Electrical Engineering ⇒ Topic : Zener Breakdown

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 V_Z, and the current should not exceed I_Z.

The Zener diode has the following applications:

• Voltage regulator
• Voltage limiters
• Waveform shaping circuits
• For calibration of voltmeters
• Protection of meters against damages

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 V_in or I_L 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

                                               V_in = I_R + V_z and I = I_Z + I_L

where I is the current through R, which is the current through Zener [I_z < (I_z)max] + I_L, the load current.

                                                        figure  (a)

When V_in is changed: If V,. increases, I also increases and thus, V_R = IR also increases such that v_z is constant. Since I_L 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 V_in. decreases, the Zener diode takes a lower current.

When the load current is changed: If V_in is kept constant and IL increases, V_R = IR is constant due to constant V_in. and V_Z Therefore, I is also fixed. Thus, l_z = I - I_L decreases when I_L 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:

                                                 .......... (1)

                                                                ............ (2)

If I_z = (1_z)_max, I_L= 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