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MOSFET: How to calculate RDSon for a particular VGS?

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cait-sith

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N.B. This is my first time designing a real-world circuit with transistors.

The goal is to switch a fully-resistive load in and out of a circuit with a MOSFET switch. The switch shall be controlled with a micro-controller.

Summary of design parameters:

* ID = 1A (max)
* TA = 50 °C (max)
* VGS = 3.0 V (min)

The DMN3404L was chosen as a candidate. It is an N-Channel Logic Level MOSFET. The datasheet is available at: http://www.diodes.com/datasheets/ds31787.pdf

From the datasheet, we find:

* VGS_th = 2.0 V (max)
* RDS_on = 28 mΩ (max) at VGS = 10V, TA = 25 °C
* TJ = 150 °C (max)
* RθJA = 90 °C/W

Maximum power dissipation, PD, is:
* PD = (TJ - TA) / RθJA = 1.11 W

Given ID = 1A (max), TA = 50 °C (max):
* RDS_on = PD / ID^2 = 1.11 Ω (max)

So, as long as our RDS_on is < 1.11 Ω, we should be okay. Therefore, we need to calculate RDS_on for our particular VGS.

From Figure 5, we find RDS_on is de-rated by 0.5% / °C above 25 °C. Therefore, for a junction temperature of 150 °C:

* RDS_on = 28 * (1 + 0.005*125) = 46 mΩ (max) at VGS = 10V.

But, we are using VGS = 3V, not 10V. How do I calculate RDS_on (max) when VGS = 3.3V?
 

alexan_e

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I can't give you a number for RDS-on but in figure1 page 2 you can see that with 3v Vgs and 2A drain current you will have about 0.25Vds *2A=0.5W , for 1A it will be much lower.

Alex
 

cait-sith

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We can estimate triode RDS_on (from Figure 1) for VGS = 3.0V as about 125 mOhm. Then, de-rating by 0.5% per degC, we get RDS_on ~= 200 mOhm at TJ = 150 degC. Therefore, we expect PD ~= 200 mW for ID = 1 A.

But... this graph is for typical characteristics, not maximum. So... is a factor of five safety margin acceptable? Where do they even get the `maximum' figure for RDS_on in the "Electrical Characteristics" table?
 

alexan_e

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According to the electrical characteristic the typical and max value for Rds-on have a difference of about 30% (for Vgs 4.5v) , even with 100% you are still fine

Alex
 

cait-sith

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Thanks for your help. So, how would you typically approach a design problem like this?
 

RCinFLA

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Thanks for your help. So, how would you typically approach a design problem like this?


Select a MOSFET that specs the Rds-ON close to your desired Vgs drive.

The Rds-ON versus Rds is dependent on physical device size. This means higher input capacitance to drive for lower Rds and higher breakdown voltage spec'd MOSFET's.

If you want to use it with lower Vgs you have to start with a physically larger MOSFET die which will have higher input capacitance drive requirements. The chart of Rds versus Vgs is typical number at 25 deg C. You should at least double the Rds number from the typical graph provided to allow for part to part variation and temperature degradation.
 

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