Workshopband als PDF - Mpc.belwue.de

MPC-WORKSHOP FEBRUAR 2013
Figure 4: IV-characteristic of green light emitting LED, measured
data (red dot) together with simulated curve (full line) after parameter
optimization and photodiode data showing the light
emission.
Figure 5: IV-characteristic of red light emitting LED, measured
data (red dot) together with simulated curve (full line) after parameter
optimization and photodiode data showing the light
emission.
The application of the general model (figure 2) on
the green light emitting diode gives the curve shown
in figure 4. Measured data can only be reproduced by
simulation based on the complete model. Due to the
fact that the emission coefficient of the parallel diode
DP2 amounts to close to N = 500, which corresponds
to a slope of the curve in this section close to zero,
diode DP2 can be replaced by a resistor RP with very
similar simulation results. This alternative model has
the advantage of less model parameters and can always
be applied when the slope of the IV characteristic
in the low voltage region is close to zero.
To model the red light emitting diode (figure 5) the
general model can be simplified as following. Only
the main diode DM together with the serial resistance
is necessary to model the active region of the LED.
The horizontal sub threshold part of the IV characteristic
can be modeled with a parallel resistance RP. An
additional serial diode DS will give only small improvement
in the reproduction of the measured data by
the model. In summary the model consists only in the
elements RS, DM and RP.
Figure 6: IV-characteristic of blue light emitting LED, measured
data (red dot) together with simulated curve (full line) after parameter
optimization and photodiode data showing the light
emission.
Figure 7: Temperature dependence of the IV-characteristic of blue
light emitting LED, measured data (dotted) together with simulated
curves (full line) at 30°C (blue), 50°C (red) and 70°C (green).
Finally, data of the blue light emitting diode are given
in figure 6. As for the green light emitting diode
two parallel electrical components in addition to the
main diode DM are necessary. The not vanishing
slope of the IV characteristic in the low-voltage region
indicates that the parallel diode DP2 cannot be replaced
here by a parallel resistor. From the sensitivity
analysis we can conclude that the serial diode DS is
not necessary to model the IV characteristic of this
LED.
B. Temperature dependence
LED’s especially used in outdoor technology will
work under very different ambient temperature conditions.
Consequently it is necessary to verify the validity
of the model for different temperatures. Main Spice
parameters describing the temperature dependence of
the saturation current IS(T) are the energy gap EG and
the saturation current temperature exponent XTI [2].
Consequently, for an accurate description of the temperature
dependence the knowledge of EG, which is
given by the material composition of the LED [1] is
necessary. This knowledge is not available for us.
However, the energy gap is related to the wavelength
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