Zener Diode

ZENER DIODE

A zener diode is also called as voltage reference, voltage regulator or breakdown diode Fig (1.33) shows a schematic symbol for a zener diode.

The zener diode is a silicon PN junction device which differs from a rectifier diode, in the sense that it is operated in the reverse breakdown region. The breakdown voltage of a zener diode is set by carefully controlling the doping level during manufacture.

Reverse Characteristic of a Zener Diode

We have just discussed that a zener diode is operated only in the reverse‒bias region. That is why we shall discuss its characteristic in this region only. Fig (1.34) shows the reverse portion of the V‒I characteristic of the zener diode.

It may be noted from this figure that as the reverse voltage (V_R) is increased, the reverse current (I_z ‒ zener current) remains negligibly small upto the 'knee' of the curve (point ‘p'). At this point, the effect of breakdown process begins. From the bottom of the knee, the breakdown voltage (v_z) remains essentially constant. This ability of a diode is called regulating ability. It maintains an essentially a constant voltage across its terminals over a specified range of zener current values. The following two points are important from the characteristic of a zener diode.

(i) There is a minimum value of zener current called breakover current designed as I_z(min) which must be maintained in order to keep the diode in breakdown (or regulation) region. When the current is reduced below the knee of the curve the voltage changes drastically and the regulation is lost.

(ii) There is a maximum value of zener current designated as I_z(max) above which the diode may be damaged.

Applications of Zener diode

• As voltage regulators.

• As fixed reference voltage in power supplies and transistor biasing.

• As clippers in wave‒shaping circuits.

Difference between the PN junction diode and zener diode.

PN Junction diode

• The PN Junction diode is a semi conductor diode which is formed when n-type and p-type semiconductor crystals are joined together.

• It is operated in forward bias region as well as reverse bias region.

• Application: (i) As a switch in logic circuits (ii) As a signal diode in communication circuits

Zener diode

• It is also a silicon special PN Junction diode which differs from a rectifier diode in the sense operated in the reverse break down.

• It is operated only in reverse bias region.

• Application: (i) As a voltage regulator (ii) As a fixed reference voltage in transistor biasing circuits.

Applications of Zener diode as voltage regulator

A voltage regulator is a circuit that maintains the output voltage of a DC power supply constant against variations in

• Input AC voltage

• Load current

The circuit diagram of a zener regulator is shown. The operation of zener regulator can be explained by separately considering the variations in input supply and the load current. These two cases are explained next.

Zener regulator with varying voltage regulator/

(i) If R_L Increases I_L decreases, to keep constant. I_z increases,

 R_L ↑ → I_L (min) ↓ → I_z (max) ↑

 I_L (min) = I ‒ I_L (max)          ... (1)

(ii) If R_L decreases I_L increases, to keep I constant I_z decreases,

 R_L ↓ → I_L (max) ↑ → I_z (min) ↓

 I_L (max) = I − I_z (min)            ….(2)

(iii) The maximum power dissipation in zener remains same as

 P_D=V_zI_z (max)            ….(3)

Regulation with Varying input voltage

The circuit diagram of a zener regulator for varying input voltage is illustrated. The input voltage V_s is shown as a variable DC supply. This input voltage is unregulated, as it varies due to the variations in the input AC supply of the rectifier. In Fig. 1.37, the zener diode is reverse biased. The necessary condition for the operation of this circuit is that the zener diode should always remain in the breakdown region. Throughout the operation of the circuit.

Zener regulator with varying input voltage

It can be seen that the output is

V_o = V_z is constant.

V_L = V_o / R_L = V_z / R_L = constant

 I = 1_L+ I_Z

Now if V_o increases then the total current I increases, But I_L is constant as V_z is constant. Hence the current Iz increases to keep I_L constant.

If V_o decreases then current I decreases. But to keep I_L constant, Iz decreases.

Regulation Varying with load

The circuit conditions for the zener regulator with varying load current is illustrated in fig. The load current I_L, will vary if the load resistor, R_L is variable. The load current increases when the load resistance decreases, and the load current decreases if the load resistance increases.

The input voltage is constant while the load resistance R_L is variable. As V_S is constant and Vo=Vz is constant, then for constant R the current I is constant.

I = [ Vin ‒ V_Z ] / R