Electrical Engineering ⇒ Topic : Zener Breakdown
In this breakdown process, doping concentration on either side is very high. Therefore, the corresponding electric field is also very high. A free electron will result when the high electric field forces a bound electron to come out of its covalent bond. A large number of carriers and a corresponding high reverse current is generated in this process. This process is known as Zener breakdown.
In avalanche breakdown, the source of carriers is the collision process of carriers with crystal ions, the covalent bonds. The breakdown voltage is higher for lightly doped diodes. The temperature coefficient of Zener breakdown voltage is negative while that for avalanche breakdown voltage, the temperature coefficient is positive
The proper operation of a Zener diode, involves placing it in a circuit such that it is reverse biased, the biasing voltage is higher than VZ, and the current should not exceed IZ.
The Zener diode has the following applications:
Voltage regulation circuits
The main target in voltage regulation is to keep the output voltage constant even when the input voltage or load current varies.
Let us study how the output voltage can be kept fixed even when either Vin or IL changes.
In this circuit, Zener anode is connected to the negative terminal of the battery and the cathode to
the positive terminal of the battery. From Figure (a), we can write
Vin = IR + Vz and I = IZ + IL
where I is the current through R, which is the current through Zener [Iz < (Iz)max] + IL, the load current.
When Vin is changed: If V,. increases, I also increases and thus, VR = IR also increases such that vz is constant. Since IL is constant, the current flowing through the Zener diode also increases. Therefore, the diode keeps the output voltage constant by allowing a higher current to flow through it. When Vin. decreases, the Zener diode takes a lower current.
When the load current is changed: If Vin is kept constant and IL increases, VR = IR is constant due to constant Vin. and VZ Therefore, I is also fixed. Thus, lz = I - IL decreases when IL increases.The output voltage is kept fixed because Zener diode accepts a lower current to flow through it. Similarly, the Zener diode maintains the output voltage fixed by accepting a higher current to flow through it, when the load current decreases. It can be obtained as:
If Iz = (1z)max, IL= O. A practical example of a circuit having two reference voltage levels is shown in Figure (b).
figure (b) Circuit having two reference voltage levels
!! OOPS Login [Click here] is required for more results / answer