--- title: "Class C and class E dc choppers" subject: "Power Electronics" chapter: 4 key: "semester_7/power_electronics:class-c-class-e-chopper" source: https://www.bctengineer.com/q/semester_7/power_electronics/class-c-class-e-chopper site: BCT Engineering --- # Class C and class E dc choppers Power Electronics · Chapter 4: DC chopper - Asked in: 2076 Asoj, 8 marks - Video: [Class C Chopper (Basics, Circuit, Working, Waveforms & Applications) Explained](https://www.youtube.com/watch?v=7yYoP52EpgM) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/class-c-class-e-chopper - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-4 ## Question Explain class C and class E dc chopper. ## Short answer Class C is a two-quadrant chopper (I and II) with two switches and diodes, enabling motoring and regenerative braking with positive voltage only. Class E is a four-quadrant H-bridge with four switches and diodes, providing reversible voltage and current for full four-quadrant dc motor drive. ## Answer DC choppers are classified (Type/Class A to E) by the quadrants of the load voltage–current ($v_0$–$i_0$) plane in which they can operate. **Class C** is a two-quadrant chopper (quadrants I and II); **Class E** is a four-quadrant chopper. #### Class C (two-quadrant, current-reversible) chopper ```text +----+------------+ | | | | CH1 ^D2 | Vs | | | | +---+---+ Load | | | | (R,L,E) | D1^ CH2 | | | | | +----+ +----+---+ ``` It is a Class A chopper (CH1 + freewheeling diode D1) in parallel with a Class B chopper (CH2 + diode D2). - **Quadrant I (motoring):** CH1 is chopped. When CH1 is ON, $v_0 = V_s$ and $i_0$ is positive; when OFF, $i_0$ freewheels through D1 with $v_0 = 0$. Power flows from source to load. - **Quadrant II (regenerative braking):** CH2 is chopped. When CH2 is ON, the load EMF $E$ drives a negative current through L and CH2, storing energy in L ($v_0 = 0$). When CH2 is OFF, $E + L\,di/dt$ exceeds $V_s$, so negative current flows through D2 back into the source ($v_0 = V_s$). Power flows from load to source. - Load voltage is always positive; load current can be positive or negative. - CH1 and CH2 must never be ON together, or the source is short-circuited. - **Use:** dc motor drives needing motoring and regenerative braking in one direction of rotation (e.g. traction). #### Class E (four-quadrant) chopper ```text +------+------------+------+ | | | | | CH1 ^D1 CH3 ^D3 | Vs | | | | | | +--+--[Load]--+-+ | | | | R,L,E | | | | CH4 ^D4 CH2 ^D2 | +------+------------+------+ ``` Four switches with anti-parallel diodes form an H-bridge, so both $v_0$ and $i_0$ can reverse. | Quadrant | Switch chopped | Switch kept ON | Diodes used | $v_0$, $i_0$ | Mode | |---|---|---|---|---|---| | I | CH1 | CH2 | D4 (freewheel) | +, + | Forward motoring | | II | CH4 | — | D1, D2 | +, − | Forward braking | | III | CH3 | CH4 | D2 (freewheel) | −, − | Reverse motoring | | IV | CH2 | — | D3, D4 | −, + | Reverse braking | - **Quadrant I:** CH2 ON continuously, CH1 chopped; $v_0 = +V_s$ when CH1 ON; current freewheels via CH2–D4 when CH1 OFF. - **Quadrant II:** with load EMF positive and current reversed, CH4 chopped; when CH4 ON, $E$ drives current through CH4–D2 storing energy in L; when OFF, current returns to the source through D1–D2. - **Quadrants III and IV** are the mirror operations with the load EMF reversed. - **Use:** reversible, regenerative dc motor drives (all four quadrants of speed–torque). | Point | Class C | Class E | |---|---|---| | Quadrants | I and II | I, II, III, IV | | Switches | 2 + 2 diodes | 4 + 4 diodes | | $v_0$ polarity | Positive only | Both | | $i_0$ direction | Both | Both | | Application | Motoring + braking, one direction | Reversible drives | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In which quadrants does a Class C chopper operate and what are the signs of v0 and i0? A. I and II; v0 always positive, i0 either direction B. I and IV; v0 either sign, i0 positive C. All four; both reversible D. II only; v0 positive, i0 negative Answer: A. Class C = Class A in parallel with Class B, so voltage is positive and current reverses. (Key point: Class C quadrants) 2. Why must CH1 and CH2 of Class C never be ON together? A. It would reverse the motor B. It would short-circuit the source C. It would remove the freewheeling path D. It would double the load voltage Answer: B. Both switches connect across Vs through each other, giving a direct short. (Key point: Short-circuit precaution) 3. How does Class C return energy to the source? A. CH1 chopped; current freewheels through D1 B. CH2 stays ON so the source feeds the load C. CH2 chopped; when OFF, E + L di/dt exceeds Vs and current flows through D2 to the source D. The load EMF is dissipated in CH2 Answer: C. Energy stored in L during CH2 ON is pushed through D2 into the source at turn-off. (Key point: Class C regeneration) 4. How does Class E differ from Class C? A. Two switches; only i0 reverses B. One switch; only v0 reverses C. Same two switches but with a filter D. Four switches and four diodes in an H-bridge; both v0 and i0 can reverse Answer: D. Class E's bridge permits all four quadrants including reverse rotation. 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