A Novel Three-Phase Six-Switch PFC Rectifier with Zero-Voltage-Switching and Zero-Current-Switching Features

A novel three-phase power-factor-correction (PFC) rectifier with zero-voltage-switching (ZVS) in six main switches and zero-current-switching (ZCS) in the auxiliary switch is proposed, analyzed, and experimentally verified. The main feature of the proposed auxiliary circuit is used to reduce the switching loss when the six main switches are turned on and the one auxiliary switch is turned off. In this paper, a detailed operating analysis of the proposed circuit is given. Modeling and analysis are verified by experimental results based on a three-phase 7 kW rectifier. The soft-switched PFC rectifier shows an improvement in efficiency of 2.25% compared to its hard-switched counterpart at 220 V under full load.


Introduction
Power electronic converters play a critical role in the energy industry due to their ability to optimally control and condition the power they deliver to a load.In addition, they are required to control and condition the power they draw from energy sources to support their optimal operation.This is achieved by compliance to EMI and harmonic standards such as EN6100-3-2 and efficiency standards such as 80Plus [1].Soft-switching technologies are a primary enabler for improving efficiency by minimizing switching losses and reducing EMI and harmonics by "soft" ending the edges of the switching transitions [2][3][4][5][6][7][8][9][10][11][12].References [8] and [9] report soft-switching techniques that includes zero-voltage-switching (ZVS) and zero-current-switching (ZCS).Three-phase rectifiers with active power-factor-correction (PFC) control achieve an improved power factor and lower harmonic content [10][11][12][13].Active PFC rectifiers using a boost (current source) front end achieve better input current wave-shaping and lower harmonic distortion compared to their buck-derived counterparts [14].Three single-phase PFC rectifiers are used in [15] to synthesize a three-phase PFC rectifier.Reference [16] reports the use of space vector modulation (SVM) to achieve a high power factor in a three-phase six-switch rectifier.Soft-switching techniques employed in three-phase rectifiers are reported in [17][18][19] to improve efficiency and EMI performance.Soft-switching using a passive lossless snubber is presented in [17].Although this approach can improve the efficiency, the circuit suffers from higher component stress.In [18], an active snubber is used to achieve soft-switching at the expense of higher control complexity and switching stress in the auxiliary switch.In [19][20][21][22], the zero-voltage-transition and control technique was applied in a three-phase PFC rectifier.Although the main switches can achieve ZVS at turn-on, the auxiliary switch was hard-switched operated at turn-off.
A conventional three-phase six-switch PFC rectifier is shown in Figure 1.A novel soft-switched three-phase active rectifier using an active auxiliary circuit is proposed in this paper.The principal performance improvement is the achievement of ZVS at turn-on for the six rectifier switches and ZCS at turn-off for the one auxiliary switch.A detailed description of the operation of the proposed soft-switched rectifier is presented in Section 2. Validation of the design through simulation and experimental results are shown in Section 3 followed by concluding remarks in Section 4.

2
A conventional three-phase six-switch PFC rectifier is shown in Figure 1.A novel soft-switched three-phase active rectifier using an active auxiliary circuit is proposed in this paper.The principal performance improvement is the achievement of ZVS at turn-on for the six rectifier switches and ZCS at turn-off for the one auxiliary switch.A detailed description of the operation of the proposed softswitched rectifier is presented in Section 2. Validation of the design through simulation and experimental results are shown in Section 3 followed by concluding remarks in Section 4.

Proposed Three-Phase Six-Switch Soft-switching PFC Rectifier
The proposed three-phase six-switch soft-switching PFC rectifier is shown in Figure 2. The circuit inside the dotted box is a soft-switching assist circuit to achieve ZVS in the main switches and ZCS in the auxiliary switch.The soft-switching assist circuit consists of the auxiliary switch SA, resonant inductor LR, transformer Tr, barrier diode DR1, clamp circuit RC-DC-CC, and resonant capacitor (the capacitance employs the parasitic capacitance of main switch).Three phase line voltages VRN, VSN, VTN for a balanced three-phase system are shown in Figure 3.The 60° symmetry in the three-phase voltages is evident from Figure 3.The operation of the threephase PFC using the 60° symmetry is described in detail in [11].

Proposed Three-Phase Six-Switch Soft-switching PFC Rectifier
The proposed three-phase six-switch soft-switching PFC rectifier is shown in Figure 2. The circuit inside the dotted box is a soft-switching assist circuit to achieve ZVS in the main switches and ZCS in the auxiliary switch.The soft-switching assist circuit consists of the auxiliary switch S A , resonant inductor L R , transformer T r , barrier diode D R1 , clamp circuit R C -D C -C C , and resonant capacitor (the capacitance employs the parasitic capacitance of main switch).

2
A conventional three-phase six-switch PFC rectifier is shown in Figure 1.A novel soft-switched three-phase active rectifier using an active auxiliary circuit is proposed in this paper.The principal performance improvement is the achievement of ZVS at turn-on for the six rectifier switches and ZCS at turn-off for the one auxiliary switch.A detailed description of the operation of the proposed softswitched rectifier is presented in Section 2. Validation of the design through simulation and experimental results are shown in Section 3 followed by concluding remarks in Section 4.

Proposed Three-Phase Six-Switch Soft-switching PFC Rectifier
The proposed three-phase six-switch soft-switching PFC rectifier is shown in Figure 2. The circuit inside the dotted box is a soft-switching assist circuit to achieve ZVS in the main switches and ZCS in the auxiliary switch.The soft-switching assist circuit consists of the auxiliary switch SA, resonant inductor LR, transformer Tr, barrier diode DR1, clamp circuit RC-DC-CC, and resonant capacitor (the capacitance employs the parasitic capacitance of main switch).Three phase line voltages VRN, VSN, VTN for a balanced three-phase system are shown in Figure 3.The 60° symmetry in the three-phase voltages is evident from Figure 3.The operation of the threephase PFC using the 60° symmetry is described in detail in [11].Three phase line voltages V RN , V SN , V TN for a balanced three-phase system are shown in Figure 3.The 60 • symmetry in the three-phase voltages is evident from Figure 3.The operation of the three-phase PFC using the 60 • symmetry is described in detail in [11].In order to simplify the analysis, Interval 1 (0°-60°) can be selected for the analysis of the switching cycles as the operation over the rectifier is identical in the other 60° segments.The following assumptions are made to support the operating analysis: (1) Input inductance LB is large enough to allow the input current to be considered as a current source over a switching period; (2) Input capacitance CL is large enough to be equivalent to the ideal voltage source VO; and (3) The output capacitance of the clamp circuit CC is large enough to allow its voltage VC to be considered a voltage source over a switching period.
Under the assumptions listed above, the simplified circuit diagram is shown in Figure 4 and the voltage polarity and current direction for each main component are defined.A detailed description of circuit operation is provided in this section.The key waveforms of the circuit for Interval 1 are shown in Figure 5, and equivalent circuits for each operating mode are shown in Figure 6.There are 12 operating modes to be analyzed over a switching cycle.

Mode 0: (t ≦ T0)
This mode is based on the analysis of the switching cycle in Interval 1 (VRN > 0, VTN > 0, and VSN < 0).Before T0, as in Figure 6a, the diode D1, D6, and D5 are in the state of conduction.The main switches S1 to S6 and auxiliary switch SA are turned off.The currents iR and iT flow through diode DB to the load and return to the AC source as the current iS.Under this condition, the voltage across the active rectifier bridge is VX = VO.In order to simplify the analysis, Interval 1 (0 • -60 • ) can be selected for the analysis of the switching cycles as the operation over the rectifier is identical in the other 60 • segments.The following assumptions are made to support the operating analysis: (1) Input inductance L B is large enough to allow the input current to be considered as a current source over a switching period; (2) Input capacitance C L is large enough to be equivalent to the ideal voltage source V O ; and (3) The output capacitance of the clamp circuit C C is large enough to allow its voltage V C to be considered a voltage source over a switching period.
Under the assumptions listed above, the simplified circuit diagram is shown in Figure 4 and the voltage polarity and current direction for each main component are defined.In order to simplify the analysis, Interval 1 (0°-60°) can be selected for the analysis of the switching cycles as the operation over the rectifier is identical in the other 60° segments.The following assumptions are made to support the operating analysis: (1) Input inductance LB is large enough to allow the input current to be considered as a current source over a switching period; (2) Input capacitance CL is large enough to be equivalent to the ideal voltage source VO; and (3) The output capacitance of the clamp circuit CC is large enough to allow its voltage VC to be considered a voltage source over a switching period.Under the assumptions listed above, the simplified circuit diagram is shown in Figure 4 and the voltage polarity and current direction for each main component are defined.A detailed description of circuit operation is provided in this section.The key waveforms of the circuit for Interval 1 are shown in Figure 5, and equivalent circuits for each operating mode are shown in Figure 6.There are 12 operating modes to be analyzed over a switching cycle.

Mode 0: (t ≦ T0)
This mode is based on the analysis of the switching cycle in Interval 1 (VRN > 0, VTN > 0, and VSN < 0).Before T0, as in Figure 6a, the diode D1, D6, and D5 are in the state of conduction.The main switches S1 to S6 and auxiliary switch SA are turned off.The currents iR and iT flow through diode DB to the load and return to the AC source as the current iS.Under this condition, the voltage across the active rectifier bridge is VX = VO.A detailed description of circuit operation is provided in this section.The key waveforms of the circuit for Interval 1 are shown in Figure 5, and equivalent circuits for each operating mode are shown in Figure 6.There are 12 operating modes to be analyzed over a switching cycle.

Mode 0: (t T 0 )
This mode is based on the analysis of the switching cycle in Interval 1 (V RN > 0, V TN > 0, and V SN < 0).Before T 0 , as in Figure 6a, the diode D 1 , D 6 , and D 5 are in the state of conduction.The main switches S 1 to S 6 and auxiliary switch S A are turned off.The currents i R and i T flow through diode D B to the load and return to the AC source as the current i S .Under this condition, the voltage across the active rectifier bridge is V X = V O .

Mode 1 (T0 < t ≦ T1)
At T0, the auxiliary switch SA is turned on to go into Mode 1.The current i1 of resonant inductor LR starts to increase, and current i1 flows through the primary winding N1 of the transformer Tr.The induced current i2 and excitation current im outflow through secondary coil N2, as shown in Figure 6b.The voltage on the seconding winding N2 is the output voltage VO.The voltage V1 and V2 across the windings of transformer Tr are obtained as follows: The current in the resonant inductor i1 increases linearly with the slope given by 1 1 (1 ) Similarly to i1, the excitation current im also displays a linear increase, and the slope is When the current i1 ascends to iS current, this mode ends.The time interval is given as below:

Mode 1 (T 0 < t T 1 )
At T 0 , the auxiliary switch S A is turned on to go into Mode 1.The current i 1 of resonant inductor L R starts to increase, and current i 1 flows through the primary winding N 1 of the transformer T r .The induced current i 2 and excitation current i m outflow through secondary coil N 2 , as shown in Figure 6b.The voltage on the seconding winding N 2 is the output voltage V O .The voltage V 1 and V 2 across the windings of transformer T r are obtained as follows: The current in the resonant inductor i 1 increases linearly with the slope given by Similarly to i 1 , the excitation current i m also displays a linear increase, and the slope is When the current i 1 ascends to i S current, this mode ends.The time interval is given as below:

Mode 2 (T1 < t ≦ T2)
At t ＝ T1, the DC link diode current ib reaches zero.The reverse recovery current of diode DB flows through diode DB in a negative direction.The resonant inductor current keeps increasing, as shown in Figure 6c.

Mode 3 (T2 < t ≦ T3)
At t ＝ T2, the parasitic capacitance of diode Db along with the resonant capacitor CR, which includes the parasitic capacitor of the main switches and the resonant inductor LR, start resonating, as shown in Figure 6d.When the equivalent voltage VX of the main switch is decreased to zero at t ＝ T3, the mode ends.The equivalent voltage VX and resonant current i1 are shown in Equations ( 6) and (7).

Mode 2 (T 1 < t T 2 )
At t = T 1 , the DC link diode current i b reaches zero.The reverse recovery current of diode D B flows through diode D B in a negative direction.The resonant inductor current keeps increasing, as shown in Figure 6c.

Mode 3 (T 2 < t T 3 )
At t = T 2 , the parasitic capacitance of diode D b along with the resonant capacitor C R , which includes the parasitic capacitor of the main switches and the resonant inductor L R , start resonating, as shown in Figure 6d.When the equivalent voltage V X of the main switch is decreased to zero at t = T 3 , the mode ends.The equivalent voltage V X and resonant current i 1 are shown in Equations ( 6) and (7).

Mode 4: (T 3 < t T 4 )
When t > T 3 , the bridge rectifier voltage V X is decreased to zero and the auxiliary switch S A continues to conduct.The corresponding equivalent circuit is shown in Figure 6e.The body diodes D 4 , D 3 , and D 2 of the main switches S 4 , S 3 , and S 2 are conducting.Turning on the main switches S 4 , S 6 , and S 2 when the bridge voltage reaches zero achieves ZVS turn-on.The detection circuitry to turn on the main switches at zero voltage also enables the minimization of duty-cycle loss and, thus, loss of efficiency.After the main switches turn on at ZVS, the resonant inductor current i 1 decreases linearly with the slope given by Equation (11).When the current of the main switches S 4 and S 2 reaches zero at t = T 4 , the mode ends. di 2.6.Mode 5: (T 4 < t T 5 ) As shown in Figure 6f, when t > T 4 , then S 4 , S 6 , and S 2 keep conducting.The current i l is continuously decreased to zero until t = T 5 .

Mode 6: (T 5 < t T 6 )
When t > T 5 , the input currents i R and i T flow through the main switches S 4 and S 2 , as shown in Figure 6g.When the current i a flows through the auxiliary switch S A , it consists mostly of the magnetizing current, i m , of the transformer.If the magnetizing inductance L m is designed to be relatively large, the current i a of the auxiliary switch S A is extremely close to zero.When T 5 < t T 6 , the auxiliary switch is set to be turned off so that it can effectively achieve the purpose of ZCS.

Mode 7: (T 6 < t T 7 )
When t = T 6 , the auxiliary switch S A is turned off, as shown in Figure 6h.Subsequently, the magnetizing current i m of the transformer charges the parasitic capacitance C oss1 of the auxiliary switch S A so that the auxiliary switch voltage will increase continuously.

Mode 8: (T 7 t T 8 )
At t = T 7 , the auxiliary switch voltage V SA increases to V O + V C and the clamp diode D C is conducting.The magnetizing current i m discharges through the clamp circuit D C -V C , as shown in Figure 6i.The slope of the excitation current in this mode is given by 2.10.Mode 10: (T 8 t T 9 ) At t = T 8 , the magnetizing current i m is decreased to zero which resets the transformer, as shown in Figure 6j.At t = T 9 , the main switch S 4 is turned off.The input current i R charges the parasitic capacitance of the main switch S 4 and the equivalent voltage V X of the main switch is increased, as shown in Figure 6k.

Mode 11: (T 10 t T 11 )
At t = T 10 , the equivalent voltage V X of the main switch is increased to V O and the diode D B is conducting as shown in Figure 6l.Subsequently, the antiparallel diode D 1 of the main switch S 1 is conducting.The input current i R flows through diodes D 1 and D B and flows back from i S through the load.

Mode 12: (T 11 t T 12 )
At t = T 11 , the main switch S 2 is turned off.The input current i T starts to charge the parasitic capacitance of the main switch S 2 , as shown in Figure 6m.

Experimental Verifications
Based on the design described, a prototype was built.When the line voltage in the three-phase input was 220 V, namely, the phase voltage was 127 V, the switching frequency 40 kHz, and the output load 7 kW, then the measured waveforms for three-phase V RN , V SN , V TN , line voltage and the line current were at low line and full load.The current waveform, as in Figure 7, is practically sinusoidal with low THD and a high power factor (the A-THD is shown in Figure 8 and power factor is shown in Figure 9).At t ＝ T8, the magnetizing current im is decreased to zero which resets the transformer, as shown in Figure 6j.

Mode 10: (T9 < t ≦ T10)
At t ＝ T9, the main switch S4 is turned off.The input current iR charges the parasitic capacitance of the main switch S4 and the equivalent voltage VX of the main switch is increased, as shown in Figure 6k.

Mode 11: (T10 ≦ t ≦ T11)
At t ＝ T10, the equivalent voltage VX of the main switch is increased to VO and the diode DB is conducting as shown in Figure 6l.Subsequently, the antiparallel diode D1 of the main switch S1 is conducting.The input current iR flows through diodes D1 and DB and flows back from iS through the load.

Mode 12: (T11 ≦ t ≦ T12)
At t ＝ T11, the main switch S2 is turned off.The input current iT starts to charge the parasitic capacitance of the main switch S2, as shown in Figure 6m.

Experimental Verifications
Based on the design described, a prototype was built.When the line voltage in the three-phase input was 220V, namely, the phase voltage was 127V, the switching frequency 40kHz, and the output load 7kW, then the measured waveforms for three-phase VRN, VSN, VTN , line voltage and the line current were at low line and full load.The current waveform, as in Figure 7, is practically sinusoidal with low THD and a high power factor (the A-THD is shown in Figure 8 and power factor is shown in Figure 9).Figure 10 shows the simulation waveforms using Isspice at half load and full load.It can be seen that the main switches S4 and S2 on the bottom sides turned on when their VX was down to zero by the resonant circuit, after the auxiliary switch SA was turned on.
Measured waveforms of the gate drive signals and voltage across the main switch are shown in Figure 11.The captured waveforms indicate the need to turn on the auxiliary switch SA before the turning on the main switches S4 and S2 to achieve ZVS.
As shown in Figure 12, when the current i1 of resonant inductor LR decreases to zero, the auxiliary switch SA can be turned off under ZCS conditions.As shown in Figure 12, when the current i1 of resonant inductor LR decreases to zero, the auxiliary switch SA can be turned off under ZCS conditions.Figure 10 shows the simulation waveforms using Isspice at half load and full load.It can be seen that the main switches S 4 and S 2 on the bottom sides turned on when their V X was down to zero by the resonant circuit, after the auxiliary switch S A was turned on.
Measured waveforms of the gate drive signals and voltage across the main switch are shown in Figure 11.The captured waveforms indicate the need to turn on the auxiliary switch S A before the turning on the main switches S 4 and S 2 to achieve ZVS.
As shown in Figure 12, when the current i 1 of resonant inductor L R decreases to zero, the auxiliary switch S A can be turned off under ZCS conditions.As shown in Figure 12, when the current i1 of resonant inductor LR decreases to zero, the auxiliary switch SA can be turned off under ZCS conditions.A comparison between the hard-switched and proposed soft-switched rectifier was performed at a load of 7kW over an input voltage range of 190-250V.The hard-switched rectifier was tested by simply disabling the soft-switch assist circuitry.Figure 13 shows the efficiency improvement from the soft-switched rectifier.The largest efficiency difference between hard-switch and soft-switch rectifier was 2.55% when the input line voltage was 220V, and Figure 14 shows the soft-switched rectifier efficiency from 1-7kW.A comparison between the hard-switched and proposed soft-switched rectifier was performed at a load of 7kW over an input voltage range of 190-250V.The hard-switched rectifier was tested by simply disabling the soft-switch assist circuitry.Figure 13 shows the efficiency improvement from the soft-switched rectifier.The largest efficiency difference between hard-switch and soft-switch rectifier was 2.55% when the input line voltage was 220V, and Figure 14 shows the soft-switched rectifier efficiency from 1-7kW.A comparison between the hard-switched and proposed soft-switched rectifier was performed at a load of 7 kW over an input voltage range of 190-250 V.The hard-switched rectifier was tested by simply disabling the soft-switch assist circuitry.Figure 13 shows the efficiency improvement from the soft-switched rectifier.The largest efficiency difference between hard-switch and soft-switch rectifier was 2.55% when the input line voltage was 220 V, and Figure 14 shows the soft-switched rectifier efficiency from 1-7 kW.

Conclusions
A novel three-phase rectifier with zero-voltage-switching and zero-current-switching features was proposed.The design has been validated with simulation and experimental data captured on a 7kW three-phase rectifier prototype.Efficiency improvement between hard-switch and soft-switch rectifiers peaks at 2.55% when the input line voltage is 220V at full load.Mathematical equations to explain circuit operation have been derived and analyzed under a sequence of operating modes.The experimental results have confirmed the proposed design of the soft-switched rectifier in achieving high efficiency, high power factor, and low THD.

Figure 2 .
Figure 2. The circuit of the proposed soft-switching PFC rectifier.

Figure 2 .
Figure 2. The circuit of the proposed soft-switching PFC rectifier.

Figure 2 .
Figure 2. The circuit of the proposed soft-switching PFC rectifier.

Figure 3 .
Figure 3.The line cycle in three-phase balance power system.

Figure 4 .
Figure 4.The simplified circuit of the proposed soft-switched rectifier.

Figure 3 .
Figure 3.The line cycle in three-phase balance power system.

Figure 3 .
Figure 3.The line cycle in three-phase balance power system.

Figure 4 .
Figure 4.The simplified circuit of the proposed soft-switched rectifier.

Figure 4 .
Figure 4.The simplified circuit of the proposed soft-switched rectifier.

Figure 5 .
Figure 5.The key waveforms of the proposed soft-switched rectifier.

Figure 5 .
Figure 5.The key waveforms of the proposed soft-switched rectifier.

Figure 6 .
Figure 6.Operation modes of the proposed soft-switched rectifier.

Figure 6 .
Figure 6.Operation modes of the proposed soft-switched rectifier.

Figure 7 .
Figure 7. Measured waveforms of input voltage and input current at full load.

Figure 7 .
Figure 7. Measured waveforms of input voltage and input current at full load.

Figure 10 .
Figure 10.Simulation of key waveforms of the main switch voltage VX and current il at (a) half load and (b) full load.

Figure 10
Figure10shows the simulation waveforms using Isspice at half load and full load.It can be seen that the main switches S4 and S2 on the bottom sides turned on when their VX was down to zero by the resonant circuit, after the auxiliary switch SA was turned on.Measured waveforms of the gate drive signals and voltage across the main switch are shown in Figure11.The captured waveforms indicate the need to turn on the auxiliary switch SA before the turning on the main switches S4 and S2 to achieve ZVS.As shown in Figure12, when the current i1 of resonant inductor LR decreases to zero, the auxiliary switch SA can be turned off under ZCS conditions.

Figure 10 .
Figure 10.Simulation of key waveforms of the main switch voltage VX and current il at (a) half load and (b) full load.

Figure 10
Figure10shows the simulation waveforms using Isspice at half load and full load.It can be seen that the main switches S4 and S2 on the bottom sides turned on when their VX was down to zero by the resonant circuit, after the auxiliary switch SA was turned on.Measured waveforms of the gate drive signals and voltage across the main switch are shown in Figure11.The captured waveforms indicate the need to turn on the auxiliary switch SA before the turning on the main switches S4 and S2 to achieve ZVS.As shown in Figure12, when the current i1 of resonant inductor LR decreases to zero, the auxiliary switch SA can be turned off under ZCS conditions.

Figure 10 .
Figure 10.Simulation of key waveforms of the main switch voltage VX and current il at (a) half load and (b) full load.

Figure 10 . 9 Figure 11 .
Figure 10.Simulation of key waveforms of the main switch voltage V X and current i l at (a) half load and (b) full load.

Figure 12 .
Figure 12.Measured waveforms of drive signal and resonant current for the main switch of the softswitched rectifier.

Figure 11 . 9 Figure 11 .
Figure 11.Measured waveforms of drive signal and voltage for the main switch of the soft-switched rectifier.

Figure 12 .
Figure 12.Measured waveforms of drive signal and resonant current for the main switch of the softswitched rectifier.

Figure 12 .
Figure 12.Measured waveforms of drive signal and resonant current for the main switch of the soft-switched rectifier.

Figure 13 .
Figure 13.The efficiency comparison between hard-switching and soft-switching rectifier at 7 kW load.

Figure 14 .
Figure 14.Measured efficiency from 1 to 7 kW with the soft-switched rectifier.