Zener Diode Zener diode
is a P-N junction diode specially designed for operation in the breakdown
region when the voltage is above a certain value known as the Zener voltage.
Zener diode may be Silicon or Germanium one but Silicon is preferred over Germanium
because of higher operating temperature and current capability. The knee point
is also sharper in the case of silicon. Zener diode is like an ordinary P-N
junction diode except that it is highly doped so as to have a sharp breakdown
voltage. These Zener diodes are special diodes which are effectively utilized
in the reverse breakdown region. It does not mean that the Zener diode is only
operable in the reverse breakdown. It can be used in the forward region also.
In the forward characteristic it will be similar to the VI characteristic of
normal diode. The Zener diode is manufactured with adequate power dissipation
capabilities so that they can operate in the breakdown region. Normal diodes
cannot operate in breakdown region because normal diodes do not have the high
power dissipation capability. That is why it is dangerous to use a normal diode
in the breakdown region. But the Zener diode has higher power dissipation
capability so that they can be used in the breakdown region without any danger
of degrading their properties. The voltage drop across the Zener diode is equal
to the Zener voltage of that diode no matter how high the reverse bias voltage
is above the Zener voltage.
The illustration above
shows this phenomenon in a Current vs. Voltage graph. With a zener diode
connected in the forward direction, it behaves exactly the same as a standard
diode – i.e. a small voltage drop of 0.3 to 0.7V with current flowing through
pretty much unrestricted. In the reverse direction however there is a very small
leakage current between 0V and the Zener voltage – i.e. just a tiny amount of
current is able to flow. Then, when the voltage reaches the breakdown voltage
(Vz), suddenly current can flow freely through it. For example if you pass a
reverse 5V through a 3V zener diode and measure the voltage across the zener
diode, that voltage will be 3V.
Zener Diode as a Voltage Regulator
Since the voltage dropped across a Zener Diode is a known and
fixed value, Zener diodes are typically used to regulate the voltage in
electronic circuits. A zener diode can be used to make a simple voltage
regulation circuit as shown in figure. The output voltage is fixed as the zener
voltage of the zener diode used and so can be used to power devices requiring a
fixed voltage of a certain value (equal to the rating of the zener diode).
Case 1: Load resistance is constant and input voltage is varying
·
Load resistance is constant means load
current (IL) will also be constant as VL = VZ is constant.
·
Now if we increase the input voltage
(Vin), current IS will increase. Due to which zener current IZ will increase as
IL is constant.
·
But the maximum current which can pass
through the zener is Iz(max). Therefore, there will·
be a limit up to which we can increase the input voltage so that zener current
will remain below Iz(max) and it will define the maximum limit of input voltage
(Vin(max)) for successful operation as voltage regulator.
·
Now if we decrease the input voltage
(Vin), current IS will decrease. Due to which zener current IZ will decrease as
IL is constant.
·
But the minimum current which can pass
through the zener is Iz(min). Therefore, there will be a limit up to which we
can decrease the input voltage so that zener current will remain above Iz(min)
and it will define the minimum limit of input voltage (Vin(min)) for successful
operation as voltage regulator.
·
To maintain a constant voltage at
output, input voltage Vin will have the following limit Vin(min) < Vin <
Vin (max).
·
This limit is based on Iz(min) and
Iz(max) which is the limit of current that can pass through the zener while
maintaining the constant zener voltage VZ.
Case 2: Input voltage
is constant and load resistance is varying
Input voltage is
constant means current IS will also be constant.
Now if we increase the
load resistance (RL), load current IL will decrease as V= = VZ is·
constant. Due to which zener current IZ will increase as IS is constant.
But the maximum current
which can pass through the zener is Iz(max). Therefore, there will be a limit
up to which we can increase the load resistance so that zener current will
remain below Iz(max) and it will define the maximum limit of load resistance
(RL(max)) for successful operation as voltage regulator.
Now if we decrease the
load resistance (RL), load current IL will increase as VL = VZ is constant. Due
to which zener current IZ will decrease as IS is constant.
But the minimum current
which can pass through the zener is Iz(min). Therefore, there will be a limit
up to which we can decrease the load resistance so that zener current will
remain above Iz(min) and it will define the minimum limit of load resistance
(RL(min)) for successful operation as voltage regulator.
To maintain a constant
voltage at output, load resistance RL will have the following limit RL(min)
< RL < RL(max)
This limit is based on
Iz(min) and Iz(max) which is the limit of current that can pass through the
zener while maintaining the constant zener voltage VZ.
