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MOSFETs and Miller plateau
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TreeOone:
Hello!

I'm looking for a suitable MOSFET which could be used to turn ON and OFF coil of electro-mechanical relay (rated current 100mA). I am intending to use +3.3V from MCU's output for charging and discharging MOSFET's gate.

I have following doubts:

MOSFETs have Miller plateau, which should be completely charged, before Vds can drop to lowest values. If in Vgs-Gate_charge diagram Miller plateau is not reached, Vds stays high.

I was happy with this understanding until I stumbled upon following datasheet:
https://hr.mouser.com/datasheet/2/308/NTD3055L104-D-114990.pdf

Now, by looking at following datasheet, two contradictory informations are presented:

1)   On page 3 there is "Figure 1. On−Region Characteristics", which clearly states that you can drive this MOSFET with 3V.  In fact it says you can also drive my electro-mechanical relay's coil. When gate
voltage is at 3V, I should expect Vds to be less than 500mV @ Id = 100mA. That's great, I found a solution. But....there is also....

2)   On page 5 there is " Figure 8. Gate−To−Source and Drain−To−SourceVoltage versus Total Charge" which states this MOSFET's gate will not even reach Miller plateau with 3V, i.e. Vds will be high, you need at least Vgs = 5V. Crap....this will not work :/

Now, could somebody explain how previously mentioned diagrams can exist in the same datasheet??????

I also have this MOSFET:

https://hr.mouser.com/datasheet/2/268/TN0604%20D080813-965142.pdf

This one has Miller plateau at 1V, but also smaller Id = 0.5A, for Vgs = 3V.

Which MOSFET should be used?

Zero999:
One question: why not use a BJT?

It's cheaper and far easier to get parts which will do this. The plain old BC338 can do this with minimal base drive. A forced Beta of 1/100, so that's IB = 1mA will give a VCE<1V (probably around 200mV), when IC = 100mA.
http://www.mouser.com/ds/2/149/BC337-193546.pdf

To answer the original question, either MOSFET will do, but they're both way overkill and way more costly, than a simple BJT. If it must be a MOSFET, then go with something like the FDN337N, which is specifically designed for 3.3V logic level operation. In any case it will still be more expensive, than a BJT.
https://www.onsemi.com/pub/Collateral/FDN337N-D.PDF
MagicSmoker:
^this is the correct answer. A MOSFET is the wrong switch for the job; use a BJT instead. If you are really lazy, use a "pre-biased" BJT which already has the base and base to emitter resistors inside such as MMUN2231 or the like.

2N3055:
NUD3124
TreeOone:
Ok, you convinced me that BJT is also acceptable option.

By looking at the datasheet:
https://www.onsemi.com/pub/Collateral/BC337-D.PDF

....if I want Ic = 100-200mA (added little redundancy), diagram "Figure 3. DC Current Gain" says I need 1mA in the base for Ic = 200mA. (h = 200 @ Ic = 200mA)

Now the question is how to feed that current to base, i.e. how big should be base resistor.

Table "ON CHARACTERISTICS" says Base−Emitter On Voltage = 1.2V @ (Ic = 300 mA, Vce = 1.0 V)

....so voltage drop on base resistor should equal 3.3V – 1.2V = 2.1V

...and resistance equals 2.1V / 1mA = 2100 Ohms

In my case this resistor should insure Vce < 1.0 V @ Ic = 100-200 mA

Is this calculation correct?

And finally, would please somebody give answer to my initial question regarding Miller plateau?
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