Natural vs forced commutation

Commutation is the process of turning OFF a conducting thyristor. The anode current is reduced below the holding current and a reverse voltage is kept across the device for longer than its turn-off time $t_q$, so that it regains its forward blocking ability.

Natural (line) commutation

  vs ~ ---[SCR]---+
                  |
                  R (load)
                  |
  ----------------+

In an a.c. circuit, the supply voltage passes through zero and reverses every half cycle. The load current falls to zero and the SCR is reverse biased by the supply itself, so it turns OFF without any extra components. Examples: phase-controlled rectifiers, AC voltage controllers, cycloconverters.

Forced commutation

        +---- C ------[TA]---+
        |  (-   +)           |
 +Vdc --+--------[T1]--------+--- load ---+
                                          |
 GND -------------------------------------+

When the auxiliary SCR $T_A$ is fired, the pre-charged capacitor $C$ is placed across $T_1$ with reverse polarity, forcing its current to zero and reverse biasing it.

In d.c. circuits the current never reaches zero naturally, so an external commutation circuit (usually a charged capacitor and an inductor, switched by an auxiliary thyristor $T_A$) forces the current to zero or applies reverse voltage. Classes:

• Class A (self/load commutation by resonant LC load)
• Class B (resonant pulse, LC across SCR)
• Class C (complementary, two SCRs with a capacitor)
• Class D (auxiliary SCR, impulse commutation)
• Class E (external pulse source)

Examples: DC choppers and inverters.

Comparison