PN junction patch diodes are often used to make electrical switches. In the forward-biased state, that is, in the on state, a small applied voltage can generate a larger current; in the reverse-biased state, that is, in the off state, only a small current exists in the PN junction.
The parameter of the switching circuit we are interested in is the switching speed of the circuit. The following content will qualitatively discuss diode switching transients and charge storage effects.
The role of SMD diodes
The diode can be used as a switch by taking advantage of the large difference between the forward and reverse currents of the diode.
When the positive pulse added to the external circuit jumps to a negative pulse at a certain moment:
The large number of minority carriers accumulated in each area in the forward direction will be pulled back to the original area by the reverse bias voltage. At the beginning, the reverse current flowing through the PN junction is very large. After a period of time, the original accumulated carriers Part of it is recombined, and part is pulled back to the original area, and the reverse current returns to the reverse leakage current value IR under normal conditions.
The phenomenon of minority carrier accumulation during forward conduction is called the charge storage effect. The reverse recovery process of the diode is caused by charge storage. The period of time during which the reverse current remains unchanged is called the storage time ts. After ts, the current on the P-N junction reaches the reverse saturation current IR, and the P-N junction reaches equilibrium. Define the time required for the reverse current flowing through the P-N junction to drop from I2 to 0.1 I2 as the falling time tf. The sum of storage time and fall time (ts+tf) is called the turn-off time of the P-N junction (that is, the reverse recovery time).
The reverse recovery time limits the switching speed of the diode:
If the pulse duration is much longer than the diode reverse recovery time, then the negative pulse can completely turn off the diode and play a good switching role;
If the pulse duration is similar to or even shorter than the reverse recovery time of the diode, the negative pulse cannot turn off the diode due to the influence of the reverse recovery process.
Therefore, to maintain a good switching effect, the pulse duration cannot be too short, which means that the repetition frequency of the pulse cannot be too high, which limits the switching speed.