Author Topic: MOSFET operating region  (Read 6956 times)

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Offline santoshgurralTopic starter

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MOSFET operating region
« on: March 16, 2020, 10:54:53 pm »
Hi

I am working on a DC electronic load circuit. Attached here is my circuit. I want to understand what region does the MOSFET operates, ohmic or saturation region when used as DC electronic load?
How do I identify in my circuit that MOSFET is in the ohmic or saturation region? In general, what are the application where MOSFETs operated in the ohmic region and saturation region specifically?

Thanks
Santosh

 

Offline Fire Doger

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Re: MOSFET operating region
« Reply #1 on: March 17, 2020, 12:07:48 am »
Ohmic

You can either calculate/simulate/measure the voltage on the gate and check it's datasheet or have a look on the circuit around it.
For example in your schematic there is only a 2m resistor between S and GND, if you drive it in saturation you have a source (pretty much) shorted to ground  :-// Do you think that it will work in saturation?
If it was driving a load (led, motor, etc) then the chances of being in saturation increases.

Ohmic is used when a voltage controlled resistor is needed. ie amplifier circuits, load circuits etc...
Saturation is used when you need a voltage controlled switch. ie driving a 12V line from 3.3 signal. Usually you will have a binary "control signal" (high - low, voltage levels depends on circuit)
« Last Edit: March 17, 2020, 12:09:20 am by Fire Doger »
 

Offline duak

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Re: MOSFET operating region
« Reply #2 on: March 17, 2020, 01:53:46 am »
I'm sorry to say that  Fire Doger is not correct.   The problem is caused by the terminology.  Go to wikipedia https://en.wikipedia.org/wiki/MOSFET#Modes_of_operation  or search for MOSFET ohmic region.

Saturation in a bipolar transistor occurs when VBE is greater than VCE and the transistor's collector to emitter conductance is at a maximum.  Saturation for MOSFETs is really Current Saturation where for a given VGS the drain current is relatively constant  for a range of VDS.  This is the region used by linear amplifiers and electronic loads.

The Ohmic region is where the MOSFET acts like a resistor, where the resistance is a function of the VGS so that the drain current is proportional to the VDS.  This is also called the Linear region but this region is used in power switching circuits.  Honest!
« Last Edit: March 17, 2020, 01:59:44 am by duak »
 

Offline Wimberleytech

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Re: MOSFET operating region
« Reply #3 on: March 17, 2020, 03:04:10 am »
Saturation: VDS > VGS - VT
Linear: VDS < VGS - VT

If you are schooled in bipolar, the terms Linear and Saturation are kinda reversed for MOS.
Better terminology for MOS is: saturation and non-saturation
« Last Edit: March 17, 2020, 12:41:27 pm by Wimberleytech »
 

Online magic

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Re: MOSFET operating region
« Reply #4 on: March 17, 2020, 06:47:42 am »
Saturation for MOSFETs is really Current Saturation where for a given VGS the drain current is relatively constant  for a range of VDS.  This is the region used by linear amplifiers and electronic loads.
If you are lucky. At low drain voltage you may end up in the linear legion too.
 

Offline Wimberleytech

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Re: MOSFET operating region
« Reply #5 on: March 17, 2020, 02:38:06 pm »
If you are lucky.
Not sure what you mean by "lucky."
 

Offline jimmc

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Re: MOSFET operating region
« Reply #6 on: March 17, 2020, 03:16:55 pm »
 

Offline santoshgurralTopic starter

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Re: MOSFET operating region
« Reply #7 on: March 17, 2020, 04:06:39 pm »
Hi

This is what exactly my doubt is also.

I just want to understand, How does it matter if MOSFET is in its linear(ohmic) region or saturation region? What difference does it make in the circuit to behave like a DC load?


Thanks
Santosh
 

Online magic

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Re: MOSFET operating region
« Reply #8 on: March 17, 2020, 05:12:06 pm »
If you are lucky.
Not sure what you mean by "lucky."
Well, in saturation you get additional input impedance for free so perhaps slightly better ripple rejection and whatnot.
 

Offline duak

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Re: MOSFET operating region
« Reply #9 on: March 17, 2020, 06:26:41 pm »
Hello Santosh,

The MOSFET will operate in the Ohmic mode only at low drain to source voltages.  For most MOSFETs, this may mean a reduction in the maximum current drawn from the test power supply when the voltage is below one volt or so.  You can see this in the Output Characteristics graph on page 3 of the data sheet.  While the resistance of the MOSFET can be varied by the op-amp at low voltages, it has a minimum value that will limit current.  Please note that the current sense resistor R1 is also in series with the MOSFET and it will add to the minimum resistance.

Your schematic shows a possible load current of 50 A.  Have you considered the power to be dissipated by the MOSFET?  Assuming the PSU is 5V, the MOSFET will have to dissipate approximately 250 W.  This is much greater than the 125 W the MOSFET is capable of.
 

Offline santoshgurralTopic starter

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Re: MOSFET operating region
« Reply #10 on: March 17, 2020, 08:17:04 pm »
Hi duak

for MOSFET to operate in linear/ohmic region VDS should be less than VGS-VTH but as you can see in below table for VGS 2.5V VDS is greater than VGS-VTH so this means the MOSFET is in saturation region for VGS of 2.5V and MOSFET is in linear/ohmic region for VGS 2.7V and 2.96V?

VGS     ID   VDS    RDS=ID/VDS   VGS-VTH    VDS < VGS-VTH
2.5           25   0.95    0.038           0.8            FALSE
2.7           50   0.9    0.018           1            TRUE
2.96           100   0.8    0.008           1.26            TRUE

also my simulation file PSU is 1V hence for 50A current 50W power to be dissipated, out of which 2miliohm resistor takes 5W dissipation and MOSFET dissipates approx 45W power.

Thanks
Santosh
 

Offline santoshgurralTopic starter

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Re: MOSFET operating region
« Reply #11 on: March 17, 2020, 08:52:22 pm »
Hi duak

for MOSFET to operate in linear/ohmic region VDS should be less than VGS-VTH but as you can see in below table for VGS 2.5V VDS is greater than VGS-VTH so this means the MOSFET is in saturation region for VGS of 2.5V and MOSFET is in linear/ohmic region for VGS 2.7V and 2.96V?

VGS     ID     VDS    RDS=ID/VDS    VGS-VTH    VDS < VGS-VTH
2.5       25     0.95    0.038                0.8            FALSE
2.7       50     0.9      0.018                  1            TRUE
2.96     100   0.8      0.008               1.26           TRUE

also my simulation file PSU is 1V hence for 50A current 50W power to be dissipated, out of which 2miliohm resistor takes 5W dissipation and MOSFET dissipates approx 45W power.

Thanks
Santosh
 

Offline santoshgurralTopic starter

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Re: MOSFET operating region
« Reply #12 on: March 17, 2020, 09:24:30 pm »
Is MOSFET resistance different in the ohmic region and saturation region for the same VGS drive voltage?
Please see the attached image?
 

Offline Wimberleytech

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Re: MOSFET operating region
« Reply #13 on: March 17, 2020, 10:26:00 pm »
Is MOSFET resistance different in the ohmic region and saturation region for the same VGS drive voltage?
Please see the attached image?
OK, you model the resistance as small-signal or large-signal.
For large-signal (which is meaningless in saturation), you simply calculate VDS/IDS.  This calculation is useful in the "linear" region because the curve can be approximated as linear in that region.

The IDS-VDS curves at various VGS values is all you need to understand terminal conditions and current.  It is all there in the plot.
 

Offline duak

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Re: MOSFET operating region
« Reply #14 on: March 17, 2020, 10:46:32 pm »
Hello Santosh,

The simple answer is that the VGS=4V curve on your graph shows the maximum ID or load current it could draw from the PSU.   The sloped R3 section of this curve shows the absolute maximum possible conductivity and lowest resistance for that VGS and VDS.  The horizontal part of the curve occurs when the MOSFET is in current saturation for that VGS.  Here, the current is a function only of the VGS.  Varying VDS does not change the current.  ie., it is in constant current mode. 

This has assumed that opamp U1 is providing 4 V VGS to the MOSFET under all conditions.  If V4 is varied then the feedback loop from the current sense resistor R1 and op-amps U1 & U2 will adjust the MOSFET VGS  to give a current corresponding to V4.  The exception to this is for the condition where VDS is too low to cause enough current to flow, that is to the upper left of the VGS=4V curve.  For example, if we set V4 to apply a VGS of 3V and VDS to 4 V a certain current will flow.   This current will remain constant as we reduce VDS until VDS is the value on the VGS=4V  curve.  If we observed VGS during this time we would see VGS increase from 3 V to 4 V.  This is a result of the feedback trying to maintain a constant current.  If we further reduce VDS, the feedback can no longer increase VGS and so the current must fall.

Does this make sense?
 

Offline Wimberleytech

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Re: MOSFET operating region
« Reply #15 on: March 18, 2020, 02:00:26 pm »
Back to saturation and "linear" regions...with an additional thought.
Another way to think about it (for symmetric MOSFETs), is to realize that when the gate voltage is greater than the drain voltage by a VT, the channel will be fully inverted (no pinchoff at the drain) and thus operating in the "linear" region.
 

Offline santoshgurralTopic starter

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Re: MOSFET operating region
« Reply #16 on: March 18, 2020, 08:59:21 pm »
My last doubt...
currently, my circuit in the attached image MOSFET is operating in linear/ohmic region, right? So in what circumstances MOSFET goes into saturation region? I mean, by making what changes would make mosfet run in the saturation region?
1)If i make VDD(PSU) from 1V to 10V?
2)If i make VGS even bigger like 10V?
all these are hypothetical values...i just want to understand when the MOSFET goes into saturation region

Thanks
Santosh
 

Offline Wimberleytech

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Re: MOSFET operating region
« Reply #17 on: March 18, 2020, 09:39:24 pm »
My last doubt...
currently, my circuit in the attached image MOSFET is operating in linear/ohmic region, right? So in what circumstances MOSFET goes into saturation region? I mean, by making what changes would make mosfet run in the saturation region?
1)If i make VDD(PSU) from 1V to 10V?
2)If i make VGS even bigger like 10V?
all these are hypothetical values...i just want to understand when the MOSFET goes into saturation region

Thanks
Santosh
please post your .asc file

!!! and model file for the devices you are using  !!!
« Last Edit: March 18, 2020, 09:48:01 pm by Wimberleytech »
 

Offline santoshgurralTopic starter

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Re: MOSFET operating region
« Reply #18 on: March 18, 2020, 09:49:59 pm »
@Wimberleytech attached .asc file and a screenshot of ckt

Thanks
Santosh
 

Offline Wimberleytech

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Re: MOSFET operating region
« Reply #19 on: March 18, 2020, 10:07:07 pm »
Here is the IV curve for your transistor.  I crudly sketched the Lin/Sat demarcation line...it is approximate.  The VGS goes from 0 to 3V in 0.1V increments.  You can see from this where your VDS needs to be for Sat or Lin.
« Last Edit: March 18, 2020, 10:15:57 pm by Wimberleytech »
 

Offline santoshgurralTopic starter

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Re: MOSFET operating region
« Reply #20 on: March 18, 2020, 10:15:19 pm »
@Wimberleytech thanks
 Could you please share how did you plot this MOSFET transfer characteristics curve (Id vgs vds) in ltspice

Thanks
Santosh
 

Offline Wimberleytech

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Re: MOSFET operating region
« Reply #21 on: March 18, 2020, 11:06:12 pm »
@Wimberleytech thanks
 Could you please share how did you plot this MOSFET transfer characteristics curve (Id vgs vds) in ltspice

Thanks
Santosh

It's in the .dc command

I have attached .asc here
 


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