High-voltage Step-up Ratio Single-phase Three-wire Inverter Based on Interleaved Boost Converter

A high-voltage step-up ratio inverter for applications using batteries as the input power source is proposed to supply the emergency power requirement from a battery pack when the grid is down. The proposed inverter is composed of an interleaved coupled-inductor boost converter and a three-leg full-bridge inverter. Four series-connected lead acid batteries are used as the input power source. When 110 and 220 V AC outputs are required by the load sides at the same time, the 48 V DC input from the batteries would be boosted up to 350 V DC for the three-leg inverter to provide AC output power. However, if only 110 V AC is required, the proposed control strategy would reduce the step-up ratio for a lower DC link voltage of 175 V DC to enhance the conversion efficiency. Moreover, to minimize damage to the batteries, the interleaved circuit topology is adopted to reduce the input current ripple. A prototype with 400 W rated output power is finally constructed. The controller is implemented with a low-cost microcontroller, HT66F50. From the experimental results, it is seen that even under unbalanced load conditions, the output AC voltage is well controlled. The total efficiency of the proposed inverter with 110/220 V AC output is 89.8%, and the efficiency of the inverter with only 110 V AC output is 91.1%.


Introduction
To avoid power supply interruption in a remote area owing to natural disasters such as earthquake, typhoon, and debris flow, the emergency power supply system has attracted much attention in recent years. In the emergency power supply system, AC power of 110 and 220 V should be available for common electrical appliances and instruments. Gasoline/diesel engine generators or renewable energy sources are usually integrated into the power supply system. A battery pack is also required in the system for energy storage and buffer. In the most common system configuration, a 48 V battery pack with four series-connected batteries is used as the energy storage module. However, a DC input power with about 156 or 312 V is required for the conventional single-phase inverter to provide the 110 or 220 V AC output power, respectively. (1)(2)(3)(4) Therefore, a step-up DC converter would be required to boost the 48 V DC power up to at least 156 or 312 V. Compared with the conventional boost converter, the coupled-inductor boost converter could more easily provide a high-voltage step-up ratio. (5)(6)(7)(8)(9)(10) However, the resulting current ripple in the low-voltage side would become higher than that in conventional topology, which would also result in additional power losses.
In this study, a high-voltage step-up ratio single-phase three-wire inverter is developed. A common 48 V battery pack is used as the input source in the proposed inverter. A two-phase coupled-inductor boost converter and a six-switch inverter are integrated into the proposed topology. The interleaved coupled-inductor boost converter would firstly convert the 48 V DC power into 175 or 350 V DC power for the six-switch inverter to properly provide 110 and 220 V AC output power for the AC side load requirement. By adopting the interleaved circuit topology, the input current ripple can therefore be reduced. Moreover, the conduction power losses can also be reduced owing to the power sharing in the two-phase topology. (11)(12)(13)(14) As to the inverter, the sinusoidal pulse width modulation (PWM) mode is used to control the output AC voltage with 110 and 220 V rms. According to the loading demand, the output power can switch into only 110 or 110/220 V modes. A low-cost microcontroller unit, HT66F50, is used to realize the digital controller. The corresponding voltage and current signals are fed back to the microcontroller unit to control the DC link voltage as well as the output AC voltages.

Circuit Topology and Operation Principles
The configuration of the proposed inverter is shown in Fig. 1. A two-phase coupled-inductor boost converter and a six-switch inverter are integrated into the proposed inverter. By using the coupled inductor, the voltage step-up ratio of the boost converter can be enhanced. Moreover, the conversion efficiency can be enhanced by sharing the power with the adopted two-phase interleaved circuit topology. The 48 V DC voltage of the battery pack would therefore easily be boosted up to the required voltage level for the six-switch inverter. The high-side and lowside switches in one arm of the six-switch inverter are driven by a set of complementary PWM signals. For a single-phase three-wire AC output application, the output terminal V shown in Fig. 1 is controlled as the neutral line. The line voltages for loads Z 1 and Z 2 are regulated to AC 110 V rms by adjusting the modulation index of the PWM gating signals of the switches S UH , S WH , S UL , and S WL .
The interleaved coupled-inductor boost converter is composed of two parallel-connected coupled-inductor boost converters as shown in Fig. 1. A set of gating signals with 180 degrees phase-shifted shown in Fig. 2 are used to control the two switches S 1 and S 2 . The duty ratio of switches S 1 and S 2 are d 1 and d 2 , respectively. It can be seen that the phase of the inductor current i L1 is also 180 degrees shifted with respect to that of the inductor current i L2 . The output current of the battery pack noted as the total input current i in is the summation of the two inductor currents. Therefore, the current ripple of the battery pack output current would be reduced. As shown in Fig. 2, the i L1m and i L2m are the magnetizing currents of the coupled inductors L 1 and L 2 , respectively. A lower current ripple would minimize damage to the battery pack as well.
The corresponding operation modes of the interleaved DC converter are shown in Fig. 3 and the operation principle in each mode is also described as follows.
• Mode 1 (t 0 ≤ t < t 1 ): Both switches S 1 and S 2 are turned on. The stored energy in the inductors L 1 and L 2 are increased by the input DC power source. The energy stored in the DC link capacitor C 0 is released to the second-stage inverter. • Mode 2 (t 1 ≤ t < t 2 ): The switch S 1 is still turned on in this mode and the switch S 2 is turned off. The diode D 2 is then turned on for releasing the energy from the inductor L 2 to the DC link. The equivalent model of the coupled inductor is shown in Fig. 4. It is seen that the   L2' : In this mode, the switch S 2 would be turned on again. Therefore, the operation principle of this mode is similar to that of mode 1. • Mode 4 (t 3 ≤ t < t 4 ): The switch S 1 is turned off in this mode, then diode D 1 would be turned on to release the energy from the inductor L 1 to the DC link. The currents in the coupled inductor L 1 are similar to those in the coupled inductor L 2 in mode 2. The current of D 1 would be 1 To ensure that the converter is operated under continuous conduction mode, the secondary side current of each coupled inductor should not be reduced to zero. As shown in Fig. 5, the design criteria of the inductor can then be derived as Eqs. (1) to (4). Moreover, the peak value of each phase input current can also be derived as Eq. (5).
As to the six-switch inverter, there are two operation modes proposed in this study according to the AC side load requirement. While the AC 220 V rms output power is required, the sixswitch inverter would be operated in mode A. Otherwise, when only 110 V rms is demanded at the load terminals, the six-switch inverter would be operated in mode B to reduce the DC link voltage. Hence, the voltage step-ratio requirement of the interleaved coupled-inductor boost converter can be reduced to enhance the conversion efficiency. (15) The PWM profiles for the proposed two operation modes are shown in Fig. 6 and the corresponding principles are described as follows.

• Mode A:
In this mode, the DC link voltage is boosted up to 350 V for the six-switch inverter to generate 220 and 110 V rms AC voltage at the load sides. The phase V is controlled to be the neutral line of the single-phase three-wire 110/220 V AC output application. The two legs (S UH , S UL and S WH , S WL ) are controlled by a set of sinusoidal PWM signals with a phase difference of 180 degrees as shown in Fig. 6(a). It is seen that the control signal of S WH is 180 degrees lagged with respect to that of S UH . Two switches on the same leg are switched by a set of complementary gating signals.

• Mode B:
In this mode, there is only 110 V AC voltage required at the load sides. The switching patterns for the arms are shown in Fig. 6(b). The phase V controlled by switches S VH and S VL is no longer controlled to be the neutral line. The switches S UH and S WH are controlled by the same PWM signal, and the control signal of S VH is 180 degrees lagged with respect to that of S UH and S WH in this mode. Therefore, there would be two 110 V AC outputs of V UV and V VW with 180 degrees phase difference. As a result, the DC link voltage is only boosted up to 175 V.

Controller Diagram of Proposed Inverter
There are two control objectives in operating the proposed inverter. Firstly, according to the load demand at the AC output terminals, the DC link voltage would be regulated to 350 or 175 V with respect to the operation mode A or mode B. The DC link voltage and two inductor currents of the two-phase boost converter are sensed and fed back to the microcontroller unit as shown in Fig. 7. Secondly, the sinusoidal PWM signals in the corresponding operation modes illustrated in Sect. 2 would be generated by the microcontroller unit to control the six switches in the three-arm inverter. The corresponding control block diagrams of mode A and mode B are shown in Figs. 8 and 9, respectively.
As shown in Fig. 7, the DC link voltage of the capacitor C 0 is denoted as V C0 and the inductor currents of inductors L 1 and L 2 are denoted as I L1 and I L2 , respectively. The command of the total input current, namely, I L_command , is obtained from the PI controller in the voltage control loop. The command of the DC link voltage V C0 * is set to 350 V for operation mode A or 175 V for mode B. Then, the current command for each inductor current in the two-phase converter is equal to the command I L_command for sharing the power flow. As a result, the PWM signals for switches S 1 and S 2 would be generated accordingly. For the control block diagram for operation mode A as shown in Fig. 8 , the line voltage V UV for load Z 1 would be 110 V rms and 180 degrees phase-shifted with respect to the line voltage V VW for load Z 2 . Therefore, the line voltage V UW for load Z 3 would be 220 V rms, which is the summation of V UV and V VW . It can be seen that the phase difference between the modulation indexes of the PWM signals for switches S UH and S WH is 180 degrees. The PWM signals of switches S UL and S WL are complementary to that of switches S UH and S WH , respectively. The    peak values of the two AC output voltages are fed back and denoted as V UV_ fb and V VW_ fb . Then, the corresponding modulation index required for the amplitude control of each AC output voltage would be calculated using the proportion-integration (PI) controller. In this mode, the output terminal V should be the controller to be the neutral node. Hence, a set of complementary PWM signals with 50% duty ratio are used to control switches S VH and S VL . Figure 9 shows the control block diagram of mode B. The modulation indexes for switches S UH and S WH are in phase to each other. A sinusoidal PWM signal with 180 degrees phase lag is adopted to control the switch S VH in this mode. As a result, only two 110 V rms voltages would be provided at the output terminals and the required DC link voltage would be twice lower than that in mode A. The step-up ratio of the converter can therefore be reduced to enhance the conversion efficiency. Figure 10 shows the constructed prototype of the proposed high-voltage step-up ratio singlephase three-wire inverter. The corresponding circuit parameters of the constructed hardware are shown in Table 1. Figure 11 shows the measured waveforms of the control signals of

PWMS2
5V/Div iL1 5A/Div iL2 5A/Div switches S 1 and S 2 and the inductor currents i L1 and i L2 . The control signals of switches S 1 and S 2 are interleaved by 180 degrees. The total input current from the battery tank is shown in Fig.  12. It can be seen that the ripple on the input current is greatly reduced. The input voltage 48 V DC is boosted up to 350 V while providing 400 W power to the DC link, where I o is the output current to the DC link. The reduced current ripple on the battery output current would also reduce the loss caused by the equivalent resistance in the battery and prevent additional damage to the battery.
The efficiency comparison between the adopted two-phase interleaved boost converter and the conventional single-phase boost converter with 400 W rated output power is shown in Fig.  13. In the operation under light to half load, the efficiency of the interleaved converter is about 3% higher. As to the heavy load condition, the interleaved topology can provide even higher efficiency of about 4%, which is because of the higher conduction losses in the single-phase boost converter. Figure 14 shows the output waveforms while the inverter is operated under mode A as mentioned in the previous section. The inverter provided 50 W to load Z 1 and 350 W to load Z 2 . It seems that there is no difference between the two output voltages on the loads Z 1 and Z 2 , i.e., V UV and V VW . The results of the inverter operated under mode B are shown in Fig. 15. The total output is 400 W, which is 200 W for load Z 1 and 200 W for load Z 2 . The total efficiency of the proposed inverter is shown in Fig. 16. It can be seen that the efficiency of the proposed topology would be better because the proposed controller can change the operation mode into Time(20μs/div)

Conclusions
In this study, a high-voltage step-up ratio single-phase three-wire inverter is proposed for providing emergency power from a battery pack. By adopting the coupled inductor, the voltage step-up ratio was increased for boosting the battery pack terminal voltage to the required level. Furthermore, an additional control strategy for the inverter with only 110 V AC loading demand is proposed. In this operation mode, the voltage step-up ratio would be only half of that in the normal operation mode with 110 and 220 V output power. The lower voltage step-up ratio would therefore increase the conversion efficiency. From the experimental results, the proposed inverter can provide good efficiency higher than 90% and up to about 91.6% while only 110 V AC is required at the load side. The efficiency can also remain up to about 89.8% while the 220 V AC is required. The experiments with unbalanced load between two output AC ports are also carried out to verify that the proposed inverter can still provide stable 110 and 220 V AC output power.