What is a Zener diode? V-I Characteristics, Working &Why is Zener Diode used a voltage regulator?

what is a zener diode -v-i characteristics of zener diode-why is zener diode used as a voltage regulator-zener diode voltage regulator-zener diode characteristics

We Discuss zener diode definition, difference between diode and zener diode, zener diode voltage regulator, zener diode characteristics, zener diode working, zener diode v-i characteristics, zener diode pdf, zener diode notes, zener diode vs diode.

What is a Zener diode?

A properly doped p-n junction diode that works in the breakdown region without damaging itself is called a Zener diode. Zener diode is also known as breakdown diode because it is designed to operate under reverse bias in the breakdown region. It is mainly used as a voltage regulator.

Symbolic representation of Zener diode is given as

Zener diode is fabricated in such a way that both n- and p- regions of the diode are heavily doped. As a result of this, the thickness of the depletion layer or region is very small (<10-6     m) A very strong electric field is set up across the junction. For Instance, if a reverse voltage of 5 V is applied across the junction,

Then electric filed, E=-dv/dr= 5*106    Vm-1    

This field is set up across the junction of the diode. This electric field is called an Ionized field.

Draw volt-ampere characteristics of a Zener Diode

The circuit diagram for studying the characteristics of a reside is shown in  22(a) and 22(b)

 V-I Characteristics of Zener Diode

The volt-ampere characteristics of the Zener diode are given in Fig. 1.36.

When the reverse voltage is applied a small reverse current I, flows until the breakdown voltage VRR is reached. The breakdown voltage has a sharp knee, followed by an almost vertical increase in current (The portion AB of the characteristic), during which period the voltage across the device remains almost constant. Thus, Izk is the minimum Zener current required to sustain breakdown and Izm is the maximum Zener current at the operating voltage Vz. Iz represents an operating current at a Zener voltage Vz.

An important parameter of a Zener diode is its dynamic resistance, which is specified at a specified operating point at a particular operating temperature. It is defined as the ratio of change in Zener voltage to the change in Zener current under reverse bias conditions. Referring to Fig. 1.36, the dynamic resistance

ZR = ∆ Vz/Iz∆.

Another important feature of a Zener diode is Power dissipation, which is the product of V, across the device and current I, through it, at a specified working temperature.

Zener diode parameters

The various parameters of the Zener diode are given at a specified temperature. Prominent among them are as follows:

(1) Zener voltage Vz : This is the normal working voltage of the device. Usually, the variation in the Zener working voltage is specified as a percentage. For example, a 12 V Zener diode with a ±10% variation has a working range from 10.8 V to 13.2 V.

(1) Zener Current Iz, : This is the normal working current at which the temperature of the device remains within safe limits.

(III) Dynamic Resistance ZR : This is the resistance of the Zener at its normal working current and voltage and is determined by taking the ratio of a small change in voltage to a small change in current.

(iv) Zener Knee Resistance ZN: This is the Zener diode resistance at the knee of the characteristic. Generally knee resistance is far more than dynamic resistance.

(5) Maximum Zener Current Izm : This is maximum current that can be handled by the device without crossing the limit of its power dissipation capability.

(vi) Minimum Zener Current Izk : This is the minimum current below which the device reverts to its normal diode operation. This is also the minimum current required to sustain breakdown.

(vii) Power Dissipation PD : This is the product of Zener voltage Vz and the Zener current at a particular operating point, i.e.,PD = Vz x lzm.

Power Supply Performance

Introduction :

Almost all electronic circuits require a d.c. source of power. For portable low power systems batteries may be used. More frequently, however, electronic equipment is energised by a power supply, a piece of equipment which converts the alternating waveform from the power line into an essentially direct voltage. Hence we need to assess the performance of the power supply arrangement. This may be done in terms of variation in the alternating line voltage and also in terms of variation in load current. For this purpose we take up the Source Effect and Load Effect as discussed below, which could be used to assess the performance of Zener Diode regulator.

Source Effect

In a d.c. power supply system, the a.c supply to the input of a transformer may not be constant and may fluctuate. A +_10% change in the ac. source voltage (VS) is quite common. Any such voltage variation will lead to variation in the output voltage V, from a power supply. Such a change in output voltage (∆V0) as a result of the change in the input is known as the Source Effect. Thus,

Source Effect = ∆V0, for a 10% change in Vs
If the output varies by 200 mV due to source voltage changes by 10%, the source effect is 200 mV, This is shown in Fig. 1.37.
Another way of expressing the change in d.c. output V0. is to show ∆V0 as a percentage of the d.c. output voltage (V0.), in which case we use the term line regulation.

Thus,

Line Regulation (AV. for a 10% change in Vs,) x 100% /V0

Load Effect :

The d.c. output voltage is also affected by changes in load current (1) When I, increases, output voltage decreases; when I, decreases, output voltage increases. The change in output voltage with the increase in load current from zero to its specified maximum level ‘[IL(max)] is known as the load select. Thus,

Load Effect = ∆V0, for ∆IL(max)

Thus, if change in load current (1) results in change in output voltage (AV) of 200 mV, the load effect is 200 mV. This load effect may also be expressed as a percentage of the output voltage, in which case it is called load regulation. Thus,

Load regulation = (∆V0 for ∆IL(max))x 100%/V0

Why is Zener Diode used a Voltage Regulator?

(a) Regulator Circuit with No Load

A Zener Diode is sometimes called a voltage regulator diode because it maintains a constant output voltage even though the current through it changes. For normal operation we have seen that we have to reverse bias the Zener diode. Furthermore, to get breakdown operation, the supply voltage Vs must be greater than the Zener breakdown voltage Vz, as shown in the simple regulator circuit of Fig. 1.38.
Fig. 1.38 Zener diode Voltage Regulator Circuit on No Load for Use as a Voltage Reference Source

A series resistor Rs is always used to limit the zener current to less than its maximum current rating. Otherwise the zener diode will burn out due to too much power dissipation.

The circuit of Fig. 1.38 is used as a voltage reference source that supplies only a very low current (much lower than Iz).

The voltage across the series or current-limiting resistor R equals the difference between the supply voltage and the zener voltage. Therefore the current through the resistor is

Iz =Vs- Vz/ Rs
The zener current could be somewhat greater than the diode knee current Izk  (see Fig. 1.36). However, the most stable reference voltage will be available if Iz, is selected as a specified test current lzt.
(b) Loaded Zener Voltage Regulator
A loaded Zener Voltage Regulator Circuit is shown in Fig. 1.40. The total supply current, which flows through the series resistor Rs is the sum of lz, and IL. It must be ensured that the minimum Zener current is sufficient to maintain the device in the breakdown region. For a Zener diode operating at a test currently of 20 mA, the minimum Zener current (Iz(min)) should be 0.5 mA. The current through the series resistor is given by

Is= lz +IL=Vs-Vz/Rs

Sometimes the load current IL may be reduced to zero. However as the voltage drop across Rs remains constant, the supply current Is remains unchanged, that is

Is= lz +IL

It is apparent that the above current flows through the zener diode when RL is disconnected. In any event the circuit should be so designed that the total current Is does not exceed the maximum Zener current lzm (see the Zener diode V-I characteristics given in Fig. 1.36). The design process using a low-power Zener diode is demonstrated in Example 1.33.

Regulator Working :

We may assess the performance of the Zener diode regulator in terms of the source and load effects as discussed in Sec. 1.12, using equation (1.13) through equation (1.16). In the event of there being an input ripple voltage, the output ripple will be severely attenuated. The ratio of the output ripple voltage to the input ripple voltage is known as the ripple rejection ratio.

We shall start by drawing the ac. equivalent circuit by replacing the Zener diode by its maximum impedance Zzt as shown in Fig.1.42(a). Hence the a,c.equivalent circuit may be taken to be a simple voltage divider. If the input voltage varies by ∆Vs, the change in the output voltage is,

∆V0, = ∆Vsx Zzt / Rs+ Z zt

The above equation is for a regulator without a load as shown in Fig. 1.42(a)
Fig. 1.42(b) gives the a.c equivalent circuit, in which a load RL, is connected In parallel with Zzt The output voltage variation is now given by

∆V0= ∆Vs, x(Zzt ||RL) /Rs+ (Zzt||RL)

The regulator source effect can be determined by using either equation (1.19) or equation (1.20) depending on whether the regulator is unloaded or loaded. If it is required to calculate the ripple rejection ratio, the above two equations may be used. Here, the input ripple voltage amplitude (Vri) and the output ripple voltage amplitude (Vr0) are substituted for the input and output voltages in eqn (1.19) and (1.20). Hence eqn (1.19) of a regulator on no load is modified to give a ripple rejection ratio equation,

Vr0/Vrl=Zzt/Rs+Zzt

Similarly, in the case of a loaded regulator, egn (1.20) becomes

Vr0/Vrl=Zzt||RL/Rs+(Zzt||RL)

We shall now find the Load Effect of the Zener diode voltage regulator First, we have to determine the output resistance, for which we will use the Thevenin’s Theorem as follows:

Step 1: Redraw Fig. 1.42. for convenience, as in Fig. 1.43(a).

Step2: Remove the voltage source and replace it by its internal resistance; remove the load resistance and look inwards as indicated in Fig. 1.43(b).

Step3: Assume zero source resistance, and the circuit become as in Fig. 1.43(c). Now the equivalent circuit looking inward shows that Rs & Zzt are in parallel hence the circuit output resistance is given by

R0 = Rs|| Zzt  —(1.23)

When the load current changes by the output voltage change is

∆V0 =∆IL( Rs|| Zzt )

Zener Diode Voltage Regulator Performance

In Section 1.13, we discussed regulator performance in terms of source and load effects, and the line and load regulations based on power supply performance (Section 1.12). We shall now study regulator performance in the usual conventional way.

Refer to Fig. 1.44 giving the circuit of a Zener Diode Voltage Regulator.
Voltage regulation is a measure of a circuit’s ability to maintain a constant output voltage when either input voltage or load current varies. A zener diode, when working in the breakdown region, can work as a voltage regulator. In Fig. 1.44, Vi, is the input d.c. voltage whose variations are to be regulated. The Zener diode is reverse connected across Vi,. When this input voltage is in excess of Vz. the diode breaks down and the current Iz, flows through it. A current IL, flows into the load RL, and the magnitude of IL is such that ILRL= Vz. This is evident since load voltage must be the same as Zener voltage. A current I= (Iz+lL) is drawn from the source.

If the supply voltage now increases, more current is drawn from the supply. Since the Zener diode is operating in the breakdown region, its current Iz, increases and, as was observed when studying the characteristics of the Zener diode, there is no appreciable voltage drop due to this increase of current. The result is that a constant voltage Vz, is maintained across its terminals, i.e., across the load, However, the series resistance R carries more current and hence dissipates more power. Thus variations of supply voltage over a certain range are effectively taken care of by the Zener voltage regulator

Now, if the supply voltage remains constant, but the load resistance keeps changing, even then, the voltage across the load would remain unchanged. If the load resistance decreases, more current flows into the load; if the load resistance increases, the load current decreases. However, since the load is in parallel with the Zener diode which is operating with a constant breakdown voltage, it is clear that the voltage across the load remains constant.

Thus, Irrespective of changes of supply voltage or load resistance, the load voltage is always maintained at a constant level.
It is easy to see that, for the Zener diode to operate in the breakdown region, the voltage across the diode must be at least equal to Vz,. We have

Vi = IR + Vz,

or     Vz = Vi-IRor

If the voltage across the diode is less than V,, the diode will be OFF and hence acts as an open circuit. In such a situation, we have

Total circuit resistance = R + RL

: Circuit current I=Vi/ R+RL

Load voltage VL = IRL =ViRL/R+RL

Also Vi = IR + VL

Let Pz  denote the power rating of the Zener diode. We have Pz = VzIz(max)

watts, where Iz(max) is the maximum value of the Zener current. Let Iz(min) denote the minimum Zener current.

If IL, is the load current corresponding to Iz(min)

total current Imin  = IL + lz(min):

Hence, voltage drop across R = Imin R

* Minimum input voltage Vi(min) = Imin R+Vz

or series resistance R = Vi (min) – Vz/ I’min

Given Vi (min), Vz, and Imin, R can be evaluated

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