Figure 9-3 shows a schematic diagram of a 22-kW high-density OBC with conventional two-stage passive EMI filter. The CM chokes and Y-capacitors provide CM filtering, whereas the leakage inductance of the CM chokes and the X-capacitors provide DM filtering. The circuit uses a three-phase power-factor correction (PFC) front-end followed by a full-bridge CLLLC topology with active synchronous rectification.

The PFC stage runs at a fixed switching frequency of 100 kHz. The CLLLC isolated DC/DC stage runs at a variable frequency from 200 kHz to 800 kHz (500-kHz nominal) and provides galvanic isolation in addition to battery voltage and current regulation. Even though the use of GaN or SiC power switches enables a high power density, the conventional passive EMI filter typically occupies over 20% of the total solution size.

Figure 9-1 Circuit Schematic of a Three-phase OBC With a Conventional Two-Stage Passive EMI Filter

Note that the DC/DC stage in particular increases the CM EMI signature based on the high dv/dt of the GaN power switches, the transformer interwinding capacitance as well as the various switch-node parasitic capacitances to chassis ground.

This application example replaces the four Y-capacitors, designated as C_Y1, C_Y2, C_Y3 and C_Y4 in Figure 9-3, with a three-phase AEF circuit using the TPSF12C3-Q1. See Figure 9-3. The AEF circuit effectively provides capacitive multiplication of the inject capacitor, which reduces the CM inductance values to maintain the target LC corner frequencies and thus the size, weight, and cost of the CM chokes, now designated as L_CM1-AEF and L_CM2-AEF. The total capacitance of the sense and inject capacitors is kept less than or equal to that of the replaced Y-capacitors, which results in the total line-frequency leakage current remaining effectively unchanged or reduced.

Figure 9-2 Circuit Schematic of a Three-phase OBC With AEF Circuit Connected