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Figure 1.1 Three‐phase converters: (a) grid converters; (b) inverter.

      The three‐phase converter is one of the most important power conversion building blocks in power electronic systems. It is widely used in various applications due to its distinct advantages as follows:

Because of these advantages, the three‐phase converters have been used almost everywhere from low power to high power such as disk drives, inverters for pumps, inverters for EV, drive inverters for bullet trains, solar inverters, wind turbines, UPS, etc. For these applications, there is an ever increasing demand for the performance of the converter. In addition to basic functions such as AC to DC or DC to AC conversion and output power quality, following demands are critical for the converter.

      First, it is expected that the converter has higher efficiency, which has become a more stringent requirement than ever before due to increasing public concern of impact of energy consumption on the environment. Besides, high efficiency can also bring economic benefits to the users. A Photovoltaic (PV) inverter with high efficiency can harvest more electricity and also improves the utilization of the PV panel. A UPS with high efficiency can save the operating cost of data centers. It also can reduce the footprint of the power supply due to lower energy loss. An EV power train with high efficiency increases the utilization of the kWh of the battery and extends mileage at the same time.

      Second, we hope that the converter has small size. It is especially critical for moving vehicles such as electric vehicles, electric railway, boats, and airplanes or aerospace applications. Smaller size means less use of material, copper, iron, isolation material, etc., which cuts down the cost. For users, it can reduce space, which is expensive in many large cities. Usually, you may hear the word power density. It means the ratio of power capacity to the volume or weight of the converter. It represents the ability of a converter to process the power at a given size. For a given power capacity of a converter, the higher the power density, the smaller is the size of the converter.

      Third, the converter is required to have good dynamics. The dynamics of a converter mainly depends on bandwidth of the close loop control systems. It is mainly constrained by the switching frequency of the converter. Generally speaking, power electronic converters have high dynamics since they use power semiconductor devices as the switch. In many applications such as ultra‐high‐speed pumps and compressors in industrial applications, power generation for aeronautics, EV, etc., it is required that the converter drives the motor to reach ultra‐high speed from tens to hundreds of thousands rpm. In some applications such as moving vehicles, the size of the electric machines can be reduced by increasing their operating frequency. To increase the fundamental frequency of the converter, it is natural to increase switching frequency. What is the limit of the switching? We will discuss it in detail later.

      Another demand is lower cost. The cost of a converter or a converter system should be optimized. A converter system is generally composed of power semiconductor devices and passive components. Size of the passive components depends on operating frequency. If we can increase the frequency, their size can be reduced.

      Actually, switching frequency is a critical parameter for the converter. It plays an important role in efficiency, power density, dynamics, and cost reduction.

      1.1.2 Switching Frequency vs. Conversion Efficiency and Power Density

In this section, we will discuss the effects of the switching frequency on the converter. First, an inverter used for UPS is taken as an example. The main circuit of UPS is back to back (BTB) converter as shown in Figure 1.2. It is composed of three‐phase grid converter as the rectifier cascaded with a three‐phase converter as the inverter to provide high‐quality power to the load. To satisfy the load requirement, output voltage of UPS needs to be an almost sinusoidal waveform. Its output voltage quality is usually described by total harmonic distortion (THD). Filters are installed in load sides. In addition, to satisfy the grid standard, filters in utility side are also installed. Filter cost ranges from 15 to 30% of total material expenditure in the UPS (excluding the battery cost). Besides, it also occupies a large footprint. Size of the passive components depends on operating switching frequency. If we can increase the switching frequency, their size will be reduced.