High-brightness light emitting diodes (HBLEDs) are rapidly gaining popularity in the automotive, consumer, and industrial markets. Brilliant colors, long life, and energy efficiency are some of the reasons why high-brightness LEDs are becoming the future of lighting applications.
In the automotive industry, the HBLED technology differentiates vehicles in terms of styling, safety, and fuel economy, going from simple switch illumination and LCD backlighting through very high-brightness headlamp applications.
Controlling luminosity of HBLEDs efficiently and reliably is not an easy task—power stage efficiency, thermal design, and electromagnetic compatibility (EMC) are some of the most critical design challenges. Typically, dedicated constant current drivers (CCD) are used for driving HBLED strings, overcoming most design issues and simplifying the design. However, CCDs are usually more expensive than a solution based on a microcontroller.
This feature describes the implementation of a smart HBLED lighting control using an 8-bit microcontroller and a low-cost discrete solution, without the need of expensive analog drivers or CCDs; the embedded closed-loop control algorithm that is implemented in the microcontroller ensures the optimal flow of current through a variable number of high-brightness LEDs, maximizing their life and avoiding undesirable visual blemishes.
The microcontroller measures the HBLED’s current and controls the discrete switched-mode power supply (SMPS) through a closed-loop PID control, while simultaneously implementing other features such as dimming, protection, and diagnostics
Important characteristics of high-brightness LEDs
As occurs in a low-intensity LED, the luminosity of a high-brightness LED is proportional to the current flowing through it. This current, typically called forward current (IF), ranges from 100 mA up to 1,000 mA for HBLEDs. At the same time a voltage drop, called Forward Voltage (VF), occurs whenever the HBLED is polarized. In HBLEDs, the luminosity and chromaticity are directly proportional to IF, therefore it is critical to have precise control of the current flowing through the HBLEDs.
Physically, HBLEDs with the same part number and same specification won’t have the exact same VF. When the current IF flowing through two HBLEDs is the same, their forward voltages, VF, might be different. Hence, controlling the LEDs intensity by means of a constant voltage might result in variations in intensity from device to device; a current control is required in order to ensure the same luminosity for all HBLEDs.
Not only does luminous intensity depend on the current flowing through the HBLED, but chromaticity as well. In order to maintain color, the HBLED must be driven with constant current. Therefore, the solution is to use a PWM (pulse width modulation), thus providing a lower average current in the HBLED (light intensity) while maintaining the same instantaneous current (LED color).
Power dissipation for applications involving HBLED is also critical. As the HBLED current increases, the power dissipation will also increase. A HBLED at 350 mA with a voltage drop of 3V will dissipate approximately one watt, without the proper thermal management, this dissipation might result in overheating of the HBLED and its long term degradation. Another important aspect of thermal design is that HBLED luminous intensity is inversely proportional to LED junction temperature, emitter colors can go to higher wavelengths as temperature increases.
Challenges when driving high-brightness LEDs
Using resistors to limit the IF current is very common for low intensity LEDs. In the case of HBLEDs the resistors must be rated for higher power, which would result in system inefficiency. Consequently, switched-mode power supplies (SMPS) are used to improve efficiency and reduce power dissipation in HBLED systems. SMPS are usually more expensive because of the need for energy storage components (inductors and capacitors); also, SMPS might create noise or EMI problems.
A group of HBLEDs can be driven together either in parallel or forming a series string; parallel driving gives the possibility of having different light intensities for each HBLED—but if a control loop is desired, each HBLED would require dedicated control, making it expensive for a large number of HBLEDs.
When connecting HBLEDs in series, forming strings, only one driver and control loop is needed per string; all of the HBLEDs in series will have the same current flowing through them giving a relatively constant brightness.
Depending on the amount of LEDs in series the strings might require voltages lower or higher than in the input voltage.
Microcontroller-based solution for high-brightness LED strings
There is a wide range of solutions in the market for driving constant current to HBLEDs; some of them are based on dedicated intelligent analog drivers while others use digital signal processors (DSPs) or microcontrollers with independent analog drivers.
There is a belief that microcontroller (MCU) based solutions are not good enough for performing the HBLED constant current control—especially because system might become unstable with a switched-mode power supply built out of discrete components, and with EMC certification as an impossible task. Freescale Semiconductor has created a design example for a dual string HBLED cighting Control based on S08MP16 8-bit microcontroller; the microcontroller is in charge of measuring the current feedback coming from the LED strings, processing it with a PID control algorithm and, as a result, controlling the operation of a discrete buck-boost switched-mode power supply, ensuring the optimal flow of current through the HBLED strings.
The microcontroller is also responsible for monitoring user inputs, battery voltage, and temperature sensors and diagnose the status of the LEDs power supply in real time; extra communication features, like LIN bus connectivity, could be also implemented in the same microcontroller.
The switched-mode power supply used to provide the power to the HBLEDs is a discrete buck-boost topology, designed to work with a variable amount of LEDs ranging from 1 to 18 LED strings (from 0 to ~65V, continuous) and up to 500 mA of output current running at a frequency of 350 kHz. The application Block Diagram is found below.
This article mentions using PWM for dimming of LEDs and some readers may think that they could use the same technique to do colour control. But there has been a long standing issue with Colour Kinetics (now part of Philips) having patents on such applications, despite PWM being used for years prior to the patent.
Those interested can find a summary at http://www.ledsmagazine.com/news/5/3/7 and http://forums.auschristmaslighting.com/index.php?topic=894.0
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