| 摘要 |
<p>An AC generator (204) has a first terminal coupled through an isolation loss detect (ILD) capacitor (206) to the positive bus of a Power-Over-Ethernet (POE) system, and has a second terminal coupled through the primary of a transformer (208) to earth ground. AC current flowing between earth ground and the positive bus (146) causes a corresponding AC voltage to be developed across the secondary of this transformer. The secondary of the transformer is coupled to an AC detector, the output of which is coupled to a comparator. The threshold of the comparator is set such that when AC current flow through the ILD capacitor exceeds a value set by this threshold, an isolation fault output is generated at the output of the comparator. In normal operation, isolation between earth ground and any conductor in the POE system coupled to the positive bus is high, so little or no AC current flows through the ILD capacitor (206). When an isolation loss occurs such as a short circuit between earth ground and any conductor in the POE system coupled to the positive bus, the AC current in the ILD capacitor and transformer increase significantly, causing the output of the AC detector to exceed the threshold of the comparator, and causing an isolation fault output. A high level of galvanic isolation is maintained even with the added ILD circuit because the DC leakage current of the ILD capacitor is negligible. In an embodiment wherein distributed bypass capacitance exists from the positive and negative buses to earth ground, the known nominal AC current caused to flow in the ILD capacitor by this distributed capacitance allows a self-check function of the ILD system. A second comparator is coupled to the output of the AC detector, and a second threshold is set which is above zero but below the voltage caused by this nominal AC current flow. If the AC generator or AC detector fails, causing the detector output to drop significantly, this second comparator changes state to show a failure of the ILD circuit.</p> |
