Hello, in my last class we discussed the need
for snubber circuits, the turn on snubber
and turn off
snubber. These circuits are required to protect
the device against di by dt and the voltage
spike across the SC. How did we limit di by
dt? We connected a small inductor in series
with the thyristor. Now, we know the current
through the inductor cannot change instantaneously
and how do we limit the voltage spike across
the SCR? We use a RC network.
The second point that we discussed was high
frequency pulse pattern is recommended to
trigger the thyristor, large number of high
frequency pulses. How do we transmit these
signals from the control circuit to the gate?
We used a pulse transformer. See here, this
is the pulse transformer.
It is a special type of transformer. Why it
is a special type? The signals or the frequency
of the signals that is transmitting from one
side to the another side is very high. The
principle of operation of this transformer
is same as that of a 50 Hertz transformer.
Only the type of core is different. See
in a 50 Hertz transformer, we use a laminated
core whereas a high frequency transformer
a ferrite core is used.
1
Ferrite core, of course this is a bigger ferrite
core, a solid ferrite core, these are suitable
for high frequency application. So, there
are 4 terminals, primary winding and the secondary
winding. On top it is written, 1 is to 1,
it implies that the number of turns in the
primary winding is same as the number of turns
in the secondary winding. So, this is a pulse
transformer used to isolate the power circuit
from the control circuit.
Now, what are the different types of SCR’s?
We had different types of diodes. Similarly
there are different types of SCR’s also.
One of them is see here, the converter grade
SCR, converter grade SCR. These are slow devices,
they are used in the circuit wherein the frequency
could be 50 Hertz and the second one is the
inverter grade SCR. So, these are fast devices,
fast devices so suitable for high frequency
application. So see, here is the module that
has two thyristors.
2
Inverter grade thyristor, the rating of each
thyristor is 45 amperes and the voltage rating
is 1200 volts. Two thyristors, see they are
connected in this fashion. See, these are
the 2 thyristors, 3 terminal, 3 power terminals
have been brought out, anode of one, anode
of one, a common point, a common point and
the cathode of another. See, these 2 terminals
for supplying the gate signal, cathode and
gate, cathode and gate, see here cathode and
gate. See the contact area, this anode, cathode
so generally the load current flows.
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So, see the surface area that is available;
whereas, see the surface area that is available
for gate and cathode. A small current is flowing
from the gate and it is mounted on a heat
sink. See, how elegant it looks.
So, that is why I told you in my introductory
lecture that the one of the main reason for
the popularity of power electronics is the
advances in power semiconductor technology.
So, the rating of this thyristors is given
here, I copied from, I downloaded this data
sheet from their site.
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See, 46F. F stands for fast 08 dot dot till
13. This stands for the various voltage ratings.
See, if it is only 08, the voltage rating
is 800 volt . If it is 13, the voltage rating
is 1300 volts. See, repetitive peak, forward
off state and reverse voltages. These are
the various voltages. See, the RMS current
rating, 120 amperes whereas, the average on
state current is 45 amperes. See, the surge
current rating, 1300 amperes. I explained
to you what is surge current when it flows,
1300 amperes and another important parameter,
I squared t, current squared time rating is
required to determine the fuse. 8450 amperes
squared s.
See, the critical rate of rise of on state
current, di by dt critical is 120 ampere per
micro second. See, the gate trigger current
IGT 150 milli amperes. So therefore, just
see the gain average current, rating is 45
amperes. Surge current is, Surge current is
1300 amperes, gate current is 150 milli amperes.
See, the gate trigger voltage 1.4 volts, holding
current 250 milli amperes, latching current
1000 milli amperes. See, I told you that latching
current is higher than the holding current.
So, these are the some of the important parameters
of the thyristor.
So, that’s about the conventional thyristor
which conducts when it is forward biased,
when it is reverse biased, it doesn’t conduct.
What if input is AC output is also AC? Like
you know fan regulators. See, I have a 50
Hertz AC supply 230 volts and there is a regulator
to regulate the voltage applied to the machine
or voltage applied to the fan to vary the
speed. Fan is again a single phase AC machine.
So, it requires a AC supply. I have an input
AC, output is also AC. So definitely, I need
to connect 2 thyristors in anti-parallel.
Now, instead of connecting 2 thyristors separately
in anti-parallel, there is a device available
what is known as the Triac. A Triac was developed
in 1964 by general electric.
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It has a complicated structure 
but then functionally it is equivalent to
2 thyristors connected anti-parallel. See
here, here is the connection, I have 1 thyristor,
thyristor 2, gates are tied together G and
I am calling this terminals as MT1 and this
a MT2. See, I cannot call anode and cathode
because anode of one is tied to cathode of
another thyristor. So, there are 2 equivalent
thyristor connected in anti-parallel. So,
now it is going to be a bidirectional device.
Remember, it is a bidirectional device, there
are 2 power terminals but then there is only
1 gate terminal.
So therefore, how do I trigger? A Triac can
be triggered by see here, making MT2 positive
with respect to MT1 and supplying a positive
gate current with respect to MT1. See, I will
repeat, MT2 should be positive with respect
to MT1 and a positive gate current with respect
to MT. It can also be triggered when MT2 is
negative with respect to MT1 and by a negative
current, negative gate current with respect
to MT1.
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So, here are the characteristics. 2 thyristors
connected in anti-parallel, say thyristor
1 and this could be thyristor 2. These are
the VI characteristics, negative resistance
region-unstable, conduction- forward blocking,
forward blocking mode for another thyristor,
conduction mode for the, for that same thyristor.
So, these are the VI haracteristics. This
triacs are very popular in fan regulators.
I told you, I showed you a very elegant fan
regulator, just the knob is brought out everything
is mounted inside a switch board.
So, there is no power dissipation. That is
why it is so small and I told you, this is
a power semiconductor device, has three legs.
MT1 MT2 and gate, this are nothing but a Triac.
This is a Triac.
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This is a Triac. So Triacs are also used in
light intensity control, room temperature
control 
but then what is the limitation of a Triac?
See I am just showing you the turn off characteristic
of a thyristor.
The current has reversed, it has attained
a peak value, the reverse recovery current
and at this point J1 and J2, J1 and J3 have
attained the voltage blocking capability.
So therefore, a negative voltage is applied
across the thyristor and this happens, this
current decays at a very fast rate. So, this
is th e turn off characteristics that we studied
for the thyristor. So, when this current is
becoming 0, the voltage across it is very
different from 0.
Now, this is for 1 thyristor, the turn off
characteristics. When it is turning off there
is a high voltage appearing across the entire
combination across the Triac. So, there is
another thyristor connected in anti-parallel.
Now, if this dv by dt is high, it may trigger
another thyristor when this current has become
.
See, this apprehension was not there when
in the case of thyristor because there is
only 1 device. It has to block in the reverse
direction as long as this voltage across it
is less than the reverse breakdown voltage.
But then in a triac, there are 2 thyristors,
the triac is equivalent to 2 thyristor sconnected
back to back. So, when a reverse voltage is
being applied to one of them, the forward
voltage appears across another thyristor.
So, the another thyristor may get triggered
because of this dv by dt or in other words,
triac has a less time than a thyristor to
recover its blocking power.
See, I have written here, it has less time
than the thyristor to recover its blocking
power or the dv by dt rating is less for the
triac. So, now let me sum up the thyristor.
It is nearly an ideal switch. Why it is an
ideal switch? It requires just a sharp pulse
to turn on. If 45 ampere thyristor, average
current rating, maximum gate current is 150
milli amperes that is required only during
this switching or turn on period. When the
current is higher than the latching, you can
withdraw the gate signal.
8
So, the gate power requirement is very small
compared to the power rating of the thyristor.
It can block both positive as well as negative
voltage. The rating of this thyristor is 1200
volts forward as well as reverse, also high
voltage, high current devices are available
in the market. The fourth point, device is
very rugged, sturdy but then what is the limitation?
It has 1 limitation. It cannot be turned off
by the application for control signal at the
thyristor gate. It cannot turn it off by applying
a negative gate current through the gate.
So see here, I have summed all the properties
of the thyristor.
So, this is the limitation, inability to turn
off by application of a control signal at
the thyristor gate. To turn off the thyristor,
current flowing through it should be reduced
to a value which is less than the holding
current. Till then it continues to conduct.
So, how do I make this device which is capable
of turning off through gate? Can I do some
modifications? Answer is yes. So, there is
a device what is known as the gate turn off
thyristor, GTO, which is capable of turning
on as well as turning off through gate. By
applying a control signal, we can turn off
as well as turn on a GTO. See here, a gate
turn off thyristor, GTO.
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A small power GTO was developed by GE, General
Electric in 1961 and in 1981, a 2.5 KV, 1
kilo ampere GTO was developed by Hitachi and
Toshiba. It can be turned on by positive IG
and can be turned off by negative IG and here
are the 2 symbolic representations of the
GTO, the same. Anode cathode structure is
the same, across here are 2 arrows, positive
IG negative IG. So, these are the 2 symbols
used to represent a GTO.
What is the structure? In what way it is structurally
different from a thyristor? It has 4 layers
similar to SCR, P1 N1 P2 N2. Then, in what
way it is different? How it is being turned
off by
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applying a negative IG to the gate? We will
see. See here P1N1 P2 N2, one of the difference
is thickness of P2 is less in the case of
a GTO. Why, I will tell you some time later.
The second difference is N2 layer is removed
by itching in place where gate contacts are
situated.
I will show you a 3D picture and it will be
very clear, see here.
See, we have P or P1 N1 P2. Now, N2 is in
small places, small islands of N2 are found,
see here. There is 1 here, see here, again
a separate N2 layer, a small island of N2
layer. See, this minus
indicates the doping level is very low and
the plus indicates doping level is very high.
So, it is similar to SCR, N2 layer is highly
doped, N1 layer is very lightly doped.
So, there are large numbers of small small
islands of cathode or in other words, what
I can say is see, there are large number of
GTO’ s. See here, P1 N1 P 2 N2, P1 N1 P
2 N2 see here also P1 N1 P 2 N2 and all the
cathodes of these GTO’s are connected to
a common heat sink and that forms a main
cathode. So, what you can say is a GTO can
be seen as a large number of small GTO’s
in parallel, as if there is a GTO here or
another GTO here, another GTO here. One GTO
can be seen as a large number of GTO’s connected
in parallel. Why parallel?
Anode is same, cathode is also same, all the
cathodes are connected to 1 heat sink and
that heat sink forms the main cathode and
another difference between a GTO and a SCR
is in a GTO gate and cathode structures are
highly inter digitized. What do you mean by
highly inter digitized? It is something like
this, this is inter digitization, this could
be inter digitization. So, what is the advantage
of doing like a inter digitization? The advantage
is that now the cathode periphery has increased,
cathode periphery has increased, also the
distance between the gate and a cathode is
very small. If this is gate and this is cathode,
this is inter digitization, cathode periphery
has increased the distance between the cathode
and gate has reduced.
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What is the advantage of this inter digitization?
I will tell you some time later. Now, you
just see the structure again. I have a P layer,
it is equivalent to P1 in SCR. Junction J1,
N layer lightly doped, junction J2 and J3.
So, when it is reverse biased, J1 should block
the voltage because though J3 is also reverse
biased, the reverse blocking voltage of J3
is very small similar to SCR and when is forward
biased J2 blocks the voltage.
Now, there is another structure. See here,
wherein a N plus layer in other words, a highly
doped N layer penetrates this P1 layer and
it is directly in contact with the N minus
layer or the N1 layer.
See here in this figure, this N plus layer
penetrates at regular intervals, regular intervals
and it is directly in contact with N1 or N
minus layer. So, in other words there is a
direct short between the anode and J1 when
it is reverse biased. See, when it is reverse
bias, entire voltage has to be blocked by
J1 because we had a P structure. Now, because
of this N plus which is in directly in contact
with anode and N1, now J1 cannot block the
negative voltage, the only 1 junction that
can block the negative voltage is J3.
The reverse voltage blocking capability is
very small. So in other words, this structure
cannot block negative voltage or a anode short
structure cannot block negative voltage. So,
this GTO is also known as asymmetrical GTO.
Why it can block only positive voltage? It
is because of junction J2 and it cannot block
the negative voltage. If at all, if it can
block is a very small voltage that is because
of J3. The another advantage of doing this
modification is that it speeds up the turn
off process. Now, how it speeds up the turn
off process? I will tell you while doing the
turn off of a GTO. See in this, a 3D figure
shown, here is same thing P1 or P plus highly
doped N plus N minus P.
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Now, coming to the inter digitization, I told
you, the remote part of a cathode is very
near to the gate. In SCR, why do we limit
di by dt during turn on? It is because the
area that is available for conduction is very
small when you turn on the device. It is a
area of the cathode adjacent to the gate electrode
is available and afterwards the conduction
spreads to the other parts. Now, what happens
in the GTO because of this inter digitization?
Even the remote part of the cathode is very
near to the gate. So in other words, a large
area is available during the turn on period
or at the instant of turn on, a large area
is available. So therefore, you can have a
very high di by dt during turn on.
Since the di by dt is very large, GTO can
be brought into conduction at a much faster
rate compared to SCR. This is because of inter
digitization. Even the remote part of the
cathode is very near to the gate, large area
is available, so you can have a very high
di by dt. So, if I can have a very high di
by dt, I can or turn on time of the GTO reduces.
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See, in a 3D it looks something like this,
I have chosen a anode short structure. So,
these are the gates, the cathode, the direction
of holes, the direction of electric. So, GTO
can be brought into conduction very rapidly
that is because of a very high di by dt is
possible.
Now, coming to the turn on characteristics,
they are similar to an SCR. There is no much
difference between the SCR characteristics
and the GTO characteristics. It is a latching
device, in the sense, in principle, gate signal
can be withdrawn once the anode current is
higher than the latching current. But it is
recommended that a positive IG is maintained
throughout during the conduction period. Why?
It is because of this reason, I told you one
of the limitations or one of
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The difference between the GTO and SCR is
that holding current for a GTO is much higher
compared to a SCR. Holding current of a GTO
is higher compared to SCR.
Now, assume that during some disturbance,
anode current has dipped momentarily. Since
the anode current has dipped momentarily,
some of parts of the GTO might turn off and
now again the current, anode current increases
very rapidly, the area that is available for
conduction is very small. So therefore, there
could be some hot spots and because of this
localized heating, a GTO may get damaged.
So therefore, it is recommended that during
conduction period, a positive IG is maintained
throughout. But then, you do not need to maintain
the same magnitude of the gate current.
So, what is being done is a high pulse of
gate current is provided during turn on, so
that the turn on time can be reduced and after
sometime you can reduce this gate current.
See the wave form, looks something like this.
A high gate current, so anode current has
attained a steady value after sometime td
what is known as the delay time. The voltage
across the device also reduced to its saturation
value. So, after sometime you can reduce the
gate current to IGT. So, this value is approximately
10% of this peak value.
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So that is about the turn on of the GTO. Now,
coming to the turn off of a GTO, so even in
the case of a thyristor when the thyristor
is on, both T1 and T2 are in saturation. That
is why the voltage drops to a very low value,
could be of the order of 1.5 volts or so.
So, T1 and T2 are in saturation. Now, you
want to turn off the GTO, so first thing that
has to be done is you have to bring that T2
out of saturation. See in this figure, this
is T1, PNP transistor and this is T2, NPN
transistor, both are in saturation.
Now, device has to be turned off. How do I
turn it off? How to bring this transistor
out of saturation? How do I bring this transistor
out of saturation? I have to reduce the base
current. Now, let us see what is the relationship
between the anode current and the gate current
that has flowing out of the gate terminal.
See, this expression that we have derived
for the SCR is still valid here. The total
saturation current is given by alpha2 into
I G plus ICBO is a sum of I CO1 and IC O2
divided by 1 minus alpha1 plus alpha2. Wherein,
alpha1 and alpha2 are common base current
gains. We have already defined them for the
SCR. When SCR is in on state, we have reduced
the gate current to a very small value. So,
in on state, I can neglect this term , so
IA can be given by ICBO divided by 1 minus
alpha1 plus alpha2.
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See here, this is the current that has to
be turned off. See, in the previous equation,
IA becomes 0 when the numerator becomes 0.
So, when can you make this numerator zero
? When, ICBO is equal to minus of alpha2 into
IG , see here. So, the relationship is IG
is equal to minus of ICBO divided by alpha2.
So, if I want I want to find out the gain
or the relationship between the anode current
and the gate, it comes like this, IA divided
by IG is equal to alpha2 divided by alpha1
plus alpha2 minus 1.
How do I improve this gain? In other words,
I make this gain as high as possible? Alpha2
should be as high as possible. What is alpha2?
It is gain of N1P2N2 transistor. Se here,
alpha2 for this transistor, this is N1P2N2.
How do I make alpha2 as high as possible?
One way is to make the thickness of P2 layer
less. P2 layer should be very thin. See, that
is one of the difference between SCR and GTO.
I told you the first difference that I told
you, layer of P2 is less compared to a SCR
and second is increase in doping level in
N2, thereby increasing the value of alpha2.
Now, how to turn off a GTO? What happens exactly
during the turn off process? IG has reversed,
IG is flowing out of the gate, IG is connected
to P2. See here, IG is P2, now IG is flowing
out of the gate terminal, so holes are extracted
from P2.
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So, as these extractions take place, the volt
age drop is developed in the P2 region. I
will tell you holes or holes from the anode
are extracted from the P base. So, during
this process, voltage drop is applied in P2
base region and eventually this voltage reverse
biases your gate and cathode junction and
both goes into cutoff. But then, entire turn
off process is not completed as yet. As the
holes extraction continues, P2 is further
depleted. See, first is gate cathode junction
gets reverse biased but then IG is still flowing
out. So, P2 gets further depleted. What happens?
Conduction area drops. Now, the current may
be flowing in the remote parts of the anode
or far away from the gate.
Now, it may so happen that the current density
in those parts may increase and if this happens
there would be a localized heating and device
may fail. So, this has to be avoided, this
has to be avoided. See, the figure that showing
the turn off process, it is something like
this.
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All the holes are diverted towards the gate.
The P2 gets further depleted, now anode current
tries to flow through the area which is far
away from the gate area. There is a reduction
in the area that is available, it may form
or in that area, the current density may increase
localized heating, eventually device may fail.
Now, the turn off of a GTO is greatly influenced
by the turn off circuit. Unfortunately, turn
off gain of GTO is very low. Turn off gain
is very low, could be of the order of say,
6 to 15, generally.
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So therefore, if anode current is 100 amperes,
by the way GTO is a high current device, high
current device, high current, high voltage
device. So, if anode current is around 100
amperes, gain is of the order of say, 6 to
15. So, gate current that is flowing out of
the terminal is 10 amperes, 10 amperes and
it is fortunately for a very short duration.
So, as the gate current starts flowing out
of the terminal, for some time anode current
remains constant. So, this period is known
as the storage time. See in this figure.
At steady state, gate current has been, we
have to reduce the gate current to a very
low value. Somewhere at this point, you desire
to turn off the GTO. So, IG reverses, though
IG has reversed, IA still remains constant
for ts duration. So, this is known as the
storage time. During storage time, anode current
remains approximately constant and this period
ts can last for a few microseconds.
Now, what happens after ts? This process has
to be studied by taking a snubber into account.
There has to be a snubber circuit to turn
off a GTO. I told you, di by dt rating of
a GTO is very high. So, you can use a very
small inductor in series with a GTO. We had
connected an inductor in series with thyristor
to limit di by dt during turn on. Similarly,
to limit di by dt, we require a very small
inductor. So, the rating of di by dt, even
for GTI is very high. Now there has to be
a turn off snubber, a RC circuit looks like
this, something like this.
Now, there is a diode, it has its own turn
on time and I have shown a small inductor
what is known as loop inductance, in dotted
lines. There is a very small inductor in dotted
lines. You wanted to turn off the thyristor,
so IG has reversed, for some time IA remains
constant and from ts onwards anode current
falls at a very fast rate.
Now, because of this loop inductance and because
of this diode, turn on time of the diode,
this current, anode current cannot start flowing
through the snubber circuit immediately 
and because of this there is going to be a
voltage spike 
because of the inductance which is there in
other parts of the circuit.
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If this inductance 0 and turn on time is very
small, then may be, immediately this anode
current finds a path, the capacitor. Now,
because of this inductor and diode, this current
gets choked and therefore because of this
a large di by dt, this spike appears a cross
the GTO. It could be dangerous, in the sense
that device may fail because of this spike.
So therefore, the snubber circuit layout is
very important. The loop inductance, the inductance
that I have shown in the dotted line should
be as small as possible. Snubber circuit layout
is very critical in the case of GTO. So, the
anode current has fallen to a very low value.
So, at the end of tf it is known as the fall
time and this is very small and from there
onwards a current what is known as the tail
current, starts flowing through this snubber
circuit.
Now, the voltage across the GTO is limited
by the dv by dt, it is determined by the dv
by dt. Now, what is this tail current? This
tail current, that period Ik is equal to zero,
cathode current is zero and the gate current
is same as the anode current. So, this tail
current is corresponding to the free charges,
they exist in N1 layer, N1 layer. The current
due to the free charges which exists in N1
which is nothing but the blocking layer, lightly
doped layer. These carriers are numerous and
they require a finite time to recombine.
See, the problem here is as the voltage rating
increases N1 layer, the thickness of the N1
layer increases. As the thickness of N1 layer
increases, time taken for these carriers to
recombine also increases. So in other words,
the tail current period increases with the
blocking voltage capability of the GTO. So,
what is the consequence of having a higher
fall time or sorry, higher tail time? See,
voltage across the device started increasing,
current in the device is still finite, the
tail current, voltage is a reasonably high
value it has attained, so therefore, the losses
that are taking place device are high. So,
turn off losses in a GTO, they are significant,
significant. So, how do I reduce the turn
off losses? So, one way to reduce is to reduce
the fall or tail current duration. How to
reduce the tail current duration? This dt
has to be reduced.
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Having decided on the voltage rating, N1 layer,
thickness of N1 layer gets decided and the
tail current depends on the thickness of N1.
Is there a way out? Yes. Why did we do anode
shorting? I said one of the advantages of
anode shorting is to reduce the turn off time.
How does it do? What it does is, see this.
Those anode shorting N layers are highly doped.
They were all were N plus, they are highly
doped. What they do is this heavily doped
N cells, they make the minority carriers trapped
in N1, recombine more quickly.
So this highly doped N cells, they helped
the minority carriers trapped in N1, recombine
more quickly. So therefore, your fall or sorry,
tail current time reduces. Sorry, it is a
tail current time reduces but then device
is no longer symmetrical. Now, it can block
only the positive voltage. Negative voltage
it cannot block. So, with this I will conclude
my today’s lecture.
Thank you.
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