Q1. What is time ratio control?
Ans: The ‘time ratio’ control refers to the ratio of the switch’s on-time to the
total switching period, commonly known as duty cycle.
𝐷=
𝑇𝑜𝑛
𝑇
where, D: duty cycle
𝑇𝑜𝑛 : On-time of the switch
T: Total switching period (T=Ton+Toff)
(a) Constant freq. Time Ratio
Control
(b) Variable freq. Time Ratio
Control
Q2. Define pulse number.
Ans: Pulse number is defined as the number of pulses of DC output voltage
generated per cycle of AC input voltage in a converter.
Q3. Define duty cycle?
Ans: Duty cycle is the ratio of the ON time of a switch to the total time period
of one complete switching cycle. It is usually expressed as a percentage.
Q4. What is the function of a chopper?
Ans: The function of a chopper is to convert a fixed DC input voltage to a
variable DC output voltage by rapidly switching the input on and off.
Q5. Classify different types of DC choppers and describe one with the help
of a circuit diagram.
Ans: Classification of Chopper
Class A
Class B
Class C
Class D
Class E
Load Voltage
positive
positive
positive
positive/negative
positive/negative
Load Current
positive
negative
positive/negative
positive
positive/negative
* https://www.info4eee.com/2022/12/classification-of-choppers.html
Type A Chopper or First–Quadrant Chopper
This type of chopper is shown in the figure. It is known as first-quadrant chopper or type A
chopper. When the chopper is on, v0 =VS as a result and the current flows in the direction of the
load. But when the chopper is off v0 is zero but I0 continues to flow in the same direction
through the freewheeling diode FD, thus average value of voltage and current V0 and I0 will
be always positive as shown in the graph.
Q6. Discuss the effect of duty cycle variation on the output voltage and
current in a DC chopper.
Ans: Chopper is the one represented by “SW” inside a dotted square which can
be turned on or off as desired.
Fig.1 Chopper circuit
Fig.2 Output voltage and current waveforms
➢ During the time period Ton, the chopper is turned on and the load voltage
is equal to source voltage Vs. During the interval Toff, the chopper is off
and the load current will be flowing though the freewheeling diode FD.
➢ The load terminals are short circuited by FD and the load voltage is
therefore zero during Toff.
➢ Thus, a chopped dc voltage is produced at the load terminals. We can see
from the graph that the load current is continuous.
➢ Average load Voltage is given by
𝑇𝑜𝑛
𝑉𝑜 =
× 𝑉𝑠 = 𝛼𝑉𝑠
𝑇𝑜𝑛 + 𝑇𝑜𝑓𝑓
Effect on duty cycle variation on voltage
In a step-down chopper, the average output voltage Vout is directly proportional
to the duty cycle.
𝑉𝑜 = 𝛼𝑉𝑠
As duty cycle increases, the output voltage increases linearly.
Effect on duty cycle variation on current
𝑉
𝐷𝑉
Assuming a resistive load 𝑅, the output current 𝐼𝑜𝑢𝑡 is 𝐼𝑜𝑢𝑡 = 𝑜𝑢𝑡 = 𝑖𝑛
𝑅
As the duty cycle increases, the output current increases linearly.
𝑅
Q7. Compare single-quadrant and four-quadrant choppers in terms of
control and application.
Ans:Single quadrant operation
A single quadrant chopper (also called a Class-A chopper) is a type of DC-DC
converter that allows power flow in only one direction, with both output voltage
and current being positive. It operates in the first quadrant of the voltage-current
(V-I) plane.
Working Principle:
•
It consists of a power switch (like a transistor or IGBT) and a
freewheeling diode.
•
The output voltage is controlled by varying the duty cycle (D), where
•
When the switch is ON, the load receives full source voltage.
•
When the switch is OFF, the diode conducts (for inductive loads) to
maintain current.
Features:
•
Output voltage and current are positive.
•
Power flow is unidirectional (source to load).
Applications:
•
Speed control of DC motors in motoring mode (e.g., electric vehicles,
elevators).
•
Used in battery-powered systems.
•
Found in switched-mode power supplies (SMPS).
Four Quadrant Chopper
A Four Quadrant Chopper is a DC-DC converter that can control the voltage
and current in all four quadrants of the voltage-current (V-I) plane. This means
it allows power flow in both directions and supports both motoring and braking
(regenerative) operations.
Quadrant Operation:
1. First Quadrant: Motoring (forward voltage, forward current)
2. Second Quadrant: Regenerative braking (positive voltage, negative
current)
3. Third Quadrant: Motoring in reverse (negative voltage, negative current)
4. Fourth Quadrant: Regenerative braking in reverse (negative voltage,
positive current)
Control Mechanism:
•
Implemented using four switches (e.g., IGBTs or MOSFETs) arranged in
an H-bridge or dual chopper configuration.
•
Duty cycles of switches are varied to control the output voltage and
direction of current.
•
Capable of reversing both the voltage polarity and current direction.
Applications:
•
DC motor drives requiring both forward and reverse motoring and
braking.
•
Electric vehicles with regenerative braking.
•
Robotics and industrial automation.
•
Elevators and cranes needing bidirectional control.
Q8. Design a basic chopper circuit to supply a 100V DC motor from a 200V
DC source.
Ans:
•
Source voltage Vs=200V
•
Desired output voltage Vo=100V
Chopper Design:
To reduce 200V to 100V using a step-down (buck) chopper, we use the
formula:
Vo=D×Vs
Where:
•
D = Duty cycle
•
Vo = Output voltage
•
Vs= Source voltage
⇒D= Vo/ Vs=100/200=0.5
So, the chopper must operate at a 50% duty cycle.
Q9. Describe the working of a Type-A chopper with waveforms.
Ans: A Class A chopper is a type of single quadrant chopper that operates in
the first quadrant of the voltage-current (V-I) plane, where both output voltage
and current are positive. It allows power to flow only from the source to the load,
making it suitable for applications like DC motor control in motoring mode.
The circuit typically consists of a power semiconductor switch (like an IGBT or
MOSFET) and a freewheeling diode. When the switch is ON, the load is directly
connected to the source and receives full voltage. When the switch is OFF, the
freewheeling diode conducts (especially for inductive loads), allowing the load
current to continue flowing. The average output voltage is controlled by adjusting
the duty cycle of the switch, using the relation Vo=D×Vs, where D is the ratio of
ON time to total switching time. This type of chopper is widely used in electric
vehicles, conveyors, and SMPS for unidirectional voltage control.
Q10. Calculate the output voltage of a buck converter operating at 60%
duty cycle with input 200V.
Ans: To calculate the output voltage of a buck converter, use the basic
formula:
𝑉𝑜 = 𝐷 × 𝑉𝑖𝑛
Where:
•
Vo = Output voltage
•
D = Duty cycle = 60% = 0.6
•
Vin = Input voltage = 200V
Vo=0.6×200=120V