Browsing videos I came across this one.

In this video the author points out that the two resistors commonly used can create a voltage divider (see Method 1) and they should be connected as in Method 2 below. However, I have only ever seen R1 and R2 represented as Method 1 in all other videos and schematics I have seen. I've never seen this mentioned in any other video/tutorial on MOSFETS before.

(R1 = 680 / R2 = 10K)




It works just fine in either method, I'm just curious if there are other reasons why Method 1 seems to be the most common method instead of method 2? In method 1, at full +5v trigger I have a V drop of 0.317 on R1 and 4.68 on R2 with 4.68v on the Gate. Method 2 I have 0v and 5v respectively and 5v on the Gate. The load appears to turn fully on in either case so the 0.317 drop probably isn't enough to be concerned with. ??

Should R2 be as close to the Gate and Source as possible and that maybe is why Method1 is so common? I have searched other schematics that use MOSFETs and they all seem to be configured as Methode 1. But Method 2 seems to make more sense.

inquiring minds want to know

 

Hello,

Most times i see method 1.
The resistor R1 could have a much lower value.
Think of values between 22 and 100 ohms.
The circuit you show will only work with "logic" gate mosfets.

Bertus

 

So type of MOSFET and it's application would determine which method is best.

 

R1 is there to reduce gate 'ringing' caused by a fast transition into a LC network, the L being the parasitic inductance of the trace and gate pin and C the gate capacitance. A typical value being 10 - 100ohm. This is why gate traces must be kept short in fast-switching circuits such as motor controllers or switch-mode power supplies, to minimise MOSFET switching losses.

R2 is to ensure the gate is not left floating. This is particularly important when the MOSFET is being driven from a MCU output, which often defaults to an input or tristate on power-up or reset. The gate, being high impedance, if left floating can accumulate a charge leading to the MOSFET turning partially on - generally not a desirable state. If the gate is driven by an dedicated gate-driver with active pull up and down states R2 isn't needed.

Method 1 is most common, I've rarely seen the second one, but it does reduce the 'voltage divider' effect though that's rarely an issue as usually R2 >> R1 by 3 or 4 orders of magnitude.

 

I'm just curious if there are other reasons why Method 1 seems to be the most common method instead of method 2?

I don't usually use an R1 but see that method 2 is better because there's no voltage divider to consider.

 

Browsing videos I came across this one.

inquiring minds want to know

Look at this for info on Mosfets! https://tahmidmc.blogspot.com/2013/01/using-high-low-side-driver-ir2110-with.html
I took to Mosfets readily due to my original, early on, Build and usage of valve (tube) circuits.
i.e. Transconductance amplifiers.

 

Method 2 is more technically correct.
If R1 is small (<30Ω) and R2 is large (>10k) which is normally the case, then Method 1 loses 0.3% of the gate drive voltage. Who cares?

 

The Gate should be connected in "some manner", or even shorted to, the Source,
while in storage, and any time that there is even a remote chance of having a "Floating-Gate".

A FET-Gate can be permanently damaged if You look at it crooked.

I make it a habit to wrap a single fine strand of Wire around the Gate and Source Pins
until the FET is soldered in place, and all other protective Components have been previously installed.
.
.
.

 

Method 1 is only used if your Mosfet’s maximum gate source voltage is lower than the drive voltage.
Older devices had a maximum Vgs of +/-10 volts, if you were to drive them using automotive-level voltages of 12 to 14 volts they could be damaged.
Most modern MOSFET have a +/-20 volts maximum gate drive and thus would not be required.
Although for the specific automotive case I still like to use it because the automotive environment has many voltage spikes.