Figure 1-2 shows a block diagram of the bidirectional DC/DC converter topology. In non-isolated topologies like that of a string converter, a bidirectional converter can be used to have the possibility of battery energy storage. Bi-directionality is necessary since the DC/DC converter needs to act as a battery charger (buck mode) in one direction and discharge the battery (boost mode) providing a higher and stable output voltage at the DC-link.

Figure 1-2 Bidirectional DC/DC Converter Block Diagram

In boost mode, since this converter supplies the inverter through the DC-link, the discharge power is limited to 4.6kW, the limitation being the maximum power rating of the inverter stage. Depending on the battery voltage, this value can go up to 30A. In the buck mode as well, there is a possibility to employ a charging current of 30A to reach higher power levels.

With a current value of 30A, just the conduction losses of a single GaN FET alone can be very high (30×30×0.03 = 27W) – which can lead to heating of the GaN device, which in turn increases the conduction losses as the R_DS(on) of the GaN FET also increases with temperature.

To reduce this effect of overheating of the devices and increased power losses due to higher conduction and switching losses of the FETs, one plausible design can be interleaving of the branches. A phase difference of 360° / 2 equals 180° is applied between the legs to reduce ripple current as seen in Figure 1-3.

Figure 1-3 Normalized Current Ripple v/s Duty Cycle

Because both the phases of a multiphase design such as this, are tied together at the output node, the inductor currents of each phase are concurrently charging and discharging the output capacitors depending on which phase is active (titled L1 and L2 Current). This charging and discharging produce an overall output current (titled Output Current), which has a lower peak-to-peak value as shown in Figure 1-4.

Figure 1-4 Output Current Waveforms

Smaller ripple current or AC current in the output capacitors lowers the overall output voltage ripple which in turn lowers the amount of capacitance needed to keep the output voltage within tolerance. Adding additional phases to a design decreases the RMS input current flowing through the decoupling capacitors thereby reducing the ripple on the input voltage.

Paralleling of the branches also helps in achieving twice the switching frequency across the output EMI filter which helps reduce the size. Symmetric interleaving involves switching the different sections with the same frequency but with interleaving of the phases to control noise output and ripple. The same current is demanded from both the branches leading to twice the output current and the duty-cycle is fixed depending on the battery voltage and the DC-link voltage. Furthermore, a dead time is inserted between the half-bridge FETs to avoid short circuit of current paths, while the switches switch in a complementary fashion. This design is therefore rated to provide a 4.6kW output for boost stage and has a capability to charge the battery nominally up to 10kW at a battery voltage of around 330V. Each interleaved stage is switched at a frequency of 65kHz, resulting in an equivalent output frequency of 130kHz.