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| Zener Diode |
We
know that "reverse biased" diode blocks current in the reverse
direction, but will suffer from premature breakdown or damage if the reverse
voltage applied across it is too high. However, the Zener Diode or "Breakdown
Diode" as they are sometimes called, are basically the same as the
standard PN junction diode but are specially designed to have a low
pre-determined Reverse Breakdown Voltage that takes advantage
of this high reverse voltage. The zener diode is the simplest types
of voltage regulator and the point at which a zener diode breaks down or
conducts is called the "Zener Breakdown Voltage" ( Vz ).
Doping
concentration of P+N DIODE > ZENER DIODE > TUNNEL DIODE>
PN DIODE
The Zener diode when biased in the
forward direction it behaves just like a normal signal diode passing the rated
current, but as soon as a reverse voltage applied across the zener diode
exceeds the rated voltage of the device, the diodes breakdown voltage VB is
reached at which point a process called Avalanche Breakdown occurs
in the semiconductor depletion layer and a current starts to flow through the
diode to limit this increase in voltage.
The current now flowing through the zener diode increases
dramatically to the maximum circuit value (which is usually limited by a series
resistor) and once achieved this reverse saturation current remains fairly
constant over a wide range of applied voltages. This breakdown voltage point, Vz is called the
"zener voltage" for zener diodes and can range from less than one
volt to hundreds of volts.
Zener
Diode I-V Characteristics:
The Zener Diode is used in its "reverse bias" or reverse breakdown mode, i.e. the diodes anode connects to the negative supply. From the I-V characteristics curve above, we can see that the zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the zener diodes current remains between the breakdown current IZ(min) and the maximum current rating IZ(max).
The Zener Diode As Regulator:
Zener Diode Acts as a Voltage Regulator only in the Reverse
Breakdown Region.A voltage regulator should maintain constant voltage across terminals of
load irrespective of fluctuation in Load or Supply.To act as a voltage
regulator the zener diode must satisfy two conditions:
(i) Current through zener diode should be grater than or
equal to IZ(min),knee current.
(ii)Voltage across terminals of
zener diode should be Vz, Breakdown voltage.
The resistor, RS is connected in series with the zener diode to limit the current flow through the diode with the voltage source, VS being connected across the combination. The stabilised output voltage Vout is taken from across the zener diode. The zener diode is connected with its cathode terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor RS is selected so to limit the maximum current flowing in the circuit.
With
no load connected to the circuit, the load current will be zero, ( IL = 0 ),
and all the circuit current passes through the zener diode which in turn
dissipates its maximum power. Also a small value of the series resistor RS will
result in a greater diode current when the load resistance RL is
connected and large as this will increase the power dissipation requirement of
the diode so care must be taken when selecting the appropriate value of series
resistance so that the zeners maximum power rating is not exceeded under this
no-load or high-impedance condition.
The
load is connected in parallel with the zener diode, so the voltage across RL is
always the same as the zener voltage, ( VR = VZ ).
There is a minimum zener current for which the stabilization of the voltage is
effective and the zener current must stay above this value operating under load
within its breakdown region at all times. The upper limit of current is of
course dependant upon the power rating of the device. The supply voltage VS must
be greater than VZ.
One
small problem with zener diode stabiliser circuits is that the diode can
sometimes generate electrical noise on top of the DC supply as it tries to
stabilise the voltage. Normally this is not a problem for most applications but
the addition of a large value decoupling capacitor across the zeners output may
be required to give additional smoothing.
Then
to summarise a little. A zener diode is always operated in its reverse biased
condition. A voltage regulator circuit can be designed using a zener diode to
maintain a constant DC output voltage across the load in spite of variations in
the input voltage or changes in the load current. The zener voltage regulator
consists of a current limiting resistor RS connected in series
with the input voltage VS with the zener diode
connected in parallel with the load RL in this reverse
biased condition. The stabilized output voltage is always selected to be the
same as the breakdown voltage VZ of the diode.
Zener Diode Voltages
As well as producing a single stabilised voltage output,
zener diodes can also be connected together in series along with normal silicon
signal diodes to produce a variety of different reference voltage output values
as shown below.
Zener Diodes
Connected in Series:
The
values of the individual Zener diodes can be chosen to suit the application
while the silicon diode will always drop about 0.6 to 0.7V in the forward bias
condition. The supply voltage, Vin must
of course be higher than the largest output reference voltage and in our
example above this is 19v.
Zener Diode Clipping Circuits:
Thus
far we have looked at how a zener diode can be used to regulate a constant DC
source but what if the input signal was not steady state DC but an alternating
AC waveform how would the zener diode react to a constantly changing signal.
Diode
clipping and clamping circuits are circuits that are used to shape or modify an
input AC waveform (or any sinusoid) producing a differently shape output
waveform depending on the circuit arrangement. Diode clipper circuits are also
called limiters because they limit or clip-off the positive (or negative) part
of an input AC signal. As zener clipper circuits limit or cut-off part of the
waveform across them, they are mainly used for circuit protection or in
waveform shaping circuits.
For
example, if we wanted to clip an output waveform at +7.5V, we would use a 7.5V
zener diode. If the output waveform tries to exceed the 7.5V limit, the zener
diode will "clip-off" the excess voltage from the input producing a
waveform with a flat top still keeping the output constant at +7.5V. Note that
in the forward bias condition a zener diode is still a diode and when the AC
waveform output goes negative below -0.7V, the zener diode turns "ON"
like any normal silicon diode would and clips the output at -0.7V as shown
below.
Square Wave Signal:
The
back to back connected zener diodes can be used as an AC regulator producing
what is jokingly called a "poor man's square wave generator". Using
this arrangement we can clip the waveform between a positive value of +8.2V and
a negative value of -8.2V for a 7.5V zener diode. If we wanted to clip an
output waveform between different minimum and maximum values for example, +8V
and -6V, use would simply use two differently rated zener diodes.
Note
that the output will actually clip the AC waveform between +8.7V and -6.7V due
to the addition of the forward biasing diode voltage, which adds another 0.7V
voltage drop to it. This type of clipper configuration is fairly common for
protecting an electronic circuit from over voltage. The two zeners are
generally placed across the power supply input terminals and during normal
operation, one of the zener diodes is "OFF" and the diodes have
little or no affect. However, if the input voltage waveform exceeds its limit,
then the zeners turn "ON" and clip the input to protect the circuit.
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