## 7 Challenges and Future Perspectives
_NIBDCs_, as converters, require no high-frequency transformers to achieve electrical isolation between the load and the source; their efficiency in low-power applicationsis high because of the ease of controlling them, as well as their light weight [55]. On the other hand, an isolated bidirectional _DC-DC_ converter is necessary for several applications for the security of the source in case of an overloaded situation, as well as for noise reduction and voltage matching between conditions; therefore, it is employed in place of _NIBDCs_[37]. * Many studies have focused on the combination of resonance network type with _IBDCs_, including, but not limited to, _SR-DAB_, _CLLC_, _CLLLC_-type _IBDC_, and _LLC_. _CLLC_ and _CLLLC_ resonant _IBDCs_ have attracted considerable attention due to their symmetrical characteristics in both the forward and reverse power flow directions. On the other hand, this structure faces difficulties because charging efficiency degrades dramatically when the battery voltage varies, and the frequency of the discharging operation varies widely [134]. In the _LLC-BDC_ structure, the _LLC_ resonant converter behaves similarly to a conventional series resonant converter with a very tight gain range, even though this type has better features than the earlier mentioned types, and is also extensively used in industry, especially in reverse power transfer [138]; this challenge must be considered in future perspectives. A bidirectional three-level _LLC_ resonant converter at fixed modulated frequency has been proposed by [178]; the proposed system can achieve a broad gain range even though the accurate analytical gain model is not derived. The system also used twelve switches, and because of this, the achievement of high efficiency over a wide gain range becomes a challenge to consider. * Multilevel inverters (_MLIs_) are added to counter the problems associated with two-level converters to meet the following criteria: (i) only sinusoidal output voltage shall be produced by the converter; (ii) the converter must have an output current of low _THD_. The advantage of _MLIs_ is that the device voltage rating and the switching frequency can be significantly lower under the same output voltage compared to those of a conventional two-level converter; consequently, switching losses can be decreased remarkably, thereby improving the efficiency [179; 32; 180; 181]. * Based on some industrial applications, such as drives, solar inverters, _UPS_, electric vehicles, and _STATCOM_, it is apparent that low-harmonic multilevel waveforms could attract the attention of industries to commercialize the most reliable and cost-effective topologies. However, it should be noted, however, that adding extra switches and auxiliary capacitors to the inverter structure increases the production cost. Adding new gate drivers, voltage balancing strategies, and switching algorithms to the control circuit will also complicate things. To be selected by a corporation to commercialize, there should be certain improvements on the topology, such as reduced size of the passive components, reduced switching frequency and switching losses, increased reliability, and efficiency. Industries are constantly attempting to find a balance between the number of levels and the complexity of design and operation [182]. It is seen that industries are looking for simplicity and high performance for their next generation of power electronic converters. The following are some of the possible future research and development challenges: (1) Extending the use of single-_DC_-source _MLI_ topologies from high-power to medium- and low-power applications, such as aerospace and aircraft, power supply, home products, electrified transportation, solid-state transformers, and so on. (2) Developing single-_DC_-source _MLI_ topologies at a higher level (more than five levels). In multiple-_DC_-source _MLIs_, one option is to replace the isolated _DC_ sources with voltage-controlled capacitors.