[SOLVED] too small ID for VGSth?

[mmitchell]

Hi,

MOSFET is voltage controlled. But why we see for this diode there is ID=250μA listed for threshold voltage?


And this is a 2A diode, but the ID=250μA listed is only 1/8000 of the rating.
**broken link removed**

Matt

 

[mtwieg]

ID refers to drain current, not diode current. It's normal to characterize Vth at a very small fraction of the rated maximum drain current. This is so that the designer knows they have to keep Vgs below that level to avoid burning the FET due to incomplete turn off.

For example if your FET operates from 400VDC and the gate driver only pulls down Vgs to 1.8V, then a few milliamps of current at 400V may cause more heating than the losses when the gate is driven high.

 

[mmitchell]

Does it actually mean
"Gate threshold voltage is the lowest VGS at which a specified small amount of ID flows. The test is run with VGS = VDS."
https://www.tek.com/support/faqs/how-can-i-test-mosfet-gate-threshold-voltage-my-curve-tracer

 

[FvM]

Vgs(th) is a well defined data sheet parameter. There shouldn't be any doubts about it's meaning.

Gate threshold voltage is the lowest VGS at which a specified small amount of ID flows. The test is run with VGS = VDS.

Why "lowest"? The curve in post #1 shows the typical value, the possible data sheet range is 1V to 3V.

 

[KlausST]

Hi,

Vgs(th) shows at wich GS voltage the FET "starts" to be conductive.

Therefore it is a relatively low ID current. Now ID of 0 uA is not a good value to specify VGS, therfore you need some value larger than 0uA but it should be far away form rated max. current.


Klaus