After looking inside the Cree 60W replacement LED bulb (here’s a link to last week’s tear down) I had several questions for Cree, so I talked with Mike Watson, Cree’s VP of corporate marketing.
My most pressing questions were about the electronic circuit design – how the heck was Cree able to build an LED driver that dimmed so well, with such an excellent power factor (.98), all the while driving an array of 80 LEDs so effectively that it is able to warranty the bulb for 10 years of heavy use?
The most common way for LED bulbs to handle triac dimmer switches is to decode the power from the triac through a high-frequency pulse-width modulation circuit. Says Mike: “We don’t use that approach in this particular bulb. We have a very proprietary type of RMS process that involves our IP [intellectual property] and phase dimming.” Hmmm. Not very informative, but Cree’s reticence is understandable: Why make it easy for your competition to copy you?
As we saw in the tear down, the bulb’s only power management IC is the STMicroelectronics 6561D transition mode power factor corrector. So back to the ST website. Here’s app note AN2711 (a pdf) 120Vac input Triac dimmable LED driver for the L6562A, a chip in the same family as the 6561, that explains the use of the 656X Chip to handle triac dimming as well as PFC in a flyback power converter topology. Granted, it’s for an isolated LED driver, but its detailed explanation of using the chip for dimming is still valid as an insight into how the Cree driver may work.
The 80 LEDs (4 LEDs per individual LED component; 20 individual LED components) are arranged in an array of two parallel strings each with 10 individual LED components, making 40 LEDs per string. I asked Mike about the long series strings of LEDs: One LED failing open would take out an entire LED string, causing half the bulb to go dark. He said the PPM (parts/million) failure for an LED in the field – especially one that would fail open — is almost vanishingly low, which gives Cree the confidence to warranty these bulbs for 10 years.
I asked Mike a question that reader Chris posed in the original tear down piece: Can you use this bulb in an enclosed fixture? As Sal Cangeloso suggested in the comments, Cree intends that you be able to use the bulb in any application you would an incandescent. Mike agreed and elaborated: As long as there is an opportunity for air to circulate, the bulb will be fine. So, in a can lighting fixture such as you’d find in a track light:
…where the fixture is open, it’s fine. If you have a totally enclosed fixture, say, one with a lens over the opening, you it would still be fine as long as the ambient temperature doesn’t rise too much. (Think: Phoenix, summertime, no air conditioning.) Watson didn’t back-pedal or hedge: My impression is that Cree wants to move people to LED bulbs as quickly as possible by showing complete confidence in LED bulb reliability.
I also got some emailed questions from readers about the manufacturability of the bulb, including:
— How is the metal-core pc board that holds the LEDs formed?
— How do the clips work? Is that a failure point compared to a soldered connection?
Here’s a photo that shows the LED matrix from a top view.
The metal-core pc board is scored between each LED pair, allowing the board to be bent into a decagon that then slips over the 10-sided column. Even though the column is metal, there is no heat conductive path, such as a heat-sink adhesive, between the metal-core pcb and the metal column. A thermal conductive compound would make for better heat transfer, but would also make for a more complex, messy and expensive assembly process.
Next up: the clips. Here’s a photo showing the plastic guide that runs up the center of the metallic column, carrying power from the driver on the pcb up through the column and then, through the clips around to the outer surface of the column.
Don’t make the mistake of thinking that clips are a dodgy way to make a power connection compared to a solder joint: Solder joints are, on average, the most likely failure point for LED bulbs, especially since these joints would be hand-soldered. A properly installed clip is more reliable than hand soldering and has the added advantage of allowing the components to snap into place cutting, thus reducing labor costs. (These bulbs are assembled in the US.)
As I said in the previous article, this bulb is my new favorite LED bulb, and at under $13 (Home Depot) it’s a great value. Is there room for another price drop? Most likely: Philips North America CEO Greg Sabansky, “…sees a “growth tipping point” with the debut this year of its 60-watt equivalent LED light bulb that will retail for about $10.” Other reports say the bulb will come out at the end of this year.