Thyristor
A thyristor is a solid-state semiconductor
device with four layers of alternating N and P-type material.
Silicon controlled
Rectifier(SCR)
•
3
terminal power semiconductor device
•
Anode
•
Cathode
•
Gate
•
Unidirectional
device
•
Current
flows from cathode and anode only
•
This
current is controlled by Gate terminal.
•
Used
in switching ac power control
•
High
Speed switch
•
Switches
between conducting and non-conducting states instantaneously .
Construction
•
Four
layer and three junction device.
•
Turned
ON by applying small voltage to gate
•
Gate
Voltage controls the current from the device
Specifications
•
Forward blocking voltage
•
The
value at which the SCR is in forward bias but not in conduction
•
Forward break-over voltage
•
The
minimum forward voltage applied to gate to turn on SCR
•
Peak reverse voltage
•
Maximum
reverse bias voltage applied to Gate
•
Holding current
•
Minimum
current to keep the SCR in conducting state
•
Latching current
•
Minimum
current to turn On the SCR
•
Maximum ON state voltage
•
Maximum
voltage appears when SCR is conducting
•
Current rating
•
Ability
of SCR to carry high magnitude current
•
Maximum gate trigger current
•
Maximum
current that can be applied to Gate terminal
•
Minimum gate trigger current
•
Minimum
current that can be applied to Gate terminal
•
Gate power loss
•
Power loss between gate and main terminal
•
Turn ON time
•
Tim
required for reaching 90% of conducting state(2 to 4 µs)
•
Turn OFF time
•
Time
required to switch OFF the SCR. (10 to 50 µs)
•
Gate reverse voltage
•
Maximum
reverse voltage that can be applied to gate
Working
•
Two states of operation:
•
ON
State
•
OFF
State
ON State
•
Gate
is positive
•
Forward
biased
•
The
device turns ON
•
After
break-over voltage the SCR starts conducting.
•
The
minimum current required to turn on SCR is called latching current.
•
The
minimum current required to keep it in
ON state is called holding current
OFF State
•
Gate
is open
•
Reverse
biased.
•
A
negligible leakage current flows in the circuit.
•
If
the value falls below holding current the SCR is turned OFF
Two transistor Model
•
SCR
has two transistors PNP and NPN connected adjacent in base.
•
When
positive GATE is applied:
•
BE
junction of Q2 is forward biased and transistor turns ON and generates
collector current.
•
Collector
current turns Q1 ON.
•
When
both transistors saturate SCR is ON and remains in ON state even after removing
GATE
V-I Characteristics:
Vertical line- forward and reverse
current in SCR
Horizontal line- forward and reverse
voltage in SCR
It has 2 characteristics:
·
Forward
characteristics
·
Reverse
characteristics
Forward characteristics:
·
When
anode is positive with respect to cathode
·
SCR
does not conduct unless reaching the forward breakover voltage VBO .
·
SCR
remains in conducting state unless current falls below the holding current
·
While
SCR is not conducting a small leakage current flows through the device.
·
When
positive gate is applied the device starts conducting.
·
The
breakover voltage decreases with increase in gate current.
·
At
sufficiently large gate current SCR behaves like a pn junction rectifier.
·
Device
will be in low impedance state while conducting and high impedance state while
not conducting
·
The
gate of the SCR controls the resistance between anode and cathode
Reverse characteristics:
·
Anode
is negative with respect to cathode.
·
A
leakage current of few microamperes flow through the device.
·
Further
increase in reverse voltage results in avalanche breakdown increasing the
reverse current sharply.
Advantages
•
Very
small amount of Gate voltage
•
Available
with high current and high voltage ratings
Disadvantages
•
Gate
terminal has no control over scr after switch on.
•
External
circuits are required to turn OFF
•
Operates
only at low frequency of order 50-60hz
•
Requires
supplementary protection circuits.
Applications
•
Power
switching
•
Phase
control
•
Choppers
•
Battery
chargers
•
Inverter
circuits
