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I ran into a problem during a circuit simulation. The circuit is rather simple: one P-MOSFET with a load. The MOSFET is acting like a high-side switch. My load is around 4-5 ohms.

I noticed that the simulator calculated (somehow) a large voltage drop on the MOSFET. Why does this keep happening? I tried other simulators too, but got a same result. Shouldn't all the 12 V be at one side of the load if the MOSFET is fully on (minus minor drop from the MOSFET itself)?

enter image description here.

The MOSFET's characteristics:

enter image description here

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    \$\begingroup\$ "open" in electronics means "not connected". I think you mean, "on". \$\endgroup\$
    – Math Keeps Me Busy
    Commented Aug 9, 2023 at 15:55
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    \$\begingroup\$ Please attach the datasheet of the MOSFET that you have used in simulation. Is the waveform that you provided the VDS of the MOSFET? If yes, why is it changing with time? \$\endgroup\$
    – sai
    Commented Aug 9, 2023 at 15:56
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    \$\begingroup\$ Probably the PMOS you have selected can't supply the current needed to drive the load. E.g. 10V over 5 Ohms is 2000 mA. Make a plot of the IV characterisitic of the PMOS to see what it can do. \$\endgroup\$
    – HKOB
    Commented Aug 9, 2023 at 15:57
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    \$\begingroup\$ Last segment is the VDS voltage others is with others load ressistance value. \$\endgroup\$
    – Luka
    Commented Aug 9, 2023 at 15:58

1 Answer 1

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The transconductance \$g_m\$ is too small at 1.14mA/V to support the load resistance. Calculating:$$(V_{GS}-V_{th})g_m=11.9\text{mA}$$

This is all the current available. So about 35mV across the load. The rest across the FET.

Choose a FET with a higher transconductance.

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