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| santoshgurral:
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 |
| santoshgurral:
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 |
| santoshgurral:
Is MOSFET resistance different in the ohmic region and saturation region for the same VGS drive voltage? Please see the attached image? |
| Wimberleytech:
--- Quote from: santoshgurral 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? --- End quote --- 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. |
| duak:
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? |
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