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A New Approach To The Design Of Driverless AC LED Light Engines

Driverless AC LED light engines have now become a commonplace item of commerce in the lighting industry. Nearly all of them use high voltage integrated circuit switching chips to intelligently change the number of LEDs in a string during a power line cycle so that the voltage of the LED string matches the instantaneous power line voltage. However, customers are demanding higher efficiency, lower cost, and especially a reduction in the flicker content of the emitted light. Peter W. Shackle, president of the consulting company, Photalume, describes some new circuits for AC LED light engines [1] that provide a non-dimming solution with reduced cost, increased efficiency and better light quality as perceived by the human eye.

Up until now, the principle practice for AC LED Light Engines was to use switches during the voltage variations of the power line voltage cycle to adjust the number of LEDs present to roughly equal the instantaneous power line voltage, and then use resistors and/or current limiters to keep the LED current at a desired level until the next switch operation. This principle has the disadvantage that some power is always being dissipated, limiting the efficiency to 75%- 80%, and inevitably the output current comes in the form of a series of half sinewaves. In particular with 50 Hz power, the presence of these half sinewave pulses of current at twice the power line frequency produces a flicker effect which some people can see out of the corner of their eye, and in the worst case may produce eye strain and headaches.

The alternative principle introduced here is to have the LED current limited by resistance for a part of the time, and limited by capacitance for the remainder. 
Thus, in every power line half cycle there are two pulses of current through the LEDs, first a pulse of displacement current and then a pulse of galvanic current. In each complete power line cycle there are four peaks, so the resulting ripple can be mainly at four times the power line frequency. For power levels below 5 W, where the US Energy Star rules do not require a minimum power factor, it is possible to have continuous DC current with the entire ripple at four times the power line frequency. At higher power levels where a minimum power factor of 0.7 is required, the output LED current ripple has a superior flicker index compared to the conventional circuits, with 87% efficiency and without using any expensive high voltage integrated circuits.

Introduction to the Field of AC LED Light Engines IAC LED light engines are attractive because of their simplicity and relatively low cost for the luminaire designer. Today they are becoming ubiquitous in hallway lighting, decorative lighting, parking lots and an increasing host of other applications. As will be explained in detail below, most of these products are using high voltage integrated circuits which, for each part of the power line voltage cycle, automatically connect the correct number of LEDs to match the power line voltage. An electronic current limiter is used to maintain a constant current between switch operations. Commonly, three stages of switching are used, although two stage and four stage products exist. Since voltage is being dropped across the current limiter, power is being dissipated, which limits the efficiency attainable. Dimming can be achieved by varying the current level programmed for each stage of switching.

The design of AC LED light engines involves numerous compromises. For example, efficiency, power factor and light quality as measured by flicker index all trade off against each other, such that if one is optimized the others usually suffer. Including dimming capabilities requires additional compromises and usually increases cost and complexity. It has become conventional to adopt a set of trade-offs that accompany the use of a high voltage integrated circuit required to allow dimming. Typical of such a compromise in today’s marketplace would be a flicker index (see below) of 0.32, efficiency of 78% and power factor of 98%. This compromise, namely poor flicker index and average efficiency, is affected heavily by the inclusionof dimming, which is not needed for a large number of applications. Without the dimming requirement, the expensive high voltage chip can be eliminated, reducing cost, the flicker index can be reduced and the efficiency can be increased to over 90%. The power levels involved dictate the best way to accomplish this. Below 5 W of input power, Energy Star in the USA does not have a power factor requirement. In this case a solution is described which has superb light quality and over 90% efficiency. When the input power is over 
5 W, the power factor for consumer products must be greater than 0.7. In this case a solution is described which also has greater than 90% efficiency and still has superior flicker performance compared to today’s conventional products.

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