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Design >> Analog Design >> Power consumption over nMos
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Message started by Gil on Apr 25^th, 2012, 5:43am

Title: Power consumption over nMos
Post by Gil on Apr 25^th, 2012, 5:43am

Hi guys (and ladies ;)),

I'm trying to find a way to calculate the power consumption over nMos during a switch (V_G turns from 0 to 1 logic).

Lets assume my circuit is made of a DC power supply V_DD, connected serially to a resistor R and an nmos transistor.

Calculating the area under the graph I_D(V_D) (using P=IV) isn't gonna work, because there's no time aspect there..

I thought I could model the transistor as a resistor R_nmos with a capacitor in parallel C_nmos,
this way I can treat R_nmos as the one who's current is I_D.
This way after (transistor's) Tau~RC the transistor reaches saturation (the capacitor C_nmos is "disconnected" after T=inf so all the current goes through R_nmos).

Is that a correct observation?

If it is, I'm not sure how can I determine C_nmos.
About the R_nmos - I can use a "zero-order" approximation by determining R_nmos = V_DD/I_saturation , since after the capacitor charges, the R, R_nmos are serial connected.

Thanks,
Gil.

Title: Re: Power consumption over nMos
Post by AnalogDE on Apr 25^th, 2012, 2:03pm

Pretty complicated to do with only hand calculation...Why not just simulate this?

Title: Re: Power consumption over nMos
Post by RobG on Apr 26^th, 2012, 8:02am

AnalogDE wrote on Apr 25^th, 2012, 2:03pm:

Pretty complicated to do with only hand calculation...Why not just simulate this?

This seems like a homework problem so I don't want to give the answer, but I think you have the right approach. If you don't have the information to calculate Cnmos you can't include the transient part. Likewise, if you don't have a time period it doesn't make sense to do the transient portion since you don't know how much it contributes to the overall power.

rg

Title: Re: Power consumption over nMos
Post by Gil on Apr 27^th, 2012, 9:02am

It's not really a homework assignment, but me trying to figure it out..

I wanna get an intuition about the POWER CONSUMPTION in MOSFET.

I wanted to get a clue about my hunch - if this is a correct way of thinking..

All that is left is to calculate somehow the R, C, equivalent to the NMOS.
This is where I need someone's advise.. :P