In tearing down the Best Buy $16.99 Insignia LED light bulb (here’s Part 1 and Part 2), it quickly became apparent that a familiar part was missing – where the heck was the transformer? I dug all the way down through the potting in the bottom of the base with no success. There was no transformer, making this the first non-isolated light bulb I’d come across.
Well, no, actually, it isn’t: The incandescent bulb design itself is non-isolated. If you broke the glass in an incandescent bulb while it was plugged in, you’d have direct access to the ac line power. And I can tell you from personal experience it’s much much easier to break an incandescent light’s bulb than it is the Insignia bulb. Plus, with the Insignia you still don’t have access to the line voltage, which is hidden away separately from the fully enclosed plastic bulb envelope, and then encased below in potting compound. Non-isolated designs can be made fully compliant with UL specifications, as well as being just plain safe**.
Let’s take another look at the LED bulb itself.
The thick, tough plastic bulb completely encircles the LEDs. The only wires that enter the compartment are the low-power wires that range from about 8-9V and carry 260 mA.
I mentioned in Part 2 of this teardown that the power management IC was the Texas Instruments LM3445, and sure enough, there’s a TI application note entitled, AN 2061 LM3445 A19 Edison Retrofit Evaluation Board (PDF) for a non-isolated buck converter. The schematic is:
The board design included in this app note is a conventionally-shaped double-sided rectangular pc board that used as-is would have probably stuck up into the bulb portion of the light bulb. In order to have the LED drive electronics all fit into the base, the system designers at Best Buy broke the single rectangular board up into the two circular-shaped pc boards (as seen in Part 2)
…that both fit into the base of the bulb, completely separated and shielded from the business-end of the bulb, with the power electronics mechanically isolated the from the light-emitting portion. The only wires that come into the bulb are the low-power DC wires that vary between approximately 8-9V.
So a re-cap of the design choices made that enable stellar price/performance of Best Buy’s $16.99 60W- equivalent, dimmable Insignia LED bulb:
- 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.
This design has my vote for replacing the incandescent light bulb icon in cartoons which show that someone just had a great idea.
**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.