Most applications for power conversion involve a raw supply to be processed to a different stabilized voltage, often with galvanic isolation. Typical scenarios may include ac mains power-converted to low-voltage dc in a laptop adapter, or a discharging battery voltage in portable equipment boosted to a constant higher value. These are one-way conversions, but with increasing interest in alternative energy schemes and electric vehicles, there’s demand for power to be able to flow in both directions to harvest excess energy.
One example would be a PV installation where dc solar-panel power can be “fed back” into the ac grid when storage batteries are fully charged, with the ac in turn providing charge to the batteries at night through a bidirectional converter/inverter. Another situation is in electric vehicles, where a bidirectional dc-dc converter drops a main 400-V traction battery voltage down to 12 V for auxiliary equipment. The dc-dc reverses power flow from 12 V to 400 V under emergency conditions when the traction battery is dangerously low in charge (Fig. 1).