace the 60W incandescent bulb. What changes in the design of LED bulbs have led to this cost reduction? The disassembly of the LED bulb allows us to see some of the design trends in LED lighting, such as how to place the LED in the bulb, and what drive architecture is used.
1. The shape of the Insignia bulb is similar to the traditional incandescent lamp we are familiar with, except that it has three metal fins and a plastic (not glass) base.
2. Use a Dremel tool to remove the outer casing of the plastic bulb, revealing six Cree white LEDs that illuminate the light source mixing chamber to produce a uniform illumination. The LEDs are mounted on metal fins that are used to support both the LED chip and the heat sink. At the bottom of the mixing chamber is an extremely thin aluminum reflector that assists in the reflection of light within the bulb. The entire circuit of the bulb is in the bulb holder under the reflector, in a separate enclosed compartment.
3. Disassemble the infused rubber compound and show two PCBs next to each other with the circuit on top. In the figure, the two PCBs are separated, next to the bulb base.
4. For the sake of comparison, a drive circuit for an LED lamp disassembled about a year ago is shown. In addition to the great differences in packaging, there are many more electronic devices. For example, the earlier design had three electrolytic capacitors and a very large transformer.
5. The two generations of drives are shown side by side in the left: Insignia drives have only two relatively small electrolytic capacitors. This structure poses a problem: Which LED driver IC is used for the bulb, how does such a small driver do it?
6. Q&A: The word “SULB†on the small IC is the mark of Texas Instruments LM3445. It does not use a transformer, indicating that the LED driver is a non-isolated design. The design of incandescent lamps is itself non-isolated. If you break the glass of an incandescent light bulb and the light bulb is plugged in, you can directly access the AC power source. Obviously, non-isolated designs can be fully compliant with UL specifications. Note: Although a non-isolated AC/DC LED driver is designed to be both safe and UL compliant, developing and testing a non-isolated offline LED driver in the lab still requires rigorous laboratory safety procedures.
Dimming of light bulbs is an important feature of the US market. I have used the LutronMaestro dimmer switch, which has programmable dimming control, and I compared it to the incandescent lamp item by item. Insignia dimming is stable and smooth, and the dimming curve is similar to incandescent lamps. Please watch the dimming test video at http://bit.ly/145ROBK.
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