Zener Diode Characteristics

When a high reverse-biased voltage applied to a diode may create a high reverse current, which can generate excessive heat and cause a diode to break down. The applied reverse voltage at which the breakdown occurs is called the breakdown voltage, or peak reverse voltage.

A special diode called a zener diode is connected to operate in the reverse-biased mode. It is designed to operate at those voltages that exceed the breakdown voltage. This breakdown region is called the zener region.

Zener Diode Characteristics

When a reverse-biased voltage is applied that is high enough to cause breakdown in a zener diode, a high reverse current (IZ) flows. The reverse current is low until breakdown occurs.

After breakdown, the reverse current increases rapidly. This occurs because the resistance of the zener diode decreases as the reverse voltage increases. The breakdown voltage of a zener diode (VZ) is determined by the resistivity of the diode.

zener diode characteristics
Figure 1. Zener Diode Characteristics

This, in turn, is controlled by the doping technique used during manufacturing. The rated breakdown voltage represents the reverse voltage at the zener test current (IZT). The zener test current is somewhat less than the maximum reverse current the diode can handle.

The breakdown voltage is typically rated with 1 to 20% tolerance. The ability of a zener diode to dissipate power decreases as the temperature increases. Therefore power dissipation ratings are given for specific temperatures. Power ratings are also based on lead lengths: shorter leads dissipate more power.

A derating factor is given by the diode manufacturer to determine the power rating at different temperatures from the ones specified in their tables. For example, a derating factor of 6 mW per degree celsius means that the diode power rating decreases 6 mW for each degree of change in temperature.

Zener diodes are packaged like PN junction diodes. Low-power zener diodes are mounted in either glass or epoxy. High-power zener diodes are stud mounted with a metal case.

Zener Diode Ratings

The maximum zener current (IZM) is the maximum reverse current that can flow in a zener diode without exceeding the power dissipation rating specified by the manufacturer.

The reverse current (IR) represents the leakage current before breakdown and is specified at a certain reverse voltage (VR). The reverse voltage is approximately 80% of the zener voltage (VZ).

Zener diodes that have a breakdown voltage of 5 V or more have a positive zener voltagetemperature coefficient, which means that the breakdown voltage increases as the temperature increases.

Zener diodes that have a breakdown voltage of less than 4 V have a negative zener voltage-temperature coefficient, which means that the breakdown voltage decreases with an increase in temperature.

Zener diodes with a breakdown voltage between 4 and 5 volts may have a positive or negative voltage-temperature coefficient.

A temperature-compensated zener diode is formed by connecting a zener diode in series with a PN junction diode, with the PN junction diode forward biased and the zener diode reverse biased. By careful selection of the diodes, temperature coefficients can be selected that are equal and opposite. More than one PN junction diode may be needed for proper compensation.

Zener Diode as Voltage Regulator

A zener diode can be used to stabilize or regulate voltage. For example, it can be used to compensate for power-line voltage changes or load-resistance changes while maintaining a constant DC output.

Zener diode as voltage regulator
Figure 2

Figure 2 shows a typical zener diode regulator circuit. The zener diode is connected in series with resistor RS. The resistor allows enough current to flow for the zener diode to operate in the zener breakdown region.

The DC input voltage must be higher than the zener diode breakdown voltage. The voltage drop across the zener diode is equal to the zener diode’s voltage rating.

Zener diodes are manufactured to have a specific breakdown voltage rating that is often referred to as the diode’s zener voltage rating (Vz). The voltage drop across the resistor is equal to the difference between the zener (breakdown) voltage and the input voltage.

The input voltage may increase or decrease. This causes the current through the zener diode to increase or decrease accordingly. When the zener diode is operating in the zener voltage, or breakdown region, a large current will flow through the zener with an increase in input voltage. However, the zener voltage remains the same.

The zener diode opposes an increase in input voltage, because when the current increases the resistance drops. This allows the zener diode’s output voltage to remain constant as the input voltage changes.

The change in the input voltage appears across the series resistor. The resistor is in series with the zener diode, and the sum of the voltage drop must equal the input voltage. The output voltage is taken across the zener diode.

The output voltage can be increased or decreased by changing the zener diode and the series resistor. The circuit just described supplies a constant voltage. When a circuit is designed, the current in the circuit must be considered as well as the voltage.

Figure 3

The external load requires a specific load current (IL) determined by the load resistance and output voltage (Figure 3). The load current and the zener current flow through the series resistor.

The series resistor must be chosen so that the zener current is adequate to keep the zener diode in the breakdown zone and allow the current to flow.

When the load resistance increases, the load current decreases, which should increase the voltage across the load resistance. But the zener diode opposes any change by conducting more current. The sum of the zener current and the load current through the series resistor remains constant. This action maintains the same voltage across the series resistor.

Similarly, when the load current increases, the zener current decreases, maintaining a constant voltage. This action allows the circuit to regulate for change in output current as well as input voltage.

Testing Zener Diodes

Zener diodes can be quickly tested for opens, shorts, or leakage with an ohmmeter. The ohmmeter is connected in forward and reverse bias in the same manner as with PN junction diodes. However, these tests do not provide information on whether the zener diode is regulating at the rated value.

For that information, a regulation test must be performed with a metered power supply that can indicate both voltage and current. Figure 4 shows the proper setup for a zener diode regulation test.

Figure 4

The output of the power supply is connected with a limiting resistor in series with a zener diode to be tested. A voltmeter is connected across the zener diode under test to monitor the zener voltage.

The output voltage is slowly increased until the specified current is flowing through the zener diode. The current is then varied on either side of the specified zener current (IZ). If the voltage remains constant, the zener diode is operating properly.

Summary

  • Zener diodes are designed to operate at voltages greater than the breakdown voltage (peak reverse voltage).
  • The breakdown voltage of a zener diode is determined by the resistivity of the diode.
  • Zener diodes are manufactured with a specific breakdown (zener) voltage.
  • Power dissipation of a zener diode is based on temperature and lead lengths.
  • Zener diodes with a breakdown voltage greater than 5 V have a positive zener voltage-temperature coefficient.
  • Zener diodes with a breakdown voltage less than 4 V have a negative zener voltage-temperature coefficient.
  • Zener diodes are used to stabilize or regulate voltage.
  • Zener diode regulators provide a constant output voltage despite changes in the input voltage or output current.
  • Zener diodes can be tested for opens, shorts, or leakage with an ohmmeter.
  • To determine whether a zener diode is regulating at the proper voltage, a regulation test must be performed.

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