## Thyristor Interview Questions & Answers

1. Question 1. What Are The Different Types Of Thyristor Protection?

• For reliable operation of SCR, it should be operated within the specific ratings.
• SCRs are very delicate devices and so they must be protected against abnormal operating conditions.

Various protection of SCR is

1. di/dt Protection
2. dv/dt Protection
3. Over voltage Protection
4. Over Current Protection
2. Question 2. Explain Briefly About Di/dt Protection?

• di/dt is the rate of change of current in a device.
• When SCR is forward biased and is turned ON by the gate signal, the anode current flows.
• The anode current requires some time to spread inside the device. (Spreading of charge carriers)
• But if the rate of rise of anode current (di/dt) is greater than the spread velocity of charge carriers then local hot spots is created near the gate due to increased current density. This localized heating may damage the device.
• Local spot heating is avoided by ensuring that the conduction spreads to the whole area very rapidly. (OR) The di/dt value must be maintained below a threshold (limiting) value.
• This is done by means of connecting an inductor in series with the thyristor.
• The inductance L opposes the high di/dt variations.
• When the current variation is high, the inductor smooths it and protects the SCR from damage. (Though di/dt variation is high, the inductor ‘L’ smooths it because it takes some time to charge). L ≥ [Vs / (di/dt)]

dv/dt Protection:-

• dv/dt is the rate of charge of voltage in SCR.
• We know that iC=C.dv/dt. ie, when dv/dt is high, iC is high.
• This high current(iC) may turn ON SCR even when gate current is zero. This is called as dv/dt turn ON or false turn ON of SCR.
• To protect the thyristor against false turn ON or against high dv/dt a “Snubber Circuit” is used.
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4. Question 3. What Is Ac Voltage Controller?

It is defined as the power electronics converter which converts the fixed AC voltage into a variable AC voltage without change in the frequency.

5. Question 4. What Is Inverter? What Are The Types Of Inverter?

A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter.

• Voltage Source Inverter (VSI)
• Current Source Inverter (CSI)
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7. Question 5. What Is Chopper?

It is equivalent to a dc transformer in ac circuit. The Chopper is a static switch which is used to get the variable dc output voltage from a constant dc input voltage.

Step down chopper:

In this circuit, the average output voltage [VO = D. VS] is less than the input supply voltage. It is also known as Buck converter.

Step up chopper:

It is also known as Boost converter. Here the average output voltage [VO = VS / 1 – D]  is more than the input supply voltage.

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9. Question 6. What Is Load Commutation?

In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.

10. Question 7. What Is Meant By Pulse Triggered Devices?

As the name indicates, to trigger these devices just a pulse is sufficient. Continuous gate voltage of entire on time is not required. The advantage of pulse triggering is, it will avoid the hard triggering.

Ex. Thyristor, GTO

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12. Question 8. What Is A Level-sensitive Device? Give Some Examples?

Some of the level sensitive devices are

• MOSFET
• IGBT
• MCT
• IGCT

In order to keep these devices in the ON state condition, we have to apply gate current/voltage continuously to these devices.

13. Question 9. What Are The Types Of Gto?

• Asymmetrical GTO
• Symmetrical GTO
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15. Question 10. What Does 10v Ac Mean? Is It The Rms Voltage Or Peak Voltage Or Average Voltage?

In general, AC voltages and currents are mentioned in RMS values only. It is sensible to compare with steady DC voltages and currents.

For example 230V AC supply means, 230Vrms AC Supply.

16. Question 11. When We Have To Connect The Scrs In A Parallel Manner?

To meet the high current demand we will connect the SCRs in a parallel manner.

Similarly to meet the high voltage demand we have to connect the SCRs in series.

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18. Question 12. Define Gate Charge In Mosfet?