Rdson of this MOSFET

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Rusttree

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I'd like to use the DMN65D8L MOSFET to activate the coil of an automotive relay (see my other thread). Oddly, there is no graph that shows Rdson as a function of Vgs. I'm concerned because I want to power my circuit at 3.3V. Technically, that's greater than the MOSFET's advertised Vgs_threshold, but that doesn't tell me what the Rdson will be at Vgs=3.3V. Am I S.O.L. with this datasheet, or is there a way to calculate it from other parameters?
 
I may have found a way to answer my own question. Diodes, Inc provides a SPICE model of the DMN65D8L (**broken link removed**). Putting that into LTSpice with a Vgs of 3.3V, I got an Rdson of 3.1Ohm. I might have to settle for that and test out hardware once I populate the PCB.
 
Not a good way to do it. The model likely uses "typical" values which the device you buy may not have. What you need is a logic level MOSFET that is designed to fully turn on a Vgs of 3.3V. The Vgs threshold voltage is of little use for this purpose since that is the voltage where the MOSFET just starts to turn on. It generally takes several volts above that to fully turn it on as a switch. The DMN65D8L is only specified down to a Vgs of 5V (minimum) when used as a switch.
 
Do I really need it to be fully turned on though? Let's say, hypothetically, the Rdson at 3.3V is 10 Ohm (which I think is a conservative over-estimate). The coil on the relay is 225 Ohm, so the total current through the coil and MOSFET is barely affected. At a typical car battery voltage of 14.5V, the MOSFET is only dissipating 38mW, which is well within its capabilities. Isn't that an acceptable situation?
 
As to your question about RDS(on), you can estimate that from Figure 1 given the anticipated current. The curves for Vgs = 3.0 and Vgs = 3.5 are given. Not much extrapolation will be needed. Although, just using the 3-V curve will show you it should work.

As for running it not fully on, for such relatively low currents and power dissipation, it should work. Here is a nice reference on how to use the thermal characteristics: https://www.electro-tech-online.com/custompdfs/2012/12/AN1832.pdf

John
 
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As mentioned if the fet doesnt turn on fully it'll operate in its linear region and you might need a heatsink.
You could get the drive voltage by winding a ferrite transformer to drive the gate, but that'd need bipolar drive so the driver for that would be more complex.
 
Since the "typical" resistance is about 3.3Ω at a Vgs of 3.0V, then a worst-case value of 10Ω at 3.3V Vgs would seem reasonable and should work as you intend.
 
Driving a lowish current relay coil from a 3.3v micro is much better suited to a NPN transistor I think.

Something like a cheap common BC337 with 2mA base drive will have a very low Vce sat, maybe under 0.1v.
 
jpanhalt said:
As to your question about RDS(on), you can estimate that from Figure 1 given the anticipated current.
Click to expand...
Oh, right, that seems obvious now. Thanks for that. Looks like for low currents, the Rdson is just over 3.1Ohm as long as Vgs is >3V. Assuming a 14.5V battery and a 225 Ohm relay coil, the temp rise on the FET will be less than 5 degrees with the worst case Rja. Should be no problem.
 
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