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

Explanation:

Thyristor or SCR (Silicon Controlled Rectifier) Behavior After Gate Pulse Removal

Definition: A thyristor, commonly known as an SCR (Silicon Controlled Rectifier), is a four-layered semiconductor device used in power electronics to control high-power applications. It operates as a bistable device, meaning it can switch between ON and OFF states based on the triggering of its gate terminal and the conditions of its anode current.

Working Principle: The SCR remains in its OFF state (blocking mode) until a gate pulse is applied. Once a sufficient gate pulse is provided to trigger the SCR, it enters the ON state (conducting mode). In the conducting mode, the SCR continues to conduct current even after the gate pulse is removed, as long as the anode current remains above a certain threshold, known as the latching current. If the anode current drops below a critical level called the holding current, the SCR turns OFF.

Correct Option Analysis:

The correct option is:

Option 1: Drops to zero

When the gate pulse is removed after firing the SCR, the current through the SCR does not immediately drop to zero. The SCR remains in the conducting state as long as the current flowing through it is above the holding current. However, if the anode current drops below the holding current level (e.g., due to circuit conditions such as a reduction in the load current or the AC supply reaching its zero-crossing point), the SCR will turn OFF, and its current will drop to zero. Hence, the option "Drops to zero" is correct but must be understood in the context of the current falling below the holding current.

Important Concepts Supporting This Behavior:

• Holding Current: This is the minimum current that must flow through the SCR to keep it in the ON state. If the current falls below this value, the SCR will turn OFF.
• Latching Current: This is the minimum current required to latch the SCR into the ON state immediately after it is triggered by the gate pulse. Once the SCR is latched, the gate pulse is no longer needed to sustain conduction.
• Gate Pulse Role: The gate pulse is only required to trigger the SCR into conduction. After firing, the gate signal has no effect on the SCR's conduction state.
• AC Circuit Operation: In AC circuits, the SCR naturally turns OFF at the zero-crossing point of the AC waveform because the current falls to zero, which is below the holding current.

Additional Information

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

Option 2: Increases

This option is incorrect. After the gate pulse is removed, the SCR's current does not increase. The current through the SCR is determined by the external circuit parameters, such as the load and supply voltage. The gate pulse only serves to trigger the SCR into conduction, and once the SCR is ON, the gate pulse has no impact on the current. An increase in current would require a change in the circuit's external conditions, not the removal of the gate pulse.

Option 3: Decreases

This option is partially correct but not entirely accurate in all scenarios. While the current through the SCR may decrease due to external circuit conditions (e.g., reduced load or the supply reaching a lower voltage), the mere removal of the gate pulse does not cause the current to decrease. The SCR remains in conduction as long as the anode current stays above the holding current. If the anode current falls below the holding current, the SCR turns OFF, and its current drops to zero rather than just decreasing.

Option 4: Remains the same

This option is incorrect in the long term. While the current through the SCR may remain the same immediately after the gate pulse is removed, it is not guaranteed to stay constant indefinitely. The current in the SCR depends on the external circuit conditions. In an AC circuit, for example, the SCR's current will naturally fall to zero during the AC waveform's zero-crossing point, causing the SCR to turn OFF.

Conclusion:

The behavior of an SCR after the gate pulse is removed is primarily governed by the anode current and its relationship to the holding current. Once the gate pulse has triggered the SCR into conduction, the gate signal has no further effect. The SCR will remain in the ON state as long as the anode current remains above the holding current. If the current drops below the holding current (e.g., due to external circuit conditions), the SCR turns OFF, and its current drops to zero. Therefore, the correct answer is "Drops to zero."

Explanation:

Subcycle Surge Current Rating of a Thyristor

Definition: The subcycle surge current rating of a thyristor refers to the maximum permissible current the thyristor can safely conduct during a short-duration (subcycle) surge event. This rating is crucial for ensuring the device's reliability and protecting it from damage during transient overcurrent conditions. The subcycle surge current occurs over a fraction of the supply frequency cycle, and its magnitude is directly related to the duration of the surge and the thermal capacity of the thyristor.

Mathematical Expression:

The subcycle surge current rating \(I_{sb}\) is mathematically expressed as:

\(I_{sb} = I \sqrt{\frac{T}{t}}\)

Where:

• \(I_{sb}\): Subcycle surge current rating
• \(I\): Normal current rating of the thyristor
• \(T\): Time for one half-cycle of the supply frequency (e.g., for a 50 Hz supply, \(T = \frac{1}{2 \times 50} = 0.01 \, \text{seconds}\))
• \(t\): Duration of the subcycle surge

Derivation and Explanation:

The subcycle surge current rating is derived from the thermal energy considerations of the thyristor. The heat generated in the thyristor during a surge event is proportional to the square of the current (\(I^2\)) and the duration of the surge (\(t\)). To ensure the thyristor does not overheat, the total energy dissipated during the surge must not exceed the thyristor's thermal capacity. The equation \(I_{sb} = I \sqrt{\frac{T}{t}}\) ensures this condition is met by relating the surge current to the normal current rating and the duration of the surge.

Key Insight: The subcycle surge current rating increases as the duration of the surge decreases because the thyristor has less time to accumulate excessive heat. The square root relationship between \(T/t\) and \(I_{sb}\) reflects this thermal behavior.

Correct Option Analysis:

The correct option is:

Option 1: \(I_{sb} = I \sqrt{\frac{T}{t}}\)

This equation correctly represents the subcycle surge current rating of a thyristor, considering the thermal capacity and the energy dissipation relationship. The surge current is proportional to the square root of the ratio of the half-cycle duration (\(T\)) to the subcycle surge duration (\(t\)), ensuring the device's safety and reliability.

Important Information

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

Option 2: \(I_{sb} = I \sqrt{\frac{t}{T}}\)

This option is incorrect because it reverses the relationship between \(T\) and \(t\). In this formulation, the surge current would decrease as the surge duration (\(t\)) decreases, which contradicts the thermal behavior of the thyristor. The correct equation shows that the surge current increases as the surge duration decreases.

Option 3: \(I_{sb} = t \sqrt{\frac{I}{T}}\)

This option is incorrect as it introduces a linear dependence on \(t\), which is not valid in the context of the thyristor's thermal behavior. The surge current rating is not directly proportional to the surge duration; instead, it depends on the square root of the ratio of \(T\) to \(t\).

Option 4: \(I_{sb} = T \sqrt{\frac{I}{t}}\)

This option is also incorrect because it incorrectly introduces a direct proportionality to \(T\) and inversely to \(t\). The correct relationship involves the square root of the ratio of \(T\) to \(t\), and \(I\) is the normal current rating, not under a square root.

Conclusion:

The subcycle surge current rating of a thyristor is a critical parameter for ensuring the device's safe operation during transient overcurrent events. The correct equation, \(I_{sb} = I \sqrt{\frac{T}{t}}\), accurately captures the thermal constraints of the thyristor and provides a reliable basis for its design and application. A proper understanding of this relationship is essential for selecting and operating thyristors in power electronic circuits.

Explanation:

Distortion Factor (DF) and Total Harmonic Distortion (THD) Relationship

Definition: Distortion factor (DF) and total harmonic distortion (THD) are metrics used to quantify the distortion in a waveform, typically an electrical signal, caused by the presence of harmonics. These metrics are vital in analyzing the quality of an electrical signal in power systems, audio applications, and communication systems.

The distortion factor (DF) is a measure of the waveform's deviation from a pure sine wave due to harmonics, while total harmonic distortion (THD) quantifies the extent of harmonic distortion as a ratio of the harmonic content to the fundamental frequency.

Correct Formula:

The relationship between DF and THD is given by the formula:

\(\mathrm{DF} = \sqrt{\frac{1}{1+\mathrm{THD}^{2}}}\)

This formula (Option 2) correctly relates DF and THD. It signifies that the distortion factor decreases as the total harmonic distortion increases, reflecting the increasing deviation of the waveform from a pure sine wave.

Derivation:

To derive the relationship between DF and THD:

• Let the fundamental frequency component of the waveform be \(V_1\), and the RMS value of the harmonic components be \(V_{\text{harmonics}}\).
• The total RMS value of the waveform, \(V_{\text{total}}\), is given by: \[ V_{\text{total}} = \sqrt{V_1^2 + V_{\text{harmonics}}^2} \]
• Total harmonic distortion (THD) is defined as: \[ \mathrm{THD} = \frac{V_{\text{harmonics}}}{V_1} \] Substituting \(V_{\text{harmonics}} = \mathrm{THD} \cdot V_1\) into the total RMS equation: \[ V_{\text{total}} = \sqrt{V_1^2 + (\mathrm{THD} \cdot V_1)^2} = V_1 \sqrt{1 + \mathrm{THD}^2} \]
• The distortion factor (DF) is defined as the ratio of \(V_1\) to \(V_{\text{total}}\): \[ \mathrm{DF} = \frac{V_1}{V_{\text{total}}} = \frac{V_1}{V_1 \sqrt{1 + \mathrm{THD}^2}} = \sqrt{\frac{1}{1 + \mathrm{THD}^2}} \]

Thus, the formula \(\mathrm{DF} = \sqrt{\frac{1}{1+\mathrm{THD}^{2}}}\) is derived, confirming Option 2 as the correct answer.

Applications:

• Analyzing power system quality in electrical grids.
• Evaluating signal quality in audio systems and communication networks.
• Designing filters and circuits to minimize distortion.

Additional Information

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

Option 1: \(\mathrm{THD} = \sqrt{\frac{1}{1+\mathrm{DF}^{2}}}\)

This formula is incorrect. The relationship between DF and THD does not support this formulation. Substituting \(V_{\text{harmonics}} = \mathrm{THD} \cdot V_1\) and \(V_{\text{total}} = V_1 \sqrt{1 + \mathrm{THD}^2}\) into the definition of DF demonstrates that Option 1 does not align with the derived formula.

Option 3: \(\mathrm{DF} = \sqrt{\frac{1}{1-\mathrm{THD}^{2}}}\)

This formula is invalid because it implies a subtraction of \(\mathrm{THD}^2\), which can yield nonsensical or negative values when \(\mathrm{THD} > 1\). The correct formula involves addition (\(1 + \mathrm{THD}^2\)) in the denominator.

Option 4: \(\mathrm{THD} = \sqrt{\frac{1}{1-\mathrm{DF}^{2}}}\)

Similar to Option 3, this formula is incorrect due to the subtraction term (\(1 - \mathrm{DF}^2\)) in the denominator. The derived relationship between DF and THD involves addition, not subtraction.

Conclusion:

The correct formula, \(\mathrm{DF} = \sqrt{\frac{1}{1+\mathrm{THD}^{2}}}\), accurately describes the relationship between distortion factor and total harmonic distortion. This formula is essential in applications where understanding and minimizing waveform distortion is critical, such as in power systems, audio engineering, and signal processing. The incorrect options fail to align with the derived mathematical relationship and can lead to erroneous interpretations if applied.

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.

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.

• Latching Current: It is the minimum anode current required to maintain the thyristor in the ON state immediately after a thyristor has been turned on and the gate signal has been removed.
• Holding Current: It is the minimum anode current to maintain the thyristor in the on-state.
• Latching current is always greater than holding current.
   

Application:

Given that, holding current = 21 mA

Latching current should be more than 21 mA