Your Electronics Open Source

by Mauro Ceresa, [Assipe] , Future Electronics
Tools and information to simplify designing products based on power LEDs.

In the myriad of parameters and curves that characterise the performance of a LED, it is necessary to define the set points from which the other major features can be gleaned, to obtain the final result: the real efficiency and reliability of our application.

By Editorial Staff [DESIGN IN]

The LED using, in lighting applications, is becoming more and more attractive and the possibility of using the powerline directly, as a source of power supply, is becoming more and more interesting. Moreover, if the circuital solution allows a compact implementation that can be placed in the same volume of a standard lamp screw, it becomes more interesting.

by Franco Musiari [ Assipe ]
Efficacy, power and costs of high brightness LEDs have got levels that make them an attractive solution in many lighting system applications.

The luminous efficacy of a light source is the ratio between the emitted luminous flux and the amount of the absorbed energy to transmit it. It is expressed in lumen/watt (lm/W). The lumen, the measure unit of the luminous flux, is equal to the luminous flux detected in a solid angle of one steradian and emitted towards all directions from a source with a luminous intensity of 1 candle. (the steradian –sr symbol- is the SI unit of solid angle and it is defined as " the solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere having an area r ²". Since the area of the sphere is 4πr², as a consequence the solid angle subtended by the whole sphere is equal to 4π sr).

by Franco Musiari [ DESIGN IN ]
Buck Regulators
This is the simplest topology - see Fig. 5 – and can directly replace the linear regulators previously described. The control circuit drives a transistor alternately in conduction status and interdiction status. During the conduction status the current flows from VIN to the diode (green dotted line on Fig. 5) and the energy initially associated to delta between VIN and the voltage fall on the LED, on the transistor and on RS is stored in the inductance. At the increasing of the stored energy in the magnetic circuit, the resistance decreases to the passage of the current, current that grows until the controller, that measures it on RS, does not drive the transistor to the interdiction status.

by Franco Musiari [ DESIGN IN ]
Driving the power LED depends on the application setting and it is not so trivial as it might appear.

The LEDs (Light Emitting Diode) are diodes whose basic characteristic is the ability to emit light when they are passed through a current that flows from P to N region. At each recombination between the charge carriers (electrons and holes), on PN junction region, a photo emission is generated, and the total quantity of emitted photons, and therefore the light intensity, is proportional to the current intensity that passes through them. The emitted light has a spectrum – wavelengths distribution – that is defined according to the materials used in the realization of the diode PN junction, although it partially depends on the current intensity and on the junction temperature. The most common materials, used in LEDs production, are those belonging to the III° and V° group of the elements periodic table: