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I am trying test my p-channel mosfet (IRF9Z34) high side switch on load. On circuit simulator I get the result I expect, Rdson (VM1) is low and load current is 1.4A. (In the target system the gate will be controlled, here the mosfet is always on for tests). T1 - P-MOSFET IRF9Z34.

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In the real circuit current is max 0.9A, so I have very big Rdson, I don't understand why. Vgs > Vth, The power supply has a current effiency > 2A.

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    \$\begingroup\$ What is the \$V_{gs}\$ used to specify the \$R_{ds}(on)\$ in the datasheet for your MOSFET, and what \$V_{gs}\$ are you applying? \$\endgroup\$
    – The Photon
    Apr 14 '20 at 18:00
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    \$\begingroup\$ It looks like the 5V is too low to fully open this MOSFET \$\endgroup\$
    – G36
    Apr 14 '20 at 18:01
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If your voltage is at least 10V you're guaranteed to have < 140 m\$\Omega\$ Rds(on) with the junction at room temperature (more when hot).

At Vgs(th) - which could be as high as 4V, you are guaranteed to have at least 250uA flowing so equivalent resistance of 16k\$\Omega\$.

If your input voltage is appreciably less than 10V you should use a different MOSFET- one that has specified and acceptable Rds(on) at a Vgs that is equal or less than the actual drive voltage. These MOSFETs are called "logic level".

For example, an AO3401A has less than 60m\$\Omega\$ Rds(on) with a 4.5V drive voltage.

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    \$\begingroup\$ Ok, I undestand. Thank you very much for help ! \$\endgroup\$
    – Pak00
    Apr 14 '20 at 18:08

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