Advice on my diagram - PWM - MOSFET - Vehicle - Page 1
Forum section: electronics / projects

author: iAmHere
author_rank: Newbie
timestamp: February 10, 2022, 06:29:33 pm
content: Hello everyoneI made a prototype of MOSFET control in PWM (120 Hz). This assembly will be installed on a vehicle.The 4 outputs can drive Resistors, LEDs and an inductive load for output 4 with a maximum current of 10A under 12V.The piloting is done thanks to a micro-controller under 3.3V.This circuit works as I want but I would like to know if "industrially" I can keep this diagram.I have doubts about:* Should "automotive" grade components be used for the entire assembly in order to limit electromagnetic interference?* Do I have to switch to P channel MOSFET so that the (+) 12v is not present on the output connectors when the MOSFETs are not driven?* I use 1 standard optocoupler, 4 outputs, is it better to take MOSFET 'drivers'?* For the 12v -> 3.3V power supply I plan to use an AP63203WU-7 Q, is this a good choice? I know tha t it can deliver 2Amp, and it's ok for my PCB.I would be happy if you could advise me of components that would be more suitable for the end use of this assembly without increasing the price of the BOM list too much

author: moffy
author_rank: Super Contributor
timestamp: February 10, 2022, 11:01:02 pm
content: On the Drain of each MOSFET I would connect a freewheeling diode to +12v to limit inductive spikes. I would also put a capacitor between +12v and Gnd to absorb some of that energy. Keep your power traces separate as much as possible from your signal traces.

author: iAmHere
author_rank: Newbie
timestamp: February 11, 2022, 12:14:07 am
content: Thank you for your interesting comments.I will provide a shottky diode on each MOSFET.The capacitor for the 12V power supply that you advise, will it be a 25 V electro-chemical capacitor 1000uF ?

author: moffy
author_rank: Super Contributor
timestamp: February 11, 2022, 03:46:39 am
content: 1000uF/25v would be fine.

author: iAmHere
author_rank: Newbie
timestamp: February 11, 2022, 07:11:48 pm
content: Quote from: moffy on February 11, 2022, 03:46:39 amIf you have other remarks on my diagram, don't hesitate to let me know

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 11, 2022, 08:17:51 pm
content: Quote from: iAmHere on February 10, 2022, 06:29:33 pmHow much current do you expect to draw from 3.3V? Do you have circuitry other than the microcontroller running at 3.3V?If your current is low (e.g. <50mA) then just use a linear regulator for simplicity. It will also avoid extra EMI issues if that is something you are concerned about.

author: iAmHere
author_rank: Newbie
timestamp: February 11, 2022, 10:23:03 pm
content: Quote from: HwAoRrDk on February 11, 2022, 08:17:51 pmThe current needed by the microcontroller is about 600/800mA. I need the assembly to do the least amount of EMI.A power supply with a current of 1A could be OK.

author: CaptDon
author_rank: Super Contributor
timestamp: February 14, 2022, 06:57:37 pm
content: I don't see any magic here, mosfets and a driver. The 100 ohm resistors for gate drive are probably too high for PWM duty which means your FET's will spend a lot of time in the linear range and heat up. What is the zener clamp diode between GND and 12vdc out there beyond mosfet circuit #4? Not being used in a useful fashion.

author: iAmHere
author_rank: Newbie
timestamp: February 15, 2022, 09:48:45 pm
content: Quote from: CaptDon on February 14, 2022, 06:57:37 pmHello CaptDon,Component D1 is a Schottky diode ( SS34 ) which is used to absorb the current peak of the load which can be inductive on this output.I put the 100 ohm resistor to limit the current draw on the mosfet gate. The gate behaves like a capacitor when you start driving it. Do you think they should be removed?https://www.student-circuit.com/learning/year2/digital-systems-design/what-is-gate-capacitance-of-mosfet/

author: CaptDon
author_rank: Super Contributor
timestamp: February 16, 2022, 02:38:33 am
content: D1 is performing no useful function! Your inductive kick will be from the top of the FET back to 12v, not between gnd and 12v. If you are truly diving the gates with 120hz PWM then yes your resistor are too high based on the very fact you stated, the gates are enormous capacitors at generally 1000pf and up so your RC circuit means slow turn on / turn off and heat in your FETs during the transition. Try more like 15 to 27 ohms.