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Topic: Verify my MOSFET selection process? (Read 176 times) previous topic - next topic

dannysauer

So, I have a pretty solid idea of how transistors work from the ECE program I was in a couple decades ago, but taking theoretical knowledge that's atrophied over 20+ years of "not designing electronics" and translating that to selecting a specific transistor...  Well, I'd like someone to help me with applied knowledge. :D

I have a nominal 12v heating pad, like a car seat heater.  It draws about 7.5 amps peak.  I'd like to make that temperature adjustable.  So, I think adjusting the on time through PWM control makes sense.  Since it's just a heating pad and not a solenoid/motor/etc, I shouldn't have any weird concerns other than "just turn it on and off", I guess?

I think I'll probably be using a 3.3v microcontroller.  I think what I want is an N-channel MOSFET with a gate voltage of 3 volts or less.  I might use a dedicated 5v microcontroller for this with a 3.3v controller running a user interface if that makes things easier; I'm not positive yet.

Clicking through Mouser's MOSFET catalog, I think maybe a TI CSD18511KCS would work (datasheet).  It's through-hole mount.  It has a gate-source voltage threshold of 1.5 volts, so should work with either MCU, right?

It'll dissipate 188 watts, so should be more than fine with a couple of these heating pads.  It has a 40v breakdown voltage, so should be fine with automotive power supply voltage (14.4v in my car).  The 3.2 mOhms RDS on resistance at 4.5 volts should be low enough that it won't get super hot switching on and off with the PWM frequency supported by any common mcu like an ESP8266 or an arduino nano or whatever, right?  Can I directly use an MCU pin either way, or do I need a second transistor to switch a higher voltage in order to get the source-drain resistance down (maybe only with the 3.3v route)?  Do I need an inline resistor to limit inrush current either way?


If someone could glance at what I just typed and let me know if I'm completely wrong or if I'm even asking the right questions, I'd really appreciate it. :)

MarkT

#1
Nov 22, 2019, 10:55 pm Last Edit: Nov 22, 2019, 11:01 pm by MarkT
The lowest gate drive voltage the CSD18511KCS is rated for is 4.5V.

Threshold is not relevant, its nothing to do with the on state. (*)
Rds(on) is the figure you want.

You are very unlikely to find a 3.3V rated MOSFET except in a surface mount package.

What you could do is use a gate-driver chip and a 10V rated MOSFET.  Gate driver chips need about 12V supply, take 3.3 or 5V logic signals in and drive a MOSFET gate.

(*) Think of a tap.  As you start turning it on and it starts to drip slowly - that's the threshold.
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dannysauer

Ok, so if I want to use a 3.3v MCU, then I'd need to use a PNP transistor like a 2N3906 to switch the 12v nominal to the gate?  That TI MOSFET says it's ok with 20v gate-to-source, so I guess it'd still be ok?

Thanks for the help. :)

wolframore

you should add a little gate resistor also, try 100 Ω to limit inrush as the gate is very sensitive.  Never leave it floating so if you're using an N Channel you will want a pull down.  

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gilshultz

Start by going to this website for some basic understanding of a MOSFET.  https://arduinodiy.wordpress.com/2012/05/02/using-mosfets-with-ttl-levels/
Then check this part: FQP30N03L  https://www.onsemi.com/products/discretes-drivers/mosfets/fqp30n06l  I believe it will do the job you want, it is in a TO220 package, avalanche protected and logic level.  My suggestion connect the arduino pin directly to the gate and use something in the range of 22K as a pull down resistor to keep it off when first reset. You can look at the threshold voltage, that is the point where it starts to enhance or quits enhancing. In your case it both, The curves show that it will give you way more amps then you need. At 2 Watts for a TO220 in free air you probably will not need a heat sink.  With a RθJA Thermal Resistance, Junction-to-Ambient -- 62.5 °C/W. It is a bit of overkill but should serve you well in many applications.  
This response is to help you get started in finding the problem, not solve it for you.
Good Luck & Have Fun!
Gil




MarkT

#5
Nov 23, 2019, 04:13 pm Last Edit: Nov 23, 2019, 04:14 pm by MarkT
you should add a little gate resistor also, try 100 Ω to limit inrush as the gate is very sensitive.  Never leave it floating so if you're using an N Channel you will want a pull down. 


The gate is only sensitive to over-voltage, it doesn't mind many amps flowing (some really big MOSFETs require amps of gate current for efficient operation).  The gate is just metal, it conducts well.

If you drive a gate from a logic chip the resistor can protect the logic chip from over current.  Gates look like large capacitors (because they are the plate of a capacitor, the M.O.S. structure is a form of capacitor)
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MarkT

Ok, so if I want to use a 3.3v MCU, then I'd need to use a PNP transistor like a 2N3906 to switch the 12v nominal to the gate?  That TI MOSFET says it's ok with 20v gate-to-source, so I guess it'd still be ok?

Thanks for the help. :)
Yes, but a gate driver chip is the preferred approach, its the tool designed for the job.  Having an NPN switch a PNP to switch a MOSFET involves more parts.  A gate driver chip plus its decoupling capacitor is simpler and gives much better protection for the MOSFET gate too.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

gilshultz

Switching the low side keeps the interface simple. In the non logic level MOSFETS you needed a gate driver chip or an appropriate circuit.  The logic level simplifies the circuit. This is the reason Automotive went to low side switching in a lot of the systems.   The gate of the MOSFET can draw amps if you switch it fast enough, remember it is the miller capacitor effect.  Try this link for a start" https://electronicsforu.com/resources/learn-electronics/mosfet-basics-working-applications.  Then this: https://www.elprocus.com/mosfet-as-a-switch-circuit-diagram-free-circuits/ and this: https://www.electronics-tutorials.ws/transistor/tran_6.html at this point you will have a basic understanding of how they work. You will find the gate is very sensitive to either + or - voltages, starting in the 15V range, ESD kills them dead.
This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil

wvmarle

#8
Nov 24, 2019, 06:11 am Last Edit: Nov 24, 2019, 06:11 am by wvmarle
You are very unlikely to find a 3.3V rated MOSFET except in a surface mount package.
Indeed. Surface mount - no problem, even for lower voltages, through hole - haven't found any yet.

The next problem is that these are only available as SOT-23 packages as far as I know, I haven't found any in larger SMD packages, and for those tiny MOSFETs 7.5A is a lot of current. You need a PCB with sufficiently large heat pad to make that work, or two MOSFETs in parallel.

So for this application a TO-220 MOSFET with gate driver IC would indeed be the best solution.

Very important is the on resistance (RDS(ON)), you want one with low on resistance to limit the heat losses (I'd aim for <10 mΩ as that's already almost 600 mW of dissipation at 7.5A, meaning you're getting into heat sink territory).
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

pert

Forum member and ATtinyCore maintainer DrAzzy/SpenceKonde sells some choice SMD MOSFETs pre-soldered on a TO-220 equivalent breakout board on their Tindie store:
https://www.tindie.com/products/drazzy/simple-sot-23-mosfet-six-pack-logic-level/

I'm not sure any of those are quite up to this particular project. DrAzzy also has some more beefy MOSFETs on breakout boards (but not the TO-220 equivalent style) available on their Tindie. Maybe this one would do the trick?:
https://www.tindie.com/products/drazzy/mosfet-switch-wfl3100a-gate-driver/

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