LED lighting control options
Light-emitting diodes (LEDs) are now a leading technology for energy efficiency and compactness, making them suitable for many applications that require high intensity illumination. Whether it's the interior and exterior lights of a car, the high ceiling metal halide lamps in a factory and workshop, or the programmable luminaires in stores and homes, LEDs are a key component.
Energy efficiency is one of the clear goals when designing LED driver circuits, but control is often considered at the same time. Both energy consumption and aesthetic factors are important. Lamp manufacturers and users want to dynamically tune the light output to meet customer needs; home users need dimming, trichromatic lights and color control; many industrial and retail users want to maximize their use. Ambient light makes artificial lighting only need to generate the required light according to the needs of different time periods in the daytime and different places in the workshop, thus reducing power consumption. To this end, the ambient sensor of the luminaire will report ambient light conditions every minute so that the LED controller can adjust the output for compensation.
In addition to selecting solutions based on the functional requirements of LED luminaires, circuit topologies such as dimming, color control, long-term reliability, time to market, and cost can be selected based on application requirements. This article details several options available to design engineers.
Pulse Width Modulation (PWM) Technology vs. Constant Current Reduction (CCR) Technology
When choosing the circuit topology used to control the LED light output, choose a solution based on pulse width modulation (PWM) or current constant reduction (CCR). If the circuit keeps the voltage substantially constant and the current flowing through the LEDs is changed according to the brightness and color requirements, both technical solutions are applicable in this case. In the former case, the current circulates in a discrete charge envelope; in the latter case, it circulates as a continuous charge flow.
The advantage of the PWM method is that the brightness and color temperature need to be controlled separately. The color temperature of LEDs optimized for lighting typically varies with current. For example, a particular LED model emits light at 2700K color temperature at 300 mA and 3000K color temperature when it rises to 600 mA. For CCR structures, the brightness will be maximized at the color temperature for a given current. When the LED is turned on and off quickly, the PWM control ensures that the luminaire will illuminate at 3000K, but the illumination will vary greatly. In general, the human eye does not feel any flicker due to the extremely high PWM frequency.
With PWM control, the light output typically varies linearly with changes in pulse width duty cycle. With CCR, the light output may not change completely linearly due to changes in current. Therefore, PWM control is an ideal choice for applications that need to dim the fixture by 50% or less while maintaining the desired color output. Therefore, RGB luminaires usually require PWM control to ensure the desired color mixing effect.
For applications where light output needs to be maximized and dimming control is relatively minor, CCR has an advantage. The power supply can transfer a higher overall voltage to the CCR luminaire, even for long-running LED strings. At the same time, it also has the advantage of lower electromagnetic radiation, supporting the installation of LED lighting equipment in environments that do not allow PWM circuit interference, such as hospital operating rooms and treatment wards.
Another application for CCR is automotive lighting, such as interior and taillights. The use of a linear current source maximizes the light output of the LED under normal operating conditions, so maximum brightness can be achieved with a small number of LED components in the headlamp assembly. However, maximizing the output in this manner may be exchanged for longer LED life. The maximum current flowing through the LED is one of the main factors affecting its service life, as it will increase the internal temperature of the device. Despite the use of short pulses, PWM controllers typically drive the LEDs at higher currents. The CCR matches the peak and average currents to reduce the pressure on the LEDs and extend the life of the device.
Designers can use a variety of options to design LED modules for PWM or CCR operation, or high configurable LED modules that are available. For example, power-based discrete designs such as linear or DC/DC converters are used in conjunction with other control circuits. Other devices can also be incorporated to provide protection against situations such as open load and other diagnostics. Discrete designs provide maximum flexibility, but design cycles and test cycles are longer.
A variety of LED driver ICs in the industry have these built-in features to greatly simplify board layout and system design. For greater time-to-market and ease of integration, designers can also use off-the-shelf subsystem kits that integrate driver electronics: they simply select and add selected LEDs.