Q2) Explain the triggering of SCR using RC exceeded then the device can be damaged.

iii)
gate triggering ? VGT - Gate voltage to trigger (VGT): Minimum
value of the gate cathode forward voltage
Ans) Triggering an SCR (Silicon-Controlled
below which reliable turn on of the thyristor
Rectifier) using an RC (Resistor-Capacitor)
cannot be guaranteed. It is specified at the
circuit is a common method to control the
same break over voltage as IGT. iv) VRRM -
conduction of the SCR. This technique
This parameter is the maximum peak voltage
involves charging a capacitor through a
that is allowed across the device. This
resistor and then discharging it rapidly to
datasheet specification parameter should not
provide a momentary gate current that
be exceeded otherwise the device may fail. It
triggers the SCR into conduction. 1) Triggering
is also always good to leave sufficient margin
Phase: At a certain point, the voltage across
to allow for transients. This parameter is
the capacitor reaches a level that can trigger
specified for conditions up to the maximum
the SCR. This voltage is typically above the
junction temperature. v) IT ( RMS ) - This
gate trigger voltage (VGT) but below the
thyristor specification is the maximum
forward breakover voltage (VBO). 2) Charging
allowed RMS current through the device. It is
Phase - Initially, the SCR is in the forward
specified for a given temperature. Different
blocking mode, and the anode voltage is
datasheet specifications may quote ambient
positive with respect to the cathode. The
temperature, Ta, case temperature, Tc, or
resistor (R) and the capacitor (C) are
even the lead temperature, Tl. The method
connected in series with the gate terminal of
used for quoting the temperature normally
the SCR. The resistor limits the charging
depends upon the type of case used for the
current of the capacitor. 3) Discharge Phase:
thyristor / SCR. vi) IGT - This is the current
As the SCR begins to conduct, a current flows
required in the gate to enables the device to
through the gate circuit, rapidly discharging
be triggered and to latch in its on-state
the capacitor. The discharge of the capacitor
provided there is sufficient anode cathode
generates a sharp and short pulse of gate
current to maintain the current flow. vii ) IGM
current ensuring the SCR turns on reliably. 4)
- This is the maximum level of gate current
Conduction Phase: Once triggered, the SCR
that should not be exceeded. viii) VRGM - This
enters the forward conduction mode and
is the maximum level of gate voltage that can
allows a larger current to flow from anode to
be applied across the gate cathode junction
cathode.
without the possibility of damage resulting. It
Q3) Explain the following specification of is wise to operate well below this voltage.
thyristor ?
Q4) With neat constructional diagram explain
i) dv/dt - dv/dt is the derivative of the voltage working of GTO. Draw its V- I Characteristic ?
with respect to time. In other words, it’s the
Working - A Gate Turn off Thyristor or GTO is
change in voltage (delta V, or ΔV) divided by
a three terminal, bipolar (current controlled
the change in time (delta t, or Δt), or the rate
minority carrier) semiconductor switching
at which the voltage changes over time. The
device. Similar to conventional thyristor, the
dv/dt rating is the maximum allowable rate of
terminals are anode, athode and gate as
rise of anode to cathode voltage that will not
shown in figure below. As the name indicates,
trigger the device without any gate signal. ii)
it has gate turn off capability. These are
di/dt - There is a maximum rate of rise for the
capable not only to turn ON the main current
on-state current during turn on. If this rate is
with a gate drive circuit, but also to turn it SCR's current flow. As the SCR's voltage
OFF. A small positive gate current triggers the becomes negative, the charged capacitor
GTO into conduction mode and also by a discharges through an inductor and diode,
negative pulse on the gate, it is capable of generating a reverse voltage that aids turn-off.
being turned off. Construction - Consider the The SCR's current drops, causing it to turn off.
below structure of GTO, which is almost
Q6) With neat circuit diagram explain how
similar to the thyristor. It is also a four layer,
overcurrent protection provided to Thyristor ?
three junction P-N-P-N device like a standard
thyristor. In this, the n+ layer at the cathode Ans) Protection of a device is an important
end is highly doped to obtain high emitter aspect for its reliable and efficient operation.
efficiency. This result the breakdown voltage Silicon Controlled Rectifier (SCR) are a very
of the junction J3 is low which is typically in delicate semiconductor device. So we have to
the range of 20 to 40 volts. The doping level of use it in its specified ratings to get desired
the p type gate is highly graded because the output. SCR may face different types of
doping level should be low to maintain high threats during its operation due to over
emitter efficiency, whereas for having a good voltages, over currents etc. There are different
turn OFF properties, doping of this region types of thyristor protection schemes
should be high. available for satisfactory operation of the
device like 1) Over voltage protection. 2) Over
Q5) What is commutation? Explain class
current protection. 3) High dv/dt protection.
C.commutation of SCR ?
4) High di/dt protection. 5) Thermal
Ans) Commutation - Commutation is the protection.
controlled process of transferring electric
Q7) Explain first quadrant chopper ? How
current from one path or component to
these choppers can be used to obtain two
another. It's crucial for proper switching and
quadrant choppers?
safe operation in devices like thyristors and in
power electronics circuits, ensuring efficient Ans) A first-quadrant chopper, or Type A
energy transfer and preventing voltage or chopper, is an electronic circuit used in power
current spikes. There are two main types: electronics for voltage control. It operates in
natural commutation, relying on AC voltage the first quadrant of the voltage-current
reversal, and forced commutation, which uses plane, controlling positive voltage and
external components or circuits to manage current. It typically includes a semiconductor
current flow and switching. Class C switch, an inductor, a diode, and a load.
Commutation - In this commutation method, During its onstate, the switch is closed,
the main SCR (which is to be commutated) is allowing current to flow from a higher voltage
connected in series with the load and an source through the inductor and into the load.
additional or complementary SCR is When turned off, the inductor's stored energy
connected in parallel with the main SCR. is discharged through the diode, maintaining
Hence, this method is also called as a positive load voltage. This chopper
Complementary Commutation. Class C configuration is suitable for step-down
commutation is a technique to turn off a voltage conversion applications. These
conducting SCR (Silicon-Controlled Rectifier). choppers can be used to obtain two quadrant
It involves using external components—a choppers - Converting a single-quadrant
capacitor and an inductor—to interrupt the chopper to a two-quadrant chopper involves
altering its circuitry to enable bidirectional varying the carrier wave frequency in
control of both voltage and current. A single- accordance with the frequency of the
quadrant chopper operates in one direction, modulating signal. As we know, a modulating
controlling positive voltages and currents. To signal is nothing but information or message
achieve two-quadrant operation, which that has to be transmitted after being
encompasses both positive and negative converted into an electronic signal. FM
voltages and currents, an inverting leg is techniques of voltage control - A voltage-
added to the existing chopper configuration. controlled oscillator or varactor diode
The process entails introducing an additional oscillator: A voltage-controlled oscillator can
switch and diode in the circuit. When the be used to form direct FM modulation by
inverting switch is turned on, the direction of directly feeding the message into the input of
energy flow reverses, allowing current to flow the oscillator. In the case of the varactor
from the load back to the input source. diode, we place this device within the tuned
circuit of an oscillator circuit.
Q9) What is the duty cycle of Chopper and
explain PWM and FM techniques of voltage Q13) Explain Type C Chopper ?
control ?
Ans ) A Type C chopper is a specialized
Ans) The duty cycle of a chopper is a measure electronic circuit used in power electronics to
of the fraction of time during which the control the average output voltage supplied
chopper switch (usually a transistor) is in the to a load. Its primary function is to step up or
ON state within one complete switching cycle. boost the input voltage. The core components
It is typically expressed as a percentage. For include a semiconductor switch, usually a
example, a duty cycle of 50% means that the transistor, an inductor, and sometimes a
switch is ON for half of the cycle and OFF for diode. When the chopper switch is closed (ON
the other half. It is the time during which the state), current flows from the input voltage
chopper is on (Ton) relative to the whole source through the inductor and into the load,
period (T = Ton+Toff). Explanation: The output storing energy in the inductor's magnetic
voltage can be changed by changing the duty field. During the off-state, the inductor
cycle (Ton/T). Explanation: The duty cycle is releases its stored energy into the load,
between 0 and 1. PWM - Pulse width effectively boosting the output voltage above
modulation is a proven effective technique the input level. Type C choppers find
that is used to control semiconductor devices. applications in voltage regulation, power
Pulse width modulation or PWM is a supplies, and systems requiring increased
commonly used control technique that voltage levels.
generates analog signals from digital devices
such as microcontrollers. The signal thus
produced will have a train of pulses, and these
pulses will be in the form of square waves.
Thus, at any given time, the wave will either
be high or low. Let us learn more about pulse
width modulation in this article. FM -
Frequency modulation is a technique or a
process of encoding information on a
particular signal (analogue or digital) by