Purpose of Inrush Limiting and the PD Bulk Capacitance
What is the purpose of inrush current limiting anyway? It is to charge the input cap of the DC-DC converter stage that follows in a controlled manner, and also do so in a reasonable amount of time. In the standard, this cap is called CPD. But to make it clearer where exactly it is positioned, we prefer to call it “CDC_DC.” Whatever the names, it is just the input bulk cap of the DC-DC stage that follows the PD’s front-end (its pass-FET section).
We remember that during detection and classification, this bulk cap is not exposed to the PSE and is kept isolated from the line by the PD’s pass-FET. So its initial condition is fully uncharged or 0 V. This FET-isolation feature is basically the ...
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