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Figure 2: LED model for DC forward characteristic
modeling [4]. In the semi logarithmic plot of the IV
characteristic of the LED, each linear segment corresponds
to the exponential behavior of one diode. A
higher current at a given voltage stands for a parallel
diode while a higher voltage at a given current stands
for a serial element (diode or resistance). This results
in the electrical circuit consisting of ideal electronic
components given in figure 2. Three parallel components,
the main diode DM and two additional diodes
DP1 and DP2. The flattening of the curve for high
currents is usually described by the series resistance
RS. The adaption of the curve at the transition from
the main diode to the series resistance makes an additional
serial diode DS necessary.
The sensitivity analysis tool included in ICCAP
gives valuable hints on the overall influence of the
individual device parameters as well as on the region
of influence of the parameters on the complete characteristic
curve. In figure 3 we show the sensitivity of
the parameters together with the IV characteristic of
the LED. We conclude that the emission coefficients
N of the main diode DM and the diode DP1 have the
strongest influence in well localized regions of the IV
characteristik. All other parameters have much less
influence. IS of the serial diode DS has the weakest
influence of all parameters. Due to the fact that the
parameters have a strongly localized influence, all
elements must be considered in this example. Consequently,
initial values for the parameters can be taken
with high accuracy from the region of influence of the
component described by the parameters. This is common
to the diode parameter determination in the semi
logarithmic IV-curve by fitting a linear curve to the
data [2]. The intersection with the y-axes gives the
logarithm of IS and the slope corresponds to
m = 1/(2.3 N Vt) with the thermal voltage Vt = kT/q.
ICCAP further allows comfortable parameter tuning
by simultaneously paying attention to the absolute or
relative RMS error. This gives accurate starting values
for all parameters. This is necessary for reliable pa-
14
DC FORWARD MODEL OF HIGH-POWER LED’S
Figure 3: Sensitivity plot for the emission coefficient N of the
main diode DM and parallel diode DP1 (upper diagram) and
saturation currents IS of all diodes, emission coefficient N of DS
and DP2 and Rs (lower diagram) together with the measured IVcurve
(dotted line).
rameter optimization in such a multi parameter analysis
problem by the Levenberg-Marquardt algorithm
available under ICCAP.
A. LED characteristics
IV. RESULTS
We have investigated the DC forward characteristic
of three LED’s of the Seoul Semiconductor F50381
RGB-LED, the red light emitting (622 nm), the green
light emitting (533 nm) and the blue light emitting
(460 nm) diode. Guided by the eye and with the help
of the sensitivity analysis, it is easy to decide whether
all electrical components of the circuit in figure 2 are
necessary. We found that the serial diode DS and/or
one parallel diode DP2 can be removed from the general
model in some cases and that the parallel diode
DP2 can be sometimes replaced by a resistor RP as
already indicated in figure 1. This will be discussed in
the following.