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Figure 1.12 Zero‐current‐switching turn‐off.

      Among the four soft‐switching techniques mentioned earlier, two methods, ZVS‐on and ZCS‐on, are used for turn‐on loss reduction while other two, ZVS‐off and ZCS‐off, are used for turn‐off loss reduction. ZVS‐on and ZCS‐off are ideal and can totally eliminate the switching loss. However, ZCS‐on and ZVS‐off are not ideal. They reduce switching loss, but the switching loss still exists.

      1.2.2 Soft‐switching Technique for Three‐phase Converters

      Soft‐switching techniques for three‐phase converters have been investigated by many predecessors. For three‐phase applications, soft‐switching converters can be divided into three classes: DC resonance converter, AC resonance converter, and soft‐switching converters with triangular current mode (TCM) control [4].

      In the DC‐side resonance converters, an auxiliary resonant circuit is installed between DC input source and DC side of three‐phase switch bridge of the converter. The fundamental philosophy of the DC‐side resonance is to use an auxiliary resonant circuit to create zero‐voltage duration at the DC side of the three‐phase switch bridge at the desired switching instant. Thus all devices of the switch bridge are turned on or turned off when the voltage on them is equal to zero so that both turn‐on loss and turn‐off loss are significantly reduced. Besides, the DC‐side resonance converter only needs one auxiliary resonant circuit regardless of the number of AC phases of the converter. This simple structure makes DC‐side resonance attractive in multiphase converter applications. The resonant DC link (RDCL) converter [5] is milestone topology in evolution of soft‐switching history. To reduce voltage stress on the devices, a revised version known as active clamped RDCL (ACRDCL) converter occurred [6, 7]. Both RDCL and ACRDCL converters are controlled with discrete pulse modulation (DPM). It is found that the soft‐switching converters with DPM require higher switching frequency than that of the PWM converter for comparable current spectral performance. Many other topologies have been developed such as the quasi‐resonant DC link (QRDCL) PWM inverter with PWM control [8–11]. They often use more complex auxiliary circuit. Zero‐voltage‐switching SVM (ZVS‐SVM) for three‐phase active clamping converters was proposed by Dehong Xu [13, 14]. The auxiliary power device only switches once in each switching cycle to realize ZVS for all the switches. It features fixed switching frequency and lower voltage stress of the power switch devices. The converter basically operates like PWM converter [15, 16]. Afterward it is generalized to edge‐aligned PWM (EA‐PWM) [17–19]. EA‐PWM is suitable to three‐phase converter, three‐phase four‐wire converter, three‐phase four‐wire BTB converter, etc.

The second class of the soft‐switching converter is AC resonance converters. Auxiliary resonant circuits are installed in AC side of the switch bridge. Distinctive advantage of the AC‐side resonance is that the auxiliary circuits are in shunt with the switch bridge and does not carry the load current. Thus the conduction loss in the auxiliary circuits is smaller. In addition, SPWM and SVM control can be applied because the converters basically operate as conventional PWM converters. Auxiliary resonant commutated pole (ARCP) converter is one of the earliest AC resonance converters [12, 20]. It achieves ZVS‐on for main switches and ZCS‐off for auxiliary switches. The inductor coupled zero‐voltage transition (ZVT) inverter achieves ZVS‐on for main switches and near‐zero current turn off for auxiliary switches [21]. DC‐side split capacitor voltage control needed for ARCP converter is avoided. The zero‐current transition (ZCT) inverter achieves zero current switching for all of the main and auxiliary switches and their antiparallel diodes [22, 23]. It is suitable to converters with IGBT devices, which can reduce turn‐off loss of IGBT due to its tail current. Other AC resonance circuits are developed [24–27]. The AC resonance converter has complex circuit because it generally needs three auxiliary resonant circuits. The number of power devices to be controlled are almost doubled in comparison to the original converter.

      The third class of the soft‐switching converter is known as the soft‐switching converters with TCM. The concept comes from critical conduction mode (CRM) in DC‐DC converter to achieve ZVS‐on [28, 29]. With TCM control, AC‐side filter inductor currents of three‐phase converters are controlled as the triangle waveform in a switching period [30, 31]. TCM is originally introduced to single‐phase totem‐pole power factor correction (PFC) converters and then extended to three‐phase converters. The distinct advantage of the soft‐switching converter with TCM is that no auxiliary resonant circuit is needed. However, there are some drawbacks such as wider range of variable switching frequency, higher rms current, and turn‐off current. Recently, there are many studies about TCM control. For readers interested in this research can read the related references on IEEE Xplore.

      As mentioned earlier, the DC‐side resonance converter only needs one auxiliary resonant circuit shared by three‐phase legs of the converter. It has issues such as variable switching frequency, higher voltage stress on devices, etc. To overcome these drawbacks, the ZVS‐SVM and EA‐PWM for three‐phase active clamping converters were proposed. The auxiliary power device only switches once in each switching cycle to realize ZVS for all the switches. It features fixed switching frequency and lower voltage stress of the power switch devices. The converter basically operates like the conventional PWM converter.

As an example of the soft‐switching technique, the concept of three‐phase active clamping converters with EA‐PWM is briefly explained. With EA‐PWM, the converter operates at the fixed switching frequency. An auxiliary circuit with auxiliary switch S₇ is installed in the DC side of the converter in Figure 1.13a. Once a high loss switch commutations in the switch bridge happens, the auxiliary switch S₇ will turn off, which starts a resonant process to create a slot with duration λ₇T_S on the DC bus voltage waveform v_bus where λ₇ is the turn‐off duty ratio of the auxiliary switch S₇ and T_S is the switching period in Figure 1.13b. During v_bus = 0, the switches in the three‐phase switch bridge make the commutation under zero voltage on their terminals, which is known as zero voltage switching. Thus the switching loss is reduced. With EA‐PWM, all high loss commutations of the switch bridge in each switch cycle are synchronized. The auxiliary power device only switches once in a switching cycle to realize ZVS for all the switches. More detailed introduction will be given in Chapter 3.

1.3 Applications of Soft‐switching to Three‐phase Converters

      1.3.1 Renewable Energy and Power Generation

      The increasing applications of renewable energy and distributed power generation have become a new driving force for application of power electronics. Three‐phase converters are playing more and more important role in the grid. More efficient and more reliable power converters are required. Soft‐switching technique can be applied to converters for renewable energy integration, energy storage systems, and Flexible AC power transmission (FACTS) devices to increase power density and dynamics and reduce size of the equipment [32].

A single‐phase PV inverter is widely used for residential PV applications as shown in Figure 1.14. The front boost converter is used to extend solar energy harvesting duration each day so that the inverter can feed power to grid with a wider PV panel output voltage. By introducing an auxiliary resonant

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