The Zener diode is a two-terminal active electronic component, whose V-I characteristics are just like PN-diode. The working of Zener diode as follows, when the diode is forward biased it acts as ON switch. When it is reverse biased the Zener diode acts as a voltage regulator that is when the input voltage is more than the Zener breakdown voltage (Vz) the output voltage (Vo) is equal to the Zener breakdown voltage.
If we recall the basics of semiconductors a PN-diode is formed by the combination of P-type semiconductor and N-type semiconductor, because of the excessive electrons (donors) in N-type material and protons (acceptors) in P-type material a junction is formed due to the combination of both materials which is termed as a potential barrier or forward voltage barrier (Vf).
A diode can be biased in three ways which are described as.
Unbiased semiconductor diode:
This is a condition where the no voltage is applied to the diode, in this condition the diode is an ideal state that is it does nothing simply an open switch. By the principle of diffusion, the holes in P-type material and electrons in N-type materials tend to go towards lower concentration. This results in forming the depletion region which is also known as a potential barrier (Vf).
In this process, the holes from the P-type material accumulate at the N-type material in the same way the electron in N-type accumulate at P-type material this results in a voltage threshold. The voltage threshold changes depending on the material used. 0.2V for Germanium and 0.6V for Silicon type.
Vth ->Vf -> 0.3V for Germanium, 0.7 for Silicon.
P-type terminal is termed as Anode (A)
N-type terminal is termed as cathode (K)
Fig 1: diode in unbiased state.
Forward biased semiconductor diode:
In this condition the anode (A) is connected to the positive rail, the cathode (K) is connected to the negative rail of the power supply (Vin). Let’s assume the power supply is a variable type. The following expression gives the overview and the behaviour of the diode.
- Vin<Vf -> diode is in OFF state therefore no conduction
- Vin=Vf -> diode starts conduction with minimal current than previous condition.
- Vin>Vf ->diode goes into saturation mode comes to ON state results to more flow of current from Anode to cathode.
From the above the first and second conditions are used to make the diode as ON,OFF switch.
Note: In this mofe the behaviour of the zener diode is similar to the PN diode.
Reverse biased Semiconductor diode:
This mode is quite opposite to the forward biased that is Anode (A) is connected to the –Ve rail of the power supply and cathode (K) is connected to the +Ve rail of the power supply. In this mode, the depletion region moves away from its origin this will continues until the depletion regions attain its maximum. After this, if the Vin starts to increase this will result in a large reverse current.
Note: If the reverse bias voltage is continued to increase the PN-diode may get diode.
For Zener diode this case differs whenever the Vin in reverse bias is increased to the diode reverse breakdown voltage the output voltage is equal to Zener reverse breakdown voltage.
After this phenomenon, the diode produces avalanche current which means the current flowing is maximum. This mode is used for developing the power supply circuits.
By observing the VI characteristics the behavior of the PN-diode and zener diode can be easily understood.
PN-diode V-I characteristics:
Fig 2: PN- diode VI characteristics
Fig 3: VI characteristics of zener diode
From both V-I characteristics, the 1st quadrant is similar for PN and Zener diode. But the 3rd quadrant changes, when the PN- diode goes into avalanche breakdown the diode may get burnt or damaged. Whereas for Zener diode after avalanche breakdown it maintains a constant voltage with an increase in current (Iz).
Note: The reverse breakdown voltage for PN- diode is given in datasheet as VRRM.
As from the previous description the Zener diode is placed in reverse biased and it is parallel to the Vin. which shown in below circuit. Here lets design a power supply using power supply using Zener diode. As in reverse biased mode after the Zener breakdown, the diode tries to draw maximum current from the source this may lead to destroying the diode. To protect it from the maximum current a source resistor is connected in series with the Zener diode to the power supply which is shown in below figure. The calculation of source resistor is given by the equation below
Fig 4: circuit diagram for zener voltage regulator.
- It is a simple circuit to design
- This requires 2 or 3 components for usage.
- Providing high current capability.
- The efficiency is poor for heavy loads because of power dissipation at series resistance and zener resistance with respective to load current.
- Change in zener resistance changes the output voltage (Vo).
- It is not suitable for larger current drawing loads, if connected the output voltage may decrease.
- Here it is not possible to vary the output voltage.