Rectifiers
Introduction
The application of rectifier circuits is in the
conversion of AC to DC power. A circuit that
accomplishes this conversion is usually called a DC
power supply.
Many familiar electrical and electronic appliances
(e.g., radios, personal computers, TVs) require DC
power to operate. For most applications, it is
desirable that the DC supply to be as steady and
ripple-free as possible.
Lecture 30 2
DC power supply
Fig.1 Schematic diagram of a DC power supply
Lecture 30 3
Classification of Rectifiers
1) Half – Wave Rectifier
2) Full Wave Rectifier
Center-Tapped Full Wave Rectifier
Bridge Rectifier
Lecture 30 4
Half Wave Rectifier
A half wave rectifier allows one half-cycle of an AC voltage waveform to pass,
blocking the other half-cycle. Half-wave rectifiers are used to convert AC voltage
to pulsating voltage, and require a single diode to construct
Fig.1 Half-wave rectifier.
labelll
Fig.2 Conduction region (0 to T/2).
Lecture 30 5
Half-wave rectifier
Fig.3 Conduction region ( T/2 to T)
Fig.4 Half wa30e
Lecture 1 rectified signal. 6
Center tapped Full Wave Rectifier
A full-wave rectifier converts an ac voltage into a pulsating dc voltage using both half
cycles of the applied ac voltage.
A full-wave rectifier appears in Fig. 5 with two diodes but requiring a center-tapped
(CT) transformer to establish the input signal across each section of the secondary of
the transformer. The diodes feed a common load R with the help of a center-tap
transformer.
Fig.5 Center-tapped transformer full-wave rectifier.
Lecture 30 7
Center tapped Full Wave Rectifier
During the positive portion of Vi applied to the primary of the transformer, the network
will appear as shown in Fig. 6
D1 - Forward biased- short-circuit
D2 – Reverse biased- open-circuit
The output voltage appears as shown in Fig.6
Fig 6 Network conditions for the positive region of vi.
Lecture 30 8
Center tapped Full Wave Rectifier
During the negative portion of the input the network appears as shown in Fig.7
D1 – Reverse biased- open-circuit
D2 - Forward biased- short-circuit
Fig 7 Network conditions for the negative region of vi.
Lecture 30 9
Center tapped Full Wave Rectifier
Fig. 8 Input and output wave forms
Lecture 30 10
Bridge Rectifier
This type of full wave rectifier uses four diodes connected in a bridge
configuration to produce the desired output.
The main advantage of this bridge circuit is that it does not require a special
centre tapped transformer, thereby reducing its size and cost. Four diodes
labeled D1 to D4 are arranged as shown in Fig 9 with only two diodes
conducting current during each half cycle
Fig 9. Full Wave Bridge Rectifier
Lecture 30 11
Bridge Rectifier
During the positive half cycle of the supply, diodes D2 and D3conduct in
series while diodes D1 and D4 are reverse biased and the current flows
through the load as shown below
Fig.10 Conduction path for the positive region of vi.
Lecture 30 12
Bridge Rectifier
During the negative half cycle of the supply, diodes D1 and D4 conduct in
series but diodes D2 and D 3 switch “OFF” as they are reverse biased.
The current flowing through the load is the same direction as before.
Fig. 11 Conduction path for the negative region of vi
Lecture 30 13
Bridge Rectifier
Over one full cycle the input and output voltages will appear
as shown in Fig.12
Fig.12 Input and output waveforms for a full-wave rectifier
Lecture 30 14
Analysis of Full-Wave Rectifier
Peak Current
The value of peak current (Imax) can be derived with the help of instantaneous value of
applied voltage and the resistance of the diodes. The value of instantaneous voltage
applied to the rectifier circuit can be given as:-
Let’s assume the forward resistance - Rf , load resistor RL then the current flowing
through the load resistor can be given as:-
.
the total current i can be obtained by the sum of i1 and i2 for the whole cycle
Lecture 30 15
Analysis of Full-Wave Rectifier
Output Current
The current through the load is the same for both the cycles of the ac signal thus, the dc
output current can be given as
DC output voltage
The average dc voltage is given as
Lecture 30 16
Analysis of Full-Wave Rectifier
RMS Current
The rms current through the load RL is given as
Lecture 30 17
Analysis of Full-Wave Rectifier
RMS Voltage
The rms value of a voltage across the load is given as
• Form factor
• The form factor is the ratio of rms value to the dc output value of current. It
is given as
Lecture 30 18
Analysis of Full-Wave Rectifier
Peak factor
It is the ratio of the peak value of current to the rms value of current
Ripple factor
Lecture 30 19
Analysis of Full-Wave Rectifier
The peak inverse voltage (PIV) of the diode is the peak value of the voltage that a
diode can withstand when it is reversed biased .The peak inverse voltage of diode in
center tapped full wave rectifier is 2 Vsmax and Bridge rectifier is Vsmax.
Rectification Efficiency: The rectification efficiency of full wave rectifier can be
obtained by the ratio of dc power delivered to load and ac power present in the output
For bridge rectifier,
Lecture 30 20
Advantages of Full Wave Rectifiers
Advantages of Full Wave Rectifiers
• The rectification efficiency of full wave rectifier is much higher than that of
half wave rectifier. It is approximately double to that of half wave rectifier
i.e. it is about 81%.
• The filtering circuit required in full wave rectifier is simple because ripple
factor in the case of full wave rectifier is very low as compared to that of
half wave rectifier. The value of ripple factor in full wave rectifier is 0.482
while in half wave rectifier it is about 1.21.
• The output voltage and output power obtained in full wave rectifiers are
much more than that of full wave rectifiers.
Disadvantages of Full Wave Rectifiers
• The full wave rectifiers need more circuit elements than half wave rectifier
which makes it costlier
Lecture 30 21
Analysis of Half Wave Rectifier---Self study
Lecture 30 22
Thank you
Lecture 1 23
