Zener Diode: Symbol, Characteristics and Applications

what is Zener Diode?

A major application for Zener diodes is as a type of voltage regulator for providing stable reference voltages for use in power supplies, voltmeters, and other instruments. In this section, you will see how the Zener maintains a nearly constant de voltage under the proper operating conditions. You will learn the conditions and illustrations for properly using the Zener diode and the factors that affect its performance.

After completing this section, you should be able to:

Characteristics of a Zener diode

• Recognize a Zener diode by its schematic symbol
• Discuss Zener breakdown
• Define avalanche breakdown
• Explain Zener breakdown characteristics
• Describe Zener regulation
• Discuss Zener equivalent circuits
• Define temperature coefficient
• Analyze Zener voltage as a function of temperature
• Discuss Zener power dissipation and derating
• Apply power derating to a Zener diode
• Interpret Zener diode datasheets

Instead of a straight line representing the cathode, the Zener diode has a bent line that reminds you of the letter Z (for Zener). A Zener diode is a silicon PN junction device that is designed for operation in the reverse breakdown region.

The breakdown region voltage of a Zener diode is set by carefully controlling the doping level during manufacture. Recall, from the discussion of the diode characteristics curve, that when a diode reaches reverse breakdown, its voltage remains almost constant even though the current changes drastically, and this is the key to Zener diode operation. This volt-ampere characteristic with the normal operating region for Zener diodes is shown as a shaded area.

Zener Breakdown:

Zener diodes are designed to operate in the reverse breakdown. Two types of reverse breakdown in a Zener diode are avalanche and Zener. The avalanche effect occurs in both rectifier and Zener diodes at a sufficiently high reverse voltage. Zener breakdown occurs in a Zener diode at low reverse voltages.  A Zener diode is heavily doped to reduce the breakdown voltage. This cause is a very thin depletion region. As a result, an intense electric flux exists within the depletion region. Near the Zener breakdown voltage (Vz), the field is intense enough to pull electrons from their valence bands and create a current.

Zener diodes with breakdown voltages of less than approximately 5 V operate predominately in avalanche breakdown. Both types, however, are called Zener diodes. Zeners are commercially available with breakdown voltages from less than 1 V to more than 250 V with specified tolerances from 1% to 20%.

Breakdown Characteristics

The reverse portion of a Zener diodes’ characteristics curve. Notice that as the reverse voltage (V_R) remains extremely small up to the “knee” of the curve. The reverse current is also called the Zener current, Iz. At this point, the breakdown effects begin; the internal Zener resistance, also called Zener impedance (Zz), begins to decrease as the reverse current increases rapidly. From the bottom of the knee, the Zener breakdown voltage (Vz) remains essentially constant although it increases slightly as the Zener current Iz, increases.

Zener Regulation:

The ability to keep the reverse voltage across its terminals essentially constant is the key feature of the Zener diode. A Zener operating in breakdown acts as a voltage regular because it maintains a nearly constant voltage across its terminals over a specified range of reverse-current values.

A minimum value of reverse current,I_zk, must be maintained in order to keep the diode in breakdown for voltage regulation. You can see on the curve that when the reverse current is reduced below the knee of the curve, the voltages decrease drastically and regulation is lost. Also, there is a maximum current, I_zm, above which the diode may be damaged due to excessive power dissipation. So, basically, the Zener diode maintains a nearly constant voltage across its terminals for values of reverse current ranging from I_zk to I_zm. A nominal Zener voltage, Vz is usually specified on a datasheet at a value of reverse current called the Zener test current.

Zener Equivalent Circuits

The ideal model (first approximation) of a Zener diode in the reverse breakdown and its ideal characteristics curve. It has a constant voltage drop equal to the nominal Zener voltage. This constant voltage drop across the Zener diode produced by reverse breakdown is represented by a de voltage symbol even though the Zener diode does not produce voltage.

Represents the practical model (second approximation) of a Zener diode, where the Zener impedance (resistance), Zz, is included. Since the actual voltage curve is not ideally vertical, a change in Zener current (ΔIz) produces a small change in Zener voltage (ΔIz), as illustrated. By Ohm’s law, the ratio of Δ Vz to Δ VIz is the impedance, as expressed in the following equation:

Normally, Zz is specified at the Zener test current. In most cases, you can assume that Zz is a small constant over the full range of Zener current values and is purely resistive. It is best to avoid operating a Zener diode near the knee of the curve because the impedance changes dramatically in that area.

                                                  

For most circuit analysis and troubleshooting work, the ideal model gives very good results and is much easier to use than more complicated models. When a Zener diode is operating normally, it will be in the reverse breakdown and you should observe the nominal breakdown voltage across it. Most schematics will indicate on the drawing that this voltage should be.

A Zener diode exhibits a certain change in Vz for a certain in Iz on the portion of the linear characteristics curve between IZM as illustrated. What is the Zener impedance?

Temperature Coefficient:

The temperature coefficient specifies the percent change in Zener voltage for each degree Celsius change in temperature. For example, a 12 V Zener diode with a positive temperature coefficient of 0.01% °C will exhibit a 1.2 mV increase in Vz when the junction temperature increases one degree Celsius. The formula for calculating the change in Zener voltage for a given junction temperature change, for a specified temperature coefficient, is

ΔVz = Vz × TC × ΔT

where Vz is the nominal Zener voltage at the reference temperature of 25 °C, TC is the temperature coefficient, and ΔT is the change in temperature from the reference temperature. A positive TC means that the Zener voltage increases with an increase, in temperature or decreases with an increase in temperature or increases with a decrease in temperature.

In some cases, the temperature coefficient is expressed in mV/°C rather than as %°C. For these cases, ΔVz is calculated as

                                 ΔVz =  TC × ΔT

Zener Power Dissipation And Derating:

Zener diodes are specified to operate at a maximum power called the maximum dc power dissipation, p_D(max) of 500 c mW, and the 1N3305A is rated at a P_D(max) of 50 W. The dc power dissipation is determined by the formula,

                                               P_D =V_ZI_Z

Power Derating:

The maximum power dissipation of a Zener diode is typically specified for temperatures at or below a certain value (50°C, for example). Above the specified temperature, the maximum power dissipation is reduced according to a derating factor. The derating factor is expressed in mW/°C. The maximum derated power can be determined with the following formula:

                                           P_D(derating) =P_D(max) – (mW/C°)ΔT

Zener Diode Datasheet Information:

The amount and type of information found on datasheets for Zener diodes (or any category of the electronic device) vary from one type of diode to the next. The datasheet for some zeners contains more information than for others. An example of the type of information you have studied that can be found on a typical datasheet. This particular information is for a Zener series, the 1N4728A-1N4764A.

Absolute Maximum Ratings:

The maximum power dissipation, P_D, is specified as 1.0 W up to 50°C. Generally, the Zener diode should be operated at least 20% below this maximum to assure reliability and longer life. The power dissipation is derated as shown on the datasheet at 6.67 mW for each degree above 50°C. For example, using the procedure illustrated, the maximum power dissipation of 60°C is.

                                   

Notice that maximum reverse current is not specified but can be determined from the maximum power dissipation for a given value of Vz. For example, at 50 °C, the maximum Zener current for a Zener voltage of 3.3 V is

                                  

The opening junction temperature, T_J , and the storage temperature, T_STG, have a range of from -65°C to 200°C.

Electrical Characteristics:

The first column in the datasheet lists the Zener type numbers,1N1728A through IN4764A.

Zener Voltage,Vz,and zener test current,Iz:

For each device type, the minimum, typical, and maximum Zener voltages are listed. Vz is measured at the specified Zener test current, Iz. For example, the Zener voltage for a 1N4728A can range from 3.315 V to 3.465 V with a typical value of 3.3 V at a test current of 76 mA.

Maximum Zener Impedance:

Zz is the maximum Zener impedance at the specified test current, Iz. For example, for a 1N4728A, Zz is 10 Ωat 76 mA. The maximum Zener impedance, Z_zk is 400 Ω at 1 mA for a 1N4728A.

Leakage Current:

Reverse leakage current is specified for a reverse voltage that is less than

the knee voltage. This means that the Zener is not in reverse breakdown for these measurements. For example, I_R is 100 μ A for a reverse voltage of 1 V in a 1N4728A.

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