This example shows a model of the zener diode used in a voltage regulator.
The Zener diode block modeled in this example presents a practical implementation that uses parameters commonly provided in datasheets. These parameters are
(1) Zener Voltage Vz
(2) Dynamic Impedance Zzt
(3) Knee Impedance Zzk
(4) Max Continuous Current Izm
(5) Forward Voltage Drop Vf
(6) On Resistance Ron
This block can effectively model three regions of operation of the zener diode I-V characteristics - forward-biased, reverse-biased before breakdown and reverse-biased after breakdown. Beyond the maximum reverse continuous current Izm, the zener is assumed to burn up and is treated as an open circuit.
The implementation of the zener diode can be seen by looking under the mask of the block, and is based on .
Zener diodes are commonly employed in applications as voltage regulators. The circuit shows an AC source fed to a step-down transformer. The output of the transformer is then rectified using a diode bridge and smoothened using a capacitive filter. The zener diode then acts to regulate the output voltage to the zener voltage 10V. The input current into the zener is limited by the resistor Rlimit to permissible values.
The programmable voltage source is setup to increase its output voltage at 0.1s. As the source output increases, so does the voltage applied at the input of the zener. However, the zener can regulate the output only as long as its input current is below the maximum specified value. This current increases as we increase the source voltage and the zener ultimately fails at about 0.112s. With the zener acting as an open-circuit at fault, voltage regulation is lost and the output of the capacitive filter gets applied to the load.
1. Wong. S, Hu. C-M, "SPICE macro model for the simulation of zener diode I-V characteristics", Circuits and Devices Magazine, IEEE® Volume 7, Issue 4, Jul 1991 Page(s): 9 - 12, 52