Class E commutation
Figure 46 shows the class E commutation circuit. In this commutation method, the reverse voltage applied to the current carrying thyristor from an external pulse source. This commutation is also known as external pulse commutation. In this case, the commutating pulse is applied through a pulse transformer which is design in such a way that there should be tight coupling between the primary and secondary windings of pulse transformer. The pulse transformer is also designed with a small air gap so that there will not be any saturation when a pulse voltage is applied to its primary. Whenever the commutation of thyristor T1 is required, the pulse duration equal to or slightly greater than the specified turn-OFF time of thyristor T1 must be applied. The voltage and current waveforms of class E commutation is illustrated in Figure 47.
MODE 1 When the thyristor T1 is triggered and turned ON, the current starts to flow through the pulse transformer and load resistance RL. The current flows through the following path:
V+ – T1 – Primary of pulse transformer – RL – V_
MODE 2 When an external pulse voltage VP is applied across the primary of the pulse transformer, a voltage will be induced in the secondary of the pulse transformer. This induced voltage in the secondary appears as reverse voltage (–VP) across the thyristor T1. Subsequently, thyristor T1 gets turned OFF. Since the frequency of induced pulse voltage is very high, the capacitor provides almost zero impedance. When the thyristor T1 is turned OFF completely, the load current decays to zero. Before the computation process, the capacitor voltage remains at a value of about 1 V. Since the minimum energy is required for commutation and both the time ratio and pulse width regulations are easily incorporated, this type of commutation method is very efficient.