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QUESTION: 1

In a type A chopper, the maximum value of Ripple current is

Solution:

__Concept:__

Steady state ripple

The peak to peak ripple current has maximum value when duty cycle ∝ = 0.5

In case 4FL ≫ R, then tan

Calculation

∴ maximum value of Ripple current is inversely proportional to chopping frequency and the circuit Inductance.

*Answer can only contain numeric values

QUESTION: 2

A boost – regulator has an Input voltage 8V and the average output voltage of 24 V the duty cycle is _

Solution:

We known Boost regulator is a step – up chopper

*Answer can only contain numeric values

QUESTION: 3

A DC chopper operates on 220 V dc and frequency of 500 Hz, feeds on RL load of the output voltage of chopper is 180 V the ON time of the chopper is –(in msec)

Solution:

V_{dc} = 220V, f = 500 Hz, V_{o} = 180 V

(2×10^{−3}) = 1.636 msec

QUESTION: 4

DC chopper feed an RLE load. If the value of E increased by 20%, the current ripple

Solution:

Current ripple does not depend on the value of E

ΔI_{L }= V_{dc} ∝ (1−∝)T

QUESTION: 5

A class C chopper is operated from 230 V battery the load is a dc motor with R = 0.15 Ω, L = 12 mH and E_{b} = 120 V. The duty cycle of the chopper to achieve regenerative braking at the rated current of 12 ampere would be equal to

Solution:

First find rated load current

For Regeneration braking load current should be negative.

-1.8 = ∝230 – 120

230∝ = 118.2

∝ = 0.5139

∴ Duty cycle of chopper = 51.39 %

*Answer can only contain numeric values

QUESTION: 6

A single - quadrant chopper is operating with the following specification: Ideal battery of 240 V; on time (Ton) = 1.5 ms, OFF time = 2 ms. The Form factor factor will be –

Solution:

__Concept:__

For Single – quadrant (type A) Chopper

Average value V_{o} = ∝ Vs

R.M.S value V_{rms} =

where ∝ = Ton/T

We known the form factor F.F =

__Calculation:__

Form factor =

∴ form factor = 1.528

*Answer can only contain numeric values

QUESTION: 7

A class C chopper is operated from a 220 V battery. The load is a dc motor with R = 0.1 Ω, L = 10 mH and E_{b }= 100 V. The rated current is 10 A. The duty cycle in motoring mode is

Solution:

QUESTION: 8

A buck converter is to be designed where V_{s} = 10V, V_{o} = 5V,R = 500Ω. The switching frequency is decided to be 20 kHz. Allowable current ripple is 5 mA. The optimum value of inductor is

Solution:

*Answer can only contain numeric values

QUESTION: 9

A step-up chopper shown in figure is to deliver 3A

In to the 10 Ω load. The battery voltage is 12 V, L = 20 μH, C = 100 μF and chopper frequency is 50 kHz. Determine the battery current variation in ampere

Solution:

__Calculation:__

V_{o} = I_{o}R_{L} = 3 × 10 = 30 V

V_{b} = V_{o} (1 – D)

1−D = 12/30 = 0.4

D = 0.6

Periodic time

On time = DT = 0.6 × 20 × 10^{-6} = 12 μs

over an on-time the change in battery current will be

__Concept:__

When the transistor is switched on, current ramps up in the inductor and energy is stored.

When the transistor is switched off, the inductor voltage reverses and acts together with the battery voltage to forward bias the diode, transferring energy to the capacitor.

When the transistor is switched on again, load current is maintained by the capacitor, energy is stored in the inductor and the cycle can start again.

The value of the load voltage is increased by increasing the duty cycle or the on-time of the transistor.

During the transistor on-period, assuming an ideal inductor and transistor

V_{b} = V_{L} = Ldi/dt

V_{b}/s = (sL) i(s)

i(s) = V_{b}/s^{2}L

i(t) = (V_{b}/L)t = kt

Over an on-time of T_{ON}, the change of battery current will be

Δi = k T_{ON}

During the off-period

V_{o }= V_{b} + V_{L }= V_{b} + L(di/dt) = V_{b} + L(Δi/Δt)

= V_{b} + L(V_{b}/L)T_{ON} /T_{off} = V_{b}(1 + T_{ON} /T_{off})

Let the duty cycle

D = T_{ON} T, and T_{off} = T – T_{ON} = T(1 - D). Now

V_{o} = V_{b} (1 – DT/T(1 – D))

which simplifies to

V_{o} = V_{b}/(1 - D)

In an ideal circuit, V_{b}I_{b }= V_{0}I_{0}. Therefore, from eq.

I_{b} = (V_{o}/V_{b})I_{b} = I_{o}/(1 – D)

I_{b} = I_{b} + Δi/2

I_{bo} = I_{b} – Δi/2

When the steady variation of battery current has been reached, the variation will be between l_{o} and I_{1}, as shown in.

QUESTION: 10

In the circuit shown below, if load R = 500 Ω, switching frequency is 25 kHz and peak to peak ripple current of inductor is limited to 0.9 A then the filter inductance L is"

Solution:

For Buck Converter

V_{0 }= αV_{dc} ⇒ α = 5/12

The peak to peak ripple current is

L = 129.62 μH

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