A step-down chopper is also called as a_____.

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First-quadrant chopper

Explanation: A step-down chopper, or buck converter, is classified as a first-quadrant chopper because it allows power flow only from the source to the load while maintaining positive output voltage and positive output current. In this configuration, both the voltage $V_0$ and the current $I_0$ remain positive, operating strictly in the first quadrant of the $V-I$ plane.

Working Principle: In a first-quadrant chopper, the switch (typically a MOSFET or IGBT) is turned ON and OFF periodically. When the switch is ON, the source voltage is applied to the load; when the switch is OFF, the load current freewheels through a diode. Because the load voltage and current always maintain the same polarity as the source, the chopper acts as a single-quadrant device.

Key Formulas: $V_0 = \alpha V_s$ — Output voltage of a step-down chopper, where $\alpha$ is the duty cycle; $\alpha = \frac{T_{on}}{T}$ — Definition of duty cycle ratio

Key Points: Operates only in the first quadrant of the V-I coordinate system.; The output voltage $V_0$ is always less than the source voltage $V_s$.; Suitable for driving passive DC loads or motor loads requiring unidirectional current.; Utilizes a freewheeling diode to maintain load current during the switch-off period.

Advantages: High efficiency due to switching operation; Smooth control of output voltage

Disadvantages: Output is strictly unidirectional; Requires high-frequency switching

Applications: DC traction motor control; Battery charging circuits; Variable speed DC drives

Additional Info: A step-down chopper is also known as a Class-A chopper.. Option B (Second-quadrant) refers to a step-up chopper (regenerative), while options C and D represent multi-quadrant converters for reversing power flow or current direction.

Answer: A is correct — A step-down chopper operates solely in the first quadrant, providing positive output voltage and current.

Exam Tip: Always remember that in chopper classification, the quadrant defines the polarity of voltage and current; Class A is 1st quadrant, Class B is 2nd, Class C is 1st and 2nd, and Class D is 1st and 4th.

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