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

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

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

Given a constant "Tj," the illuminance of HP LED varies linearly with its "If" at small values and tends to saturate at higher values. Otherwise, if a constant current (CC) drive is forced, the light emitted moves down as "Tj" increases. This is caused by the variation relationship between "Vf" and "Tj," which is approximately proportional [8]. For the XT-E LED by CREE, the typical relationship is −2.5 mV/°C. This effect makes itself visible in the displacement of its working curves. These "Vf"-"If"-"Tj"-"Luminous flux" relationship curves of the mentioned LED are shown in **Figure 2**.

Consequently, if a CC is applied on the LED, the increase in the junction temperature causes only a slight decrease in its "Vf" drop and, at the same time, in the illumination obtained, creating a natural mechanism of negative feedback that maintains the stability of the system.

However, if we use a constant "Vf" with LEDs, its power consumption increases along with its "Tj," produced by a significant increment in the forward current, creating a positive feedback that will finally conduct to a catastrophic burn failure. This phenomenon is generally known as thermal runaway and is the reason why the standard method of driving LEDs is the use for DC current sources.
