Class C and class E dc choppers

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

  +----+------------+
  |    |            |
  |   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

  +------+------------+------+
  |      |            |      |
  |     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 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).