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The Next Generation of LED Filament Bulbs

 Until now, the basicdesign of an LED lamphas remained fundamentally unchanged. The common formula has been to use asingle, high power GaN chip (or a few larger chips), add phosphor (remotely ordirectly), and drive the chip(s) with as much power as possible to yield themaximum amount of white light. The key to the longevity of the lamps is toeffectively dissipate the heat that is generated by the large LED chips. Thisis why so many LED lamps have large heat sinks added to the base of the lamps.

 

A new technology trend has emerged,Filament LED. The use of heat sinks for heat dissipation has been eliminatedcompletely, resulting in a lighter weight lamp that costs less, and has thelook of a classic incandescent lamp. 

In the Beginning

First introduced in 2008 by UshioLighting, the initial goal of Filament LED lamp was to preserve the look of theclassic Edison light bulb as illustrated in Picture 2.  Unfortunately, theproduct did not gain widespread market acceptance due to poor thermal dissipationand incomplete flux geometry. 


The response by lamp makers was to go toa single large LED chip (or large matrix of chips), and add massive heat sinksto address the thermal issues created by using the large chips. Sadly, nearlyall of the lamps created over the next several years rely heavily on the use ofbulky heat sinks to handle the thermal management issue, and therefore provideonly 180 degrees of flux geometry. Clearly, this is not the ideal solution.

Scientific Study + Market Reaction

The Swedish government, with supportfrom the Belgian government, CLASP’s European Programme and the EuropeanCouncil for an Energy Efficient Economy submitted a report on November 19,2014, to the European Commission and the Consultation Forum to highlight theemerging trend of the LED Filament approach as used in a new breed of LED lighting products.

Some of these new LED products have beenintroduced into the N.A. and European markets at low prices.  The fanfarecaught the attention of the Swedish Energy Agency and its affiliates in Europewho decided to investigate the LED filament A19 in Q3 of 2014.  The reportconfirms the LED filaments used in the new breed of LED lamps have a greatpotential as they are more efficient than many conventional LED lamps, and thenew lamps provide better flux geometry. 

Even with this positive report, FilamentLED products have been slow to be adopted in the North American market for tworeasons. Only two (AXP Lighting and Lighting Science Group) of the dozens ofFilament LED lamp manufacturers are certified by UL, and only one (AXPLighting) has received both UL and Energy Star certification. 

Filament LED lamps – Not all are createdequal

This article will analyze “Filament LEDlamps” like the example shown in Picture 3.  The study also benchmarksseveral clear LED lamp designs,  those based around optical light guidesoffered by companies including IKEA, OSRAM and Philips.


The most common use of filament stylelamps is in retro lamps, i.e., clear glass lamps that allow the “filaments” tobe seen. These look particularly good in flame-tip chandelier lamps. While thelook may appeal too many as a throw-back to the old-style, classic incandescentlamps, a practical advantage of the filament is that the LEDs can be configuredfor omnidirectional light, the same as an incandescent lamp.  This type of360 degrees flux geometry has been lacking in most LED lamps on the markettoday.

Key Design + Construction Techniques

Beyond their similarity to the originalUshio filament lamps, there are actually many novel and effective designelements in the new wave of Filament LED lamps, which make them superior tofirst generation LED lamps.  For starters, the LED “Filament” typicallyconsists of many (sometimes hundreds) of tiny unpackaged LED chips mounted on atransparent substrate instead of metal substrate.  This is commonlyreferred to as Chip-On-Glass (COG). These transparent substrates are made ofglass or sapphire materials.  This transparency allows the emitted lightto disperse evenly and uniformly without any interference. This enhances theflux geometry of the lamp.  The LED filament is then encapsulated in aresin made up of a silicone and phosphor mixture that performs the usualtransformation of the LED chips’ blue light into white light.  Thisinnovative design allows the use both of both blue LEDs as well as red LEDs tomodulate the color temperature.  Most manufacturers rely solely on thephosphor to set the color temperature. This added degree of control allows thelamp maker to provide a more accurate level of color temperature, andflexibility during the manufacturing process. The downside is that the CRIperformance will not be as consistent when using the combination of both blueLED and red LED chips.

One area to note is that using low-gradesilicone to cut cost is a common practice among many of the LED filament lampmakers in China. Low-grade silicone will become brittle after 200 hours ofoperation. It will cause the filament structure to fracture and to break thewire-bonded LED string. This is why the use of high-grade silicone is vital tothe long life of the LED lamps. Each end of the filament has a metal electrode forfurther assembly.   Figure 1 shows the step-by-step manufacturingprocess of the LED filament.


The phosphor coating is critical to theCRI performance and the safety of the LED filament lamps.   Impropercoating of the phosphor can induce leakage of the blue LED light, which can beharmful to the retina as some reports have suggested. One Filament lamps maker,AXP Lighting, has developed a novel phosphor deposition process whicheliminates blue light leakage.

First generation LED lamps are typicallybuilt with large size LEDs, then they are driven with high current for maximumperformance.  Filament LED designs achieve even better performance thantheir traditional LED lamp counterparts by under driving many smaller, lowpower LED chips. The result is less heat, better efficacies, and sleekerdesigns with no heat sink needed.  First generation LED lamps, on theother hand, need large heat sinks, which in addition to adding cost, alsointerrupt the flux geometry, which is why traditional LED lamps can neverprovide a true 360 degree flux geometry. 

It’s all about the Heat

High quality Filament LED lightingproducts pay extra attention to the thermal management.  The highvoltage-low current scheme is an ideal combination for controlling theheat.  Nevertheless, a reliable LED filament lamp needs to have multipleheat dissipation paths.  It is a fact that the junction temperature (Tj)directly correlates to the life expectancy of the LED chips.  Figure 2shows the LED light decay rates under different junction temperatures(Tj).  This phenomenon is commonly called the “Droop Effect”.  Tohave a 30,000 hour life expectancy, while maintaining 90% luminous flux, thejunction temperature needs to be maintained below 85° C.


One of the novel approaches to thermalmanagement is to use a special gas mixture inside the glass lamp to facilitatethe heat transfer to the glass surface more efficiently than just relying onconvection alone.  There are other creative ways to dissipate the heatwithout sacrificing the omnidirectional light.  One secret is how you arrange the LED filaments.  As we study the commercially available LED filament products on the market today. 

Changing Direction

It is noteworthy that even traditionalLED lamp makers like Cree recognizes the advantages of a classic incandescentlamp appearance, which is why they have introduced the 4-Flow product lineincorporating an incandescent lamp shape with simple air convection forcooling. This does confirm that lamp manufacturers acknowledge the desire for aclassic Edison style lamp which is what consumers really want in their homes.Filament LED lamps appear to have that figured out.

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