series pass element

[electros6]

Hi guys,
          I am working of a power supply. It works nice well, but I have some dought. I used N channel mosfet a series pass element. The gate is driven by a microcontroller via a transistor amplifier for a gain of 3. The amplifier is fed with a 18v via a dc to dc converter. by varying the gate through the mosfet conduct nicely. The mosfet is fed with 25V from a rectifier.
          my question is the gate voltage in the datasheet is given with respect to source Vgs, but at full conduction the mosfet source is at around 24V , but by giving around 15V from the amplifier with respect to ground the mosfet conducts pratically , but I am not able to understand it. somebody please explain

[sleemanj]

Have you confused drain and source?  It's easy to make that mistake.  In the normal configuration, for an N-channel, the Source is closest to ground (and for a P-channel, source is closest to V+)

[nickm]

If the mosfet was truly on the drain and source would be at the same voltage referenced to ground (25V) and the gate voltage would need to be around 5V higher than the source voltage (30V with respect to ground).  It sounds like your gate voltage will never be high enough to turn on the fet so it is probably conducting through the intrinsic diode which would explain the 1V drop.