Understand LED driver topology selection

Over the past few years, high-brightness light-emitting diodes (HB- LEDs ) have rapidly improved performance in lumens per package and efficacy (in lm/w). The commercial 1W LED has been provided with a cool color temperature LED (color temperature 5000K) with a lumen output of more than 1001m per package, and a luminous efficacy of 1001m/W, while the warm color temperature white LED (color temperature 3000~3500K) of the same power level also exceeds 70~801m. . These performance levels have increased by 30% to 40% compared to two years ago. With this performance, LEDs are now evolving into a viable alternative to traditional incandescent, halogen and fluorescent lamps in high-performance applications. As a result, solid-state lighting (ssL) has penetrated considerably into automotive, commercial and landscape lighting, as well as urban street lighting. LEDs can also be used for new applications, such as remote lighting based on actual solar panels, as it is easy to generate the DC drive current required by the LED from a rechargeable battery. In addition, if properly designed, LED luminaires have a working life of 3 to 595 hours, significantly reducing the maintenance costs typically associated with replacing bulbs.

However, there are many contradictions in the use and control of these LEDs. For example, many existing lighting solutions used or modified by lighting system designs do not adequately address the unique drive requirements of HB-LEDs. If designers are to optimize the benefits of LED lighting, careful consideration must be given to the technologies used to drive and control these devices to provide an energy efficient and cost effective solution. The main components of the HB-LED lighting system include LED emitters, power conversion, control and drive, thermal management, and optical components that are involved in many applications. If all these components are not fully considered, the corresponding LED lighting system is unlikely to be optimized. Unlike most light bulbs, LEDs are directional light sources, so in many applications, the use of lenses, mirrors or diffusers to provide the desired illuminating pattern and the luminaire's luminaire appearance is critical. Similarly, if thermal management issues are not properly addressed, the operating life of the lighting system will be significantly reduced, offsetting the main advantages of using long-life LEDs. Power and drive aspects are equally important for the long-term work of lighting systems. The supply voltage source in the HB- LED lighting design depends on the type of application being used. For buildings and indoor lighting , the voltage source is usually the AC mains. Outdoor lighting may use a widely regulated power source such as a low voltage AC power source, a solar panel with a backup battery, or an AC mains power source. In automotive applications, the power source is typically a lead-acid battery (12Vdc).

If there is no form of power conversion, the voltage source should be avoided to drive the LED emitter, as normal voltage fluctuations can cause large changes in LED current because the voltage/current (V/I) curve of the LED is very steep; Under different driving currents, temperatures, and production (different batches), the LED forward voltage varies widely. In addition, for safety reasons, most AC mains applications have isolated power conversions based on electronic switching power supplies or magnetic transformers that convert high line voltages into safe low voltages suitable for driving LEDs.

One of the main functions of the LED driver circuit is to stabilize the current under a variety of operating conditions, regardless of the input conditions and how the forward voltage changes. The drive circuit must meet application requirements for energy efficiency, capacitance tolerance, form factor, cost, and safety. At the same time, the chosen method must be easy to use and strong enough to adapt to the extreme environment of a particular application.

There are several different methods of steady flow. A resistor that is powered by a fixed voltage source is the simplest and lowest cost method of current regulation. In fact, they do not steadily flow, but simply limit the maximum current when the LED forward voltage changes and the source voltage changes and causes a current change that causes brightness variations. This may be acceptable for low current indicator applications, but as the current increases and the number of LEDs in series increases, the problem arises. To overcome this problem, it takes a lot of money and time-consuming to encode the LEDs and select the appropriate resistors to match the LED string forward voltage. Even with these steps, there is still a problem of brightness variation caused by input voltage variations.

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