Diodes are the most basic and important component in electronic engineering, they are used in a variety of applications in the electronics industry. Of those applications, rectifiers are the most popular one.
Rectifier is the circuit that converts AC voltage into DC voltage by employing different types of switches, which have unidirectional current conduction properties. As diodes have this property that’s why they are most widely used in rectifier circuits. In rectifier circuits, diodes act as the one-way switch and they do not allow the conduction of current in reverse direction.
There are
various types of rectifier circuit used in modern electronics like half wave
rectifier, full wave rectifier, bridge rectifier, center tapped rectifier etc.
As the
title indicates, in this article we will delve into the core concept of diode
bridge full wave rectifier. But before moving further in this article, let us
understand some basic terminology and concepts related to this article.
A Full Wave Rectifier circuit is a type of rectifier circuit that converts AC voltage into
DC voltage by conducting for both half of AC supply i.e. positive half and
negative half. This rectifier circuit ensures the conversion of full waveform
of AC voltage into DC voltage.
For the
conversion of full waveform of AC voltage into DC voltage, a full wave
rectifier circuit used two types of configurations i.e. Centre Tapped
Rectifier and Bridge Rectifier configuration.
A bridge
rectifier circuit configuration used four switches connected in bridge
arrangement as shown in figure.
Above
figure shows the generalized full wave bridge rectifier circuit diagram in
which four switches S1, S2, S3 & S4
are connected in bridge-like arrangement.
These four
switches can be any of the electronic devices that have unidirectional current
conduction property like Diode, SCR, MOSFET etc.
Diode Bridge Full Wave Rectifier
As the name
implies, a Diode Bridge Full Wave Rectifier is a type of full wave rectifier
circuit that converts AC voltage into DC voltage by using four diodes that are
connected in a bridge like arrangement.
Diodes are
the semiconductor devices that allow current to flow in one direction only. In
case of diode, we cannot control the conduction of current by any other
external means that's why it is considered as an uncontrolled switch and any
rectifier circuit that employs only diodes is called an uncontrolled
rectifier.
An
uncontrolled rectifier is a type of rectifier in which we cannot control or
regulate the DC output voltage unlike a controlled rectifier. Means an uncontrolled
rectifier converts AC input voltage into fixed DC voltage.
Diode
Rectifiers are the fundamental application of semiconductor diodes like signal
diode and power diode. For low power applications like in Analog Electronics or
Digital Electronics, signal diodes are used while for high power applications
like in Power Electronics power diodes are used.
Diode Bridge Rectifier Circuit Diagram
A diode bridge rectifier uses four diodes connected in a bridge rectifier configuration as shown in the given figure. The one side of the AC supply having voltage Vs = Vm.sin(ωt) is connected in between D1 and D4 at point a and other side of AC supply is connected in between D3 and D2 at point b and the load resistance connected across the terminal c and d having terminal voltage Vo and draws a current Io.
Diode
Bridge Rectifier Working
A full wave
rectifier circuit converts both half of AC input supply into DC voltage. For
the efficient conversion of both half of AC input supply, in a diode bridge
rectifier circuit, two diagonally opposite diodes conduct for positive half and
the other two diagonally opposite switches conduct for negative half of AC
input supply.
Let us
understand the working diode bridge full wave rectifier step by step.
For the
sake of simplicity, the diodes are considered to be ideal. By “ideal" we
mean that the reverse time and the forward voltage drop are negligible.
As we know,
the AC waveform is a sinusoidal waveform that changes its magnitude and
direction periodically as shown in figure
In the
given figure we see that, Vm is the peak value of AC, Vs
is the instantaneous value of the sinusoidal waveform and it has the
fundamental time period 2Ï€.
So,
When the positive half of AC
supply is given to the rectifier circuit, then point a is positive with respect
to b. In this condition diodes D1 and D2 are forward
biased and D3 and D4 are reverse biased. As D1
and D2 are forward biased they start conduction and behave like a
short circuit and D3, D4 does not allow conduction and
behave like open circuit. In that case the above circuit can be redrawn as
shown in the given figure.
In the above figure we see that
if we apply positive half of AC input then the load voltage Vo is
equal to positive half of applied voltage Vs.
When negative half of AC supply
is given then point b is positive with respect to a. In this condition diodes D3
and D4 are forward biased and diodes D1 and D2
are Reverse biased. As D3 and D4 are forward they start
conduction and behave like a short circuit and D1, D2
does not allow conduction and behave like open circuit. So, in that case the
above circuit can be redrawn as shown in the given figure and the direction of the flow of current is shown
by the red lines in the given figure.
In the
above figure, we see that for the negative half of AC supply the load voltage
Vo is equal to -VS.
Diode Bridge Rectifier Output Waveform
Above we
discussed that if the positive half of the AC input supply is given to the
rectifier, then the average output voltage Vo is equal to Vs.
And when
the negative of AC input supply is given then the average output voltage Vo is
equal to -Vs.
So, if we
combinedly observe these above statements then we will conclude that the output
voltage waveforms for the diode bridge rectifiers for the pure resistive load
shown in the given figure.
Diode Bridge Rectifier Formulas
Average Value of the Output Waveform / Average Output Voltage of Diode Bridge Rectifier
Average
value of the output waveform for the Diode Bridge Rectifier.
Vo = 1/T 0ʃT Vm.
sin(ωt).
Vo = 1/π 0 ʃπ Vm.
sin(ωt).
So, the average output current for resistive load is
Root Mean Square (R.M.S) Value for the output waveforms for Diode Bridge Full Wave Rectifier
Vo rms =
(1/T 0ʃT (Vm. sinwt)2
)1/2
Vo rms =
(1/π 0 ʃπ (Vm.
sinwt)2 )1/2
Input Power Factor of Diode Bridge Rectifier
Power
Factor is defined as the ratio of active power to apparent power supply to the
rectifier.
Apparent
power supplied to the rectifier Pac = Vs.rms .
Is.rms
Active
power of the rectifier Pdc = Vo rms . Io rms
As we discussed above
Vo
rms = Vs.rms
Io
rms = Is.rms
So,
the input power factor for diode bridge rectifier for the pure resistive load
is 1.
Note :-Unity power factor is not
possible practically, above power factor comes unity because assumption have
taken while doing this analysis.
Diode Bridge Rectifier with Capacitor Filter
If we observe the output voltage waveform of
the full bridge diode rectifier discussed above, then we will find that it is
not the constant waveform like a DC signal. However, it is desired that the
output voltage of the rectifier circuit is constant with no ripple in it. But
it is not true, the rectified Dc output voltage is pulsating in nature and
contains some harmonics.
So, to obtain a
smoother DC output waveform, a filter capacitor is connected parallel to the
load. This filter capacitance reduces the ripple in the output voltage waveform
by charging and discharging itself.
To get the understanding of how the filter capacitance reduces the
ripple in output voltage we are taking the help of Matlab simulation.
Given
figure shows the diode bridge rectifier circuit with filter capacitor using
Matlab simulation. In this Matlab simulink model we have taken Vm
(Peak Amplitude) for the input signal is 10V and the supply frequency is 50Hz.
To ease the understanding of the effect of filter capacitance in a diode
bridge rectifier we take the output waveform for three conditions.
In the first condition we take the output voltage waveform without
filter capacitance. If we observe the output voltage waveform of the rectifier
without filter capacitance then we will find that the output voltage goes to
zero at each fundamental time period i.e. 0.01.
In the second condition we take the output voltage waveform with 10mf
filter capacitance.
In this condition during 0 to 0.005 sec, while the rectified voltage
goes from 0 to its peak value the capacitor is charged up to the peak value of
the input voltage.
And during 0.005 to 0.01 sec, while the input voltage goes to 0 from its
peak value the capacitor discharges to the load. The discharging speed of the
capacitor is slower due to its time constant. So, the output voltage waveform
in this case goes to zero slowly as compared to the previous case.
But before going to zero or fully discharging the capacitor the second
pulse of the rectified output waveform again charges the capacitor to its peak
value. And this process of charging and discharging of capacitor repeats again
and again resulting in the output voltage waveform never going to zero. We can
observe this phenomenon in the output waveform of the given figure.
In the third condition we take the output voltage waveform with 50mf
filter capacitance.
In this condition, we increase the value of the capacitance from 10 mf
to 50 mf. The higher the value of the capacitance, the higher the time constant
of the capacitor. As we know that the time constant of the capacitor is T = RC.
This will result in a decrease in the discharging speed of the capacitor. Means
the capacitor takes more time to discharge. As result less ripple in output
waveform.
Above discussion can be concluded that the introduction of
the sufficiently large capacitor in the rectifier circuit results in a smoother
output waveform.
Advantages of Diode Bridge Full Wave Rectifier
There are certain advantages of using diode bridge full wave
rectifier over other rectifier circuits these are
- Diode bridge full-wave rectifiers are more efficient compared to half-wave rectifiers because they utilize both halves of the AC input waveform.
- Diode Bridge full wave rectifiers are economically cheaper as compared to other full wave rectifier circuits such as controlled rectifier and half controlled rectifier because these circuits use controlled switches such as SCR, MOSFET etc. which are a bit expensive.
- A diode bridge rectifier full wave circuit is preferably used over center tapped full wave rectifier circuit because center tapped rectifier requires a center tap transformer which makes this circuit bulkier and costlier. And diodes used in center tap rectifier circuits have high peak inverse voltage and high PIV diodes are costlier.
Applications of Diode Bridge Rectifier
It is
important to note that diode bridge full wave rectifiers are uncontrolled
rectifiers, means they lack the ability to control or regulate the DC output
voltage. Despite this drawback diode bridge full wave rectifiers are most
widely used in those applications of modern electronics where control of DC
output voltage is not necessary like in battery chargers for mobile, laptop, or
any other electronics equipment, in TVs, LED driver circuits etc.
Above
mentioned applications are low power applications so signal diodes are used in
such types of applications.
For high
power applications like in UPS, Household Inverters, Electric Welding Machines
power diodes are used in rectifier circuits.
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