Notes on INDUSTRIAL AUTOMATION
Module I
Introduction to Power Electronics
• Power electronics deals with controlling of flow of current and voltage in any electric system.
• It converts electric power to a form desired by user load.
Applications
Converters, Inverters, Battery charges, UPS, SMPS, HVDC
transmission, Cyclo-converters, Industrial motor drives,
Electric Vehicles etc.
Need for power Electronic Devices
• Higher Voltage Ratings
• Higher Current Rating
Examples for power electronic devices
Power-Diodes, Power-MOSFET, Power-BJT, IGBT, GTO, SCR etc.
Power Diode is uncontrollable…it turns ON if it is forward biased
P-MOSFET (Power MOSFET)
MOSFET is Metal-oxide semiconductor field effect transistor P-MOSFET
Construction of P-MOSFET
• It has a n+ substrate (heavily dopped n layer)
• The lightly dopped n- region increases voltage rating
• The p region is epitaxially grown
• To the p region n+ layers are diffused
• These n+ layers act as Source (S) terminal
• Gate (G) is placed above n-,p and n+ regions separated by
SiO2.
Working of P-MOSFET
When Drain (D) to Source (S) is forward biased by giving
supply VDD, the n- to p layer gets reverse biased so device wont
conduct. When Gate (G) is supplied with positive voltage, electrons in
p layer creates a field between n- and n+ layers. Thus, effectively the
device acts like a pure n material semi-conductor. So, it is called n-
channel MOSFET.
Characteristics of P-MOSFET
P-MOSFET has Transfer Characteristics (ID v/s VGS) and Output Characteristics (ID v/s VDS). As VGS given to gate is
increased it is easier for device to turn ON. Ohmic, Saturation and cut off regions are shown. Once device is ON gate
current ID depends on Gate voltage VGS and not on supply voltage VDS.
• MOSFET is a voltage-
controlled device
• Drain current depends
on gate supply voltage
• Minimum gate voltage
needed for a MOSFET
to start conduction is
known as threshold
voltage (VT)
• Maximum VDS which
can be given without
avalanche breakdown
is called break down
voltage.
,IGBT (Insulated Gate Bipolar Transistor)
MOSFET has high switching speed but it has high conduction loss and low power rating. A Bipolar junction
transistor (BJT) on the other hand has less conduction loss and high power rating. The goodness of both these devices
are combined to get IGBT.
Construction of IGBT
• IGBT has an injecting layer p+
• Buffer layer n+
• Drift layer n-
• Body layer p
• Layer of n+ is diffused to p
• Emitter(E) which is common terminal connected
to this n+ and p layers.
• Gate(G) is insulated with SiO2 and is connected
to n-, p and n+ layers.
• Third terminal Collector (C) is for supply voltage.
Working of IGBT
When supply VCC is given between C and E terminal
with positive on C and negative on E, the junctions J1 and
J3 gets forward biased, but junction J2 is reverse biased so
there is no conduction between drift layer to body layer.
Now if a positive voltage is given to Gate due to
capacitance effect an electron field is formed in p-layer
near Gate. This is similar to P-MOSFET.
Since the p region has electron field, the device now acts as a normal
P-N junction diode which is forward biased. Now the device start conduction.
Characteristics of IGBT
There are two types of characteristics for IGBT
1. Transfer Characteristics (IC v/s VGE)
2. Output Characteristics (IC v/s VCE)
Transfer Characteristics
When a supply
voltage VCC is given, to start
conduction and produce the
collector current a positive
gate voltage should be
given. The gate to emitter
voltage must be above
certain value called
threshold voltage (VGET).
Above threshold voltage
the device starts
conducting.
Output Characteristics
Output voltage of
the device is VCE. As VCE is
increased there is no conduction till gate voltage is given. When
gate voltage is given device starts conducting and the output current depends on magnitude of gate voltage given.
Since collector current is controlled by gate voltage, IGBT is called a voltage controlled device. The cut off, active and
saturation region are similar to that of P-MOSFET.
• IGBT has faster switching speed (turn on turn off speed) than P-BJT but is slower than P-MOSFET
• IGBT has lesser conduction losses than P-MOSFET but is not as good as P-BJT
• IGBT can be used for high power application
,SCR (SILICON CONTROLLED RECTIFIER)
Bell labs invented SCR. SCR is a 4 layered device made of P-
N-P-N junctions. SCR is a latching type device that means once it is
turned ON, it stays ON till we turn it OFF. Turn ON process of SCR is
called firing and turn OFF process is called commutation of SCR.
Hence it is a semi-controlled device.
Construction of SCR
SCR is made of PNPN layer. Outer P layer is called Anode (A)
and Outer N layer is called cathode (K). The inner P layer acts as
Gate (G).
WORKING OF SCR
1. Reverse Blocking Mode
When Supply is connected such that –ve is given to anode and +ve to
cathode, it is called reverse biased. Junction J1 and J3 are reverse biased.
Junction J2 is forward biased. The device wont conduct. If voltage given is
above ‘reverse break down voltage’ then the device breaks down.
2. Forward Blocking Mode
When Supply is connected such that +ve is given to anode and -ve to
cathode, it is called reverse biased. Junction J1 and J3 are forward biased.
But Junction J2 is reverse biased. Potential barrier gets formed at J2
junction. The device wont conduct. If voltage given is above forward break
over voltage the device conducts
3. Forward Conduction Mode
When in Forward blocking mode if we give a positive voltage to Gate with
respect to cathode, the potential barrier gets reduced and J2 starts
conducting. This method is known as gate triggering of SCR and mostly
used method of triggering SCR.
Characteristics of SCR
As magnitude of Gate current is increased in
forward blocking mode, it is easier for SCR to
turn on. Hence SCR is current controlled device
Latching Current
Once SCR is turned ON, it remains ON. Gate can now be stopped. But a minimum anode current should start
to flow through SCR before withdrawing gate pulse to ensure successful turn ON of SCR is called latching current. If
not latched, SCR turns off as soon as gate pulse is turned OFF. Time duration of Gate pulse that should be given
depends on Latching current of SCR.
Holding Current
While turning OFF SCR, anode current should reduce below a certain level known as holding current, then
only SCR can be turned OFF. Latching current is always lesser than holding current. I L = (2 to 3 times) IH.
Summary
SCR is a Bipolar switch. It is Unidirectional. SCR is a minority carrier device. SCR has highest rating (10KV, 3KA) and
lowest speed. Once Latched to ON position SCR does not turns OFF. So, Gate supply can be turned OFF after SCR gets
ON. So Gate current of SCR is given as pulse. To Latch to ON state anode current should be above Latching Current To
turn OFF SCR anode current should be below holding current.
, TWO TRANSISTOR ANALOGY OF SCR
SCR can be viewed as two BJTs connected as shown in figure below
In a BJT
𝐼𝐶 = 𝐶𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝐼𝐵 = 𝐵𝑎𝑠𝑒 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝐼𝐸 = 𝐸𝑚𝑖𝑡𝑡𝑒𝑟 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝛼 = 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝐺𝑎𝑖𝑛 = 𝐼𝐶 / 𝐼𝐸
𝐴𝑙𝑠𝑜, 𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
hence 𝑰𝒌 = 𝑰𝒂 + 𝑰g
The two-transistor equivalent circuit shows that the collector current of the NPN transistor Q2 is directly from
the base of the PNP transistor Q1, while the collector current of Q1 feeds into the base of Q2. These two inter-
connected transistors rely upon each other for conduction as each transistor gets its base current from the other’s
collector current. So, until one of the transistors is given some base current nothing can happen even if an Anode-to-
Cathode voltage is present.
When the thyristors Anode terminal is negative with respect to the Cathode, the center N-P junction is
forward biased, but the two outer P-N junctions are reversed biased and it behaves very much like an ordinary diode.
Therefore, a thyristor blocks the flow of reverse current until at some high voltage level the breakdown voltage point
of the two outer junctions is exceeded and the thyristor conducts without the application of a Gate signal.
If the Anode terminal is made positive with respect to the Cathode, the two outer P-N junctions are now
forward biased but the center N-P junction is reverse biased. Therefore, forward current is also blocked. If a positive
current is injected into the base of the NPN transistor Q2, the resulting collector current flows in the base of
transistor Q1. This in turn causes a collector current to flow in the PNP transistor, Q1 which increases the base
current of Q2 and so on.
SCR TRIGGERING METHODS
The process of turning ON thyristor is called Triggering. Firing is process of giving signal to trigger SCR. Firing angle is
angle measured from the instant SCR is forward biased to instant SCR is fired. Methods to trigger SCR are:
1. Forward Voltage Triggering
2. Gate Triggering
3. dv/dt Triggering
4. Temperature Triggering
5. Light Triggering
Methods of Gate Triggering – R, RC & UJT Triggering
Module I
Introduction to Power Electronics
• Power electronics deals with controlling of flow of current and voltage in any electric system.
• It converts electric power to a form desired by user load.
Applications
Converters, Inverters, Battery charges, UPS, SMPS, HVDC
transmission, Cyclo-converters, Industrial motor drives,
Electric Vehicles etc.
Need for power Electronic Devices
• Higher Voltage Ratings
• Higher Current Rating
Examples for power electronic devices
Power-Diodes, Power-MOSFET, Power-BJT, IGBT, GTO, SCR etc.
Power Diode is uncontrollable…it turns ON if it is forward biased
P-MOSFET (Power MOSFET)
MOSFET is Metal-oxide semiconductor field effect transistor P-MOSFET
Construction of P-MOSFET
• It has a n+ substrate (heavily dopped n layer)
• The lightly dopped n- region increases voltage rating
• The p region is epitaxially grown
• To the p region n+ layers are diffused
• These n+ layers act as Source (S) terminal
• Gate (G) is placed above n-,p and n+ regions separated by
SiO2.
Working of P-MOSFET
When Drain (D) to Source (S) is forward biased by giving
supply VDD, the n- to p layer gets reverse biased so device wont
conduct. When Gate (G) is supplied with positive voltage, electrons in
p layer creates a field between n- and n+ layers. Thus, effectively the
device acts like a pure n material semi-conductor. So, it is called n-
channel MOSFET.
Characteristics of P-MOSFET
P-MOSFET has Transfer Characteristics (ID v/s VGS) and Output Characteristics (ID v/s VDS). As VGS given to gate is
increased it is easier for device to turn ON. Ohmic, Saturation and cut off regions are shown. Once device is ON gate
current ID depends on Gate voltage VGS and not on supply voltage VDS.
• MOSFET is a voltage-
controlled device
• Drain current depends
on gate supply voltage
• Minimum gate voltage
needed for a MOSFET
to start conduction is
known as threshold
voltage (VT)
• Maximum VDS which
can be given without
avalanche breakdown
is called break down
voltage.
,IGBT (Insulated Gate Bipolar Transistor)
MOSFET has high switching speed but it has high conduction loss and low power rating. A Bipolar junction
transistor (BJT) on the other hand has less conduction loss and high power rating. The goodness of both these devices
are combined to get IGBT.
Construction of IGBT
• IGBT has an injecting layer p+
• Buffer layer n+
• Drift layer n-
• Body layer p
• Layer of n+ is diffused to p
• Emitter(E) which is common terminal connected
to this n+ and p layers.
• Gate(G) is insulated with SiO2 and is connected
to n-, p and n+ layers.
• Third terminal Collector (C) is for supply voltage.
Working of IGBT
When supply VCC is given between C and E terminal
with positive on C and negative on E, the junctions J1 and
J3 gets forward biased, but junction J2 is reverse biased so
there is no conduction between drift layer to body layer.
Now if a positive voltage is given to Gate due to
capacitance effect an electron field is formed in p-layer
near Gate. This is similar to P-MOSFET.
Since the p region has electron field, the device now acts as a normal
P-N junction diode which is forward biased. Now the device start conduction.
Characteristics of IGBT
There are two types of characteristics for IGBT
1. Transfer Characteristics (IC v/s VGE)
2. Output Characteristics (IC v/s VCE)
Transfer Characteristics
When a supply
voltage VCC is given, to start
conduction and produce the
collector current a positive
gate voltage should be
given. The gate to emitter
voltage must be above
certain value called
threshold voltage (VGET).
Above threshold voltage
the device starts
conducting.
Output Characteristics
Output voltage of
the device is VCE. As VCE is
increased there is no conduction till gate voltage is given. When
gate voltage is given device starts conducting and the output current depends on magnitude of gate voltage given.
Since collector current is controlled by gate voltage, IGBT is called a voltage controlled device. The cut off, active and
saturation region are similar to that of P-MOSFET.
• IGBT has faster switching speed (turn on turn off speed) than P-BJT but is slower than P-MOSFET
• IGBT has lesser conduction losses than P-MOSFET but is not as good as P-BJT
• IGBT can be used for high power application
,SCR (SILICON CONTROLLED RECTIFIER)
Bell labs invented SCR. SCR is a 4 layered device made of P-
N-P-N junctions. SCR is a latching type device that means once it is
turned ON, it stays ON till we turn it OFF. Turn ON process of SCR is
called firing and turn OFF process is called commutation of SCR.
Hence it is a semi-controlled device.
Construction of SCR
SCR is made of PNPN layer. Outer P layer is called Anode (A)
and Outer N layer is called cathode (K). The inner P layer acts as
Gate (G).
WORKING OF SCR
1. Reverse Blocking Mode
When Supply is connected such that –ve is given to anode and +ve to
cathode, it is called reverse biased. Junction J1 and J3 are reverse biased.
Junction J2 is forward biased. The device wont conduct. If voltage given is
above ‘reverse break down voltage’ then the device breaks down.
2. Forward Blocking Mode
When Supply is connected such that +ve is given to anode and -ve to
cathode, it is called reverse biased. Junction J1 and J3 are forward biased.
But Junction J2 is reverse biased. Potential barrier gets formed at J2
junction. The device wont conduct. If voltage given is above forward break
over voltage the device conducts
3. Forward Conduction Mode
When in Forward blocking mode if we give a positive voltage to Gate with
respect to cathode, the potential barrier gets reduced and J2 starts
conducting. This method is known as gate triggering of SCR and mostly
used method of triggering SCR.
Characteristics of SCR
As magnitude of Gate current is increased in
forward blocking mode, it is easier for SCR to
turn on. Hence SCR is current controlled device
Latching Current
Once SCR is turned ON, it remains ON. Gate can now be stopped. But a minimum anode current should start
to flow through SCR before withdrawing gate pulse to ensure successful turn ON of SCR is called latching current. If
not latched, SCR turns off as soon as gate pulse is turned OFF. Time duration of Gate pulse that should be given
depends on Latching current of SCR.
Holding Current
While turning OFF SCR, anode current should reduce below a certain level known as holding current, then
only SCR can be turned OFF. Latching current is always lesser than holding current. I L = (2 to 3 times) IH.
Summary
SCR is a Bipolar switch. It is Unidirectional. SCR is a minority carrier device. SCR has highest rating (10KV, 3KA) and
lowest speed. Once Latched to ON position SCR does not turns OFF. So, Gate supply can be turned OFF after SCR gets
ON. So Gate current of SCR is given as pulse. To Latch to ON state anode current should be above Latching Current To
turn OFF SCR anode current should be below holding current.
, TWO TRANSISTOR ANALOGY OF SCR
SCR can be viewed as two BJTs connected as shown in figure below
In a BJT
𝐼𝐶 = 𝐶𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝐼𝐵 = 𝐵𝑎𝑠𝑒 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝐼𝐸 = 𝐸𝑚𝑖𝑡𝑡𝑒𝑟 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝛼 = 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝐺𝑎𝑖𝑛 = 𝐼𝐶 / 𝐼𝐸
𝐴𝑙𝑠𝑜, 𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
hence 𝑰𝒌 = 𝑰𝒂 + 𝑰g
The two-transistor equivalent circuit shows that the collector current of the NPN transistor Q2 is directly from
the base of the PNP transistor Q1, while the collector current of Q1 feeds into the base of Q2. These two inter-
connected transistors rely upon each other for conduction as each transistor gets its base current from the other’s
collector current. So, until one of the transistors is given some base current nothing can happen even if an Anode-to-
Cathode voltage is present.
When the thyristors Anode terminal is negative with respect to the Cathode, the center N-P junction is
forward biased, but the two outer P-N junctions are reversed biased and it behaves very much like an ordinary diode.
Therefore, a thyristor blocks the flow of reverse current until at some high voltage level the breakdown voltage point
of the two outer junctions is exceeded and the thyristor conducts without the application of a Gate signal.
If the Anode terminal is made positive with respect to the Cathode, the two outer P-N junctions are now
forward biased but the center N-P junction is reverse biased. Therefore, forward current is also blocked. If a positive
current is injected into the base of the NPN transistor Q2, the resulting collector current flows in the base of
transistor Q1. This in turn causes a collector current to flow in the PNP transistor, Q1 which increases the base
current of Q2 and so on.
SCR TRIGGERING METHODS
The process of turning ON thyristor is called Triggering. Firing is process of giving signal to trigger SCR. Firing angle is
angle measured from the instant SCR is forward biased to instant SCR is fired. Methods to trigger SCR are:
1. Forward Voltage Triggering
2. Gate Triggering
3. dv/dt Triggering
4. Temperature Triggering
5. Light Triggering
Methods of Gate Triggering – R, RC & UJT Triggering
