Switching Thyristor On
The dynamics of the transition from the off-state to the on-state is characterized by the turn-on time of the power semiconductor thyristor, usually calculated from the beginning of the gate current pulse until the anode current reaches the specified value or the moment the anode voltage drops to the specified small value. In this case, the turn-on time is usually divided into two phases: the turn-on delay time and the rise time.
During the delay time, there are no significant changes in the anode current and voltage. If the gate current has a sharp leading edge, i.e., the gate action on the thyristor can be considered pulsed, then the delay time is approximately equal to the accumulation time in the lightly doped layers of the charge structure of the excess electron-hole pairs, which equal to Qcrit, plus the time of travel of electrons and holes through the heavily doped layers.
During the rise time, the actual transition of the thyristor to the on-state occurs, after the completion of this phase, the equipment in which the thyristor is located behaves like this solid-state switch is closed. Power semiconductor thyristors, as a rule, operate in circuits with an active-inductive load, in which the current rise time, after the thyristor switch is closed, is determined by the current rise time, where usually they do not use the moment of the anode current rise to a specified value, but the moment of the anode voltage drop on the thyristor to a specified value, much less than the source voltage, i.e., the moment when the thyristor switch can be conditionally considered closed.
It should be noted that the end moment of the turn-on, as a rule, does not correspond to the physical switching of the thyristor on over the entire area. The process of propagation of the turn-on state is longer and can take up to several milliseconds, whereas the turn-on time of the thyristor, determined by the rules described above, takes only a few microseconds.
Switching Thyristor Off
The dynamics of the transition from the on-state to the off-state are characterized by two physical processes: recovery of the reverse and forward blocking characteristics.
The reverse recovery of the power semiconductor thyristor is generally similar to the process of reverse recovery of the power semiconductor diode and is described by similar characteristics. In the case of the power semiconductor thyristor, the reverse recovery of its high-voltage emitter p-n junction, which is usually an anode p-emitter, occurs, accompanied by the expansion of its space charge region and the removal of part of the excess electrons and holes accumulated in the lightly doped layers.
Since the thyristor conducting the anode current of the operation density has n- and p-base layers flooded with excess electron-hole pairs and is similar in its state to the power semiconductor diode, the phase of the reverse voltage delay during its reverse recovery is very close in the processes to that in the diode.
At the end of this phase, a part of the p-base is released from the excess electron-hole pairs. As a result, an obstacle is created for the removal of the electrons into the n+ emitter, and the forward-biased collector p-n junction begins to inject holes into the n-base.
Due to this transistor effect, there is a slight increase in the current and the reverse recovery charge in the thyristor compared to a similar diode structure. However, this effect, as a rule, becomes significant only at the end of the phase of the reverse recovery current decay, which leads to a more significant reverse recovery tail current than that of the power semiconductor diodes.
This transistor phase of the reverse current flow continues even after the reverse voltage across the power semiconductor thyristor reaches the source voltage. The removal of the excess electrons from the n-base is greatly reduced, and the flow of excess holes leaving through the reverse-biased anode emitter p-n junction is compensated by the flow of holes injected by the forward-biased collector p-n junction. As a result, the number of excess electron-hole pairs in the n-base is reduced mainly only due to their recombination.
Thus, the recovery of the reverse blocking has already occurred, i.e., a reverse voltage is applied to the power semiconductor thyristor, however, a significant charge of excess electron-hole pairs still remains in the n-base. If the polarity of the voltage applied to the thyristor changes, then the process of reverse recovery of the collector p-n junction will occur, since it will be under reverse bias.
The pulse of the recovery current of this p-n junction can lead to the re-activation of the thyristor structure, i.e., the forward blocking has not yet been restored.
To prevent the thyristor structure from switching, it is necessary that the reverse recovery charge of the collector p-n junction stays less than Qcrit. But this charge is proportional to the charge of the excess electron-hole pairs at the moment of the voltage polarity reversal.
Since the charge of excess electron-hole pairs in the n-base decreases with time due to recombination, this condition will be fulfilled at some point in time. The shortest time interval between the moment of changing the direction of the anode current during reverse recovery and the moment of changing the polarity of the anode voltage when the power semiconductor thyristor does not switch is called the turn-off time – tq.
Find more information about power semiconductors in Power Semiconductors section of the website.