Over the past few years I have bought a number of GU10 mains voltage LED bulbs and have generally been disappointed with their brightness. The 9W LED bulbs I bought (supposedly 460-590 Lumen) produce far less light than the 50W Halogen bulbs they replace (specified as 350 Lumen on the box, but I think 800 Lumen would be more typical give 16L/W).
Testing them with my Fluke I found that they draw far less current than I would expect from their "specification", so I have always assumed that the limiting factor was the mains power circuit and that if this circuit were permanently being driven at 100% it would be far more likely to fail than the LEDs themselves.
True to form, over the past few months, two of these 9w (3x3w) bulbs have died, the first almost 12 months to the day from when it was first fitted. I estimate that this bulb was used for approximately 2000 hours over that year, far less than I would expect from a LED bulb. In both cases, the failure manifested as the bulb pulsing light at between 1 and 3 Hz rather than providing continuous illumination.
Opening up one of the failed bulbs, I found that it contained a BP3102 High Precision PSR Constant Current LED Driver and from what I can see of the PCB, the circuit is the pretty much the reference circuit in the data sheet. Given that the data sheet says explicitly that "the output power of system should be less than 5W", I cannot see that it is honest to call this a 12W bulb.
Looking for datasheets for the LED itself, it looks like each of the three LEDs should be putting out 160-190 lm at 3W, so 480-570 lm is pretty close to the specified 460-590 lm), however the power draw suggests that the actual light output at 5W should be more like 266-300 lm, which seems about right.
Putting aside the dishonesty of sellers rating a bulb on the potential output of the LEDs rather than on the actual output of the bulb, I wonder if there might be a technical reason for overrating the LEDs but under rating the power circuit.
There is an obvious cost advantage to the manufacturer to using lower spec parts, but I would have thought that failing to meet the advertised specifications and reduced lifetime would not be considered a good tradeoff.
Similarly, I would have expected that the LEDs would only draw as much power as they need, so overrating the power supply would never be a problem, and the ideal would be for LEDs and power supply to be matched in their power requirement and output.
Is there something I am missing about how a circuit like this should be designed, or is my cynicism just realism?
Best Answer
The LED power may be limited by thermal considerations. Both the power supply and the LED are prone to failure. The former because of possible shoddy transient protection and inevitable death of filter capacitors (exacerbated by the typical high temperatures). LED life will also be greatly foreshortened by high junction temperature.
These things are a trade-off. My suspicion is that the LED rating of 3W you mention is not really achievable (with any reasonable lifetime) in the environment of the light bulb, and everything (driver and LED) is really close to the line. Here's a snippet from a Chinese "3W" LED datasheet:
The Power Dissipation is absolute maximum 3W at \$T_A = 25°C \$. It will have to be derated above 25°C at the bulb, so I think you can see that calling that thing a "3W" LED is somewhere between irrationally optimistic and an outright lie. While the bulb maker might be perfectly happy to run it right at 3W, if they die so fast that they are getting 40' container loads of lamps dumped back at them, they'll not be so happy.
The only thing I really expect to be true of the bulb is the (initial) lumen output (you can expect them to fade over time). If the lumen output falls short, especially out of the box, that is false advertising and I would return them. So far, from reputable makers such as GE, I have not been disappointed. I can't directly measure lumens, but I use lux meter to check light levels on work surfaces and so on, and can reasonably well compare halogen and LED bulbs with similar angles.