device descriptions
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Overview of electric drivesConverter
Types of power electronic switches and their symbols
Diodes
Thyristors or SCRs
BJTs
MOSFETsIGBTs
GTOs
IGCTs
Other devices
Summary of Capabilities
Presentation summary
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Overview of electrical drives
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Converter
As the figure above suggests, a converter is a combination of power electronic
switches, inductors, capacitors, resistors (as snubbers) and transformers.
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Types of power electronic switches
Note:Most of these switches carry current only in one direction.
However in some switches (eg. MOSFET) there may be a diode to carry currentin the opposite direction.Most of the controlled switches are normally off.
However, there may be some switches (eg. Static induction transistor SIT)
that may be normally on.
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Device symbols
General symbol of a switch
Diode A K
A K
G
T2T1
G
C
E
B
TRIAC
SCR
BJT(NPN)
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Device Symbols (Continued)
D
S
GMOSFET (N- channel)C
E
GIGBT
A K
G
GTO
Note: AAnode, B- Base, C-Collector, D-Drain , G-Gate ,E-Emitter, KCathode, S-
Source, T1-Terminal 1, T2- Terminal 2
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Diodes
iD
vD
infinity
infinity
Ideal Characteristics
iD
vD
Reverse
blocking
region
Vrated
VFForward
Voltage
drop
Actual Characteristics
A K
Symbol
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Different types of diodes
1. Schottky diodes
They have low forward voltage drop ( 0.3 V).
They are used in low voltage high current circuits.
They have low reverse voltage capabilities (50100 V).
They are also very fast switching type of diodes with very less reverse
recovery time (trr).
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2. Fast/ Ultra-fast/ Hyper fast recovery diodes
These diodes have very low recovery time, typically 30ns trr 1 s.
Thus these diodes are used in high frequency circuits.
Different types of diodes (2)
3. Line frequency diodes
These diodes have larger recovery time trr 100 s.
They are available with large current/ voltage ratings (several kAs and kVs).
They are typically used for line frequency (50, 60 Hz) rectifiers.
Large on-state voltage drops up to 3 volts.
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Reverse recovery time of a diode
iD
trr(reverse recovery time)
In the circuit shown above (left) ,when S is turned on the current through D falls in
the manner shown (above, right). The reverse recovery time trr depends upon the speedat which the free electrons at the diode p-n junction is swept away.
Vbus
ID
S
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Thyristor or SCR (Silicon Controlled
Rectifier)
P1
P2
N1
N2
DC
K
A
L
O
A
D
DC
G
VG
Equivalent SCR structure
A=Anode, G=Gate, K= Cathode
A K
G
Symbol
( )
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SCR (2)They have four layer PNPN junctions.
Can be turned on by
(i) Applying +VGK(continuous or pulsed)
(ii)Light activation (very good for isolation of driver circuit from
power circuit)
(iii) Applying +VAK VBO
(iii) Applying large dVAK/dt
(iv) With high junction operating temperature.
To start conduction, SCR anode cathode current has to go above IL
(latching current) before the gate pulse can be removed. Otherwise
SCR stops conduction.
Once latched, the SCR current has to be maintained above IH
(holding current). Otherwise SCR stops conducting.
IH < IL.
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SCR Characteristics
G
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Commutation of SCRs
Commutation means stoppage of current conduction.
SCRs cannot be commutated by just removing the gate drive.
It can only be stopped from conduction by applying a negative voltage
across anode and cathode for a specified period of time (tq). This is called voltage/ line
commutation.
Any positive voltage reapplied within tq (recovery time) may cause the SCR
to start conduction again.
By forcing the current in SCR to go to zero. As current falls below holding current (IH)SCR switches off. This is called current/ load / forced commutation. This normally
requires an additional SCR, inductor and capacitor.
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Examples of SCR Commutation
(i) Example : Line Commutation (Halfwave controlled rectified DC drive)
(ii) Example : Load Commutation (Series inverter)
Note: R,L,C form an under-damped
circuit
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di/dt & dv/dt limitations for SCRsAt turn on, localized hot spots near gate connection may destroy an SCR unless di/dt
(rate of rise of anode-cathode current) is restricted. This can be achieved by including
an inductor in series with a SCR.High values of dVAK/dt can trigger SCR even in the absence of gate current.
Example:
. SCR is represented by a resistor RAKwhen it is off. Just after switch sw is closeddVAK/dt @ t =0 is equal to V/ (see below), where is the time constant. = L/RAK.Remedy: Connect Rs, Cs across SCR. Typical values of Rs, Cs (15 , 0.1 F).
V
RAK
RS CS
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Bipolar Junction Transistors (BJTs) and
Power DarlingtonsThe BJT is operated either in cut off or in saturation region.
To operate the device in saturation region, IB> IC/hFE where hFE is the DC
current gain of the device (hFE = 5 to 10 in saturation region for BJT).
Low hFE implies high base current requirements. Hence BJTs are not usedextensively for high power level switches.
Instead Darlington transistor is used reduce drive power (base current)rs are
used.
hFE of a Darlington transistor is around 75 to 100.
Typical operating frequency is between 1- 10 kHz.
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BJT and Darlington symbols and
CharacteristicsC
E
B
C
E
B B
C
E
NPN PNP Darlington
NPN CharacteristicsIdeal BJT Charatersistics
iC
vCE
iB4
iB1
iB2
iB3
vCE(sat)
IC
iB1
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Reverse or Forward Bias Safe Operating
Area
Under both transient (switch on & off) and steady state operating conditions the power
dissipation of the device has to be within the safe operating area. The limits of the safe
operating areas are imposed by
ICM (maximum collector current)
TJ max (maximum junction temperature)
Second breakdown (only in BJTs)
BVCEO (maximum collector emitter breakdown voltage)
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Second Breakdown
Second breakdown : It is the thermal runaway caused by localized hot spots
due to current crowding. This is because a BJT is a minority carrier device
having a negative temperature coefficient of resistance. Increased current will
thus cause higher temperature leading to lower resistance and hence more
current.
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Power MOSFETs
Comes with intrinsic body diode (Figure above, left) which is not very fast.
Can be replaced by a faster diode (Figure above, right) in the following way.
Recently some MOSFETs are available with fast body diode.
N Channel MOSFET
with body diode
Body diode bypassedP Channel MOSFET
with body diode
D
S
G
D
S
G
D
S
G
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MOSFET Characteristics
iD
vDS
infinity
infinity
Ideal MOSFET
Characteristics N-Channel MOSFET Characteristics
Note: BVDSS is forward breakdown voltage
iD
vGSvGS(th)
iD
vDS
vGS4
vGS3
vGS2
vGS1
vGS1< vGS2
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Other MOSFET Characteristics
Operating frequency 1-10 MHz
Rds(on) : The on state channel resistance between Drain and Source. This
is a very important parameter.
Rds(on) K.(BVDSS)2.5-2.7. Thus higher the higher the breakdown voltage
higher is Rds(on) .
Rds(on) is usually specified at room temperature. It can increase 2-3 times
once the device is heated up.
Recent development (COOLMOS) has higher voltage but lower Rds(on) .
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Insulated Gate Bipolar Transistor (IGBT)
IGBT is a hybrid device.
Has a comparable on-state conduction loss as a BJT.
Can switch faster than a BJT but is slower than a MOSFET.
Initially IGBTs used to have latch up problems due to a parasitic thyristor
(top fig. right) existing due to the nature of the IGBT construction
(M1, T1 are responsible for normal operation. T1, T2 cause latch up).
Once latched, gate control is lost. Then the only way to stop IGBT from conduction
was to commutate the device like an SCR.
D
S
G
M1
T1
T2
IGBT symbol Latch-up caused by parasitic thyristor
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Actual IGBT Characteristics
iC
vGEvGE(th)
Gate-emitter CharacteristicsCollector-emitter Characteristics
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Other Devices
1) Gate Turn-off Thyristor (GTO):
A K
G
The device has no reverse blocking capability.
The device can be turned-off by applying large negative gate current.Maximum controllable anode current: Maximum value of device current with
which the device can be successfully turned-off. This value is less than the
maximum current a GTO can carry.
2) Integrated Gate Commutated Thyristor (IGCT):
Similar to GTO with much less gate drive requirement to turn off the device.
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Other devices (2)
3) Power junction field effect transistor or JFET or static induction transistor (SIT). They
are not very popular as they are normally ON devices.
4) Field Controlled Thyristor (FCT) is also a normally ON device.
5) Mos Controlled Thyristor (MCT) failed device.
Summary of power electronic device
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Summary of power electronic device
capabilities
Ref: Mohan, Undeland and Robbins.Power Electronics, 2nd ed., 1995, John Wiley)
Note: Even though MCT (MOS Controlled Thyristor) had been predicted as the
device of the future by the arrow and the dotted box, the technology never matured.
Instead, IGCT (Insulated Gate Commutated Thyristor) technology seems more promising.
Check the following website for more details on this device:
http://www05.abb.com/global/scot/scot232.nsf/veritydisplay/
ae8a8e1d424af97ec12576c40053ca34/$file/igct.pdf
http://www05.abb.com/global/scot/scot232.nsf/veritydisplay/http://www05.abb.com/global/scot/scot232.nsf/veritydisplay/
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