Fast Characterization of Power LEDs: Circuit Design and Experimental Results

A precise measurement of optical power, forward voltage, and junction temperature of light-emitting diodes (LEDs) is the key for characterization and health monitoring of these devices. In many cases, LED characterization is carried out with relatively long (10 ms and longer) pulses, that is, in conditions in which self-heating can significantly impact measurement results. To overcome this limitation, this article proposes a fast and versatile measurement approach based on a specifically designed current source, with a maximum current of about 1 A, high stability (variations under 0.1%) and settling time <20 mu s, and demonstrates its applicability to pulsed and transient characterization of power LEDs. The proposed system has the inherent advantages of 1) permitting a fast pulsed characterization of the devices, which-as we demonstrate-is much more accurate than quasipulsed or dc analysis; 2) allowing isothermal characterization of LEDs without requiring long settling times, with beneficial impact on the throughput of LED characterization; 3) allowing characterization of the voltage heating transient (during constant current operation), which is the key for junction temperature and thermal resistance extraction, as well as for the development of compact models; 4) monitoring the optical power during the self-heating transient; and 5) the spectrum of the device providing additional information, such as the peak-shift or the phosphor behavior. The efficacy of the proposed approach has been demonstrated by testing commercial LEDs: the results clearly indicate that a fast (< 20 mu s) LED characterization is necessary for a proper extraction of the main spectral parameters and of the related temperature dependence.