Transcript Diode Circuits or Uncontrolled Rectifier

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Chapter 1
Review of Power
Electronics Converters
• Power electronics refers to control and conversion of
electrical power by power semiconductor devices wherein
these devices operate as switches.
• "Electronic power converter" is the term that is used to refer
to a power electronic circuit that converts voltage and
current from one form to another.
• Rectifier converting an AC voltage to a DC voltage,
• Inverter converting a DC voltage to an AC
voltage,
• Chopper or a switch-mode power supply that converts a DC
voltage to another DC.
• Cycloconverter and cycloinverter converting an AC voltage
to another AC voltage.
Harmonics
• It can be defined as a sinusoidal component of a periodic waves or quality
having frequencies that are an integral multiple of the fundamental frequency.
f ( t )  a0 
a0 
1
2
2


 an cos(nt )  bn sin( nt ) 
n 1
f (t ) dt
1
2
an 
 0
an 
1
0

f (t ) sin ( nt ) dt
 0
1
bn 
a0  an  0
1
bn 
2

f (t ) cos (nt ) dt bn   f (t ) sin (nt ) dt
 
 
For Odd Functions:
For Even Functions: a0 
f (t ) cos ( nt ) dt
1

f (t ) dt
 0
an 
2
2
f (t ) sin (nt ) dt
 0
f (t ) cos (nt ) dt

0
bn  0
f(x)
f (t )   f ( t )
L
Odd Function
x
-L
L
f(x)
2
f (t )  f ( t )
Even Function
x
-2
2
f(x)
Not Odd or
Even Function
x


• The amount of distortion in the voltage or current waveform is qualified by
means of a Total Harmonic Distortion (THD).
Vsn2

 I sn2
2
2
2
2
V V
THDi  100 *
•
I s  I s1
 100 *
I s1
n 1
I s1
THDv  100 *
s
s1
Vs1
 100 *
n n
Vs1
Table (1.1) IEEE 519-1992 current distortion limits for general distribution systems (120 to 69kV) the maximum
harmonic current distortion in percent of I L
Individual Harmonic order (Odd Harmonics)
I SC / I L
35<n
% THD
0.6
0.3
5.0
2.5
1.0
0.5
8.0
4.5
4.0
1.5
0.7
12.0
12.0
5.5
5.0
2.0
1.0
15.0
15.0
7.0
6.0
2.5
1.4
20.0
n<11
11< n<17
17< n<23 23< n<35
<20
4.0
2.0
1.5
20<50
7.0
3.5
50<100
10.0
100<1000
>1000
Bus voltage at PCC
Individual voltage distortion (%)
THDv(%)
69 kV and blow
3.0
5.0
69.001 kV through 161kV
1.5
2.5
161.001kV and above
1
1.5
Semiconductors Switch types
• Diode
Forward voltage drop
=0.7 Volts for Silicon
= 0.4 Volts for Germanium
Fig.1.1 The diode iv characteristics
Peak Inverse voltage (PIV): Is the maximum voltage that a diode can
withstand only so much voltage before it breaks down.
Maximum Average Current: Is the average current that the diode can carry.
Thyristor
Invented in 1957
consists of four layers of
semiconductor materials
VJ
(p-n-p-n)
1AK
32
Gate Pulse
J1
J2
J3
State
+ve
+ve
Forward
Forward
Forward
ON
+ve
0
Forward
Reverse
Reverse
OFF
+ve
-ve
Forward
Reverse
Reverse
OFF
-ve
+ve
Reverse
Forward
Reverse
OFF
-ve
0
Reverse
Reverse
Reverse
OFF
-ve
-ve
Reverse
Reverse
Reverse
OFF
Fig.1.2
Thyristor types:
•
•
•
•
•
•
Phase controlled thyristor(SCR)
Fast switching thyristor (SCR)
Gate-turn-off thyristor (GTO)
Bidirectional triode thyristor (TRIAC)
DIAC
Light activated silicon-controlled rectifier
(LASCR)
Gate Turn Off thyristor (GTO).
• A GTO thyristor can be turned on by a single
pulse of positive gate current like conventional
thyristor
• It can be turned off by a pulse of negative gate
current.
The GTO has the following advantage over
thyristor:
• Elimination of commutating components in
forced commutation resulting in reduction in
cost, weight and volume,
•
Reduction in acoustic and electromagnetic
noise due to the elimination of commutation
chokes,
•
Faster turn OFF permitting high switching
frequency,
•
Improved converters efficiency, and,
•
It has more di/dt rating at turn ON.
The thyristor has the following
advantage over GTO.
•
ON state voltage drop and associated losses
are higher in GTO than thyristor,
•
Triggering gate current required for GTOs is
more than those of thyristor,
•
Latching and holding current is more in GTO
than those of thyristor,
•
Gate drive circuit loss is more than those of
thyristor, and,
•
Its reverse voltage block capability is less than
its forward blocking capability.
Bi-Directional thyristor (TRIAC).
• TRIAC is used for the
control of power in AC
circuits.
• It is equivalent of two
reverse parallelconnected SCRs with
one common gate.
• Conduction can be
achieved in either
direction with an
appropriate gate current.
it is thus a bi-directional
gate controlled thyristorFig.1.4 The electric circuit symbols of each type of thyristors.
with three terminals.
Fig.1.6 Operating characteristics of TRIAC.
DIAC
• DIAC is like a TRIAC
without a gate terminal.
• DIAC conducts current in
both directions
depending on the voltage
connected to its
terminals.
• When the voltage
between the two
terminals greater than
the break down voltage,
the DIAC conducts and
the current goes in the
direction from the higher
voltage point to the lower
voltage one.
• The DIAC used in firing
circuits of thyristors since
its breakdown voltage
used to determine the
firing angle of the
thyristor.
Current and future power semiconductor
devices development direction.
Bipolar Junction Transistor (BJT)
MOSFETs
1.7 General Power Semiconductor Switch Requirements
1. Maximum current carrying capability.
2. Maximum voltage blocking capability.
3. Forward voltage drop during ON and its temperature dependency
4. Leakage current during OFF
5. Thermal capability
6. Switching transition times during both turn-on and turn-off
7. Capability to stand dV/dt when the switch is OFF or during turn-off
8. Capability to stand dI/dt when the switch is ON or during turn-on
9. Controllable dI/dt or dV/dt capability during switching transition
10. Ability to withstand both high current and voltage simultaneously
11. Switching losses
12. Control power requirement and control circuit complexity
.
Insulated Gate Bipolar Transistors
The IGBT fundamentally changes the BJT current control into voltage
control while maintaining the advantages of the BJT.
The MOSFET has excellent dynamic and static performance. It dominates low
voltage applications below 600 V. The IGBT is slower than the MOSFET but has
better forward voltage drop above 600 V. It dominates applications from 600 to
3000 V.