Characteristics of high-frequency rectifier diodes

The rectifier diode in a switching power supply must have the characteristics of forward voltage reduction and fast recovery, as well as 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. Fast recovery and ultra fast recovery rectifier diodes have moderate and high forward voltage drops, ranging from 0.8 to 1.2V. These two rectifier diodes also have high cutoff voltage parameters. Therefore, they are particularly suitable for use in low-power auxiliary power circuits with an output voltage of around 12V.

Due to the operating frequency of modern switching power supplies being above 20kHz, the reverse recovery time of fast recovery rectifier diodes and ultra fast recovery rectifier diodes is reduced to nanoseconds compared to general rectifier diodes, greatly improving the efficiency of the power supply. According to experience, when choosing a fast recovery rectifier diode, its reverse recovery time should be at least one-third of the rise time of the switching transistor. These two types of rectifier diodes also reduce switch voltage spikes, which directly affect the ripple of the output DC voltage. In addition, although some rectifier diodes called soft recovery type have lower noise, their reverse recovery time is longer and the reverse current is also larger, which increases the switching loss and cannot meet the working requirements of switching power supplies.

Whether a heat sink is required for the use of fast recovery rectifier diodes and ultra fast recovery rectifier diodes as rectifier devices in switching power supplies depends on the maximum power of the circuit. In general, these diodes are allowed to have a junction temperature of 175 ℃ during manufacturing, and manufacturers have technical specifications for this indicator to provide designers with the ability to calculate the maximum output operating current, voltage, and housing temperature.

Even under high forward current, the forward voltage drop of Schottky rectifier diodes is very low, only about 0.4V. Moreover, as the junction temperature increases, the forward voltage drop is even lower, making Schottky rectifier diodes particularly suitable for low voltage output circuits around 5V. The reverse recovery time of Schottky rectifier diode can be ignored because this device is a majority carrier semiconductor device, and there is no problem of clearing the stored charge of minority carriers during the switching process of the device.

Schottky rectifier diodes have two major drawbacks: firstly, their ability to withstand reverse cut-off voltage is relatively low, with current products being around 100V; secondly, their high reverse leakage current makes them more susceptible to thermal breakdown compared to other types of rectifier devices. Of course, these drawbacks can also be overcome by adding instantaneous overvoltage protection circuits and appropriately controlling junction temperature. It shows the characteristics and parameters of typical high-speed rectifier diodes.