# Linear Integrated Circuit Questions and Answers – Switching Regulator

This set of Linear Integrated Circuit Multiple Choice Questions & Answers (MCQs) focuses on “Switching Regulator”.

1. What is done in switching regulators to minimize its power dissipation during switching?
a) Uses external transistor
b) Uses 1mH choke
c) Uses external transistor and 1mH choke
d) None of the mentioned

Explanation: To minimize power dissipation during switching, the external transistor must be a switching power transistor and a 1mH choke smooth out the current pulses delivered to the load.

2. Fixed voltage regulators and adjustable regulators are often called as
a) Series dissipative regulators
b) Shunt dissipative regulators
c) Stray dissipative regulators
d) All the mentioned

Explanation: Series dissipative regulators simulate a variable resistance between the input voltage & the load and hence functions in a linear mode.

3. Linear series regulators are suited for application with
a) High current
b) Medium current
c) Low current
d) None of the mentioned

Explanation: In series dissipative regulator, conversion efficiency decreases as the input or output voltage differential increases and vice versa. So, linear series regulators are suited for medium current application with a small voltage differential.

4. A series switching regulators
a) Improves the efficiency of regulators
b) Improves the flexibility of switching
c) Enhance the response of regulators
d) All of the mentioned

Explanation: A series switching regulators is constructed such that, a series pass transistor is used as a switch rather than as a variable resistance in linear mode.

5. The switching regulators can operate in
a) Step up
b) Step down
c) Polarity inverting
d) All the mentioned

Explanation: The switching regulators can operate in any one of the three modes depending on the way in which the components are connected.
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6. Find the diagrammatic representation of basic switching regulator?
a)

b)

c)

d)

Explanation: Basic switching regulators consist of four major components;
1) Voltage source
2) Switch
3) Pulse generator
4) Filters as mentioned in the diagram.

7. What are the conditions to be satisfied by a voltage source for using it in switching regulator.
i. It must supply the required output power and the losses associated with the switching regulator
ii. It must be large enough to supply sufficient dynamic range line and load variations
iii. It must be sufficient high to meet the minimum requirement of the regulator system to be designed
iv. It may be required to store energy for a specified amount of time during power failure especially if the system is designed for a computer power supply.
a) i and iii
b) i, ii, iii and iv
c) ii, iii and iv
d) i, iii and iv

Explanation: A voltage source must satisfy the entire given requirement to be used in switching regulator.

8. Which among the following act as a switch in switching regulator?
a) Rectifiers
b) Diode
c) Transistors
d) Relays

Explanation: A transistor is connected as power switch and is operated in the saturated mode.Thus, the pulse generator output alternatively turns the switch ON and OFF in switching regulator.

9. What should be the frequency range of pulse generator?
a) 250 kHz
b) 40 kHz
c) 120 kHz
d) 20 kHz

Explanation: The most effective frequency range for pulse generator for optimum efficiency and component size is 20kHz.

10. Filter used in switching regulator’s are also as called
a) DC – AC transformers
b) AC – DC transformers
c) DC transformer
d) AC transformer

Explanation: Filter converts the pulse waveform from the output of the switch into a dc voltage. Since this switching mechanism allows a conversion similar to transformers, the switching regulators is often referred to as a DC transformer.

11. Which of the following is considered to be the most important components of the switching regulator?
a) RC or RLC filter
b) RL or RLC filter
c) ORC or RL filter
d) RC, RLC or RL filter

Explanation: RL or RLC filter is the most important components of the switching regulator, because there are several areas that are affected by the choke of inductor including energy storage for the regulators output ripple, transient, response etc.

12. Which is the most commonly used low voltage switching regulators?
a) Powdered Permalloy toroids
b) Fermite EI, U and toroid cores
c) Silicon steel EI butt stacks
d) None of the mentioned

Explanation: The silicon steel EI butt stack exhibits high permeability high flux density and ease of construction and mounting therefore, it is most commonly used in low voltage switching regulators.

13. Find the value of Rsc, L and Co for a µA7840 switching regulator to provide +5 v at 3A, using the following specifications: toff= 24µs, ripple voltage = 400mA and ton=26µs.
a) Rsc = 55 mΩ , L = 25µH & Co = 750µF
b) Rsc = 550 mΩ , L = 25µH & Co = 75µF
c) Rsc = 650 mΩ , L = 25µH & Co = 65µF
d) Rsc = 720 mΩ , L = 25µH & Co = 250µF

Explanation: Peak current,Ipk= 400mA× 1.5 (since Ipk = 1.5 A for peak current)
∴ Rsc = 0.33ohm/Ipk = 0.33ohm/6 = 0.055ohm.
=> L= [(Vo +Vp) / I pk]×toff =[(5+1.25) /6 ]× 24×10-6 =25µH.
=> Co = [Ipk (Ton +Toff)]/[8×Vripple] ∵T = [ton + toff] = 26µs + 24µs = 50µs
=> Co = [ (6×50µs)]/(8×50mA) = 7.5×10-4 = 750µF.

14. Calculate the efficiency of the step down switching regulator given the input voltage Vin= 13.5v and output voltage =6v. Assume the saturating Voltage Vs=1.1v and the forward voltage drop Vd = 1.257v
a) η = 75%
b) η = 48.5%
c) η = 63.9%
d) η = 80.5%

Explanation: Efficiency of the step down switching regulator, η = {[(Vin-Vs+Vd)]/ [Vin]}×(Vo) / [(Vo+Vd)] = {[(13.5v-1.1v+1.257v)/13.5v]} ×[(6/(6 ×1.257)] => Efficiency of switching regulator, η = (1.012×0.7955)×100 = 0.8051×100 = 80.5%.

15. Match the characteristics for various switching regulators.

 Switching regulator Characteristics 1. Inverting (i) [ton / toff ] = [ Vo + Vd] / [Vin -Vs -Vd]; Rsc = ( 0.33/ Ipk) 2. step down (ii) [ton / toff ] = [ Vo + Vd -Vin] / [Vin-Vs] ; Rsc = ( 0.33/ Ipk) 3. step up (iii) [ton / toff ] = [ Modulus Vo +Vd] / (Vin-Vs); Rsc = ( 0.33/ Ipk

a) 1- iii , 2- i , 3- ii
b) 1- i , 2- ii , 3- iii
c) 1- iii , 2- ii , 3- i
d) 1- iii , 2- ii , 3- i

Explanation: Characteristics and design formula for step up, step down and converting mode of switching regulator.

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