An LED Driver is an electronic device which regulates the power to an LED or a string (or strings) of LEDs. In contrast to conventional light sources that runs directly from an alternating current (AC) power supply, LEDs operate on DC power. Hence, a constant supply of DC electrical current at a fixed output or a variable output within an allowed range must be applied to an LED array for stable, non-flickering lighting.
Functions of LED Driver
LED drivers provide an interface between the utility power (line) and the LED (load), converting the incoming 50 Hz or 60 Hz AC utility power at line voltages such as 120 Volts, 220 Volts, 240 Volts, 277 Volts or 480 Volts to the regulated DC output current. An LED driver circuit should have immunity against voltage spikes, surge and other noise on the AC line while also filtering out harmonics in the output current to prevent them from affecting the output quality of the LED light source. The driver is not merely a power converter. Some types of LED drivers have additional electronics to enable precise control of the light output or to support smart lighting.
LED Driver Design Considerations
The design of an LED driver involves many variables, including cost, size, parts count, efficiency, power factor (PF), total harmonic distortion (THD), input voltage range, flicker, dimming capabilities, electromagnetic interference (EMI), and operating temperature. LED tubes run off a constant current source which is typically configured as a switching mode power supply (SMPS), it may be of a single-stage type or of a two-stage type. An SMPS LED driver that is used to operate an LED tube require a low cost, low parts count solution to provide regulated DC output power with low ripple, good power factor and moderate line current distortion. A single-stage LED driver performs both power factor correction (PFC) and switching regulation in one circuit. The two-stage system uses two separate circuits for PFC and DC-DC conversion.
Input Voltage Range
Most LED drivers are designed to operate at specific voltages. Today’s increasing demands on electrical utilities can cause wide line voltage variations during load demand changes, which in turn cause light output from lamps operated on dedicated drivers to vary with the input voltage changes. With line regulation technology, many LED drivers can maintain constant light output through nominal input voltage ranges of 120 to 277 volts, thereby compensating for any change in input voltage. With the latest patented technology, LUMIXESS® LED ballast that allows wide input voltage from 120V to 347V, offer much greater flexibility and other advantages such as inventory reduction
Power Factor (PF)
Power factor is the measurement of how effectively a driver converts the voltage and current supplied by the utility power into watts of usable power delivered to the driver and lamps. Perfect power utilization would result in a power factor of one.
Power factor measurements pertain only to the effective use of utility power supplied to the driver. They are not an indication of the driver’s ability to supply light through the lamps. Because low power factor drivers require about twice the current needed by high power factor drivers, they allows fewer fixtures per utility unit and have to added wiring costs. High power factor drivers are generally required for all commercial lighting applications, and the PF should be above 0.9 as DLC specified.
Total Harmonic Distortion (THD)
Harmonic distortion occurs when the wave-shape of current or voltage varies from a pure sine wave. Except for a simple resistor, all electronic devices, including electromagnetic and electronic ballasts, contribute to power-line distortion. For LED drivers, THD is generally considered the percent of harmonic current the driver adds to the utility power system. The ANSI standard for electronic ballasts specifies a maximum THD of 32% for commercial applications. However, most electric utilities now require that the THD of LED driver / ballast should be less than 20% as DLC specified.
EMI / RFI
Because they operate at high frequency, LED drivers may produce electromagnetic interference (EMI) or radio frequency interference (RFI). RFI frequencies are a subset of EMI frequencies. EMI issues cover all possible operating frequencies while RFI is only concerned with radio and television frequencies. This interference could affect the operation of sensitive electrical equipment, such as radios, televisions or medical equipment.
Output Ripple
The output ripple is magnitude of AC voltage appearing superimposed on the DC output specified in peak to peak volts or expressed as a percent of the nominal output voltage. During the conversion of the AC to DC in a power supply, the resulting output DC pulsates in rhythm with the mains supply frequency which is either 50 or 60Hz depending on location.
Single-stage drivers do not provide complete suppression of the alternating waveform after rectification. The output current may contain large ripple that results in light flicker at twice the line frequency. While two-stage drivers have front-stage act as PFC to correct phase errors and reduce harmonics, and back-stage as DC-DC converter to provide tight regulation with minimal ripples in the output current provided to the LED load. The extra output stage substantially increases the cost and size of the driver and causes additional efficiency loss. Nevertheless, the two-stage design offers a fair trade for better quality light output (less flicker) and allows smooth, deep dimming in almost every output current range.
Generally Type B tube applied internal single-stage driver and while Type C tube applied external two-stage driver. Therefore flicker is inherent issue in Type B tube lamp regardless it is single-end or double-end Direct-Wire (Ballast-Bypass) lamp.
1 thought on “The ABC of LED Driver for Tube Lights”
Thanks for your blog, nice to read. Do not stop.