Using constant current regulator to realize high brightness LED lighting solution

　　In recent years, the LED field has seen advances such as high-brightness LEDs (HB-LEDs), which have been quickly applied to end markets such as automotive, building and street lighting. Moreover, with the latest driver technology, high-performance LED lighting has become more reliable and efficient than traditional lighting forms, and is becoming more and more popular in the market.

　　However, due to the lack of standardization, there are many different ways to drive and control LEDs. Many applications use solutions that do not take into account the special needs of LEDs. Although some methods can meet the new certification requirements, they do not provide excellent system solutions. Therefore, there is an opportunity and demand for application-specific solutions in the market.

　　System Solution

　　Dr. Roland Haitz, a retired scientist at Agilent Technologies, once emphasized that the lighting field is essentially a focus of the entire electronics industry. He said, "Although Edison was only the 38th person to invent the filament-based light bulb, he was the first to provide the entire lighting system." The existence of HB-LED has created a huge opportunity to replace the existing lighting technology with many shortcomings. However, in line with Dr. Haitz's point of view, we believe that a systematic solution is needed to realize this concept, that is, to let the HB-LED system replace traditional lighting systems such as filaments, fluorescent tubes, and even halogen and xenon lighting.

　　The main components of a solid-state HB-LED lighting system can be simply divided into power conversion, control and drive, thermal management, optics, and of course the LED itself. Without the complete provision and application of any of these components, a given HB-LED lighting system cannot work efficiently. For example, without the use of lenses and light guides to focus and process the light source, the lighting specifications of the application cannot be met. Similarly, if thermal management issues are not carefully considered and addressed, the system's operating life will be seriously affected as the LED junction temperature rises sharply to far above the component's maximum rated operating temperature.

　　The voltage source in HB-LED lighting systems varies depending on the type of application. For architectural and building applications, we can usually expect the supply voltage to be AC ​​power. Meanwhile, outdoor lighting may use AC power, unregulated power sources such as 12V lead-acid batteries, or solar power. For automotive applications, the power source is usually a 12V battery.

　　Although it is possible to drive LEDs from a voltage source without using some type of power conversion, this is not a good idea because normal voltage fluctuations will cause large changes in LED current. Considering the extremely steep voltage/current (V/I) curve and the large differences in forward voltage (usually above 1V) between different batches of LEDs, the use of an isolated or non-isolated power conversion stage is very necessary.

　　LED current regulation

　　The main function of the LED driver is to limit the current, regardless of the input conditions and how the forward voltage changes under various operating conditions. The driver itself and the overall system solution must meet the application requirements in terms of energy efficiency, current tolerance, form factor, size, cost and safety. The selected solution must also be easy to apply and strong enough to meet the extreme environmental conditions of the specific application.

　　Designers can choose from three different basic regulator topologies, depending on the specifics of their application:

　　● Buck—Used when the minimum input voltage (Vin) under all operating conditions is always greater than the maximum operating voltage of the LED string.

Figure 1 Typical circuit diagram of a buck converter

　　● Boost—Used when the maximum input voltage (Vin) under all operating conditions is always less than the minimum operating voltage of the LED string.

Figure 2 Typical circuit diagram of boost converter

　　● Buck-Boost or Single-Ended Primary Inductor Converter ( SEPIC )—Used when there is overlap between the input and output voltages. Advances in coupled inductors have made these solutions easier to implement in buck or boost topologies of the same size. Once mastered, the SEPIC topology offers many advantages over other commonly used topologies, including higher efficiency, smaller form factor, and lower cost.

Figure 3 Typical circuit diagram of buck-boost converter

　　Classification of LED current stabilization solutions

　　1 Resistor

　　Resistors are the simplest and lowest cost solution for current regulation. In practice, they are not a practical solution because they rely on the battery voltage, cause LED brightness variations, are inefficient and necessarily expensive, and require time-consuming and labor-intensive LED coding.

　　2 Linear Regulators

　　Linear regulators are easy to design, provide effective current regulation and overcurrent protection, and provide an external current set point, making them a "medium" HB-LED system current regulation solution. However, in today's energy-conscious era, designers may consider them to be too power-hungry and unacceptably inefficient for many installations, especially street lighting, buildings, and battery-powered applications. Inefficient linear regulators almost always have thermal management issues, which usually require some form of heat sink, but increase the size and cost of the overall design.

　　3 Switching Regulators

　　Switching regulators are the most expensive and technically complex solution for controlling LED current. Unlike linear regulators and simple resistor-based current-regulation solutions, they are susceptible to electromagnetic interference (EMI), which presents another challenge for designers to overcome. However, switching regulators are highly efficient and can provide brightness control capabilities for applications. For medium to high power solutions, or applications that need to handle a wide input voltage range, switching regulators are the only viable option.

　　4 Constant current regulator

　　2-pin and 3-pin constant current regulators (such as those developed by ON Semiconductor) can provide a simpler and lower-cost solution than linear regulators and switching regulators, and have performance advantages over resistor solutions. The 2-pin components provide a fixed output, while the 3-pin version provides the ability to set the output using a simple external resistor. Its output current value ranges from 20 to 150mA, and the maximum operating voltage is 45V, ensuring that it can withstand the battery load dump voltage.

Figure 4 Typical circuit diagram of constant current regulator

　　Using constant current regulators is like using linear and switching regulators, ensuring constant brightness over the wide LED voltage range they support. They also protect the LED from overdriving at higher input voltages and significantly reduce or completely eliminate costly and problematic LED inventory coding issues. A wide input voltage range of up to 40V supports operation under a variety of application conditions and withstands related supply voltage fluctuations. Constant current regulators can be configured as buck, boost, or SEPIC topologies. If the current of the LED string being driven is higher than a single constant current regulator can support, these components can be connected in parallel to provide a solution.

　　The light emitted by an LED is proportional to its average output current. Constant current regulators can also control this current to provide additional light output regulation. Dimming can be provided using analog dimming or digital pulse modulation techniques. Analog dimming schemes combine the input PWM signal with the feedback voltage to reduce the average output current. Digital dimming schemes use the input PWM signal to inhibit the switching of the regulator and reduce the average output current. Typical dimming frequencies are 200 to 1000 Hz because the human eye cannot see subtle changes at frequencies above 200 Hz, but can perceive subtle changes below this frequency.

　　Conclusion

　　HB-LED has the potential to become the main force in a variety of lighting applications. When comparing HB-LED lighting with other lighting technologies, power efficiency, long life and design diversity are only part of our positive parity. With the development of specific constant current regulators, better all-round system solutions will be born.