device S1. Thus the overlapping of voltage and current of the power device during turn‐on process is reduced. The integral of multiplication of uS1 and iS1 during turn‐on duration becomes smaller than that of the hard switch. Thus turn‐on loss of the device is reduced. However, ZCS‐on is not ideal turn‐on process because turn‐on loss still exists. It still has the overlapping of the voltage and current in turn‐on transient process.
Figure 1.9 Zero‐voltage‐switching turn‐on.
Figure 1.10 Zero‐current‐switching turn‐on.
Zero‐Voltage‐Switching Turn‐off (ZVS‐off): During a power device turn‐off process, its voltage gradually increases from zero value while its current quickly goes down. Thus the voltage and current overlapping of the device during turn‐off transient process is reduced. As shown in Figure 1.11, when gate drive signal steps down for turning off the device, the voltage uS1 of the device gradually increases from zero while the current iS1 quickly goes down. Typically, there is capacitance paralleled with the power device, which suppresses the voltage increasing rate when the device turns off. Thus the overlapping of voltage and current of the power device during turn‐off process is reduced. The integral of multiplication of uS1 and iS1 during turn‐off process becomes smaller. Thus turn‐off loss of the device is reduced. However, ZVS‐off is also not ideal. There still exists turn‐off loss.
Zero‐Current‐Switching Turn‐off (ZCS‐off): Current through a power device already decreases to zero before the gate drive signal steps down to the lower level for turning off the device. Thus the voltage and current overlapping of the device during turn‐off transient process is eliminated. As shown in Figure 1.12, current iS1 through the power device S1 is set to zero before its gate drive signal ug1 goes to the low level for turning off the device. Typically, there is an external inductor or L‐C resonance branch serially connected with the power device S1, which causes the current of the device S1 decrease to zero automatically before the turn‐off signal is applied to the gate drive. Thus the overlapping of voltage and current of the power device during turn‐off process is eliminated. The integral of multiplication of uS1 and iS1 in turn‐off duration becomes zero. Thus turn‐off loss of the device is avoided. ZCS‐off is ideal turn‐off. There is no turn‐off loss.
Figure 1.11 Zero‐voltage‐switching turn‐off.
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
