For comparison purposes, Figure 4 shows the driver electronics for a LED light teardown from about a year ago. Not only is the packaging quite different, but there are a lot more electronics. For example, there are three electrolytic capacitors in the older design and a very large transformer.
shows the two different generations of drivers side-by-side: The Insignia driver has just two relatively small e-caps. This begs the question: What LED driver IC does the bulb use, and how does it allow such a tiny driver?
answers this question: The “SUL B” on the tiny IC is the marking for the Texas Instruments LM3445. There is no transformer, indicating that the LED driver design is non-isolated.
There is certainly precedent for non-isolated light bulbs: The incandescent bulb design itself is non-isolated. If you broke the glass in an incandescent bulb while it was plugged in, you would have direct access to the AC line power. Clearly, non-isolated designs can be made fully compliant with UL specifications. (While a non-isolated AC-DC LED driver design can be both safe and meet UL specifications, keep in mind that developing and testing a non-isolated offline LED driver in the lab requires stringent lab safety procedures.)
To get an idea for the actual LED driver circuit design, we can turn to the TI application note AN 2061 LM3445 A19 Edison Retrofit Evaluation Board (PDF)
for a non-isolated buck converter. The schematic is shown in Figure 7
Click on image to enlarge
In order to have the LED drive electronics all fit into the base, the Insignia's designers broke the single rectangular board up into the two circular-shaped pc boards (shown back in Figure 3
) which fit into the base of the bulb, separate and mechanically isolated from the LEDs. The only wires that come into the bulb are the low-power DC wires that vary between approximately 8-9V. More details and photos on the dimming control performance are here
- Placing the Cree LEDs on the three heat sinks that were part of the surface of the bulb. This quickly gets rid of the heat and keeps it separate from the power electronics.
- Forcing the bulb to meet the same, familiar envelope of the incandescent light, which in turn forced the power control electronics to fit into the tiny base.
- Going with a non-isolated buck topology to lower the parts count. Fewer parts = lower cost, smaller size, and greater reliability.
The Insignia bulb is currently one of the best values for an LED bulb on the market.
About the author
Margery Conner has covered the LED industry since its beginnings. She has a BSEE from the University of California, Irvine, and produces www.designingwithLEDs.com
. You can read in detail about the Insignia (Part 1
, Part 2
, Part 3
) and other lighting tear-downs there.
. LED performance boosted by wireless bonding
. White LEDs printed on paper
. OneLED - Waterproof LEDs advance design of outdoor displays
. LEDs tailor automotive lighting, improve safety
. Power and controlling high brightness LEDs
. LED drivers provide control, circuit-protection design options
. Controlling multiple LED strings with C2000 MCUs
. Understanding LM-80 to evaluate LEDs
. Efficient method for interfacing TRIAC dimmers and LEDs
. Compatibility per watt, meaningful metric for dimmable LEDs