Traction Inverter Design with a Direct Bypass to Boost Converter

Direct bypass to DC-DC boost converter in traction inverter increases converter's capability and efficiency significantly by providing a lower loss path for power flow between the battery and DC-link terminal. A bypass using diode is an excellent solution to achieve this capability at low cost and system complexity. Bypass diode operates in the linear operating region (DC Q-point) when the battery discharges through the bypass diode to drive the electric motors. Therefore, thermal stress on the DC-link capacitor is shared between the input and DC-link capacitors through the bypass diode. On the other hand, inverters introduce voltage oscillation in the DC-link terminal which results in unwanted energy oscillation through the bypass diode during battery charging. Both of these phenomena have been explained in details. It is possible to eliminate this power oscillation during battery charging using minimum voltage level boosting at a reduced frequency or using a bi-directional switch (i.e., IGBT/diode or MOSFET/diode pair). The control strategy to achieve minimum voltage level boosting has been described in details. Moreover, it is possible to further reduce power loss of the boost converter by completely bypassing the inductor using a bi-directional switch during high voltage battery charging.