"Smart home", "smart metering" or "smart grid" are the actual keywords for a new type of intelligent technology. From supply of energy, water and heat to intelligent automation and lighting solutions, an efficient, environmentally friendly but also more comfortable operation can be achieved in many areas with the help of new concepts and advanced components. Especially the lighting sector benefits from these new technologies in several ways.
Thanks to their energy efficiency, compactness, high reliability and durability, LEDs have been established in almost all market areas which require lighting, as they deliver intense brightness and high color brilliance while consuming less power. However, they must feature high reliability, low interference, excellent heat management and integrated safety mechanisms. All this is highly demanding to the power supply, the LED driver.
LED drivers play an important role in the overall design of the lighting, by regulating the "power output" and relieve the microcontroller from a huge workload with integrated functionality such as GPO, RAM and amplifier circuits. Built-in PWM control results in improved dimming performance and reduced need for external components. In order to reduce the power consumption, Rohm has implemented advanced low-power amplifiers and a proprietary oscillator technology in its LED drivers. The following article describes an intelligent lighting system featuring a TRIAC dimming-based example.
LEDs replacing traditional incandescent lamps are usually arranged as an array, which together form an evenly illuminated surface. The brightness of the LEDs driven in series depends on the operating current. The total voltage drop is between 10 and 30 V. The LED chain should be powered by a precisely controlled constant current source, and the current must be regulated accurately so that neighboring lamps have the same brightness. Therefore, the main task of an LED driver is a constant light output, means a steady power supply to the LEDs, despite possible power variations. This constant current is held in a corridor of 2.5 percent over a wide voltage range (Figure 1
Figure 1: Circuit scheme of LED voltage
Other requirements to quality lighting concepts are high precision, constant output voltage and various dimming functions. A variety of dimmers including transistor or triac-based dimmers can be found in the market, all working with a phase angle detection of the alternating current. This article describes a LED power supply with a triac-based dimmer solution which is able to realize LED light control without great effort and avoid undesirable variations caused by the electrical instability of the dimmer. Depending on the application, other solutions such as PWM or linear dimming can be supported with the respective external circuitry because the internal blocks of the LED driver are designed in a way to ensure a high degree of flexibility.
A challenge of dimming LED lamps is the fact that a normal driver cannot identify the phase cut angle of the alternating current. In order to enable an efficient LED dimming solution, the LED driver has to determine the power supply to the LEDs by analyzing the phase angle of the dimmer (Figure 2
). Wrong, premature off or on switching of the TRIAC dimmer caused by a power drop has to be avoided. Since LEDs are designed to be highly efficient even with reduced operating current, this is not easy to achieve. As important is a certain profile of the current modulation in order to avoid issues such as flickering or flashing. The phase cut angle to the LED current transfer curve, should be adjusted to the sensitivity curve of the human eye through an internal logarithmic equation. This way, a uniform and natural dimming behavior is generated which is similar to the one of a conventional bulb. Ideally, the emitted brightness should approximately correspond to the dimming of a 60W bulb.
Figure 2: Dimming characteristics