See part 1 here: http://everycircuit.com/circuit/6594722429403136
Here is the same Nmos that we were looking at in my previous presentation: IRF7201n from International Rectifier datasheet.
Datasheet test details:
Rds(on) = 0.03 Ohm
Vgs = 10 volts
VTO = 1 volt
Id = 7.3 amp
In EC here on the left we can see that in this setup:
Vds = 0.359 volts
Id = 1.2 amp
So ... Rds(on) = Vds/Id as per V = IR
Then ... Rds(on) = 0.359/1.2
so Rds(on) = 0.03 Ohm just like the datasheet
We can create it to be that Rds(on) by adjusting the Beta, as we saw in my previous presentation, until the Id and Vds seen, give us the value which we are after for Rds(on) ... try it in the circuit on the right.
Now here is an important new formula for you:
Rds(on) = 1/[Vov × Beta]
We can manipulate that to read as:
Beta = 1/[Vov × Rds(on)]
From the given datasheet details:
We know that Vov = 9 (ie 10-1)
We know that Rds(on) = 0.03
So ...
Beta = 1/[9 × 0.03]
Beta =1/0.27
Beta = 3.7 A/V² expect some rounding errors in EC
Let's check that using Beta = KP(W/L) each part as seen in these Nmos settings.
Beta = 0.037 × (100/1)
Beta = 3.7 A/V² just the same as our new formula.
We now have this very handy way of calculating Beta, simply by using this formula.
All we need is the Vov and the Rds(on) which are given on the datasheet.
We can then put our calculated Beta value in to our EC settings just as we did in my previous presentation.
Here is that formula again:
Beta = 1/[Vov × Rds(on)]
Now go back to the four setups on my previous presentation and see if you can find Beta for each mosfet if you were to alter each VTO as being 1, 2, 3, 4
See which one gives you the lowest result for Rds(on) ?
Your answer will be the one with the highest Vov
And that means the one with the lowest VTO
If you were selling your mosfets as a manufacturer, you would want to boast about it having as low a Rds(on) as possible ...
The lowest Rds(on) which you could rightly offer is the one which you tested at the lowest VTO
If you were to test your factory mosfets which had a higher VTO, then your Rds(on) value would be higher and you would lose sales where people were buying from your competitor with his stated lower Rds(on) ... so in my opinion, every manufacturer is likely to test the mosfets with as low a VTO as possible to get the lowest Rds(on) result for their datasheet 'Flagship' presentation ... There is no law which states that they must test at a specific VTO, as long as it is within the true stated range. A specific test VTO [ie: Vgs(th) ] is not usually stated on a datasheet, so opinion reigns for this issue without validation.... but the Rds(on) for each different VTO value will be likewise different. Note: This particular mosfet has just one unambiguous VTO stated on the datasheet of 1v
One extra thing to note with this formula:
It only applies if the mosfet is in its Ohmic state.
Very low Vds gives a more accurate result.
Q: Is the datasheet mosfet in its Ohmic state?
A: I'll leave you to think about that for now.
Ask if you are stuck.
Finally: Be careful with units etc ...
eg: 0.037 A/V² which I used above, may be seen in the EC settings as 37mA/V² for the KP
When you are familiar with setting up a mosfet from a datasheet, you may want to take a few meter measurements of a real bought mosfet on a board simple setup to obtain the same necessary values eg Vgs, VTO, Vds and Id to calculate your actual bought mosfet Rds(on), then calculate Beta with that new formula above ready to use in EC for circuit testing etc.
