The main types of power diodes and an overview of thyristors

The main types of power diodes and an overview of thyristors

The main types of power diodes

Power diodes are widely used in many power electronic circuits. In the AC-DC conversion circuit, the power diode is used as a rectifier element, in a circuit where the inductance element needs to release electric energy, as a freewheeling element, and as a voltage isolation, clamping or protection element in various converter circuits. In practical applications, different types of power diodes should be selected according to the different requirements of different occasions.

According to the working characteristics of power diodes, several commonly used power diodes are introduced below.

1. Ordinary diode

General Purpose Diode, also known as Rectifier Diode, is mostly used in rectifier circuits with low switching frequency (below 1kHz). Its reverse recovery time is longer, generally above 5μs, but its forward current rating and reverse voltage rating can reach very high, reaching thousands of amperes and thousands of volts respectively.

2. Fast recovery diode

Fast recovery diode (FRD) refers to a diode with a short reverse recovery time (less than 5μs). In the process, the gold doping process is mostly adopted, and some of the internal structure adopts a PN junction structure, and some adopts an improved PIN structure. The forward voltage drop is 1~2V, which is slightly higher than that of ordinary diodes, and the reverse withstand voltage is mostly below 1200V. In terms of performance, it can be divided into two levels: fast recovery and ultra-fast recovery. The reverse recovery time of the former is hundreds of nanoseconds or longer, and the latter is below 100ns, even reaching 20~30ns.

Fast recovery diodes are mostly used in high frequency circuits with controllable switches and requiring short reverse recovery time.

3. Schottky diode

Schottky diode is a diode based on the barrier formed by the contact between metal and semiconductor, referred to as Schottky Barrier Diode (SBD), which has the characteristics of reduced forward voltage (0.4~0.5V) and short reverse recovery time (10~40ns). However, its reverse leakage current is large, withstand voltage is low, and it is sensitive to temperature. Therefore, the reverse steady-state loss cannot be ignored, and its operating temperature must be strictly limited, and it is mostly used in applications where the voltage is lower than 150V.

Semi-controlled device  thyristor

Thyristors are also called silicon controlled rectifiers (SCR), formerly referred to as silicon controlled rectifiers. In 1957, General Electric developed the world’s first thyristor product, which developed rapidly in the 1960s and 1970s, and gradually formed an emerging discipline-power electronics technology.

At present, in addition to the continuous improvement of performance, voltage and current capacity of thyristors, fast thyristors, bidirectional thyristors, gate-off thyristors, reverse conducting thyristors and light-controlled thyristors have also been derived to form a thyristor series.

The name thyristor generally refers to a basic type of thyristor – ordinary thyristor. The following articles will mainly introduce the working principle, basic characteristics and main parameters of ordinary thyristors, and then briefly introduce its various derivative devices.

The external structure of thyristor

The thyristor is a power semiconductor device with 3 lead-out electrodes: anode A, cathode K and gate (control terminal) G. There are two commonly used packaging structures, bolt type and flat plate type. The appearance and symbols are shown in Figure 1. Figure 1a is a low-current plastic package type, such as when the current is slightly larger, it needs to be fastened to the heat sink; Figure 1b and Figure 1c are bolt-type, which must be tightly screwed on the radiator when in use; Figure 1d is a flat plate, the two planes are the anode and the cathode respectively, and the elongated terminal drawn out is the gate, which is clamped tightly between two radiators that are insulated from each other during use.

The main types of power diodes and an overview of thyristors
Figure 1 – Shape and symbol of thyristor

Figure 2 is a thyristor heat sink, Figure 2a is suitable for a bolt type, and Figures 2b and c are suitable for a flat plate. The flat-panel packaged thyristor has a good heat dissipation effect, and the thyristor with a rated current above 200A adopts a flat-panel structure.

The main types of power diodes and an overview of thyristors
Figure 2 – Heat sink of thyristor