In the 1950s, the main power electronic devices were mercury arc rectifier tubes and high-power vacuum tubes. In the 1960s, the thyristor was developed and became widely used in power electronic circuits due to its reliable operation, long lifespan, small size, and fast switching speed. By the early 1970s, it had gradually replaced the mercury arc rectifier tubes. In the 1980s, the switching current of common thyristors reached several thousand amperes, and the positive and reverse working voltages they could withstand reached several thousand volts. Based on this, a series of derivative devices such as gate-turn-off thyristors, bidirectional thyristors, light-controlled thyristors, reverse-conducting thyristors, and a variety of other new power electronic devices were developed, including unipolar MOS power field-effect transistors, bipolar power transistors, electrostatic induction thyristors, functional combination modules, and power integrated circuits.
All power electronic devices have two working characteristics: conduction and blocking. Power diodes are two-terminal (anode and cathode) devices, and their current is determined by the voltage-voltage characteristics. Apart from changing the voltage between the two terminals, their anode current cannot be controlled, so they are called non-controlled devices. Ordinary thyristors are three-terminal devices, and their gate signal can control the conduction of the device but not its turn-off, so they are called semi-controlled devices. Gate-turn-off thyristors, power transistors, and other devices can control both the conduction and turn-off of the device with their gate signals, so they are called fully-controlled devices. The latter two types of devices are more flexible in control, have simpler circuits, and have fast switching speeds, and are widely used in rectification, inversion, and chopper circuits, being the core components of power electronic devices such as motor speed regulation, generator excitation, induction heating, electroplating, electrolytic power supply, and direct power transmission. These devices constitute the devices not only with small size and reliable operation, but also with very obvious energy-saving effects (generally 10% to 40%).
The positive and reverse voltages that a single power electronic device can withstand are fixed, and the current it can pass is also fixed. Therefore, the capacity of power electronic devices composed of a single power electronic device is limited. Therefore, in practice, multiple power electronic devices are connected in series or parallel to form components, whose voltage and current carrying capacity can be increased by several times, thereby greatly increasing the capacity of power electronic devices. When devices are connected in series, it is hoped that each component can withstand the same positive and reverse voltages; when connected in parallel, it is hoped that each component can share the same current. However, due to the individuality of the devices, when connected in series or parallel, each device cannot evenly share the voltage and current. Therefore, when power electronic devices are connected in series, measures to equalize the voltage should be taken; when connected in parallel, measures to equalize the current should be taken.
When power electronic devices are working, they will generate heat due to power loss. Excessive device temperature will shorten the lifespan and even burn the device. This is the main reason limiting the current and voltage capacity of power electronic devices. Therefore, the cooling problem of the devices must be considered. Common cooling methods include self-cooling, air cooling, liquid cooling (including oil cooling and water cooling), and evaporative cooling.