Categories Bulb

Reliability and Lifetime Matter Most for Bulbs – Part 2

The short answer is yes. MTBF is an expression of the likelihood of failure during the product’s lifetime. Put simply, the longer the MTBF is, the less likely it is that the device will fail during its lifetime. For example, the brakes in your car have a relatively short lifetime, perhaps three years with average use, but you want them to be very reliable during that time. End-of life is defined as failure – but what is ‘failure’ anyway?

There are lots of phrases that describe this – perhaps the best one for lighting applications is also one of the simplest: “if the product can no longer perform the application for which it is intended then it has failed”. The phrase ‘no longer fit-for-purpose’ should be relatively easy to define for an LED bulb – if you no longer get enough light out of it, then that is a failure. Moreover, when the light flickers and hiccups for a long time during start-up that could also be considered a failure. However, the typical specification for an LED bulb runs to perhaps  20 or more line items (a lot more if it is a smart bulb), but an aging bulb that fails to meet many of them would not fit the average user’s definition of failure as described above.

This is important. If a bulb at the end of its life was required to deliver output that met the complete specification at the beginning of its life, electrolytic output capacitors would not be used to limit current ripple (for example). Replacing the aluminum electrolytic output capacitors with ceramics (which have an extremely long life) would insure that output capacitance stayed high, but the capacitance per unit volume and price of ceramics makes their use impracticable.


Replacing an aluminum Electrolytic capacitor with ceramics in a bulb would be challenging.


So the engineer has the challenge of deciding which parts of the specification the bulb needs to meet as it begins to age and which parts it does not.  For most applications, increasing output ripple will not be seen as a cause of failure by the user, so the engineer can accept the reduction in performance that comes with using components (such as electrolytic output capacitors) that are necessary for a practical design.

Yet electrolytic capacitors cannot be used everywhere in the circuit because they can cause the type of failure that users will not accept. The use of single stage LED drivers, PFC and constant current (CC) combined into a single switching stage, eliminates electrolytic bulk capacitors from the input stage. (A weakness of two stage converters, which have separate PFC s and CC driver stages, is that they need a bulk -capacitor which, as it ages, reduces capacitance and eventually results in a bulb that may be reluctant to start or may fail completely.)


2-stage converters use a bulk capacitor in a location, which will eventually cause a hard failure.


Single-stage converters have no lifetime-ending bulk capacitor but accept non-lifetime ending higher output current ripple.


Perhaps the best known trade-off in performance against time is in the drop off in light output (lumens) that is associated with the degradation of the phosphors in the LED. Figures like L-70 (a subject for another discussion) are used to describe how well the LEDs in an application perform above a minimum light output specification.

If a light dims over time, often  it isn’t a problem as the user will typically notice; however if a bulb in the set fails and a replacement is added (assuming you can still find one of the same model and type) then the light output may be noticeably different. Fortunately, the human eye is poor at detecting relative light intensity, so light output requirements (at end-of life) that are implicit in the L-70 figure exploit this fact. It is possible to design a lamp that increases the drive current to compensate for a reduction in lumens per watt from the LEDs. This is not a common design requirement due to the cost of the necessary detection circuitry. The rise of smart lighting (especially in Europe and North America) may lead to the introduction of this kind of feature in more lighting applications in the future.

It is clear that the parts of the specification that a customer can accept are dependent on how the human eye perceives light, so in order to determine what parts of a lamp’s operation are critical to maintain over-time, it is  useful to understand what the human eye can distinguish and tolerate– therefore the next topic we will look at is Macadam ellipsis, intensity perception and frequency/amplitude considerations for output ripple.

Categories Bulb

Latest 60 W Equivalent LED A19 Bulb for $2.49

For a limited time, one major local home improvement retailer in the United States is offering a non-dimming 60 W equivalent LED A19 type bulb for $2.49 each, sold in a two bulb value pack manufactured by a major LED maker. Similar types of bulbs were about 6x the cost a few years ago. With price drops like this, we tend to see articles and discussions about the fall in LED bulb prices over the past few years, performance comparisons to previous versions on lumens/watt trends, or teardowns and investigations into the cost of materials etc. I decided to do a teardown to look at the design changes that went into this bulb to drive down cost versus an LED bulb from just a few years ago.   The trends to note from this perspective have been ongoing, but then the question to ask ourselves as designers is where will the basic design of an LED bulb step to next?

Figure 1 is a picture of the $2.49 bulb and as you can tell I was not trying to be pretty about the heat sink and casing as I was more interested in the driver at first.  It requires 8.5 W for a 60 W equivalent at 800 Lumens A19 bulb.


Figure 1. $2.49 A19 60 W Equivalent LED Bulb


Figure 2. Older A19 60 W Equivalent LED Bulb for comparison


Figure 2 shows the teardown from a bulb taken apart a few years ago, it cost $15.99 but I promised not to get into a LED pricing reduction analysis.  By the way, it required 8 W for only a 40 W equivalent at 450 Lumens A19 bulb.

To begin with, removing the plastic “optics” used to require a heat gun, but now days most of the time you can pry the plastic cover from the heat sink base.  Then simply remove a few screws and release the driver inside the casing. In older bulbs, there was still potting material inside the casing surrounding the driver along with a series of interlocks holding the casing to the heat sink.  Potting material is still used today, but I am considering general trends in the construction of LED bulbs.  The new bulb weighs around 49 g total, the old bulb heat sink alone is 70 g so for the same input power to the bulb, the heat sink is smaller and there is no potting around the driver. The number of LEDs went from 18 to 11 with the lumen output almost double. I did not measure the LED voltage on the old string, but I measured over the 60 V SELV limit on the new bulb as safety is now achieved from the mechanical housing not through electrical isolation from the driver.  Therefore, it’s obvious that lower weight, less LED counts, better materials or materials that function good enough can all contribute to lower cost.

The driver is more interesting.  The old driver was an isolated flyback design.  The controller has power factor correction, but what is not shown in Figure 2 is the added expense of having to cut the printed circuit board along the primary to secondary isolation boundary for safety creepage and clearance. The new driver is non-isolated and does not use a controller, but a set of discrete components.  The new driver has a smaller EMI filter and even the transformer is smaller. The BOM count was 53 or so in the past versus 27 or so today – this will reduce cost and improve reliability in the driver. [1]

Clearly, the trend is to reduce the driver electronics besides the material changes and efficiency of LEDs.  So where are drivers for LED bulbs going next?  Integration: but there are discrete designs that use the low cost components without the need for an integrated analog controller that provides good enough performance. NPN transistors in place of MOSFET transistors, better EMI filtering, fuses part of the printed circuit board, etc.  Non-isolated designs: when thinking about this trend and reducing component count the idea of Direct AC LED topologies can reduce the component count further and possibly eliminate some of the lower reliability electrolytic capacitors.

But how do these observations enable a smart LED solution?  Lighting is evolving from something used to see or read, but is becoming a user experience: warm dimming, color tuning, communication to control, visible light communication to guide us, sensors that interpret implicit human movement or environment changes, etc.  The latest low cost LED Bulb is a pathway to enable the technologies to take lighting into the future by reducing not just cost, but by pushing the boundaries on how we think about the construction of the bulb and how we challenge conventional thinking about consumer electronic design.  As a semiconductor supplier, the need is to stay ahead of the curve by providing the building blocks to make these future lighting applications realized, how many suppliers are there that can provide these building blocks? I know of one that is developing the building blocks to advance lighting into the smart future. [2]

Categories Bulb

What’s Happening with the “California Quality LED Bulb” Initiative?

Beyond the phase out of 40 and 60 W standard general service incandescent lamps as part of the Energy Independence and Security Act of 2007, at the start of 2014 the Voluntary California Quality Bulb standard has now been in place for a year so this seemed like an appropriate point to see what has occurred in the market in response to this legislation. Slightly more than a year ago, the California Energy Commission (CEC) finalized guidelines for investor owned utilities on what type of LED replacement bulbs would be eligible for rebates in the residential space. They established a < 1 year transition time where utilities could continue to offer incentives for products which did not meet the new CEC “California Quality Bulb” specification but still meet the current ENERGY STAR LED Bulb requirements. The table below is a quick summary of some key CEC requirements and how they compare with the updated ENERGY STAR bulb specification, which will become effective this September.


In a nutshell, the CEC raised the bar to qualify for an incentive with a significant increase in the Color Rendering Index (CRI) and color control as well as adding 2 years to the minimum warranty period. At the time this legislation was being publicly reviewed, the main concerns raised by some manufacturers were that these requirements would increase power consumption and cost as described in a prior LED Journal article. Using the latest LED Lighting Facts database, I did a search of how many new 60 W equivalent (>800 lumens) A19 medium screw base lamps with a CRI greater than 90 and a color temperature of 2,700 or 3,000 K had been introduced. This lamp was selected as it is one of the most common types used in the residential segment. Recently, four new bulbs were added, all from different suppliers, indicating manufacturers are responding to the market opportunity. This is not surprising since California represents > 10 percent of the US households and tends to be a bellwether for energy efficiency trends in the country. Since the data entered into the LED Lighting Facts is incomplete, it is not clear if all the bulbs will meet all the constraints of the new CEC requirements as some are not released on the market.

One of the bulbs introduced last fall targeting the CEC requirement was the 2700K 800 lumen Cree TW (True White). Since Cree already had an 80+ CRI ENERGY STAR 800 lumen lamp, this offers a vehicle to compare power consumption and price. In this case, the added power for the high CRI implementation increased consumption by 4 W (~ 40 percent power increase to 13.5 W) and the Cree TW product is priced $7 higher; $19.97 versus $12.97 for their 80+ CRI version.

The remaining aspect to consider is on the incentive side. California has a patchwork of public utilities as well as three large investor owned utilities. The latter must have their energy efficiency incentive programs reviewed by the utility commission and all have taken a different approach for the “Quality Bulb” initiative. Based on a random retail pricing sampling in the three utility regions, the incentives specifically on the Cree high CRI lamp versus the comparable 80+ CRI lamp were $10, $7 and $0. So in some areas the full price difference is borne by the consumer, while at the other extreme, the CEC compliant bulb is $3 less than the 80+ CRI version.

As more CEC compliant bulbs come on the market, the situation will no doubt become more dynamic, but it would be interesting to analyze comparable store sales to see how different price points influence market adoption. This would be very useful in seeing how consumers respond to the higher performance “Quality Bulb” and if it encourages retailers to introduce these bulbs in other regions of the country. Of course the most interesting test is replacing an existing bulb with a CEC bulb in a normal household environment and seeing if anyone in the family notices the difference.