Author Topic: Mosfet Source Voltage based on the Gate Voltage  (Read 1538 times)

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Offline DallasNYC

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Mosfet Source Voltage based on the Gate Voltage
« on: May 06, 2017, 10:23:27 am »
Hi, First time post. I am just getting started with electronics. watching alot of youtube and reading alot.
My Initial understanding of a mosfet was that once the Gate threshold voltage was met than the gate was fully opened.
Than I watched this video about Electronic Speed Control:

If you skip to the 12 minute mark, the author says that the Source Voltage can never be higher than the Gate Voltage and it is reduced by the threshold voltage?
This threw me for a loop??
So I took a look at the data sheet for the mosfet he is using, IRF3205:
http://www.irf.com/product-info/datasheets/data/irf3205.pdf
Am I right by looking at the Fig 1 chart "Typical Output Characteristics" @ T=25C, that a Gate voltage of 5 volts would allow a source voltage of around 20 volts and an Amperage around 30 amps?
The Bold curves representing the values of the gate voltage. The 2nd lowest line representing 5 volts, and following that line to the end, the bottom 'Vds' graph takes us just past 20 volts, and the side 'Id' graph takes us just above 30 amps.
So even my initial understanding was wrong, and it appears that the amount of voltage at the gate does effect the source. (Although it appears to effect the amps more).
So is my understanding right?
Which would mean the author of the video is wrong?
If not, can someone explain it to me?

That said, the first 12 minutes were very good and helped me get a grasp on E.S.C. with EMF control.

Thanks
D
 

Offline w2aew

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Re: Mosfet Source Voltage based on the Gate Voltage
« Reply #1 on: May 06, 2017, 10:32:26 am »
Here are two videos I made about MOSFETs. The first is an introduction to the various types of FETs, and the second shows a few ways to do basic testing of MOSFETs, which gives some insight into their operation:





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Offline Audioguru

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Re: Mosfet Source Voltage based on the Gate Voltage
« Reply #2 on: May 06, 2017, 10:34:31 am »
No. The datasheet for every Mosfet shows that for the range of threshold voltage the current is very low because the Mosfet is barely turned on, FAR from being fully opened.
The very low on-resistance is shown when the gate to source voltage is 10V for most Mosfets and is 4.5V for "logic level" Mosfets.

The graphs on a datasheet show a "typical" one that you cannot buy. Some are less sensitive and some are more sensitive even if they have the same part number as show in the "ranges" in the text of the specifications.
 

Online Mechatrommer

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Re: Mosfet Source Voltage based on the Gate Voltage
« Reply #3 on: May 06, 2017, 11:58:55 am »
without looking into detail. he's using all n-mosfet in Rdson mode. once the mosfet is turned on Vs
My Initial understanding of a mosfet was that once the Gate threshold voltage was met than the gate was fully opened.
no, once the Gate threshold voltage was met, the drain to source will start conducting current, but not that much. the more you put voltage to the gate compared to this "threshold voltage", the more current will be able to pass drain to source. the way you read it imo. look at how voltage you supply to the gate and select the corresponding line in the graph, and then look how much voltage connected across the DS and you draw a vertical line up to the corresponding graph line. the graph has 2 region, one is high slope, one is low slope. if the intersection meet the high slope, your mosfet will work as "fully open" mode (well very close to it, if not it), if your vertical line meet the low slope region, your mosfet will work in saturation region (linear region term in bjt), that is the mosfet is in current limit region or higher mosfet DS resistance mode. hope that is right, the way i read it. and remember, gate voltage is with respect to source pin, not "circuit ground" that i was confusing much when first playing with this stuff. the source pin can be near ground level, or near V+ level depending on your circuit. but the circuit you showed is bipolar driver which is the high side mosfet's source should be close to V+ rail. and the lower side mosfet's source is circuit ground. there is a whole difference between lower and higher side mosfets.
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Offline DallasNYC

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Re: Mosfet Source Voltage based on the Gate Voltage
« Reply #4 on: May 09, 2017, 01:15:46 pm »
Hi, firstly thanks for the quick responses, and sorry for my slow response.
I was away for the weekend mostly and trying to figure some of this out on my own.
After watching w2aew's videos and reading everyone's posts.
I feel my understanding of how to read the Typical Output characteristics chart was generally correct.
I do understand that the chart is a range and one would stay below a certain percentage of the range when choosing a part.
I am mostly interested in micro-controllers, so it is the logic level mosfets I'm interested in learning about.
Mechatrommer, you are right. I was thinking of the gate voltage in respect to ground and not the source pin.
The speed controller video make much more since to me now.

I plan to get some mosfets this week to play around with..

A question in the mean time:

If I have a 12 volt motor that I want to run from a 12 volt source, if the load is on the drain side of the mosfet, than the drain to source voltage at the mosfet would essentially be 0 volts or very minimal.
So in this case, meeting the threshold voltage of say 5 volts would open the mosfet?
But would it open sufficiently for the amperage to run the motor?
 

Online Mechatrommer

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Re: Mosfet Source Voltage based on the Gate Voltage
« Reply #5 on: May 09, 2017, 02:59:35 pm »
the max current for irf3205 is 110A, looking at the graph, giving Vgs of 6V or more will enable the fet to supply that. to be certain the mosfet is utilized to the max, you can give more up to 15-20V Vgs. to be precise about that, someone may explain it better. you can also use Rdson spec as the initial ballpark as to what Vds the mosfet will be in operation in fully on state.
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Offline DallasNYC

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Re: Mosfet Source Voltage based on the Gate Voltage
« Reply #6 on: May 10, 2017, 07:19:56 am »
O.K. yes, I see the Rds(on) spec, and as im looking at various mosfet data sheets I'm seeing as Audioguru mentioned, the conditions column for the Rds(on) usually gives a 10v or 4.5v Condition (or sometimes both 10V and 4.5V w/ the different amps for each).

High Side and Low Side were new terms to me, but now I understand they are in reference to the location of the mosfet with regards to the load. So my 12v motor example would be a low side condition, but the original esc video requires some mosfets at the high side, and that makes all the difference.

Thanks so much guys,
D

 

Offline BrianHG

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Re: Mosfet Source Voltage based on the Gate Voltage
« Reply #7 on: May 10, 2017, 09:06:37 am »
If all you want to do is turn the 12v DC motor off and on, including simple PWM speed control, with your MCU, all you need is 1 low side n-channel logic level mosfet + i would recommend a diode and cap for protection of inductive surge during the switching.  If you want to go bi-directional, then things get more complicated & you will need to look at low & high side mosfets + a high side gate driver.
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