Electronic – Understanding MOSFET gate characteristics

mosfet

I am just starting to use MOSFETs more frequently. Rather than use suggested parts in circuits that others design, I want to understand them better so I can design my own circuits. I typically use them to drive inductive loads from microcontroller pins.

I'd like to see if my understanding is correct with the following:

  1. The \$V_{GSS}\$ (Gate-Source Voltage) listed under Absolute Maximum Ratings is the maximum voltage that can be applied to the gate with respect to the source. (And thus should be avoided.)

  2. \$V_{GS}\$ (Gate Threshold Voltage) is the voltage at which the gate starts to turn on, but does not mean that the MOSFET (as a switch) is fully conducting. It is given with min and max values, which means that the starting point may vary from part to part.

  3. To determine the gate voltage at which the MOSFET is fully "on" the \$R_{DS(on)}\$ (Drain-Source On-Resistance) value can be examined: the "test conditions" show voltage values at which the FET is fully on.

I don't think my understanding of #3 is quite right. Olin provided a great answer which is where I derived that point from.

Consider a Fairchild FQP30N06L; the datasheet shows two test voltages (10 and 5V) for the \$R_{DS(on)}\$ at different resistances (varying only by <10 mΩ).

Based on this, I'm confused about how to determine what voltage I should supply at the gate for the FET to be considered fully on. Can you explain what I need to look at on the datasheet to calculate this properly?

Best Answer

Point 1 and point 2 are quite accurate I would say. For point 3, I always search for the following graph when I'm looking for the likely conduction capabilities of FETs: -

enter image description here

It tells me that if I put 3.5V on the gate and I want to pass 10A drain current there is likely to be 0.5V dropped across the device leading to a power dissipation of 5W. This is an on-resistance of 50 m\$\Omega\$. However, if i used a gate drive of 10V, I can expect the voltage dropped by the device to be about 0.25V i.e. a power dissipation of 2.5W or an on-resistance of 25 m\$\Omega\$.

Regarding the state of "fully-on" you can see that there is no magic state that is reached at some arbitrary gate voltage but I'd say that at about 4.5V gate drive, the benefits of driving with a higher gate voltage are diminishing.

Figure 3 in the data sheet (which shows the graph which you deduced the on resistance variation of 10 m\$\Omega\$) is a derivation of the graph (figure 1) in my answer. Obviously, Figure 1 carries more information because it covers a wider range of gate voltages.