## 1. Introduction

The three-phase CSR, also known as the buck-type rectifier, is widely used in AC/DC conversion systems, such as fast electric vehicle chargers, energy storage devices, communication power supplies, adjustable speed drives, wind power generation systems, high power applications, etc. [

1,

2,

3,

4,

5,

6,

7,

8]. Compared with the conventional boost-type converter [

9,

10,

11], the aforementioned buck-type CSR systems provide a smaller AC input filter, inrush current limiting capability, and controllable step-down voltage conversion with a power factor correction (PFC) function for the abovementioned industrial applications [

12,

13]. Hence, the three-phase CSR has been a popular research area and has attracted a lot of attention over the past few years.

There are several three-phase CSRs introduced in most of the literature, including the six-switch CSR [

14], three-phase four-wire CSR [

15], current doubler CSR [

16], matrix-type CSR [

17], three-switch CSR [

18], swiss-type CSR [

19], delta-type CSR [

20], split freewheeling diode CSR [

21], etc. Another kind of isolated CSR is achieved with a high-frequency transformer [

22,

23,

24,

25,

26,

27]. It could provide electrical isolation between the input and output to ensure safe operation, but it has a higher cost and the power density could be decreased. Meanwhile, the design of high-frequency transformers and modulation schemes is more difficult for researchers. Therefore, the isolated CSR is not suitable for most of industrial applications. Moreover, both CSRs could obtain a sinusoidal input current and constant DC output voltage, as well as high stress on semiconductor devices, which is not expected in practice.

Generally, CSRs usually use the transistor (IGBT or MOSFET) in series with a diode to form switches, so the switches would have a reverse blocking capability and can block the AC current. Inevitably, there would be a reversed body diode in the transistors due to the production process [

28,

29]. Although a reverse blocking IGBT (RB-IGBT) has been developed in recent years [

30,

31,

32], it has a higher switching loss. Unlike the boost-type voltage source rectifier (VSR), the body diodes of the transistors are ignored and are not utilized in most applications of the conventional CSRs. If we also consider the body diode as a current flowing device in the CSR circuit, the circuit will exhibit other superior characteristics that are distinct from the conventional topological structures. Therefore, a new current path with the body diode is obtained by changing the inflow terminal or outflow terminal to restructure the CSR topology in this paper. The proposed CSR features an asymmetrical topological structure and would have reduced stress on semiconductor devices. It means that half of transistors on low voltage stress can be achieved in PFC operation, and the proposed converter could have a higher efficiency at a low modulation index due to the multiple freewheeling paths. Compared with the conventional CSR, the detailed advantages of the proposed CSR are summarized as follows:

(1) Low cost without additional hardware;

(2) Half of transistors on lower voltage stress 1/2 V_{L_im};

(3) Low current stress 1/3 i_{o} in freewheeling period;

(4) High efficiency at low modulation index;

(5) Smaller output filter for CSR system.

According to the above analysis, the rest of the paper is organized into five sections. In

Section 2, the proposed CSR structure is introduced and compared to the conventional CSR. Then the basic operation principle and stress characteristics are analyzed in

Section 3. Detailed discussions are carried out in

Section 4. As a proof of concept, the proposed CSR is performed on a prototype in

Section 5 and the conclusion is drawn in

Section 6. All theoretical analysis and experimental results show that the proposed CSR is a suitable topology for step-down voltage applications.

## 2. Topological Structure

Figure 1a shows the conventional standard six-switch CSR topological structure, there are three bridge arms and each arm can be divided into symmetrical upper and lower switch parts. Taking the arm of A phase as an example, the outflow terminal of the A phase current is the same with the inflow terminal and they are both at the symmetrical point.

Different from the conventional CSR structure, the outflow terminal of the proposed CSR in

Figure 1b is not the same with the inflow terminal but is connected between the diode and the transistor on the upper arms. This is an asymmetric CSR, while the two topological structures have the same number of devices. As can be seen, compared with the conventional CSR in

Figure 1a, the body diodes on the upper arms are added to the current path of the proposed CSR. With the modified structure, the current path in the proposed CSR has a minor change.

It should be noted that another topological structure can be constructed by changing the inflow terminal rather than the outflow terminal and the structure would have similar characteristics, but this is omitted for the sake of brevity.

## 4. Discussions

#### 4.1. Influence of Input Displacement Angle

Due to the existence of input filters, the filter capacitor

C_{i} consumes reactive power and an input displacement angle appears between input voltage and current. To solve this problem, the new modulation signals, represented as dotted lines with the compensation angle

φ in

Figure 9, could be applied to the proposed CSR.

Note that the voltage stress on different transistors would change with the compensation angle. For the transistors in the upper arm, the maximum voltage stress is still input line-to-line voltage amplitude

V_{L_im}. However, the voltage stress on the transistors in the lower arm would increase together with the compensation angle. As shown in

Figure 9, the maximum voltage stress also reaches

V_{L_im} when the compensation angle is set as

π/3. It should be noted that the maximum output voltage is achieved at unity power factor operation, so the input displacement angle is always designed as zero to obtain a wide range of output voltage. Therefore, half of the transistors would withstand the voltage stress of nearly

V_{L_im}/2 since the input displacement angle is not large in practice.

Considering the input phase voltage amplitude V_{im} as a boundary, due to V_{L_im}/2 = 0.866 V_{im} < V_{im}, there is enough margin, 13.4%, to satisfy the voltage stress change resulting from the input displacement angle. Therefore, it can be concluded that a low voltage rating V_{im} could be achieved in half of the transistors in the proposed CSR system.

#### 4.2. Power Loss Analysis

The power loss is related to the switching loss

P_{s} and conduction loss

P_{c}. From the operation modes in

Figure 5, compared with the conventional CSR, it is clear that a turn-on is added when mode 2 changes to mode 3, and a turn-off is added when mode 3 changes to mode 1 for the proposed CSR. However, the turn-on and turn-off are on low voltage and current stress. Hence, the switching loss of the proposed CSR only has a slight increase compared with the conventional CSR.

Assuming that all semiconductor devices are in a healthy state, the conduction loss

P_{p, c} of the proposed CSR is divided into two types:

From

Figure 6, the current of the conventional CSR flows through two transistors and two diodes at any time, and the conduction loss

P_{c, c} of the conventional CSR can be expressed in the same form for different vectors:

As can be seen in the above equations, compared with the conventional CSR, a body diode is added in the current path when active vectors act, but a transistor is reduced in the current path when the zero vector acts in the proposed CSR system.

Moreover, the conduction loss of a single device is expressed as the sum of two parts:

where

V_{on} is the forward voltage and

R_{on} is the on-resistance.

Since there are three current paths in freewheeling mode in the proposed CSR, the average current

i_{avg} and rms current

i_{rms} have a significant reduction at this time, so the current stress has a greater effect on conduction loss than other factors. In the proposed CSR, although the conduction loss of the active vector is slightly increased with the high number of conduction devices, the conduction loss of the zero vector is significantly reduced due to the lower number of conduction devices and lower current stress in the freewheeling period. It means that the zero vector has an important role for the proposed CSR to reduce conduction loss. On the other hand, the conduction loss of the CSR is much larger than the switching loss in practice [

6,

33], so the slightly increased switching loss has little effect on total loss when there is a longer period in freewheeling mode.

In summary, compared with the conventional CSR, the proposed CSR has a slightly increased switching loss P_{s}, and the conduction loss P_{c} is significantly reduced at a low modulation index. The total loss at a high modulation index is increased slightly but a decreased total loss is achieved at a low modulation index. Therefore, the proposed CSR has a higher efficiency at a low modulation index and it is more suitable for low power applications compared to the conventional CSR.

#### 4.3. Comparative Analysis of Other Conventional CSRs

This section presents a brief comparative review of CSRs, including the number of devices, stress on transistors, gain of the converter, PFC function, and other characteristics.

Figure 10 summarizes the existing conventional CSR topological structures.

Table 4 illustrates the characteristics of the abovementioned conventional CSRs and the proposed CSR.

As can be seen in

Figure 10 and

Table 4, unlike the current doubler CSR in [

16] and matrix-type CSR in [

17], a standard six-switch CSR has six transistors and six diodes, as well as the proposed one. Although a three-switch CSR is designed in [

18], the converter has the maximum number of diodes and the conduction loss is high. The Swiss-type CSR in [

19] can reduce the switching loss, but it also has a higher number of devices and conduction loss. The delta-type CSR in [

20] could be used to reduce the conduction loss due to the low current stress, but the effect is significant only at a high modulation index. The Current doubler CSR in [

16] also could reduce the conduction loss, but there is a high cost and low gain, and the design of the switching commutation process is more difficult. In addition, all the mentioned CSRs have high voltage stress on transistors. To solve this problem, a CSR with the split-diode connection was introduced in [

21]. However, this converter is restricted in applications since it can only operate at unity power factor. Meanwhile, the transistors still withstand the voltage stress

V_{im} rather than 0.866

V_{im}. For the CSRs, a freewheeling diode on the DC side is the most common method to reduce the conduction loss in the freewheeling period, but the additional hardware could increase costs and reduce power density.

Due to the low current stress in the freewheeling period, half of transistors with a low voltage rating, and no need of additional hardware, the proposed CSR is one of the optimal solutions for high step-down voltage applications.

## 6. Conclusions

A novel three-phase CSR based on an asymmetrical structure to reduce stress on semiconductor devices is proposed in this paper. Compared with the conventional standard six-switch CSR, the proposed CSR topological structure only has a minor change and no additional hardware is required. With the corresponding SVPWM scheme, half of the transistors could achieve both the lower voltage stress 1/2 V_{L_im} and low current stress 1/3 i_{o} in the freewheeling period. Owing to the reduced stress, the proposed CSR has a higher efficiency at a low modulation index, and a smaller output filter could be used in the CSR system. In addition, the CSR was evaluated and compared by an experimental prototype. The comparative experimental results indicate that the proposed CSR has a higher performance in low power output applications. With the reduced stress and low cost, the proposed asymmetrical CSR is a very suitable topology for the implementation of a buck-type power factor correction mains interface, especially for communication power supplies or the integration of fast electric vehicle charging stations within smart grids.