3v MCU to N-Channel MOSFET gate

[dksoba]

What is the best way to drive an N-Channel MOSFET gate using 3 or 3.3V (Stellaris ARM Cortex-M3 micro's from TI) digital signal? I've used these transistors from Vishay:

https://www.vishay.com/docs/68998/si4174dy.pdf

in conjunction with a PIC18F4550 at 5V digital signals. I'm afraid that 3V won't be enough, so I need to have some sort of driver. I'm looking at low frequency switching, so it's not really critical. Maybe 20khz at most.

If I used a PNP transistor and a 5V or 10V supply rail, logic low would cause the transitor to be fully on, and allow current to flow into the gate capacitor, but logic high wouldn't stop the current from flowing, right? I'm a bit confused on PNP transistors. And then I'd need an NPN transistor for draining the gate voltage, right? Kind of like that push-pull post that was on this board earlier?

Anyways, I'm curious how other people have solved this problem before.

Thanks,
Matt

 

[keith1200rs]

I am confused about the two micros, but the FET has a threshold of 2.2V max and a high gm so I would have thought would be ok depending on what current you want out of it. Why do you think you need a level shifting driver?

Keith

 

[dksoba]

At 3.3V won't the on resistance be too high? IE If I'm running multiple amps through at 3.3V, the chip will get very hot, yes?

Matt

 

[keith1200rs]

How much current do you need the FET to switch?

Keith

 

[dksoba]

5 amps max. And I need the FET to be as small form factor as possible. The ones in the link I provided above I've used before and don't even get warm when switching 10A @ Vgs=5V.

Matt

 

[keith1200rs]

Unfortunately my mobile Acrobat reader isn't showing me the graphs in the datasheet so I will have to look later, but you could check the resistance at 3V Vgs at 5A on the graphs.

Keith

 

[dksoba]

RdsON is only given for 4.5V and 10V.

There's a graph of Vgs curves (3-10V, in 1V increments) vs Vds and Ids. Ids for 3V is only given up to a Vds of 3V, but Vds should never get THAT high, right?

Matt

 

[keith1200rs]

OK, I am back at a computer and can read the data sheet graphs now. The first graph on page 3 shows drain current versus drain-source voltage for 3V, so if you look up your drain current you can see what the volts drop will be. At 5A it looks like around 0.2V drop - not perfect at around 40m ohms, so it will dissipate a watt. You would be able to halve that with a gate driver I would guess. So your options are accept the power dissipation, use a higher voltage gate drive or pick a transistor with lower ON resistance with 3V gate drive.

Keith.

 

[dksoba]

The MOSFET has a gate capacitance (Input capacitance) of around 1 nF. If I drive the MOSFET by using a pull up resistor at 5V, say 1kohm, and pull it down using an npn transistor, then I could drive the MOSFET w/a higher gate voltage. However, this will limit my switching speed...

To have a 5V drop over a 1kohm resistor, the npn transistor must sink at least 5mA.

With the resistor/capacitor, I'd have an RC constant of 1e3Ohms*1e-9F = 1uS. If I want to switch at 20khz, this has a period of 50uS, which means that my switching time (fall/rise time) would be about 2-4% of the entire period. If I switched even slower, this would work better.

Does this sound reasonable?

Matt

 

[keith1200rs]

Yes that sounds OK. Not as good as a push pull drive but then you would need three transistors for that. I have simulated it for comparison and attached it. Even with a 3V drive it looks OK. but then the model will use typical not worst case figures.

Keith.

 

[dksoba]

Thanks for the help! This makes sense. It looks like in the SPICE simulation the resistor only case is only about half as efficient as the transistor gate drive case.

What program did you use for simulation? This looks different than LTSPice but at the same time has a lot of similarities..

Matt

 

[keith1200rs]

It is SIMetrix https://www.simetrix.co.uk/

All Spice based simulators should be able to produce similar results (Hspice, Pspice, TINA, Multisim, LTspice, Winspice etc).

Keith.

 

[donfrench]

Matt,

I am curious what you eventually did to solve this problem as I have the same need. Is there a MOSFET with 3V Vgs? That would be the ideal solution for me but I haven't been able to find such a device.

 

[alexan_e]

Use **broken link removed**
you will find plenty of results

 

[donfrench]

Matt, I wish that was helpful. I am too inexperienced to figure out how to narrow that list of 841 devices down to the ones that will work for me. Which device did you use?

 

[alexan_e]

There are two filers you can use:

RDS(on) Max 2.5V (mOhms) which is the maximum ON resistance for a Vgs=2.5v
and
RDS(on) Max 2.7V (mOhms)