---
title: "Chopper classification; step-up converter"
subject: "Advanced Electronics"
chapter: 7
key: "semester_4/advanced_electronics:chopper-classification-step-up"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/chopper-classification-step-up
site: BCT Engineering
---

# Chopper classification; step-up converter

Advanced Electronics · Chapter 7: Introduction to Power Electronics

- Asked in: 2070 Chaitra, 2+5 marks
- Video: [Step Up Chopper( Introduction,Basic Principle ,Modes of Operations & Expression for output Voltage)](https://www.youtube.com/watch?v=USDpo3ikvos) — EngineeringBoom
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/chopper-classification-step-up
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-7

## Question

Classify chopper on the basis of power flow. Explain the principle of step-up converter with resistive load.

## Answer

#### Classification of chopper on the basis of power flow (quadrant of operation)

| Class | Quadrant(s) | Vo | Io | Power flow |
|---|---|---|---|---|
| A | I | + | + | Source → load (motoring) |
| B | II | + | − | Load → source (regenerating) |
| C | I and II | + | ± | Both directions |
| D | I and IV | ± | + | Both directions |
| E | All four | ± | ± | Both directions |

So by power flow, choppers are **unidirectional** (Class A, B) or **bidirectional** (Class C, D, E). By output level they are also classed as step-down (Vo < Vs) and step-up (Vo > Vs).

#### Principle of step-up converter with resistive load

```text
       L     iL           D
 +Vs--uuuu--+----------->|---+-----+
            |                |     |
            S (switch)       C     R   Vo
            |                |     |
 −  --------+----------------+-----+
```

1. **Switch ON (0 < t < Ton):** The inductor is placed directly across Vs; current rises linearly and energy is stored in L. The diode is reverse biased, and the capacitor supplies the resistive load.
```text
vL = Vs  →  ΔI = Vs·Ton/L
```
2. **Switch OFF (Ton < t < T):** Inductor current cannot change suddenly; its voltage reverses and adds to Vs. Current flows through D into C and R. Load voltage is now higher than Vs.
```text
vL = Vs − Vo (negative)  →  ΔI = (Vo − Vs)·Toff/L
```
3. **Steady state:** current rise during ON equals fall during OFF:
```text
Vs·Ton = (Vo − Vs)·Toff
Vo = Vs·T/Toff = Vs/(1 − k),  k = Ton/T
```

Load current Io = Vo/R and, ignoring losses, input current Is = Io/(1 − k).

**Waveforms:**

```text
S    |‾‾‾‾‾‾|___|‾‾‾‾‾‾|___
iL   |  /‾‾\   /‾‾\
     | /    \_/    \_   rises ON, falls OFF
vo   |‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾  ≈ Vs/(1−k), small ripple
```

Example: Vs = 12 V, k = 0.6 → Vo = 12/0.4 = 30 V. Since 0 < k < 1, Vo is always greater than Vs.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Class A chopper operates in which quadrant?
   A. Quadrant I, source to load
   B. Quadrant II, load to source
   C. Quadrants I and II
   D. All four
   Answer: A. Vo and Io are both positive. (Key point: Class A)

2. Which class operates in all four quadrants?
   A. Class A
   B. Class E
   C. Class B
   D. Class C
   Answer: B. Class E has ±Vo and ±Io. (Key point: Class E)

3. During switch ON in the boost converter?
   A. Inductor discharges via D
   B. Diode conducts
   C. Inductor is across Vs and stores energy; capacitor supplies load
   D. Load is shorted to ground
   Answer: C. The diode is reverse biased. (Key point: Boost ON state)

4. What is the steady-state output voltage?
   A. Vo = k·Vs
   B. Vo = Vs·(1 − k)
   C. Vo = Vs/k
   D. Vo = Vs/(1 − k)
   Answer: D. For Vs = 12 V and k = 0.6, Vo = 30 V. (Key point: Boost output)

## Related in Advanced Electronics

- [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
- [Chopper classification; step-down chopper RL load](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aclassify-chopper-step-down-rl) — Advanced Electronics, chapter 7
- [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 numerical: 220 V, 10 Ω, 50% duty](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Achopper-220v-10ohm) — Advanced Electronics, chapter 7, asked 4×
- [Inverters; chopper numerical 220 V, 10 Ω](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ainverters-chopper-220v-10ohm) — Advanced Electronics, chapter 7, asked 4×

## Related in other subjects

- [Step-up chopper and controllable power transfer](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Astep-up-chopper-power-transfer) — Power Electronics, chapter 4
- [Step-up chopper operation and output voltage](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Astep-up-chopper-principle) — Power Electronics, chapter 4, asked 8×
- [Step-up chopper average and rms output](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Astep-up-chopper-avg-rms) — Power Electronics, chapter 4
- [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