Imagine this: You have a ProBook 4540s with no power. The DC jack gets 19V, but no LED. Common fault: the charging IC or a blown PQ101 MOSFET.
You suspect the 3.3V and 5V coil stages. On a schematic, you see the PWM controller (usually a BQ series chip). But on the board, these coils are surrounded by a forest of tiny capacitors. hp probook 4540s boardview portable