**3. DC LED drivers**

**2. Working behavior of a high-power LED**

**Figure 2.** Working parameters of a high-power XT-E white LED by CREE.

The light output of a high-power (HP) LED depends basically on two factors: the electric inputs—forward voltage ("Vf") and current ("If") values—and its crystal junction tempera-

ture ("Tj"). This last value inevitably increases as electrons and holes combine.

**Figure 1.** High power 3000 K white XTE LED by CREE. Blue emitter with phosphorus cover.

18 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

Because of this electrical behavior of HP LEDs, in case of DC supply, we need to have a current control device in series with the LEDs to guarantee stable working conditions. Creating a regulated current source can go from a very simple concept, using a passive polarization resistor, to a more complex solution using active current regulator circuits. Here, we summarize the general trends found in the study and the market to achieve this objective, accounting nonsteady (batteries) and steady (switching converter outputs) DC sources.

### **3.1. Passive control methods: polarization resistors**

The simplest driver is a current-limiting resistor placed along with the powered LEDs. The reduction in the polarization voltage of LEDs due to temperature increment is compensated with the larger drop in the resistor (negative feedback effect). If the supply voltage is very well defined and stable, close to "Vf," this is the simplest, most reliable, and long-lasting LED driver. However, it is not significantly efficient as all the required current is driven through these impedances generating a significant amount of heat, and a substantial resistance value is required to keep current within an adequate range. If the voltage source is not stable, the brightness of the LED would vary remarkably because a small voltage variation would already lead to large changes of the output power. This system is only able to correct the variations of the behavior of the LED due to the changes of its "Tj."
