Figure 9-2 shows a schematic diagram of a 3-kW high-density server rack power supply 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 achieve DM attenuation. Similar to TI reference designs PMP23069, TIDA-010062 and PMP41006, the circuit uses a single-phase bridgeless power-factor correction (PFC) front-end. The DC/DC stage, which provides galvanic isolation and step-down voltage regulation, is a phase-shift full-bridge (PSFB) topology with fixed switching frequency. Alternatively, the DC/DC stage may be an LLC-based topology with variable switching frequency.

In this particular example, the TTPL PFC stage and isolated DC/DC stage run at fixed switching freqeuncies of 100 kHz and 200 kHz, respectively. Even though the use of GaN power switches (such as LMG3522-Q1 with top-side cooling) enables a high power density, the conventional passive EMI filter still occupies at least 20% of the overall solution size.

Figure 9-1 Simplified Circuit Schematic of a Server Rack Power Supply With a Conventional Two-Stage Input EMI Filter

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

This application example replaces the two Y-capacitors, designated as C_Y3 and C_Y4 in Figure 9-2, with a single-phase AEF circuit using the TPSF12C1. See Figure 9-3. The AEF circuit provides capacitive multiplication of the inject capacitor, which reduces the CM inductance values 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 similar to that of the replaced Y-capacitors, which results in the line-frequency leakage current remaining effectively unchanged.

Figure 9-2 Circuit Schematic of a Single-phase Server Power Supply With AEF Circuit Connected