I measure in common source connected mosfet (19N10L) Vds about 2.4V in on state. The current is 2A.
In datasheet is defined Rds 0.1ohm. Why is Vds in this mosfet so high (2.4V)? Shouldnt be Vds = 0.1 x 2A = 0.2V?
Vcc is 18V.
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Rds = 100mOhm @ Vgs = 10V
Are you driving the gate voltage high enough? If Vgs is not high enough Rds will be higher.
The FQD19N10L is a logic-gate MOSFET, and will turn on completely with only a few volts on the gate. Rated current is nearly 16 amps, so 2 amps is not even close to the device limit.
Maximum gate voltage is 20 volts, so 18 volts is not an issue.
There is one possibility I can think of. Are you sure you have not connected your circuit like
Then 2.4 volts is a reasonable number, being about the gate voltage a logic-level device needs.
If so, you need to change your circuit to