I have been studying MOSFETs and have a question regarding saturation mode. To my understanding, the drain current is constant in saturation mode. Does this mean that the drain-source resistance increases with drain-source voltage (to keep drain current constant)? If so, does this mean that the voltage drop across the transistor increases with drain-source voltage? For instance, if I were applying a high voltage (say 20V, and the transistor was saturated at 10V) across the drain-source and had a load in series with the transistor, would I get a voltage drop much less than 20V across the load (because of the high resistance of the transistor)? I am not sure if my interpretation is too "ohmic". Any help/resources would be appreciated. Thank you!
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\$\begingroup\$ AFAIK, RDS is not tied to VDS, except possibly with minor thermal effect \$\endgroup\$– dandavisCommented Jul 5, 2017 at 20:25
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1\$\begingroup\$ I suggest checking out this question \$\endgroup\$– RedjaCommented Jul 5, 2017 at 20:28
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\$\begingroup\$ Thank you for the responses! I just want to make sure my understanding is correct. If the load I am using has a resistance of 200 ohms, would it be a good approximation that the load drops about 20 V and draws about 0.1 A (assuming the MOSFET is saturated and can handle a maximum drain current of 5 A)? \$\endgroup\$– dtsCommented Jul 5, 2017 at 22:40
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\$\begingroup\$ I guess what I am really struggling with is the concepts of the drain current. Most datasheets have a graph comparing drain-source voltage to drain current. When the graph says the transistors draw 3 A, for example, at saturation is this the max current it can draw or the amount of current it will draw? Is the current drawn determined by the transistor or the load itself? \$\endgroup\$– dtsCommented Jul 6, 2017 at 0:05
1 Answer
For a given gate-source voltage and above a certain drain-source voltage, the MOSFET behaves like a constant current sink. Say you have 3 volts applied to the gate with respect to the source - if if the drain-source voltage rises to 2 volts, the drain current reaches a plateau of about 5 milliamps. If you increase the drain-source voltage (whilst keeping gate voltage at 3 volts), the drain current remains largely at 5 mA.
This is broadly referred to as "constant current" mode of operation.
Is the current drawn determined by the transistor or the load itself?
The current is determined by the transistor mainly but, because there is a slight slope there is a small change in current with a big change in drain load resistance.
for example, at saturation is this the max current it can draw or the amount of current it will draw?
This is the typical current it will draw for a given gate-source voltage (3 volts in my example).
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\$\begingroup\$ Thank you for the response! So, if I attach a load to the transistor, would the load have 5 amps going through it (in the example you gave)? \$\endgroup\$– dtsCommented Jul 6, 2017 at 11:32
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\$\begingroup\$ Providing the resistance of the load does not cause the voltage across the MOSFET to drop below about 2 volts then yes. If R is 10 ohms, 5 amps drops 50 volts hence you need a Vcc of at least 52 volts. Clearly if R is 5 ohms you only need a Vcc of 27 volts (or more). \$\endgroup\$– Andy akaCommented Jul 6, 2017 at 11:35
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\$\begingroup\$ I think I understand. I think I just misinterpeted the meaning of the drain-source voltage as meaning the voltage of the drain relative to ground instead of relative to source. Thank you again! \$\endgroup\$– dtsCommented Jul 6, 2017 at 11:39
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1\$\begingroup\$ Thanks on SE = upvoting/answer acceptance! You may not be aware of this. \$\endgroup\$– Andy akaCommented Jul 6, 2017 at 11:42