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5-1
Diode Rectifiers
EE 442-642
5-2
Half-Bridge Rectifier Circuit: R and R-L Load
Current continues to flow for a while even after
the input voltage has gone negative.
5-3
Half Bridge Rectifier Circuit: Load with dc back-emf
• Current begins to flow when the input voltage exceeds the dc back-emf.
• Current continues to flows for a while even after the input voltage has gone
below the dc back-emf.
5-4
Full-wave Rectifier with Center tap Transformer
• The DC currents of the two half- wave rectifiers
are equal and opposite, Hence, there is no DC
current for creating a transformer core
saturation problem.
• Each diode carries half of the load average
current, but the same peak load current.
• Note that the VRRM rating of the diodes must be
chosen to be higher than 2Vm to avoid reverse
breakdown.
• The center-tap transformer is considered bulky
with additional losses.
5-5
Full Bridge Rectifier – Simple R Load
Average value of output voltage:
where Vs and Vm are the RMS and peak values of input voltage.
Rectification Ratio = Pdc/Pac = 0.81 or 81%
Form Factor (FF) of DC side voltage (or current) = Vrms/Vdc = 1.11
Ripple Factor = rms value of AC component/DC component = (FF2-1)1/2 = 0.48
mmssdo VVVVV 637.0)/2(9.0)/22(
5-6
Filters
• Filters are employed in rectifier circuits for smoothing out the dc output
voltage of the load.
• Ripple Reduction Factor at fr:
5-7
Inductor DC Filter
• Minimum value of inductance required to
maintain a continuous current is known as
the critical inductance LC:
• The choice of the input inductance depends
on the required ripple factor . Ripple voltage
of a rectifier without filtering:
• Considering only the lowest-order harmonic
(n = 2), the output ripple factor of a simple
inductor-input dc filter is
5-8
Full Bridge Rectifier with dc-side Voltage
Zero current corresponds to
dc voltage equal to the peak
of the input ac voltage
Average value of DC-side current
(obtained numerically):
5-9
Full Bridge Rectifier – Simple Constant Load Current
9.0
1
%43.48]1)8/([100
,...7,5,3,/
9.0)/22(
2
1
1
PF
DPF
THD
hhII
III
II
ssh
dds
ds
RSM value of source current
RMS value of fundamental current
RMS value of harmonic current
Current THD
Displacement Power Factor
Power Factor
5-10
Full Bridge Rectifier – Simple Constant Load Current
(AC-side filtering)
RMS value of harmonic source current
THD of Source Current:
5-11
Diode-Rectifier with a Capacitor Filter
• Approximate value of peak-to-peak ripple:
• Average output voltage:
• RMS value of output ripple voltage:
• Ripple Factor:
• Inrush resistance Rinrush is sometimes needed
to limit the initial inrush current to a value
below that of the diodes. It is usually placed
on the DC side, then shorted out afterwards.
• In many cases, Rinrush is not needed if the
Equivalent Series Resistance of the capacitor
and cable/transformer wire resistance are
sufficiently large.
5-12
Diode-Rectifier Bridge with AC-Side Inductance
Commutation angle:
Average of DC-side voltage:
dssd
s
ds
ILVV
V
IL
29.0
2
21cos
5-13
Voltage Distortion at PCC
• PCC is the point of common coupling
• Distorted current flow results in distorted voltage
5-14
Dual Voltage Rectifier
• In 115-V position, one capacitor at-a-time is charged from the input.
5-15
Three-Phase, Four-Wire System
• A common neutral wire is assumed
• The current in the neutral wire is composed mainly of the
third harmonic and can be higher than the phase currents
5-16
3-Phase Rectifier Circuit
• Star Connection: direct currents in the
secondary windings cause transformer
core saturation problem.
• The transformer core saturation
problem in the three-phase star rectifier
can be avoided by zig-zag (or inter-
star) connection.
• Three-phase Bridge (or 6-pulse)
Rectifier – most commonly used in
industry applications.
5-17
Three-Phase Bridge Rectifier with R Load
• Average value of DC-side
voltage:
where Vm is the peak value of
the phase voltage.
• RMS value of DC-side voltage:
mmdo VVV 654.133
mmrms VVV 655.14
39
2
3
5-18
Six-Phase Bridge Rectifier (12-Pulse)
• Average and RMS values of
DC-side voltage:
where Vm is the peak value of
the phase voltage.
mrmsdo VVV 712.1
5-19
Three-Phase Full-Bridge Rectifier with DC Current
5-20
Three-Phase, Full-Bridge Rectifier with DC Current
RSM value of source current
RMS value of fundamental current
RMS value of harmonic current
Current THD
Displacement Power Factor
Power Factor
955.0
3
1
%31]1)9/([100
,...7,5,3,/
78.0)/6(
816.03/2
2
1
1
PF
DPF
THD
hhII
III
III
ssh
dds
dds
5-21
3-Phase, Full-Bridge Rectifier with Inductance
Commutation angle:
Average of DC-side voltage:
dsLLd
LL
ds
ILVV
V
IL
335.1
2
21cos
5-22
3-Phase Rectifier with DC Source
5-23
Three-Phase Rectifier with Capacitor Filter
• PSpice-based analysis
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