Figure 2-3 shows a block diagram of the bidirectional DC-DC converter topology. In non-isolated topologies like this, a bidirectional converter can be used in systems with the possibility of battery energy storage. Bidirectionality is necessary since the 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 2-3 Bidirectional DC-DC Converter Block Diagram

In boost mode, since this converter supplies the inverter through the DC link, the discharge current is limited to 30 A. In the boost mode as well, there is a possibility to employ a charging current of 30 A to reach higher power levels. As a result, this leads to higher conduction and switching losses of the FETs, so interleaving of the branches can be carried out. Paralleling of the branches also aids in achieving twice the switching frequency across the output EMI filter which helps reduce the size. A phase difference of 360° / 2 equals 180° is applied between the legs to reduce ripple current. The same current is demanded from both the branches leading to 2 × 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. Both the interleaved stages have individual input capacitor and Bourns inductor 145452 (D6755) which is 186-μH rated, and a common output capacitor to minimize the output ripple voltage. All the passive components are designed to match the requirements for worst-case ripple and EMI conditions.

This design is therefore rated to provide a 3.6-kW output for buck stage and has a capability to charge the battery up to 7.2 kW. Each interleaved stage is switched at a frequency of 60 kHz, resulting in an equivalent output frequency of 120 kHz.