--- title: "Chopper classification; step-down chopper RL load" subject: "Advanced Electronics" chapter: 7 key: "semester_4/advanced_electronics:classify-chopper-step-down-rl" source: https://www.bctengineer.com/q/semester_4/advanced_electronics/classify-chopper-step-down-rl site: BCT Engineering --- # Chopper classification; step-down chopper RL load Advanced Electronics · Chapter 7: Introduction to Power Electronics - Asked in: 2068 Chaitra, 2+5 marks - Video: [Step down chopper with R L load](https://www.youtube.com/watch?v=FTO3yxfOIRI) — EngineeringBoom - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/classify-chopper-step-down-rl - Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-7 ## Question Classify chopper. Explain principle of step-down chopper with RL load. ## Short answer Choppers are classed by output voltage (buck, boost, buck-boost), quadrant (A–E), and commutation. A step-down chopper with RL load alternately connects supply via a switch and freewheels inductor current through a diode; average output voltage Vo = k·Vs, and ripple depends on L and frequency. ## Answer #### Classification of choppers 1. **By output voltage level:** - Step-down chopper (buck): Vo = k·Vs < Vs - Step-up chopper (boost): Vo = Vs/(1 − k) > Vs - Step-up/down chopper (buck-boost) 2. **By quadrant of operation (direction of Vo and Io):** - Class A: first quadrant (+V, +I) - Class B: second quadrant (+V, −I), regenerative - Class C: two-quadrant, I and II (current reversible) - Class D: two-quadrant, I and IV (voltage reversible) - Class E: four-quadrant 3. **By commutation:** voltage-commutated, current-commutated, load-commutated, impulse-commutated. #### Step-down chopper with RL load ```text CH +Vs --/ --+--------+ | R FD L | E (0 if pure RL) − ------+--------+ ``` **Mode 1 – CH ON (0 < t < Ton):** Load is connected to Vs; FD reverse biased. Current rises exponentially from I1 to I2: ```text Vs = R·i + L·di/dt + E i = I1·e^(−t/τ) + ((Vs − E)/R)(1 − e^(−t/τ)), τ = L/R ``` **Mode 2 – CH OFF (0 < t < Toff):** Inductor energy keeps current flowing through FD; vo = 0. Current decays from I2 to I1: ```text 0 = R·i + L·di/dt + E i = I2·e^(−t/τ) − (E/R)(1 − e^(−t/τ)) ``` **Waveforms (continuous current):** ```text vo Vs |‾‾‾‾|____|‾‾‾‾|____ 0 Ton T io I2 | /\ /\ I1 |__/ \___/ \___ (rises: CH)(falls: FD) ``` **Output relations:** ```text Vo = (1/T)∫₀^(Ton) Vs dt = k·Vs Io = (k·Vs − E)/R ΔI = I2 − I1, maximum at k = 0.5: ΔImax = (Vs/R)·tanh(R/(4fL)) ≈ Vs/(4fL) ``` Higher chopping frequency or larger L gives smoother current. Used for DC motor speed control. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In which quadrant does a Class A chopper operate? A. First quadrant (+V, +I) B. Second quadrant (+V, −I) C. Fourth quadrant D. All four quadrants Answer: A. Class A has both load voltage and current positive, so it works only in the first quadrant. (Key point: Class A chopper) 2. In a step-down chopper with RL load, what happens when the chopper CH turns OFF? A. Load current drops to zero instantly B. The inductor energy keeps current flowing through the freewheeling diode and vo = 0 C. The supply voltage is applied in reverse across the load D. The diode is reverse biased and the load is open Answer: B. The inductor opposes the change of current, so current continues through FD, making vo = 0 while the current decays. (Key point: Freewheeling diode role) 3. What is the average output voltage of the step-down chopper (duty ratio k)? A. Vs/(1 − k) B. k·Vs C. (1 − k)Vs D. Vs/k Answer: B. Vo = (1/T)∫0^Ton Vs dt = Ton·Vs/T = k·Vs, which is less than Vs. (Key point: Average output voltage) 4. How can the load current ripple be reduced? A. Lower chopping frequency or smaller L B. Higher chopping frequency or larger L C. Operate at k close to 0.5 only D. Remove the freewheeling diode Answer: B. The ripple is about Vs/(4fL) at maximum, so increasing f or L makes the current smoother. (Key point: Current ripple) ## Related in Advanced Electronics - [Step-down chopper with RL load](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Astep-down-chopper-rl) — Advanced Electronics, chapter 7 - [Chopper classification; step-up converter](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Achopper-classification-step-up) — Advanced Electronics, chapter 7 - [Freewheeling diode; step-up chopper principle](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Afreewheeling-diode-step-up-chopper) — Advanced Electronics, chapter 7 ## Related in other subjects - [Step-down chopper RLE load, 5 kHz](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Astep-down-chopper-rle-5khz) — Power Electronics, chapter 4 - [Step-down chopper RLE load continuity](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Astep-down-chopper-rle-200v) — Power Electronics, chapter 4 - [Step-down chopper: fundamental of load current](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Astep-down-chopper-fundamental-load-current) — Power Electronics, chapter 4 - [Step-down chopper with DC motor load](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Astep-down-chopper-dc-motor) — Power Electronics, chapter 4