|
N Mosfet switch ....How to set the Rds₍ₒₙ₎ in EC
The schematic looks to be working nicely.....but it's not quite right for a real mosfet:
A real mosfet has a tiny resistance built in which by the nature of V=IR and P=IV means that wasted heat power will be produced there. There is no 'real heat' produced on EC, so that resistance Rds₍ₒₙ₎ is not really needed on EC....instead, just aim to get the gate bar working fully with max current flow.
Although the Rds₍ₒₙ₎ is not important in EC, it is important in a real mosfet. We need to calculate how much heat will be dissipated and know when a heat sink is required. For interest sake, we can play with creating a mosfet Rds₍ₒₙ₎ which can help us to understand what is happening visually and give us practice with the real heatsink calculation....just remember, we don't need it on EC...better to aim for max current flow with good gate bar movement, and use a calculation for real heat.
Now look at the schematic...
The load resistor is 8Ω, the supply for that is 10v, so the current should ideally be 10/8 = 1.25A
But see now the current is shown as 1A instead of 1.25A
Tap on the wire just above the mosfet drain and now see the volt reading at the bottom line of oscilloscope.
That voltage never drops below 2V....... we would like it to drop to zero just as a mechanical switch would do.
Hence there is a 2v drop across the mosfet (drain source). 2v / 1A = 2Ω ie there is a hidden 2Ω resistance in the mosfet.....we can call this our Rds₍ₒₙ₎ (R_esistance at d_rain s_ource whilst turned on)
Real Nmosfets may have an Rds₍ₒₙ₎ of around 0.05Ω as per its datasheet, so let's alter this mosfet to that:
Look at the schematic again....Consider it as a simple resistance divider with R1=8 and R2=0.05 (not R2=2Ω as presently deduced above in the schematic).
If we call the mid point voltage Vm, then Vm=[(R2/(R1+R2)) x (Vsupply)]
So Vm=(0.05/8.05) * 10 ie Vm=0.0621v ie 62.1mV is the required volt drop across the mosfet if Rds₍ₒₙ₎ is to be 0.05Ω
Let us now then alter our Nmos settings so that we achieve a volt drop shown as approx 62mV for that oscilloscope min reading ... ignore the max 10v reading which refers to the switched off situation ... Rds₍off₎
Alter the KP mosfet setting carefully whilst looking at the drain wire volts (to source zero) on the bottom left of the oscilloscope...When it holds at 62.1mV you are done. You have now set that hidden Rds₍ₒₙ₎ to 0.05Ω and simulated your datasheet spec. As a double check....V=IR so 0.0621/1.24 = 0.05Ω where the current through load and drain source is now seen as ≈ 1.24A if you take a look. If you can't turn the KP dial any further, then use the Width or Length dials....it's the bar movement and circuit current etc that we are concerned with remember, not any specific dial value for this simulation.
So there we have it....the mosfet bar looking good, a full current flow and a Rds₍ₒₙ₎ set to whatever we wish.
Now alter the supply to any volts you wish, and likewise the load R1......use the same method to create a Rds₍ₒₙ₎ of 0.06 or whatever your datasheet states.....good messing....keep an eye on that gate bar movement.
Now see here for real mosfet heatsink decision http://everycircuit.com/circuit/5293393395843072
|
