How can we accurately select Schottky diodes?
In switching power supplies, we often use Schottky diodes, but due to different manufacturers and other reasons, the performance varies greatly. When choosing Schottky diodes, we must consider the following parameters:
1. Conduction voltage drop VF: VF refers to the voltage drop at both ends of the diode during forward conduction. Choosing a Schottky diode is to try to choose a diode with a smaller VF.
2. Reverse saturated leakage current IR: IR refers to the current flowing through the diode when a reverse voltage is applied at both ends of the diode. The Schottky diode has a higher reverse leakage current, so choosing a Schottky diode is to try to choose a diode with a smaller IR.
3. Rated current IF: Refers to the average current value calculated based on the allowable temperature rise during long-term operation of the diode.
4. Maximum surge current IFSM: The excessive forward current allowed to flow. It is not a normal current, but an instantaneous current, which is quite large.
5. Maximum reverse peak voltage VRM: Even without reverse current, as long as the reverse voltage is continuously increased, sooner or later the diode will be damaged. The reverse voltage that can be applied is not an instantaneous voltage, but a repeatedly applied forward and reverse voltage. Due to the AC voltage applied to the rectifier, its maximum value is a specified important factor. The maximum reverse peak voltage VRM refers to the maximum reverse voltage that can be applied to avoid breakdown.
6. Maximum DC reverse voltage VR: The maximum reverse peak voltage mentioned above is the peak voltage repeatedly applied, and VR is the value obtained by continuously applying DC voltage. For DC circuits, the maximum DC reverse voltage is crucial for determining the allowable and upper limits.
7. Maximum operating frequency fM: Due to the presence of the junction capacitance of the PN junction, its unidirectional conductivity will deteriorate when the operating frequency exceeds a certain value. The fM value of Schottky diodes is high, with a maximum of 100GHz.
8. Reverse recovery time Trr: When the working voltage changes from forward voltage to reverse voltage, the ideal situation for diode operation is that the current can be instantly cut off. In fact, it usually takes a little time to delay. The amount that determines the current cutoff delay is the reverse recovery time. Although it directly affects the switching speed of the diode, it does not necessarily mean that a small value is good. That is to say, when the Schottky diode suddenly reverses from conduction, the reverse current decays greatly to the time required to approach IR. This indicator is crucial when high-power switching tubes operate in high-frequency switching states.