Thyristors MCQ Quiz - Objective Question with Answer for Thyristors - Download Free PDF

Explanation:

Metal Oxide Varistor (MOV)

Definition: A Metal Oxide Varistor (MOV) is a type of electronic component used for protecting electrical and electronic circuits from transient voltage spikes. It is a non-linear resistor whose resistance decreases significantly as the voltage across it increases beyond a certain threshold. MOVs are widely utilized in surge protection devices due to their ability to absorb and divert excess energy caused by voltage surges.

Working Principle: MOVs are made of metal oxide grains (usually zinc oxide) embedded in a ceramic matrix. These grains form multiple junctions that exhibit non-linear behavior. When the voltage across the MOV remains below its threshold (clamping voltage), it behaves like a high-resistance component, allowing negligible current to flow through it. However, when the voltage exceeds the threshold, the MOV's resistance drops drastically, allowing it to conduct current and divert the excess energy away from the protected circuit.

Advantages:

• Effective protection against voltage surges and spikes.
• Compact design suitable for a wide range of applications.
• Quick response time to transient events.
• Cost-effective solution for over-voltage protection.

Disadvantages:

• Degradation over time due to repeated exposure to voltage surges.
• Limited energy absorption capacity, requiring careful selection based on application.

Applications: MOVs are commonly used in:

• Surge protection devices for power distribution systems.
• Protecting sensitive electronic equipment such as computers, televisions, and telecommunication devices.
• Industrial equipment and motor control circuits.
• Gate circuits in power electronics to safeguard them against over-voltage conditions.

Correct Option Analysis:

The correct option is:

Option 1: Gate circuit against over currents.

This option correctly identifies one of the applications of MOVs. In gate circuits, MOVs are specifically used to protect against over-voltage conditions that could damage the gate or associated components. Since MOVs respond quickly to voltage surges, they are ideal for safeguarding delicate electronics in gate circuits.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Gate circuit against over voltages.

This option is incorrect because MOVs are designed to protect circuits against transient voltage spikes, not specifically over currents. Gate circuits in electronic devices can be sensitive to voltage surges, but the MOV's role is primarily focused on handling over-voltage conditions rather than over-current issues.

Option 3: Anode circuit against over currents.

This option is incorrect because MOVs are not typically used for protecting anode circuits against over currents. Anode circuits in power electronics are more likely to be protected using current-limiting devices such as fuses or circuit breakers. MOVs are designed for handling transient voltage spikes rather than current-related issues.

Option 4: Anode circuit against over voltages.

While MOVs can be used for over-voltage protection, this option does not correctly specify the primary application of MOVs in gate circuits, which was the focus of the question. The anode circuit's protection would depend on the specific application and device requirements, but MOVs are more commonly associated with gate circuit protection.

Option 5: None of the above.

This option is incorrect because MOVs are indeed used for protecting gate circuits against over-voltage conditions, as explained above. Therefore, the correct answer cannot be "None of the above."

Conclusion:

Metal Oxide Varistors (MOVs) play a critical role in protecting sensitive electronic circuits from transient voltage spikes. They are widely used in gate circuits to safeguard against over-voltage conditions, ensuring the reliability and longevity of the components. While MOVs are effective for surge protection, their limitations, such as degradation over time and energy capacity, must be considered during the design and selection process. Understanding the applications and working principles of MOVs is essential for their correct usage in various industries and electronic systems.

Explanation:

Thyristor Power Converter in Discontinuous Mode

Definition: A thyristor power converter is said to operate in discontinuous mode when the load current becomes zero for a part of the output cycle, even though the load voltage may still be present. This typically occurs under light load conditions or in systems where the energy demand from the load is intermittent. The discontinuous mode is a critical operational condition to understand in power electronics as it affects the design, control, and performance of the converter.

Working Principle: In a thyristor-based converter, when the load current drops to zero during a portion of the cycle, the thyristors are not conducting, and the load is temporarily disconnected from the power source. This is referred to as the discontinuous conduction mode (DCM). The presence of load voltage even when the load current is zero is indicative of the stored energy in the reactive components (like inductors or capacitors) being released, maintaining a voltage across the load.

Correct Option Analysis:

The correct option is:

Option 1: The load current is zero even though the load voltage is present.

In the discontinuous mode, the load current ceases to flow for a part of the cycle, while the load voltage may still be present due to energy stored in inductive or capacitive components. This matches the behavior described in Option 1, making it the correct answer. Discontinuous conduction mode (DCM) is a common phenomenon in circuits with reactive elements, where the current waveform becomes discontinuous, but the voltage waveform remains continuous due to the energy stored in the reactive components.

Advantages of Discontinuous Mode:

• Reduced conduction losses as the thyristors are not conducting for the entire cycle.
• Better thermal management of thyristors due to reduced conduction duration.

Disadvantages of Discontinuous Mode:

• Higher voltage and current stresses on the thyristors and other components during transitions.
• Increased electromagnetic interference (EMI) due to rapid switching events.
• Complex control mechanisms required to manage the transitions between continuous and discontinuous modes.

Applications: Discontinuous mode operation is commonly observed in power converters used in scenarios where the load demand fluctuates significantly, such as in motor drives, power supplies, and renewable energy systems.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Both load voltage and load current are zero simultaneously.

This option is incorrect as it describes a state where neither voltage nor current is present, which typically indicates a complete disconnection or shutdown of the system. This is not characteristic of the discontinuous conduction mode, where the load voltage can still be present even when the load current is zero.

Option 3: The load current is present even though load voltage is zero.

This scenario is not possible in a properly functioning thyristor power converter. For the load current to flow, there must be a voltage across the load to drive the current. Hence, this option is incorrect.

Option 4: When load current is ripple free.

This option is incorrect as well. Ripple-free load current typically occurs in continuous conduction mode (CCM), where the load current does not drop to zero during the operation. Discontinuous conduction mode, by definition, involves a load current that becomes zero for a part of the cycle, which is the opposite of ripple-free behavior.

Conclusion:

Understanding the operational modes of thyristor power converters is essential for designing efficient and reliable power electronics systems. The discontinuous conduction mode occurs when the load current becomes zero for part of the cycle while the load voltage remains present. This mode has specific implications for the performance, control, and design of the converter, as highlighted in the explanation. Option 1 correctly describes this phenomenon, distinguishing it from the other options, which are either incorrect or describe different operational conditions.

Explanation:

Maximum Surge Current Rating of an SCR

Definition: The maximum surge current rating of an SCR (Silicon Controlled Rectifier) specifies the highest instantaneous current that the SCR can safely conduct for a short duration without being damaged. This rating is crucial for ensuring the SCR's reliability under transient conditions, such as during startup or fault scenarios.

Correct Option Analysis:

The correct option is:

Option 3: Non-repetitive current with sine wave.

This option correctly defines the maximum surge current rating of an SCR. The surge current rating is typically specified for a non-repetitive sine wave current. This means that the SCR can handle a single transient current pulse with a sinusoidal waveform for a short duration. The sinusoidal waveform is commonly used in specifications because it represents the typical AC current waveforms encountered in power systems. The non-repetitive nature of the surge current indicates that this is not a continuous or repetitive current but rather a single event or transient condition.

Importance: The maximum surge current rating is critical for applications where the SCR may encounter high inrush currents, such as during the charging of capacitors, motor startups, or short circuit conditions. Exceeding this rating can lead to thermal and mechanical stress on the SCR, potentially causing permanent damage or failure. Hence, it is essential to design circuits such that the surge current remains within the specified limits.

Factors Affecting Surge Current Rating:

• Pulse Duration: The allowable surge current depends on the duration of the current pulse. Longer pulse durations result in higher thermal stress, reducing the surge current rating.
• Junction Temperature: The surge current rating is specified at a certain junction temperature. Higher junction temperatures reduce the SCR's ability to withstand surge currents.
• Waveform Shape: The rating is typically given for a sinusoidal waveform. Other waveform shapes, such as rectangular or triangular, may have different effects on the SCR's performance.

Applications:

• Power converters, such as rectifiers and inverters, where transient conditions may occur.
• Motor control circuits, where high inrush currents are common during startup.
• Protection circuits, where the SCR must handle fault currents before disconnecting the load.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Repetitive current with sine wave.

This option is incorrect because the maximum surge current rating of an SCR does not refer to repetitive currents. Repetitive currents are the continuous currents that the SCR can handle during normal operation, and they are specified separately as part of the SCR's maximum RMS current or average current ratings.

Option 2: Non-repetitive current with rectangular wave.

This option is incorrect because the surge current rating of an SCR is typically specified for a sinusoidal waveform, not a rectangular wave. While rectangular waveforms may occur in certain applications, the thermal and mechanical stress on the SCR under rectangular wave conditions can differ significantly from those under sinusoidal wave conditions.

Option 4: Repetitive current with rectangular wave.

This option is incorrect for similar reasons as option 1. The surge current rating does not pertain to repetitive currents, whether sinusoidal or rectangular. Repetitive currents are part of the SCR's normal operating specifications and are not related to transient conditions.

Conclusion:

The maximum surge current rating of an SCR is a critical parameter that defines its ability to handle transient currents safely. It is specified for non-repetitive sine wave currents because this waveform closely represents the typical transient conditions in AC power systems. Proper understanding and application of this rating are essential to ensure the reliable operation of SCRs in various circuits, especially under conditions of high inrush or fault currents.

Switching Characteristics of SCR

SCR or Thyristor Turn-on Time: 

Thyristor turn-on time may be defined as the time required by the SCR to change its state from forward blocking mode to forward conduction mode when a gate pulse is applied.

The total turn-on time of SCR comprises three different time intervals:

Delay Time:

• It is the time between the instant at which the gate current reaches 90% of its final value and the instant at which the anode current reaches 10% of its final value.

Rise Time:

• Rise time is the time taken by the anode current to rise from 10% of its final value to 90% of its final value. 

Spread Time:

• It is the time taken by the anode current to rise from 90% of its final value to 100%. 
• It is the time required for the forward blocking voltage to fall from 10% of an anode voltage to the on-state voltage drop of the device.

Concept

The current through an inductor is given by:

\(I_L={V_L\over L}\times t\)

\(t={I_L\over V_L}\times L\)

where, t = time period

I_L = Latching current

V_L = Voltage

L = Inductor

Calculation

Given, IL = 4 mA

V_L = 200 V

L = 0.2 H

\(t={4\times 10^{-3}\over 200}\times 0.2\)

t = 4 μs

Construction:

• The SCR is a four-layer and three-terminal device.
• The four layers made of P and N layers are arranged alternately such that they form three junctions J₁, J_2, and J₃.
• These junctions are either alloyed or diffused based on the type of construction.

Doping level:

• The level of doping varies between the different layers of the thyristor.
• Out of these four layers, the first layer (P1 or P+) and Last layer (N2 or N+) are heavily doped layers.
• The second layer (N1 or N-) is a lightly doped layer and the third layer (P2 or P+) is a moderately doped layer.
• The junction J1 is formed by the P+ layer and N- layer.
• Junction J2 is formed by the N- layer and P+ layer
• Junction J3 is formed by P+ layer and N+ layer.
• Thinner layers would mean that the device would break down at lower voltages.

Key Points

Holding Current: It is the minimum anode current to maintain the thyristor in the on-state. 

Latching current is always greater than holding current.

Additional Information The thyristor or SCR is a power semiconductor device which is used in power electronic circuits.

They work like a bistable switch and it operates from nonconducting to conducting.

The designing of thyristors can be done with 3-PN junctions and 4 layers.

It includes three terminals namely anode, gate, and cathode. 

Concept:

• Need of series connection of SCR is required when we want to meet the increased voltage requirement by using various SCR’s.
• When the required voltage rating exceeds the SCR voltage rating, a number of SCR’s are required to be connected in series to share the forward and reverse voltage.
• When the load current exceeds the SCR current rating, SCR are connected in parallel to share the load current.

Application:

According to the question:

Given that

SCR 1 voltage = 350 V; current = 6 A

SCR 2 voltage = 300 V; current = 9 A

SCR 3 voltage = 250 V; current = 12 A

Let us take the total current to be ‘I’

Current through resistor R in shunt with SCR 1 is

I₁ = I – 6

Similarly, current through resistor R is shunt with SCR 2

I₂ = I – 9

And, current through resistor R in shunt with SCR 3

I₃ = I – 12

Now, the string voltage becomes

V_s = I₁R + I₂R + I₃R

Vs = (I - 6) R + (I - 9) R + (I - 12) R

Vs = (I - 6) R + (I - 6) R – 3 R + (I - 6) R – 6 R …..(1)

Note that

We should consider the extreme case from the calculation of resistance R for voltage equalization in string of SCR.

In extreme case, the voltage drop across SCR 1 (or the one having highest voltage drop) will be maximum forward blocking voltage.

∴ V­_max = 350 = (I - 6) R     ----(2)

Put (I - 6) R = 350 in equation (1), we get

V_s = 350 + 350 – 3 R + 350 – 6 R

(350 + 300 + 250) = 1050 – 9 R

900 = 1050 – 9 R

9 R = 1050 – 900

\(R = \frac{{150}}{9}{\rm{\Omega }}\)

R = 16.66 Ω

Turn on time: thyristor takes some transition time to go from forward blocking mode to forward conduction mode. This transition time is called turn on time of SCR

Turn off time: time during which a reverse voltage is applied across the thyristor during its commutation process.

Turn off time of a thyristor is greater than turn-on time.

Two thyristors of the same rating and same specifications may have equal or unequal turn-on and turn-off periods. 

dv/dt protection:

• When the SCR is forward biased, junctions J1 and J3 are forward biased and junction J2 is reverse biased. This reverse-biased junction J2 exhibits the characteristics of a capacitor.
• If the rate of the forward voltage applied is very high across the SCR, charging current flows through the junction J2 is high. This current is enough to turn ON the SCR even without any gate signal.
• This is called as dv/dt triggering of the SCR. 
• dv/dt rating of thyristor indicates the maximum rate of rise of anode voltage that will not trigger the device without any gate signal. We use a snubber circuit to control this limit.
• A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor.
• False turn – ON of an SCR by large dv/dt, even without application of a gate signal can be prevented by using a snubber circuit.
• Snubber limits the dv/dt across the switching device during the turnoff of the device.

Advantages of SCR:

• It can handle large voltages, currents, and power.
• The voltage drop across conducting SCR is small. This will reduce the power dissipation in the SCR.
• Easy to turn on.
• The operation does not produce harmonics.
• Triggering circuits are simple.
• It has no moving parts.
• It gives noiseless operation at high efficiency.
• We can control the power delivered to the load.
   

Drawbacks of SCR:

• It can conduct only in one direction. So it can control power only during the one-half cycle of ac.
• It can turn on accidentally due to the high dv/dt of the source voltage.
• It is not easy to turn off the conducting SCR. We have to use special circuits called commutation circuits to turn off a conducting SCR.
• SCR cannot be used at high frequencies or perform high-speed operations. The maximum frequency of its operation is 400 Hz.
• Gate current cannot be negative.
   

Applications of SCR: Controlled rectifiers, DC to DC converters or choppers, DC to AC converters or inverters, As a static switch, Battery chargers, Speed control of DC and AC motors, Lamp dimmers, fan speed regulators, AC voltage stabilizers.

• The Gate turn off thyristor (GTO) is a four-layer PNPN power semiconductor switching device that can be turned on by a short pulse of gate current and can be turned off by a reverse gate pulse.
• The magnitude of latching, holding currents is more. The latching current of the GTO is several times more as compared to conventional thyristors of the same rating.
• On state voltage drop and the associated loss is more.
• Due to the multi-cathode structure of GTO, triggering gate current is higher than that required for normal SCR.
• Gate drive circuit losses are more. Its reverse voltage blocking capability is less than the forward voltage blocking capability.
• GTO has the capability of being turned off by a negative gate – current pulse
• For low power applications, it has higher blocking voltage capability.

I2t rating is used to determine the thermal energy absorption of the device. This rating is required in the choice of a fuse or other protective equipment employed for the SCR. This is the measure of the thermal energy that the SCR can absorb for a short period of time before clearing the fault by the fuse.

Important Point:

di/dt rating of thyristor indicates the maximum rate of rising of the anode to cathode current. We use a series reactor to control this limit

dv/dt rating of thyristor indicates the maximum rate of rising of anode voltage that will not trigger the device without any gate signal. We use a snubber circuit to control this limit

The silicon control rectifier (SCR) consists of four layers of semiconductors, which form NPNP or PNPN structures, having three P-N junctions labeled J1, J2 and J3, and three terminals.

• Thermal runaway is a self-destruction process in which an increase in temperature creates such a condition which in turn increases the temperature again.
• This uncontrolled rise in temperature causes the component to get damaged.
• A thermal runaway of a thyristor occurs because negative resistance coefficient of the junction.
• A negative coefficient for a material means that its resistance decreases with an increase in temperature.