Design of a Modiﬁed-Bridge Circuit with a Master-Slave Input Supply Mechanism for Ozone-Driven System Applications

: This study proposes a design of a modiﬁed-bridge circuit with a master–slave input supply mechanism for ozone-driven system applications. Because the single-source supply design is becoming the mainstream choice in the existing ozone-driven systems, the input supply reliability of the ozone-driven system is crucial. Therefore, this proposed design involves a modiﬁed-bridge circuit combined with inductors and transistors, which can be augmented with the energy storage device as a backup source to improve the reliability of the input supply for the ozone-driven system. In addition, considering that the original source directly connected to DC BUS can re-charge the energy storage device, the energy recycling operation mode is designed in this proposed system to extend the duration of the energy storage device, which improves the supply reliability of the ozone-driven system further. To validate this proposed system, both model formulation and hardware realization are assessed through different test scenarios. Experimental outcomes of these tests conﬁrm the practicality of the proposed design.


Introduction
Ozone technologies have seen wide application in various fields, such as agriculture, sterilization [1][2][3], water purification [4][5][6], portable air cleaners [7,8], and surface cleaning [9,10]. In ozone technology applications, signals with both high-voltage and high-frequency features must be generated to achieve proper electrical discharge in the ozone chip. Therefore, a suitable driving circuit or system circuit design is crucial. In some studies, advanced components with a high-frequency capacity have been used to build the ozone load driving resonant circuit [11][12][13][14]. In addition, resonant circuit designs such as LCC type or hybrid type have been adopted, due to the high voltage-gain property, to generate a high-voltage signal in a high-frequency range [15][16][17][18]. The existing ozone-driven systems are most often designed with a single input port. If a fault occurs in the power source during operation, the ozone-driven system must stop working and wait for power recovery, which significantly affects the ozone quality. Adopting designs using a single power source inevitably impairs the operational reliability of the ozone-driven system in the above application fields. To improve the reliability of the input source, connecting directly the energy storage devices to the ozone-driven system may seem to be a quick fix. However, there are issues that remain to be resolved to achieve the voltage balance between the energy storage device and the input power source.
To solve the voltage balance problem, energy storage devices are often integrated with the main input power for better power quality. Therefore, we believe that adopting a dual-input port structure in the construction of ozone-driven systems will provide a viable solution. In [19], Saleh et al. connected the multi-level converter to the energy storage devices to drive the DBD plasma load adopting multi-level switching architecture and using multiple DC converters, which provide multiple input ports for connecting energy storage devices to overcome the problem resulting from the single power source. In addition, some studies have proposed a multi-input converter with a special control strategy to extend the input amount when connecting to energy storage devices. For example, the integration of energy storage devices and supercapacitors provides the energy for reducing the numbers of DC converters through a novel multiple-input converter [20]. Furthermore, a triple-input converter is designed for three kinds of power sources, adopting an interleaved full-bridge converter that integrates each power source to improve the supply reliability [21]. The above studies have proposed novel architectures to integrate two or more power sources and achieve encouraging results. Next, because the energy storage device is adopted as another source, the re-charging function of the energy storage device is also essential. Therefore, some studies have discussed bi-directional power transmission. For instance, an isolated converter with a bi-directional power transmission is proposed for a two-power-source input and power control in [22]. In addition, these studies have discussed the interleaved buck/boost converter with zero-voltage switching control for some bi-directional converter designs [23,24]. Furthermore, to obtain a high voltage gain with high efficiency to drive the ozone chip, some resonant circuits have been proposed [15][16][17][18]. Such a circuit consists of magnetic inductors to reach the resonance while satisfying the requirement of soft switching. Yet, the characteristics of this resonant circuit were easily affected under the bi-directional power transmission since its architecture was asymmetrically connected. Therefore, some studies have adopted the symmetrical resonant circuit for the bi-directional power transmission to achieve the stabilization of the bi-directional power transmission [25][26][27]. The aforementioned methods reveal that encouraging results were observed, yet each of them came with some inherent demerits in the ozone-driven applications. Moreover, considering that the ozone-driven system is usually applied to a cleansing function, the reliability of the ozone-driven system will affect directly the cleansing quality. Ozone-driven systems, which have been applied widely, are costly. Therefore, substituting the novel ozone-driven equipment for the traditional one is a difficult mission. As mentioned above, considering the replacement issues, the design of the extra-installing circuit is an excellent choice, which lets the original ozone-driven system continuously provide a stable voltage by increasing the reliability of the power supply in order to maintain a stable ozone generator. To clearly understand the relationship between each aforementioned reference and the corresponding study subject, the features of the above references are as listed in Table 1.
In this paper, a modified-bridge circuit is proposed for the ozone-driven system, which modifies a bridge circuit with inductors isolated to provide dual-input ports. Here, the master source and the slave source are connected to different input ports to independently provide the energy while maintaining the source status of each port. In addition, a symmetrical resonant tank is adopted and designed for the bi-directional power transmission, letting the resonant property of the bi-directional power transmission be similar. Once the original source is connected directly to DC BUS, the energy can be re-charged to the slave source through the proposed system. Thus, delivering or re-charging the energy storage device becomes much easier by the proposed system and the discharging time of the energy-storage device is also improved. Moreover, through the voltage of each port being regulated by the controller, different operation modes can be selected by simple frequency modulation. The features of this proposed system are listed as follows: (1) The proposed modified-bridge circuit accomplished in this paper emphasizes the benefits of using two input power sources that are supplying coordinately to improve the supplied reliability and reduce the component cost for the ozone-driven system. (2) The proposed system can be extra-installed in the original ozone-driven system, which means that it is not necessary to replace the original one, keeping the ozone quality and reducing the replacement cost.
Electronics 2022, 11, 254 3 of 23 (3) The resonant circuit design is of a symmetrical type, which achieves a similar resonant property under the bi-directional power transmission. Difficult control and a complicated algorithm are unnecessary to enhance the stability of the proposed system. (4) The voltage status of each port can be detected through the control system operation mode setting by judging whether the energy is being provided through DC BUS or re-charged to the slave source (energy storage device).
The rest of this article is organized as follows: Section 2 introduces operations of the proposed system, Section 3 discusses the parameter design and analysis of the symmetrical resonant circuit, Section 4 describes the control mechanism analysis, Section 5 reports the experimental results, and Section 6 presents the conclusion. Table 1. References of feature analysis and the corresponding subjects.

No.
Features Reference [11] An accurate analytical converter-level model is introduced. The model evaluates and optimizes the switching solution of a SiC MOSFET for the pulsed power converter.
SiC MOSFET application of the ozone-driven circuit design for increasing the component reliability [12] A high-power high-frequency ICP drive system that achieves high efficiency and durability is proposed. A 10 kW 4 MHz switching inverter and a novel control method are proposed for ICP with a high Q factor and highly variable impedance. The proposed phase lead sensing circuit can always guarantee ZVS operation of a high-power high-frequency switching inverter. Through the proposed system, ICP with a much higher plasma density can be safely driven. [13] An optimized version of a gate-boosting driver circuit for SiC MOSFETs for ultrafast switching is proposed. A MOSFET turn-on time of below 2 ns is achievable at a high operating voltage and moderate current and below 3 ns for a wider range of load-current and operating voltage conditions. [14] A high-voltage SiC MOSFETs module can serve as the main switch in a repetitive high-voltage nanosecond pulse generator. A series-connected MOSFETs module with only a single external gate driver requiring few components is suitable for a compact assembly. A 10 kV SiC MOSFETs module with a turn-on transition time~10 ns is developed. A compact and high-voltage pulse generator composed of three 10 kV SiC MOSFETs module is tailored, with a typical rise time of~40 ns and peak a voltage of~30 kV. [15] A 2.0 kW high-voltage rep-rate charging power supply based on a non-isolated inductor-capacitor-capacitor (LCC)-type resonant converter operating in continuous conduction mode is proposed. It provides a constant charging current with high control accuracy/efficiency. The design of the proposed capacitor charging power supply (CCPS) is a low-cost, simpler, compact, and efficient driver for the miniature pulsed plasma sources.
Different resonant architecture analysis for acquiring a higher voltage gain [16] A hybrid LLC resonant converter design is proposed. There are three modes of operation: As a full-bridge converter, as a dual-phase half-bridge LLC converter, or as a single-phase half-bridge LLC converter. Using GaN devices with a power density of 65 W/in 3 has been designed. [17] A comparative half-bridge LLC resonant converter using the integrated LCLC resonant transformer is proposed.
To reduce the demanded leakage inductance, a resonant capacitor can be connected in parallel to the magnetizing inductor through auxiliary winding as an LCLC resonant transformer. An integrated LCLC resonant transformer with lower leakage inductance for efficiency comparison is implemented.

No.
Features Reference [18] The light-load efficiency of the LLC resonant converters is an improvement. The simplified optimal trajectory control (SOTC) for burst mode can solve this issue by using an optimal switching pattern for burst-on time.
The digital implementation of SOTC for burst mode and its limitation in a high-frequency LLC converter are discussed. [19] Multistage and multilevel switch-mode converters are employed to construct the power supply. The proposed MSML-PEC power supply uses boost dc-dc PECs without high-frequency transformers. The removal of high-frequency transformers has improved the average efficiency and reduced the size of each dc-dc PEC. Modular, lightweight, and compact size power supply has been achieved.
Multiple input design for adding the input port to increase the supply reliability [20] A novel multi-input converter (MIC) is proposed in this article for interfacing a battery-SC combination. The SC converter is mostly rated for the full surge power. The battery is operated in current control mode under normal loading conditions. The proposed system balances the flow of power between the load and the HESS. [21] Modeling and two controller design techniques for a triple active bridge (TAB) three-port dc-dc converter comprising the fuel cell, the battery, and the load are proposed.
A new decoupling matrix-based proportional-integral controller design method reduces the design complexity and improves the system dynamic performance in comparison to similar three-port converters reported in the paper.

[22]
A multiple-input configuration of an isolated bidirectional dual-active bridge dc-dc converter (MIBDC) is proposed for power flow control in a combinational battery storage.
The proposed system can be operated in an independent source or a combinational source mode of operation to control the power transfer. It has the capability of bidirectional power flow and smoother transition. The proposed configuration can also be used with unequal voltage level sources by connecting them in series on the multi-input side. [23] A ZVS interleaved buck/boost bidirectional converter using the active-clamp technique is introduced in this paper. Applying a simple auxiliary circuit comprised of an auxiliary switch, an auxiliary inductor, and a clamp capacitor, ZVS condition is achieved by the converter operating duty cycle or load condition. The converter can operate with the conventional fixed-frequency pulse-width modulation (PWM) control.
Bidirectional converters and symmetrical resonant circuit discussion for delivering and re-charging of energy regulation [24] An interleaved bi-directional buck-boost is proposed for achieving zero-voltage switching. Applying a coupled inductor with a variable coupling coefficient achieves a wide input-output voltage operating range. The variable coupled inductor significantly improves the soft-switching range and reduces the circulating energy.The gallium nitride interleaved bi-directional buck-boost converter is applied in this paper. [25] An integrated half-bridge CLLC (IHBCLLC) resonant bidirectional dc-dc converter is proposed in various applications, including energy-storage systems. A half-bridge CLLC resonant circuit and a buck/boost circuit are integrated. The proposed converter obtains the high voltage gain by pulse-width modulation.
The CLLC resonant circuit can help MOSFETs to achieve soft switching and high voltage gain.

No.
Features Reference [26] The total power loss optimization and the magnetic design of the CLLC resonant converter based on the artificial intelligence (AI) algorithm are proposed in this paper. The parameters of the magnetic component and the resonant component are derived by the AI algorithm. The parameters of the transformer are optimized by the AI algorithm. A planar transformer is used for CLLC designing in this paper. [27] The CLLC bidirectional resonant converter is discussed in this paper. Application fields are battery chargers and dc microgrids. Asymmetric parameters methodology (APM) can design gain curves for charging and discharging modes separately. Figure 1a shows the application illustration of the proposed system in this paper. As shown in this application illustration, the two parts are the original system and the extra-installed system. In the original system, the energy of the ozone-driven system is directly supplied by the original supply through DC BUS [28,29]. It is noted that the output voltage of the original supply fits the voltage of DC BUS. To avoid ozone-driven system malfunction when a fault occurs in the original supply, the proposed system is connected to DC BUS. Moreover, both sources are included in the proposed system, the low-voltage PFC source [30][31][32][33] and energy storage devices. As shown in Figure 1, when the main supply delivers the power to DC BUS, the main supply not only provides the energy for the ozonedriven system through DC BUS but also charges the energy storage device simultaneously. When the main supply fails, the energy of DC BUS will be supplied by the proposed system. The master and the slave sources can both provide the energy. If the master source is normal, it can provide the energy for DC BUS and re-charge the energy storage devices through the proposed system at the same time. If the master source provides insufficient energy and causes abnormality, the energy storage devices will provide independently the energy for DC BUS through the proposed system to regulate the energy of each source and maintain the input voltage for the ozone-driven system. Figure 1b shows the extra-installed system illustration of the proposed system. As shown in this block diagram, the proposed system feeds the energy to the original ozone-driven system through DC BUS. When the energy is provided by the master source or the slave source, the voltage signal of each port transfers to the feedback circuit, the controller, and the isolated gate driver to deliver suitable driving signals of the power transistors of the power stage and allow the output voltage of the proposed system to be stabilized. In Figure 1b, the voltage feedback of the ozone load transfers to the original ozone-driven system for stabilizing the ozone generation exciting and the proposed system does not intervene in the ozone control, which means that the proposed system can be applied to any existing ozone-driven system. The application compatibility of the proposed system is higher than any solutions.

System and Circuit Description
The architecture of the proposed system is shown in Figure 2. There are five parts: a master-slave input supply circuit, a symmetrical resonant tank, a full-bridge circuit, a control circuit, and an ozone generator. The proposed system uses a master-slave input supply circuit formed by six power transistors (Q 1 -Q 6 ), two inductors (L 1 and L 2 ), and two diodes (D 1 and D 2 ). D 1 and D 2 provide current paths when L 1 and L 2 release the energy in order to avoid a higher voltage spike across the two inductors, which will occur when instantaneous current variation without current freewheeling path emerges. Furthermore, two input ports are designed to let two power sources be applied to this proposed system. The inductors are applied to isolate and store the energy. To smoothly release the energy of the inductors through any path and avoid the energy being delivered to the master source, Q 5 and Q 6 are located upside down and different as a conventional full-bridge circuit. The proposed system also includes a symmetrical resonant tank composed of two resonant inductors (L r1 and L r2 ), two resonant capacitors (C r1 and C r2 ), and a step-up transformer (T). In addition, the proposed system possesses a full-bridge circuit consisting of power transistors (Q 7 -Q 10 ) and an output capacitor (C D ). The proposed system not only flexibly delivers the energy to DC BUS for the ozone generator but also opportunely recovers the energy from DC BUS. First, the master power supply (Master source) and the energy storage device (Slave source) are connected to the master-slave input supply circuit. Both the output of the full-bridge circuit and the ozone generator are connected to DC BUS. When the original source connects directly to DC BUS and provides the ozone generator and the proposed system with the energy, the frequency of Q 7 -Q 10 will be controlled to convert the energy of DC BUS to the voltage at high frequency, which is transferred through the symmetrical resonant tank and the transformer, and the high-frequency voltage signal is rectified and stabilized by the body diodes of Q 1 -Q 4 and capacitor C s , respectively. Finally, the energy will be charged to the energy storage device (the slave source). When the original source fails, the energy of the ozone generator must be provided through the proposed system. The master source (V m ) delivers the energy through the masterslave input supply circuit with the high-frequency square AC voltage being converted by controlling the frequency of Q 1 -Q 6 . This high-frequency square AC voltage passes through the symmetrical resonant tank and the transformer and is rectified and stabilized by the body diodes of Q 7 -Q 10 and capacitor C D , respectively. Next, the energy of C D is transferred to the ozone generator through DC BUS. In addition, to ensure that the energy storage device (the slave source) maintains high voltage levels for full charging, the master source provides the energy to the energy storage device by controlling the frequency of Q 1 -Q 6 . If a fault occurs in the master source, the energy will be provided by the slave source (V s ) by controlling the frequency of Q 1 -Q 6 to convert the energy of the slave source to the high-frequency square AC voltage, which was transferred through the symmetrical resonant tank and the transformer and is rectified and stabilized by the body diodes of Q 7 -Q 10 and the capacitor C D , respectively. Subsequently, the energy of C D is transferred to the ozone generator through DC BUS.

System and Circuit Description
The architecture of the proposed system is shown in Figure 2. There are five parts: a master-slave input supply circuit, a symmetrical resonant tank, a full-bridge circuit, a control circuit, and an ozone generator. The proposed system uses a master-slave input

Proposed System
Feedback circuit Controller

Operation Mode and Circuit Analysis
Since this proposed system can be applied in three operations, the circuit operations must be analyzed for each operation mode. Figures 3-5 depict the circuit operation for three operations, and Figure 6 shows their corresponding waveforms. First, the circuit operations will be analyzed for the master source supply operation, as shown in Figures  3 and 6a. The operation is described as follows: (1) Mode I (t0-t1): As shown in Figure 3a, Q2 and Q6 are turned off and Q4 and Q5 are turned on, achieving zero-voltage switching in this mode; the energy of the master source is stored in inductor L2 by turning on Q4 and releasing the energy of inductor L1, which is stored in the energy storage device and Cs by turning on Q5 and the diode of Q1. Therefore, the currents of L1 and L2 (iL1 and iL2) linearly decrease and increase, respectively. At this time, since the resonant voltage vr1 of the primary side is a positive square wave, the resonant current ir1 of the primary side flowing through Q1 and Q4 is positive and the resonant current ir2 of the secondary side flows through the body diode of Q7 and Q10 to transfer the energy to DC BUS with output capacitor CD. It is noted that the current im of the master source has low ripples because iL1 and iL2 are interleaved to store and release energy, which can contribute to mitigating the

Ozone generator
Ozone-driven circuit  To improve the input supply reliability of the ozone generator, the master-slave input supply circuit is adopted. However, when two sources are applied to provide the energy for DC BUS, the voltage of each source must be considered. Therefore, the inductors of the master-slave input supply circuit (L 1 and L 2 ) are applied to regulate the energy of each source in this study. Here, to design these two inductors, the duty cycles of all transistors are 50% and the boundary between continuous and discontinuous inductor current I Li-min must be 0. L 1 and L 2 are as follows: where I Li is the average current of the inductor; D is the duty cycle of the power transistors; which is set up at 0.5; and T s is the time period. From Equation (3), the minimum inductor for continuous current can be written as below: From Equation (4), since the duty cycles of all transistors are about 50%, the voltage and the current of the storage device are only taken into consideration in the inductor design.
If the current of the inductor is designed in the continuous mode for lower charging power of the storage device, the larger inductor must be designed; otherwise, the smaller inductor is enough in this proposed system. Table 2 shows the circuit specification of the proposed system.

Operation Mode and Circuit Analysis
Since this proposed system can be applied in three operations, the circuit operations must be analyzed for each operation mode. Figures 3-5 depict the circuit operation for three operations, and Figure 6 shows their corresponding waveforms. First, the circuit operations will be analyzed for the master source supply operation, as shown in Figures 3 and 6a. The operation is described as follows: (1) Mode I (t 0 -t 1 ): As shown in Figure 3a, Q 2 and Q 6 are turned off and Q 4 and Q 5 are turned on, achieving zero-voltage switching in this mode; the energy of the master source is stored in inductor L 2 by turning on Q 4 and releasing the energy of inductor L 1 , which is stored in the energy storage device and C s by turning on Q 5 and the diode of Q 1 . Therefore, the currents of L 1 and L 2 (i L1 and i L2 ) linearly decrease and increase, respectively. At this time, since the resonant voltage v r1 of the primary side is a positive square wave, the resonant current i r1 of the primary side flowing through Q 1 and Q 4 is positive and the resonant current i r2 of the secondary side flows through the body diode of Q 7 and Q 10 to transfer the energy to DC BUS with output capacitor C D . It is noted that the current i m of the master source has low ripples because i L1 and i L2 are interleaved to store and release energy, which can contribute to mitigating the burden of the master source supply. This operation enters the next mode until Q 4 and Q 5 are turned off. (2) Mode II (t 1 -t 2 ): In this mode, all power transistors Q 1 -Q 6 are turned off, as shown in Figure 3b. The energy of L 2 begins to be released and i L2 linearly decreases; thus, the energy of L 2 is stored in the energy storage device and C s through the body diode of Q 3 and i r1 is freewheeling through the body diodes of Q 2 and Q 3 at the same time.
When the voltage of the parasitic capacitor of Q 2 and Q 3 drops to zero, the body diodes of Q 2 and Q 3 provide a path for current freewheeling. Here, i L1 still decreases and i L2 begins to decrease. When Q 2 , Q 3 , and Q 6 are turned on, this operation enters the next mode. (3) Mode III (t 2 -t 3 ): As shown in Figure 3c, Q 2 and Q 6 are turned on and achieve zerovoltage switching in this mode. The energy of inductor L 2 is charged to the energy storage device and C s by turning on Q 6 and the body diodes of Q 3 , and the energy of the master source is stored in inductor L 1 by turning on Q 2 . Therefore, i L1 and i L2 linearly increase and decrease, respectively. Since the resonant voltage v r1 of the primary side becomes a negative square wave in this time, the current i r1 becomes negative and flows through Q 2 and Q 3 . i r2 also becomes negative and flows through the body diode of Q 8 and Q 9 and transfers the energy to DC BUS with output capacitor C D . When Q 2 and Q 6 are turned off, this operation enters the next mode. (4) Mode IV (t 3 -t 4 ): In this mode, all power transistors Q 1 -Q 6 are turned off again, as shown in Figure 3d; the energy of L 1 begins to be released; and i L1 linearly decreases. Thus, the energy of L 1 is stored in the energy storage device and C s through the body diode of Q 1 . Meanwhile, i r1 is freewheeling through the body diodes of Q 1 and Q 4 . When the voltage of the parasitic capacitor of Q 2 and Q 3 drops to zero, the body diodes of Q 1 and Q 4 provide a path for current freewheeling. Here, i L1 begins to (4) Mode IV (t3-t4): In this mode, all power transistors Q1-Q6 are turned off again, as shown in Figure 4d. When the voltage of parasitic capacitors of Q1 and Q4 drops to zero, the body diodes of Q1 and Q4 provide a path for current freewheeling. Therefore, ir1 is freewheeling through body diodes of Q1 and Q4 at this time. In addition, because no load can consume the surplus energies of L1 and L2, both iL1 and iL2 linearly decrease slowly through D1 and D2. When Q1 and Q4 are turned on again and the circuit returns to Mode I, the aforementioned modes of operation continue. (4) Mode IV (t3-t4): In this mode, all power transistors Q1-Q6 are turned off again, as shown in Figure 4d. When the voltage of parasitic capacitors of Q1 and Q4 drops to zero, the body diodes of Q1 and Q4 provide a path for current freewheeling. Therefore, ir1 is freewheeling through body diodes of Q1 and Q4 at this time. In addition, because no load can consume the surplus energies of L1 and L2, both iL1 and iL2 linearly decrease slowly through D1 and D2. When Q1 and Q4 are turned on again and the circuit returns to Mode I, the aforementioned modes of operation continue.  When the original source directly connects and provides the energy for DC BUS, the energy will be recovered from DC BUS to the energy storage device through the proposed system. Finally, the circuit operations will be analyzed for and the energy is transferred from DC BUS, as shown in Figures 5 and 6c. Therefore, this operation is described as follows: (1) Mode I (t0-t1): As shown in Figure 5a, because the voltage of Q7 and Q10 drops to zero in the last mode, Q7 and Q10 are turned on and achieve the zero-voltage switching in   When the master source cannot normally provide the energy, the slave source provides the energy for the proposed system. Next, the circuit operations will be analyzed for the slave source supply operation, as shown in Figures 4 and 6b, with this operation described as follows: (1) Mode I (t 0 -t 1 ): As shown in Figure 4a, Q 2 and Q 3 are turned off, Q 1 and Q 4 are turned on, and the zero-voltage switching of Q 1 and Q 4 is achieved in this mode. Because Q 5 and Q 6 are always turned off in this mode, if there are surplus energies in these inductors (L 1 and L 2 ), i L1 and i L2 must be freewheeling through D 1 and D 2 until both i L1 and i L2 linearly decrease to zero. Since v r1 is a positive square wave, i r1 is also positive and flows through Q 1 and Q 4 . The current i r2 flows through the body diode of Q 7 and Q 10 , and the energy is transferred to DC BUS with output capacitor C D . This operation enters the next mode when Q 1 and Q 4 are turned off. (2) Mode II (t 1 -t 2 ): As shown in Figure 4b, all power transistors Q 1 -Q 6 are turned off.
When the voltage of the parasitic capacitor of Q 2 and Q 3 drops to zero, the body diodes of Q 2 and Q 3 provide a path for current freewheeling. i r1 is still positive and freewheeling through the body diodes of Q 2 and Q 3 . i L1 and i L2 are still freewheeling through D 1 and D 2 if there are surplus energies in L 1 and L 2 . When Q 2 and Q 3 are turned on, this operation enters the next mode. (3) Mode III (t 2 -t 3 ): As shown in Figure 4c, because the voltage of Q 2 and Q 3 drops to zero in the last mode, Q 2 and Q 3 are turned on in this mode and zero-voltage switching of Q 2 and Q 3 is achieved. Since v r1 becomes a negative square wave at this time, i r1 becomes negative and flows through Q 2 and Q 3 ; i r2 also becomes negative and flows through the body diode of Q 8 and Q 9 and transfers the energy to DC BUS with output capacitor C D . Because no load can consume the surplus energies of L 1 and L 2 , both i L1 and i L2 linearly decrease slowly through D 1 and D 2 . When Q 2 and Q 3 are turned off, this operation enters the next mode. (4) Mode IV (t 3 -t 4 ): In this mode, all power transistors Q 1 -Q 6 are turned off again, as shown in Figure 4d. When the voltage of parasitic capacitors of Q 1 and Q 4 drops to zero, the body diodes of Q 1 and Q 4 provide a path for current freewheeling. Therefore, i r1 is freewheeling through body diodes of Q 1 and Q 4 at this time. In addition, because no load can consume the surplus energies of L 1 and L 2 , both i L1 and i L2 linearly decrease slowly through D 1 and D 2 . When Q 1 and Q 4 are turned on again and the circuit returns to Mode I, the aforementioned modes of operation continue.
When the original source directly connects and provides the energy for DC BUS, the energy will be recovered from DC BUS to the energy storage device through the proposed system. Finally, the circuit operations will be analyzed for the energy is transferred from DC BUS, as shown in Figures 5 and 6c. Therefore, this operation is described as follows: (1) Mode I (t 0 -t 1 ): As shown in Figure 5a, because the voltage of Q 7 and Q 10 drops to zero in the last mode, Q 7 and Q 10 are turned on and achieve the zero-voltage switching in this mode by turning off Q 8 and Q 9 . The resonant voltage v r2 is a positive square wave because this voltage is converted by the switching of Q 7 and Q 10 . The resonant current i r2 is positive and flowing through Q 7 and Q 10 , and the resonant current i r1 that is induced by transformer T is also positive and flows through the body diode of Q 1 and Q 4 to transfer the energy to the energy storage device with capacitor C s . This operation enters the next mode until Q 7 and Q 10 are turned off. (2) Mode II (t 1 -t 2 ): As shown in Figure 5b, all power transistors Q 7 -Q 10 are turned off because the voltage of the parasitic capacitor of Q 8 and Q 9 drops to zero. i r1 is freewheeling through the body diode of Q 8 and Q 9 . When Q 8 and Q 9 are turned on, this operation enters the next mode. (3) Mode III (t 2 -t 3 ): As shown in Figure 5c, because the voltage of Q 8 and Q 9 drops to zero in the previous mode, Q 8 and Q 9 are turned on in this mode and the zero-voltage switching of Q 8 and Q 9 is achieved. The resonant voltage v r2 becomes a negative square wave because this voltage is converted by switching Q 8 and Q 9 . The resonant current i r2 is positive and flows through Q 8 and Q 9 , and the resonant current i r1 that is induced by transformer T is also positive and flows through the body diode of Q 2 and Q 3 to transfer the energy to the energy storage device with capacitor C s . This operation enters the next mode until Q 8 and Q 9 are turned off. (4) Mode IV (t 3 -t 4 ): In this mode, all power transistors Q 7 -Q 10 are turned off, as shown in Figure 5d, because the voltage of the parasitic capacitor of Q 7 and Q 10 drops to zero. i r1 is freewheeling through the body diode of Q 7 and Q 10 . When Q 7 and Q 10 are turned on again, the circuit returns to Mode I for the aforementioned cycle to continue.

Symmetrical Resonant Circuit Analysis
To enhance the transmission efficiency, the resonant circuit and the transformer are usually applied to the power conversion. However, resonant characteristics are easily affected because the power transmission is not unidirectional. The symmetrical design of the resonant tank must be taken into consideration for reducing the variation in resonant characteristics in the different power transmission directions. The symmetrical resonant tank analysis and transformer design are described in detail in this section. Figure 7 depicts the configuration of the symmetrical resonant tank, where L r1 and L r2 are the resonant inductors, C r1 and C r2 are the resonant capacitors, T is the transformer, and L m is the magnetic inductor of the transformer. v r1 is the resonant voltage of the primary side, which is generated by the front stage circuit. v r2 is the resonant voltage of the secondary side, which is generated by the symmetrical resonant tank and input into the rear circuit. R L is the equivalent load of DC BUS and V o is the output voltage after rectification and stabilization by the rear stage circuit such as V DC BUS of Figure 2. To select the resonant capacitor, the operation frequency f s must be defined first, and the resonant inductor can be determined from Equation (5), as follows:

Operation Mode and Circuit Analysis
Due to the symmetrical structure of this resonant tank, the resonant parameters of both sides can be determined from Equation (5). To ensure the symmetrical configuration of the resonant tank, the resonant parameters of the primary side L r1 and C r1 are determined first and the resonant parameters of the secondary side L r2 and C r2 are calculated by using Equation (6), as follows: where n is the turn ratio of the transformer. When all resonant parameters are selected, the resonant characteristics must be analyzed for the different operations. The process will be explored in the next section.

Symmetrical Resonant Circuit Analysis
To enhance the transmission efficiency, the resonant circuit and the transformer are usually applied to the power conversion. However, resonant characteristics are easily affected because the power transmission is not unidirectional. The symmetrical design of the resonant tank must be taken into consideration for reducing the variation in resonant characteristics in the different power transmission directions. The symmetrical resonant tank analysis and transformer design are described in detail in this section. and Lm is the magnetic inductor of the transformer. vr1 is the resonant voltage of the primary side, which is generated by the front stage circuit. vr2 is the resonant voltage of the secondary side, which is generated by the symmetrical resonant tank and input into the rear circuit. RL is the equivalent load of DC BUS and Vo is the output voltage after rectification and stabilization by the rear stage circuit such as VDC BUS of Figure 2. To select the resonant capacitor, the operation frequency fs must be defined first, and the resonant inductor can be determined from Equation (5), as follows:

Operation Mode and Circuit Analysis
Due to the symmetrical structure of this resonant tank, the resonant parameters of both sides can be determined from Equation (5). To ensure the symmetrical configuration of the resonant tank, the resonant parameters of the primary side Lr1 and Cr1 are determined first and the resonant parameters of the secondary side Lr2 and Cr2 are calculated by using Equation (6), as follows: where n is the turn ratio of the transformer. When all resonant parameters are selected, the resonant characteristics must be analyzed for the different operations. The process will be explored in the next section.

The Energy Is Provided by the Master or the Slave Source
Next, the analysis of voltage gain and impedance phase is described when the energy is provided from the master source or the slave source, as shown in Figure 8. As shown in Figure 8b, the equivalent circuit of Figure 8a, to consider the variation of RL, the impedance of the transformer is mapped to the primary side of the transformer, where Lr2/n 2 , n 2 Cr2, and 8RL/n 2 π 2 are the resonant parameters that mapped to the primary side from the secondary side, and these resonant parameters are in parallel connection with magnetic inductor Lm. This resonant equivalent impedance Zp_1 is calculated as

The Energy Is Provided by the Master or the Slave Source
Next, the analysis of voltage gain and impedance phase is described when the energy is provided from the master source or the slave source, as shown in Figure 8. As shown in Figure 8b, the equivalent circuit of Figure 8a, to consider the variation of R L , the impedance of the transformer is mapped to the primary side of the transformer, where L r2 /n 2 , n 2 C r2 , and 8R L /n 2 π 2 are the resonant parameters that mapped to the primary side from the secondary side, and these resonant parameters are in parallel connection with magnetic inductor L m . This resonant equivalent impedance Z p_1 is calculated as Electronics 2022, 11, x FOR PEER REVIEW 14 of 25 π ω ω π ω ω ω π ω π ω π − − + = − − + + Based on Equations (8) and (9), Figure 9 shows the curves of voltage gain and input impedance under different load conditions. When load variation occurs, the system frequency is adjusted to maintain the voltage gain, as shown in Figure 9a. Since the voltage gain is larger than unity, the operating frequency must be adjusted to meet the requirements of the system output. Then, as depicted in Figure 9b, where the phase of input impedance is larger than zero on the right-hand side of resonant frequency, it means that the condition of zero-voltage switching is achieved. Therefore, the operating frequency will be larger than the resonant frequency (fn > 1).  Consequently, the input impedance Z in_1 and the voltage gain G v1 (jω) are derived below.
Based on Equations (8) and (9), Figure 9 shows the curves of voltage gain and input impedance under different load conditions. When load variation occurs, the system frequency is adjusted to maintain the voltage gain, as shown in Figure 9a. Since the voltage gain is larger than unity, the operating frequency must be adjusted to meet the requirements of the system output. Then, as depicted in Figure 9b, where the phase of input impedance is larger than zero on the right-hand side of resonant frequency, it means that the condition of zero-voltage switching is achieved. Therefore, the operating frequency will be larger than the resonant frequency (f n > 1).

The Energy Is Provided from DC BUS
In this section, the study discusses the scenario in which the energy is provided by DC BUS. As seen from Figure 10a, which presents the equivalent circuit of energy reversal, the residual load power of DC BUS is first converted to an AC squared signal and then delivered to the resonant circuit. As shown in Figure 10b, to consider the variation in R s , the impedance of the transformer is mapped to the primary side of the transformer, where C r1 /n 2 , n 2 L r1 , and 8R s /n 2 π 2 are the resonant parameters that mapped to the secondary side from the primary side, and these resonant parameters are in parallel connection with magnetic inductor L m . This resonant equivalent impedance Z p_2 is calculated as Z p_2 (jω) = −n 4 ω 2 L r1 C r1 + n 4 8jωC r1 R s + 1 n 2 πjωL m −n 4 ω 2 πL r1 C r1 − n 4 ω 2 πL m C r1 + 8n 4 jωC r1 R s + π

The Energy Is Provided from DC BUS
In this section, the study discusses the scenario in which the energy is provided by DC BUS. As seen from Figure 10a, which presents the equivalent circuit of energy reversal, the residual load power of DC BUS is first converted to an AC squared signal and then delivered to the resonant circuit. As shown in Figure 10b, to consider the variation in Rs, the impedance of the transformer is mapped to the primary side of the transformer, where Cr1/n 2 , n 2 Lr1, and 8Rs/n 2 π 2 are the resonant parameters that mapped to the secondary side from the primary side, and these resonant parameters are in parallel connection with magnetic inductor Lm. This resonant equivalent impedance Zp_2 is calculated as ( )

The Energy Is Provided from DC BUS
In this section, the study discusses the scenario in which the energy is provided by DC BUS. As seen from Figure 10a, which presents the equivalent circuit of energy reversal, the residual load power of DC BUS is first converted to an AC squared signal and then delivered to the resonant circuit. As shown in Figure 10b, to consider the variation in Rs, the impedance of the transformer is mapped to the primary side of the transformer, where Cr1/n 2 , n 2 Lr1, and 8Rs/n 2 π 2 are the resonant parameters that mapped to the secondary side from the primary side, and these resonant parameters are in parallel connection with magnetic inductor Lm. This resonant equivalent impedance Zp_2 is calculated as ( ) Given Equation (10), the transfer function of voltage gain Gv2(jω) and input impedance Zin_2(jω) when the energy is provided from DC BUS are then derived as follows:  Given Equation (10), the transfer function of voltage gain G v2 (jω) and input impedance Z in_2 (jω) when the energy is provided from DC BUS are then derived as follows: Z in_2 (jω) = −n 4 πjω 3 C r1 C r2 (L r1 + L m ) + (jωC r2 ) π + 8n 4 jωC r1 R s −1 × π + ω 4 n 4 πC r1 C r2 L m L r2 + n 2 L m L r1 + L r1 L r2 −ω 2 πC r2 (L r2 + L m ) + n 4 πC r1 (L r1 + L m ) +8jωC r1 C r2 R s n 6 L m + L r2 + 8n 4 jωC r1 R s (11) |G v2 (jω)| = |V s | |v r2 (jω)| = 2jωL m R s n 2 jωL m + 8R s n 2 π + n −2 jωC r1 + n 2 jωL r1 Next, Figure 11 shows the simulation results of transfer gain G v2 (jω) and input impedance Z in_2 (jω) under load variations. In Figure 11a, since the voltage gain is lower than 1, this means that the voltage steps down in this operation mode. Figure 11b shows the curve of the phase angle for input impedance. To achieve zero-voltage switching and considering that the intended resonant property requires inductance, the operating frequency must be larger than the resonant frequency (f n > 1). Let the phase angle of impedance be higher than 0 degrees. When the parameters of the resonant tank are selected based on the above consideration, the parameters of the resonant tank of the proposed system are shown in Table 3.
1, this means that the voltage steps down in this operation mode. Figure 11b shows the curve of the phase angle for input impedance. To achieve zero-voltage switching and considering that the intended resonant property requires inductance, the operating frequency must be larger than the resonant frequency (fn > 1). Let the phase angle of impedance be higher than 0 degrees. When the parameters of the resonant tank are selected based on the above consideration, the parameters of the resonant tank of the proposed system are shown in Table 3.   If Vm is larger than or equal to Vm-min, it means that the energy of the master source is sufficient. Yet, if VDC BUS is still lower than VDC BUS-ref, it implies that the energy is insufficient for the ozone-driven system. Next, the controller   If V m is larger than or equal to V m-min , it means that the energy of the master source is sufficient. Yet, if V DC BUS is still lower than V DC BUS-ref , it implies that the energy is insufficient for the ozone-driven system. Next, the controller decreases frequencies of Q 2 , Q 4 , Q 5 , and Q 6 (f Q2 , f Q4 , f Q5 , and f Q6 ) to raise the voltage gain and detects repeatedly V DC BUS until V DC BUS is larger than or equal to V DC BUS-ref . Subsequently, the frequencies of Q 2 , Q 4 , Q 5 , and Q 6 (f Q2 , f Q4 , f Q5 , and f Q6 ) are increased to reduce the voltage gain. If V DC BUS is lower than or equal to V DC BUS-ref , the process is back to detecting V m . Conversely, the operation mode is changed. This operation mode is called the master source supply mode in the flowchart in Figure 12.

Control Mechanism Analysis
Following the above operation, if V m is smaller than V m-min , it means that the energy of the master source is insufficient and the voltage of the slave source V s must be detected at this time. If V s is larger than or equal to V s-min , it means that the energy of the slave source still needs to be replenished because V DC BUS is still lower than V DC BUS-ref . As a result, the controller decreases the frequencies of Q 1 -Q 4 (f Q1 -f Q4 ) to raise the voltage gain and then V DC BUS is detected repeatedly until V DC BUS is larger than or equal to V DC BUS-ref .
The controller then increases the frequencies of Q 1 -Q 4 (f Q1 -f Q4 ) to reduce the voltage gain and detects V m again. If V m is still lower than V m-min , the same operation will continue. Conversely, the operation mode is back to the master source supply mode. However, if V s is also smaller than V s-min , it means that the master source and the slave source will both fail to provide the energy and then the process goes to stop all operations. This operation mode is named the slave supply mode, as shown in Figure 12. decreases frequencies of Q2, Q4, Q5, and Q6 (fQ2, fQ4, fQ5, and fQ6) to raise the voltage gain and detects repeatedly VDC BUS until VDC BUS is larger than or equal to VDC BUS-ref. Subsequently,  the frequencies of Q2, Q4, Q5, and Q6 (fQ2, fQ4, fQ5, and fQ6) are increased to reduce the voltage gain. If VDC BUS is lower than or equal to VDC BUS-ref, the process is back to detecting Vm. Conversely, the operation mode is changed. This operation mode is called the master source supply mode in the flowchart in Figure 12. Following the above operation, if Vm is smaller than Vm-min, it means that the energy of the master source is insufficient and the voltage of the slave source Vs must be detected at this time. If Vs is larger than or equal to Vs-min, it means that the energy of the slave source still needs to be replenished because VDC BUS is still lower than VDC BUS-ref. As a result, the controller decreases the frequencies of Q1-Q4 (fQ1-fQ4) to raise the voltage gain and then VDC BUS is detected repeatedly until VDC BUS is larger than or equal to VDC BUS-ref. The controller then increases the frequencies of Q1-Q4 (fQ1-fQ4) to reduce the voltage gain and detects Vm again. If Vm is still lower than Vm-min, the same operation will continue. Conversely, the operation mode is back to the master source supply mode. However, if Vs is also smaller than Vs-min, it means that the master source and the slave source will both fail to provide the energy and then the process goes to stop all operations. This operation mode is named the slave supply mode, as shown in Figure 12.
Finally, when the controller begins with the acquisition of VDC BUS from the feedback circuit and VDC BUS is larger than VDC BUS-ref, it implies that the energy of DC BUS is generated by the original source that connects directly to DC BUS. Then, the controller detects Vs. If Vs is less than Vs-max, this means that the energy can be charged to the slave source (energy storage device). Next, the controller decreases frequencies of Q7-Q10 (fQ7-fQ10) to raise the voltage gain and detects Vs repeatedly until Vs is larger than or equal to Vs-max. The controller increases the frequencies of Q7-Q10 (fQ7-fQ10) to reduce the voltage gain and detect VDC Finally, when the controller begins with the acquisition of V DC BUS from the feedback circuit and V DC BUS is larger than V DC BUS-ref , it implies that the energy of DC BUS is generated by the original source that connects directly to DC BUS. Then, the controller detects V s . If V s is less than V s-max , this means that the energy can be charged to the slave source (energy storage device). Next, the controller decreases frequencies of Q 7 -Q 10 (f Q7 -f Q10 ) to raise the voltage gain and detects V s repeatedly until V s is larger than or equal to V s-max . The controller increases the frequencies of Q 7 -Q 10 (f Q7 -f Q10 ) to reduce the voltage gain and detect V DC BUS again. If V DC BUS is lower than or equal to V DC BUS-ref , the process is back to detecting V m . Conversely, the operation mode processes at the same mode. This operation mode is called the DC BUS supply mode in the flowchart in Figure 12.

Experimental Results
This section validates the feasibility and performance of a modified-bridge circuit with a master-slave input supply mechanism. Figure 13 shows the photograph of the hardware circuit. This proposed system can be separated into two parts. The first is the main circuit, which consists of a master-slave input supply circuit, a symmetrical resonant tank, a full-bridge circuit, a controller (MCU), and two auxiliary powers. The other is the ozone generator, which includes an ozone load, a driven circuit, an auxiliary power, and a controller. It is noted that the terminal connectors for the DC BUS of the main circuit and the ozone generator are connected, allowing the energy to be exchanged between them.
To validate the capability of the modified-bridge circuit with a master-slave input supply mechanism, the voltages of the master power supply (Vm), the voltages of the energy storage device (Vs), the voltage and current of the resonant tank (vr and ir), and the output status (Vo and Io) are discussed under different scenarios. Experimental results and corresponding waveforms are also analyzed below.

Input and Output Status
Here, the input and output validations start with different modes of testing, as shown in Figure 14. When the operation mode is the master source supply, it implies that the energy is provided solely from the master source. In Figure 14a, the voltage of the master source (Vm) is about 48 V and the voltage of the slave source (Vs) is about 30 V, which means that the voltage of the master source is ready to provide the energy and the voltage of the slave source is insufficient. In this operation mode, the voltage and the current of DC BUS (VDC BUS and IDC BUS) are about 400 V and 2 A, respectively. According to this waveform in Figure 14a, when the consumption of DC BUS reaches 2 A, the master source not only provides independently the energy for the ozone generator and stabilizes VDC BUS but also charges the slave source through the proposed system. When the operation mode is the slave source supply, it means that the energy is provided from the slave source. In Figure 14b, Vm is about 30 V and Vs is about 50 V, which means that the master source cannot provide sufficient energy and the slave source must substitute for the master source. In this operation mode, when IDC BUS is about 2 A and VDC BUS is still maintained at To validate the capability of the modified-bridge circuit with a master-slave input supply mechanism, the voltages of the master power supply (V m ), the voltages of the energy storage device (V s ), the voltage and current of the resonant tank (v r and i r ), and the output status (V o and I o ) are discussed under different scenarios. Experimental results and corresponding waveforms are also analyzed below.

Input and Output Status
Here, the input and output validations start with different modes of testing, as shown in Figure 14. When the operation mode is the master source supply, it implies that the energy is provided solely from the master source. In Figure 14a, the voltage of the master source (V m ) is about 48 V and the voltage of the slave source (V s ) is about 30 V, which means that the voltage of the master source is ready to provide the energy and the voltage of the slave source is insufficient. In this operation mode, the voltage and the current of DC BUS (V DC BUS and I DC BUS ) are about 400 V and 2 A, respectively. According to this waveform in Figure 14a, when the consumption of DC BUS reaches 2 A, the master source not only provides independently the energy for the ozone generator and stabilizes V DC BUS but also charges the slave source through the proposed system. When the operation mode is the slave source supply, it means that the energy is provided from the slave source. In Figure 14b, V m is about 30 V and V s is about 50 V, which means that the master source cannot provide sufficient energy and the slave source must substitute for the master source. In this operation mode, when I DC BUS is about 2 A and V DC BUS is still maintained at about 400 V, it means that the slave source provides independently the energy for the ozone generator and it can stabilize V DC BUS through the proposed system. Operating in the DC BUS supply mode implies that the original source directly connects to DC BUS to supply the energy. It is noted that the slave source is an energy storage device, so the energy from DC BUS charges the slave source. In Figure 14c, V DC BUS and I DC BUS are about 420 V and 2 A, respectively, and the waveform of I DC BUS is negative, which means the energy is delivered from DC BUS because the original source directly connects to DC BUS. In addition, when V DC BUS is higher than 400 V, the energy can be provided from DC BUS to the slave source through the proposed system. In this waveform of Figure 14c, V s is about 40 V and V m is 0 V, which means that the energy from DC BUS is only charged to the slave source and the master source cannot be charged, because the slave source is an energy storage device and the master source is a general power supply. ozone generator and it can stabilize VDC BUS through the proposed system. Operating in the DC BUS supply mode implies that the original source directly connects to DC BUS to supply the energy. It is noted that the slave source is an energy storage device, so the energy from DC BUS charges the slave source. In Figure 14c, VDC BUS and IDC BUS are about 420 V and 2 A, respectively, and the waveform of IDC BUS is negative, which means the energy is delivered from DC BUS because the original source directly connects to DC BUS. In addition, when VDC BUS is higher than 400 V, the energy can be provided from DC BUS to the slave source through the proposed system. In this waveform of Figure 14c, Vs is about 40 V and Vm is 0 V, which means that the energy from DC BUS is only charged to the slave source and the master source cannot be charged, because the slave source is an energy storage device and the master source is a general power supply.

Resonant Property Verification
Here, the resonant property with the voltage and the current of the resonant circuit is verified under different operation modes, as shown in Figure 15. According to Figure  15a, the proposed system operates in the master source supply mode. The input voltage of the resonant circuit (vr1) can be observed. vr1 is seen to lead the input and the output current (ir1 and ir2) with a phase difference of θm, which means that the resonant property is exhibiting the inductive characteristics to help in the achievement of zero-voltage switching (ZVS). In Figure 15b, the proposed system is in the slave source supply mode and vr1 still leads ir1 and ir2 with a phase difference of θs, which implies that the resonant property is exhibiting the inductive characteristics to help achieve ZVS. It is noted that though the front of the proposed system is an integrating circuit design, the power transistors of the front of the proposed system can still achieve the condition of ZVS. In Figure  15c, the DC BUS supply mode is operated in the proposed system and vr2 also leads ir1 and

Resonant Property Verification
Here, the resonant property with the voltage and the current of the resonant circuit is verified under different operation modes, as shown in Figure 15. According to Figure 15a, the proposed system operates in the master source supply mode. The input voltage of the resonant circuit (v r1 ) can be observed. v r1 is seen to lead the input and the output current (i r1 and i r2 ) with a phase difference of θ m , which means that the resonant property is exhibiting the inductive characteristics to help in the achievement of zero-voltage switching (ZVS). In Figure 15b, the proposed system is in the slave source supply mode and v r1 still leads i r1 and i r2 with a phase difference of θ s , which implies that the resonant property is exhibiting the inductive characteristics to help achieve ZVS. It is noted that though the front of the proposed system is an integrating circuit design, the power transistors of the front of the proposed system can still achieve the condition of ZVS. In Figure 15c, the DC BUS supply mode is operated in the proposed system and v r2 also leads i r1 and i r2 with a phase difference of θ D , which means that the inductive characteristic is exhibited in this resonant circuit even if the energy is provided from DC BUS for the output of the resonant circuit, which helps to achieve ZVS. Through the resonant property verification, the test results of the resonant property can meet the design of the resonant circuit, which can achieve the inductive property to help achieve ZVS irrespective of the supply mode under which the proposed system is operating.
ir2 with a phase difference of θD, which means that the inductive characteristic is exhibited in this resonant circuit even if the energy is provided from DC BUS for the output of the resonant circuit, which helps to achieve ZVS. Through the resonant property verification, the test results of the resonant property can meet the design of the resonant circuit, which can achieve the inductive property to help achieve ZVS irrespective of the supply mode under which the proposed system is operating.

Zero-Voltage Switching Verification
In this section, zero-voltage switching (ZVS) achievement is verified in the test results. Please refer to Figure 16 for an explanation. Here, the voltage and the current for the power transistors of the proposed system must be observed while power transistors turn on. In Figure 16a, when the operation mode is the master source supply, negative currents (ids2 and ids5) flow through the body diodes of Q2 and Q5 before the voltages (vds2 and vds5) become zero. When vds2 and vds5 become zero because Q2 and Q5 are turned on, ids2 and ids5 flow through Q2 and Q5, realizing zero-voltage switching. In Figure 16b, when the operation mode is the slave source supply, the currents (ids1 and ids2) are negative and flow through body diodes of Q1 and Q2 before Q1 and Q2 are turned on. Until the voltages (vds1 and vds2) become zero because Q1 and Q2 were turned on, ids1 and ids2 become positive and flow through Q1 and Q2 to achieve ZVS. In Figure 16c, the system is operating in the DC BUS supply mode and negative currents (ids7 and ids8) flow through body diodes of Q7 and Q8 before Q7 and Q8 are turned on. When the voltages (vds1 and vds2) become zero since Q7 and Q8 are turned on, ids7 and ids8 become positive and flow through Q7 and Q8 because of ZVS. These testing results show that the power transistors can be operated under ZVS regardless of the operation mode this proposed system is operating in, implying that the switching loss can be reduced effectively.

Zero-Voltage Switching Verification
In this section, zero-voltage switching (ZVS) achievement is verified in the test results. Please refer to Figure 16 for an explanation. Here, the voltage and the current for the power transistors of the proposed system must be observed while power transistors turn on. In Figure 16a, when the operation mode is the master source supply, negative currents (i ds2 and i ds5 ) flow through the body diodes of Q 2 and Q 5 before the voltages (v ds2 and v ds5 ) become zero. When v ds2 and v ds5 become zero because Q 2 and Q 5 are turned on, i ds2 and i ds5 flow through Q 2 and Q 5 , realizing zero-voltage switching. In Figure 16b, when the operation mode is the slave source supply, the currents (i ds1 and i ds2 ) are negative and flow through body diodes of Q 1 and Q 2 before Q 1 and Q 2 are turned on. Until the voltages (v ds1 and v ds2 ) become zero because Q 1 and Q 2 were turned on, i ds1 and i ds2 become positive and flow through Q 1 and Q 2 to achieve ZVS. In Figure 16c, the system is operating in the DC BUS supply mode and negative currents (i ds7 and i ds8 ) flow through body diodes of Q 7 and Q 8 before Q 7 and Q 8 are turned on. When the voltages (v ds1 and v ds2 ) become zero since Q 7 and Q 8 are turned on, i ds7 and i ds8 become positive and flow through Q 7 and Q 8 because of ZVS. These testing results show that the power transistors can be operated under ZVS regardless of the operation mode this proposed system is operating in, implying that the switching loss can be reduced effectively.

Operation Mode Change Verification
Since there are three operation modes in this proposed system and the operation mode changes according to the voltage of each port, the operation mode change must be verified in this section. In this experiment, DC power supplies are used in each port to simulate the master source, the slave source, and the original power source, which directly connects to DC BUS. The voltage of DC power is adjusted to simulate the voltage variety of each port, to observe the operation mode change situation of the proposed system as shown in Figure 17. Figure 17a,b shows the voltage variation in each port and the driving signal when the master source supply mode changes to the slave source supply mode. In Figure 17a, the voltage of the master source gradually drops to 40 V and the driving signal of Q 5 is turned off, which means that the energy of the master source is insufficient to provide energy and the driving signals of the corresponding power transistors (Q 5 and Q 6 ) Electronics 2022, 11, 254 20 of 23 must be stopped. After 20 ms, the driving signal of Q 1 is turned on and the voltage of the slave source drops a little bit, which means that the driving signals of the corresponding power transistors (Q 1 and Q 2 ) start to operate, letting the slave source substitute for the master source to provide the energy. In Figure 17b, there is a roughly 40 ms time difference between the voltage of the master source starting to gradually drop and the voltage of the slave source dropping a little bit. The voltage of DC BUS can be stabilized. Figure 17c,d shows the voltage variation in each port and the driving signal variation when the slave source supply mode switches to the master source supply mode. In Figure 17c, the voltage of the slave source drops to 36 V and the driving signal of Q 1 is turned off, which simulates the situation when the energy storage device (the slave source) is cut off and the driving signals of the corresponding power transistors (Q 1 and Q 2 ) are turned off. After 20 ms, the driving signal of Q 5 is turned on and the voltage of the master source drops a little bit, which means that the driving signals of corresponding power transistors (Q 5 and Q 6 ) start to operate and let the master source provide the energy. In Figure 17d, the voltage of DC BUS is still stable during operation mode change, which is about 60 ms. Finally, both the master source and the slave source cannot provide the energy and the energy is re-charged to the slave source (the energy storage device). As shown in Figure 17e, the voltage of DC BUS is stepped up to 420 V and the driving signal of Q 1 is turned off, which simulates that the original source directly connects to DC BUS and the energy will be provided through DC BUS from the original source. The driving signals of corresponding power transistors (Q 1 and Q 2 ) must be turned off. After 20 ms, the driving signal of Q 7 starts to operate and the voltage of the slave source is stepped up, which means that the driving signals of the corresponding power transistors (Q 7 -Q 10 ) start to operate to provide the energy from DC BUS and re-charge the energy storage device (the slave source). Through these experimental results, it can be observed that the proposed system is functioning well regardless of the operation mode.

Operation Mode Change Verification
Since there are three operation modes in this proposed system and the operation mode changes according to the voltage of each port, the operation mode change must be verified in this section. In this experiment, DC power supplies are used in each port to simulate the master source, the slave source, and the original power source, which directly connects to DC BUS. The voltage of DC power is adjusted to simulate the voltage variety of each port, to observe the operation mode change situation of the proposed system as shown in Figure 17. Figure 17a,b shows the voltage variation in each port and the driving signal when the master source supply mode changes to the slave source supply mode. In Figure 17a, the voltage of the master source gradually drops to 40 V and the driving signal

Comparison with the References
Finally, a comparison with the reference focuses on some indexes, such as input port amount, input voltage, used converter amount, supply coordination function, and output voltage in this section as listed in Table 4. First, the proposed system has two input ports that connect the master source and the slave source (energy storage devices) by applying circuit integration design, which significantly improves the supply reliability. Other references have only one input port that connects a single source. If the source failure occurs, this kind of ozone-driven circuit will stop operating. It is noted that [15] has five input ports that connect five batteries to supply, yet six circuits are applied in this reference, which means complicated control and a higher circuit cost for this design. Furthermore, the supply coordination function must be discussed because the energy is provided by two sources in this paper and the proposed system has three operation modes to decide which one source delivers or re-charges, and other references have no supply coordination function because only a single source provides the energy. From the indexes of the input voltage and the output voltage, the proposed system can be extra-installed with the original ozone-driven system [11], which implies that the original system remains the same to maintain the ozone quality and minimize the system change to reduce the installation cost. the voltage of DC BUS is still stable during operation mode change, which is about 60 ms. Finally, both the master source and the slave source cannot provide the energy and the energy is re-charged to the slave source (the energy storage device). As shown in Figure  17e, the voltage of DC BUS is stepped up to 420 V and the driving signal of Q1 is turned off, which simulates that the original source directly connects to DC BUS and the energy will be provided through DC BUS from the original source. The driving signals of corresponding power transistors (Q1 and Q2) must be turned off. After 20 ms, the driving signal of Q7 starts to operate and the voltage of the slave source is stepped up, which means that the driving signals of the corresponding power transistors (Q7-Q10) start to operate to provide the energy from DC BUS and re-charge the energy storage device (the slave source). Through these experimental results, it can be observed that the proposed system is functioning well regardless of the operation mode.
(a) (b) (c) (d) (e) Figure 17. Experiment waveforms of (a) show the driving signal verification when the master source supply mode changes to the slave source supply mode, (b) depict the voltage of each port when the master source supply mode changes to the slave source supply mode, (c) elucidate the driving signal verification when the slave source supply mode changes to the master source supply mode, (d) show the voltage of each port when the slave source supply mode changes to the master source supply mode, and (e) show the driving signal verification when the slave source supply mode changes to the DC BUS supply mode, (Vs: 20 V/div, Vm: 20 V/div, VDC BUS: 150 V/div, vgs_Q1: 5 V/div, vgs_Q5: 5 V/div, vgs_Q7: 5 V/div, and time: 100 ms/div). Figure 17. Experiment waveforms of (a) show the driving signal verification when the master source supply mode changes to the slave source supply mode, (b) depict the voltage of each port when the master source supply mode changes to the slave source supply mode, (c) elucidate the driving signal verification when the slave source supply mode changes to the master source supply mode, (d) show the voltage of each port when the slave source supply mode changes to the master source supply mode, and (e) show the driving signal verification when the slave source supply mode changes to the DC BUS supply mode, (V s : 20 V/div, V m : 20 V/div, V DC BUS : 150 V/div, v gs_Q1 : 5 V/div, v gs_Q5 : 5 V/div, v gs_Q7 : 5 V/div, and time: 100 ms/div).

Conclusions
This paper presents the realization of a modified-bridge circuit with a master-slave input supply mechanism for ozone-driven system applications. Through the masterslave input supply circuit, the completed circuit exhibits better reliability during different operating conditions. In this study, a symmetrical resonant circuit that achieves zeroswitching conditions under each operation mode is designed. The hardware prototype is realized in the laboratory, and experiments are executed under different scenarios. The experimental results validate the practicality of the proposed system, which should be most beneficial to improve the reliability of the ozone-driven system.