3 Bias Power in Common Drain Configuration of Back-to-Back and Bidirectional FETs

The common drain configuration of back-to-back FETs and bidirectional switches affects the architecture of the isolated bias power supply on the primary side of single-stage OBCs immensely. The switches do not have a common source, therefore, supplying both FETs using the same isolated bias power output is not possible. In the bidirectional switches, two FETs inside a single package can be driven independently using two single-channel gate drivers or a dual-channel gate driver. Therefore, two different isolated bias power supplies are needed for the bidirectional switch. In Figure 3-1, the same color shaded FETs can share the same isolated bias power supply output.

Figure 3-1 Block Diagram of the 3-Phase Single-Stage OBC With Common-Drain Configuration

There are a total of nine isolated bias power supply outputs required in the common-drain configuration, which includes six in the AC side and three in the HV battery side. The top four FETs and the bottom four FETs have a common-source and can share the same isolated bias power supply output. Similarly, the two FETs at the switch nodes (SW1, SW2, SW3, N) also have a common source, which enables the FETs to share the same isolated bias power supply output.

The common-drain configuration (which is more common in cases of bidirectional switches) requires two less isolated bias supply power outputs when compared with the common source configuration shown in Section 2. In the common-source architecture, two power switches are placed closely back-to-back. Therefore, the common-source architecture encounters less issues with the differences in Vgs at the gates of the two back-to-back switches due to the parasitic in the PCB layout. However, in the common-drain configuration, give more attention to the differences in Vgs at these gates of switches, especially in the case of SW1, SW2, SW3, and N, since the switch node (with a common source) can have higher parasitic effects (for example, stray inductance) between the two switches.