This document discusses power semiconductor devices and related topics over multiple pages. It covers SCR firing circuits, commutation circuits, heat sinks, snubber circuits, fuse selection, and the series and parallel operation of SCRs. It also discusses switching losses that occur in semiconductor devices when they are turned on and off. Key points covered include how SCRs are fired, the purpose of commutation and snubber circuits, heat dissipation methods, fuse types and ratings, and factors that influence power losses in semiconductor switches.

1. Power Semiconductor Devices
Part-2

4. Power Semiconductor Devices:
• Firing circuits and commutation circuits.
• Cooling and heat sink.
• Snubber circuits
• Fuse selection.
• Series and parallel operation of SCR
• Switching Losses in Semiconductor Devices
This Module will cover the following topics

5. SCR Firing Circuits (Manual)

6. Firing Circuits
As the capacitor charges up, the potential
difference across its plates slowly increases
with the actual time taken for the charge on
the capacitor to reach 63%of its maximum
possible voltage, in our curve 0.63Vs being
known as one Time Constant, ( T ).

7. SCR Firing Circuits (UJT)

8. SCR Firing Circuits

9. Brushless motors rely on semiconductor switches to turn stator
windings on and off at the appropriate time.
The process is called electronic commutation, borrowing on
terminology used for the mechanism in dc motors, called a
commutator, that switches current into the rotarcoiles.
Commutation circuits
As we know that once a thyristor starts conducting then it continue to
conduct till the current flowing through it reduced below the holding
current. In commutation we mainly reduce thyristor's current from
holding current.
Also Read: Triggering Methods Of Thyristor or SCR

10. There are two methods of commutation of thyristors
:
Natural Commutation
Forced Commutation.
1. Natural Commutation:
The process of turning off a thyristor without using any external circuits is
known as Natural commutation. This type of commutation is only possible in
AC Applications.
• When using A.C. Supply, the current passing through the device is
alternating.
• This alternating current go to peak positive value , passes through its zero
and then go through peak negative value. When the alternating quantity at
zero then the current passing through the device also zero and at same
time a reverse polarity develops across the thyristor.
• This will quickly Turn OFF the thyristor. This process of commutation is
known as Natural Commutation.

11. 2. Forced Commutation :
Classification of Forced Commutation Methods
The forced commutation can be classified into different methods as follows:
Class A: Self commutated by a resonating load
Class B: Self commutated by an LC circuit
Class C: Cor L-C switched by another load carrying SCR
Class D: C or L-C switched by an auxiliary SCR
Class E: An external pulse source for commutation
Class F: AC line commutation
Forced commutation can be observed while using DC supply; hence it is also
called as DC commutation. The external circuit used for forced commutation
process is called as commutation circuit and the elements used in this circuit
are called as commutating elements

14. Class C, C or L-C switched by
another load–carrying SCR
This configuration has two SCRs. One of them
may be the main SCR and the other auxiliary.
Both may be load current carrying main SCRs.
The configuration may have four SCR with the
load across the capacitor, with the integral
converter supplied from a current source.
Assume SCR2 is conducting. C then charges up
in the polarity shown. When SCR1 is triggered,
C is switched across SCR 2 via SCR 1 and the
discharge current of C opposes the flow of load
current in SCR2.

15. Natural VS. Forced Commutation of thyristors

16. Heat Sink
Losses in power electronic converters produce heat that must be
transferred away and dissipated to the surroundings
The major losses occur in semiconductor power switches, while their small
size limits their thermal capacity
High temperatures of the semiconductor structures cause degradation
of electrical characteristics of switches, such as the maximum blocked
voltage or the turn-off time
Serious overheating can lead to destruction of a semiconductor device in
a short time

17. • Natural convection cooling
• Forced air cooling
• Liquid cooling
Heat Sink

18. Heat Sink

19. Snubbers Circuits

20. 20
Rugged, reliable, efficient, long lived, but not very fast
A switch

21. 21
Snubbers are energy-absorbing circuits used to suppress
the voltage spikes caused by the circuit's inductance
when a switch, electrical or mechanical, opens up.
The most common snubber circuit is a capacitor and
resistor connected in series across the switch (transistor)
Snubbers Circuits
Voltage Spikes

22. Switch is making/On
V = 0,
At the moment the switch is
opening (contact breaking)
the voltage will increase and
overshoot (spike)will occur
then settled to source value
At this moment the inductor
will behave like a source and
will provide the energy to
CAP to remain energized is
reflected by the –ve dip in
the current.
The moment Switch is making/On
the voltages will reduce across the
SW and instantaneous current will
rush find the reassessment then
settled down.

23. • Reduce or eliminate voltage or current spikes
• Limit dI/dt or dV/dt
• Shape the load line to keep it within the safe operating area (SOA)
• Transfer power dissipation from the switch to a resistor or a
useful load
• Reduce total losses due to switching
• Reduce EMI by damping voltage and current ringing
Snubbers are circuits which are placed
across semiconductor devices for
protection and to improve performance.
Snubbers can do many things:
Two most common snubbers are:
The resistor capacitor (RC) damping network
The resistor-capacitor-diode (RCD) turn-off snubber.
RC Snubber Circuits

24. RC Snubber Circuits

25. RC Snubber Circuits

26. Circuit Protections

27. Circuit Protection
Circuit protection requires :
Over Current protection
Over voltages Protections
Fuses are use d in between the source and the load.
The fuse is a reliable over current
protective device, primarily used as
a circuit protection device for over
currents, overloads and short
circuits
Fuse possesses an element that is
directly heated by the passage of
current and is destroyed when the
current exceeds a predetermined
value
Overloads
Short-Circuits

28. Normal load vs. Overload
Fuse reaction time.
Fuse Reaction time

30. Fuse Reaction time
Normal load vs. Overload
Fuse reaction time.
Pre-arc time tpa
This refers to the time range
between the onset of a current
that is large enough to melt the
fusible element.
Arcing time ta
This refers to the time lapse
between the generation of the
arc and its (final) extinction.
Operating time top
This is the sum of the pre-arcing
time and the arcing time.
tT

31. In distribution systems on bases of speed ratio
Fast blow : K Type
Slow blow: T Type
Fast Below and Slow Blow Fuses

32. There are a number of standards to classify fuses
according to the rated voltages, rated currents,
time/current characteristics, manufacturing
features and other considerations
Circuit Protection
ANSI/UL 198‐1982 standards:
Low voltage fuses are 600 V or less.
Medium and high voltage fuses are
in range 2.3‐138 kV
fuse cutout and a backup current-limiting fuse
A Fault operated can be easily
identified from the ground since it
drops open.
Both the fuse tube and the backup
limiter can be quickly removed from
the mounting with a telescoping hot
stick.
American National Standards Institute

33. Working

34. Overload protection requires a current transformer which simply measures the current
in a circuit.
Overload
CT for operation on a 110 kV grid

35. TOC protection operates based on a current vs time curve. Based on this
curve if the measured current exceeds a given level for the preset amount
of time,
There are two types of overload protection:
Instantaneous overcurrent
Time over current (TOC).
Instantaneous overcurrent requires that the current exceeds a
predetermined level for the circuit breaker to operate.

36. MOV Technology
• Contains a ceramic mass of zinc
oxide grains combined with small
quantities of bismuth, cobalt and
manganese sandwiched between
two metal plates
• The boundary between each grain
and its neighbor forms a diode
junction, allowing current to only
flow in one direction
• Equivalent to a mass of back-to-
back diode pairs, each in parallel
Schematic Symbols

37. Surge Protective Device (SPD)
• A surge protective device, or SPD, reduces the
magnitude of a voltage transient surge thus
protecting equipment from damaging effects.
SPD’s were commonly known in the past as TVSS
(Transient Voltage Surge Suppressor)
• A SPD tries to:
– Send surge away (to ground)
– Acts as a momentary ‘short circuit’
• ‘short circuit’ ≈ voltage equalization ≈ no overvoltage ≈
protected load

38. How a SPD Works
• The SPD acts as a pressure relief valve
• The pressure relief valve (SPD) does nothing until an over-
pressure pulse (voltage surge) occurs in the water (power) supply
SPD
Transient
Voltage
SPD Shunt Path
Surge Protective Device

39. Metal Oxide Varistors (MOV) Contains a ceramic mass of zinc oxide grains,
combined with other metal oxides sandwiched
between two metal plates forming a network of
back-to-back diode pairs
Silicon Junction Diode The diode is installed reverse-biased under
normal conditions. When the voltage rises
above normal conditions the diode becomes
forward-biased
Spark Gap If a voltage surge is experienced a spark ignites
gases creating an arc across the gap
Gas Tube Arrestor Commonly used for telephone lines as they
enter a building
Sophisticated spark gap that safely shunts the
surge to ground
Types of SPD Technologies Surge Protective Device

40. MOV Failure Modes
• There are two types of
MOV failure modes:
1. High energy over-voltages
2. Lower energy repetitive
pulses

41. MOV Failure due to High Energy Over Voltages
• Event: Large single energy event
spike or transient beyond the
rated capacity of the device
– Failure: Device will rupture or
explode
• Event: Sustained over-voltage
condition building up energy
– Failure: Device will go into
thermal overheating and
rupture (thermal overload)
• Event: Repeated lower level
spikes or transient over-voltage
conditions
– Failure: Device will slowly
degrade until failure
Due to the destructive nature of this failure
surge rated fuses are required for all MOV
installations. (Except TPMOV®)
MOV Failure
Every time an MOV switches, it’s life
is slightly degraded. The greater the
transient hit, the greater the
degradation of the MOV

42. Series and parallel operation
of SCR
SCRs are available of ratings up to 10 KV and 3 KA.
Sometimes system face demand, more than these ratings.
That requires combination of more than one SCRs .
Connecting SCRs in Series meets high voltage demand.
Connecting SCRs in parallel meets high current demand.

43. Series and Parallel Connection of SCR or Thyristor
Series Operation of SCR
When the operating voltage is more than the
rating of one SCR the multiple SCRs of same
ratings are used in series.
Parallel Operation of SCR
When the operating current is more than the
individual current ratings of SCRs then we use
more than one SCRs in parallel.

44. Switching Losses in Semiconductor Devices
Power dissipate is common in all semiconductor devices.
Device fail and destroy themselves if it dissipate too much power, that may also
damage the other system components.
What happened when switch
is turned ON and OFF
Switch is ON
Entire current Io flows through the
switch and the diode is reverse biased
Switch is turned OFF
I0 flows through the diode and a voltage equal
to the input voltage Vd appears across the
switch. (assuming a zero voltage drop across the ideal diode)

45. Switching Losses in Semiconductor Devices
When the switch is turned ON by applying
positive control signal to the switch, as is
shown During the turn-on transition of this
generic switch
The current buildup consists of a short
delay time td(on) followed by the current
rise time tri.
The current Io flows entirely through the
switch can the diode become reverse
biased and the switch voltage fall to a
small on-state value of Von, voltage fall
time of tfv .
The waveforms in Fig indicate that large
values of switch voltage and current are
present simultaneously during the turn-on
crossover interval tc(on)
Voltages across switch

46. Switching Losses in Semiconductor Devices
The energy dissipated in the device during this turn-on
transition can be approximated from Fig
switching power loss Ps in the switch is due to
these transitions can be approximated from
Eqs
The energy dissipation Won in the switch during this
on-state interval can be approximated as

47. Switching Losses in Semiconductor Devices
The other major contribution to the power loss in the
switch is the average power dissipated during the on-
state Pon , which varies in proportion to the on-state
voltage.
From Eq.
which shows that the on-state voltage across the
switch should be as small as possible
Therefore, the total average power dissipation
PT in a switch equals the sum of Ps and Pon .
PT = Ps + Pon