Test: Step Down Choppers - Electrical Engineering (EE) MCQ

Concept:

In a buck converter, the output voltage is given by

V_o = δV_in

and I_in = δI_o

Where V_o is the output voltage

V_in is the input voltage

I_in is the input current

I_o is the output current

δ is the duty cycle

Calculation:

Given that, input current (I_in) = 2.4 A

Output current (I_o) = 6 A

Duty cycle, 

Concept:

The average output voltage in step down chopper is given by:

where, V_S = Supply voltage

Calculation:

Given, V_S = 15V

Concept:

In a buck converter (step down chopper)

Where,

D = Duty ratio

V₀ = Average out-put voltage or DC output voltage

V_i = Input voltage

t_on = On time period

T = total time period

Calculation:

V₀ = 150 V, V_i = 200 V

D = 0.75 = 75%

Note:

RMS output voltage of buck converter is given by

Concept:

In a buck converter (step down chopper)

Where,

D = Duty ratio

V₀ = Average out-put voltage or DC output voltage

V_i = Input voltage

t_on = On time period

T = total time period

I₀ = output current

Calculation:

Given that, input voltage (V_S) = 48 V

Resistive load (R) = 24 Ω

Switching frequency (f) = 250 Hz

Switch on duration = 1 ms

Duty cycle ratio δ = ON duration / Time period

δ = 1 / 4 = 0.25

In a buck converter,

Step-down chopper:

It is also called a Buck converter.

The output voltage steps down compared to the input voltage.

The circuit is shown below.

Formula:

The output voltage can be calculated as

Vo = D. Vs

D = Duty cycle

Vs = Source voltage

Calculation:

Given

D = 55% = 0.55

V_s = 100 V

Vo = 0.55 × 100 = 55 V

During 0 < t < t_r, the load is connected to the source and the voltage across the switch is V_s and load current increases.

During t_r < t < t_f, the load current decreases. This implies that the back emf is connected to the load inductance and voltage across the switch is zero as the complete back emf is dropped overload inductance.

During t_f < t < t₀, the load current is zero, so back emf appears across the switch.

Concept:

So, when D increases, also increases.

 is the average load voltage.

D is the duty cycle.

 is the input voltage.

 is the voltage drop across the switch in the ON condition.

Step-down chopper:

Case 1: When the switch is ON

The diode is reverse biased.

V_O = V_S

Case 2: When the switch is OFF

The diode is reverse biased.

V_O = 0

The r.m.s. value of output voltage is given by:

DC chopper

It is a power electronics device that is used to convert pure DC into pulsating DC.

The average value of output voltage (pulsating value):

where, D is the duty cycle

Calculation

Source voltage (V_s) = 220 V

V_ch = 2 V

When the chopper is in an ON state:

When the chopper is in an OFF state:

V_o = 0V

The waveform is given below:

The average value of output voltage is:

Concept:

Buck converter: It is a switch mode DC to DC electronic converter in which the output voltage will be transformed to a level less than the input voltage. It is also called a step-down converter.

The output voltage of buck converter is given as:

V_o = D V_in

Where,

V_o is the output voltage

V_in is the input voltage

I_in is the input current

I_o is the output current

D is the duty cycle

Calculation:

During ON time:

During OFF time: