Talking about the myth of AC-LED
(Author: Liu Kent)
Since the work of the AC- LED application research and development alliance between the ITRI and more than a dozen manufacturers in October 2008, the media has been competing to report, and all parties praised it, and there is a hidden momentum to create an unlimited business opportunity.
In fact, the concept of AC-LED has existed for a long time. Professor Nishizawa Ryoichi proposed the concept of AC-LED in the US Patent No. 4,329,625 in 1982. Seoul Semiconductor of South Korea is the first to invest in the development of single crystal AC-LED. In recent years, the market is also filled with Discrete AC- LED products with multiple LEDs in series and a current limiting resistor. But whether it is single crystal AC-LED or Discrete AC-LED, it can only be used for small wattage applications, such as night light, god light and so on. High-power applications, such as LED street lights above 100W, use normal high-power LEDs without the use of AC-LEDs.
Why is there such a gap? I think it is because the basic concept of AC-LED advocates is: free AC/DC converter, as long as a current limiting resistor is used, it can be driven directly by AC, avoiding 15%~30% of AC/DC converter. Additional loss. But this high-efficiency assumption seems to be seriously embarrassing.
Under ideal operating conditions, AC-LEDs can achieve 95% efficiency, but at a high cost. The main trick to achieving high efficiency is to connect many LEDs in series while reducing the voltage drop of the current limiting resistor. However, the result of connecting a large number of LEDs is that the current conduction angle becomes very narrow. The power factor may fall below 0.50 and cannot pass the US Energy Department's Energy Star specification (Note: Energy Star's power factor requirements for solid state lighting fixtures are above 0.9.
To make matters worse, reducing the voltage drop of the current limiting resistor will expose the AC-LED to the risk of being burnt out. Electronic motor engineers should know that the AC voltage will fluctuate. And depending on the region and environment, some places may be as high as 130Vrms and some may be as low as 100Vrms. Moreover, the VF of the LED varies greatly with temperature and the technology of the manufacturer. Deliberately reducing the resistance of the current limiting resistor to improve efficiency is at risk of burning out when the AC voltage is high or the VF is low. When the AC voltage is low, there is a problem of insufficient brightness.
The semiconductor design of Seoul Semiconductor is relatively conservative. Only 20 LEDs are connected in series, and the voltage drop of the current limiting resistor is also much increased. However, the circuit efficiency is as low as 60%. This number is hard to accept, why?
Because the biggest challenge of LED lighting is the heat dissipation problem. 60% of the circuit efficiency means that 10 watts is input, only 6 watts are effectively used to drive the LED lighting components, and the other 4 watts are consumed above the current limiting resistor. At present, designing LED lighting, it is already a headache to deal with the heat dissipation problem of the LED component itself, not to mention the additional current limiting resistor with an additional 67% heat loss.
In short, no matter how you design, you will find that AC-LED cannot pass the multiple specifications of Energy Star. If efficiency is emphasized, there is no way to pass the cause, and there is no way to guarantee the life. If safety, durability and power are emphasized, efficiency cannot be passed, and the cost of heat dissipation of products is also greatly increased.
Nowadays, high-power LED street lamp design uses AC-DC power supply with high power factor of 0.95 or higher to drive LED module, the efficiency is over 87%, and most LED street lamp designs can accept 90Vac to 260Vac wide input voltage. range. The high-efficiency AC-LED seems to be trying to prove the correctness of its basic concept, and it is still far from the practical stage of challenging LED street lights or general lighting.
In fact, the entire power supply (in fact, the entire power electronics) industry is based on a basic principle. The principle is that no current-limiting resistors are used (because the resistors consume power), and electronic circuits and inductors and capacitor components are used to achieve a constant voltage or constant current output. AC-LED relies on a large current-limiting resistor to maintain LED current fluctuations. It is contrary to the basic principles of power electronics. Its low efficiency is an inevitable result. Therefore, it is not surprising that AC-LED is unable to break through its practical dilemma.
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