The rectifier diode in the switching power supply must have the characteristics of forward voltage reduction and fast recovery, and also have sufficient output power. The following three types of rectifier diodes can be used: fast recovery rectifier diode; Ultra fast recovery rectifier diode; Schottky rectifier diode. The fast recovery and ultra fast recovery rectifier diodes have moderate and high forward voltage drop, ranging from 0.8 to 1.2V. These two rectifying diodes also have high cut-off voltage parameters. Therefore, they are particularly suitable for use in low-power auxiliary power supply circuits with an output voltage of about 12V.
Since the operating frequency of modern switching power supply is above 20kHz, the reverse recovery time of the fast recovery rectifier diode and the ultra fast recovery rectifier diode is reduced to nanosecond level compared with the general rectifier diode, so the efficiency of the power supply is greatly improved. According to experience, when selecting the fast recovery rectifier diode, its reverse recovery time should be at least 1 / 3 of the rise time of the switching transistor. These two rectifying diodes also reduce the switching voltage spike, which directly affects the ripple of the output DC voltage. In addition, although the noise of some rectifying diodes called soft recovery type is small, their reverse recovery time is long, and the reverse current is also large, which increases the switching loss and cannot meet the working requirements of the switching power supply.
Whether the fast recovery rectifier diode and the ultra fast recovery rectifier diode need a heat sink when they are used as rectifying devices in the switching power supply depends on the maximum power of the circuit. In general, the allowable junction temperature of these diodes during manufacturing is 175 ℃, and the manufacturer has technical specifications for this indicator to provide the designer with the maximum output working current, voltage and shell temperature. Even under the action of large forward current, the forward voltage drop of Schottky rectifier diode is very low, only about 0.4V. Moreover, with the increase of junction temperature, the forward voltage drop is lower. Therefore, Schottky rectifier diode is particularly suitable for low-voltage output circuits of about 5V. The reverse recovery time of Schottky rectifying diode is negligible, because this device is a semiconductor device with most carriers. During the switching process of the device, there is no problem of clearing the stored charge of a few carriers.
Schottky rectifying diodes have two major disadvantages: first, the withstand capacity of reverse cut-off voltage is low, and the product is about 100V; Second, the reverse leakage current is large, which makes the device more vulnerable to thermal breakdown than other types of rectifier devices. Of course, these disadvantages can also be overcome by adding instantaneous overvoltage protection circuit and properly controlling junction temperature. The characteristics and parameters of a typical high-speed rectifier diode are shown.
