I want to control an N-channel MOSFET from a 3 V power source.
The thing is, I have difficulties understanding how to know the minimum Vgs value is to be applied for the MOSFET to be saturated. For example, what about the MOSFET CSD19501KCS (80 V N-Channel NexFET)?
Answer
The first indication of how low Vgs you can apply is the Vgs-th (gate-source threshold voltage)
In this case Vgs-th is 3.2V (max value) so anything below that is a no go. Also note that the Vgs-th is specified for VGS=VDS and ID=250uA, so when you apply Vgs=3.2V, you will get an equal voltage drop across drain-source with a drain current of just 250uA , in other words you can't really use the mosfet with that low Vgs.
To find a proper Vgs, you should check the Vgs vs Rds-on graph and find an appropriate value of gate-source bias that has a drain-source resistance that is low enough for your application.
Your specific device has this graph
so the lowest you can go based on that is a Vgs=4.5V for about 18 \$ m\Omega\$ resistance (estimated values).
There is another graph where you can get info from.
The graph is for VDS=5V so for Vgs=3.9V the Rds-on will be 5V/20A = 0.25 Ohm, if that level of resistance suits your application then you can use a Vgs that low but to get the best of the specific device you need to go higher.
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