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How to Model an LED
Modeling LEDs is generally a simple proposition. Modeling is done with the diode primitive and the primary objective is to match the LED forward characteristic. The normal forward drop of the diode with typical parameters is about 0.7 volts for currents in the 10m to 100ma range. In an LED the drop will be much larger. Typically the forward drop is 2.5 to 4 volts To achieve the larger drop you must decrease the diode IS model parameter value from its typical value of 1E-15 to as low as 1E-55. The smaller the IS the larger the forward drop.
To simplify the process, consider the following test circuit.
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This circuit, when run in DC analysis with the analysis limits of Fig 2, plots the diode's current vs. its voltage, stepping the value of IS from 1E-20 to 1E-52 to cover a broad range of possible LED forward characteristics.
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Fig. 3 shows what the DC analysis looks like. All you have to do is find the closest fit to the characteristics of the LED you are modeling and select that value of the IS parameter in your diode model for the LED.
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This plot shows the Id vs Vd curve for 33 values of IS from 1E-20 to 1E-52. The forward resistance is that of the intrinsic diode alone which, for these currents, is usually below 10 ohms. If you need a higher incremental resistance you should make RS, the series diode resistance, larger. Of course, doing so will alter the above curves so some adjustment in IS will probably be needed.
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