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Topic: FET recommendation (Read 646 times) previous topic - next topic

luxy

Hello,

i am looking for some widely used FET which is fully open at 5V. If i would control them with microcontroller it should be fully open if i use "digitalWrite HIGH" function.

I need it mostly for controlling smaller motors and LED strips with PWM. Up till now i used IRL2505 but is quite expensive and i can't get them from our local electro shop (i had to order them from Farnell).

Can you suggest some cheaper and widely used alternative?

Btw, i saw that in some circuits GATE of FET is connected directly to microcontroller I/O while in some others small resistor (about 50 ohm) is put between GATE and microcontroller. How is better or correct?


oric_dan

IRL540s are inexpensive, but will have a larger Rds(on) than the newer, more expensive parts.
Shouldn't matter, if the currents aren't too high.

Using a small value R in the gate ckt is usually a good idea. Helps isolate the uC I/O pin from
the largish gate capacitance, as well as spikes fed back to the gate via the drain-gate parasitic
capacitance. Good to also have a large value R between the gate and gnd to hold the Mosfet
off in case the input is floating.

luxy

Thank you for quick response..

Previously i read this topic http://arduino.cc/forum/index.php?PHPSESSID=3b975fffaa8d0f704526b230271c2771&topic=70635.0

I don't understand which information in datasheet is telling above which voltage FET is fully open. If i am running it from Arduino it should be fully open at 5V.

So, for example.. V(dss) of FET is 100V, I(d) is 33A and R(ds_on) is 44 mili ohms. If i am planning to drive 12V motor with 200mA current consumption with this fet, i can understand this informations that way:

12V and 200mA of motor is not even close to 100V of V(dss) and 33A I(d) so that's ok.. Is that kind of understanding correct?

If i use resistor between arduino I/O pin and gate of FET because i don't want to give more than 25mA from I/O pin i can use about 200 ohms resisotor (5V / 0,025A = 200 ohms).. Ok?

And how to calculate or which data should i look to know at which voltage is FET fully open?

fungus

#3
Jan 28, 2013, 11:47 am Last Edit: Jan 28, 2013, 12:36 pm by fungus Reason: 1

I don't understand which information in datasheet is telling above which voltage FET is fully open. If i am running it from Arduino it should be fully open at 5V.


I don't think MOSFETs are ever "fully open" (do you want zero resistance??)

Usually there's a graph on the datasheet with a power curve, eg. On this datasheet it's figure 3:

http://www.vishay.com/docs/91300/91300.pdf

Fig. 3. says that with 5V on the gate and 50V at the drain you'll get about 60 amps going through it at 25 degrees Celsius. From that you can calculate resistance, power dissipation, etc.

The datasheet also mentions "Rds" on page 2. This is a logic level MOSFET so it gives you a figure for resistance with a gate voltage of 5V, ie. 0.077 Ohms (with a 17 Amp test load).

0.077 Ohms isn't "fully open" but it should be low enough for most jobs.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

MarkT

There is confusion here - in electricity and electronics "open" means "open circuit" means infinite resistance, _always_.   This is
confusing if you are new - its the opposite from taps and valves used for fluids!

Avoid this confusion by using "on" and "off"!   There are many many "logic-level n-channel MOSFETs", this is the more
official term.  Logic level means the Rds(on) is quoted for Vgs=4.5V (or sometimes 5V).  Always ignore Vthr in datasheets
for switching applications, its of no use and causes a lot of confusion... The important parameters are Vgs/Rds(on),
total gate charge, Vds and switching speed.

Generally a 150 ohm gate resistor will protect the Arduino pin when connected to a high current MOSFET (they can
have many nF of gate capacitance, the resistor means the current rating for the Arduino pin won't be exceeded during switching.)

If you need fast PWM though you'll need a MOSFET driver chip to boost the current to the gate (otherwise the capacitance
limits the speed of switching leading to large switching losses - PWM at 10kHz ideally wants switching times to be
sub-microsecond, for instance, to keep switching losses sensible.)
[ I won't respond to messages, use the forum please ]

luxy

Ok, i think i understand it now..

If FET is rated as "Logic-Level Gate Drive" that means that it can be controlled directly from microcontroller (because logic 1 is about 5V) and there is no need for extra FET controller (only if you need high PWM frequency, because it depends of GATE current).

Voltage on gate change resistance of FET but it's never zero.. Higher voltage means lover resistance. If it is near to zero that's mean that it is turned ON, if i can say it that way.

V(ds - drain to source) means maximum voltage for supplying (motor for example)
V(gs - gate to source) means maximum voltage for driving FET

Is that correct?

For example, if i use 12V motor with 200mA and want to run it with PWM through FET without MOSFET driver chip. How can i calculate maximum PWM frequency for stable running?

I saw Fig. 3 but i cant find 60 A of current on ordinate axis.. How did you read that from 10^0 and 10^1?

fungus


Ok, i think i understand it now..

If FET is rated as "Logic-Level Gate Drive" that means that it can be controlled directly from microcontroller (because logic 1 is about 5V) and there is no need for extra FET controller (only if you need high PWM frequency, because it depends of GATE current).

Voltage on gate change resistance of FET but it's never zero.. Higher voltage means lover resistance. If it is near to zero that's mean that it is turned ON, if i can say it that way.

V(ds - drain to source) means maximum voltage for supplying (motor for example)
V(gs - gate to source) means maximum voltage for driving FET

Is that correct?


It's asymptotically close to correct...




For example, if i use 12V motor with 200mA and want to run it with PWM through FET without MOSFET driver chip. How can i calculate maximum PWM frequency for stable running?


That's a difficult calculation. A MOSFET gate acts like a capacitor which has to be charged/discharged. The faster you can do this, the cooler the MOSFET will run. At the end of the day you'll probably have to try it and see. If it gets too hot then add a heatsink or decrease the PWM rate (or figure out how to charge/discharge the gate faster).

Remember: An Arduino pin only has 40mA of current available for this and a capacitor has near-zero resistance at the start of a charge/discharge. This means you always need a current-limiting resistor between the Arduino pin and the MOSFET gate (so you don't draw more than 40mA from the Arduino).

40mA at 5V gives 125 Ohms (although you should probably use the next value up from that just to be safe, eg. 133, 150...whatever you have in your resistor box)
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

luxy


The faster you can do this, the cooler the MOSFET will run.


Intresting.. I was pretty sure that higher frequency of switching cause hotter MOSFET ;)

I think i found cheap and widely used FET for my purpose with low Rds(on).. It's IRF540N
http://www.irf.com/product-info/datasheets/data/irf540n.pdf

fungus



The faster you can do this, the cooler the MOSFET will run.


Intresting.. I was pretty sure that higher frequency of switching cause hotter MOSFET ;)


"More frequent" switching isn't the same as "faster" switching.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

KeithRB

fungus:
FET's do become "fully on", generally Rds(on) is just the parasitic resistance of the contacts and bulk silicon. "Fully on" to me, means that the channel is completely inverted and any increase of gate voltage does not result in an increase of channel conductivity. (Whether that happens at the Rds(on) measurement point is another question!)

retrolefty

You might be interested in an ebay offering that another forum member found recently, but the sellers seems to be running low, only 4 more sets of 10 available. I wouldn't expect to be able to run them at their 12 amp max rating without some serious heatsinking, but for up to a couple of amps or so you can't beat the 20 cent each cost.

http://www.ebay.com/itm/300848355748?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

Lefty

oric_dan



The faster you can do this, the cooler the MOSFET will run.


Intresting.. I was pretty sure that higher frequency of switching cause hotter MOSFET ;)

I think i found cheap and widely used FET for my purpose with low Rds(on).. It's IRF540N
http://www.irf.com/product-info/datasheets/data/irf540n.pdf

IRF540 is "not" logic-level, but the IRL540 that I cited in post #2 is.

retrolefty




The faster you can do this, the cooler the MOSFET will run.


Intresting.. I was pretty sure that higher frequency of switching cause hotter MOSFET ;)


"More frequent" switching isn't the same as "faster" switching.




Your both correct but talking of two different things. The faster one can drive (charge or discharge) the gate capacitance the less time the device will take to switch from full on to full off or visa versa. However the faster the PWM frequency is being used the lomger the time the device will be in switching transition per unit of time, so the higher the pwm frequency the higher the device dissipation will be, compared to lower PWM frequency.

Lefty

arduinohabib

What is man's best friend? The breadboard!

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