HB LED (High Brightness Light Emitting Diode) is the fastest growing light source in the world. Its luminous efficiency is 35~50Lm/W, which exceeds incandescent and halogen lamps. It has been improved to produce devices with luminous efficiencies greater than 100 Lm/W, which exceeds the luminous efficiency of fluorescent lamps and is comparable to HID lamps.
Figure 1 Basic LED converter provides constant current for LED series configuration
Designers are increasingly using HBLEDs in buildings, street and decorative lighting, signage, high-end televisions, and displays. These applications require an AC voltage or a DC supply voltage to provide a rectified DC current. A constant current source is implemented by the controller for non-isolated applications, such as traffic lights, where low voltage isolated power supplies are present or where the LEDs are held in hard-to-reach locations by class-2 fixtures.
The Buck converter structure is suitable for applications where the input voltage exceeds the output voltage, including most signage, decoration, and architectural lighting. A common array uses LEDs in series to ensure equal current through each device (see Figure 1). This avoids the use of additional circuitry in the parallel configuration to maintain equal current. However, in the case where the forward voltage across a single series stack exceeds the maximum voltage available or the available output voltage of the converter, a large array requires a parallel structure.
If a LED in the series structure is shorted, the remaining LEDs will continue to operate, but failure of the LED in the parallel structure will cause the remaining LEDs to fail.
Dimming is required in many applications. The blending of primary LEDs can produce any color in the spectrum by adjusting the intensity of each color, providing many possibilities for displaying illumination, billboards, and scene lighting. Taking IR's IRS2540 as an example, the IRS2540-based Buck rectifier system is driven by a low-frequency logic-level PWM control signal that does not cause visible flicker to the human eye, and is adjustable over the entire brightness range. The signal determines the average LED current by letting the converter operate in burst mode, changing the intensity of the light (see Figure 2). Another dimming method simply adjusts the DC current of the LED and it also changes the color of the LED.
Figure 2 Burst mode dimming provides excellent control of the LED string's light output while maintaining excellent color fidelity
HBLED enables architects, designers, specifiers, and manufacturers to create lighting effects for theaters, studios, nightclubs, restaurants, and other high-visibility venues. Digital scene control provides a compelling and ever-changing lighting display.
Figure 3 IRS2540 provides continuous time-varying current monitoring for true average current control
LED light sources can also be used for landscape and outdoor lighting . They have a long life span, require less maintenance, and are less affected by moisture intrusion. Unlike traditional light bulbs, LEDs do not have easily broken parts, even if they are not handled with care.
By integrating a floating high side driver, the IRS2540 is able to continuously monitor the LED current using current sensing mode, see Figure 3.
The IRS2540 achieves average current control rather than peak current control and provides an inherently stable rectification that can operate over a wider range of voltages and loads without encountering design constraints. The delay of the circuit creates a current ripple in the inductor, but this ripple can be easily reduced by using an output capacitor in parallel with the load. This can also be done by increasing the delay in the current feedback to reduce the frequency and increasing the efficiency by reducing the switching losses. This structure has inherent overload and short circuit protection.
There are two variants of the high voltage LED converter IC driver, 200V and 600V. With PWM duty cycles up to 100%, converter instability does not exceed 50% as in peak current control systems. An example shows that at 175 kHz, its efficiency exceeds 85%, see Figure 4.
Figure 4 IRS2540 and similar products IRS2541 support bus voltages up to 200V and 600V respectively.
Manufacturers of LCD TVs, monitors, laptop monitors, and custom panels are using LED backlights instead of CCFLs (cold cathode fluorescent lamps) to extend the palette of reproducible tones by up to 45%, and have achieved NTSC color space 105. % reproduction, and the best CCFL lighting LCDTV can only achieve 65% to 75%. Even CRT (Cathode Ray Tube) displays cannot match the color reproduction and brightness of LED-based systems. (small soup)
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