The rectifier diode in the switching power supply must have the characteristics of reduced forward voltage and fast recovery. It should also have sufficient output power. The following three types of rectifier diodes can be used: fast recovery rectifier diode; ultra-fast recovery rectifier diode; Base rectifier diode. Fast recovery and ultrafast recovery rectifier diodes have moderate and high forward voltage drops ranging from 0.8 to 1.2V. These two rectifier diodes also have higher cutoff voltage parameters. Therefore, they are particularly suitable for use in an auxiliary power supply circuit of low power with an output voltage of around 12V.
Since the modern switching power supply operating frequency is above 20 kHz, the reverse recovery time of the fast recovery rectifier diode and the ultra-fast recovery rectifier diode is reduced to the nanosecond level compared to the general rectifier diode, thus greatly improving the power supply. s efficiency. As a rule of thumb, when choosing a fast recovery rectifier diode, the reverse recovery time should be at least 1/3 of the rise time of the switching transistor. These two rectifier diodes also reduce the switching voltage spike, which directly affects the ripple of the output DC voltage. In addition, although some noises called soft recovery rectifier diodes are small, their reverse recovery time is long and the reverse current is large, which makes the switching loss increase and cannot meet the working requirements of the switching power supply.
The fast recovery rectifier diode and the ultra-fast recovery rectifier diode need to be used as a rectifying device in the switching power supply, depending on the maximum power of the circuit. In general, these diodes are allowed to have a junction temperature of 175 ° C during manufacturing. Manufacturers have technical specifications for this specification to provide the designer with the maximum output operating current, voltage and case temperature.
Schottky rectifier diodes have a low forward voltage drop even under large forward currents, only about 0.4V, and their forward voltage drop is lower as the junction temperature increases. Schottky rectifier diodes are especially suitable for low voltage output circuits around 5V. The reverse recovery time of the Schottky rectifier diode is negligible because the device is a majority carrier semiconductor device that does not remove the minority carrier charge during the switching process of the device.
Schottky rectifier diodes have two major drawbacks: First, the reverse-cutoff voltage is less tolerant, and current products are about 100V. Second, the reverse leakage current is large, making the device more reliable than other types of rectifier devices. It is easily broken by heat. Of course, these shortcomings can also be overcome by increasing the transient overvoltage protection circuit and properly controlling the junction temperature. The characteristics and parameters of a typical high speed rectifier diode are shown.
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