Bidirectional Flyback Converter

The above diagram depicts a compact bidirectional fly-back converter. The operation of the converter can be explained as follows: while drawing power from the battery to the load the switch T1 is driven by the PWM signal of appropriate duty cycle and T2 will be permanently off. The secondary current flows through the diode D2. The current path for this condition is shown with a solid red line in the figure. When the current flows through T1 (during active state of PWM signal) energy will be stored in L2. When T1 is off (during inactive state of PWM), because of the polarity of stored potential in L2 current flows through D2.

The dashed blue line shows the current flow when energy is put back into the battery i.e. when there is energy transfer in the reverse direction. In this case switch T2 and the diode D1 comes into action. Duty cycle and/or frequency of PWM signal can be varied to control the battery recharge voltage/current.

Advantages:

1. More than one flyback converter can be connected in parallel at the DC bus with all the battery terminals floating w.r.t each other.

2. The transistor-diode configuration (or IGBT-diode) is available in market as a single piece.

3. The flyback structure is very compact

You can also refer to the ieee paper "Multiphase Bidirectional Flyback Converter Topology for Hybrid Electric Vehicles" for further reference.