Structure of Zener diode is same as that of a P-N junction diode with some minor modifications. In forward direction, current is proportional to the applied voltage. And in reverse direction, no current flows through the diode until applied voltage is greater than breakdown voltage. Difference between this and normal P-N junction is at breakdown region.
How Zener Diode Works?
For a normal diode, during reverse biased condition, the applied voltage is greater than the breakdown voltage which permits a very large amount of current to flow through the diode. This current is due to both, avalanche multiplication and tunneling. Avalanche breakdown is uncontrollable and the current due to this will be very high. Heat thus produced may destroy the device permanently. These diodes are used in the reverse breakdown region typically as voltage regulators, since they regulate voltage by increasing current flow.
Fabrication of the diode is tuned such that breakdown current is dominated by tunneling current. Both P and N region of a the diode is highly doped so the depletion region formed at the P-N junction will be very narrow. When reverse bias voltage is applied, the electric field developed across the P-N junction cause charge carriers to tunnel through the depletion region. For a small increase in voltage, there is a large increase in the current. No resistance is offered by the Zener diode for the increase in voltage. Therefore voltage across the diode always remains the same.
Applications of Zener Diode
Today, Zener Diodes are used in circuits of different applications. The most common application of Zener Diode is Voltage Regulation. Other than voltage regulation some other applications are also discussed below. They are:
- As Shunt Regulator
Zener diode shunt regulator consists of a the diode, a resistor to limit the current entering and a load resistor. The resistor used in the circuit will limit the current entering it and it will also ensure the operating position of the diode.This is known as shunt regulator circuit because the regulating element i.e., the diode is placed in parallel with the load. For low powers shunt regulators are efficient. Today this diode is used good to produce a stable voltage and this application is used widely in analog regulated power supplies.
When no load resistor is connected, the current through the load will be zero and all the current will pass through the diode giving out large power. When the load is being connected, the voltage across the load is same as the voltage across the zener diode. One of the major disadvantages of the circuit is that it creates electrical noise while stabilizing the voltage.
- For Overvoltage Protection
Zener Diode is a good circuit for overvoltage protection. Fuse, transistors, zener diode, SCR is the basic components of the circuit shown below. When there is a presence of overvoltage, the SCR will turn ON and it holds the output voltage and blows the fuse and disconnects the power supply. In normal conditions, no current flows through zener diode and the SCR is not fired. While when the voltage rises, the diode will conduct and the SCR will fire blowing out the fuse.
- As a Peak Clipping Circuit
Zener Diode is widely used in clipping circuits. Sometimes back to back connected zener diodes are used in clip circuits to produce the square wave.
- Audio or RF Applications
Zener Diode is widely used for audio or RF applications. Mainly these applications require the need of a stable voltage source. To satisfy this feature zener diodes are used in these applications.