This pre-regulated isolated open-loop LLC transformer driver converter provides four 18-V outputs up to a total of 6 W for traction-inverter applications. The LLC topology allows the transformer to have significant leakage inductance, but a much smaller primary-secondary capacitance, which significantly reduces common-mode current injection through the bias transformer. The boost pre-regulator is designed to provide 7.5 W to the LLC converter, which can support a maximum of 6-W output.

Figure 1-1 Top Photo

• Resistive loads
• Power supply capable of 30 V, 10 W minimum
• Oscilloscope and probes
• Digital multimeters

The efficiency of the boost converter and the open-loop LLC were measured separately. For the boost measurements the LLC was disabled by shorting the DIS/FLT pin of the UCC25800-Q1 (TP11) to GND and a resistive load was applied across the connector labeled VBOOST (J2). Efficiency curves were measured for inputs of 6 V, 12 V, 24 V, and 28 V.

Figure 3-1 Boost Efficiency and Power Dissipation

The LLC measurements were taken with a regulated 30 V applied at the connector labeled VBOOST (J2) and with the boost input (J1) disconnected.

Figure 3-2 LLC Efficiency and Power Dissipation

The efficiency and power dissipation data for the boost and LLC stages is shown in the following tables.

Figure 3-3 Boost Converter Efficiency, 6 V_IN

Figure 3-4 Boost Converter Efficiency, 12 V_IN

Figure 3-5 Boost Converter Efficiency, 24V_IN

Figure 3-6 Boost Converter Efficiency, 28 V_IN

Figure 3-7 LLC Converter Efficiency, 30 V_IN

The following thermal image shows the board running with 6 V_IN and 6 W being drawn from the 18-V LLC outputs (3 × 1 W and 1 × 3 W).

Figure 3-8 Top Thermal Image

The loop stability of the boost converter is shown in the following plot. The plot was obtained with the LLC converter disabled and a resistive load applied across J2.

Figure 3-9 Loop Stability, 12 V_IN, 250-mA Load