MOSFETS

[HardCoil]

Hey all,

I am trying to understand, what is the difference between having a single mosfet with a 5A push led switch than a dual mosfet? Because i have been reading and reading and some say its to protect the switch from frying as the switch only supports up to 5A when u are subohming up to 20-30A. But then how do you calculate that even? How do you know the how much power does the mosfet regulates to the switch for the switch to handle such currents without damaging the chip? I also heard that its much more safer to have a mech switch like the classy nemesis mechs, no wiring just straight up circuits. But what if i want to make my own box mod? To source a high amp switch is not only expensive but its too big!

So atm i am wrapping my head around this MOSFET and switching issue because i dont want it to fry and end up my mod going full "ON" and not turning off since the switch failed. I am of course planing to use an on off switch. With a volt meter and a mosfet. But i am trying to understand this MOSFET and switching issue.

Some / most say its fine with a 3A switch with a mosfet or dual mosfet even to protect that switch and you'll be fine. But some say it will still fry. So most of the threads or articles i read are all based on one is good two is better. 3a is ok 5a is better but best go for 20a switch? There are no calculations whatsoever its just all based on, "yep i tried and it was cool!" "Works great for me!" I dont want to know if its just "FINE". Or its "BETTER". I want to know how do you KNOW its fine and how do you KNOW duals protects the switch's amperage ratings? There has to be a calculation right?

I just need some light on this before i start up something and end up having to replace faulty wirings or components later on. I want to do it right all the way. Much help would be greatly appreciated folks.


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[Nikea Tiber]

A MOSFET is a type of transistor. It uses a small current (what goes through the switch, MUCH less than 5 amps) to control a large current (what goes to your atty... Potentially up to 100 amps for a parallel 26650 box).
Any transistor will have a max current rating. If you need to handle more current than a single transistor is capable of you can use more than one in parallel. Two 20 amp transistors in parallel can handle 40 amps. Three can handle 60, etc.

Let me elaborate as to why MOSFETs are used. Mechanical switches capable of handling large amounts of current ( > 5 amps) have to be pretty large to dissipate the power (I.e. carry the current without heating dangerously). If you look at any switch on a popular mech mod you will notice it is pretty large. Even a "lighter" contact switch like the leaf blade switch on the castigador or the bent copper rod of the Luna Sea are 15-20x (at least) as massive as the contacts in a 5A momentary switch. The contacts on the 5A will heat up at the current load a subohm atomizer can pull. If they heat up too much they can fail safely and the switch will stop carrying current, or they can fail where the contacts fuse and the switch is just a hard short.

 

[HardCoil]

Thanks for the reply, yes i understand all that shorting part on the switches. So, the MOSFETS you refer to are there any specifics? Because considering a 20A N-channel MOSFET would handle 20A but then i am planning to sub ohm up to 0.08ohms, the current that i would probably hit is about 43A-45A. So by doubling the MOSFET and including a resistor should protect the mosfet and my switch as it handles 40A now? Or it has to be more or exact? And how do you know which mosfet too? The ones i read and try my best to learn and understand are PSMN1R9 , PSMN1R6 , IRLB3034. The IRL seems to be popular.

Sorry for the question load. I understand the circuitry references and the reason for mosfets to be there is just that i wanna know how to calculate them. Any mosfets should do or u need a soecific one?

Today you have helped me understand that mosfets has a max current to handle, no one has ever clarrified to me about the max current it could handle. Thanks! I find your reply very helpful to me.

 

[duc916]

I also heard that its much more safer to have a mech switch like the classy nemesis mechs, no wiring just straight up circuits.

You heard right. In any mosfet, a sliver of glass is the only thing preventing a dead short. It's not a matter of IF electromigration is going to happen in a high-heat application, it's WHEN.

 

[HardCoil]

Oooooooh... Now u cleared my head a little. Its not a matter of wether the MOSFET is could survive the amperage circuit or not its just a matter time that the mosfet fails because of the electromigration which is in lame man's term static electricity yes?


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[duc916]

Wikipedia can explain it better than I ever could.

https://en.wikipedia.org/wiki/Electromigration#Failure_mechanisms

Electromigration is exacerbated by high current densities and the Joule heating of the conductor (see electrical resistance), and can lead to eventual failure of electrical components. Localized increase of current density is known as current crowding

 

[HardCoil]

Them math.... My head hurts... Lmao


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[duc916]

Check out this chip node failing from electromigration viewed in an electron microscope... Pretty cool. Not so cool when it's catching fire on your dining table.

 

[HardCoil]

Nice! Thanks for the video! In this case i am going to make a tri tube. No need mosfets. Atleast i only need to maintain the tube for longer vape life. Then having a mosfet after a few hopefully years the thing burns up lol


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[Mooch]

You heard right. In any mosfet, a sliver of glass is the only thing preventing a dead short. It's not a matter of IF electromigration is going to happen in a high-heat application, it's WHEN.

What's the failure scenario for a dead short? The coil resistance would typically limit current
Hmm...though If the gate failed to the source it would short the battery, but it would blow open instantly.

While electromigration is present at all temperatures the risk of failure can be kept quite low by a combination of a well spec'd MOSFET along with paralleling if required (with current sharing derating). There's no reason why the MOSFET(s) can't last for many years...assuming a moderate derating across the specs.

 

[HardCoil]

Thats what i was thinking as well in the first place broooo!! But the thing is no one is teaching me how to calculate the mosfets specs.. From what i am reading the specs for the mosfets i have seen is pretty decent...


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[Mooch]

There are a lot of frakkin' mosfet specs. Teaching that can take days
Ok, here's how to calculate max amps for any TO-220 case mosfet...

The max power (heat) one of these can dissipate is about 2.3W. Calculating this is a whole different matter I won't cover here. Drop it to 2.0W for a little extra reliability. Find the max Rds-on (drain-source static resistance) from the datasheet and multiply by 1.5. This is needed because the resistance of the mosfet goes up as it gets hotter. Now use this equation...

Max Amps = square-root(2W / (Rds-on * 1.5))

For the IRLB3034 you get a bit over 25A as the max you can put through it continuously. You can go higher than 25A since it's pulsed current. Since the mosfet is sealed in heat shrink though and next to warm/hot batteries I prefer fo stick with 25A.

Lots of other things you need to be aware of though when it comes to mosfets. Check out the Box Modders FB group's file section...lots of info there. And you don't need to join the group to read the ciles.

 

[duc916]

I should have clarified what I meant by a dead short. A short across the MOSFET, essentially closing the switch and firing the atty (indefinitely if unattended).

There's no guarantee it will only fail in to an open circuit. They could as easily fail closed.

 

[HardCoil]

THANK YOU MOOCH, that there is what i wanted!!!! That calculator magickiee!!! I will join now!! Thank you!! For once in my life someone UNDERSTANDS WHAT I WANT!!! LMAO


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[duc916]

Seems to me a chassis-mounted mosfet with a coat of thermal grease would have a much better chance of a long life than one wrapped in heatshrink. Ive always thought that was kinda... crazy.

 

[HardCoil]

Heard of the grease too. Heat shrinks is another story. I read some say heat shrinks are not goo to wrap around the mosfet as it retains the heat longer rather than dissipating it...


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[duc916]

If you ever find enough room to mount mosfets to the inside wall of a metal box mod, Arctic Silver 5 is the best thermal paste you can buy.

 

[HardCoil]

Googling now... Very interested.... Sounds very cool and sounds soooo practical to dissipate the heat..


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[HardCoil]

It looks expensive duc lol


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[duc916]

Look forward to your build!