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 percent. 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 on image.
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 on image.
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
Courtesy of EETimes Europe
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