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

The voltage regulation of the LLC converter outputs is shown in the following figures.

Figure 3-10 3-W Rail Regulation Graph

Figure 3-11 3-W Rail Regulation Table

Figure 3-12 1-W Rail Regulation

Figure 3-13 1-W Rail Regulation Table

The cross regulation was measured to showcase the effects of varying load on the other outputs of the LLC converter. The 3-W rail (Vo3W) and one of the 1-W rails (Vo1W3) were varied while the other 2 rails (Vo1W1 and Vo1W2) were held constant.

Figure 3-14 Cross Regulation Table

The switching behavior of both converters is shown in the following figures. As in previous sections, the two converters were evaluated separately.

Figure 4-1 Boost Converter Switch Node, 6 V_IN, Light Load

Figure 4-2 Boost Converter Switch Node, 6 V_IN, Maximum Load

Figure 4-3 Boost Converter Switch Node, 28 V_IN, Light Load

Figure 4-4 Boost Converter Switch Node, 28 V_IN, Maximum Load

Figure 4-5 LLC Converter Primary Side Switch Node, 30 V_IN, No Load

Figure 4-6 LLC Converter Primary Side Switch Node, 30 V_IN, 50% Load

Figure 4-7 LLC Converter Primary Side Switch Node, 30 V_IN, Max Load

Figure 4-8 LLC Converter Secondary Side Resonant Capacitor, 30 V_IN, Max Load

The output voltage ripple of each converter is shown in the following figures.

Figure 4-9 Boost Converter Output Ripple, 6 V_IN, Maximum Load

Figure 4-10 Boost Converter Output Ripple, 28 V_IN, Maximum Load

The LLC output ripple was measured on the 3-W rail to show the worst-case ripple.

Figure 4-11 LLC Converter Output Ripple, 30 V_IN, Maximum Load

The start-up behavior of each converter is shown in the following figures. The boost converter start-up was measured on controller enable, the controller was disabled by shorting the COMP pin to GND. V_OUT is shown in yellow.

Figure 4-12 Boost Converter Start-up, 13.5 V_IN, Maximum Load

For the LLC converter, a 3-W output (yellow) and a 1-W output (blue) are shown.

Figure 4-13 LLC Converter Start-up, 30 V_IN, No Load

Figure 4-14 LLC Converter Start-up, 30 V_IN, 50% Load

Figure 4-15 LLC Converter Start-up, 30 V_IN, Maximum Load