Comprehensive Guide to Zener Diode Operation and Voltage Regulation

in this lecture I will introduce Xena diode Xena diode is a special diode and it is widely used in voltage regulation

to maintain a constant voltage across a varying load it is widely used in voltage regulation if we want a constant

voltage across a varing load we use xener diode for this purpose let's say RL is the ver load RL is the varying

load I will connect Xeno diode like this this is the symbol of xener diode and if this xener diode is reverse biased if

this xener diode is reverse biased then we can replace the Xena diode we can replace the Xena

diode by a constant voltage source and the voltage is equal to VZ and this voltage is given in the specific sheet

of the xener diode we have different voltages for different xener diode this is the varying load RL and we can have

the varying current with constant voltage so the xener diode is used for voltage regulation before moving to the

equivalent circuits of Xena diode I will explain two types of breakdown the first one is Avalanche breakdown the second

one is xener breakdown in case of avalanche breakdown we apply a high reverse Bas potential across the diode

the electrons gain High kinetic energy and break the calent bond and they also free more electrons due to the

collisions and we have a chain reaction and because of this large current is produced the avalanch breakdown occurs

in lightly doped diodes lightly doped diodes lightly doped diodes are

the normal diodes normal diodes Xena diodes are highly dope diodes and in case of lightly dope

diodes when you apply High reverse bass potential the electrons will gain the kinetic energy and they'll break the

calent bond and collide with other electrons this process will free more electrons and we have a chain reaction

and because of this large current is produced but in case of xener breakdown we can achieve the breakdown much

earlier let me write this down breakdown break down is

achieved is achieved much earlier much earlier in case of xener diode and

this is because we increase the doping level on both the sides on N side and P side we increase the doping levels and

we have the breakdown earlier as compared to the normal diodes in case of xener breakdown breakdown is initiated

by direct rupture of covalent bonds by direct rupture of covalent bonds by direct rupture of calent bonds I mean

tearing of covalent bonds this means steering steering of calent bonds and this happens due to strong electric

field at the junction developed by high reverse bias potential so there is difference between Avalanche breakdown

and xener break down in case of avalanche breakdown electrons gain the high kinetic energy and they break the

calent bonds on the other hand in case of xener breakdown there is direct rupture of covalent bonds because of

high electric field I hope Avalanche and zener breakdowns are now clear to you now we will move to the next point the

power dissipation capability Zena diodes are special diodes whose power dissipation capabilities are much more

than the normal diode and because of this they can easily work in Breakdown reason so xener diodes have high power

dissipation capabilities as compared to the normal diodes when we use the normal diode in Reverse Bas condition their

properties will degrade but on the other hand if we use the zener diode in Reverse bass condition the properties

will not degrade Zena diode in forward bias reason is similar to the normal diode in forward bias reason so this is

very important point because most of the people relate Zena diode to the reverse bias condition only but xener diode can

easily be used in forward bias reason also there is no difference between the zener diode and normal diode in case of

forward bias reason we can also use Zer diode in forward bias reason it will have similar characteristics as normal

diode this is one important point the most important use of xener diode is voltage regulation and xener

diode acts as voltage regulator when we operate it in reverse bias reason but we can easily operate zener diode as normal

diodes in forward bias reason now we will move to the next point that is the symbol of xener diode the

symbol of xener diode first we have to make the symbol of normal diode this is the symbol of

normal diode P side n side and after this I will make crooked n The Crooked n like this and this is the symbol of

xener diode the voltage is represented by v subscript Z this is the xener voltage now I will try to obtain the

equivalent models for xener diode using the vi characteristics of xener diode this is the forward bias reason and this

is the reverse bias reason in case of forward bias reason I have already explained the xener diode is similar to

the normal diode this is the symbol of xener diode I will connect P side to the positive terminal and N side

to the negative terminal now the xener diode is forward biased and I can replace this symbol I can replace this

symbol by the barrier potential 0.7 Volts for silicon and the diode resistance the diode resistance r z this

is positive negative and if we consider RZ to be very small we can have the equivalent

model like this in case of forward bias reason very simple now we will move to the reverse bias reason and I will

consider this reason from here to here and this is the reason when there is no breakdown this is the reason before the

breakdown and in this reason all also the xener diode will act as the normal diode and I will try to obtain the

equivalent model this is plus this is minus because we have to reverse bias the diode n side is connected to the

positive terminal and the P side is connected to the negative terminal and when this happens the current through

the diode is very small and when current is very small we replace the diode with high resistance

and we can consider it as open circuit so this is the equivalent model for the reason before the breakdown now we will

move to the most important part of this lecture we will try to obtain the equivalent model of the xener diode when

there is breakdown this voltage is VZ K vzk is the voltage after which the breakdown occurs then we have another

voltage VZ KN we can obtain VZ KN if we extend if we extend this linear reason this will cut the voltage axis at this

point and this voltage is VZ KN and let's take a random point on this linear reason and the corresponding current the

corresponding current is i z the corresponding voltage the corresponding voltage is v z and this current here is

i z k this is the knee current now we have everything to obtain the equivalent model of xener diode in

this reason I will first draw the symbol we have to reverse bias the diode so I will connect this side to the

positive terminal and this side to the negative terminal then we have the potential

v z KN + minus and then we have the resistance of diode RZ and this is equal to this is equal to v z VZ is simply

equal to VZ KN VZ KN plus this voltage and this voltage is equal to this voltage is equal to RZ multiplied by i z

the resistance of the zena diode and the current flowing through the DI so we have VZ KN plus i z r z and if you

want to calculate the value of RZ then it is equal to 1 by slope of the curve and we can easily obtain the zener

voltage if we have this values if we consider RZ to be very small if we consider RZ to be very small then we

have the model like this plus minus VZ kn and this is v

z so VZ is equal to VZ KN if we neglect the diode resistance r z and this is the equivalent model of the zener diode and

this is very important here you can clearly see this is VZ KN here you can clearly see the zener voltage is equal

to VZ KN and VZ KN is already mentioned in the specification sheet so if you need five volts or six volt PS of

constant voltage across any resistance then you can easily connect the zener diode across the resistance and if we

apply reverse bias potential across this diode we have 5 volts or six volts across the resistance because you can

see VZ is equal to VZ KN a constant voltage and we can say that the xener diode is acting as a voltage regulator

VZ and power dissipation is mentioned in the data sheet they are mentioned in the data sheet and these two values are the

most important values and you must have to consider them before any use and if we draw if we

draw the diagram for normal diode and xener diode then you can see the difference in the depletion layer this

is the depletion layer of normal diode this is p side this is n side and this is the depletion layer and in case of

xener diode in case of xener diode the depletion layer is much narrow this is the depletion layer this is the

depletion layer in case of zener diode and this depletion layer is narrow if you compare the depletion layer in case

of normal diode I will quickly revise the equivalent model of xener diode first thing we have to do is to connect

the N side of the diode the N side of the diode to the positive terminal and the P side of the diode to the negative

terminal so that the diode is reverse biased and once the diode is reverse biased we have the equivalent model like

this this voltage is VZ KN r z is the resistance and if you neglect r r z if we neglect RZ then we

only have we only have VZ KN as the value of xer voltage in this case VZ is equal to VZ KN plus the current is i

z i z RZ this is the value of VZ in this symbol also I will represent the current i z this is the

direction of the current i z because this is the reverse bias current and in this case when we neglect the

RZ VZ is simply equal to VZ KN in numerical problems we are going to use this model this model is mostly used we

neglect RZ and we have VZ not as the value of VZ this is very important model because in most of the numerical

problems you have to use this so it is better if you draw this model somewhere and you must also know how to obtain the

models using the vi characteristics this is all for this lecture see you in the next one