Highly efficient LED drivers provide intelligent lighting solutions
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% over a wide voltage range (Fig. 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 (Fig. 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
To avoid flickering during operation, the built-in dimmer capacitor has to be completely discharged when the triac is non-conductive. To avoid a premature shutdown of the TRIAC, a constant low current flow has to be maintained when the triac is conductive. The load of the LED lamp mimics the resistance of the load of a light bulb, which in contrary to an LED, can easily maintain a holding current of 8 to 40 mA. In order to efficiently support the TRIACs ROHM has included a dynamic load controller into its LED driver solutions for interior lighting which allows a dimming accuracy up to 5%. In addition there is a high LED performance and voltage tolerance in order to achieve optimum performance and high reliability.
Figure 3 shows a summary of all these functions in an application diagram
Figure 3 BD555BKFV application diagram. For full resolution click here.
The BD555-family of LED current regulator ICs in a compact SOP14 packaging can be deployed for LED solutions up to 25W. The high efficiency is provided by an auxiliary winding with high voltage tolerance on the basis of a coupling coil, which supplies the IC with an additional voltage of up to 30 volts. Configured as a network-powered flyback converter with a selectable frequency of 50 to 400 kHz it generates high efficiency in a compact SOP 14 package with low weight, while stabilizing network and supply voltage fluctuations and providing higher reliability. Combined with the scanning of the actual LED current, the precise LED current detection delivers the necessary accuracy for a multitude of applications.
These measures guarantee smooth operation and prevent brightness fluctuations. Together with the external switching transistor, the new BD555 drivers can encompass a wide range of LED performance. The patented weak and strong bleeder circuit (see Figure 4 - Area A) reduces power dissipation and also exposes a high quality, flicker-free dimming light output when used with conventional dimmer switches on a transistor or triac basis. The precise dimming features based on the mentioned proprietary solution can be achieved with a real phase angle detection of 35-140 degrees.
Figure 4 BD555BKFV Block Diagram. For full resolution click here.
The curve after which the conversion of in the phase angles in LED currents takes place, is adjusted by an internal logarithmic adaption to the sensitivity curve of the human eye, thus delivering a pleasantly smooth and natural dimming performance. A pulse width modulation controller or a linear DC signal may alternatively replace the phase angle. An intelligent, on-chip power management system with low startup current and low operating current also controls the start-up and normal operation for optimized system energy consumption. The soft start behavior can be defined with the same capacitor which is also used for dimming detection
With the new LED driver family, the user can choose between different, simple to sophisticated power supply options to achieve maximum energy efficiency. He also receives a variety of configuration options, such as adaptation to 240V or 100V power grids, HB-LED configurations and adjustable soft start or emergency light functions. In addition, a wide range of external HV semiconductors is supported.
Various protection circuits such as the bleeder control, under voltage lockout, -, and protection against over current (induction saturation), short circuit and overheating ensure reliability and fault tolerance. Customers expect a much longer lifetime from LEDs than from standard lamps which also justifies a higher purchase price. These integrated features help to achieve this goal without major development effort. A protective mechanism against the false activation of the LEDs by a leakage current is included as well. With an operating temperature of -40 to +110° C, the drivers cover a wide range of application areas.
The LED driver family BD555 is a flexible building block solution for demanding dimmable LED lamps, and not only creates the conditions for the design of highly efficient systems with low power consumption, but also gives the designers the chance to abandon expensive and complex multi-component solutions.
About the author: Raimund Wagner is Product Manager LED drivers at ROHM Semiconductor GmbH
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