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Bookmark | Report 2017-03-07 21:42   Visits: 137   Answers: 1

Why do LED lights flicker?

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2017-03-07 23:02
Flicker and visual fluctuation can be noticed when there are sufficiently large ripples in the direct current (DC) delivered to the LED lamp. A residual of the alternating current (AC) input may come up on the output as a variation or ripple which may correspond to the AC frequency, like for example 120 or 100 Hz. The AC sinusoidal input voltage would typically be rectified by a full-wave or a half-wave rectifier into a rectified sinusoidal input voltage before being delivered to power the LEDs. The LEDs can't be forward-biased to illuminate and the flicker phenomenon takes place during the dead time (near the beginning and end of each DC pulse cycle in which the input voltage is less than the combined forward voltage drop of the LEDs) at a repetition rate of twice the AC sinusoidal frequency. The ripple current is often twice the input AC line frequency, as an illustration, if the AC sinusoidal frequency is 60 Hz, the rectified sinusoidal frequency will double as 120 Hz.

LEDs are current driven rather than voltage driven devices. Line voltage sources are AC waveforms and the voltage at the line source varies with time. A flicker of the LED takes place when a current waveform of the power provided to the LED is imbalanced. An LED driver is a self-contained power supply which includes outputs matched to the electrical characteristics of the LEDs. The driver circuitry is designed to convert the AC mains voltage into the constant load voltage and constant load current to ensure that flicker is not visible to the human eye. The ripple current in the LED load may be reduced by utilizing an electrolytic capacitor across the diode bridge in the driver circuitry. However, there's an increasing trend to use linear driver or AC direct drivers in LED lamps because of their simple design, low cost, compact size, and immunity to electromagnetic interference (EMI). The main shortcoming of this circuit is the high current ripples of the output current as it uses only MOSFET transistors and integrated circuits to control both stage and does not use electrolytic capacitors to decrease current ripple in the LEDs as the manufacturers think electrolytic capacitors are too large and expensive, and may degrade circuit reliability due to their temperature-sensitive property. Therefore the linear driver powered LED lamps are more likely to flicker if the circuit design does not have features to smooth out the large output current ripple that causes the flickers.

Flickering in LED lamps can occur in the use of a Triac circuit for analog LED dimming or phase angle dimming. A Triac is a bidirectional thyristor device that behaves as a controlled AC switch that can conduct current in either direction. Triac dimmers work especially well when used to dim incandescent and other resistive lighting devices. Nevertheless, flickering or blinking can occur when these circuits are used to dim LED luminaires due to the switching current fluctuations caused by interaction with an EMI input filter or premature turnoff during current reversals.
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