4. In forward voltage triggering method, the SCR is forward biased i.e., anode is more
positive than cathode but this voltage is increased significantly. The gate terminal is kept
open.
As the voltage increases, junction J2‘s depletion layer width increases, which in turn
increases the accelerating-voltage of minority carriers at this junction. At a particular
voltage, there will be an Avalanche Breakdown at the inner junction J2 as a result of
minority charge carriers colliding with atoms and releasing even more minority charge
carriers.
This voltage is known as Forward Break over Voltage VBO. At this voltage, the junction
J2 becomes forward biased and the SCR turns into conduction state. A large current flows
through the SCR (from Anode to Cathode, which is limited by the load resistance) with a
very low voltage drop across it.
During the turn ON state, the forward voltage drop across the SCR is in the range of 1 to
1.5 volts and this may be increased with the load current.
In practice this method is not employed because it needs a very large anode to cathode
voltage. And also once the voltage is more than the VBO, the SCR turns on and a very
high current flows through it instantly, which may cause damage to the SCR. Therefore,
most of the cases this type of triggering is avoided.
7. Temperature Triggering
◇ This type of triggering is also known as Thermal
Triggering as the SCR is turned by heating it. 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 and further it increases the
current gains of the SCR. 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, the width of
the depletion layer decreases. So, when the forward
bias voltage is near to VBO, we can turn ON the SCR by
increasing the junction temperature (J2). At a particular
temperature, the reverse bias of the junction breaks
down an the device starts to conduct.
8. ◇ This triggering occur in some circumstances particularly when it the device
temperature is more (also called false triggering). This type of triggering is
practically not employed because it causes the thermal runaway and hence
the device or SCR may be damaged.
15. ◇ Dv/dt triggering is the technique in which SCR is turned ON by changing the forward bias
voltage with respect to time. Dv/dt itself means the rate of change of voltage w.r.t time. It is
independent of the magnitude of voltage. The voltage may be low, but the rate of its rise should
be high enough to turn SCR ON.
◇ In this type of triggering, whenever the SCR is in forwarding bias, then two junctions like J1 & J3
are in forwarding bias and J2 junction will be in reverse bias. Here, J2 junction performs like a
capacitor because of the existing charge across the junction. If the ‘V’ is the voltage across the
SCR, then the charge (Q) and capacitance can be written as
◇ ic = dQ/dt
◇ Q = CV
◇ ic = d (CV) /dt = C. dV/dt + V.dC/dt
◇ When dC/dt = 0
◇ ic = C. dV/dtas
◇ Thus, as the change of voltage rate across the SCR turns into high or low, then the SCR may
trigger.
17. Defination
◇ Light Triggering
◇ An SCR turned ON by light radiation is also called as
Light Activated SCR (LASCR). Hence, Light
Triggering is also known as Radiation Triggering.
Generally, this type of triggering is employed in
phase controlled converters in HVDC transmission
systems.
19. Working of light Triggering..
◇ In this method, light rays with appropriate
wavelength and intensity are allowed to strike the
junction J2.
◇ The bombarded energy particles from the light
(neutrons or photons) causes to break electron
bonds as as result, new electron – hole pairs are
formed in the device.
20. Advantages of Light Triggering..
◇ The advantage of using light triggering
for the thyristor is the prevention of
electrical noise disturbances.
◇ Thus, LASCR is considered to be one of
the best devices.
21. Disadvantages of Light Triggering..
◇ The SCR (silicon controlled rectifier) is unidirectional
devices, so it can control power only in DC power during
positive half cycle of AC supply, thus only DC power is
controlled with the help of SCR.
◇ The gate current cannot be negative.
◇ In AC circuit, it needs to be turned on each cycle.
◇ It cannot be used at higher frequencies
23. An SCR is said to be ‘turned OFF’ if there is no flow of forward current
and even if the SCR is once again forward biased (positive voltage at
anode), the SCR will not conduct without any Gate Signal (using one of
the SCR Turn ON Methods).
The reverse voltage, which causes to commutate the SCR, is called the
Commutation Voltage. Depending on the type of switching of SCR
(Cyclic or Sequential), the commutation methods are classified into two
major types. They are:
Natural Commutation
Forced Commutation
Before discussing about different types of ACR Turn OFF Methods,
there is an important quantity known as the Turn OFF Time of SCR
which we have to understand.
Turn OFF Time tOFF of an SCR is the time between the moment anode
current becomes zero and the moment SCR starts to block the forward
voltage.
SCR Turn OFF Methods
24. Natural Commutation
In natural commutation, the source of commutation voltage is the supply source
itself. 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.
25.
26. During the positive half cycle of the AC Supply, the load current flows normally.
But, during the negative cycle, the SCR will turn OFF (due to momentary zero
current and immediate negative polarity). For successful natural commutation, the
turn OFF time tOFF must be less than the duration of half cycle of the supply.
27. When the anode current is reduced below the level of the
holding current, the SCR turns off. However, it must be noted
that rated anode current is usually larger than 1,000 times
the holding value. Since the anode voltage remains positive
with respect to the cathode in a dc circuit, the anode current
can only be reduced by opening the line switch S, increasing
the load impedance RL or shunting part of the load current
through a circuit parallel to the SCR, i.e. short-circuiting the
device.
Natural Commutation
29. “
What is Commutation ?
Commutation means transfer of current from one
circuit to another.
29
30. 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. They are: 30
31. Classification of Forced
Commutation
31
•Class A – Self Commutation by Resonating the Load
•Class B – Self Commutation by Resonating the Load
•Class C – Complementary Commutation
•Class D – Auxiliary Commutation
•Class E – Pulse Commutation
such as class A, B, C, D, and E. This commutation is mainly used in
chopper and inverter circuits.
34. Class A commutation is a sub-classification of forced
commutation sometimes called self or resonant
commutation. To commutate the thyristor, two necessary
actions must be considered:
~ IA > IH i.e., anode current must be less than
holding current.
~ The potential at anode must be lower than the
cathode.
37. DEFINATION
Class B Commutation of Thyristo
Class B commutation of thyristor is another type of forced commutation technique that is
used to turn off the SCR. In this commutation technique, the device gets turned off by
building reverse current in a gradual manner from cathode to anode.
39. WORKING
◇ In class B thyristor commutation LC is connected in parallel with thyristor . Before triggering on the
SCR, the capacitor is charged up (dot indicates positive). If the SCR is triggered or given a
triggering pulse, then the resulting current has two components.
◇ The constant load current flowing through the R-L load is ensured by the large reactance connected
in series with the load which is clamped with a freewheeling diode.
◇ If sinusoidal current flows through the resonant L-C circuit, then the capacitor C is charged up with
dot as negative at the end of the half-cycle
◇ The total current flowing through the SCR becomes zero with the reverse current flowing through
the SCR opposing the load current for a small fraction of the negative swing. If the resonant circuit
current or reverse current becomes just greater than the load current, then the SCR will be turned
OFF
40. ADVANTAGES OF CLASS B COMMUTATION
The advantages of class-B commutation method of an SCR are as under
1. It is simple.
2. The commutating components L and C do not carry load current.
41. APPLICATIONS OF CLASS B COMMUTATION
The application of class-B commutation method is as under
1. It is used in d.c. chopper circuit.
42. Class C:
C or L-C Switched by another Load
Carrying SCR
43. ◇ In the above thyristor commutation techniques we observed
only one SCR but in these class C commutation techniques
of thyristor there will be two SCRs.
◇ One SCR is considered as main thyristor and the other as
auxiliary thyristor.
◇ In this classification both may act as main SCRs carrying
load current and they can be designed with four SCRs with
load across the capacitor by using a current source
forsupplying an integral converter.
◇ If the thyristor T2 is triggered, then the capacitor will be
charged up.
44. ◇ If the thyristor T1 is triggered, then the capacitor will
discharge and this discharge current of C will oppose
the flow of load current in T2 as the capacitor is
switched across T2 via TI.
► Mainly used in single-
phase inverters with centre
tapped transformers.
► Viz. Mc Murray Bedford
inverter.
▸ It is useful even at frequencies below 1000 Hz...
