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I would like to know why in most of Mosfet Datasheets there are parameters like Maximum Vgs or Maximum Vds but never Vdg, the maximum voltage acceptable between Drain and Gate.

Maybe because it's similar to Vgs or Vds?

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...but never Vdg...

I just opened the datasheet of the 2N7002 which does list a maximum rating for \$V_{DGR}\$. As I would expect it has the same value as \$V_{DSS}\$, both 60 V.

That makes sense as \$V_{GSS}\$ has a much smaller maximum value, that is because the values of \$V_{GS}\$ is limited to what the gate oxide of the MOSFET can withstand. For the 2N7002: \$-20V < V_{GS} < + 20 V\$

The maximum \$V_{DS}\$ can be much larger, many MOSFETs are constructed such that \$V_{DS}\$ can be much larger than the maximum \$V_{GS}\$. This is done by constructing the drain in such a way that it can handle a higher voltage. See the article on power MOSFETs on Wikipedia. Without this "high voltage" drain construction the applications for MOSFETs would be limited.

So if \$V_{DS}\$ can be much larger than \$V_{GS}\$ then the maximum \$V_{DG}\$ must also increase, it cannot be the same as the maximum \$V_{GS}\$ as that would limit the maximum \$V_{DS}\$.

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