Zener diode is a bi-terminal and bilateral heavily doped PN junction diode that can work in both the forward biasing and the reverse biasing. The symbol for a Zener diode is shown in figure 1.1. and its physical appearance in figure 1.2.
Fig. 1.1 Zener Diode Symbol
Fig. 1.2: Physical appearance of a Zener diode
Construction of Zener Diode:
Zener diode is a heavily doped PN junction diode whose current-voltage characteristics are controlled by controlling the doping level of PN junction. As opposed to normal rectifier diode, a Zener diode can work in both forward and reverse bias condition. A Zener diode is specially designed to operate in reverse bias, although it can work in forward bias similar to a rectifier diode. A basic structural view of a Zener diode is shown in figure 1.3.
Fig 1.3: Constructution of Zener diode
Breakdown in Zener Diode
Two types of breakdowns are occurred in a Zener diode i.e
At high reverse voltage, avalanche breakdown occurs in both conventional and Zener diodes. When a large amount of reverse voltage is provided to the PN junction, the free electrons obtain enough energy to accelerate rapidly. These high-velocity free electrons collide with other atoms, knocking off more electrons. As the electric current in the diode gradually increases as a result of this continual collision, a significant number of free electrons are released. A typical diode may be permanently destroyed by this abrupt increase in electric current, but a Zener diode is intended to work under avalanche breakdown and can withstand the quick rise of current. In Zener diodes with a Zener voltage (Vz) greater than 6V, avalanche breakdown occurs.
The electric field in the depletion area becomes strong enough to draw electrons from their valence band when the applied reverse bias voltage approaches the Zener voltage. Valence electrons that obtain enough energy from the depletion region’s strong electric field escape out from the parent atom. A slight rise in voltage causes the electric current to rapidly increase at the Zener breakdown region.
Fig 1.4: Breakdown in Zener diodes
Operation of Zener Diode
Typical characteristic curve of a Zener diode is shown in figure 1.5. with reference to this curve, it is cleared that when a Zener diode is forward bias, current is increased in proportion to increase in voltage after the barrier potential is overcome. When Zener diode is reverse bias, a very small amount of current starts to flow through the circuit due to minority charge carriers, but current is drastically increased when the reverse voltage reached the breakdown value where due to avalanche effect, a bunch of minority charge carriers are escaped form the valence band of Zener diode. Before the avalanche, a stage called the Zener breakdown, occur when in increase in reverse voltage increase the current but no or very small change occurs in voltage, at this point the verse voltage is termed as the Zener voltage, VZ. This Zener voltage value for a Zener diode can be controlled using the doping level of the diode and its value may vary from 1V to 200V for a typical Zener diode. the working region of a Zener diode is between the VZ and the VBR, the breakdown voltage.
Fig 1.5: Zener diode V-I characteristics
Ratings of Zener Diode
The ratings for each type of a Zener diode are given on the specification sheet provided by the manufacturer with each Zener diode. these ratings include Zener voltage (VZ), tolerance range of Zener voltage, Zener current limits, max. power dissipation, max. operating temperature, Zener impedance, thermal derating factor in milliwatts per degree Celsius and the reverse leakage current. Some specifications for a typical Zener diode are provided below:
Zener/Breakdown Voltage — The reverse breakdown voltage, also known as a Zener, ranges from 2.4 V to 200 V, with some devices hitting 1 kV, while the maximum for a surface-mounted device is 47 V.
Max. Current — The maximum current at the rated Zener Voltage (Vz – 200 A to 200 A) is called Iz (max).
Current Iz (min) – This is the smallest amount of current that the diode needs to break down.
Max. Power — The maximum power that a Zener diode can dissipate is indicated by its power rating. The product of the diode’s voltage and the current passing through it gives it.
Temperature Stability – Diodes with a voltage of 5 V have the best temperature stability.
Zener Resistance (Rz) – This is the resistance to the Zener diode.
Voltage Tolerance – It is normally 5% Zener Resistance (Rz) – It is the resistance to the Zener diode.
NOTE: The destruction of the diode occurs when current crossed the power limit of the diode which depends upon current through Zener diode and the junction temperature.
Applications of a Zener diode
Zener Diode as a Shunt Voltage Regulator
To limit the current into the diode, a series resistor is attached to the circuit. It is connected to the d.c positive .’s terminal. It’s designed in such a way that it can work in breakdown situations as well. We don’t utilise a standard junction diode since the diode’s low power rating can be destroyed if reverse bias is applied over its breakdown voltage. The Zener diode current should always be minimal when the minimum input voltage and maximum load current are used. Because the input voltage and needed output voltage are known, selecting a Zener diode with a voltage that is close to the load voltage, i.e. VZ = VL, is easier. Zener diode is also termed as a voltage regulator diode. the circuit diagram of a Zener diode as a shunt regulator is shown in figure 1.6. Here Zener diode is connected across the load to regulate the load voltage to admissible value.
Fig 1.6: Zener diode as shunt voltage regulator
Working of the circuit shown in figure 1.6 is divided into two halves:
- When the voltage source VAA is constant but load current IL changes:
Consider the voltage source VAA is constant, but load current is intended to change. if VOUT be the required output voltage, then current through the circuit are IL = VOUT / RL and IZ = VOUT / RZ which form the total current through circuit as:
IT = IL +IZ … 1.1
Since VR, the voltage drop across resistor R is
VR = IT x R … 1.2
VAA = VR + VOUT … 1.3
Therefore, VOUT will only remain constant, if VAA and VR remain constant, the condition may only be fulfilled when IT remain constant which is only possible when the IZ is compensated with the IL as shown in equation 1.1. It is also cleared that the VZ should be equal the required VOUT so as to regulate the voltage up to the mark.
- When the load current IL is constant but voltage source VAA changes:
Now consider the load is not varying and the supply voltage is intended to change. in this scenario, the increase in supply voltage VAA would increase the VOUT. As a result, IZ and hence the IT would increase by virtue of which VR will increase across the series resistor R. If the regulator is properly designed, the increase in voltage across the R , i.e., VR should approximately equal to the change in the supply voltage VOUT which in result keep the output voltage VOUT to constant value as shown in equation 1.3. The value of a series resistor R and its power rating for the regulator circuit can be found from the equation 1.4 and equation 1.5 respectively as given below: