Part of Lecture series on EE321N, Power Electronics-I delivered by me during Fifth Semester of B.Tech. Electrical Engg., 2012 Z H College of Engg. & Technology, Aligarh Muslim University, Aligarh Please comment and feel free to ask anything related. Thanks!

1. EE-321 N
Lecture-3
Thyristor Characteristics Contd
&
Thyristor Turn ON Methods

2. One Request
Please arrange for the projector cord and pull
out the screen in advance
Please save the time!

3. Supplying Gate Current
• The forward break over voltage can be
reduced if a current is supplied through the
gate terminal
• Higher the magnitude, lower will be the
forward break over voltage
• This is illustrated next
5-Sep-12 3

4. Effect of Gate current on forward BO
voltage
5-Sep-12 4
> >

5. Reverse Blocking Mode
• Anode –ve w. r. t. cathode. Junctions J1 & J3
are reverse biased while J2 is forward biased
• Only a small leakage current flows (~µA)
• Exactly similar to RB mode of a pn junction
diode
• Device is said to be in OFF state
• Avalanche BD of J1 & J3 occurs at a voltage
known as reverse breakdown voltage VBR
55-Sep-12

6. SCR switching Summary
Turn ON Conditions
1. VAK > 0
2. IDevice > IL
3. IG > 0
Turn OFF Conditions
1. IDevice < IH
2. Reverse bias only
adds to reliability
65-Sep-12
Device will continue to conduct
even if IG is just one pulse
Thyristor turns OFF and remains
OFF until turning ON conditions
are satisfied

7. Two Transistor Model & Turn ON
Mechanism of SCR
5-Sep-12 7

8. • The collector current IC of a thyristor is related to
the emitter current IE and the leakage current of
the collector-base junction, ICBO.
• The common-base current gain is defined as:
CBOEC III += α
E
C
I
I
=α
85-Sep-12

9. Therefore,
and
(Using KCL at T1)
I I IC A CBO1 1 1= +α
I I IC K CBO2 2 2= +α
)1(221121 CBOKCBOACCA IIIIIII +++=+= αα
95-Sep-12

10. But,
(Using KCL at T2)
Substituting (2) into (1) and solving for IA:
)2(GAK III +=
)(1 21
212
αα
α
+−
++
= CBOCBOG
A
III
I
105-Sep-12

11. •On application of small magnitude of IG, IK
increases. Since IK = IE2, α2 also increases which in
turn increases IC2.
•Thus IB1 and hence IE1 increases leading to
increase in α1. Ultimately, α1 + α2 → 1 and IA → ∞
•Each transistor drives another one into saturation
and device reaches conduction state.
•IA is practically limited by external circuit
impedance and the voltage drop is approx. that of
a single pn junction
115-Sep-12