21. Calculate the De-rating factor if the string efficiency is 90 %.
A. .4
B. .1
C. .7
D. .8
Answer: A
The de-rating factor is used to measure the reliability of a string. The value of the De-rating factor is
1-(string efficiency)=1-.9=.1.
22. Calculate the string efficiency if the de-rating factor is .22.
A. 78 %
B. 42 %
C. 20 %
D. 90 %
Answer: A
The string efficiency is calculated for the series and parallel connection of SCRs. The value of string efficiency is
1-(De-rating factor)=1-.22=78 %.
23. 25 V-rated n SCRs are connected in series. The operation voltage of the string is 300 V and the string efficiency is 40 %. Calculate the value of n.
A. 27
B. 30
C. 34
D. 36
Answer: B
The string efficiency can be calculated using the formula operation
voltage÷(Number of SCRs×Rated voltage).
The value of n is 300÷(25×.4)=30.
24. The most reliable method for thyristor triggering is thermal triggering.
A. True
B. False
Answer: B
Gate triggering is more reliable than any other triggering method. The risk of false triggering of SCR reduces in the case of gate triggering.
25. SCR is unipolar.
A. True
B. False
Answer: B
The SCR is a uni-directional, bipolar power electronics device. It is bipolar in nature because of its reverse and forward mode blocking capability.
26. What is the formula for thermal voltage?
A. K×e÷T
B. K×T÷e
C. K2×e÷T
D. K×e2÷T
Answer: A
The formula for thermal voltage is K×e÷T
where
K is the Boltzmann constant
e is the charge on the electron
T is the absolute temperature.
27. Device with highest d(i)÷d(t) and d(v)÷d(t) capability is _____________
A. GTO
B. BJT
C. SCR
D. SITH
Answer: D
SITH stands for static induction thyristor. It has a buried gate structure in which the gate electrode is placed in the n-base region. They have high d(i)÷d(t) and d(v)÷d(t) capability.
28. ____________ is the switching frequency of SITH.
A. 55 kHz
B. 100 kHz
C. 91 kHz
D. 105 kHz
Answer: B
100 kHz is the switching frequency of the static induction thyristor. Static Induction Thyristor has a low frequency because of lower turn-off current gain.
29. Calculate the value of the Crest factor if Vpeak=1.5 V and Vr.m.s=2.5 V.
A. .4
B. .6
C. .5
D. .7
Answer: B
The value of the crest factor is
Vpeak÷Vr.m.s=1.5÷2.5=.6 V.
It signifies the peak value is .6 times than the r.m.s value.
30. Calculate the output voltage of the Boost converter if the supply voltage is 1 V and the duty cycle value is .9.
A. 10 V
B. 20 V
C. 40 V
D. 60 V
Answer: A
The output voltage of the buck converter is
Vo = Vin×(D)
The value of the duty cycle is less than one which makes the Vo < Vin. Vo = Vin×(1-D)=10 V.
