June 5, 2017: With the rise of a number of specialty LED lighting applications such as horticultural lighting, displays and back lighting, general lighting, automotive lighting, etc, and with LED lighting network growing at a tremendous pace, AC-driven LEDs and arrays are one such LED technology that is gaining popularity as it helps to reduce cost and complexity of the driver electronics.
However, there is a lack of standardized testing methods to certify the AC devices as safe. The absence of standard testing methods for AC LEDs has always been a concern for the industry. Illuminating Engineering Society (IES) and CIE committees are actively engaged in formulating testing standards for AC LEDs. These standards will cover temperature controls, optical measurements that are needed to make different varieties of products in the market safe and usable.
What is an AC LED?
An AC LED is an LED that operates directly off AC line voltage. The typical characteristics include dual strings of multiple LEDs (to achieve high voltages) arranged in opposite polarity (or configured with a full-bridge rectifier) to accept the AC waveform and a bias resistor to limit input current. While simple and inexpensive the AC LEDs suffer from many drawbacks including flicker, high total harmonic distortion (and subsequently poor power factor) and low efficacy.
Importance of AC-LED testing
AC LEDs are designed to function at a steady AC voltage, and in order to characterize and measure such LED devices, researchers at the Taiwan Industrial Technology Research Institute (ITRI) and the National Institute of Standards and Technology (NIST) carried out studies. After conducting such studies, the researchers published a number of technical papers in order to demonstrate methods for performing the essential measurements.
In 2010, an operational group at the IES Testing Procedures Committee (TPC) was established to develop the standardized AC LED measurement procedures–IES LM-88. Around the same year, another operational group at CIE (International Commission on Illumination) Division 2 also established to develop the technical report to characterize AC-driven LEDs.
After several years of dedication, both standard-writing authorities are in the concluding stages of setting up their documents. The document describes the methods that need to be followed along with necessary precautions to be observed in performing exact measurements of entire luminous flux, radiant flux, photon flux, electrical power, luminous efficacy, chromaticity, and wavelength features of high-power AC driven light emitting diodes, according to the IES LM-88 scope. This accepted process takes into account AC-LED modules, remote-phosphor AC-LED packages and AC-LED packages.
ES LM-88 is intended particularly at characterizing high-power AC LEDs. Such LEDs are those that need a heat sink for their regular functioning. At present, most of the LEDs fall under this category. The AC LEDs are operated on an AC power supply with no extra external electronics. With increasing number of AC-LED products in the market, the standardized methods of calculations can offer producers and users with reliable and consistent means of gathering and using information.
Temperature control conditions
IES LM-88 identifies the temperature control condition, which is always significant to get precise results. During the calculation, the temperature for a device under test (DUT) is controlled by the ambient temperature or by the device level. The temperature is the basis for both the cases, which is supposed to be directly concurrent to the average junction temperature. LM-88 offers two measurement methods.
The initial method is to measure the AC LEDs with the help of a single cycle operation. As soon as the temperature is under control, the initial step is to alleviate the DUT with the intended junction temperature, which is controlled by the ambient temperature or by an active cooling system at the DUT. The subsequent step is applying the electrical input and then calculating the optical output. The dimension shall be inclusive for one AC cycle.
The second method is to use continuous AC operation. Here, the active cooling system is used in which the DUT is mounted, and needs to be set up at the required junction temperature. As soon as the AC power to the DUT is turned on, the primary current needs to be noted; it will rapidly alter owing to the junction temperature increase. LM-88 specifies that the electric power to the active cooling system be accustomed so the LED’s functioning current is sustained at the primary level, followed by the visual measurements.
The technical drive
CIE is actively engaged in establishing a technical report to characterize the AC LEDs, where the report aims to offer assistance for characterizing visual measurement of AC LEDs for testing laboratories, with importance in reproducible and smaller measurement doubts. Such characterization consists of the purpose of AC LEDs’ electrical, thermal, and visual properties. In addition, the document offers suggestions for operating conditions, measurement conditions, and calibration of the measurement system.
With the extra effects of AC LEDs inflict, IES’ and CIE’s pioneering standard documents will significantly benefit the LED lighting industry. As AC LEDs play a significant role in the contemporary lighting market, a methodical approach to test, report, measure and characterization of the devices with reliable and consistent results will definitely help the industry to grow in a healthy manner and at the same time, encourage specifiers and regulators to use more objective methods for ultimate safety of the users.