Turn-off characteristics

All PN junction diodes will store charge in the form of minority carriers when conducting forward current. Minority carrier injection is the mechanism of conductivity modulation, which leads to the decrease of forward voltage drop (VF). In this sense, it is advantageous. However, when a reverse voltage is suddenly applied to the conducting diode, because a large number of minority carriers are stored in the PN junction during conduction, it will take some time to completely withdraw or neutralize these minority carriers by the cut-off time, that is, it will take a period of time to recover the reverse blocking ability. The reverse recovery process is defined as the reverse recovery time (trr). It is noteworthy that the diode is equivalent to a short circuit state before the blocking capability is restored.

Softness factor S is used to describe the rate at which the reverse recovery current disappears from the maximum IRM. Dirr/dt is an important parameter for reverse recovery current. If the dirr/dt is too large, the reverse peak voltage URM will be too high due to the existence of inductance L in the line, and sometimes strong oscillation will occur, which will damage the diode. The influence of dirr/dt on the reverse characteristic can be expressed by the concepts of soft characteristic and hard characteristic. The softening coefficient S can also be expressed as the amplitude of the reverse peak voltage can be predicted by the above formula. Among them, L is the peak voltage applied to the active device when the total inductance URM of the circuit is reversed recovery of the diode. Its value must be less than the voltage rating of the active device, so it is more practical to use di (rec)/dt to express the softness factor. The total time required to deplete charge storage is defined as reverse recovery time trr. As a measure of switching speed, it is a very important parameter when selecting diodes. Trr for general purpose is about 25s. It can be used in rectifiers and circuits with frequencies below 1kHz, but it can be used in choppers and inverters. In the circuit, the fast recovery diode with TRR below 5S must be selected. In some absorption circuits, fast switching on and soft recovery diodes are required.

From the definition of softness factor, it can be seen that it reflects how long the base minority disappears due to recombination in the reverse recovery TB process of the diode. Therefore, the softness factor is closely related to minority carrier lifetime control method, base width and diffusion concentration distribution, component structure and structural parameters. The softness factor will be improved if more residual charges are stationed in the residual base area after the expansion of the space charge area and longer time is stationed.

2: Ways to Improve Performance

Although PIN transistor has good reverse voltage withstanding ability, it produces large reverse peak voltage during reverse recovery because of its poor reverse recovery characteristic, which affects the normal operation of the whole circuit. Secondly, a practical PIN rectifier has an order of magnitude higher forward voltage drop at the instant of switching on than the steady-state voltage drop. The peak voltage can exceed 30V, which is mainly caused by the limited diffusion velocity of minority carriers. It is related to the resistivity and width of N-base material. In order to improve the working characteristics of the diode, related technologies are used in the design and fabrication of the device.