9.2.1.3 Input Diodes or Diode Bridges

The IEEE 802.3af requires the PD to accept power on either set of input pairs in either polarity. This requirement is satisfied by using two full-wave input bridge rectifiers as shown in Figure 13. The full configuration may not be required when a custom high-power system is implemented. Silicon p-n diodes with a 1-A or 1.5-A rating and a minimum breakdown of 100 V are recommended, however Schottky diodes will yield a somewhat lower power loss. Diodes exhibit large dynamic resistance under low-current operating conditions such as in detection. The diodes should be tested for their behavior under this condition. The total forward drops must be less than 1.5 V at 500 μA and at the lowest operating temperature.