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-2
Diodes (Contd) &
Thyristors

2. Ex. 2.2 MHR
The reverse recovery time of a diode is trr = 3 µs
and the rate of fall of the diode current is
di/dt = 30 A/µs. Determine
(a) the storage charge QRR, and
(b) the peak reverse current IRR
5-Sep-12 2

3. Solution
(a)
(b)
( )
2
2
6
2 1
/ 2
0.5 30 3 10 135μC
RR
rr RR rr
Q di
t Q t
di dt dt
−
= ⇒=
= × × × =
6 6
2 2 135 10 30 10
90 A
RR RR
di
I Q
dt
−
= = × × × ×
=
5-Sep-12 3

4. See Prob. 2.1 MHR
45-Sep-12

5. Power Diode Types
55-Sep-12

6. 1. General Purpose Diode
• Also called line frequency or mains diode
• ON state voltage very low (below 1 V)
• Large trr (about 25 µs)
• Very high current (up to 5 kA) and voltage
(5 kV) ratings
• Used in line-frequency (50/60 Hz) applications
such as rectifiers for electroplating, welding,
traction
65-Sep-12

7. 2. Fast Recovery Diode
• Very low trr (<1 µs)
• Power levels at several hundred volts and
several hundred amps
• Normally used in high frequency circuits like
choppers, inverters, induction heating
75-Sep-12

8. 3. Schottky Diode
• Very low forward voltage drop (typical 0.3 V)
• Limited blocking voltage (50-100 V)
• Used in low voltage, high current application
such as switched mode power supplies.
85-Sep-12

9. See SiC Diodes in MHR
5-Sep-12 9

10. Today’s Task
Find out
• A list of power electronic device manufacturers
• Any magazine (Non IEEE) pertaining to PE
• Relevant analysis softwares (other than
MATLAB)
• Read out an article on significance of Power
Electronics
105-Sep-12

11. Thyristors
(SCRs)
5-Sep-12 11

12. Thyristors
• Four layer, three terminal, minority carrier
based semi-controlled device
• Available in a wide range of ratings ranging
from few A to several kA and few tens of volts
to several kV
• One typical rating is 4500 V, 4000 A, 10 kHz for
a Gate Turn Off Thyristor (GTO)
• Thyristor family consists of a large number of
devices (to be discussed later)
5-Sep-12 12

13. Thyristors
• Needs to be forward biased + a positive
current pulse supplied through the gate
terminal to turn ON and conduct
• Once it latches into ON condition, the gate
terminal loses its control and the device
continues to conduct just like a diode
• Gate does not play any role in turning OFF
• That is why it is a semi-controlled device
5-Sep-12 13

14. Device Structure
5-Sep-12 14

15. Physical Layout
15
Thyratron Tube Thyristor
Device Symbol
A
(+)
G, iG
K
(−)
A
(+)
K
(−)
G, iG
5-Sep-12

16. Operation & Characteristics
165-Sep-12
G
iG

17. Ideal Characteristics
175-Sep-12
Forward
Conduction

18. SCR Operation
Three modes:
1. Reverse Blocking Mode
2. Forward Blocking Mode
3. Forward Conduction Mode
185-Sep-12

19. SCR Junctions
5-Sep-12 19

20. Forward Blocking Mode
• Anode is +ve w. r. t. cathode
• Junctions J1 & J3 are forward biased while J2 is
reverse biased
• Only a small leakage current flows from anode
to cathode
• SCR does not conduct and this is known as the
forward blocking mode
• Junction J2 breaks down at a sufficiently high
value of VAK called breakover voltage VBO
205-Sep-12

21. Forward Conduction Mode
• At a sufficiently high value of VAK called breakover
voltage VBO junction J2 suffers avalanche
breakdown
• Since J1 and J3 are already forward biased, there
will be free movement of carriers across all three
junctions, resulting in a large forward anode
current.
• The device will be in a conducting state or ON-
state.
• The voltage drop would be due to the ohmic drop
in four layers and typically 1 V
215-Sep-12

22. Contd...
• In the ON-state, the current is limited by the
external impedance.
• The current must be greater than the latching
current IL in order for the device to conduct;
otherwise, the device will go into the blocking
mode as the anode-cathode voltage is
reduced.
22

23. Contd...
• If the current is reduced below a value which
is known as the holding current IH the
thyristor will go into the blocking state.
• The holding current is in the range of
milliamperes.
• Devices ceases to conduct if the current falls
below holding current level
23
2 to 3 times ofL HI I=