Analyze LED characteristics with PSpice
Recent advances in LED technology have lead to LEDs' widespread use in outdoor-signal applications, such as in traffic and railroad signals. A typical LED signal consists of an LED array and a power supply. When a low-voltage power supply is either desirable or mandatory, series/parallel combinations of LEDs become inevitable. However, analyzing and optimizing series/parallel combinations of LEDs with varying forward characteristics can be complicated. Using the parametric and Monte Carlo capabilities of PSpice greatly simplifies this task.
To model an LED in PSpice, use the diode model. You can set the IFK and ISR parameters in the diode model to zero; Figure 1 shows the resultant PSpice diode forward-current model and corresponding equations. As the equations in the figure show, you can express the forward voltage across the diode model, or VFWD, as the sum of the voltage across the series resistance and the voltage across the intrinsic diode.
The dominant term in the VFWD equation of Figure 1, assuming RS is less than 10?, is the logarithmic term. Therefore, if you vary the model parameter N in Monte Carlo or parametric analyses, then the VFWD varies accordingly. A helpful hint: When creating an LED model using programs such as Parts (www.microsim.com), use curve-tracer plots or an enlarged photocopy of the VI curve from data books to extrapolate data points along the VI curve.
Figure 2 shows an example for which N varies linearly between 2.07 and 2.53, or 2.3 ± 10%. The forward voltage at 20 mA varies from 1.59 to 1.94V, or 1.765 ± 9.9%. By editing the "N=2.3299" statement in the LED model to "N=2.3299 DEV 10%" assigns a 10% device tolerance to the LED model. Therefore, when you execute a Monte Carlo analysis, the forward characteristics of each LED in the circuit vary randomly. Figure 3's example performs 20 Monte Carlo sweeps at 1V/sec, with N set for a 10% tolerance.
The final example is the analysis of a simple circuit (Figure 4a). The input consisted of a 60-mA pulse, and the simulations determine the peak current through D1 for 0, 10, and 100?. The model statement assigned a 10% tolerance to N, and the example executes 50 Monte Carlo runs. The results for R=0 reveal a large standard deviation of 10 mA. The results for R=10 reveals a smaller standard deviation of about 5 mA (Figure 4b). The results for R=100 reveals a small standard deviation of only 1 mA.