Power electronics devices operate at high power levels and require high efficiency compared to linear electronics. Some key power electronic devices include thyristors, IGBTs, MOSFETs, and integrated power circuits. Thyristors can be turned on through forward voltage triggering or gate triggering and turned off through natural commutation in AC circuits or forced commutation using resonant circuits, complementary devices, or external pulses. Proper protection of power devices includes using snubber circuits, overvoltage and overcurrent protection, and gate protection.

1. How is Power electronics
distinct from
linear electronics?
Mrs Shimi S L
Assistant Professor, NITTTR,
Chandigarh, India

2. Power Electronics
Power Engineering Electronics Engineering
(i)High power level Signal Level
(ii) High Efficiency Efficiency not a major
concern
(iii) EMI Movement of holes
and electrons

4. Doping

5. Power Diode

6. Free charge density
Metals (Electrons) 10^23 per cubic cm
Insulators (Electrons) < 10^3 per cubic cm
Semiconductors (10^8 – 10^19 per cubic cm)
P + (holes) N+ (Electrons) 10^19 per cubic cm
P - (holes) N- (Electrons) 10^14 per cubic cm
P (holes) N (Electrons) 10^17 per cubic cm

7. IGCT - Integrated Gate-Commutated Thyristor
PIC - Power Integrated Circuit

8. Comparison of BJT , MOSFET and IGBT
Switch
type
Base/
Gate
Control
Variabl
e
Switching
Freq
On
state
Voltage
drop
Max.
voltage
rating,
Vs
Max.
current
rating, Is
Advantages Limitations
BJT Current Medium Low 1.5 kV 1 kA Simple, low-on state
voltage drop
Needs high base
current to turn-on &
sustain on-state
current. Base drive
power loss. High
switching losses.
MOSFET Voltage Very High High 1 kV 150 A Higher switching
speed. Low switching
loss. Facilitates
parallel operation.
Low gate loss
High on-state voltage
drop, as high as 10 V.
Lower off-state voltage
capability.
IGBT Voltage High Medium 3.5 kV 2 kA Low on-state voltage.
Little gate power
Lower off-state voltage
capability.
Cool
MOS
Voltage Very High Low 1 kV 100 A Low gate drive
requirement; Low on-
state voltage drop
Low V & I ratings

12. MOS-controlled thyristor
MOS turn-off thyristor
Emitter Turn Off Thyristor
Static Induction Thyristor

13. Static induction transistor

15. Diode Rectifiers: A diode rectifier circuit converts AC voltage into a
fixed DC voltage. The input voltage to rectifier could be either single
phase or three phase.
AC to DC converter : An AC to DC converter circuit can convert AC voltage into
a DC voltage. The DC output voltage can be controlled by varying the firing
angle of the thyristors. The AC input voltage could be a single phase or three
phase.
AC to AC converter :This converters can convert from a fixed ac input voltage
into variable AC output voltage. The output voltage is controlled by varying
firing angle of TRIAC. These type converters are known as AC voltage
regulator.
DC to DC converter : These converters can converter a fixed DC input voltage
into variable DC voltage or vice versa. The DC output voltage is controlled by
varying of duty cycle.
Static Switches : Because the power devices can be operated as static
switches or contactors, the supply to these switches could be either AC or DC
and the switches are called as AC static switches or DC static switches.

17. Thyristor

18. • Vertical cross section of SCR
Two transistor equivalent circuit

19. Steady State Characteristics of a Thyristor

20. Gate Characteristics of a Thyristor

21. Switching Characteristics of a Thyristor

24. Thyristor Firing
Techniques

25. Triggering Techniques
• Forward - Voltage Triggering
• dv/dt Triggering
• Temperature Triggering
• Light Triggering
• GATE Triggering

26. GATE Triggering
• Resistance and RC Firing Circuit
• UJT triggering
• Pulse Transformer in Firing Circuit

27. Simple Resistor and RC Trigger Circuit
RC Trigger Circuit
Resistor Trigger Circuit

28. UJT triggering
Unijunction transistor: (a) Construction (b) Model (c) Symbol

29. Unijunction Transistor: (a) Emitter Characteristic Curve (b) Model for VP .

30. Unijunction Transistor Relaxation Oscillator
and Waveforms

31. Pulse Transformer in Firing Circuit

32. Triac Firing Circuit

35. Thyristor Protection

36.  Snubber Circuit
 Overvoltage Protection
 Overcurrent Protection
 Gate Protection

37. Heat Removal Mechanism
SCR (stud-type) on air-
cooled kits
Fin-type Heat Sink SCR (hokey-puck-type) on
power pak kits
Assembly of power converters

38. Snubber Circuit
Rs Cs
LoadVs
+
_
L

39. Crowbar Protection
_

40. Gate Protection
R1 C1
R2
Rs
Cs
di/dt inductor
Gate Protection
ZD
CB- Circuit breaker ; FACLF- Fast acting current limiting fuse ;
HS- Heat sink ; ZB- Zener diode

41. Thyristor Commutation
Techniques

42. Introduction
• Commutation
– Process of turning off a conducting thyristor.
• Natural Commutation
• Forced Commutation
Current Commutation
Voltage Commutation

43. Natural Commutation
• Occurs in AC circuits
~
T
+
−
v ov s
R↑ ↑
ω t
ω t
ω t
ω t
S u p p l y v o l t a g e v s
S i n u s o i d a l
V o l ta g e a c r o s s S C R
L o a d v o l t a g e v o
T u r n o ff
o c c u r s h e r e
0
0
π
π
2 π
2 π
3 π
3 π
α
t c
G a t e P u l s e
π + α
α π + α

44. Natural Commutation of Thyristors takes place
in
–AC voltage controllers.
–Phase controlled rectifiers.
–Cyclo converters.

45. Forced Commutation
• Applied to dc circuits
• Commutation achieved by reverse biasing the
SCR or by reducing the SCR current below
holding current value.
• Commutating elements such as inductance
and capacitance are used for commutation
purpose.

46. Methods of Forced Commutation
• Self commutation, Load commutation.
• Resonant pulse commutation.
• Complementary commutation.
• Impulse commutation.
• External pulse commutation.

47. Forced Commutation
is applied to
• Choppers
• Inverters

48. Self Commutation
Or
Load Commutation
Or
Class A Commutation
(Commutation By Resonating The Load)

49. • Circuit is under damped by including suitable
values of L & C in series with load.
• Oscillating current flows.
• SCR is turned off when current is zero.
V
R L V ( 0 )c
C
T i
L o a d
+ -

50. C u r r e n t i
C a p a c i t o r v o l t a g e
G a t e p u l s e
V o l ta g e a c r o s s S C R
0 ππ / 2
ω t
ω t
ω t
ω t
V
− V
2 V
Conduction time of SCR
π
ω
=

51. Resonant Pulse Commutation
(Class B Commutation)

52. • Initially C charged with polarity as shown in
figure.
• T1 is conducting & IL is constant.
• To turn off T1, T2 is fired.
• iC(t) flows opposite to IL& T1 turns off at iC(t) =
IL

53. Resonant Pulse Commutation
waveform

54. Complementary Commutation
(Class C Commutation,
Parallel Capacitor Commutation)
V
R 1 R 2
T 1 T 2
I L
i C
C
a b

55. • Two SCRs are used, turning ON one SCR turns
off the other.
• T1 is fired, IL flows through R1.
• At same time ‘C’ charges towards ‘V’ through
R2 with plate ‘b’ positive.
• To turn off T1, T2 is fired resulting in capacitor
voltage reverse biasing T1 and turning it off.
• When T2 is fired current through load shoots
up as voltage across load is V+VC

56. G a t e p u l s e
o f T 1
G a t e p u l s e
o f T 2
C u r r e n t t h r o u g h R 1
π
I L
V
t
t
t
t
C u r r e n t t h r o u g h T 1
C u r r e n t t h r o u g h T 2
2
2
V
R
2
1
V
R
V
R 1
V
R 2
V
R 1
2
1
V
R
t
t
t
V o l t a g e a c r o s s
c a p a c i t o r v a b
V o l t a g e a c r o s s T 1
C u r r e n t
t h r o u g h R 2
t C t C
tC
V
- V

57. Voltage Commutation

58. External Pulse Commutation
(Class E Commutation)
V S V A U X
L
C
T 1 T 3T 2
R L
2 V A U X
+
−