MOSFET Gate Voltage - Query - Page 1
Forum section: electronics / beginners

author: Freshman
author_rank: Regular Contributor
timestamp: April 09, 2024, 04:58:15 pm
content: There's this MOSFET (https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/414/SH8K5.pdf)The gate threshold range for this MOSFET is given as 1V to 2.5V from the table (but from the table I see this value is for a drain current of only 1mA).My question is, I want to get a drive current of 1A. In that case, What should be the maximum allowable gate-source threshold voltage I can give?Seeing the graph below, Gate-Source voltage vs Drain current, the maximum gate-source voltage that is shown is 3V.So my question is:1. What is the maximum gate-source voltage that I can give to get a drain current of 1A?2. What if I give 10V as gate-source voltage? - I know that I won't damage the MOSFET as the maximum gate-source voltage that is mentioned in the datasheet is 20V. So, what would happen if I give 10V as Vgs?

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 09, 2024, 05:32:32 pm
content: We usually don't calculate gate voltage for specifed current because a dependence is very uncertain and unreliable (temperature dependent etc). But if it a school task then you may use a datasheet.

author: BennoG
author_rank: Frequent Contributor
timestamp: April 09, 2024, 05:33:37 pm
content: the max GS voltage is 20V so 10V is no problem.Benno

author: MarkT
author_rank: Frequent Contributor
timestamp: April 09, 2024, 10:04:35 pm
content: That MOSFET is rated for 4.5V gate drive (i.e. there is an Rds(on) entry for that).The threshold voltage is not relevant to turning on the MOSFET, only for guaranteeing when it is fully _off_. This specification always confuses beginners, best to completely ignore it unless you are using the MOSFET as an amplifier (rather than a switch). Look for Rds(on).

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2024, 10:45:14 am
content: The answer is on page 3 of the data sheet: figures 4 and 5. Note these graphs are often the typical values, so increase the actual gate drive by >50% to ensures it turns on hard.

author: CaptDon
author_rank: Super Contributor
timestamp: April 10, 2024, 01:43:08 pm
content: You don't normally try to predict a gate voltage vs. drain current because it varies greatly. If your circuit were designed to maintain a steady current across varying loads and drain voltage then you would use a control loop to operate the gate. In most cases you want to use the MOSFET as a switch. Full on or full off. They are showing you a typical gate voltage for 1ma. considered the OFF threshold. What you would be more interested in is most likely the required gate voltage to turn the device fully on so that the RDS is low and the MOSFET dissipates very little heat. MOSFETS are a non-linear device so when used in a linear amplifier application a surrounding negative feedback circuit is required to create a linear transfer function. If your course of study says "What gate voltage is required for 1 amp of drain current" then the course is total crap as they fail to understand that the particular parameter in question varies too greatly to predict!!! And this is often how poorly these courses are presented and why my junior engineers make the same mistakes over and over.

author: CaptDon
author_rank: Super Contributor
timestamp: April 10, 2024, 02:56:22 pm
content: Most power MOSFET's when used at currents below 30% of their maximum rated current have very little need or advantage for gate voltages above 4.5 to 5.5. The gate behaves electrically like a capacitor of .001uf to .005uf for large MOSFET's and what ever charge you push into the gate you will also have to remove to turn the device off. Increasing the gate drive voltage means you need a more robust drive circuit capable of pushing and pulling possibly several amps of current in and out of the gate when switching at high speeds. Often we see a sensible limit of 10 volts on gates rated for 20 volts maximum. This keeps the device out of hard saturation and provides faster switching speeds. I have seen gates pulled to voltages below the source terminal to get very fast turn off times. In most cases the term 'maximum gate voltage' applies to both the forward and reverse polarities of the gate voltage unless there are internal clamp diodes on the gate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 10, 2024, 03:42:32 pm
content: Quote from: Zero999 on April 10, 2024, 10:45:14 amThis.More specifically, notice how the plot goes down to 1mA. Notice the test value for Vgs(th): Vds = 10V, Id = 1mA. This plot is also at Vds = 10, so we have exactly the conditions for Vgs(th), and we can see that, for the particular specimen tested, Vgs(th) was exactly 1.6V.Generally, this curve shifts left or right due to manufacturing differences (doping level, gate oxide thickness).We can constrain rotation/skew of these curves, by the Rds(on) spec. At Vgs(on) = 4V, Id = 3.5A, we have Rds(on) max 150mΩ. This is Vds = 0.525V, much less than the 10V test case, but Id vs. Vds is a one-to-one function, i.e. it only goes down as Vds goes down. If we move up to 3.5A following the curves, we find Vgs(on)(typ) = 3.2V or so. Finally, we know Yfs = 2S, so if we go up by 0.8V, we would extrapolate this curve by 1.6A or to 5.1A, well above the test current (which of course then comes down, as Vds is saturated). So we are sure the curve isn't tilted by more than 0.8V, and probably is a lot less than this, supporting the assumption that Vgs(th) shift is a valid approach.Walking the curve back down to 1A, we drop about 0.5, 0.8 gate volts, so presumably a safe Vgs(on) at 1A would be 3.2 to 3.5V. Better to stay on the high side of that, then.A 5V level shifter or gate driver, trivially handles the problem, and a lower-voltage logic output may not have sufficient drive strength for whatever switching speed is desired anyway.Other than that, there are MOSFETs with Vgs(th) in the 0.5-1.5V range, with guaranteed 3.3V Rds(on) parameters, which can be used with LVCMOS logic (again, assuming switching speed is acceptable).If your purpose is to use 3.3V logic, I would keep shopping.As for higher, anything up to the rating will do, but you waste extra gate charge and drive power in the process, and I would avoid pushing the rating exactly. 12V is perfectly safe to use, it's just way more than needed; 5 or 6V is probably optimal, to minimize both Rds(on) and drive power.Tim

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2024, 06:49:29 pm
content: Quote from: CaptDon on April 10, 2024, 02:56:22 pmYou forgot the Miller effect which vastly increases the gate capacitance, just as the MOSFET starts to conduct.For fun, I measured the gate capacitance of a SPICE model, with the MOSFET conducting 1A vs off.M1 is on. The gate is biased to the drain and is forced to conduct 1A. So V1 doesn't affect the DC bias, it's AC coupled via C1, a huge 1F capacitor.M2 is off, so its gate can be directly connected to V1.C1 is so huge, it makes no difference to the gate current of M1.Run AC analysis at 1Hz, to make it easy:Ig(M1): mag: 1.16312e-05 phase: 89.9995° device_currentIg(M2): mag: 1.77878e-07 phase: 90° device_currentThe phase angles are 90°, so they're both purely capacitive, which makes it easier:Calculate the impedance of M1's gate:Z = V/I = 1/(1.16312*10-05) = 85.97*103The capacitance:The formula for impedance is:Z = 1/(2ᴨFC)Rearrange for C:C = 1/(2ᴨFZ)C = 1/(2ᴨFZ) = 1/(2ᴨ*1*1.16312*10-05) = 1.85*10-6FM2's gate impedance:Z = 1/(1.77878*10-07) = 5.622*106The capacitance:C = 1/(2ᴨ*1*5.622*106) = 28.31*10-9FThe Miller effect multiplies the capacitance by 65, under these conditions.