A single-phase full-bridge soft-switching inverter circuit with an auxiliary resonant circuit

A new topological structure is proposed in this article for the traditional single-phase full-bridge inverter circuit by adding an auxiliary synchronizing resonant circuit. Without extra switching transistors, the complexity of the control circuit is simplified while enabling zero voltage switching (ZVS) functionality. The significance of this proposed topology is that the switching transistors of all bridge arms are sharing one common synchronizing unit which is constructed with an absorbing inductor and capacitor. In this way, the added-on components can be substantially reduced for the main switching transistor circuit. By utilizing a big-volume capacitor as the constant voltage source, the sourceless energy feedback unit can minimize the switching losses. In the meantime, the proportional-resonant feedback circuit can modify the sinusoidal pulse width modulation (SPWM) duty ratio to stabilize the output current. In the PSIM power electronic simulation software environment, the operation of the soft-switching inverter circuit is analyzed to demonstrate our proposed topology can reduce the on-off switching losses as expected for the switching transistors of the soft-switching inverter circuit.


Introduction
As the fundamentals of the economy are progressively growing and improving, gross energy consumption is continuously increasing worldwide.The issues regarding lack of resources and energy saving with efficiency have become critical challenges.Confronting the emerging scenario, the national chairman of China proposed to control the overall energy consumption and to promote the industries of new energy or low-carbon emission.In the report of the 19th National Congress Communist Assembly, it is pointed out that there must be policies regarding energy-saving and resource recycling.In the meantime, with advanced science and technology, most efforts should come together at constructing an energy system with security, reliability, efficiency, and no pollution.Hence, a solid foundation could be established for the next transformation and upgrade of energy generation and utilization.As electrics have come into the sight of the general public, electricity becomes a major source among others in the energy business.In the current electric power system, alternating current (AC) is used as the standard for transmission and generation.An inverter that converts direct current (DC) into AC is widely applied in every respect of our daily life.With the technological development in the semiconductor field, the insulated gate bipolar transistor (IGBT) has been applied for power switching devices.The IGBT has the merits like high switching frequency, the low voltage drop at onstage, low driving power, and a high rating of the module's voltage and current level.Hence, it is the device being considered first while designing a medium to small power rating inverter.There is also a tendency to make the inverter smaller, and lighter with a higher switching frequency [1][2][3].On the other hand, the high-frequency switching will make the switching power loss increase, as well as the electric stress over the device.To alleviate the problem, the soft-switching technique was proposed to maintain a much lower loss with the same high-frequency on-off operation.It is also a desirable topic to study the application of soft-switching techniques at power supply with inverters in recent years.Hence, the soft-switching technique drives the switching frequency even higher within a much smaller device than possible.The overall conversion efficiency of the power electric inverter is progressively improved [4].In this paper, an auxiliary resonant circuit is added to the traditional single-phase full-bridge inverter circuit to smooth the change of current on the four arms of the bridge during the inversion process through the auxiliary resonant circuit respectively, to complete the ZVS turn-on and implement soft switching [5][6][7][8][9][10].

Methodology 2.1. Inverter basic structure
The general structure of an inverter is depicted in Figure 1.The input power takes a 300V DC assembly and converts the DC to the AC output through a single-phase full-bridge soft-switching inverter circuit with an auxiliary resonant circuit.The control signal of the switching device in the inverter circuit is driven by the sine pulse width modulation (SPWM) signal, while the feedback current is drawn in the output circuit.The duty cycle is adjusted in real time by the proportional-resonant (PR) controller for the SPWM signal.Hence, the output current converges to the preset nominal value.

Single-phase passive soft-switching inverter circuit
The circuit topology of a high-efficiency passive soft-switching inverter is shown in Figure 2(a) and Figure 2(b).An auxiliary resonant network is appended to the conventional single-phase full-bridge inverter circuit.The auxiliary resonant circuit consists of sourceless passive components Lr, Ls, Cr1, and Cr2, in addition to an electrolyte capacitor Cs which can be treated equivalently as a voltage source, and the energy feedback diodes, Ds1 to Ds4.The full-bridge circuit is composed of the main switch components S1 to S4 together with the diodes D1 to D4, respectively.There is no need to set up the dead zone period in this topology due to the short period and the combined function of the large inductance Lr and two clamping diodes that prevent the power supply from a short circuit.The circuit operates with an auxiliary resonant loop to reduce the change rate of current while S1∼S4 is on, and the change rate of voltage while it is off, respectively.Thus, zero voltage switching (ZVS) is achieved with switching losses reduced.The proposed inverter topology in this paper takes advantage of a resonant circuit composed of passive components with a simpler structure while comparing to the inverter topology from the other works of literature [11,12,13].2√2 cos ∅ where ϕ is the power factor of the load.

PR feedback control
The PR controller began to receive attention after the year 2001 and was first used in the active filter and harmonic compensation control.Afterward, it was gradually applied to single-phase and three-phase current control in 2004.The transfer function is as follows: The intrinsic idea is to add two fixed-frequency closed-loop poles to the jω axis of the controller transfer function to generate a resonance at the particular frequency, as well as use the resonance to increase the gain at that fixed frequency.Theoretically, the resonance will make the gain at the design frequency tend to be infinity.Thus, the control signal at that frequency can be tracked very closely.In this paper, the PR controller is used to detect the output current.When the output current is lower or higher than the setup value 10 2 A, the duty cycle of the SPWM is adjusted in real-time to ensure that the output current is within the setup range [13][14][15][16][17][18].

Total harmonic distortion (THD)
Harmonics are the power contained in the output voltage at a frequency that is an integer multiple of the fundamental, i.e., the power spectrum generated by a Fourier series decomposition of a periodic waveform.The total harmonic coefficient characterizes the proximity of an actual waveform by referencing the ideal fundamental sine wave [19].The calculation is defined as: where Q and Q 1 are the total effective value and the fundamental mode effective value, respectively, to be either voltage or current.

Results
Based on the previously described method, the simulation results by PSIM are compared with those from the conventional inverter.Our simulation results indicate that the power supply implemented with a soft switching technique can reduce the switching on/off losses.Within the software simulation environment of PSIM, the proposed parallel resonant inverse soft-switching inverter power supply is simulated with the driving signal of 20 kHz switching frequency.In this way, the IGBT turn-off loss can be minimized on the one hand and by considering the overall physical dimensions of the power transformer and filter on the other hand.

Simulation setups
To ensure the rapid and complete saturation of the IGBT turning on, the component being selected is AUIRGP4063D for the IR company.The load resistor R is 18 Ω, and the load inductor is 1 mH.

Soft and hard switching comparison
As shown in Figure 4, we take one of the IGBTs as an example.When the IGBT receives a high level, which is the rising edge of the drive signal, the current flowing through the IBGT slowly rises from zero to a steady-state value and then gradually decreases.The voltage crossover area with the IGBT is very small, i.e., the turn-on loss is very small.At the same time, the voltage across the IGBT is always zero during a complete high-level period, and the current is also maintained at a small value to complete the ZVS, i.e., zero voltage turn-on.On the contrary, in Figure 5, the circuit is a conventional inverter circuit, and there is no auxiliary resonant circuit.The switching signal frequency is the same as 20 kHz.We can clearly see that during the falling and rising edges of the driving signal of the IGBT, the current flowing through the IGBT and the voltage at both ends of the IGBT have a large overlap area, which means that the switching loss is larger for the conventional hard switching in terms of energy loss in one switching cycle of the switching device.By comparing the proposed inverter circuit with the conventional inverter circuit, the soft-switching inverter in this paper is also simpler than the conventional inverter circuit.

Single-phase full-bridge inverter output waveform analysis
The simulated output waveforms from the single-phase full-bridge inverter circuit were analyzed to examine the performance.From Figure 6, the output waveform is shown on the left, and there are 5 full cycles for the period of 0.01s that is equivalent to 50 Hz which is identical to the number from the oscilloscope inside the PSIM.The output result from the conventional circuit is as shown in Figure 7.As shown in Figure 6, the output waveform is shown on the left.The Fourier analysis of the output waveform by PSIM software is shown on the right.The total harmonic coefficient THD of the output without softswitching is 15.9 %.

Discussions
This paper studied a soft-switching topology for a single-phase full-bridge inverter circuit.In today's inverter circuits, soft-switching technology generally is implemented with passive components such as inductors and capacitors to generate resonance to realize soft-switching.Nevertheless, there are also additional auxiliary switching transistors for delicate control of the main switch to be on and off.From Figures 4 and 5, during one on-off cycle period, the switching loss is reduced for the inverter with the auxiliary resonant circuit in contrast to the conventional inverter.
It is an advantageous feature especially for a miniature with a higher switching frequency.Due to the inductive nature of an inverter, from Figures 6 and 7, there are certain amounts of THD.The THDs for the proposed and the conventional inverter circuit are 14.7% and 15.9% with DC input voltage 300V and switching frequency 20 kHz, respectively.Hence, the THD is improved with a slightly better efficiency.

Conclusion
According to the simulation results and analysis, our proposed inverter architecture with auxiliary passive resonant soft-switching is a practical design with better performance and lower cost in contrast to the conventional inverter design.The key feature is that the switch transistor at each branch of the bridge shares the same resonant circuit for the control signal.Under the regulation of the PR resonant circuit, when the output current is either higher or lower than the target 10 2 A, the duty cycle of SPWM is adjusted accordingly to ensure the output current is within a certain range.The overall architecture is also simpler than the conventional inverter with an easier control strategy and without extra auxiliary transistors for switching control.The performance is also better as described in the previous section.Therefore, it is very positive to investigate further according to this preliminary study.

Figure 1 .
Figure 1.The block diagram of basic inverter structure.

Figure 2 (
Figure 2 (b).Single-phase passive soft-switching inverter circuit.The effective value of the fundamental mode from the result of Fourier transformation on the output current can be obtained by  1 4  2  The effective value of the load voltage Uo can be found by the equivalence between the input and output power:     ∅ Therefore, we have

Figure 6 .
Figure 6.The time domain and frequency domain analysis of the output waveform from the proposed circuit simulation.

Figure 7 .
Figure 7.The time domain and frequency domain analysis of the output waveform from the conventional circuit simulation.
The resonant inductors L r and L s are 15 µH, respectively.The resonant capacitors C r1 and C r2 are 33 nF.The setup values for specific components are listed in Table1.The IGBT driving signals are shown in Figure3.