LED Driver implements power factor regulation and dimming function

With the advancement of new materials and manufacturing processes, a new lighting source, high brightness LEDs are now attracting increasing attention from academia and industry. LEDs are both environmentally friendly and energy efficient. Usually phosphor material is used to convert monochromatic light from a blue or UV LED to broad-spectrum white light. Compared with all the other commonly used lighting sources, LEDs provide 30 percent efficiency with much room to improve. Unlike the CFL, the LED contains no mercury, and is not easy to break. LEDs have very long lifespans (100,000 hours which is more than 10 times the typical CFL). LEDs can endure very high numbers of on/off cycles. And LEDs are easy to drive compared with traditional sources. The circuits are compact and can be installed on PCB. We'll present a power-factor-regulated circuit that can also implement a dimming function.

Typically the operating current of the high brightness white (HBW) LED ranges from 200 to 700mA. The typical drive voltage is about 2.5V to 4V. Strings of LEDs are connected in series and/or parallel to increase the flux. The number of LED for residential lighting is around 10 to 20. For street and parking lot lighting, 30 to 50 LEDs are used. While for commercial lighting, hundreds of units are used for high flux level. The voltage of a series of LEDs is the voltage drop of individual LED times the number of LEDs in series. The current of a series in parallel is the sum of the LED currents of all the strings. LED brightness is dependent on the current. For a wide range of operating currents, the lumen is approximately proportional to the average current in the LED.

There are two approaches for LED dimming. One is to regulate the LED current, which can be implemented by current feedback control through the power converters. It is cheap and easy to implement, and can be used for the lighting applications. However, since the color tune of the lighting depends on the LED current, such a dimming method is rarely used in display applications, where accurate color tune is needed. Another method is dimming through pulse width modulation (PWM) because the LED can be turned on and off very fast and without any negative effect. The PWM method is widely used for display applications since these applications require accurate color tune which is dependent on the current of the LED.

A number of isolated/non-isolated power conversion topologies serve the functions that are necessary for driving the high brightness LED for various power levels. There are several single stage offline ac/dc power converters with power factor pre-regulation for LED lighting applications using voltage mode PWM controllers, including boost converter, single-ended primary inductance converter (SEPIC) converter, and flyback converter. These topologies are suitable for different power capacity and different customer requirements.

The flyback converter is one of the frequently used topologies for low power, low cost ac/dc conversions for residential lighting applications. Running it in discontinuous conduction mode, the input harmonics normally meets the standards such as IEC 1000-3-2 (Figure 8 near the end of this article shows a flyback circuit.) The LED current is dimmed by the ac line voltage. Provided that the duty ratio, D<1/(1+V₁/NV₀), the current in the output diode always decays to zero before the end of the switching period.

Figure 1: A flyback converter for LED lighting application with ac line dimming, power factor versus ac line voltage. Current and voltage with (c) 220V input and (d) 140V input Blue: rectified line voltage (50V/div), Red: Line current (100mA/div).
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Proper snubber circuitry is needed to suppress the ringing caused by the leakage inductance of the transformer. A prototype flyback converter board with ac line voltage dimming can be used for applications with less than 10 LEDs for current up to 500mA and 220V (RMS) ac line voltage. The current waveforms for different line voltages are shown in Fig. 1. The current waveform will be distorted if the transformer current goes to continuous. The relation of the LED current versus the ac line voltage is shown below, as is the power factor curve. For most of the operating conditions, the power factor is higher than 0.92.

Figure 2: A flyback converter for LED lighting application, power factor versus ac line voltage.
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Figure 3: A flyback converter for LED lighting application, LED current versus ac line voltage.
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