In switching power supply, the main function of diode is rectification. According to the frequency of alternating current in the rectifying circuit, the rectifying circuit in the switching power supply can be divided into a primary rectifying circuit (low-frequency rectification) and a secondary rectifying circuit (high-frequency rectification).
Among them, the primary rectification circuit refers to rectifying the mains alternating current (the frequency is 50Hz or 60Hz), and the secondary rectification circuit refers to rectifying the high-frequency alternating current (the frequency is tens of kHz to hundreds of kHz or even higher). The principle of primary rectifier circuit and secondary rectifier circuit is to use the unidirectional conductivity of diodes to change alternating current into unidirectional pulsating direct current, but their requirements on diode parameters are different. Power loss and reverse blocking ability are mainly considered when selecting diodes for primary rectifier circuit, that is, forward voltage drop UF and reverse current IR in diode parameters should be as small as possible.
When selecting diodes for the secondary rectifier circuit, in addition to considering that the forward voltage drop UF and the reverse current IR should be small, the reverse recovery time TRR must meet the needs of the circuit, and the reverse recovery time becomes a priority issue in selecting diode parameters. Fast recovery diodes and Schottky diodes are commonly used in switching power supply circuits.
Fast recovery diode
The reverse recovery process of fast recovery diode is very short, which is called fast diode for short. Most of the fast diodes adopt gold doping measures in technology, some adopt PN junction structure, and some adopt improved pin structure. In terms of performance, the fast recovery diode can be divided into two levels: fast recovery and ultra fast recovery. The reverse recovery time of the fast recovery diode is generally less than 5 µ s, and the minimum reverse recovery time of the ultra fast recovery diode can be less than 50ns. The fast recovery diode and ultra fast recovery diode are generally 0.8 ~ 1.2V, but the maximum reverse working voltage is small, mostly below 400V or 600V.
Schottky diode
Schottky diodes are made by the principle of metal semiconductor junction formed by metal and semiconductor contact, so Schottky diodes are also shaped as metal semiconductor (contact) diodes or surface barrier diodes. The most significant advantage of the Schottky diode is that the reverse recovery time is extremely short (can be as small as several nanoseconds), and the forward conduction voltage drop is only about 0.4V. However, the disadvantage of the Schottky diode is that the highest reverse working voltage is very low, generally below 100V.
