The Philips SlimStyle design is very well thought-out and shows an awareness of nuances that seems to escape many lamp and luminaire designers.
The new Philips SlimStyle 60W equivalent lamp is notable for its flat appearance. Many speculate about how and why this design came to be and to what extent it is better or can be indicative of future directions. There is no certainty about whether others will follow, but some things seem clear. The surprising thing is how unsurprising some of the design details are.
Let's first get a few simple things out of the way. Some speculative postings on this site and elsewhere have suggested that this lamp does its thing and magically achieves its thermal management objectives without a heat sink, by using only what seems to be a plastic case. Not so. The internal LEDs are actually mounted on a 2.6" diameter traditional aluminum metal core board (MCB). That 5.3 sq. inch MCB is indeed the heat sink.
Heat sink
The fact that the 26 SMD emitters are placed near the periphery creates excellent heat spreading and the lowest thermal resistance to air -- better than a typical extruded or cast heatsink. The solder mask on both sides of the MCB increases emissivity by 10% (i.e., increases heat-sink cooling effectiveness by 10%). Counter-intuitively perhaps, the plastic shells over the MCB do not meaningfully impede heat transfer from MCB to ambient (I'll omit the math here).
The bulb employs 26 LEDs nominally rated about 500 mW each but used here at about 380 mW. Interestingly, Philips used a double-sided MCB with 13 LEDs on each side in a mirrored arrangement. Double-sided MCBs are rare in lighting products, but not entirely unknown. Philips can use one here because they are not affixing to another heat-sink surface.
Operating temperature
All 26 LEDs are driven as a series string from a circuit having a nominal output, at full brightness, of just under 80 VDC -- an approach tailored for maximum efficiency. The driver pushes the LEDs quite hard. After stabilizing at full brightness, the LED packages are operating at, or slightly above, 90°C in a 25°C ambient. That is a kind of no-no in commercial LED luminaires, where 80°-85°C is considered the edge of the danger zone. But the SlimStyle is a consumer item, rated for 25,000 hours, with great competitive price pressures from folks who often bend rules -- so I get it about the 90°C.
Foldback
A key thing here -- I noticed that lumen output dropped about 15%-20% as LED temperature passed through 70°C to the 90°C range on the way to equilibrium. The driver circuit has an NTC protection for thermal "foldback" (i.e., dimming) in the event of MCB overheating. I suspect that the threshold was set such that I was seeing some of that protective dimming just starting to kick in as the lamp stabilized at maximum temperature. It's hard to say if the lamp would have gone to 850-900 lumens and then "folded back" to its rated 800 lumens, or instead gone to 800 and folded back to 720-750. I think the former. I can test relative lumens against a reference but do not have an integrating sphere to make exact lab-type total lumen measurements of an A-19 type.
The point is that Philips incorporates thermal protection. It probably wanted to set its knife-edge on the conservative side, in case somebody sticks the lamp into a base-up socket or in an enclosed fixture where things could cook.
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