Unit 2 power electronics converter and alternator

Power electronics

1. Unit 2: Turn-on and Turn-off
Methods of Thyristors.
T. Mukhopadhyay

2. SCR Turn-On methods
•There are various method to turn on
the SCR which are listed below:
Forward voltage triggering.
Thermal triggering.
dv/dttriggering.
Gate triggering.
Light triggering.
•Forward voltage triggering: By
increasing the anode cathode voltage
in forward bias kept the gate terminal
open the width of the depletion
region changes at J2. At a certain
value of the anode cathode voltage
(Forward break over voltage) an
avalanche breakdown happen at J2
and the depletion region vanishes.
Then SCR stars conducting. In practice
this method is not employed because
it needs a very large anode to cathode
voltage. Once the voltage is more than
the VBO, it generates very high
currents which may cause damage to
the SCR.

3. SCR turn on methods
•Thermal triggering: The reverse leakage current depends on the temperature. If the
temperature is increased to a certain value, the number of hole-pairs also increases. This causes
to increase the leakage current. This starts the regenerative action inside the SCR since the (α1 +
α2) value approaches to unity (as the current gains increases). By increasing the temperature at
junction J2 causes the breakdown of the junction and hence it conducts.
•dv/dtTriggering: In forward blocking state junctions J1 and J3 are forward biased and J2 is
reverse biased. So the junction J2 behaves as a capacitor (of two conducting plates J1 and J3 with
a dielectric J2) due to the space charges in the depletion region. The charging current of the
capacitor is given as I = C dv/ dt.Where, dv/dtis the rate of change of applied voltage and C is the
junction capacitance. From the above equation, if the rate of change of the applied voltage is
large that leads to increase the charging current which is enough to increase the value of alpha.
So the SCR becomes turned ON without a gate signal. It is a false turn ON process and also this
can produce very high voltage spikes across the SCR so there will be considerable damage to it.
•Light Triggering : An SCR turned ON by light radiation is also called as Light Activated SCR
(LASCR). This type of triggering is employed for phase controlled converters in HVDC transmission
systems. In this method, light rays with appropriate wavelength and intensity are allowed to strike
the junction J2. when the light struck electron-hole pairs are generated at the junction J2 which
provides additional charge carriers at the junction leads to turn ON the SCR.

4. SCR turn on methods (Contd……)
•Gate Triggering: This is most common and efficient method to turn ON the
SCR. When the SCR is forward biased, a sufficient voltage at the gate
terminal injects some electrons into the junction J2. This result to the
breakdown of junction J2 even at the voltage lower than the VBO. In gate
triggering method, a positive voltage applied between the gate and the
cathode terminals. The train of pulse signal is used at gate terminal for
triggering purpose. The main advantage of this method is that gate drive is
discontinuous or doesn’t need continuous pulses to turn the SCR and
hence gate losses are reduced in greater amount by train of pulses. For
isolating the gate drive from the main supply, a pulse transformer is used. A
pulse transformer is basically a 1:1 transformer. Higher the frequency of
the gate signal lower the size of the gate drive component.

5. Resistance triggering circuit
•Resistance triggering circuit is the simplest and most
economical.
•The range of the firing angle for this type of triggering is 0 to
90 degree.
•Here R2 is the variable resistance which is used to control the
gate current. R1 is the stabilizing resistance which is used to
limit the maximum gate current.
•During the positive half of the supply voltage diode allows
the flow of current. Initially current flows through R1, R2,
Diode, R since the thyristoris off.
•The voltage across the R appears across the gate terminal of
SCR. When the voltage across the R reaches to the threshold
value the SCR turns on. The value of R should be selected in
such a way that the gate voltage must not exceed the
maximum permissible limit. The instant at which SCR turns
ON is called as firing angle.
•By changing R2 the gate current changes simultaneously the
firing angle changes. The firing angle is proportional to the R2
but the firing angle can not go beyond 90 degree in this case.
•As the firing angle increases from 0 to 90 degree the output
can be controlled from 100% down to 50 %.

6. R-C triggering circuit
•In this triggering the firing angle can be controlled
up to 180 degree.
•During positive half cycle Capacitor gets charge
through variable resistance R with upper plate
positive. When the Vcreaches to the gate triggered
voltage D1 is forward bias and as a result SCR is
turned on. During this period capacitor discharge
until negative voltage cycle appears across
capacitor. By changing the value of R the charging
time (RC) changes so accordingly firing angle
changes. As R increases firing angle increases.
•During negative half cycle capacitor charges
through D2 with lower plate positive to the peak
supply voltage Vmat Ѡt=270. After that supply
voltage increases from –Vmto zero at Ѡt=360.
During this period the capacitor voltage Vcmay fall
slightly. In this case D1 is reverse bias and prevent
the SCR to turn on during this period.

7. SCR triggering circuit using UJT
•UJT has a lightly doped n-type silicon layer to which a
heavily doped p-type emitter is embedded. It has three
terminals (B2,B1and E)
•When the R2 & R3 are much less than the inter base
resistance (5-10kΩ) the UJT act as relaxation oscillator.
The output pulse is used to trigger the SCR.
•Here when the switch is ON the capacitor stars charging
through R1. When the Vccharges up to the peak point or
VpUJT starts conducting and capacitor discharging
through R3 (47Ω). R3 is selected based on voltage level
required to trigger the SCR.
•The pulse voltage between B1 and ground is used to
trigger the SCR.
•In this case firing angle can be controlled in a range of 0
to 180 degree.
•The firing angle can be varied by varying the value of R1.

8. Synchronized UJT triggering circuit
•Here the Diode D1-D4 rectify ac to dc. Zenerdiode
is to clip the rectified voltage to a standard level Vz
which remain constant. This voltage Vzis applied
to the charging circuit RC. The capacitor charges at
a rate determined by R. When the capacitor
voltage reaches up to unijunction threshold
voltage (peak voltage) the emitter and B junction
break down and capacitor starts discharging
through the primary winding of pulse transformer.
As the current is in the form of pulse the voltage at
the secondary windings of pulse transformer is also
pulse in nature. Pulses at two secondary windings
feed the same in phase pulse to two SCRs of a full
wave rectifier. The firing angle can be controlled by
controlling the value of R.

9. Programmable Unijunction Transistor
•Programmable Unijunction Transistor (PUT) is a
three-terminal four-layer device like theSCR, the
difference being that the gate terminal is
connected to then-type layer near the anode. It
operates like SCR It will block current until
triggered on. When triggered it will latch on if the
current is high enough and remain on until the
anode current falls below the holding current. To
trigger the PUT into the on state, current is passed
from the anode to the gate, therefore the anode–
gate junction must be forward biased. It’s
characteristics is similar to UJT. The transition from
the off to the on state is also faster in the PUT than
in the UJT. The most significant advantage of the
PUT over the UJT is that the standoff ratio is
variable, externally programmable and not
determined by the structure or manufacture of the
device. The peak point voltage may be varied by
varying the potential on the gate of the PUT
relative to the anode.

10. Programmable UJT as relaxation oscillator
•The PUT is used in a relaxation oscillator circuit similarly
to the UJT. The distinction is that the standoff ratio is
programmable.
•Resistors R
1and R
2set up a bias voltage on the gate
terminal of the PUT. The voltage is dependent on the
supply voltage and the respective values of R
1and R
2.
These resistors program the standoff ratio.
•When the supply is connected, the capacitor charges via
RV
1. The rate at which it charges is fixed by the time
constant of the RC circuit.
•When the anode voltage (the voltage across the
capacitor) reaches the peak value, the PUT turns on.
•When the PUT turns on, the capacitor discharges via the
PUT and R
3. The resistance of this discharge path is very
low, so the discharge time is very short.
•The resulting output pulse will have a high peak value
with a very fast rise time, ideal for triggering an SCR or a
TRIAC.

11. Pulse transformer based triggering circuit
•The pulse transformer’s main function is to produce a signal for SCR as well
as to give electrical isolation.
•This transformer has usually two secondary. The urns ratio from primary to
two secondary is either 2:1:1 or 1:1:1.
•It has low winding resistance, low leakage reactance and low inter-winding
capacitance.
•When square waveform is applied to the transistor T1 which acts as a
switch. When the input pulse is high the transistor is ON the DC voltage is
applied to the primary of pulse transformer and transferred to the
secondary.
•When the amplitude of the supplied pulse is low the T1 is OFF and diode D
F
starts to operate and carry the demagnetizing current of primary winding
and no voltage appears across secondary.
•RL limits the current in the primary of the pulse transformer.
•Diode D in secondary prevent the reverse flow of gate current.
•For a high value of magnetizing inductance the input pulse is faithfully
transmitted as a square pulse at the output terminal of pulse transformer.
•For low value of magnetizing inductance the input pulse is transmitted in the
form of exponentially decaying pulse at the secondary.

12. Optocouplerbased triggering circuit
•An optocoupler consists of a photo-
transistor and an LED.
•When current flows in the LED, the emitted
light is directed to the phototransistor,
producing current flow in the transistor. The
coupler may be operated as a switch, in which
case both the LED and the phototransistor are
normally off. A pulse of current through the
LED causes the transistor to be switched on
for the duration of the pulse.
•there is a high degree of electrical isolation
between the input and output terminals and
so the termoptoisolatoris sometimes used.
•Here when the opto-coupler is ON the
current flows through R1 and R2. The voltage
across the R2 is used to trigger the SCR.

13. SCR turn off methods
•The process used for Turn-OFFa SCR is called as
Commutation. By the use of Commutation Process,
the SCR operating mode is changed from Forward
Conducting Mode to Forward Blocking Mode (OFF-
State).
•So, to turn OFF a conducting SCR properly the
anode or forward current of SCR must be reduced
to zero or below the level of holding current and
sufficient reverse voltage must be applied across
the SCR to regain its forward blocking state. When
the SCR is turned OFF by reducing forward current
to zero, excess charge carriers exists in different
layers. To regain the forward blocking state of an
SCR, these excess carriers must be recombined.
Therefore, in order to accelerate this
recombination process, a reverse voltage is applied
across the SCR.
•Commutation methods are classified into two
categories: 1. Natural commutation 2. Forced
Commutation.
•Natural commutation: If the SCR is connected to
an AC supply, at every end of the positive half
cycle, the anode current naturally becomes zero
(due to the alternating nature of the AC Supply). As
the current in the circuit goes through the natural
zero, a reverse voltage is applied immediately
across the SCR (due to the negative half cycle).
These conditions turn OFF the SCR. This method of
commutation is also called as Source Commutation
or AC Line Commutation or Class F Commutation.
This commutation is possible with line
commutated inverters, controlled rectifiers, cyclo
converters and AC voltage regulators because the
supply is the AC source in all these converters.
During the positive half cycle of the AC Supply, the
load current flows normally. But, during the
negative cycle, the SCR will turn OFF. For successful
natural commutation, the turn OFF time t
OFFmust
be less than the duration of half cycle of the
supply.

14. Forced commutation
•In case of DC circuits, there is no natural current zero to turn OFF
the SCR. In such circuits, forward current must be forced to zero
with an external circuit (known as Commutating Circuit) to
commutate the SCR. Hence the name, Forced Commutation.
•This commutating circuit consist of components like inductors and
capacitors and they are called Commutating Components. These
commutating components cause to apply a reverse voltage across
the SCR that immediately bring the current in the SCR to zero.
•Depending on the process for achieving zero current in the SCR
and the arrangement of the commutating components, Forced
Commutation is classified into different types.
•Class A-series resonant commutation circuit.
•Class B-shunt resonant commutation circuit.
•Class C-complementary commutation.
•Class D-Auxiliary Commutation.
•Class E-External pulse commutation.
•This commutation is mainly used in chopper andinverter circuits.
•Class A commutation: This is also known as the Resonant
Commutation. The commutating components are L and C and
the Capacitor can be connected either in parallel or in series
with the load resistance R
L. The value of load resistance and
the commutating components are selected in such a way that
they form an under-damped RLC resonant circuit. When the
circuit is applied with a DC Source, the forward currents
starts flowing through the SCR and during this period, the
capacitor is charged up to the value of V
dc.The resonant
frequency of the circuit, which depends on the Commutation
Components L and C and also on the load resistance,
determines the time for switching OFF the SCR. This type of
commutation is generally used in Series Inverters. To make
the circuit underdamped the relation between R, L,C must
satisfy R
2
<(4L/C) and the value of C should be selected in
such a way that load voltage passes through zero before the
current passes through zero. So the load current must lead
the load voltage by an angle α=tan
−1
(
????????????−????????????
??????
). So, it must be a
leading power factor circuit.
•The turn off time of circuit is α/Ѡ
d.

15. Class B commutation or resonant pulse
commutation
•This type of commutation is also called a current commutation.
•The commutation circuit comprises ofC,L and an auxiliary thyristor
TA. Initially main thyristorT
1and TA are in off state and capacitor C is
charged to voltage V
swith left hand plate positive.
•Now, at t=0, the main thyristor/ SCR is gated and turned on. Load
current equal to I
0starts flowing through the main thyristorT
1and
Load. Now, we want to turn this thyristoroff. To do this, we fire the
auxiliary thyristorTA at t=t
1. Till time t=t
1, the capacitor is charged with
source voltage V
si.e. v
c= V
s, capacitor current i
c= 0 and current
through main thyristorT
1i.e. i
0= I
0.
•When auxiliary thyristorTA is fired, it starts conducting and provides a
path for the discharge of capacitor C. L, C and TA forms a resonating
circuit.??????
??????=-??????
??????
Τ
??????
??????sin(Ѡ??????). Negative sign in the above expression of
resonating current is given as the actual current flows in a direction
opposite to the direction of current i
c. after half cycle, the value of
i
creduces to zero at t=t
2. This means, the current through the auxiliary
thuristorTA reduces to zero & voltage across the TA is also reversed at
t=t
2then TA will get turned off.
•Now, TA is OFF and capacitor C is charged up to source voltage Vswith
its right hand plate positive. This means, the diode D is now forward
biased and hence resonating current i
cwill now flow through least
resistive path i.e. through C, L, D and main thyristorT
1. But this
resonating current icwill flow through the main SCR T
1from cathode
to anode i.e. in reverse direction. This simply means, the current I
through the main thyristorT
1will be given as
•I = I
0–i
c
•When the magnitude of i
creaches I
0, the current through the SCR
T
1will become zero. This can be seen at t=t
3. At this time current
through main thyristoris zero and voltage across it is reversed biased
so main thyristorwill be turned off.

16. Class C commutation
•This commutation technique makes use of two thyristors, one is main and
another is auxiliary (complimentary). Here the main thyristoris denoted as
T
1while the complementary thyristoris T
2. The main thyristoris serially
connected with the load while the auxiliary one is parallellyconnected with
the main thyristor. It is used in current source inverter.
•It is given the name complementary commutation due to the reason that
the two thyristorsin the circuit get on alternatively. This means that to turn
the thyristoroff, the auxiliary thyristoris triggered whereas to turn off the
auxiliary thyristor, T
1is triggered.
•there are three modes of operation.
•Mode 0:This mode of operation corresponds to the initial state of the circuit
when both the thyristorsare in off state and so the voltage across the
capacitor is also 0. This means,T
1= off;T
2= off;V
C= 0.
•Mode I: In this mode of operation, the circuit is provided with dc supply
input and along with that thyristorT
1is triggered with a gate signal. Due to
this, T
1will come in conducting state. This leads to two currents one will be
the load current while the other will be the charging current of the capacitor
to flow through the whole circuit. Hence, the overall current that flows
through SCR T
1will be the sum of load current and charging current
•??????=
??????
????????????
+
??????
??????
. Due to the flow of the charging current, the capacitor gets
charged up to the peak of the supply input. However, the charging current
reduces to 0, once the capacitor gets fully charged up to the supply input,
and thus the only current that continues to flow through T
1is the load
current. Hence, T
1= on;T
2= off;V
C= V.
•Mode II: In this mode of operation is such that by providing a triggering pulse
to T
2the path becomes short-circuited. Once this happens, the polarity of
the charge stored by the capacitor reverse biases the thyristorT
1. This
reverse biased condition leads to turning off of the thyristorT
1.The overall
current flowing through thyristorT
2is
• ??????=
2??????
????????????
+
??????
??????
. So, by triggering T
2i.e., complementary SCR, the main SCR
T
1will get turned off. Hence, mode II operation provides, T
1= off;T
2=
on;V
C1= -V.

17. Class D commutation
•This type of commutation used in step down chopper circuit.
•Here, T
1is the main thyristorthat is required to be commutated while T
xis
an auxiliary thyristorwhich is part of external circuitry that turns off the
device. However, the circuit also includes elements such as a capacitor,
diode, and inductor combinedlythat operate to perform the commutation of
the thyristor.
•Initially, on applying voltage V across the circuit but in the absence of gate
triggering pulse both the thyristorsare in the off state. Due to the supply
input, the diode is in reverse biased condition thus, no flow of current takes
place through the circuit hence the voltage across the capacitor will be 0.
Further, out of the two thyristors, the auxiliary thyristoris provided gate
triggering pulse which brings it to conducting state. Thus, the current starts
to flowthrough capacitor. This flow of current charges the capacitor C with a
similar polarity as that of supply input and it gets charged up to the peak of
the supply value i.e., V
dc.once the capacitor gets fully charged then the
polarity across the capacitor reverse biases the auxiliary thyristori.e.,
T
xleading to turn it off. Hence, in this case, the capacitor will store the
charge.
•once T
xgets turned off then gate triggering pulse is given to thyristorT
1thus
it will get turned on under the presence of supply input. As T
1comes into the
conducting state, then there will be two loop currents that will flow through
the circuit. The current I
Lcorresponds to the load current and we have
already assumed it to be constant at the beginning itself. While
I
Ccorresponds to the current flowing through the series LC circuit which
gives rise to LC oscillations within that loop. This current will be the
discharging current of the capacitor. As the negative half of the signal will
reverse biases the diode. but as it is the property of the inductor that it
opposes the current that produced it hence the inductor starts releasing the
stored energy. Due to the release of the stored energy of the inductor, the
capacitor in the circuit gets charged again but this time with opposite
polarity than the former case.Now, as T
1is in conducting state and we have
to commutate T
1thus, for this we need to trigger T
xi.e., the auxiliary
thyristor. Once, T
xbegins to conduct along with T
1then the polarity across
the capacitor will bring T
1to a reverse-biased state and this will lead to
turning the thyristoroff.

18. Class E commutation
•An external current pulse is used in this technique to
commutateSCR. It is also called as external pulse
commutation.
•Initially thyristorT1 is conducting and hence, load is being fed
by main voltage source V
sthrough T1. Now, we want to turn
off main SCR T1. For this, thyristorT3 is fired or gated at t=0.
Once T3 is ON, it starts conducting at t=0 sec. A resonating
circuit consisting of V
1, L and C is formed and resonating
current starts flowing. Due to resonating current, capacitor C
gets charged up to 2V
1at t = π√(LC) with its upper plate
positive. After t= π√(LC) capacitor will not allow any flow of
resonating current as it is fully charged. Thus, the current
through T3 gets reduced to zero and hence commutated.
•Now, thyristorT2 is fired or gated to turn it ON. Once T2 is
ON, it starts conducting. With T2 ON, main thyristorT1 is
subjected to a reverse voltage. The magnitude of his reverse
voltage is (V
s–2V
1). Therefore, main thyristorT1 is turned
OFF. After T1 is turned off or commutated, capacitor
dissipated its stored energy through the load.