Why is the Zener Diode Often Used as a Reference Diode: A Closer Look

Zener diodes are a popular choice for use as reference diodes in electronic circuits due to their unique properties and advantages. These semiconductor devices exhibit a stable reverse breakdown voltage, making them ideal for providing a fixed voltage reference in a wide range of applications. In this comprehensive guide, we will delve deeper into the reasons why Zener diodes are often the go-to solution for reference diode applications.

Understanding Zener Diode Characteristics

Zener diodes are a type of semiconductor diode that are designed to operate in the reverse breakdown region. Unlike regular diodes, which are typically used in the forward-biased region, Zener diodes are specifically engineered to have a well-defined reverse breakdown voltage, known as the Zener voltage (VZ).

The Zener voltage is the voltage at which the diode starts to conduct current in the reverse direction. This voltage remains relatively constant over a wide range of current levels, making Zener diodes excellent candidates for voltage reference applications.

The two primary mechanisms that contribute to the Zener breakdown in these diodes are:

1. Zener Breakdown: This occurs in heavily doped p-n junctions, where the electric field across the junction is high enough to cause electrons to tunnel through the potential barrier, resulting in a breakdown.

2. Avalanche Breakdown: This occurs in lightly doped p-n junctions, where the high electric field accelerates charge carriers to the point where they can create additional electron-hole pairs through impact ionization, leading to a breakdown.

The choice between Zener breakdown and avalanche breakdown depends on the doping concentration and the reverse voltage applied to the diode. Zener diodes can be designed to exhibit either of these breakdown mechanisms, with the Zener breakdown typically occurring at lower voltages (below 5V) and the avalanche breakdown occurring at higher voltages (above 5V).

Advantages of Zener Diodes as Reference Diodes

1. Stable Voltage Reference: The primary advantage of using Zener diodes as reference diodes is their ability to provide a stable and well-defined voltage reference. The Zener voltage remains relatively constant over a wide range of current levels, making it an ideal choice for applications that require a fixed voltage reference.

2. Wide Voltage Range: Zener diodes are available in a wide range of voltage ratings, typically from around 2.4V to 200V, allowing designers to select the appropriate Zener voltage for their specific circuit requirements.

3. Power Handling Capability: Zener diodes come in various power ratings, ranging from milliwatts to several watts, enabling their use in a variety of applications with different power requirements.

4. Temperature Stability: Zener diodes exhibit a relatively low temperature coefficient, meaning their Zener voltage remains stable over a wide range of operating temperatures. This makes them suitable for use in temperature-sensitive circuits.

5. Cost-Effectiveness: Zener diodes are generally inexpensive and readily available, making them a cost-effective solution for voltage reference applications.

6. Simplicity of Use: Zener diodes are easy to implement in electronic circuits, requiring only a current-limiting resistor to operate as a voltage reference.

Zener Diode Voltage Series and Selection

To cater to the diverse needs of electronic circuit designers, Zener diodes are available in a standardized voltage series known as the E24 series. This series provides a range of Zener voltage values, with each value differing from the previous one by a factor of √10 (approximately 1.22).

The E24 Zener Diode Voltage Series includes the following standard voltage values:

Voltage (V)
2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, 10, 11, 12, 13, 15, 16, 18, 20, 22

When selecting a Zener diode for a voltage reference application, it is important to consider the following factors:

1. Zener Voltage (VZ): Choose a Zener voltage that is close to the desired reference voltage, but slightly higher to account for the voltage drop across the current-limiting resistor.

2. Zener Current (IZ): Determine the appropriate Zener current based on the circuit’s requirements and the Zener diode’s power rating. Typically, Zener diodes are operated at a current between 5% to 20% of their maximum power rating.

3. Power Rating: Select a Zener diode with a power rating that can handle the maximum power dissipation in the circuit. The power dissipation can be calculated as P = VZ × IZ.

4. Temperature Coefficient: Consider the temperature coefficient of the Zener diode, which can affect the stability of the reference voltage over the operating temperature range.

By carefully selecting the appropriate Zener diode based on these factors, designers can ensure a stable and reliable voltage reference in their electronic circuits.

Zener Diode Voltage Reference Circuit Design

To use a Zener diode as a voltage reference, a simple circuit configuration is typically employed. The basic Zener diode voltage reference circuit consists of a Zener diode and a current-limiting resistor connected in series between the power supply and ground.

The current-limiting resistor is used to set the Zener diode’s operating current (IZ) within the recommended range, ensuring stable voltage regulation. The value of the current-limiting resistor can be calculated using Ohm’s law:

R = (Vcc – VZ) / IZ

Where:
– R is the current-limiting resistor value
– Vcc is the supply voltage
– VZ is the Zener voltage
– IZ is the Zener diode’s operating current

For example, if the supply voltage (Vcc) is 12V, the desired Zener voltage (VZ) is 5.1V, and the Zener diode’s operating current (IZ) is 10mA, the current-limiting resistor value can be calculated as:

R = (12V – 5.1V) / 0.01A = 690 Ω

The nearest standard resistor value (E12 series) would be 680 Ω.

It’s important to note that the Zener diode should be operated within its specified power rating to ensure reliable and stable voltage regulation. Exceeding the power rating can lead to overheating and potential damage to the Zener diode.

Advanced Zener Diode Voltage Reference Circuits

While the basic Zener diode voltage reference circuit is widely used, there are more advanced circuit configurations that can further improve the stability and performance of the voltage reference.

1. Temperature-Compensated Zener Diode Reference: To compensate for the temperature coefficient of the Zener diode, a temperature-compensated Zener diode reference circuit can be used. This typically involves connecting a second Zener diode with a complementary temperature coefficient in series with the main Zener diode, effectively canceling out the temperature-induced voltage variations.

2. Buffered Zener Diode Reference: In this configuration, a buffer amplifier, such as an operational amplifier (op-amp), is used to isolate the Zener diode from the load, ensuring that the reference voltage remains stable even with varying load conditions.

3. Shunt Regulator Zener Diode Reference: The shunt regulator configuration uses a Zener diode in parallel with the load, providing a constant voltage reference while also regulating the output voltage. This approach is particularly useful when the load current varies significantly.

4. Series Regulator Zener Diode Reference: In this configuration, the Zener diode is connected in series with the load, acting as a series voltage regulator. This circuit is suitable for applications where the load current is relatively constant, and the input voltage may fluctuate.

These advanced Zener diode voltage reference circuits offer enhanced stability, improved temperature compensation, and better load regulation, making them suitable for more demanding applications that require a highly stable and precise voltage reference.

Conclusion

Zener diodes are widely used as reference diodes due to their unique characteristics, including a stable reverse breakdown voltage, wide voltage and power ratings, and ease of use. Their ability to provide a fixed voltage reference, even in the presence of varying supply voltages or load conditions, makes them an indispensable component in a wide range of electronic circuits.

By understanding the principles of Zener diode operation, the standardized voltage series, and the various circuit configurations, designers can effectively leverage the advantages of Zener diodes to create reliable and stable voltage references in their electronic designs.

References:

1. Zener Diode Characteristics and Applications
2. Zener Diode Voltage Regulator Circuits
3. Temperature Compensation of Zener Diode Voltage References
4. Zener Diode Voltage Reference Design