What are the differences between power semiconductor devices used in DC-to-AC power conversion systems and those used in AC-to-DC power conversion systems?

What are AC and DC?

We are surrounded by AC and DC power sources, and in the experience of the power system, they constantly convert to each other,we are often asked how these converters work.

1. The current, voltage, and electromotive force whose direction does not change with time are all called direct current(DC).

2. Alternating current refers to the variation of the magnitude and direction of electromotive force, voltage, or current over time. 

The difference is that alternating current refers to the magnitude and direction of electromotive force, voltage, or current that change over time, while the direction of direct current does not change over time.(AC)

Direct current (DC) can be generated from various sources, including batteries, solar cells, fuel cells, and certain advanced alternators. DC supplies offer a more consistent voltage output, making them the preferred choice for many electronic devices that rely on DC power sources like batteries.

However, not all electrical equipment operates on DC . Many devices, especially common household appliances such as lamps, washing machines, and refrigerators, run on alternating current (AC). AC is supplied directly from the electrical grid through wall outlets and is the dominant form of electricity in the market. 

Even though some equipment may require immediate conversion from AC to DC, all power outlets in buildings deliver electricity in the form of AC. This prevalence of AC distribution is due to its superior long-distance transmission capabilities when compared to DC. 

AC generation is more straightforward and cost-effective because it can be easily produced.

Rectifiers and inverters

There is not an insurmountable threshold between alternating current and direct current. The alternating current can be rectified to output direct current, and then filtered to form a stable direct current; And direct current can output alternating current through inverter.

This allows us to easily switch between the two, which are key components of many electrical systems worldwide: rectifiers and inverters.

What is a rectifier?

Rectifier is a part of the power supply, which is a device or device that converts alternating current into direct current, and is also used as a detector for radio signals; A rectifier is a type of converter. Rectifiers can be made of solid-state diodes, vacuum tube diodes, mercury arc tubes, or stacks of copper oxide and selenium.

The principle of a rectifier is to use electronic components to convert alternating current into direct current. 

When AC power is applied to the input of the rectifier, the transformer converts it into the required voltage, which is then converted into direct current through the rectifier circuit. 

Due to the rapid changes in voltage and current of alternating current, the output of direct current may contain a large amount of high-frequency noise and fluctuations. To reduce the impact of these noises and fluctuations, it is necessary to use a filtering circuit for filtering.

Classification of rectifiers: The design of rectifiers depends on various factors, such as power supply type, bridge configuration, components used, control properties, etc. There are mainly two types, single-phase and three-phase rectifiers. If further divided, rectifiers can be divided into three types, namely non controlled, semi controlled, and fully controlled rectifiers.

Rectifiers are widely used in various electronic devices and industrial control systems. The following are several common rectifier applications:

Electronic equipment: Rectifiers are used in electronic equipment to convert alternating current into direct current to meet the operational requirements of the equipment. For example, computer power supply, TV power supply, audio power supply, etc.

Electric tools: Rectifiers are used in electric tools to control the speed and direction of the motor. For example, electric drills, hammers, saws, etc.

Lighting equipment: Rectifiers are used in lighting equipment to control the brightness and color of light bulbs. For example, LED lights, fluorescent lights, etc.

Communication equipment: Rectifiers are used in communication equipment to provide stable power supply to ensure the normal operation of the equipment. For example, mobile phones, routers, modems, etc.

Recommended fuses for use in rectifiers:

CBX3M15 Series，CBX4M15 Series，CBX4M15 Series，CBX5M15 Series

What is an inverter?

The equipment used for converting direct current to alternating current is called an inverter. The principle is to send direct current to a transistor used for inverter output, and use transformers and other components connected to the circuit of the transistor to form positive feedback on the tube, causing the tube to generate "oscillation" current (oscillation) and become alternating current output. If a "strict" current output waveform is required, relevant electronic components must also be connected, Compose a circuit for shaping the output waveform.

The operation of a rectifier inverter differs significantly. The inverter serves as a DC to AC transformer, essentially involving a voltage inversion process in conjunction with the converter.

Instead of employing diode converters or transistors, these electronic switches are responsible for modulating the input direct current through specific pulse combinations.

This modulation, known as pulse width modulation (PWM), aims to create a waveform resembling AC, close to a sine wave, by utilizing a DC power source. 

The greater the complexity of your transistor network and the sophistication of the switching algorithm, the closer you can come to replicating an actual sine wave.

PWM is a fundamental technology applicable across various domains, and it finds widespread use in power systems.

In an era of 'mobility', mobile office, mobile communication, mobile leisure and entertainment. In the mobile state, not only does it require low-voltage direct current supplied by batteries or batteries, but also the indispensable 220 V AC power in daily environments, which can be met by inverters.

VFD and UPS

Now that we have an understanding of the functions of rectifiers and inverters, let's delve into two crucial devices: the variable frequency drive (VFD) and the uninterruptible power supply (UPS).

Both of these devices leverage rectifiers and inverters to offer innovative solutions to common problems. Let's start by examining the VFD.

A typical VFD comprises three primary components: a rectifier on the left, a DC connection in the center, and an inverter on the right. 

It takes input AC power on the left, which can be sourced from a single-phase power outlet, like a wall socket, or a three-phase power supply from a commercial or industrial source.

The rectifier network converts the input power into a smooth, ripple-free DC output, which is then used to drive the inverter transistor. Using pulse width modulation (PWM), the inverter recreates a sinusoidal spectrum at a chosen frequency. 

This output is employed to drive a motor, meeting the demand for faster applications.

In essence, the VFD takes the input power, disassembles it using a rectifier to achieve nominal speeds below 60 Hz, and restructures it into an alternate form of communication. 

There are three main advantages to employing this conversion VFD: the ability to convert a standard 60 Hz power supply to any desired frequency, the capacity to softly initiate the motor load to prevent surges and voltage drops, and an enhancement of system efficiency leading to substantial energy savings.

It's worth noting that the rectifier is isolated from the DC bus in the VFD system, ensuring its proper operation.

UPS:There is another scenario where a single-phase input is fed into a drive system, resulting in a three-phase output using the same rectifier-inverter UPS topology. In fact, the only significant difference from a standard UPS in this case is the inclusion of batteries, which allows it to address backup power concerns.

In the simplest UPS, known as an offline UPS, the primary AC power source supplies the load and charges the batteries. So, if the main power source is interrupted, the system can switch to battery power and then switch back to AC power by using the inverter to provide power to the load.

Another version of the UPS is the online or double conversion UPS. This type provides the primary load power through a rectifier and inverter system, similar to a VFD. However, it also includes batteries on the DC bus. 

This configuration achieves complete electrical isolation and can even offer faster backup response because the backup system is already running at a fixed 60 Hz.

Typically, we connect power from an AC source to a load via a DC link. In these cases, the rectifier often employs diodes, as they are more cost-effective and do not require control in some specific applications, such as regenerative VFDs used in elevators or active power management UPS systems. However, there are situations where we need to push and pull power from a DC source, and in these cases, power must be regulated through the rectifier.

Studying the roles of rectifiers and inverters, and how we can integrate them more deeply, is crucial. In this context, various systems like solar and wind energy can synchronize with the power grid. 

Additionally, they play a significant role in industrial automation equipment, such as medium-voltage drives, to generate appropriate control signals.

High-power rectifiers are essential for welding equipment to ensure the quality of the DC power supply. On the other hand, electric vehicles rely on inverters to drive their motors and do not require rectifiers to charge their batteries.

This list barely scratches the surface of applications ranging from high voltage to medium voltage to low voltage. Understanding the roles played by rectifiers and inverters can significantly enhance your comprehension of diverse electrical systems.

The following are some related companies that produce and sell Transfer Switches and have a good reputation in the power field:

Eaton has the following series of VFD:

DV1 series: DV1-341D5NB-C20CX1,DV1-34013NB-C20CX1,DV1-12010NB-C20CX1

DF1 series: DF1-343D0NB-C20CX1,DF1-34060NN-C20CX1,DF1-324D0NB-C20CX1,DF1-34210DB-C20CX1

DG1 series: DG1-342D2EB-C20C,DG1-34031FB-C21C,DG1-34072FB-C54C