The isolated DC/DC converter system block is responsible for providing regulated and isolated 24 V to the connected field equipment. To include this functionality, the PD includes a flyback design, which takes advantage of the LM5155 flyback controller with the CSD19531Q5A external N-channel MOSFET.

Figure 3-11 PD Power Supply Start-up

On the captured oscilloscope screenshot in Figure 3-11, the start-up of the PD power supply is seen starting at completion of the detection without SCCP (Fast Start-up). The SCCP line voltage rises with the slew rate controlled by the PSE. Once this voltage rises above the eFuse undervoltage lockout (UVLO) threshold, the eFuse turns on and the eFuse output voltage rises with slew rate set by its DVDT capacitor. Finally, once the UVLO threshold of the LM5155 controller is crossed, the DC/DC converter turns on with the slew rate controlled by its soft-start capacitor. The TPS2660 and LM5155 slew rate controls on the PD were tuned so that the inrush current would not exceed the overcurrent protection (I_LIM) in the eFuse.

The scalability and adaptability of this design were both very important, because of the various power classes from the IEEE 802.3bu and 802.3cg standards.

The following list shows the PD compatible power classes:

By changing UVLO, OVP, and other slew rate or protection passive components, the PD can operate with input voltages from 12 V to 36 V and currents of up to at least 632 mA and therefore support all 24 V classes.