Turning on a FET with logic

Thread Starter

MikeJacobs

Joined Dec 7, 2019
95
Given the fact that fets are voltage controlled devices and require essentially zero current at the gate, what are the major considerations when picking out logic components to drive them?
For example say you want to drive the gate with an OR gate or a buffer or an inverting buffet?
Assuming the logic gate has a reasonably fast turn on time to keep the fet from dissipating high power for very long, what else should one look at when trying to drive fets from Logic? Obviously you need to make sure your biasing is correct but assuming that is taken care of?
 

SamR

Joined Mar 19, 2019
1,622
There is an entire class of FETs considered as Logic Level Devices due to their less than 5Vgs required to open the gate fully.
 

crutschow

Joined Mar 14, 2008
24,074
what else should one look at when trying to drive fets from Logic?
Two other important parameters are its on-resistance/current rating, and its voltage rating.
If you need to switch it rapidly, then its gate charge is also of interest, since that charge needs to be added and removed from the gate to switch a MOSFET.
 

OBW0549

Joined Mar 2, 2015
3,141
Given the fact that fets are voltage controlled devices and require essentially zero current at the gate, what are the major considerations when picking out logic components to drive them?
One thing to consider is the Vgs threshold voltage, and understanding that Vgs(th) is the gate-to-source voltage at which the FET barely begins to conduct. Many newcomers mistakenly assume that Vgs(th) is the voltage at which the FET conducts fully; this is not so. Consult the device datasheet to find how much Vgs is required for the FET to conduct the amount of drain current you need.

Another thing to consider is that although FETs require essentially zero static gate current, their gates appear as fairly large capacitors (several thousand pF in some cases) and considerable drive current is needed to make them switch fast. Ordinary logic gates/buffers, although fast when driving low-capacitance loads, may not be able to supply the current needed for fast switching. MOSFET driver ICs are special devices designed to do just that, and many have maximum output currents exceeding an ampere. The MCP1406/7 is one I've used successfully.
 

Audioguru again

Joined Oct 21, 2019
674
I wish that Vgs(th) is never mentioned since no circuit ever operates at it, where the Mosfet is barely on and is almost off.
All Mosfets have a high gate-source capacitance so for fast switching you need a gate driver that has enough output current to charge and discharge the capacitance quickly.
 

Audioguru again

Joined Oct 21, 2019
674
Why would a linear Mosfet operate very close to cutoff? Near cutoff it is very non-linear and distorted.
Datasheets list Vgs(th) so that you can avoid it.
 

crutschow

Joined Mar 14, 2008
24,074
Why would a linear Mosfet operate very close to cutoff?
But it has to operate close to Vgs(ths) due to the typical high transconductance of a MOSFET, such as when used in an audio amp.
The value is useful when estimating the linear bias point of a MOSFET.

For example, below is the Vgs vs current for a MOSFET with a Vgs(th) of 1V.
Obviously it has to operate close to Vgh(th) for typical analog operating currents (particularly for the low bias currents of a Class AB).

1578253704160.png
 
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Thread Starter

MikeJacobs

Joined Dec 7, 2019
95
Interesting comments here
So I see many buffers and gates have a typically high current source Capability but not obviously at a high voltage
One or the other which has been discussed here previously

So what specs should you be looking at in the data sheets of a mosfet and a buffer to match the capacitive requirements


Also, I think many many many people
Operate fets near VGSth with success
So much so it’s a semi common practice
Perhaps not right at VGSth but near a volt or of VGSth
 

Audioguru again

Joined Oct 21, 2019
674
The gate-source threshold voltage for most Mosfets is 2V to 4V and is when it conducts only 0.25mA which is way too low for a class-AB audio amplifier and is extremely low for a Mosfet with a max current of 33A (IRF540) or more. A range of 132 thousand times.

An ordinary CD4xxx Cmos logic device has a very low output current but a 74HCxxxx Cmos logic device powered from 5V has a much higher output current and can drive a logic-level Mosfet quickly.
 
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