Reader's Question About "Short-Pulse Testing"
In reference to the article "Short-Pulse Testing Eliminates Self-Heating Errors to Produce True L-I Graphs" in issue LpR#83, a reader has asked why one measures with these short pulses, since this differs very much to the conditions in real application. Read the author's answer below.
In General: A basic problem in characterizing LEDs is an exact knowledge of the junction temperature during a measurement due to the measurement induced self-heating effect. The shorter the pulses, the better one knows the junction temperature during the measurement: the shorter the pulse is, the less is the self-heating effect and the junction temperature is very close to the temperature set prior to the measurement/start of current supply.
Case a) Pure colour LEDs w/o any conversion element: If the junction temperature of the same device measured at the same current under short pulse condition and DC condition, respectively, is the same, then the short pulsed measurements gives the same result as the DC-measurements.
In practice, deviations typically origin from not exactly equal junction temperature.
Requirement: the measurements are performed using the same measurement system, otherwise deviations in absolute values must be expected due to measurement uncertainties.
Case b) LEDs with phosphor conversion
In addition to the dependency of the efficiency of the pure LED chip on temperarure also the phosphors usually show a decrease in conversion efficiency due to increased phosphor temperature. The increased phosphor temperature is caused by heating the phosphor due to the heat transfer of the chip to the phosphor.
Additionally, the phosphor is heated by the conversion losses: the conversion efficiency is not 100% and – more important – photons emitted by the chip are converted to photons of lower energy; the energy difference is compensated by heat energy.
The self-heating effect of the phosphor is also dependent on the amount of photons (or radiant flux), which means a brighter chip causes more heating of the phosphor due to the conversion losses. Consequently, the phosphor converted LED packages performance depends also on the supplied electrical current.
The consequence: there are so many different conditions used in real applications that it is simply not possible to provide data for each specific application case. The so-called “special conditions” (short-pulsed measurements) have been widely established in the market: they allow at least a fair comparison of different LEDs’ performance under quasi-standardized conditions, which are used by LED package manufacturers.
As long as there is no standardized application relevant procedure, which defines well the operating conditions that shall be used for performing the measurements, the current status is the most reasonable, despite some weaknesses.
I fully understand the problem at application level, but there is currently no generally accepted solution which enables LED manufacturers to perform the corresponding measurements. And even if there was a standardized procedure, it would be only a compromise.
How a DC measurement should be performed with high precision of the output (optical properties at a set junction temperature and electrical current under DC operation) is described in CIE TR 225, but the standardized operating conditions (the input parameters) are not available.
Dr. Markus Schneider
Verification & Characterization
OSRAM Opto Semiconductor
