Reverse recovery process of rectifier diode

火辣西米秀

Reverse recovery process of rectifier diode [Copy link]

1. The diode has a reverse recovery process from forward conduction to cutoff

Add an input voltage as shown in the figure below to the silicon diode circuit shown in the figure above. During the time 0-t1, the input is +VF, the diode is turned on, and current flows in the circuit.

Assume VD is the forward voltage drop of the diode (about 0.7V for silicon tube). When VF is much larger than VD, VD can be ignored.

At t1, V1 suddenly changes from +VF to -VR. Ideally, the diode will be cut off immediately, and there should be only a small reverse current in the circuit. But in reality, the diode does not cut off immediately, but first changes from the forward IF to a large reverse current IR=VR/RL. This current maintains for a period of time tS before gradually decreasing. After tt, it drops to a very small value of 0.1IR, and then the diode enters the reverse cutoff state, as shown in the figure below.

The conversion process of a diode from forward conduction to reverse cutoff is usually called the reverse recovery process. Among them, tS is called the storage time, tt is called the transit time, and tre=ts+tt is called the reverse recovery time. Due to the existence of the reverse recovery time, the switching speed of the diode is limited.
2. The reason for the reverse recovery process-charge storage effect
The reason for the above phenomenon is that when the forward voltage VF is applied to the diode, the carriers continue to diffuse and store. When the forward voltage is applied, the holes in the P region diffuse to the N region, and the electrons in the N region diffuse to the P region. In this way, not only the barrier region (depletion region) becomes narrower, but also a considerable number of carriers are stored. Electrons are stored in the P region, and holes are stored in the N region. They are all unbalanced minority carriers, as shown in the figure below.

After the holes diffuse from the P region to the N region, they do not immediately recombine with the electrons in the N region and disappear, but continue to diffuse within a certain distance LP (diffusion length), while recombine with the electrons and disappear. In this way, a certain number of holes will be stored within the LP range, and a certain hole concentration distribution will be established. The concentration is the largest near the junction edge, and the farther away from the junction, the smaller the concentration. The larger the forward current, the more holes are stored, and the greater the gradient of the concentration distribution. The situation of electrons diffusing to the P region is similar. The figure below shows the distribution of stored charges in the diode.

We call the phenomenon of non-equilibrium minority carrier accumulation during forward conduction the charge storage effect.

When the input voltage suddenly changes from +VF to -VR, the electrons stored in the P region and the holes stored in the N region will not disappear immediately, but they will gradually decrease through the following two ways: ① Under the action of the reverse electric field, the electrons in the P region are pulled back to the N region, and the holes in the N region are pulled back to the P region, forming a reverse drift current IR, as shown in the figure below;

②与多数载流子复合。
在这些存储电荷消失之前，ＰＮ结仍处于正向偏置，即势垒区仍然很窄，ＰＮ结的电阻仍很小，与RL相比可以忽略，所以此时反向电流IR=（VR＋VD）/RL。VD表示ＰＮ结两端的正向压降，一般 VR>>VD，即 IR＝VR／RL。在这段期间，IR基本上保持不变，主要由VR和RL所决定。经过时间ts后Ｐ区和Ｎ区所存储的电荷已显著减小，势垒区逐渐变宽，反向电流IR逐渐减小到正常反向饱和电流的数值，经过时间tt，二极管转为截止。

From the above, we can see that the reverse recovery process of the diode during the switching process is actually caused by the charge storage effect, and the reverse recovery time is the time required for the stored charge to disappear.
The difference between a diode and a general switch is that the "on" and "off" are determined by the polarity of the applied voltage, and there is a small voltage drop Vf in the "on" state and a small current i0 in the "off" state. When the voltage changes from forward to reverse, the current does not immediately become (- i0), but within a period of time ts, the reverse current is always large, and the diode is not turned off.

After ts, the reverse current gradually decreases, and after tf, the current of the diode becomes (- i0). ts is called the storage time, and tf is called the fall time. tr= ts+ tf is called the reverse recovery time, and the above process is called the reverse recovery process. This is actually caused by the charge storage effect, and the reverse recovery time is the time required for the stored charge to be exhausted. This process prevents the diode from being used as a switch under fast continuous pulses. If the duration of the reverse pulse is shorter than tr, the diode can be turned on in both the forward and reverse directions, and cannot function as a switch.