Power Consumption of MOSFET in Active & Saturation Regions

[pavan garate]

I know that a MOSFET behaves like a resistor in Active Region, and has constant power consumption in Saturation Region.

My queries are-

1) Whether the MOSFET consumes more power in Active region or Saturation region?

2) If a MOSFET works in alternating Active & Saturation regions, what will be the effects on its power consumption?

3) Are the effects on power consumption different for NMOS & PMOS?

 

[goldsmith]

1) Whether the MOSFET consumes more power in Active region or Saturation region?

Hi pavan
A mosfet in linear region deals with a lot of dissipation .

2) If a MOSFET works in alternating Active & Saturation regions, what will be the effects on its power consumption?

More dissipation !

3) Are the effects on power consumption different for NMOS & PMOS?

Pmosfet usually deals with more dissipation rather than N mos

Best Wishes
Goldsmith

 

[Audioguru]

Most people are confused with "saturated" and "active" regions for a Mosfet because they are the opposite to a bipolar transistor.

Power= the voltage across it times the current flowing in it.
When a Mosfet is fully turned on it has high current but almost no voltage then its power dissipation is very low.
When a Mosfet is turned off it has full voltage across it but no current then its power dissipation is zero.

When a Mosfet alternates between being fully turned on and turned off, then again its power dissipation is very low. But if it switches slowly then it dissipates power.

When a Mosfet has half the supply voltage across it and half the maximum load current in it then its power dissipation is fairly high and can be easily calculated.

 

[pavan garate]

1) But if we consider the output I-V curves of MOSFET, when Vds becomes sufficiently large, the MOSFET enters Saturation Region & Id is almost constant but greater than Id in Active region. That means, the product "Vds x Id" is always greater in Saturation region as compared to Active region. Since Power = Voltage x Current , the MOSFET should dissipate more power in Saturation region as compared to Active region. Isn't it..??

2) If a MOSFET works in alternating Active & Saturation regions, whether the power consumption will be simply alternately increasing & decreasing, or some different behavior will be followed?

 

[Audioguru]

When the Mosfet is fully turned on then it has low voltage across it but its current is high. It dissipates low power.
When the Mosfet is turned on so that it has half the voltage across it and conducts half the current then it dissipates high power.

If a Mosfet switches on and off slowly then it has a lot of time where it has voltage across it and current in it at the same times so it dissipates high power.
If the Mosfet switches on and off quickly then it dissipates low power. But its gate has high capacitance so a driver circuit is needed to charge and discharge the gate capacitance quickly.

 

[goldsmith]

Hi again

When the Mosfet is fully turned on then it has low voltage across it but its current is high. It dissipates low power

In addition to this :
Vds while mosfet is turned on is given by : Rdson*Id as Rdson is pretty low so the wasted power is pretty low too .

If a Mosfet switches on and off slowly then it has a lot of time where it has voltage across it and current in it at the same times so it dissipates high power.

In a short time it's in linear region which means linear loss power too !

If the Mosfet switches on and off quickly then it dissipates low power. But its gate has high capacitance so a driver circuit is needed to charge and discharge the gate capacitance quickly.

Then Ploss is given by : (Rdson*Id^2)*ton/T====>Rdson*Id^2*D.C

Gate loss is some sort of reactive loss so we can improve it somehow :smile:
Audioguru @ i hope you don't consider that i'm saying this to you , i know you know these things as well i tried to describe what you said for the original poster ;-)


Best Wishes to all
Goldsmith

 

[Tunelabguy]

1) But if we consider the output I-V curves of MOSFET, when Vds becomes sufficiently large,..

What is going to make Vds "sufficiently large"? In most switching applications, when the MOSFET goes into saturation, it has so much less resistance than the load it is switching that Vds becomes very low. It would be a very unusual application that imposed a sufficiently large Vds and then turned on the gate all the way to saturate the device. But if that is what you mean by saturation, then of course you are right. The power dissipation would be out of sight.

 

[goldsmith]

when the MOSFET goes into saturation, it has so much less resistance than the load it is switching that Vds becomes very low

Hi Tunelabguy
Please do not call it saturation ! because of a historical reason it is vise versa than BJT ! so you should call it turned on or turned off !

Best Wishes
Goldsmith

 

[Audioguru]

Hi Tunelabguy
Please do not call it saturation ! because of a historical reason it is vise versa than BJT ! so you should call it turned on or turned off !

Best Wishes
Goldsmith

You beat me because that is exactly what I was going to say.

 

[Tunelabguy]

Hi Tunelabguy
Please do not call it saturation ! because of a historical reason it is vise versa than BJT ! so you should call it turned on or turned off !

Best Wishes
Goldsmith

Normally I would not call it saturation for the reasons you mentioned. But when I respond to someone who uses certain words, I like to use those same terms if possible, just to be understood by that person, even if the words they use are not quite right, unless the use of those words is so terribly incorrect as to be totally misleading as to the facts. This was a borderline case.

 

[pavan garate]

You all say that there is no Saturation Region for MOSFET. But I found lots of PDFs & PPTs on net describing Triode (i.e. Active) Region & Saturation Region of MOSFET. See the attachment.

Goldsmith & Audioguru, please don't blame Tunelabguy for calling it "Saturation" region. He is right that the confused newbies like me should be addressed in that way, even if it's somewhat wrong. But I also agree that the experts like you should tell us the right part.

Thank you.

 

[Audioguru]

When a bipolar transistor is turned on hard so that its collector to emitter voltage is low it is said to be SATURATED. When it is a linear amplifier with some voltage across it and some voltage across its load it is said to be LINEAR.

A Mosfet is the opposite. When it is turned on hard it is said to be Ohmic, Triode or LINEAR. When it is an active amplifier it is said to be SATURATED.

So the words SATURATED and LINEAR should not be used when talking about a Mosfet mode. Instead it is either a Turned On Switch or it is a Partially Turned On Amplifier.