MOSFET operating region - Page 1
Forum section: electronics / projects

author: santoshgurral
author_rank: Contributor
timestamp: March 16, 2020, 10:54:53 pm
content: HiI 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?ThanksSantosh

author: Fire Doger
author_rank: Regular Contributor
timestamp: March 17, 2020, 12:07:48 am
content: OhmicYou 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 groundDo 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 controlledresistoris needed. ie amplifier circuits, load circuits etc...Saturation is used when you need a voltage controlledswitch. ie driving a 12V line from 3.3 signal. Usually you will have a binary "control signal" (high - low, voltage levels depends on circuit)

author: duak
author_rank: Super Contributor
timestamp: March 17, 2020, 01:53:46 am
content: I'm sorry to say that Fire Doger is not correct. The problem is caused by the terminology. Go to wikipediahttps://en.wikipedia.org/wiki/MOSFET#Modes_of_operationor 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!

author: Wimberleytech
author_rank: Super Contributor
timestamp: March 17, 2020, 03:04:10 am
content: Saturation: VDS > VGS - VTLinear: VDS < VGS - VTIf you are schooled in bipolar, the terms Linear and Saturation are kinda reversed for MOS.Better terminology for MOS is: saturation and non-saturation

author: magic
author_rank: Super Contributor
timestamp: March 17, 2020, 06:47:42 am
content: Quote from: duak on March 17, 2020, 01:53:46 amIf you are lucky. At low drain voltage you may end up in the linear legion too.

author: Wimberleytech
author_rank: Super Contributor
timestamp: March 17, 2020, 02:38:06 pm
content: Quote from: magic on March 17, 2020, 06:47:42 amNot sure what you mean by "lucky."

author: jimmc
author_rank: Frequent Contributor
timestamp: March 17, 2020, 03:16:55 pm
content: This thread from a few years ago discussed the same question.https://www.eevblog.com/forum/chat/how-does-this-power-mosfet-work-in-a-constant-current-load/msg22553/#msg22553Jim

author: santoshgurral
author_rank: Contributor
timestamp: March 17, 2020, 04:06:39 pm
content: HiThis 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?ThanksSantosh

author: magic
author_rank: Super Contributor
timestamp: March 17, 2020, 05:12:06 pm
content: Quote from: Wimberleytech on March 17, 2020, 02:38:06 pmWell, in saturation you get additional input impedance for free so perhaps slightly better ripple rejection and whatnot.

author: duak
author_rank: Super Contributor
timestamp: March 17, 2020, 06:26:41 pm
content: 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.

author: santoshgurral
author_rank: Contributor
timestamp: March 17, 2020, 08:17:04 pm
content: Hi duakfor 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-VTH2.5 25 0.95 0.038 0.8 FALSE2.7 50 0.9 0.018 1 TRUE2.96 100 0.8 0.008 1.26 TRUEalso 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.ThanksSantosh

author: santoshgurral
author_rank: Contributor
timestamp: March 17, 2020, 08:52:22 pm
content: Hi duakfor 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-VTH2.5 25 0.95 0.038 0.8 FALSE2.7 50 0.9 0.018 1 TRUE2.96 100 0.8 0.008 1.26 TRUEalso 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.ThanksSantosh

author: santoshgurral
author_rank: Contributor
timestamp: March 17, 2020, 09:24:30 pm
content: Is MOSFET resistance different in the ohmic region and saturation region for the same VGS drive voltage?Please see the attached image?

author: Wimberleytech
author_rank: Super Contributor
timestamp: March 17, 2020, 10:26:00 pm
content: Quote from: santoshgurral on March 17, 2020, 09:24:30 pmOK, 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.

author: duak
author_rank: Super Contributor
timestamp: March 17, 2020, 10:46:32 pm
content: 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?

author: Wimberleytech
author_rank: Super Contributor
timestamp: March 18, 2020, 02:00:26 pm
content: 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.

author: santoshgurral
author_rank: Contributor
timestamp: March 18, 2020, 08:59:21 pm
content: 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 regionThanksSantosh

author: Wimberleytech
author_rank: Super Contributor
timestamp: March 18, 2020, 09:39:24 pm
content: Quote from: santoshgurral on March 18, 2020, 08:59:21 pmplease post your .asc file!!! and model file for the devices you are using !!!

author: santoshgurral
author_rank: Contributor
timestamp: March 18, 2020, 09:49:59 pm
content: @Wimberleytech attached .asc file and a screenshot of cktThanksSantosh

author: Wimberleytech
author_rank: Super Contributor
timestamp: March 18, 2020, 10:07:07 pm
content: 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.

author: santoshgurral
author_rank: Contributor
timestamp: March 18, 2020, 10:15:19 pm
content: @Wimberleytech thanksCould you please share how did you plot this MOSFET transfer characteristics curve (Id vgs vds) in ltspiceThanksSantosh

author: Wimberleytech
author_rank: Super Contributor
timestamp: March 18, 2020, 11:06:12 pm
content: Quote from: santoshgurral on March 18, 2020, 10:15:19 pmIt's in the .dc commandI have attached .asc here