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# Are drain and source interchangeable for power MOSFETs?

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#### ALU1AB

##### Newbie level 2
Hi all,

To my understanding, unlike normal MOSFETs, discrete power MOSFETS have asymmetric structure.
But is it still ok to swap drain and source? (source at higher potential than drain)
Will I_D equations for triode(linear) & saturation region still apply?

Thanks

Hi all,

To my understanding, unlike normal MOSFETs, discrete power MOSFETS have asymmetric structure.
But is it still ok to swap drain and source? (source at higher potential than drain)
Will I_D equations for triode(linear) & saturation region still apply?

Thanks

obvious question is --- why do you want to do this ?
obvious answer is ----- try it out and let us know what you find

Any discrete MOSFET works in linear mode, so question about saturation mode is not correct.
Saying MOSFETS have asymmetric structure what do you mean (bulk connects to source)?
I think You have forgotten take account inner bulk-drain diode, that doesn't allow to work NMOS with drain voltage less then source (bulk) voltage.

Any discrete MOSFET works in linear mode
You'll remember that saturation region is defined by the limit line Vds > Vgs - Vth. There's no reason why a "discrete MOSFET" shouldn't work in saturation mode. Most MOSFETs in linear amplifiers actually do. You possibly meaned to say that a MOSFET switch should avoid saturation region.

The substrate diode point is right, of course. Due to the substrate diode, only a small part of the MOSFET reverse characteristic can be utilized. It's nevertheless essential for synchronous power switches.

This is where I got confused,

You can see that the Power NMOS's source is connected to the Power supply and drain is connected to the load and the current will be flowing from the source to drain. (so that means negative vds?).

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This is where I got confused

You can see the Power NMOS's source is connected to the PS, drain is connected to the load, and current is flowing from the source to drain. Is VSD positive?
I know it's being used as a switch so does linear(triode) region equation still apply here?

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