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Topic: mosfet choice (Read 707 times) previous topic - next topic

pelide

Hi everyone, I'm new in the forum, so I apologize if I made the wrong section, I'm developing a project according to which I need to take advantage of the PWM output of the fans of the motherboard of my PC to power a large load, this to do in so that the PC automatically manages the voltage based on the processor temperature, I know that a square wave with a maximum of 12v comes out of that pin, so I would need a mosfet to be controlled with this square wave to always manage the voltage of 12v, but with about 40a. The problem is that I don't know which mosfet to choose, I would choose one from 12v 50a to avoid problems, so I need it to use it as a "pwm relay", in order to modulate the output voltage thanks to the square wave coming from the pwm pins of the motherboard, can you advise me on some suitable mosfet, or even a connection diagram (in case it may need resistances, control chips, etc.) I would like to make sure that the motherboard manages the voltage directly without the arduino in between, but if necessary, I will have no problems exploiting arduino. Thanks.

MarkT

40A is a large load, an array of MOSFETs might be the best choice for that sort of current, driven
from a MOSFET driver chip.

MOSFETs are never selected on their max current ratings in practice, since that is a completely impractical
specification (for many devices the package bond-wires will melt before the chip's maximum current rating,
and you'd need highly efficient water-cooling system to reach it anyway).   I've seen datasheets quoting a
TO220 MOSFET as having 260A maximum, despite 25A being pushing it for the TO220 package pins!

MOSFETs are selected by power dissipation, using the on-resistance.  40A though a 10 milliohm device
would dissipate 16W, requiring a large heatsink and perhaps a fan.  Use two devices and each sees 20A,
so the total dissipation is only 8W.  There are also switching losses which means you want to drive the
MOSFETs quickly with PWM - again a gate-driver chip is the answer.

A gate driver chip like the MIC4422 will drive several MOSFETs easily, give each a small value gate resistor
something like 10 to 50 ohms.  Such chips normally expect logic-level signals, but many like the MIC4422 will
accept upto their 12V supply voltage on the input.  MOSFET driver chip needs about 12V supply and thus
you don't need logic-level MOSFETs.

With high currents being switched rapidly you'll need to pay attention to the layout and decoupling.
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UKHeliBob

Welcome to the forum

A suggestion for you.  Please edit your post and break it down into smaller paragraphs to make it easier to read.  As it is there is one slab of text which makes it harder to read and understand than it need be.  

Bear in mind that we know nothing about your project, hardware, software or what you are doing so have to pick that up whilst reading your post
Please do not send me PMs asking for help.  Post in the forum then everyone will benefit from seeing the questions and answers.

pelide

Excuse me if I have not replied, and excuse me if I have written badly, but English is not my main language, my project is to manage Peltier cells (heat pumps) to be put in the computer directly from the pin of the fans of the card PC motherboard, this is because a square wave comes out of the pwm pin of the fans, if I'm not mistaken with peaks of 5v, thanks to this / these mosfets I would like to manage the cell voltage directly from the motherboard according to the temperature. How about the IRF540N?

pelide

so what mosfet I will chose? I thinked to use some IRF540N, but I don't known if they are ok, I will use it whit Arduino, all it's more easy if i use it.

MarkT

You didn't mention Peltier devices in your first post, that immediate flags up the issue that they
will be less efficient driven with PWM than a constant DC (they conduct heat backwards immediately
the voltage is removed).

You can use an inductor to smooth the output though, but some experience designing switch-mode
power circuitry would be advantageous for that.

BTW this is a good example of why is never a good idea to hide details in your first post.  http://xyproblem.info/
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pelide

Sorry if I don't specified this, I thinked that will be not usefull. So I will use an inductor whit a mosfet? I buyed an irlz44n.

pelide

#7
May 27, 2020, 10:42 pm Last Edit: May 27, 2020, 10:43 pm by pelide
Can I use every type of inductor? I want to take it from an atx power supply, but I don't know if they are good for me. And how I will connect it to the peltier module? Only on a cable, or in the middle of the cables?

Wawa

Is this Peltier project meant to cool a CPU? Did you do the calculations?
Many before you have failed, because of the inefficiency of Peltier cooling.
You need a heatsink the size of a filing cabinet with a bank of roaring fans to cool a 100watt CPU.
Leo..


pelide

No problem for this, I have a water loop with 4 radiator to delete any problems!

MarkT

Can I use every type of inductor? I want to take it from an atx power supply, but I don't know if they are good for me. And how I will connect it to the peltier module? Only on a cable, or in the middle of the cables?
If you don't have experience in designing switch-mode supplies, you will be lucky to succeed, an understanding
of power electronics and basic test equipment (a 'scope, basically) are ideally needed. 

No you cannot use every type of inductor, you need to understand magnetics and their behaviour,
inductors are much more complicated than other passive components, you need to know about saturation,
frequency ratings, losses to pick a suitable inductor (or be lucky).
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

pelide

Thanks, there are any problems if I use the peltier device whit pwm directly connected? The frequency is from 21 to 28khz, I don't think that in this small time the module will burn or also

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