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MONDO ARC

AC or DC - that is the question

issue 42 Apr / May 2008


It was one of Hamlet’s most famous soliloquies that reminded Dr. Geoff Archenhold of the increasing strength of a new Solid-State Lighting technology that could eventually take its place alongside conventional LED technology. However, will AC driven LEDs actually displace DC driven LEDs – that is the real question!

The impact of Light Emitting Diodes (LEDs) on the general lighting industry has only just started with tremendous revenue growth seen during 2007 for white LEDs (see the SSL roundup article in December 2007). However the rapid pace of LED development continues unabated in early 2008 with the release of Seoul Semiconductors Z-Power P7 single LED emitter package emitting up to a maximum of 900 lumens from just 10 electrical watts and up to 995 lumens maximum from a 15W device from LEDEngin Inc.
This month’s article will briefly touch upon the latest LED emitter packages to be launched onto the lighting market and to look at the new class of AC driven LEDs that do not require AC/DC LED drivers and offers the promise of simplifying LED luminaire design.
The latest LEDs are more efficient, usable and environmentally friendly than CFLs.
At the end of the first quarter of 2008 several manufacturers have launch new LED emitter packages onto the market all exhibiting common characteristics of increased light output, greater efficiency and lower LED thermal resistance. Indeed, these latest “digital” LEDs have now exceeded the raw lumen output of most typical Compact Fluorescent Lighting devices (see table 1 for comparison) and at a much higher efficacy at much higher levels of light quality and with a lower carbon footprint meaning LEDs should now become the light source of choice.
As shown in Table 1 the P7 Series compared to general 60-watt incandescent lamps, which provides an efficacy of approximately 11 lumens per watt, emits light at up to 900 lumens and has a maximum efficacy of 90 lumens per watt. The colour temperature of the P7 is 6300K meaning that it is only available in cool white and the colour rendering index (Ra) is low at 70. However this lighting breakthrough is happening at a time when oil prices have exceeded $107 per barrel and the environmental waste aspects are fuelling interest for energy efficient systems all around the world.
In addition, the P7 Series shows remarkable performance compared to compact fluorescent lamps with a typical CFL consuming 15 watts and emitting light at 930 lumens delivering an efficacy of 62 lumens per watt, while the P7 Series’ efficacy is nearly one-and-a-half times higher at 90 lumens per watt. Therefore, for the first time a single LED emitter package in a space of only 18.4mm x 12mm x 6.54mm can produce the same raw lumens as a 15W CFL bulb which is pretty impressive!
As discussed in many of the previous LED articles, the maximum performance of a P7 (derived from a manufacturer’s datasheet) should not be used to predict the total system efficacy due to thermal performance in standard operating conditions but even the typical luminous flux for a P7 is rated at 700 lumens providing a typical efficacy of 70 lumens per watt (still better than that of a CFL).
Figure 1 shows an enlarged picture of the P7 and reveals the secret of how the P7 LED emitter can produce so much light output, on closer inspection the photo reveals four individual chips inside the package. The data sheet gives the total forward current as 2.8A, with forward voltage of 3.6V, so one can assume each chip is driven at 700 mA (or around 2.5W). When the device is driven at 1400 mA (forward voltage = 3.3V), or 350 mA per chip, the typical output is 400lm, corresponding to an efficacy of around 86 lm/W.

The configuration of the LED chips within the P7 is structured so that they operated in parallel (as shown in figure 2) which is rather unusual for two reasons:
1. Running LED chips in parallel without any voltage or current balancing can create uneven current paths that could result in a variety of issues from uneven chip illumination where one LED has more current flowing through it so it has a high lumen output compared to the others through to rapid failure if one LED chip fails because the current through the other 3 LEDs has to take the current meant for 4 LEDs.
2. There is currently very little choice for high current LED drivers especially those that can drive currents up to 2.8A so the industry will need to wait until a large number of drivers become available at these higher currents.

Perhaps, it may have been more prudent to design the P7 so that all of the LED chips were connected in series which would mitigate any of the issues highlighted and would provide a choice of drivers from 350mA up to 700mA. In fact, the LEDEngin LZ4 10W and 15W LED devices have gone one further by providing direct access to each LED chip (see figure 2) within the package enabling the lighting manufacturer to decide whether to control each LED individually, connect them in series or similar to the P7 in parallel.
Although the luminous efficacy of the LEDEngin product range is lower than that indicated by the P7 it offers a smaller package of just 7mm by 7mm and comes in a wide range of colours and cool, neutral and warm white colour temperatures with a CRI as high as 90 as shown in Tables 2 and 3.

Referring to Figure 3 which overlays the P7 4 chip LED emitter with the equivalent sized single chip emitters there is clearly a significant step change in performance. This performance increase is due to improved LED packaging resulting in a lower thermal resistance of just 3°C/W for the P7 and the LED chips are operated at a lower forward current.
It is clear that the P7 will create a new trend of multi-die, high luminous output emitter packages at affordable costs and it is certain other LED manufacturers will follow with low-cost variants ensuring the rapid deployment of LEDs in general lighting markets.

AC LEDs – What are they?
The vast majority of LEDs sold in the market place are DC driven LEDs however a new class of LEDs that are driven directly from the mains supply are being touted as an alternative to traditional DC LEDs. DC-driven LEDs have several disadvantages including:

  • AC/DC power supplies add extra cost and require more space.
  • Design difficulties to accommodate space and thermal dissipation requirements for DC-LEDs make it less viable as a replacement for conventional incandescent bulbs or building structure lightin
  • Waste materials from the converter causes environmental pollution
  • 80% efficiency of AC/DC converter causes 20% of electricity loss
  • When using an AC/DC converter in small or enclosed area the heat from LED and converter interaction can accumulate heat causing a reduction in lifespan
  • Overheating can cause a fire concern which will require safety plans
  • Cannot be easily configured or linked together in long chains


While the lifespan of DC-driven LEDs sold in the market is 50,000 to 100,000 hours, the AC-DC converter needed for its application to lighting fixtures only has a lifespan of 20,000 hours. The need to change the AC-DC converter several times over the life of the LED is a major shortcoming of DC-driving LED technology, and can limit its appeal for lighting fixtures.
In classic textbooks on electronics or physics it will state all LEDs are DC devices so how can an LED be driven directly from an AC supply at either 220 or 240V? The answer is fairly straight forward; the AC LED device is actually made up of two strings of series-connected die, connected in different directions; one string is illuminated during the positive half of the AC cycle, the other during the negative half. Thus, the device is essentially non-polar. Since the strings fabricated on the substrate are formed from many p-n junctions in series (as shown in figure 4), the total forward voltage of each string is very high, and approaching the AC mains input voltage.
Therefore, an AC LED must be designed for a specific voltage range and at this stage cannot be interchangeable between say 110V or 220V.
Like any LED, proper mounting to a thermally conductive surface is critical. You’ll also need to be mindful that the leads and traces will be carrying high voltages and so care must be taken for fixture design.
The reason AC LED emitters have not been successful to date is due to several factors including:

  • Not as bright as DC LEDs
  • Not as efficient as DC LEDs
  • Low number of manufacturers so reduced availability and choice
  • Reliability is susceptible to voltage variations
  • Intensity variation with over or under voltage on AC mains
  • Switching frequency is limited to mains frequency (50/60Hz) so not suitable for many applications eg; TV lighting
  • Systems are designed around mains voltages
  • The AC LEDs cannot be dimmed easily
  • Available in only low wattage emitters (<10W)

Due to significant research by two companies, Seoul Semiconductor in Korea and Lynk Labs in the US, several of the disadvantages holding back AC LEDs are now being addressed. For example, Seoul Semi announced in February that their AC LED known as the Acriche (shown in figure 6) had achieved a lumens per watt efficacy level of 80 lm/W although the system is still only available in 2W or 4W versions.
The high reliability of the Acriche AC LED is shown in figure 5 where 70% of initial lamp lumens is reached after 20,000 hours of operation at a junction temperature of 80°C. Similarly to DC LEDs the lifetime of the AC LEDs increases as the junction temperature decreases.
A second company Lynk Labs offers a range of AC driven technology evolved from a core technology layer called No Return Path “NRPTM” technology. NRP and other AC LED technology has been integrated into Lynk Labs AC-LED devices, assemblies, drivers and system solutions for solid-state lighting applications.

Conclusions
The rapid increase in LED luminous efficacy is continuing unabated and today the total light output from a small 10W LED emitter package is essentially equal to that of a 15W Compact Fluorescent ensuring that it is a matter of time before the LED will displace the majority of conventional light sources. The technical barriers are rapidly being overcome one by one and soon a single emitter package will break the 1000 lumen output in a cost effective manner (eg; <$5).
It is less clear to predict the impact of AC driven LEDs against a market dominated by DC LEDs especially as the technology is still so young however AC LEDs are ideal for applications where LED drivers are impractical to install, quality mains voltages are available and no dimming is required such as signage illumination.
I believe the future will see both AC and DC LED systems co-exist within the general lighting market as both techniques have advantages and disadvantages however as the market has already adopted DC LED systems it will be a significant time before AC systems can claim the number of AC LED shipped.
If you have any further comments on AC LED technology or would like to know more about any aspects of designing LED systems then please contact the author by email.

garchenhold@hotmail.co.uk

 

Table 1: Comparison of the Seoul Semi P7 and LEDEngin LZ4 versus Incandescent and CFL Lamps


  • Figure 1: A 10W Seoul Semiconductor P7 Series LED emitter


  • Figure 2: Showing the electrical configuration of the P7 and LZ4


  • Table 2: The LEDEngin emitter product range from 3-15W single packages


  • Table 3: The LEDEngin emitter Lumen output for the 3-15W single LED packages


  • Figure 3: Typical luminous flux per LED package showing the performance of the P7 4-chip system compared to similar sized single chip equivalents


  • Figure 5: The output degradation of an Acriche AC LED over time and junction temperature


  • Figure 6: The Seoul Semiconductor 4W Acriche AC driven LED

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