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By way of disclaimer, I do not know a whole lot about MOSFETs, so there is a good likelihood for errors in my question.

I am debugging a PCB where a voltage line is too high and I am trying to identify the faulty component. I have replaced the voltage regulator that regulates this 1.8V line twice, but the line still is measuring ~2.4V (on multiple meters) so I don't think the voltage regulator is at fault.

This 1.8V line is used as the gate for multiple MOSFETs throughout the PCB. I have been doing a lot of research and have read some things about the relationship between the drain and gate of MOSFETs. Namely, that the drain-source current increases as the gate voltage increases, and that Vds = Vg - Vt (where Vt is the threshold voltage).

So, are the inverses of these statements true? If the drain-source current was being increased by some other component, would that cause Vg (my faulty 1.8V line) to increase? If Vds is somehow increased, would that cause Vg to increase or Vt to decrease?

Thanks for the help. I will do my best to clarify if need be.

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Your equation Vds = Vg - Vt is not correct. The relationship between Vds and Vg will actually determine which region the transistor is operating in, if Vg is greater than Vt:

  • Vds < Vg − Vt =>The linear region, where it acts like there's a resistor between the drain and the source (as Vds increases, Ids increases linearly).
  • Vds ≥ (Vg – Vt) =>The saturation region, where an increase in Vds only results in a small increase in Ids.

In your case, Vg is unexpectedly at the same high voltage as Vds. If Vds is dragging Vg up, it's because there's been permanent damage to the transistor. This doesn't sound like the most likely cause of your problem, or at least the root cause. The board shouldn't have been designed to operate with Vds much greater than Vg, and it probably wasn't.

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  • \$\begingroup\$ This saves me from chasing false leads. Thank you! \$\endgroup\$ – Taylor Davison May 17 '18 at 22:33
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Yes the equation may be rearranged as such:

Vg = Vds+Vt

However a mosfet will not generate energy, think of it like a voltage controlled resistor where the votlage of the gate controls the resistance.

I would remove the mosfet completely and measure the voltage, if it is coming from somewhere else, it will still register 2.4V

There is a possibility that the gate got damaged, and you could get bleed through that way, but not as likely as other scenarios

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  • \$\begingroup\$ Thanks for the answer! So in that equation, is Vg, Vds, or Vt the dependent? Meaning which of those voltages is driven by the other two? \$\endgroup\$ – Taylor Davison May 17 '18 at 20:15

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