This gives the detailed characteristics of SCR, MOSFET and IGBT. It also includes the Firing Circuit, Commutation Circuit and Snubber Circuit of SCR

1. POWER ELECTRONICS
More About Power Semiconductor
Devices
D.Poornima,
Assistant Professor (Sr.Gr),
Department of EEE,
Sri Ramakrishna Institute of Technology,
Coimbatore

2. Static and Switching
Characteristics: SCR,
MOSFET and IGBT

3. Static
Characteristics
: SCR

4. Switching Characteristics: SCR
● Thyristor is subjected to different voltages across it and different
current flows through it during turnon and turn-OFF process.
● Due to time variation of voltage across thyristor and current through it
during turn-ON and turn-OFF process, the switching characteristics of
thyristor will be dynamic.
● There are two types of switching characteristics of thyristor namely
1. Turn-ON characteristics of thyristor
2. Turn-OFF characteristics of thyristor

6. • When a positive gate signal is applied to a forward biased SCR, the
transition of SCR from blocking state to conducting state is called as turn
ON mechanism.
• The time taken for SCR to traverse from the blocking state to conducting
state is called as turn on time.
• Total turn-on time can be divided into three intervals
1.Delay time td
2.Rise time tr
3.Spread time tp
Turn on Characteristics

7. Delay time td
• The delay time td is the required time interval from
initial anode current, i.e., forward leakage current
to reach 10% of final value of anode current
(0.1IA) where Ig is the final value of gate current
IA is the final value of anode current.
• The delay td can also be measured by the time
interval during which anode voltage falls from VA
to 0.9VA where VA is the initial value of anode to
cathode voltage.
• As the gate signal is applied the flow of gate
current has a non-uniform distribution of current
density over the cathode surface to the p layer

8. Delay time td
• Delay time is the time taken for the current density to be uniform over
the cathode surface
• Delay time can be reduced by – applying high gate current, higher
VAK

9. Rise time tr
• Rise time tr is the time interval during which the anode
current increases from 10% to 90% of final anode
current.
• During rise time the forward blocking OFF-state
voltage decreases,
• Rise time tr can be measured from the instant of 90%
of forward blocking OFF-state voltage (0.9VA) to the
instant at which forward blocking OFF-state voltage
reaches to 0.1VA.
• It can also be measured from the instant of the gate
current reaches 0.9Ig to the instant at which gate
current I

10. Rise time tr
• From the beginning of Rise time tr , anode current starts spreading from
the narrow conduction region near the gate.
• The rise time is inversely proportional to the amplitude of gate current and
its build up rate.
• The rise time deceases when high and steep current pulse is applied to
gate of SCR.

11. Spread time tp
• The spread time tp is the time interval during
which the forward blocking voltage falls from
10% of its value (0.1VA) to the ON-state
voltage drop about 1 V to 1.5 V.
• It can also be defined as the time taken by
the anode current to rise from 0.9IA to IA.
• During this time, the conduction spreads over
the entire cross section of the SCR cathode.
• The spreading time depends on the area of
cathode and gate structure of SCR.
Spread time tp

12. • After spread time,
1. SCR is completely turned on ,
2. Final steady state anode current flows through the device ,
3. Voltage drop across SCR is about 1 V to 1.5 V.
• The total turn-ON time is equal to 1 to 4 µs and depends on
1. anode circuit parameters
2. amplitude of gate current and its wave shape.
• With the increase of gate current, the turn-ON time will be decreased
• The amplitude of gate current is 3 to 5 times of minimum gate current to trigger
thyristor.
• During design of gate triggering circuit,
1. The gate must be removed just after turn ON of thyristor.
2. When the thyristor is reverse biased, the gate signal should not be applied.
3. The pulse width of gate pulse must be sufficient so that the anode current must
increase to latching current and it must be grater than turn-ON time.

13. • Thyristor can be turned OFF if the anode current is reduced slowly
below the holding current IH.
• The turn-off process of thyristor is called commutation.
• There are two different methods of thyristor turn-off
1.Natural commutation
2.Forced commutation.
• The turn-off time tq can be defined as the time interval between the
instant at which anode current through the device becomes zero and the
instant at which SCR regain its forward blocking capability.
• It is sum of the reverse recovery time trr and gate recovery time tgr
tq = trr + tgr
Turn off Characteristics

14. Reverse Recovery Time trr
• At time t = t1, the anode current becomes zero.
• After t = t1, anode current starts to build up in the reverse direction due to
presence of charge carriers in the four layers
• This reverse recovery current removes the excess charge carriers from
junctions J1 and J3 between t = t1 and t = t3.
• Holes are swept out from top p-layer and the electrons are sweeping out
from bottom n-layer.
• At t = t2, about 60% of the stored
charge carriers are removed from outer
two layers
• Carrier density in junctions J1 and J3
decreases, reverse recovery current
also decreases

15. Reverse Recovery Time trr
• The rate of decrease of reverse recovery current is very fast but it is
gradual thereafter.
• This causes a reverse surge voltage to appear across the SCR and the
device may be damaged.
• This condition may be avoided by using RC snubber circuit across SCR.

16. Gate Recovery Time tgr
• At time t = t3, reverse recovery current is about zero
• Thyristor is able to block the reverse voltage.
• Between t = t1 and t = t3, all excess charge carriers
are removed from outer junctions J1 and J3.
• But middle junction J2 still consists of charge carriers
and it cannot block the forward voltage.
• These charge carriers cannot flow to
external circuit, can be removed by
recombination only.
• The recombination is possible when a
reverse voltage is applied across SCR

17. Gate Recovery Time tgr
• The recombination happens between t = t3 and t = t4.
• This time interval is called gate recovery time tgr.
• At t = t4, thyristor operates in OFF state.
• Thyristor turn-off time is in the order of 3 µs to 100 µs.
• The turn-OFF time depends on
1. Amplitude of forward current
2. Junction temperature
3. di/dt during commutation process.
• With the increase of forward current,
junction temperature and di/dt,
thyristor turn-OFF time increases.

18. STATIC CHARACTERISTICS /
IV CHARACTERISTICS:
MOSFET

19. • There are two types of Static Characteristics for MOSFET
1.Transfer Characteristics
2. Output Characteristics

20. Transfer Characteristics of MOSFET
• The n-channel power MOSFET has
three terminals namely gate (G), drain
(D) and source (S)
• Input signal VGS is applied across gate
to source and the output signal VDS is
obtained from drain.
• The current flow from drain to source ID
is controlled by gate signal.
• Source terminal is common between
the input and output

21. Transfer Characteristics of MOSFET
• The transfer characteristic is the plot of
drain
• current ID as a function of gate-to-source
voltage VGS.
• When VGS is less than threshold voltage
VGS(th), the current flow is zero.
• VGS(th) for power MOSFET is about 2 to 3
V.
• ID is equal to zero, the drain to source is
open circuit and the device should able to
hold the supply voltage VDD.

22. Output characteristics of a power MOSFET
• Out put characteristic is the plot of ID as
a function of VDS when VGS is constant.
• It consists of three regions such as
1. cut-off region
2. active region
3. ohmic region
• When VDS is small, the relation between
ID and VDS is linear
• MOSFET operates in the ohmic region

23. Output characteristics of a power MOSFET
• The device turns off in the cut-off
region and turns on in the ohmic
region.
• In active region ID does not depend on
VDS but it varies with VGS.
• This region is also called saturation
region.
• VGS(max) is the maximum allowable
gate-to-source voltage when the gate
oxide will not be broken
• Usually it can be break at electric field
about 5–10 Mega Volt/cm.

24. Switching Characteristics
: Mosfet

25. • The three internal capacitances are
• gate-to-source capacitance Cgs,
• gate-to-drain capacitance Cgd
• drain-to-source Cgd.
• During turning on Cgd and Cgs are to be
charged through gate.
• This charging takes a finite time.
• So, the turn on cannot be
instantaneous.

27. • The turn-on delay tdn is the time that
is required to charge the input
capacitance to threshold voltage level.
• The rise time tr is the gate charging
time from the threshold level to the full-
gate voltage VGSP.
• During rise time, drain current rises
from zero to full-on current ID.
• Thus, the total turn-on time
ton = tdn + tr.
• The turn-on time can be reduced by
using low-impedance gate-drive
source.

28. • MOSFET is the majority carrier device
• Turn-off process is initiated soon after
removal of gate voltage at time t1.
• The turn-off delay time tdf is the time
required for the input capacitance to
discharge from the overdrive gate
voltage V1 to the pinch-off-region.
• The fall time, tf is the time that is
required for the input capacitance to
discharge from the pinch-off region to
threshold voltage VGST.
• If VGS ≤ VGST, the transistor turns off.
• During fall time, drain current reduces
from ID to zero.

29. STATIC CHARACTERISTICS /
V-I CHARACTERISTICS: IGBT

30. • There are two types of Static Characteristics for IGBT
1.Transfer Characteristics
2. Output Characteristics

31. Output and Transfer characteristics of IGBT

32. Output characteristics of IGBT
• It is similar to that of a BJT except that the parameter which is kept constant for a
plot is VGE because IGBT is a voltage controlled device unlike BJT which is a
current controlled device.
• When the device is in OFF mode (VCE is positive and VGE < VGET) the reverse
voltage is blocked by J2 and when it is reverse biased, i.e. VCE is negative, J1
blocks the voltage.

33. Transfer characteristics of IGBT
• When gate-emitter voltage is less than a minimum voltage (VGET), no current
flows through the IGBT.
• This means , a minimum amount of forward voltage is required to make IGBT
turn ON.

34. Switching Characteristics
: IGBT

36. • Turn on time ton is composed of two components
• Delay time (tdn)
• Rise time (tr).
• Delay time is defined as the time in which collector current rises from leakage
current ICE to 0.1 IC (final collector current) and collector emitter voltage falls from
VCE to 0.9VCE.
• Rise time is defined as the time in which collector current rises from 0.1 IC to IC
and collector emitter voltage falls from 0.9VCE to 0.1 VCE.

37. • The turn off time toff consists of three components
• Delay Time (Tdf)
• Initial Fall Time (Tf1)
• Final Fall Time (Tf2).
• Delay time is defined as time when collector current falls from IC to 0.9 IC and VCE
begins to rise.
• Initial fall time is the time during which collector current falls from 0.9 IC to 0.2 IC
and collector emitter voltage rises to 0.1 VCE.
• The final fall time is defined as time during which collector current falls from 0.2 IC
to 0.1 IC and 0.1VCE rises to final value VCE.

38. Triggering and
Commutation Circuit for
SCR

39. • When the SCR is forward biased, it can be turned on
(triggered) by using any of following methods:
• When VAK >VBO (Forward voltage triggering)
• When gate drive is applied (Gate triggering)
• When dv/dt exceeds permissible value (dv/dt triggering)
• When gate cathode junction is exposed to light (light triggering)

40. Forward Voltage Triggering
When VAK >VBO
• When VAK >VBO (forward break-over voltage), the SCR is driven
in forward conduction even if gate is open.
• J2 is reverse biased during forward blocking mode
• When VAK exceeds VBO, the avalanche break-down of junction
J2 takes place.
• Anode to cathode voltage falls to very small value (2V).
• Heavy anode to cathode current flows and is only limited by the
load.
IAK = V/load
• Not preferred because during turn on, it is associated with large
voltage and large current which results in huge power loss and

41. Gate triggering
When gate drive is applied
• A +ve gate to cathode signal is applied whenever the SCR
is to be driven into forward conduction mode
• The +ve gate pulse will flood the P layer with electrons
from cathode
• Cathode N layer is heavily doped
• Some of the electrons will reach junction J2, reduce
depletion region width
• When Ig increases, avalanche break-down of junction J2 takes place
• SCR is driven into forward conduction mode (ON state) even if VAK < VBO.

42. Forward Conduction Region(2.1)
When gate drive is applied
• Most convenient, efficient, simple way of
triggering the SCR.
• Gate has no control over the conduction once it
is turned on
• The current IAK is only limited by the load, i.e.,
IAK = V/load

44. dv/dt triggering
When dv/dt exceeds permissible value
• dv/dt - rate of change of anode to cathode voltage
with respect to time.
• In forward blocking state - equivalent internal
capacitor is formed inside the SCR from anode to
gate and gate to cathode.
• Whenever the voltage applied across the SCR
changes rapidly, a transient current flows due to the
internal capacitance.
• This current adds to the forward leakage current.
• And hence the SCR turns on even if VAK < VBO or gate drive is not applied.

45. Forward Conduction Region(3.1)
When dv/dt exceeds permissible value
• The dv/dt turn-on may cause false triggering
(unwanted) of the SCR.
• Never used for triggering.
• dv/dt rating is given in volts per microseconds (V/μs).
• The voltage variations across SCR must be kept less
than permissible value of dv/dt
• A small resistance is connected between gate and cathode to avoid false
triggering of SCR due to dv/dt.
• Acts as a external path for leakage current generated by the internal capacitor.

46. Forward Conduction Region(4)
When a gate cathode junction is exposed to light
• When the gate cathode junction is exposed to a beam of light, the
current flows in the junction due to photons of light.
• This current acts as a gate drive to the SCR and it is driven into
conduction.
• This type of triggering is normally used in light activated SCRs
(LASCR).

47. Firing Circuits of SCR

48. • A thyristor can be turned on by forward-voltage triggering , dv/dt triggering,
temperature triggering, light triggering and gate triggering
• Gate triggering method is very efficient and most reliable method.
• A gate signal of proper wave shape and frequency must be applied between gate and
cathode.
• There is no requirement of gate signal after successful triggering.
• The gate voltage signal is generated by a gate-drive which is called triggering or
firing circuit
• The different triggering circuits are:
1. Resistance (R) triggering circuit
2. Resistance capacitance (RC) triggering circuit
3. Uni-junction transistor triggering circuit or UJT relaxation oscillator triggering
circuit
4. Half-wave controlled rectifier

49. Resistance (R) Triggering Circuit
• Simplest resistance triggering circuit, but not used anymore
• R1 is the variable resistance, R2 is the stabilizing resistance
• When R1 is zero, the gate current flows through Rmin, D, gate-cathode, load and
source.
• This current should not be greater than maximum permissible gate current Igm.

50. Resistance (R) Triggering Circuit
• During +ve half-cycle of input voltage at ωt = α, the voltage applied at gate
terminal is greater than VgT.
• Before turn-on of SCR, the input voltage is applied to SCR and operates in
forward blocking state.
• The value of R2 should be such that the maximum voltage across it is not greater
than gate voltage Vgm.
• Since R1 and R2 are very large, the gate trigger circuit draws small current.
• Due to diode D, the current flows in positive half cycle only.

51. Resistance (R) Triggering Circuit
• When R1 is reduced, ig current increases, Vg exceeds the VgT and thyristor will be
turn ON.
• Hence the firing angle is directly proportional to R1.
• As R1 increases, firing angle a increases.
• The maximum limit of firing angle is α = 90°.
• Since α= 0° is not possible, the range of firing angle is represented by 90° ≥ α >
0°.
• Depends on temperature and there is difference in performance between SCRs

52. RC Triggering Circuit
• In -ve half cycle of supply voltage, capacitor C charges through diode D2 to -ve peak
value of supply voltage, –Vm.
• At ωt = –90°, Vc = –Vm.
• After ωt = –90°, the supply voltage starts to decrease from –Vm to zero at ωt = 0°
• In –90° ≤ ωt ≤ 0°, the capacitor voltage decreases and finally fall to OA.
• At ωt = 0°, Vc = OA.
• After ωt > 0°, the supply voltage is +ve
and capacitor starts to charge through R.
• When Vc reaches B , it holds the +ve
voltage of the supply voltage, the
capacitor voltage Vc >VgT and thyristor
will be turned on.

53. RC Triggering Circuit
• Hence, the firing angle can be controlled by varying resistance R.
• Diode D1 is used to flow current in positive direction only.
• Hence, it prevents the breakdown of cathode to gate junction during negative half
cycle.
• In this method, firing angle will be never 0° and 180°.
• The control range of firing angle is 0° < α < 180°.
• In a RC firing circuit, the following condition must be satisfied:
•
• Thyristor will be turned on when Vc = VgT + Vd where Vd is voltage drop across diode
D1.
• The maximum value of R is given by

54. RC Triggering Circuit
• If the R value is less, firing angle is less and conduction angle is more.
• When R is increased, firing angle increases and conduction angle decreases.

55. UJT Triggering Circuit
• R and RC triggering circuits give prolonged pulses.
• Power dissipation in the circuit is large
• Cannot be used in automatic or feedback control systems
• UJT can provide pulse triggering which is more efficient
• The resistor RE is chosen so that the load line determined by
RE passes through the device characteristic in the negative
resistance region i.e., to the right of the peak point but to the
left of the valley point.
• If the load line does not pass to the right of the peak point P,
the device cannot turn on.

56. UJT Triggering Circuit
• The gate of the SCR connected to B1 region of the UJT.
• Cathode is grounded, anode is connected to the load.
• VBB is applied, emitter current is zero, capacitor will start charging through RE.
• V&I through the capacitor at this time is called peak voltage VP and peak current
IP.
• For ensuring turn-on of UJT
• Consider the peak point at which IRE = IP and VE = VP ,
Then VE = VBB – IRERE.
• So

57. UJT Triggering Circuit
• To trigger the SCR, the fully charged capacitor will make the emitter current flow
and the capacitor starts discharging through R.
• This will be a trigger pulse which can turn on the SCR.

58. Commutation Circuits of SCR

59. • A thyristor (SCR) can be turned OFF when
• its forward current IA is reduced below the holding current IH
• a reverse voltage is applied across the thyristor for a specified time so that the device
recover to the blocking state.
• The process of turning OFF the thyristor is called commutation.
• There are two types of commutation:
1. Natural or line commutation
2. Forced commutation

60. Natural Commutation
• In ac circuits, when the current in the SCR
goes through a natural zero and a reverse
voltage appears across the SCR, the SCR will
be turned OFF.
• This is called natural commutation. In
natural commutation
• No requirement of external circuits
• Used in single phase and three phase
controlled rectifiers, ac voltage controllers
and cyclo-converters.

61. Forced Commutation
• In dc circuits, to turn off the SCR,
the forward current must be reduced to zero forcefully by an external circuit
• This is known as forced commutation.
• Separate commutation circuits are required, need more control components, cost
will be more.
• Based on arrangement of commutation circuit, components and the manner in
which zero current is obtained, forced commutations are classified as:
1. Class A Commutation or Resonant Commutation
2. Class B Commutation
3. Class C Commutation
4. Class D Commutation
5. Class E Commutation

62. Class A /Resonant/Load/Self Commutation

63. ● Capacitor C & Inductor L are used as commutating element.
● L and C are connected in series with the load resistance if the value of load resistance is
low.
● If the value of load resistance is high, L is connected in series with resistor whereas C is
connected across the load.
● The main idea behind load commutation technique is to make an under-damped circuit.
● When the circuit is energized from DC source, current is rises to maximum and then
begins to fall, decays to zero and tends to reverse.
● During the time when SCR conducts, voltage across SCR (VT) is zero.
● But as soon as current reaches zero and tend to reverse, a reverse voltage equal to the
source voltage is applied across the terminals of SCR.
● Thus, after time A, anode current of SCR is zero and it is reversed biased and hence SCR /
thyristor is turned off on its own at instant A.
Class A /Resonant/Load Commutation

64. Class B/Resonant-Pulse
Commutation

65. Class B/Resonant-Pulse Commutation
• Commutation circuit comprises of Capacitor C, Inductor L and an auxiliary
thyristor TA.
• LC series circuit is connected across the SCR.
• Commutation circuit has negligible resistance, circuit is always under-damped
• Initially T1 and TA are in off state and C is charged to voltage Vs with left hand
plate +ve.
• At t=0, T1 is gated and turned on.
• Load current I0 starts flowing through T1 and Load.
• To turn off T1 , TA is fired at t=t1.
• Till time t=t1, the capacitor is charged with source voltage Vs i.e. Vc = Vs,
capacitor current ic = 0 and current through main thyristor T1 i.e. i0 = I0

66. Class B/Resonant-Pulse Commutation
• When auxiliary thyristor TA is fired at t1, it starts conducting and provides a path for
the discharge of capacitor C.
• L, C and TA forms a resonating circuit.
• The resonating current ic for this circuit is given as
• Negative sign is given as the actual current flows in a direction opposite to the
direction of current ic
• The capacitor starts discharging through TA and L.
• Vc decreases, ic increases in the opposite direction, till the capacitor discharges
fully.
• When capacitor is completely discharged, vc = 0, capacitor current ic =Ip.

67. Class B/Resonant-Pulse Commutation
• After this capacitor starts charging in the opposite direction, with right plate as
positive.
• During this time, vc starts increasing in the opposite direction, ic decreases to go to
zero.
• At t=t2, ic reduces to zero, a reverse voltage Vab appears across TA and it gets
turned off.
• TA is OFF, C is charged up to Vs with its right-hand plate +ve.
• D is now forward biased, ic will flow through least resistive path i.e., through C, L, D
and T1.
• ic flows through T1 from cathode to anode i.e., in reverse direction
• This reduces the current through T1 and at t-t3, the current through T1 will become
zero.

68. Class C/ Complementary Commutation

69. Class C/
Complementary
Commutation

70. Class C /Complementary Commutation
• Commutation circuit consists of two thyristors, main thyristor T1 and auxiliary
thyristor T2 and a commutating capacitor.
• The resistance R1 is connected in series with main thyristor T1 and R2 is
connected to T2.
• Initially, both T1 and T2 are in OFF state and Vc is zero.
• When triggering pulse is applied to T1 at t = t1, thyristor T1 will be turned on
• Two currents namely load current I1 and capacitor charging current iC flows
through the circuit.
• The load current i1 follows though the following path: V+ - R1 – T1 – V- and the
capacitor charging current flows through the path V+ - R2 – C - T1 – V-

71. Class C /Complementary Commutation
• At steady state condition, capacitor is fully charged to the supply voltage V
• At t=t2, a triggering pulse is applied to auxiliary thyristor T2 at t = t2, it will be turned
on.
• A -ve polarity voltage of the capacitor C is applied to anode of thyristor T1 w.r.t
cathode.
• T1 will be reverse biased and turned OFF immediately.
• The capacitor C is charged through the load and its polarity becomes reverse.
• The charging path of capacitor is V+ - R1 – C – T2 – V-.
• Again, thyristor T1 is triggered and turned ON at t = t2.
• Then auxiliary thyristor will be turned OFF immediately as reverse bias voltage is
applied across T2 and capacitor starts to charge in reverse direction.

72. Class D/ Impulse
Commutation

73. • Circuit consists of two thyristors such as main thyristor T1 and auxiliary thyristor
TA, inductor L, diode D and a commutation capacitor C.
• The main thyristor T1 and load resistance RL act as a power circuit but inductor L,
diode D and auxiliary thyristor TA are used to form the commutation circuit.
• Initially, the dc voltage V is applied to circuit, the thyristor T1 and TA are in OFF-
state.
• Triggering pulse is applied to TA at t=t1, it turns ON and capacitor C gets charged.
• The capacitor charging current flows through the path V+ - C+ - C- -TA -Load -V-.
• Since the voltage across the capacitor C increases gradually, the current flow
through the thyristor TA decreases slowly.
• Whenever the capacitor is fully charged to V, the auxiliary thyristor TA gets turned
Class D/ Impulse Commutation

74. • When the triggering pulse is applied to T1, the current flows in the two different
paths:
• The load current IL follows through V+ - T1 - RL -V- and
• commutation current (capacitor discharging current) through C+ - T1 – L- D- C-.
• When the capacitor is fully discharged, its polarity will be reversed.
• The discharging of capacitor C in reverse direction is not possible due to
presence of diode D.
• Whenever the thyristor TA is triggered and turned on, capacitor C starts to
discharge through C+ - TA – T1- C-.
• When the commutating current becomes more than IL, T1 gets turned OFF.
• C will again charge to the supply voltage V.
Class D/ Impulse Commutation

75. • Vs is the main voltage source
• T1 is main thyristor which carry the load current,
• V1 is the external voltage source
• T2 and T3 are auxiliary thyristors used for commutation of main thyristor T1
• Initially T1 is conducting and load is being fed by main voltage source Vs through T1.
• To turn off T1, T3 is fired at any time ,assume it to be t=0 sec.
• Once T3 is ON, a resonating circuit
consisting of V1, L and C is formed and
resonating current starts flowing.
Class E/ External Pulse Commutation

76. • Due to resonating current, C gets charged up to 2V1 with its upper plate positive
• After t= π√(LC), capacitor will not allow any flow of resonating current as it is fully
charged.
• Current through T3 gets reduced to zero and hence commutated.
• Thyristor T2 is fired or gated to turn it ON.
• With T2 ON, main thyristor T1 is subjected to a reverse voltage.
• The magnitude of his reverse voltage is (Vs – 2V1).
• Main thyristor T1 is turned OFF.
• After T1 is turned off or commutated,
capacitor dissipated its stored energy
through the load.
Class E/ External Pulse Commutation

77. Snubber Circuits

78. • Thyristors may be subjected to over voltage and over current in some situations.
• Junction temperature may exceed the maximum allowable temperature and device
may be damaged permanently.
• Different efficient cooling methods are used to dissipate the excess heat into
atmosphere.
• During turn-ON and turn-OFF process, there is a lot of power loss within the device
and temperature increases
• During turn ON of thyristor,di/dt may be large, local heating within the device is
possible, device may be destroyed
• False triggering of thyristor is possible due to high dv/ dt and noise signal across
gate to cathode terminals
• For normal operation of thyristors, the following protections should be taken care:
1. di/dt protection
2. dv/dt protection
3. Over-voltage protection
4. Over-current protection
5. Gate protection

79. di/dt Protection
• If a thyristor is forward biased and a gate pulse is applied to gate cathode, anode
current starts to flow in the region nearest to gate cathode junction.
• After that the current spreads across the whole area of junction.
• When the rate of rise of anode current, di/dt is large compared to spreading
velocity of carries 50 m/s, the local hot spots will be developed near the gate
connection due to high current density and thyristor will be damaged permanently.
• Consequently, the rate of rise anode current must be kept within the specified
value during turn on of thyristor.
• To maintain the di/dt within a specified value, a di/dt inductor may be connected in
series with the anode circuit.
• The di/dt rating varies from few tens of A/µs to 500 A/µs.

80. dv/dt Protection
• When rate of rise of dv/dt across thyristor is high, charging current flows through
thyristor and the device will be turned ON without any gate signal.
• This is called dv/dt triggering of thyristor.
• Actually, this type of triggering is called false or abnormal triggering.
• For proper and reliable operation of thyristor, the rate of rise of dv/dt must be kept
within the specified limit.
• The typical value of dv/dt is about 20 V/µs to 500 V/µs.
• For dv/dt protection, a snubber circuit, a series combination of resistance Rs and
a capacitance Cs, is connected across thyristor.

81. Over voltage Protection
• The transient over-voltage across thyristor may turn ON the device without any
gate signal and perform malfunction.
• Generally, thyristor should be able to withstand external over-voltage and internal
over-voltage.
• External over-voltage: Happens when current flow in an inductive circuit is
interrupted, when the lightning strikes on the lines, when transformer is energised
or de-energised.
• Due to transient over-voltage, thyristor will be turn ON abruptly and over-voltage
appear across load.
• Consequently, a large fault current flows though the converter circuit and thyristors
may also be damaged partially or completely.

82. Over voltage Protection
• Internal over-voltage: Generated during turn-OFF process of thyristor.
• After the anode current becomes zero, current starts to flow in reverse direction
due to stored charges and reaches the peak.
• After that current starts to fall abruptly with large di/dt.
• As a series inductance L is present in the converter circuit, a large transient L di/dt
voltage is generated and thyristor may be damaged due to this transient over-
voltage.
• Therefore, voltage clamping devices such as varistor may be used to protect
thyristors from over-voltage.

83. Over Current Protection
• SCRs possess low value of thermal time constants.
• Faulty conditions lead to overcurrent due to which the temperature at the junction
becomes higher than the specified value, destroying the device.
• Circuit breakers or fuses are required to resists the flow of current more than the
rated value.
• Type of system where the measures have to be applied is to be properly
considered.
• For a weak supply network, the fault current is limited, by keeping the source
impedance below the surge current rating of the SCR.
• This means conventional fuses and circuit breakers are employed to deal with
overcurrent.
• Operation must occur in a coordinated way before the overcurrent could damage
the device,
• Over current must be controlled and the branches that are faulty must be isolated.

84. Gate Protection
• Protection of the gate circuit from overvoltage and overcurrent is important.
• Spurious signals appear at the gate terminal when there exist transients in the
power circuit.
• SCR gets on by the unwanted gate triggering.
• Shielded cables are used for gate protection.
• Such cables lowers the chances of inducing emf thus, the unwanted triggering of
the thyristors is minimized to large extent.

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