High-brightness LEDs (HBLEDs) are rapidly gaining popularity in the automotive, consumer electronics and industrial markets. Bright colors, long life and high energy efficiency are some of the reasons why high-brightness LEDs are the future of lighting applications.
In the automotive industry, HBLED technology differentiates vehicles from styling, safety and fuel economy, from simple switching lighting, LCD backlighting to extremely high brightness headlight applications. However, efficient and reliable control of the brightness of HBLEDs is not an easy task; power stage efficiency, thermal design and EMC are the most critical design challenges in applications involving HBLEDs. Typically, a dedicated constant current driver (CCD) is used to drive the HBLED string to solve most important design problems and simplify the design. However, CCDs are usually more expensive than microcontroller-based solutions. This article describes the use of 8-bit microcontrollers (MCUs) and low-cost separation solutions to implement intelligent HBLED lighting control, thereby avoiding the use of expensive analog drives or CCDs.
Important features of high-brightness LEDs As is the case with low-intensity LEDs, the luminous intensity of a high-brightness LED is proportional to the electrical flow that passes through it. This current is often referred to as forward current (IF) and is in the range of 100 mA to 1000 mA in the HBLED. At the same time, whenever the HBLED is polarized, a voltage drop occurs, called the forward voltage (VF). In HBLEDs, luminosity and chromaticity are directly proportional to IF, so precise control of the current through the HBLED is critical.
HBLEDs with the same part number and technical specifications do not necessarily have exactly the same VF value. When the current IF through the two HBLEDs is the same, their backward voltage VF may be different. Therefore, controlling the intensity of the LED by means of a constant voltage may result in a difference in density between the HBLED and the HBLED, and to ensure that all HBLEDs have the same brightness, a current control must be provided.
Not only is the luminous intensity related to the current through the HBLED, but the chromaticity is also related to the HBLED current. In order to maintain the HBLED color, the HBLED must be driven with a constant current. This solution will use PWM (Pulse Width Modulation) to provide a lower average current in HBLED (light intensity) while maintaining the same instantaneous current (LED color).
As the HBLED current increases, power consumption will also increase. The current is 350mA. HBLEDs with a voltage drop of 3V consume approximately 1 watt of power. This dissipation can cause HBLED overheating and long-term performance degradation if proper thermal management is not performed. Another important aspect of thermal design is that the intensity of HBLED illumination is inversely proportional to the junction temperature of the LED. As the temperature increases, the color of the emitter enters a higher wavelength.
In low-intensity LEDs, it is very common to use resistors to limit IF current. In HBLEDs, the rated power of the resistor must be higher, which can lead to inefficient systems. Therefore, in HBLED systems, Switch Mode Power Supplies (SMPS) are used to increase efficiency and reduce power consumption. Since SMPS requires energy storage components (inductors and capacitors), the price is usually more expensive; at the same time, SMPS can also cause noise or EMI problems.
A group of HBLEDs can be driven simultaneously in parallel or in series. Parallel driving allows each HBLED to have a different illumination intensity. However, if a control loop is required, each HBLED will require a dedicated control, so the cost is too high for a large number of HBLEDs.
When the HBLEDs are connected in parallel, each string requires only one drive and control loop, and the current through all of the HBLEDs in the series is the same, providing them with a relatively constant brightness. Depending on the number of LEDs in series, the voltage required for line penetration may be lower or higher than the input voltage.
There are a number of solutions on the market that drive HBLED constant current, some based on dedicated intelligent analog drivers and the other using digital signal processors (DSPs) or microcontrollers with independent analog drivers.
It is widely believed that MCU-based solutions are not the best way to perform HBLED constant current control, especially when the system uses a switched-mode power supply with discrete components to become unstable and impossible to pass EMC certification. Freescale Semiconductor has created a design example for dual string HBLED lighting control. The HBLED is based on the S08MP16 eight-bit microcontroller. The MCU is responsible for measuring the current feed from the LED string and processing it using the PID control algorithm to control the operation of the independent buck-boost switching power supply, ensuring the best through the HBLED string. Current flow.
The microcontroller is also responsible for monitoring user input, battery voltage and temperature sensors, diagnosing real-time LED power supply status, and special communication functions such as LIN functions can be implemented in the same microcontroller.
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