Type-B and Type-C dc choppers

Choppers are classified by the quadrants of the $v_0$–$i_0$ plane they work in. Type B works in the second quadrant only (regeneration); Type C works in the first and second quadrants (motoring and regeneration).

Type-B chopper (second quadrant)

     D
 +---|<|----+--------+
 |          |        |
 Vs        CH        L
 |          |       (E)  load
 |          |        |
 +----------+--------+

The load must contain an EMF $E$ (e.g. a dc motor during braking or a battery).

• CH ON: the load is short-circuited through CH; $v_0 = 0$. $E$ drives current through L and CH (current direction is out of the load's positive terminal, i.e. $i_0$ negative); energy is stored in L: $E = L\,di/dt + Ri$.
• CH OFF: the inductor voltage $L\,di/dt$ adds to $E$, so $v_0 = E + L\,di/dt$ exceeds $V_s$; diode D conducts and the current flows back into the source. Energy is returned to $V_s$.
• Load voltage is positive, load current is negative, so power flows from load to source.
• Average output voltage: $V_0 = (1-k)V_s$, where $k$ is the duty cycle of CH. It works as a step-up chopper from $E$ to $V_s$.

 v0  Vs |    ____      ____
        |   |    |    |    |
      0 |___|    |____|    |__ t
 |i0|   |   /\        /\
        |  /  \      /  \
        | /    \____/    \__ t
         Ton Toff

Use: regenerative braking of dc motors.

Type-C chopper (first and second quadrants)

 +-----+--------------+
 |     |              |
 |    CH1   ^D2       |
 Vs    |    |         |
 |     +----+-----+  load
 |     |    |     |  R,L,E
 |    D1^  CH2    |   |
 |     |    |     +---+
 +-----+----+

It combines a Type-A chopper (CH1, D1) and a Type-B chopper (CH2, D2) in parallel.

• First quadrant (motoring): CH1 is switched. CH1 ON: $v_0 = V_s$, $i_0$ positive, power to load. CH1 OFF: $i_0$ freewheels through D1, $v_0 = 0$.
• Second quadrant (braking): CH2 is switched. CH2 ON: load shorted, $E$ drives negative current through CH2, energy stored in L. CH2 OFF: negative current flows through D2 into the source ($v_0 = V_s$), returning energy.
• Load voltage is always positive; load current reverses. CH1 and CH2 must never be ON together (supply short).
• Use: dc motor drives with motoring and regenerative braking in one direction (traction, electric vehicles).