Thyristor Ratings And Protection (1).pdf

1. The voltage safety factor (V​SF​) for an SCR is the ratio of a) peak working voltage & peak reverse repetitive voltage b) dv/dt & di/dt c) peak repetitive reverse voltage & maximum value of input voltage d) peak repetitive reverse voltage & rms value of input voltage View Answer

6. For a SCR the maximum rms on-state current is 35 A. If the SCR is used in a resistive circuit for a rectangular wave with conduction angle of 90°. Calculate the average & rms currents respectively. a) I/4, I/2 b) I/2, I/√2 c) I/4, I​2​/2 d) I/4, I/√2 

Answer: c  Explanation: It is peak repetitive reverse voltage/ the maximum value of input  voltage(Vm). 

2. The forward dv/dt rating of an SCR a) increases with increase in the junction temperature b) decreases with increase in the junction temperature c) increases with decrease in the rms value of forward anode-cathode voltage d) decreases with decrease in the rms value of forward anode-cathode voltage

View Answer Answer: a Explanation: First convert the conduction angle into N for  rectangular wave. N = 360°/Conduction angle = 4 Therefore, Iavg = I/4 

Irms = I/√4 = I/2. 7. For an SCR the average & rms values of current are I/4 & I/2 respectively. Calculate the average on-state current rating (I​TAV​). Take maximum RMS on-state current = 35 A. a) 8.78 A b) 10.10 A c) 17.5 A d) 24.74 A

View Answer View Answer Answer: a  Explanation: If the temperature is high, lesser dv/dt is required to turn on the  device as the higher temperature has already excited few of the holes &  electrons. 

3. The finger voltage of an SCR is a) minimum value of Vak to turn on the device with gate triggering b) maximum value of Vak to turn on the device with gate triggering c) minimum value of Vak to turn on the device without gate triggering d) maximum value of Vak to turn on the device without gate triggering View Answer Answer: a  Explanation: Finger voltage is the minimum value of Vak (anode to cathode  voltage) to turn on the device with gate triggering, it is to be avoided for  accidental turn-on of the device. 

4. Which among the following anode current waveforms will have the minimum junction temperature? a) 100 % DC b) 25 % DC c) 50 % DC d) AC View Answer Answer: b  Explanation: N % DC is nothing but a wave with n % duty cycle. Lower the Duty  cycle lesser is the current flowing & lesser is the temperature dissipation. 

5. An SCR has half cycle surge current rating of 3000 A for 50 Hz. Calculate its one-cycle surge current rating a) 3121.32 A b) 2121.32 A c) 3131.32 A d) 2131.32 A View Answer Answer: b Explanation: By equating the energies involved in one cycle &  subcycle 

I​2​.T = Isb​2​.t Isb = 3000 A T = 1/50 

t = T/2.

Answer: c Explanation: Form factor = RMS/Average vaules of current  Therefore, FF = 2 

I​TAV​ = 35/2 = 17.5. 8. The amp​2​-sec rating of the SCR specifies a) The power dissipated by the device when fault occurs b) The energy dissipated by the device when fault occurs c) The energy that the device can absorb before the fault is cleared d) The energy that the device can absorb while operating in the forward blocking mode. View Answer Answer: c  Explanation: The amp​2​-sec is the energy that the device can handle before the  fault is cleared. Its value decides how fast the fault has to be cleared to avoid  damage to the device. 

9. The maximum rms current of an SCR is 50 A. For a 120° sine wave conduction the form factor (FF) = 1.878 Find the average on-state current rating (I​TAV​). a) 93.9 A b) 174 A c) 26.62 A d) 68.52 A View Answer Answer: c  Explanation: I​TAV​ = 50/FF. 

10. The thermal resistance between junction & the SCR (θ​jc​) has the unit a) Ω/°C b) W/Ω c) °C/W d) ΩW/°C View Answer Answer: c  Explanation: Thermal resistance always has the unit degree temperature per  watt.  --------------------------------------------------------

1. di/dt protection is provided to the thryistor by a) connecting an inductor in parallel across the load b) connecting an inductor in series with the load c) connecting an inductor in parallel across the gate terminal d) connecting an inductor in series with the gate

b) I(1 + e​-t/τ​) c) I(1 – e​t/τ​) d) I(1 + e​-t/τ​) View Answer

View Answer Answer: b  Explanation: By placing the di/dt inductor (L) in series with the load, the  change in the anode current can be limited to a small value. 

2. The local hot spot formation in the cross-section of the SCR is avoided by a) reducing the junction temperature b) applying gate current nearer to the maximum gate current c) using only R loads d) proper mounting of the SCR on heat sink View Answer Answer: b  Explanation: Applying the higher gate current spreads the ions quickly and  avoids hotspot formation. 

3. The dv/dt protection is provided in order to a) limit the power loss b) reduce the junction temperature c) avoid accidental turn-on of the device d) avoiding sudden large voltage across the load View Answer Answer: c  Explanation: Accidentally some voltage spike or noise may occur in the  vicinity of the device, if the magnitude is large enough it may turn on the SCR. 

4. dv/dt protection is provided to the SCR by a) connecting a capacitor in parallel with the load b) connecting an inductor in series with the load c) connecting a capacitor & resister in parallel with the device d) connecting an inductor & resister in parallel with the device View Answer

Answer: a Explanation: As soon as the switch is closed, C acts like a S.C & the  voltage equation gives Vs = (Rs + Rl) I + L di/dt 

Solve the above D.E. 6. The effect of over-voltages on SCR are minimized by using a) RL circuits b) Circuit breakers c) Varistors d) di/dt inductor View Answer Answer: c  Explanation: Varistors are non-linear voltage clamping devices, RC circuits  across the loads can also be used. 

7. Over-current protection in SCRs is achieved through the use of a) Varistors b) Snubber Circuits c) F.A.C.L.F & C.B. d) Zener diodes View Answer Answer: c  Explanation: FACLF stands for Fast Acting Current Limiting Fuse. 

8. False triggering of the SCRs by varying flux & noise is avoided by using a) F.A.C.L.F & C.B b) Shielded cables & twisted gate leads c) Snubber circuits d) di/dt inductor in series with the gate terminal View Answer Answer: b  Explanation: Shielded cables provide isolation from outside noise. 

9. The thyristor has the following specifications Vs = 400 V

Answer: c  Explanation: Snubber circuit R-C in parallel with SCR is connected for dv/dt  protection. 

(di/dt)​max​ = 25 A/μsec

5. Figure below shows SCR having dv/dt and di/dt protection, when the switch(shown in green) is closed the current through Rl =

Find the value of L 

  a) 8 μH b) 80 μH c) 16 μH d) 160 μH 

View Answer a) I(1 – e​-t/τ​)

Answer: c Explanation: As soon as the switch is closed, C acts like a S.C & the  voltage equation gives, Vs = (Rs + Rl) I + L di/dt Solve the above D.E. 

di/dt = Vs/L e​-t/τ di/dt is maximum at t = 0, substitute the above given values & find L 

10. Thyristors are used in electronic crowbar protection circuits because it possesses a) high surge current capabilities

5. Which of the following thermal resistance parameters depends on the size of the device and the clamping pressure? a) θ​sA

b) high amp ​2​-sec rating

b) θ​cs

c) less switching losses d) voltage clamping properties 

c) θ​jc​ d) None

View Answer Answer: a  Explanation: Crowbar protection circuits have high surge current capabilities.    -----------------------------------------------------------------------------------------------------------------------------   

1. The usual way to accomplish higher gate current for improved di/dt rating is by using a) varistors b) pilot thyristors c) twisted cables d) op-amps View Answer Answer: b  Explanation: Pilot SCR is an SCR which is fired which activates the  firing circuit and fires the main SCR. 

2. Inter-digitating of gate-cathode regions in SCR devices improves the a) I​2​t rating b) di/dt rating c) dv/dt rating d) thermal resistance View Answer Answer: b  Explanation: Inter-digitating is the inter-mixing of the gate-cathode  area to improve the di/dt ratings. di/dt rating is improved by  providing more cathode conduction area during the delay and rise  time. 

3. The dv/dt rating of SCR can be improved by using a) cathode-short structure b) anode-short structure c) gate-short structure d) centre gate thyristor

View Answer Answer: b  Explanation: The case-to-sink thermal resistance depends on the  size of the device, pressure, grease between the interface, etc. 

6. The sink to ambient thermal resistance of SCR θ​sA​ a) depends on the flatness of the device b) depends on the length of the device c) depends on the current carrying capabilities d) is independent on thyristor configuration View Answer Answer: d  Explanation: It does not depend on any of the device  configurations. 

7. P​av​ x (θ​jc​ + θ​cs​ + θ​sA​) = a) Maximum specified temperature b) Energy lost c) Difference in temperature between junction & ambient d) Sum of junction & ambient temperature View Answer Answer: c  Explanation: P​av​ = (T​j​ – T​a)​ /θ​jA  θ​jA​ = (θ​jc​ + θ​cs​ + θ​sA​). 

8. Heat dissipation from heat sink mainly takes place by a) radiation b) convection c) absorption d) none of the mentioned View Answer

View Answer Answer: a  Explanation: Cathode shorts are realized by overlapping metal on  cathode n+ layers with a narrow p-region in between. 

4. The total thermal resistance between junction and ambient θ​jA​ is 10°C/W. θ​jc​ is 2°C/W. θ​cs​ is 4°C/W. θ​sA​ = ? a) 4°C/W b) 2°C/W c) 10°C/W d) 16°C/W

Answer: b  Explanation: Heat DISSIPATION (rejection of heat to the  atmosphere) takes place through convention only. 

9. For low power SCRs (about 1 Ampere current) _____________ type of mounting is used a) lead b) stud c) bolt-down d) press-fit View Answer

View Answer

Answer: a  Explanation: Lead mounting is a very simple time of mounting  used for low power devices. 

Answer: a  Explanation: θ​jA​ = θ​jc​+ θ​cs​+ θ​sA​. 

10. In the ___________ type of mounting the SCR is pressed between two heat sinks a)

bolt-down mounting b) stud-mounting c) press-pak mounting d) cross-fit mounting View Answer Answer: c  Explanation: In the press-pak type the device is pressed or  clamped between two heat-sinks & external pressure is applied  from both the sides.