Abstract
Many examples of capacitorless and inductorless drivers used to power LED lamps directly from the AC mains use complicated circuitry in the form of a dedicated integrated circuit which is designed for a particular multistring lamp in order to obtain the value of the power factor very close to 1 and high efficiency of the whole unit. However, the authors of such descriptions do not provide any details concerning the optimum choice of the number of strings in a LED lamp and the number of LEDs in each string. The aim of this work is to show that acceptable values of both electrical parameters mentioned above can be obtained using a LED lamp with just one or two strings. Some formulae and results of their numerical analysis for the 230 V 50 Hz mains are presented, thanks to which designers of LED lamps and their AC direct drivers may easily estimate the optimum conduction voltage of a LED string or the number of single LEDs in a string.
Highlights
Solid-state lighting, based on light emitting diodes (LED), has been rapidly replacing conventional sources of light such as incandescent bulbs or compact fluorescent lamps (CFL)
Whereas incandescent bulbs can be directly connected to the AC supply network and CFLs require only some simple circuitry to periodically ignite gas discharge in the lamp, feeding of LED lamps is much more complicated
In this case the power factor reaches its maximum value PFmax = 0.961 when we use a string with the turn-on voltage VLED = 128 V (t1 = 1.29 ms) and the efficiency becomes 50 %
Summary
Solid-state lighting, based on light emitting diodes (LED), has been rapidly replacing conventional sources of light such as incandescent bulbs or compact fluorescent lamps (CFL). Whereas incandescent bulbs can be directly connected to the AC supply network and CFLs require only some simple circuitry to periodically ignite gas discharge in the lamp, feeding of LED lamps is much more complicated. As they generate light when they are forward biased and their current versus voltage characteristics are steep, they require feeding with a constant, or periodically constant current – like in the case of pulse width modulated (PWM) waveform. If LEDs were fed from voltage sources, a small variation of voltage would cause much greater change in current. They have been withdrawn from common use only because of their low efficiency
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