What are diodes? – Comparison of Features of Rectified Diodes

The diodes discussed in this chapter are devices with high withstand voltages and the ability to handle large currents, but they can be classified into several categories according to their characteristics, features, and manufacturing processes. Diode characteristics and performance are basically optimized for the intended application.

Comparison of the Features of Rectified Diodes

Here we have classified rectified diodes into four types: general rectification applications, general switching applications, Schottky barrier diodes, and fast-recovery diodes. Their features are summarized in the table below.

General-purpose diodes have as their primary purpose general rectification, that is, conversion of an alternating current into direct current. A diode bridge combines diodes to rectify a current. In addition, diodes are also used for protection so that overcurrents do not flow in the event of reverse-connection of a power supply or battery. The forward voltage VF differs depending on the current that can be handled, but is typically around 1 V. This is the general VF for diodes formed from a silicon PN junction. Rectification of a 50/60 Hz commercial power supply is generally assumed, and so the reverse recovery time trr is usually not particularly short.

The main application of switching-type diodes is, as the name implies, for switching of power supplies. Such devices typically have a VF equal to those of general-purpose diodes. Because the main purpose is switching applications, trr is shorter than that of general-purpose diodes. However, these devices do not have the speed of Schottky barrier diodes or fast-recovery diodes, but their characteristics are fast compared with general-purpose diodes.

Schottky barrier diodes (SBDs) do not have PN junctions; instead, they use Schottky barriers, which occur at the junction between a metal and a semiconductor such as N-type silicon. Compared with PN junction diodes they feature a low VF and fast switching characteristics. However, the reverse leakage current IR is large, and depending on conditions, care must be taken to prevent thermal runaway. Even when a large current of for example 10 A is passed, the VF is about 0.8 V, and at several amperes it is around 0.5 V, and so a representative application of these devices is in DC-DC converters and the secondary side of AC-DC converters, where high efficiency is demanded.

Fast-recovery diodes (FRDs) are PN junction diodes, but are fast diodes with a greatly improved trr. And while the rated voltage (reverse current VR) of SBDs is 200 V or lower, FRDs can have high voltages of 800 V or higher. However, in general the VF is higher than for general-purpose diodes, and is typically around 2 V for a device with high voltage, large-current specifications. In recent years however there has been an increase in devices with reduced VFs. Due to their high voltages and fast operation, they are often employed in AC-DC converters and inverter circuits.

The following diagram presents the above characteristics visually. Although not something that is limited to these four diode types, the basic temperature characteristics of Si diodes are also shown.

To repeat once more, the figure on the left indicates that SBDs have a lower VF and higher IR than the other three types.

The figure in the middle indicates that trr is much shorter for SBDs and FRDs than for the other two diode types, and that if the IR flowing during the time trr is large, losses are large.

The figure on the right is the basic temperature characteristic for Si diodes. At higher temperatures the VF falls and the IR increases.

Next time, we will explain SBDs in somewhat more detail.

• Schottky barrier diode