Full Bridge Configuration
The history of the full bridge (FB) inverter configuration presented in started close to 1980, when this type of topology was proposed for railroad applications [
57] and afterwards for uninterruptible power supplies [
58]. At that time, the developments of the new types of the ferrite materials and possibilities to increase the switching frequency with new types of semiconductors allowed for a decrease of losses and sizes of the magnetic components, resulting in the improvement of the efficiency of the system in general. The topology consists of four main parts: the DC-side bridge, the AC-side bridge, the high-frequency transformer (HFT) and the output grid filter. The DC-side bridge consists of four switches that work as an inverter, which forms the high-frequency pulses. The transformer provides galvanic isolation between the two sides and allows them to step up/step down voltage. The AC-side bridge can form the sinusoidal voltage output or work as a rectifier, depending on the modulation method. The output AC-side LCL filter is used to filter the high-frequency pulses. Given the realization of bidirectional switches in the AC side using two discrete devices, this leads to a distinct disadvantage–a relatively high number of active semiconductor devices required.
Figure 6. Full-bridge (FB) configuration (
a) and modulation(
b) proposed in [
52].
Another major challenge for the isolated FB configuration inverter topology is that the transformer leakage inductance and the output filter inductor both serve as current sources for the AC-side bridge. Thus, the AC-side bridge operates as a current breaker, which endures high-voltage spikes due to the cutoff of the leakage inductor current. The natural commutation principle for this converter was proposed in [
59]. In addition, various other modulation methods using DC- or AC-side switches were presented for the current topology [
46,
47,
48,
49,
50,
51,
52]. These methods typically allow utilizing energy stored in the leakage inductance to create soft switching conditions for semiconductors, which also particularly or fully solves the mentioned problem of high-voltage overshoots. The topology features the capability of bidirectional power flow without any additional auxiliary circuit.
In [
46], the 40 kVA bidirectional converter for medium voltage converter (MVC) application was proposed. This work presents a special multi-mode quasi-resonant phase-shift modulation (PSM) method, which allows achieving soft switching in all semiconductor devices. The additional snubber capacitor is added for DC-side semiconductors and RCD for the AC side to reduce turn-off losses and ringing. Special multi-mode modulation allows the creation of the additional quasi-resonant interval, which is activated near the zero-crossing point. It is utilizing the resonant circuit formed by the DC-side snubber capacitor and transformer leakage inductance to achieve soft switching at the current zero-crossing point. The details of the modulation method are described in [
65]. The experimental waveforms confirm that the method is verified, and the converter can operate without any additional damping circuit for the leakage inductance energy. At the same time, the optimization of the transformer and snubber capacitance can be a point of the discussion, because this can significantly influence of the converter efficiency, especially at the higher step-up ratio of the transformer.
Two unipolar pulse-width modulation (PWM) based modulation methods for the FB isolated matrix inverter are reported in [
47], both of these methods allow achieving the soft switching for the AC-side switches. In contrast, the DC side has only the zero voltage switching (ZVS) turn-on transient. The first method allows operating four switches at the AC side at line frequency, further reducing the remaining switching loss and improving the efficiency. The authors applied the hybrid structure of the switches at the AC side –four of them are IGBT with grid frequency switching and four MOSFET with high working frequency. This allows price reduction of the converter, which was the authors’ aim. The second modulation method is proposed for the full MOSFET bridge configuration. In this method, two of the switches working with the line frequency, two–as synchronous switches, and all the other are forming the output voltage. The authors verified the experimental 1.2-kW prototype with 400-V DC input voltage and 240-V AC RMS output using one to one isolation transformer. The peak efficiency of the system is 96% for the full silicon carbide (SiC) MOSFET design. In [
108], the same group of authors presented applications for this converter in the high-frequency distributed power delivery system. This utilization allows elimination of the traditional low-frequency transformer and its replacement it with HFT. The potential target application is air and ground transportation and renewable energy systems.
In some studies, the leakage inductor energy is redirected to the additional auxiliary circuit, which allows the clamp of the voltage overshoot. In [
48], the new modulation method based on the DC-side PSM is addressed for the FB inverter operation. For the minimization of voltage overshoot across AC-side switches, the clamping circuit with the rectifier bridge, capacitor and DC–DC converter is applied. The energy from the AC side is returned to the DC input and reused again in the converter. The topology is verified with 1 kW with 270 V of input and 100-V RMS at the output as UPS inverter. Other clamping circuit approaches were addressed in [
49,
50,
51,
109]. These papers present different types of regenerative flyback snubber. Since this snubber is technically a separate converter, in addition to voltage clamping, it can provide additional features for the topology: reduce the amount of the circulation current through the main switches, improve the soft-start procedure or improve the quality of the current in the case of non-unity power factor. The energy stored in the snubber capacitor can be utilized in two ways–being reused in the next switching cycle of the conversion, as proposed in [
49,
50] or transferred back to the DC side and reused again as shown in [
109]. In addition, it is claimed to allow improving the efficiency of the converter in general. In [
110], different snubbers for the minimization of the influence of high step-up transformer are analyzed for the case of the integration of low-voltage DC source to the grid. On the other hand, the utilization of the snubbers requires an additional transformer or two flyback transformers, switches, diodes, capacitors and additional control channels and isolated drivers. The utilization of this snubber can be more suitable for the high-power converter. For the lower-power converters, this solution can be less optimal, considering the price and overall complexity.
Many high-power converters commonly use bridge-type topologies due to the better utilization of the switches and lower stresses. The FB topologies can be extended to the three-phase grid system, as presented in [
53,
59,
76,
111]. In this case, at the AC side, an additional leg is supplied and the modulation method is extended for the three-phase system ().
Figure 7. Three-phase FB configuration.
LL (dual inductor) HB Configuration
The dual inductor half-bridge (LL HB) converter type is a popular solution in the DC–DC converters due to the low current ripple. presents the bidirectional converter topology proposed for electric vehicle battery charging applications in [
97,
114]. The original paper describes it as “single-stage bidirectional converter for EV”; however, it will be referred here as LL HB due to the topology configuration. The converter features a full-bridge on the DC side with an additional filter inductor L
DC and LL-type HB at the AC side. The converter with the input of 120-V RMS AC is connected to a 220–336 VDC battery through a converter that has a high-frequency transformer with a turn ratio of 0.5. In the rectifier mode, the converter operates as a two-phase boost converter, while in the inverter mode, it acts similar to a FB converter with the current doubler rectifier the output voltage regulation is achieved by PSM. The converter features reduced current ripple due to the presence of two inductances at the AC side, allowing the ZCS to be achieved at the AC side, and ZVS turn-on at the DC side. Moreover, it can operate in four quadrants with active and reactive power regulation for both directions of the power flow. In the inverter mode, the PSM is implemented for the DC-side switches, while the AC-side switches are continuously on or off depending on the polarity of the grid voltage. Peak efficiency of 96.5% at full power is reported for the 1.5-kW prototype. At the same time, the topology requires an additional inductor, the voltage stress on the switches is increased, and it is estimated to have substantial energy circulation at partial load. The simulation results of the three-phase system shown in [
98].
Figure 12. Single-stage bidirectional converter for EV (LL HB) configuration (
a) and modulation (
b) proposed in [
114].