General MOSFET Question

[mithrandiir42]

Hello,

I'm trying to find the right MOSFET for my application, I've tried about a half dozen different ones and they don't act quite like I'd want them to. I figure it's just because I don't really know what I'm looking for when I'm reading the datasheets.. so here's my application

my project runs between 2.7V and 4V and I'm using an AVR microcontroller to switch on/off some LEDS and I'd like to use an N-Channel MOSFET to drive them.

Now so far everything works, (almost) some MOSFETs work better than others. It looks like of the ones I've tried, the best performing one only really switches on the LEDS at about 60% brightness when the gate voltage is high. If i crank up my power supply to 5V, then it comes pretty close. So right now I'm guessing either my gate voltage isn't high enough to switch the transistor fully on (even though the gate threshold is about 1.4V (i thought that would cover it) - or there's a voltage drop of about 1.5V from Drain to Source. I've also tried P-Channel MOSFETs and i get the inverse, I get a nice fully ON led when the gate voltage is low, but then when it's high, it doesn't fully switch off.

So i'm confused about what I should be looking for in a MOSFET
Low Gate Threshold?
Low RDS (I don't really understand what that is)?
or something else?

In a perfect world, I'd like to the source voltage for my LEDs to be as close to my supply voltage as possible, and to be able to switch the transistor fully on and fully off while my supply voltage is anywhere between 2.7V and 4V


If there's no way to do this, i guess my next option is to boost my voltage to 5V so if my voltage drops by 1.4V across the transistor, i still get 3.6V at my LEDs. But I'd like to avoid that if at all possible because space on my PCB is very limited.. and it means more parts/complications.


Thanks

 

[audioguru]

The threshold voltage of a Mosfet's gate is when it barely turns on, or it barely turns off. Its current is only 0.25mA.
Look at the on-resistance spec (RDS) that tells you the gate voltage to fully turn it on.
Most Mosfets need a 10V gate voltage but a few are "logic-level" and turn on pretty well when their gate is 4.5V and more.
Some special tiny Mosfets need a very low gate voltage to fully turn on.

 

[MikeMl]

For low voltage switching (ie. from a micro-controller port-pin powered on 3.3 to 5V) you are better off using a simple NPN transistor or if you need high currents, use an NPN Darlington. Most port pins can source ~10mA when high, so even a simple 2N3904 with a just a base resistor to limit the base current can sink 200mA.

 

[Hero999]

How many LEDs do you have?

If it's just one, connect it to the output of the MCU via a suitable resistor.

 

[smanches]

Are you placing the MOSFETs on the high side or low side of the LEDs. Sounds like you're trying to drive them on the high side (that is, switch the power before the LED.) Instead put the mosfet on the low side (switching the GND side of the LED.) This is because the specified gate voltage is referenced to the source pin of the MOSFET. If the source pin is not at GND, then you have to raise the gate even higher to get it to turn on.

The gate threshold is when it STARTS to turn on. You have to look at the notes for the Rds_on parameter on the datasheet and see at what Vgs voltage they used for that parameter. This is the true ON voltage that must be applied to the gate. Most MOSFETs are 10V, although Logic level MOSFETs are good at 5V and even lower.

 

[Speakerguy]

Some special tiny Mosfets need a very low gate voltage to fully turn on.

Yes, there are in fact MOSFETs that are meant to turn on at voltages as low as 3.3V, 2.5V, and even 1.8V. Here's one that turns on reasonably well by 1.5V Vgs:

https://www.fairchildsemi.com/ds/FD/FDS6064N3.pdf

Crazy some of the stuff they are coming out with now that everyone wants to direct interface power electronics to FPGAs and DSPs. The downside with these devices is typically huge input capacitances - this 20V, 23A device has 7nF input capacitance!

 

[mithrandiir42]

How many LEDs do you have?

If it's just one, connect it to the output of the MCU via a suitable resistor.

about 10x 20mA leds so i need to be able to source at least 200ma.

I tried a regular old NPN, that works fine but I'd also like to be able to do this with some high power leds totaling about 2.5A so that's why I'm trying to find a way to do this MOSFETs.

 

[mithrandiir42]

Are you placing the MOSFETs on the high side or low side of the LEDs. Sounds like you're trying to drive them on the high side (that is, switch the power before the LED.) Instead put the mosfet on the low side (switching the GND side of the LED.) This is because the specified gate voltage is referenced to the source pin of the MOSFET. If the source pin is not at GND, then you have to raise the gate even higher to get it to turn on.

The gate threshold is when it STARTS to turn on. You have to look at the notes for the Rds_on parameter on the datasheet and see at what Vgs voltage they used for that parameter. This is the true ON voltage that must be applied to the gate. Most MOSFETs are 10V, although Logic level MOSFETs are good at 5V and even lower.

You're correct, I'm placing the transistors high-side. the reason being was my LEDs are actually Common-Cathode RGB leds, but I have some Common Anode ones that I will try out later tonight, placing the transistors low-side.

 

[mithrandiir42]

Yes, there are in fact MOSFETs that are meant to turn on at voltages as low as 3.3V, 2.5V, and even 1.8V. Here's one that turns on reasonably well by 1.5V Vgs:

https://www.fairchildsemi.com/ds/FD/FDS6064N3.pdf

Crazy some of the stuff they are coming out with now that everyone wants to direct interface power electronics to FPGAs and DSPs. The downside with these devices is typically huge input capacitances - this 20V, 23A device has 7nF input capacitance!

Ah, I have a very similar MOSFET that I haven't tried yet.. mainly because I've been hand soldering to these all these little components to test them out, and although I'm getting very good at it, it's a little time consuming, OnceI find one that I plan on using, i'll try it out on a PCB. I think i should really etch a bunch of breakout boards for these common packages to make my life easier.

This is the one that I have:
https://www.fairchildsemi.com/ds/FD/FDS6298.pdf


what is the downside to having large input capacitance? slower switching time? wasted power?

 

[smanches]

Yes, large input capacitance will cause slower rise/fall times, which induces more wasted power as heat.

 

[mithrandiir42]

Well I put the MOSFET on the low side and viola it works beautifully. I'll probably end up using this solution and use common anode LEDs.

But out of curiosity, if I just *HAD* to drive the Common Cathode LEDs with a MOSFET on the high side, would a P-Channel MOSFET be more suited to the task?

 

[smanches]

Yes, PFETs are often used to drive things on the high side. Although then you drive the voltage lower than the source to turn it on.