10.2 DC/DC Converter Startup

The PSE and TPS2376-H are power and current limited sources, which imposes certain constraints on the PD power supply design. Improper design of the system can prevent PD startup with some combinations of Ethernet lines and PSE sources. The root of most startup problems revolves around the dc/dc converter.

Dc/dc converters have a constant input power characteristic that causes them to draw high currents at low voltage. Also, a converter may draw in excess of 125% of its rated power during startup when the output voltage approaches its regulated value, and the output capacitors are charging while the load draws its full power. These characteristics lead to two undesired events. First, if the converter starts up during inrush, it can draw more current than available from the TPS2376-H and cause the startup cycle to fail. Second, if the converter startup current exceeds the TPS2376-H current limit, it may discharge the bulk capacitor until V_(RTN-VSS) exceeds 10 V and forces the TPS2376-H into inrush.

The following guidelines should be used:

1. Set the TPS2376-H inrush to a moderate value such as 140 mA.
2. Hold the dc/dc converter off during inrush using PG.
3. Implement a softstart that keeps the peak start-up current below 600 mA, and preferably only a modest amount over the operating current, at the minimum PSE voltage and maximum feed resistance.
4. If step 3 cannot be met, the bulk input capacitor should not discharge more than 8 V during start-up at the minimum PSE voltage and maximum feed resistance. Start-up must be completed in less than 50 ms.

Step 4 requires a balance between the converter output capacitance, load, and input bulk capacitance. While there are some cases which may not require all these measures, it is always a good practice to follow them.

Downstream converters that use PG control are turned off during a hard fault or thermal cycle, and will go through an orderly restart once the bulk capacitor is recharged. Converters that do not use PG need to permit a restart by either drawing less current than the inrush current limit provides, or by disabling long enough to allow the bulk capacitor to recharge. A converter that has bootstrap startup can be designed to accomplish this goal.