Author Topic: Is my selection of N channel MOSFET look okay?  (Read 291 times)

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Offline Heisen

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Is my selection of N channel MOSFET look okay?
« on: May 04, 2021, 07:07:28 pm »
Greetings everyone,

I'd like to say I am inexperienced in this kind of stuff.

I wanted to select a N channel mosfet for switching a load (which is a LED or LEDs in series) at 12v with maximum current of 300mA. The switching signal is PWM from a microcontroller with 100ohm resistor in series and a 10k pull down (no mosfet driver), the voltage level can be at 5v or 3.3v (haven't decided yet, mosfet must work with both) at switching frequency of less than 1 kHz. I need to use a lot of these mosfet's on single board each with individual PWM signal, all parts on the board are automotive grade so these mosfet must be AEC-Q101 qualified. I am making a prototype board so I need something that I can hand solder so I choose SOT-23-3 package size.

Note :- I will be using potting epoxy on my boards, so there will be no airflow to these mosfets. I used ambient temperature of 85°C for calculations, these tiny boards will sit behind the taillights.

Summing up mosfet requirements  :-

1. Can handle guaranteed 12v 300mA load. (3.6 watt)
2. Should be logic level, fully turns on at 3V.
3. Can handle switching losses at 1 kHz, no mosfet driver required.
4. Automotive grade AEC-Q101.
5. SOT-23-3 package.

I don't know what Rds on value I am looking at for these mosfet's since there is no airflow, I have no idea how would I calculate the power dissipation of the mosfet.

With my limited knowledge I went to mouser and started narrowing down with the above requirements  :-

There were 133 results left and at last I sorted them with lowest Rds On and this mosfet comes up at the top.

Nexperia PMV15ENEAR https://www.mouser.in/ProductDetail/Nexperia/PMV15ENEAR?qs=T3oQrply3y80fkWq77yzJw%3D%3D

I attached the datasheet.

Considering my load, is this mosfet okay? What current level can this handle?

Thank you for your time.

« Last Edit: May 05, 2021, 09:10:23 pm by Heisen »
 

Offline SuzyC

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Re: Is my selection of N channel MOSFET look okay?
« Reply #1 on: May 05, 2021, 11:01:37 pm »
It would work just fine at Vg-s >3V.

 

Offline Heisen

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Re: Is my selection of N channel MOSFET look okay?
« Reply #2 on: May 06, 2021, 08:34:33 pm »
It would work just fine at Vg-s >3V.

I have a confusion. Say I have about 32 of these on the single board and all are connected to individual MCU pins with a 100 ohm resistor in series. 100 ohm resistor limits the current to 50mA per pin. So if I turn on all the 32 outputs of the MCU high, for a brief moment there will be a sudden current source from MCU of about (50mA x 32) 1.6A due to gate capacitance (total gate charge is 14nC) of the mosfet. Obviously the MCU can not supply that much of current.

To mitigate this issue I can increase the resistor value to limit the current to mosfet, but now this will result in higher switching time for the mosfet's. This means high switching loses, hence power dissipation will go up in mosfet. I will be potting my board, so power dissipation is a concern.

How to balance this out? Knowing my switching frequency is 1kHz.

My voltage regulator for the MCU is LM2936MP-5.0/NOPB which has maximum output current of just 50mA. I choose this because of it's low quiescent Current of 15 uA and it's linear and automotive grade.
« Last Edit: May 06, 2021, 08:41:43 pm by Heisen »
 

Offline SuzyC

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Re: Is my selection of N channel MOSFET look okay?
« Reply #3 on: May 07, 2021, 10:07:04 am »
Things are not adding up here.

What kind of MCU are you using that can source/sink 50mA per output?

You are going to need a higher current output LDO to accomplish your goal than a '2936.

Turn smaller groups of outputs on in sequence, with a short delay between turn ons, else use buffer ICs.
« Last Edit: May 07, 2021, 10:30:24 am by SuzyC »
 


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