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Author Topic: MOSFET recommendations  (Read 2560 times)

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Offline airman00Topic starter

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MOSFET recommendations
« on: February 01, 2008, 03:52:30 PM »
I need MOSFETs for an H bridge which controls a 12V 10A motor

any recommendations for MOSFETs , but they should be PWM optimized also

Preferably a free sample or from a place that sells in bulk  , since I need to test out MOSFETs for a robot platform I plan to manufacture. Once the prototype of the platform is done , ill post info , but right now I need to know where I can get such MOSFETs from.



Thanks so much!
,Eric

P.S.
If someone can find complementary pairs of the MOSFETs I need, I would appreciate that immensely
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Offline Rebelgium

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Re: MOSFET recommendations
« Reply #1 on: February 02, 2008, 07:50:06 AM »
Lately I've also been searching for MOSFETs a few times.
It's so hard to find the right MOSFET for your purpose, does anyone know a good method to search for them?

Sorry I couldn't reply to your post airman. :-\
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Offline Admin

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Re: MOSFET recommendations
« Reply #2 on: February 04, 2008, 10:53:26 PM »
to search, just go to digikey.com or mouser.com and type in MOSFET

I used to get my free mosfets from fairchildsemi many years ago . . . not sure if they still give out samples . . .

and for the kids reading this, don't abuse the free sample system!

Offline airman00Topic starter

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Re: MOSFET recommendations
« Reply #3 on: February 04, 2008, 10:56:48 PM »
Yes but at fairchild it says like 30V mosfet

I need a mosfet to switch a 12V 10A motor, please just link me to a MOSFET fairchild sells that fits my needs

please, i will use that as a model for other MOSFET orders
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Offline Admin

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Re: MOSFET recommendations
« Reply #4 on: February 04, 2008, 11:20:17 PM »
30V is the maximum rating. You can use any voltage below that.

Search for fairchild to see what they have  ;)

Offline SmAsH

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Re: MOSFET recommendations
« Reply #5 on: February 04, 2008, 11:31:23 PM »
umm what? if you cant use any below 30 wouldnt it be the min?
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Offline krich

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Re: MOSFET recommendations
« Reply #6 on: February 05, 2008, 06:01:08 PM »
I think what he's asking (and I've wondered myself) is what is the defining characteristic for choosing a MOSFET?  For resistors its Ohms, for caps its Farads, pretty simple.  But when you're looking at a list of 500 MOSFETs on digikey, what criteria do you use to make the right choice?

I've wondered the same for Diodes and Transistors too.  I'm showing my electronics newb-ness.  :(

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Re: MOSFET recommendations
« Reply #7 on: February 05, 2008, 10:03:25 PM »
Well, it depends on what the MOSFET is being used for . . .

If you want to use it for an H-Bridge or motor control, you want it PWM optimized.

You also want it rated for the voltage and power (voltage * current) that you expect to go through it.

For example, suppose you are going to use 12V motors that use 5A of current.

That means your mosfets should be rated higher than 12V and higher than 60W of power.

Of course you probably can get one rated for 40W of power, and put a big heat sink on your mosfet too . . .

Offline Soeren

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Re: MOSFET recommendations
« Reply #8 on: February 05, 2008, 10:42:03 PM »
Hi,

If you want to use it for an H-Bridge or motor control, you want it PWM optimized.

For example, suppose you are going to use 12V motors that use 5A of current.

That means your mosfets should be rated higher than 12V and higher than 60W of power.

Of course you probably can get one rated for 40W of power, and put a big heat sink on your mosfet too . . .

For 12V at 5A, you'd want one that can handle say 25V and 10A. However, with PWM, the power dissipated in the MOSFET is NOWHERE near 60W, but dependant on the R_DS_on and switching times.
If eg. the R_DS_on is 0.01 Ohm (10 mOhm), it would dissipate 5A^2 * 0.01 = 250 mW when open and (usually) a larger amount depending on switching speed which can't be calculated before the M/S-ratio, the switching time, the total gate capacitance and the driving circuit is known, but it won't be anywhere 60W (or 40W for that matter).

A good reason to not go for the beefiest MOSFET available, as it will also have the highest total gate capacitance.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
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Offline gamefreak

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Re: MOSFET recommendations
« Reply #9 on: February 06, 2008, 10:20:50 AM »
one thing you could do is getting many smaller kinds and run them in parallel, FIRST robotics has included in its kit of parts a few speed controller H-bridges, made of of 12 MOSFETs(3 on each side) with a large capicitor, and a small computer fan on top for cooling, it runs 12 volt motors off of an 18 amp batery.

According to the book in front of me it is also very wise to hook up a pull down or pull up resistor so that when you power up there is a default state, another thing about MOSFETs is that there gate must always be connected to either ground or source since there gate uses a capacitor, if you leave it floating after you power it up many unknown things could happen. You also want to have a minimum of Rds( resistance between drain and source) so that a minimum of power is wasted. If you waste to much power then the MOSFET will heat up, making more resistance, and more heat until it shutsdown or breaks.

So back to paralleling MOSFETs,
Quote
The resistance of two equal MOSFETs is half the original value
equivalent resistance in ohms =Rds / 2

So the more you have in parallel the more the heat/resistance/current draw are split up among them and the better it runs.

It may be cheaper to buy many lesser ones than one really good one



All of this(except the stuff about FIRST) comes from the book: "Intermediate Robot Building" by David Cook
« Last Edit: February 06, 2008, 10:32:33 AM by gamefreak »
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