A Novel Three-Phase Current Source Rectifier Based on an Asymmetrical Structure to Reduce Stress on Semiconductor Devices

Abstract

This paper presents a novel three-phase current source rectifier (CSR) for AC/DC step-down voltage conversion to reduce voltage and current stress. The proposed converter features an asymmetrical connection between upper and lower arms compared with conventional CSRs, but has the same number of devices. With the proposed asymmetrical structure and modified space vector pulse width modulation (SVPWM) scheme, half of transistors only need to withstand half of the line-to-line voltage rather than the full line-to-line voltage, and its DC link current can be shared by multiple switches in freewheeling periods. Therefore, it is able to bring about a significant reduction in voltage and current stress, allowing for an improvement in the converter without additional cost. The topological structure, operation principles, and comparative analysis are specifically presented. Finally, an experimental prototype is built up to verify the performance of the proposed converter.

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.

3. Basic Operation Principle and Stress Characteristics of the Proposed CSR

3.1. Modulation Scheme

SVPWM is one of the most popular modulation schemes for CSRs. Similar to three-phase VSRs, the core idea of the SVPWM for three-phase CSRs is the input reference current space vector I_ref synthesis. Firstly, in order to ensure that the output side is not opened and the input side is not shorted at any time for the proposed CSR, there exist seven switching states, as listed in

Table 1. The switching states corresponding to the space vectors of the proposed CSR.

Vectors		Upper Arm			Lower Arm			VPN	iA	iB	iC
		S11	S12	S13	S21	S22	S23
Active vectors	I1	√	×	×	×	×	√	VAC	io	0	−io
	I2	×	√	×	×	×	√	VBC	0	io	−io
	I3	×	√	×	√	×	×	VBA	−io	io	0
	I4	×	×	√	√	×	×	VCA	−io	0	io
	I5	×	×	√	×	√	×	VCB	0	−io	io
	I6	√	×	×	×	√	×	VAB	io	−io	0
Zero vector	I0	×	×	×	√	√	√	0	0	0	0

. The existing switching states can be classified into six active vectors and one zero vector, where I₁–I₆ are the active vectors, and I₀ is the zero vector.

In order to obtain the given sine current waveforms, the input reference current space vector I_ref must be constructed as a space rotating current vector with angular velocity w and modulus length I_im, and the running trajectory of the corresponding input reference current vector would be a circular trajectory. Therefore, for the sake of the above objectives, it is very important to select the appropriate current vector in Table 1 to synthesize the input reference current vector I_ref during one switching period.

To analyze the principle of vector synthesis, an ideal three-phase voltage is assumed in Figure 2. In each input cycle exists six sectors and every sector is further divided into two regions. Figure 3 shows the SVPWM schematic diagram of the proposed CSR.

According to [13], the input reference current space vector I_ref is synthesized by two active vectors and one zero vector. It can be calculated as:

$$I_{\mathrm{ref}}=d_{\alpha }{I}_{\alpha }+d_{\beta }{I}_{\beta }+d_0{I}_0$$

(1)

where d_α, d_β, and d₀ are the duty cycles for different vectors, respectively. During one switching period T_s, the duration formulas of all the vectors are expressed as:

$$\begin{array}{l}{T}_{\alpha }=d_{\alpha }{T}_{\mathrm{s}}=m{T}_{\mathrm{s}}\sin (\pi /3-\theta )\\ T_{\beta }=d_{\beta }{T}_{\mathrm{s}}=m{T}_{\mathrm{s}}\sin \theta \\ T_0=T_{\mathrm{s}}-T_{\alpha }-T_{\beta }\end{array}$$

(2)

where m is the modulation index and m $\in$ [0, 1]; θ is the sector angle. Then the average DC output voltage in one switching period can be calculated as:

$$u_{\mathrm{o}}=1.5m{V}_{\mathrm{im}}\cos \varphi$$

(3)

where V_im is the amplitude of the input phase voltage, φ is the input displacement angle. Therefore, when m = 1, the maximum DC output voltage 1.5 V_im can be achieved at unity power factor operation.

3.2. Operation Modes

With the rapidly increasing switching frequency, the CSRs have been gradually focused on switching loss to improve the converter efficiency. To ensure the minimum number of switching actions in one switching period, Figure 4 shows the switching pattern with three segments in sector I for the proposed CSR. For each segment, Figure 5 shows that the switching states corresponds to different vectors in sector 1.

For the proposed CSR, the complement operation mode in sector 1 during one switching period is divided into three modes. The detailed content is presented as follows:

Mode-1 (t₀~t₁): At this interval, S₁₁ and S₂₂ are turned on. Due to V_A > V_B, the diode D₁₂ would remain off, and the body diode of S₁₂ conducts. With the corresponding switching state, the current would flow through S₁₁, D₁₁, S₂₂, D₂₂, and the body diode of S₁₂, resulting in V_AB at the output side. The equations during this operation mode are given as:

$$V_{\mathrm{P}}=V_{\mathrm{A}},V_{\mathrm{N}}=V_{\mathrm{B}},i_{\mathrm{o}}=i_{\mathrm{AB}}$$

(4)

Mode-2 (t₁~t₂): At this interval, S₁₁ and S₂₃ are turned on. Due to V_A > V_C, the diode D₁₃ would remain off, and the body diode of S₁₃ conducts. The current would flow through S₁₁, D₁₁, S₂₃, D₂₃, and the body diode of S₁₃, resulting in V_AC at the output side. The equations during this operation mode are given as:

$$V_{\mathrm{P}}=V_{\mathrm{A}},V_{\mathrm{N}}=V_{\mathrm{C}},i_{\mathrm{o}}=i_{\mathrm{AC}}$$

(5)

Mode-3 (t₂~t₃): At this interval, S₂₁, S₂₂, and S₂₃ are turned on. This is freewheeling mode, and no power transfer happens at the output side. Due to V_A > V_B > V_C, only the body diode of S₁₃ at minimum C phase conducts, the others remain off. The DC current would be shared equally among three paths, and each path consists of two diodes and one transistor. The equations during this operation mode are given as:

$$V_{\mathrm{P}}=V_{\mathrm{N}}=V_{\mathrm{C}},i_{\mathrm{o}}=\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.i_{\mathrm{o}}+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.i_{\mathrm{o}}+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.i_{\mathrm{o}}$$

(6)

For the purpose of comparison, Figure 6 shows the switching states of the conventional CSR in sector 1. As can be seen, compared with the conventional CSR, the active vectors in the proposed CSR have the same switching states, while the zero vector will turn on all switches in the lower arm.

The operation modes of the proposed CSR have two differences compared to the conventional one. The first is that a body diode is added in the current path when active vectors act. Another is that three current paths can be obtained in the freewheeling period, and a transistor in each path can be reduced. With the differences from the modified current path, several characteristics could be obtained, especially the low stress on the transistors. Based on the operation modes, the stress on the transistors in the proposed CSR is described in next section.

3.3. Voltage Stress

For the following analysis of the voltage stress, due to the diode conduction characteristics of CSR, summarized in [21], several conclusions are emphasized, as follows:

(1)

Upper arm: If V_x > V_P (x = A, B, C), the transistors withstand the voltage stress V_x−V_P, otherwise, the transistors withstand zero voltage stress.

(2)

Lower arm: If V_x > V_N (x = A, B, C), the transistors withstand zero voltage stress, otherwise, the transistors withstand the voltage stress V_N−V_x.

Firstly, taking mode 1 (t₀~t₁) of the proposed CSR in Figure 5a as an example, there will be V_P = V_A and V_N = V_B during this time. The transistors in the upper arm both withstand zero voltage stress, while only the transistor S₂₃ in the lower arm would withstand the voltage stress V_B−V_C.

In mode 2 (t₁~t₂) of Figure 5b, the transistor S₂₃ does not withstand the voltage stress since the transistor turns on at this time, and the others still withstand zero voltage stress.

In mode 3 (t₂~t₃) of Figure 5c, all the switches in the lower arm turn on, so the corresponding transistors do not withstand voltage stress. Due to V_A > V_B > V_C in sector 1, the bus voltage is the minimum phase voltage V_C. Therefore, the transistors S₁₁, S₁₂, and S₁₃ of the upper arm will withstand the voltage stress V_A−V_B, V_B−V_C, and 0, respectively.

Table 2. Voltage stress of different CSRs in sector 1.

		Conventional CSR			Proposed CSR
		Mode 1	Mode 2	Mode 3	Mode 1	Mode 2	Mode 3
Upper arm	S11	0	0	0	0	0	VA−VC
	S12	0	0	0	0	0	VB−VC
	S13	0	0	0	0	0	0
Lower arm	S21	0	0	0	0	0	0
	S22	0	0	VA−VB	0	0	0
	S23	VB−VC	0	VA−VC	VB−VC	0	0

summarizes the voltage stress of the proposed CSR in sector 1. From the above input voltage analysis in Figure 2, the maximum voltage stress in the upper arm is the input line-to-line voltage amplitude (V_A−V_C)_max = V_{L_im}, and the maximum voltage stress in the lower arm would equal half of the input line-to-line voltage amplitude (V_B−V_C)_max = V_{L_im}/2. Similarly, the same phenomenon can be found in sector 2 in

Table 3. Voltage stress of different CSRs in sector 2.

		Conventional CSR			Proposed CSR
		Mode 1	Mode 2	Mode 3	Mode 1	Mode 2	Mode 3
Upper arm	S11	0	VA−VB	VA−VC	0	VA−VB	VA−VC
	S12	0	0	VB−VC	0	0	VB−VC
	S13	0	0	0	0	0	0
Lower arm	S21	0	0	0	0	0	0
	S22	0	0	0	0	0	0
	S23	0	0	0	0	0	0

.

For the sake of comparative analysis, the voltage stress of the conventional CSR also can be calculated in the same way and is described in Table 2 and Table 3. To compare the voltage stress between the proposed CSR and the conventional CSR more intuitively, taking S₁₁ in the upper arm and S₂₁ in the lower arm as an example, Figure 7 shows the corresponding voltage stress during the entire input cycle. Based on the comparative analysis, it can be seen that half of the transistors for the proposed CSR have lower voltage stress V_{L_im}/2.

3.4. Current Stress

Except for the reduction of voltage stress on the transistors, the proposed CSR has the same function to reduce the current stress due to the asymmetric topological structure and modified modulation strategy. From Figure 8, in the freewheeling mode, the switches of the proposed CSR in the lower arm are all turned on. As expected, three current paths are simultaneously obtained in the proposed CSR system, and each current path only has two diodes and one transistor. Hence, the flowing current for S_21~23 is equal to 1/3 of the DC output current. Compared with the conventional CSR, this would cause a significant reduction in current stress. This means that a higher efficiency could be achieved for the proposed CSR when there is a longer freewheeling period at a low modulation index.

A mentioned method that adds a freewheeling diode to the DC side can be commonly used in the CSR circuit. However, the proposed CSR has no additional hardware, and the volume and cost are decreased, so the power density will increase.

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:

$$\{\begin{array}{l}{P}_{\mathrm{p},\mathrm{c},\mathrm{active}\mathrm{vector}}=2P_{\mathrm{p},\mathrm{c},\mathrm{transistor}}+3P_{\mathrm{p},\mathrm{c},\mathrm{diode}}\\ P_{\mathrm{p},\mathrm{c},\mathrm{zero}\mathrm{vector}}=P_{\mathrm{p},\mathrm{c},\mathrm{transistor}}+2P_{\mathrm{p},\mathrm{c},\mathrm{diode}}\end{array}$$

(7)

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:

$$\{\begin{array}{l}{P}_{\mathrm{c},\mathrm{c},\mathrm{active}\mathrm{vector}}=2P_{\mathrm{c},\mathrm{c},\mathrm{transistor}}+2P_{\mathrm{c},\mathrm{c},\mathrm{diode}}\\ P_{\mathrm{c},\mathrm{c},\mathrm{zero}\mathrm{vector}}=2P_{\mathrm{c},\mathrm{c},\mathrm{transistor}}+2P_{\mathrm{c},\mathrm{c},\mathrm{diode}}\end{array}$$

(8)

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:

$$P_{\mathrm{c},\mathrm{device}}=i_{\mathrm{avg}}{V}_{\mathrm{on}}+i_{\mathrm{rms}}^2{R}_{\mathrm{on}}$$

(9)

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. Comparative analysis of the CSRs.

	Number of Devices		Stress on Transistors		Gain	PFC
	Transistor	Diode	Current	Voltage
Proposed	6	6	Low	Low	1.5 Vim	Yes
Six-switch [14]	6	6	High	High	1.5 Vim	Yes
Four-wire [15]	8	8	High	High	1.5 Vim	Yes
Current doubler [16]	12	2	Low	High	0.75 Vim	Yes
Matrix-type [17]	12	0	High	High	1.5 Vim	Yes
Three-switch [18]	3	12	High	High	1.5 Vim	Yes
Swiss-type [19]	8	8	High	High	1.5 Vim	Yes
Delta-type [20]	6	12	Low	High	1.5 Vim	Yes
Split diode [21]	6	8	High	Medium	1.5 Vim	NO

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.

5. Experimental Result

In order to verify the effectiveness of the proposed CSR, comparative experiments between the standard six-switch symmetric CSR and the proposed asymmetric CSR were carried out and the prototype is shown in Figure 11. In particular, the diode–transistor series combination is composed of the IGBT and diode. The input L_iC_i filter, with a damping resistor R_i, is used to suppress the high-frequency harmonics, and a resistor R_o = 50 Ω serves as the load. Other experimental parameters are summarized in

Table 5. Parameters of the experimental prototype.

Parameter	Symbol	Value/Types
Input voltage	Vrms	55 V
Input frequency	fg	50 Hz
Input filter resistor	Ri	5 Ω
Input filter inductor	Li	380 µH
Input filter capacitor	Ci	10 µF
Output filter inductor	Lo	2.5 mH
Output filter capacitor	Co	33 µF
Switching frequency	fs	10 kHz
Modulation index	m	0.8
Output power	Po	200 W
Transistors	S11–S23	FGA25N120
Diodes	D11–D23	RHRG30120
Control circuit	DSP + FPGA	TMS320F28335/ CoreEP4CE6d

.

5.1. Voltage Stress

The comparative experimental waveforms between the standard six-switch CSR and the proposed CSR in this paper are shown in Figure 12. Both CSRs could obtain a sinusoidal input current and constant DC output voltage. This means that the proposed CSR has similar input and output characteristics to the conventional CSR. However, there would be a minor displacement angle between the input phase voltage and the current due to the existence of the input filter. This could be solved by a compensation angle that has been mentioned in most of the literature.

In Figure 13a, the voltage stress on all the transistors in the conventional CSR is about V_GE = 134 V, which is equal to the line-to-line voltage amplitude V_{L_im}. Additionally, the measured result of the proposed CSR is shown in Figure 13b. Although the transistor S₁₁ in the upper arm still withstands the line-to-line voltage amplitude, the transistor S₂₁ in the lower arm could withstand the voltage stress V_GE = 67 V. Hence, half of the transistors in the proposed CSR only need to withstand the voltage stress 1/2 V_{L_im}, which is consistent with the theoretical analysis.

5.2. Unity Power Factor Operation

The input filter could cause a minor displacement angle between the input voltage and the current. In order to operate at unity power factor, a compensation angle should be adopted in the CSR system. The corresponding experimental waveforms under unity power factor operation is shown in Figure 14a. With the compensation angle, the voltage stress has a slight increase in this case. However, the angle is too small in practice, so the voltage stress does not exceed the phase voltage amplitude. Moreover, the total harmonic distortion (THD) of the input current is 2.2%.

The THDs of two CSRs under different modulation indexes are given in Figure 14b. As can be seen, the THDs are quite close in the full modulation range and the values are low in most of the modulation range compared with 5.0% of IEC 61000-3-2. This means that the input performance of the proposed CSR is not affected and is similar to the conventional one. The same input filter could be used in both CSR systems, and the cost and volume of the proposed asymmetrical CSR do not need to change compared with the conventional symmetrical CSR.

5.3. Current Stress and Efficiency

Figure 15 shows the detailed experimental waveforms of the flowing current for S₂₁. Two zoomed areas would exist, i.e., Figure 15a for sector I and Figure 15b for sector IV. As can be seen, when the zero vector acts, the flowing current i_f is increased or decreased to nearly 1/3 of the DC output current i_o in different sectors. The current stress is significantly reduced in freewheeling mode. Due to the reduced current stress, another experimental phenomenon should be emphasized. It can be found that the amplitude of the output current i_o in Figure 13b is lower than the one in Figure 13a, so a small output filter could be used in the proposed CSR.

The measured efficiency of different CSR systems is illustrated in Figure 16. As expected, with the long freewheeling period, the proposed CSR has a higher efficiency at a low modulation index compared to the conventional CSR.

5.4. Temperature Rising

Figure 17 shows the measured temperature of transistor S₁₁ and S₂₁ in the proposed CSR system. Since the current of the proposed CSR could pass through the body diode of transistor S₁₁ in the upper arm, and the transistor S₂₁ in the lower arm withstands low voltage stress. Therefore, the temperature of transistor S₁₁ is a little higher than that of S₂₁, and the heat distribution is uneven for the proposed CSR system. However, all the transistors can be operated in a safe operating area even though the heat dissipating devices are not used in practice. According to the abovementioned experimental results, the system performance of the proposed CSR is not further affected by the inconsistent temperature rise compared to the conventional one.

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.