*Bidirectional (V2G/G2V) On-Board Charging Systems*

For opting for the off-board charger in the power electronic converter there are a few main factors that must be studied, such as high reliability, distortion-free operation, less grid interference, lower system size, weight, high efficiency, and, last but certainly not least, high efficiency. In order to be portable, light, and effective, wide band gap devices have contributed a lot, and they made switching frequency optimal as required in this domain by low gate charge and output capacitance. This was possible all because of the GaN-based power transistors. The devices such as capacitors, inductors, and transformers, which are passive in nature, were also changed to be lighter weight and smaller in size [107,108] because of the advent of WBG technology. It is noted that GaN-based transistors are high electron mobility transistors (HEMT), thus we call them GaN-HEMT in short, and they have a voltage rating up to 660 V, whereas the current ranges from 20–50 A [104,107]. These components are mostly deployed in off-board chargers (OBCs) with the output power ranging in between 3.0 kW and 20 kW. Figure 6 is illustrated in order to show two singlephase bidirectional off-board chargers with a special DC to DC stage structure as well as the same identical AC to DC structure, which is a totem pole PFC. One may also see the dual active bridge that is functional because of the galvanic-nature-based isolation and bidirectional power transformation, including zero-voltage detector and switching at both primary as well as secondary sides. This has compact-size-based components and a fixed-frequency operation, as referred to in [109].

**Figure 6.** GaN-switch-based bidirectional OBC system topologies [107].

It is tough to achieve the full range of ZVS due to the wide range in load power. The resonant bidirectional CLLC architecture (where C is capacitance and L is inductance) shown in Figure 7b is incredibly efficient because of the zero-current switching (ZCS) on the secondary side and the zero-voltage switching (ZVS) in the main bridge. The CLLC architecture has the drawback of not being able to adjust output voltage using the series resonant frequency when it is being used for charging.

In order to solve this issue, reference [70] advises switching from frequency modulation in the DC-DC stage to DC bus voltage modulation in the PFC stage. The resonant CLLC stage will be able to function at its most effective level as a result [109,110]. A modular converter method is a suitable replacement for the development of ultra-fast charging systems.

Four AFE converters are combined and connected in parallel to generate the current design, which is suggested for the 600 kW DC ultra-fast charger [111]. This is seen in Figure 7. A comparison of silicon-based and silicon-carbide-based semiconductors has been performed for each module with a 150 kW power rating. In order to examine the effectiveness of Si (SKM400GB12T4) and SiC (CAS300M12BM2) devices at various power levels, a non-linear electro-thermal simulation model was adopted. The simulations for both scenarios contain the pertinent datasheet information. Figure 8 illustrates how SiC devices are substantially more effective as chargers than silicon-based ones. Wide band gap devices can save energy in this way because Si has a larger loss than SiC.

**Figure 7.** (**a**,**b**) Modular 600 kW DC ultra-fast charger [111].

**Figure 8.** Si- and SiC-based high-power off-board charging system efficiency map.
