3. Two transistor Analogy and Turn on methods.pptx - SCR...

SCR turn-on methods
•
Two Transistor Analogy
•
Turn-on methods
•
Dynamic Characteristics
•
Important specifications

Typical ratings-10KV,3000A,30MW
-1A,10V,10W

Two transistor model

Turn-On process of SCR
• For Finite I
G
^-----I
A
^

1
^ with I
A
^->I
E1

2
^ with I
K
^>I
E2
(I
K
=I
A
+I
G
)
So,( 
1
+ 
2
)^Further I
A
^
Regenerative or Positive f/b…. ,( 
1
+ 
2
)~1
I
A
is very large ….SCR ON.
•
Under Transient conditions , SCR can turn ON
Capacitance of PN jn influence the thyristor.
SCR in Blocking state(I
G
=0)
High dv/dt------Current ^ in The jn capacitors
So,High dv/dt---- i
C
^--CB01,CB02^--
I
A
^-- ,( 
1
+ 
2
)^=1---I
A
very large
So, if dv/dt is high….Undesirable turn on can take place

Thyristor turn-ON methods
•
Thyristor turning ON is also known as Triggering .
With anode positive with respect to cathode, a thyristor can
be turned ON by any one of the following techniques : –
•
Forward voltage triggering
•
Gate triggering
•
dv/dt triggering
•
Temperature triggering
•
Light triggering

Forward Voltage Triggering
• When break-over voltage (VBO) across a thyristor is exceeded
than the rated maximum voltage of the device, thyristor turns
ON.
• At the break-over voltage the value of the thyristor anode
current is called the latching current (IL) .
• Break-over voltage triggering is not normally used as a
triggering method, and most circuit designs attempt to avoid its
occurrence.
• When a thyristor is triggered by exceeding VBO, the fall time of
the forward voltage is quite low (about 1/20th of the time taken
when the thyristor is gate-triggered).
• However, a thyristor switches faster with VBO turn-ON than
with gate turn-ON, so permitted di/dt for breakover voltage
turn-on is lower.

Gate Triggering
•
Turning ON of thyristors by gate triggering is simple and efficient method of
firing the forward biased SCRs.
•
In Gate Triggering, thyristor with forward breakover voltage (VBO), higher
than the normal working voltage is chosen . – This means that thyristor will
remain in forward blocking state with normal working voltage across anode
and cathode with gate open.
•
Whenever thyristor’s turn-ON is required, a positive gate voltage b/w gate
and cathode is applied.
•
With gate current established, charges are injected into the inner p layer and
voltage at which forward break over occurs is reduced. Forward voltage at
which device switches to on-state depends upon the magnitude of gate
current. – Higher the gate current, lower is the forward break over voltage .
When positive gate current is applied, gate P layer is flooded with electrons
from cathode, as cathode N layer is heavily doped as compared to gate P
layer. As the thyristor is forward biased, some of these electrons reach
junction J2. As a result, width of depletion layer around junction J2 is
reduced . – This causes junction J2 to breakdown at an applied voltage lower