Research and Analysis on Active Soft Switching Technology Based on Multilevel Inverter

introduction

Multi-level conversion technology has attracted more attention due to its advantages such as reducing the voltage stress of devices, eliminating the problem of voltage balancing without connecting devices in series, reducing the harmonic content of the output voltage, and reducing the electromagnetic interference caused by dv/dt and di/dt. Its emergence has opened up a new idea for the development of high-voltage and high-power converters. After years of research and development, there are three main topological structures of multi-level converters:

1) Diode clamped;

2) Capacitor clamping type (Flying Capacitors);

3) Cascade inverter with independent DC power supply

(Cascaded?InverterswithSeparatedDCSources).

Multilevel conversion technology has become the most active branch in the field of power electronics in terms of high voltage and high power conversion. Multilevel converters are mainly used in high voltage and high power applications. The voltage stress and current stress on their switching devices are relatively large. Therefore, with the increase of switching frequency, the switching loss caused by hard switching of multilevel converters is considerable, which greatly reduces the efficiency of the circuit and the ability to handle power. At the same time, the high dv/dt and di/dt caused by the multilevel converter working in the hard switching state will produce more serious electromagnetic interference. In order to solve the many problems caused by the high frequency of multilevel converters and hard switching, in recent years, there have been many reports on the application of active soft switching technology to multilevel converters, and good results have been achieved. This paper will analyze and compare various active soft switching technologies for multilevel converters found in the literature, and point out their respective advantages and disadvantages as well as application prospects.

Active Soft Switching Technology of Diode Clamped Multilevel Inverter

After years of efforts, researchers have proposed a variety of active soft switching topologies for two-level inverters , mainly focusing on DC link resonant inverters and pole resonant inverters. So far, the research on active soft switching technology for multi-level inverters is mainly to expand the two soft switching topologies of DC link resonant inverters and pole resonant inverters to multi-level circuits.

1. Modular clamped DC link three-level soft switching inverter

The structure of the modular clamped DC link three-level soft switching inverter is shown in Figure 1. Cr1 and Cr2 are resonant capacitors.

Figure 1 Modular clamped DC link three-level soft switching inverter

Figure 2 Modular clamped DC link three-level soft switching conversion

The resonant capacitor, resonant inductor and auxiliary switch form a soft switching conversion module in the DC link. Cr1=Cr2, Lr1=Lr2, so the two quasi-resonant tanks form a mirror-symmetrical module in the DC link, as shown in the dotted line part in the figure.

During the switching period, the clamp switches S1′ and S2′ are in the off state, releasing the bus voltage of the inverter from the DC link, so that the voltage at points P and M drops to zero through resonance, providing conditions for the realization of soft switching. At this time, the main switch device of the three-level inverter can realize soft switching operation under zero voltage conditions. When the switch is completed, the resonance process is ended by opening the clamp switches S1′ and S2′, and the voltage of the DC link is added between the positive and negative poles of the bus. Because the upper resonant tank circuit and the lower resonant tank circuit are mirror-symmetrical, their working principles are the same. However, in the actual system, the conversion current Io1 of the upper resonant tank circuit and the conversion current Io2 of the lower resonant tank circuit may not be equal due to the effect of the midpoint current IN, so the time required for the positive bus voltage and the negative bus voltage of the inverter to resonate to the midpoint voltage may not be equal, which will affect the zero voltage switching condition. In order to ensure the realization of the zero voltage condition, it is necessary to synchronize the upper resonant tank circuit and the lower resonant tank circuit with synchronous logic. In order to trigger the resonance of the upper resonant tank circuit and the lower resonant tank circuit in an appropriate order, the delay time between triggering the upper resonant tank circuit and triggering the lower resonant tank circuit is defined as td, which is proportional to the midpoint current IN, td==. The direction of the midpoint current determines which resonant tank circuit's resonance process is delayed and triggered, as shown in Figure 1. If the midpoint current is positive, that is, IN>0, the resonance process of the upper resonant tank circuit is delayed by td before being triggered, otherwise, the resonance process of the lower resonant tank circuit will be triggered after a delay of td.

Figure 2 is a simplified control logic block diagram of the resonant tank circuit. The soft switching conversion module receives the PWM pattern from the inverter controller. When the PWM pattern changes, the detection circuit will generate a signal to trigger the zero voltage conversion process. The synchronization circuit is used to ensure that the upper resonant tank circuit and the lower resonant tank circuit reach zero voltage at the same time. Once the zero voltage condition is established, the main switch can be switched at zero voltage, and the PWM rearrangement module then sends a new PWM pattern to the gate circuit.

The advantages of this circuit are:

1) Modular design: Fewer components are used in the auxiliary conversion circuit.

2) The voltage and current stresses borne by the main switching device are equal to the voltage and current stresses borne by the hard-switching inverter.

3) The main switch device of the inverter and the clamp switch of the auxiliary circuit are turned on at zero voltage; the auxiliary switch in the auxiliary circuit is turned off at zero current. The disadvantages of this circuit are:

Due to the influence of the midpoint current, the time required for the positive bus voltage and the negative bus voltage of the inverter to resonate to the midpoint voltage may not be equal, and external control logic is required to synchronize the two, which increases the complexity of the circuit and reduces the reliability of the circuit .

Ultra-soft switching topology

In high voltage and high power applications, various soft switching topologies have been proposed, among which the auxiliary resonant commutated pole inverter is a relatively successful one. Recent literature shows that it is theoretically feasible to extend the auxiliary resonant commutated pole topology to the diode clamped multi-level inverter. Figures 3 and 4 summarize the proposed auxiliary resonant commutated pole soft switching topologies for three-level inverters.

The circuit shown in FIG3 has an auxiliary resonant conversion circuit composed of auxiliary switches Saux1 and Saux2, a resonant inductor Laux, and turn-off absorption capacitors C1, C2, and C3. The auxiliary switch Saux1 helps the main switches Sa1 and Sa1′ to complete the conversion under soft switching conditions, and the auxiliary switch Saux2 helps the main switches Sa2 and Sa2′ to complete the conversion under soft switching conditions. C1 serves as the turn-off absorption capacitor of switch Sa1, C2 serves as the turn-off absorption capacitor of switch Sa2′, and C3 serves as the turn-off absorption capacitor of the internal switch tubes Sa2 and Sa1′ and the clamping diodes Da and Da′.

The advantages of this circuit are:

1) The number of additional components required is minimal;

2) It can realize zero voltage switching of the main switch tube and zero current switching of the auxiliary switch tube.

The disadvantages of this circuit are:

Research on Active Soft Switching Technology of Multilevel Inverter

Figure 3 Auxiliary resonant conversion three-level inverter

Figure 4 Auxiliary resonant conversion three-level inverter

Figure 5 Auxiliary resonant conversion three-level inverter

1) The blocking voltage borne by the auxiliary switch is 0.75Udc, which is a relatively high value.

2) The effective value of the current flowing through the absorption capacitor C3 is 1.4 times the effective value of the current flowing through the absorption capacitors C1 and C2.

3) When the internal switch tubes Sa2 and Sa1′ and the clamping diodes Da and Da′ are turned off, their energy is indirectly absorbed by the capacitor C3, which will cause a large stray inductance and lead to parasitic oscillation during soft conversion. In order to overcome the shortcomings of the circuit in Figure 3, the circuits shown in Figures 4 and 5 are proposed respectively.

The auxiliary resonant conversion circuit of the circuit shown in FIG4 is composed of auxiliary switches Saux1 and Saux2, resonant inductors Laux1 and Laux2, and turn-off absorption capacitors C1, C2, C3, and C4. When the switch tube Sa2 is turned on and Sa2′ is turned off, the auxiliary switch Saux1 helps the main switches Sa1 and Sa1′ to complete the conversion under the soft switching condition, so that the output voltage is alternately connected to the positive bus and the midpoint; when the switch tube Sa1 is turned on and Sa1′ is turned off, the auxiliary switch Saux2 helps the main switches Sa2 and Sa2′ to complete the conversion under the soft switching condition, so that the output voltage is alternately connected to the negative bus and the midpoint. C1 serves as the turn-off absorption capacitor of the switch tube Sa1, C2 serves as the turn-off absorption capacitor of the switch tube Sa2 and the clamping diode Da, C3 serves as the turn-off absorption capacitor of the switch tube Sa1′ and the clamping diode Da′, and C4 serves as the turn-off absorption capacitor of the switch tube Sa2′.

The advantages of this circuit are:

1) The blocking voltage borne by the auxiliary switch is reduced to 0.5Udc.

2) It can realize zero voltage switching of the main switch tube and zero current switching of the auxiliary switch tube.

3) The energy of the clamping diode when it is turned off is directly absorbed by the absorption capacitor, which reduces the stray inductance and also reduces the loss caused by the reverse recovery characteristics of the diode when it is turned off.

The disadvantages of this circuit are:

1) Two resonant inductors are used, and the number of components in the circuit is large.

2) For soft switching conversion, when the product of the average output voltage and the average output current (i.e., the average output power) within a switching cycle is greater than zero, the layout of the absorption components is optimal, because at this time, the conversion occurs between the directly absorbed devices Sa1 and Da and between Sa2′ and Da′; when the average output power within a switching cycle is less than zero, the conversion occurs on the internal switching tube Sa2 or Sa1′ that is indirectly absorbed, which will cause a large stray inductance and lead to parasitic oscillations during soft conversion.

In order to overcome the shortcomings of the circuit shown in FIG4 and make the arrangement of the absorption components optimal, the literature [10] proposed the soft switching topology shown in FIG5. The auxiliary resonant conversion circuit of the circuit is composed of auxiliary switches Saux1 and Saux2, resonant inductors Laux1 and Laux2, active clamping switches composed of Sa and Da, Da′ and Sa′, and turn-off absorption capacitors C1, C2, C3, and C4. However, the arrangement of the absorption capacitors in this circuit is different from that in the circuit shown in FIG4. Each main switch device is connected in parallel with a direct absorption capacitor. Such a structure minimizes the stray inductance and reduces the loss caused by the conversion of the average output power less than zero. However, the circuit requires an active clamping switch to control the charging process of the absorption capacitor during the soft switching conversion. The positive effect of the active clamping switch is to improve the balance of the static voltage and the loss balance between the switching devices during low speed and high torque operation.

The advantages of this circuit are:

1) Reduce the stray inductance to a minimum, greatly reducing the probability of parasitic oscillation.

2) It can realize zero voltage switching of the main switch tube and zero current switching of the auxiliary switch tube.

3) When operating at low speed and high torque, the balance of static voltage and loss balance between switching devices are improved. The disadvantages of this circuit are: adding two active clamping switch tubes and using two resonant inductors increase the number of components in the circuit, increase the complexity of the circuit, and reduce the reliability of the circuit.

In practical applications, the above three circuits still have the following problems:

1) Midpoint stability problem. Since the DC link of the three-level auxiliary resonant conversion pole soft switching topology has four capacitors, there are two midpoints (1 and 2). During the ARCP conversion, the charging balance of the two midpoints is determined by the direction, magnitude and duration of the auxiliary currents iaux1 and iaux2. Only when the phase shift between the output current and the output voltage is around ±90°, that is, when the load is a pure reactive load, for the auxiliary currents iaux1 and iaux2, the ampere-second values ​​of the currents flowing into midpoints 1 and 2 respectively are equal within one output cycle. In other cases, the midpoint will be offset. Therefore, in the diode-clamped three-level auxiliary resonant conversion pole inverter, due to the existence of two independent midpoints, the problem of midpoint stability caused by charging balance is inevitable.

Figure 6 Auxiliary resonant conversion extremely soft switching topology of capacitor clamped three-level inverter

Figure 7 Auxiliary resonant conversion extremely soft switching topology of capacitor clamped three-level inverter

2) When the anti-parallel diode of the auxiliary switch is turned off, an overvoltage will be caused at both ends of the auxiliary switch due to its reverse recovery characteristics. Although various solutions to reduce the overvoltage have been proposed, they all greatly increase the complexity of the circuit.

Topological Discussion

The concept of the diode-clamped three-level auxiliary resonant conversion extremely soft switching topology is extended to the N-level inverter . Obviously, the N-level auxiliary resonant conversion extremely soft switching topology still has the problem of midpoint stability; moreover, the number of additional components is also greatly increased; the reliability of the system and the complexity of control also hinder the application of the N-level auxiliary resonant conversion extremely soft switching topology to industry. It should be emphasized that the midpoint stability problem of the N-level auxiliary resonant conversion extremely soft switching topology is not caused by the fluctuation of component parameters or the influence of parasitic parameters, but is caused by the shortcomings of the circuit topology itself. It is not difficult to conclude that the midpoint stability problem of the N-level auxiliary resonant conversion extremely soft switch and the complexity of the topology make it very unlikely that this circuit topology will be applied to actual industrial systems. Active soft switching technology of capacitor-clamped multi-level inverter

So far, the research on active soft switching technology of capacitor-clamped multilevel inverters is very limited.

1. The auxiliary resonant conversion of capacitor clamped three-level inverter is extremely soft

Switching Topology

The auxiliary resonant conversion extremely soft switching topology of the capacitor clamped three-level inverter is shown in Figure 6.

The auxiliary switch of this circuit is connected between the positive and negative poles of the DC bus, and the resonant inductor Laux and capacitors C1, C2, C3, and C4 form a resonant path. This circuit has a fatal weakness, that is, the blocking voltage borne by the auxiliary switch is equal to Udc, which makes the circuit meaningless in practical application.

Another auxiliary resonant conversion extremely soft switching topology of a capacitor clamped three-level inverter is shown in Figure 7. One auxiliary switch is connected between the output end (via Laux2) and the midpoint of the clamping capacitor, and the other auxiliary switch is connected between the midpoint of the clamping capacitor (via Laux1) and the midpoint of the DC link. The blocking voltage borne by the auxiliary switch of this circuit is only Udc/4. Compared with the auxiliary resonant conversion extremely soft switching topology of the diode clamped three-level inverter, the voltages of the two midpoints (1 and 2) of this circuit are stable. The midpoint of the clamping capacitor is determined by the auxiliary current, which changes direction alternately every switching cycle and is not affected by the power factor. This circuit adopts the method used in the hard-switched capacitor clamped three-level inverter to stabilize the clamping capacitor voltage, that is, alternately using the two possible zero states of the three-level converter to stabilize the midpoint voltage of the clamping capacitor. In addition, the direction of the auxiliary currents iaux1 and iaux2 changes every 180° of the output current to balance the asymmetry of the output current caused by the modulation strategy and the power factor. Since the output current is three-phase symmetrical in a three-phase system, the midpoint of the DC link can be stabilized in the same way as the midpoint of a traditional two-level auxiliary resonant converter voltage source inverter.

However, this circuit has the following disadvantages:

Research on Active Soft Switching Technology of Multilevel Inverter

Figure 8 Auxiliary resonant conversion extremely soft switching topology of capacitor clamped N-level inverter

1) Like the auxiliary resonant conversion extremely soft switching topology of the diode clamped three-level inverter, when the anti-parallel diode of the auxiliary switch is turned off, an overvoltage will be caused at both ends of the auxiliary switch due to its reverse recovery characteristics. 2) It is very sensitive to parasitic inductance parameters. If it is not handled properly, it will cause parasitic oscillation.

2. Discussion on the Capacitor Clamped N-Level Assisted Resonant Converter Ultra-Soft Switching Topology

The auxiliary resonant conversion extremely soft switching topology of the capacitor clamped three-level inverter is extended to the capacitor clamped N-level inverter, as shown in Figure 8. The blocking voltages of all auxiliary switches are equal, which is equal to Udc/〔2(N-1)〕.

The auxiliary resonant conversion and extremely soft switching topology of capacitor-clamped N-level inverter has only been discussed in theory. The midpoint balance problem needs further research, and there is still a lot of work to be done before it can be applied in practice.

Conclusion

The so-called active soft switching technology of multi-level inverter is to add some auxiliary active switching devices and inductors and capacitors to the original main circuit structure of the multi-level inverter, and soften the switching process of the power device through appropriate detection and timing control to realize the function of soft switching. At present, the research on the soft switching technology of multi-level inverter is mostly focused on active soft switching, and some multi-level inverter active soft switching circuit topologies have been obtained. From the above analysis, it can be seen that the commutation process of these circuit topologies is relatively simple; however, all multi-level active soft switching topologies are all added with active auxiliary switches and corresponding detection and control circuits. Since the topological structure and control of the multi-level converter itself are already quite complex, the addition of active auxiliary switches and corresponding detection and control circuits increases the complexity of the system, thereby reducing the reliability of the system. Therefore, in situations where reliability requirements are high, the application of active soft switching technology of multi-level inverters still has certain difficulties, and there is still a lot of work to be done. Compared with active soft switching technology, passive soft switching technology does not require active auxiliary switches and corresponding detection and control circuits, so it has great advantages in circuit complexity and reliability. At present, there is little research on passive soft switching technology of multi-level inverters. Therefore, applying passive soft switching technology to multi-level inverters is a direction worthy of attention in the field of multi-level inverter soft switching technology.