12 V system with many MOSFETs in parallel. Decoupling/Reduce Ground Bounce

Hi all,

I am trying to build the following system, which will contain many MOSFETs (see attached). The modules will work simultaneously (except module 1 and 2). For now, I have module 1.

I am aware of ground bounce problem and I would like it to minimize it.

I could also reduce the interactions between modules through programming, but that will lower the control quality of the system.

What formula should I use to calculate the capacitance of each capacitor I will need?

I have been suggested to separate the Arduino with another power source. Would putting it all in one “plug” be such a bad idea even with capacitors?

What modules are these?

What are the individual loads? What current do they each take? (forget power, current is what determines
the switching device specifications) Are any loads inductive?

What are the power supplies?

Capacitor - you mean decoupling for the supply, on the modules, on the loads?

Hi
OPs pic.

Tom... :slight_smile:

As TomGeorge kindly pointed out, I made an sketch of it:

TomGeorge:

MarkT:
What modules are these?

What are the individual loads? What current do they each take? (forget power, current is what determines
the switching device specifications) Are any loads inductive?

What are the power supplies?

Capacitor - you mean decoupling for the supply, on the modules, on the loads?

-I call them modules, as each is in charge of doing a specific function
-The modules 1, 2 and 5 are resistive loads in conjunction with small inductive loads (ventilators)
-In module 3, all of them are inductive loads
-Module 4 is a resistive load

-As it is a 12 V system:
Module 1: 6,1 A
Module 2: 1 A
Module 3: 3 A max. Between 0,3-0,5 A per unit
Module 5: 0,7 A

-The power supplies would be 12 V wall adapter/s. As asked, I might need 1 or more.

-Capacitor: I am unfortunately no expert (hence asking). I have read and being advised that working with several MOSFETs can cause disturbances in the system (ground bounce), that would prevent the system from working correctly.

Therefore, my questions are:
-Should I really use 2 power adapters (one for Arduino, the other for the modules) (please see sketch)?
-How can I minimize the "ground bounce" phenomena? How big, where and how many would I need? For this, I asked for a calculating formula so I could calculate it from the data supplied.

I've not thought of grounding issues as ground bounce but I have a couple of suggestions.

You are on the right track that a single point ground is usually optimum.

Question:

Are you planning to PWM any of the output modules? Its important because noise generated is occurs when things are changing.

russelcrowe_:
As TomGeorge kindly pointed out, I made an sketch of it:
-I call them modules, as each is in charge of doing a specific function
-The modules 1, 2 and 5 are resistive loads in conjunction with small inductive loads (ventilators)
-In module 3, all of them are inductive loads
-Module 4 is a resistive load

Any inductive load needs free-wheel diode across it - with a pulse rating adequate for the load current.

-As it is a 12 V system:
Module 1: 6,1 A
Module 2: 1 A
Module 3: 3 A max. Between 0,3-0,5 A per unit
Module 5: 0,7 A

These current values decide the on-resistances needed for the mosfets, which determine the
heat dissipation (and thus whether heatsinking is needed). Heat dissipation = I^2 R
(So for instance 6A with 20milliohm device dissipates 0.72W, which is borderline for needing
a heatsink).

If controlling direct from 5V logic the MOSFETS need to be logic-level, this is vital.

-The power supplies would be 12 V wall adapter/s. As asked, I might need 1 or more.

One for the arduino supply, one for all the mosfet modules is fine - you won't find a 12V wall wart for
that current level (you omitted module 4's current requirement BTW, but its over 11A for the others).

Make the supply capable of at least 25% more than the total current rating, never a good idea to
stress things to the limit.

-Capacitor: I am unfortunately no expert (hence asking). I have read and being advised that working with several MOSFETs can cause disturbances in the system (ground bounce), that would prevent the system from working correctly.

Yes supply decoupling of 100µF or more would be good at each module. That will substantially reduce
spikes in supply and ground on switching. 16V rated or 25V rated electrolytics are suitable.

Therefore, my questions are:
-Should I really use 2 power adapters (one for Arduino, the other for the modules) (please see sketch)?
-How can I minimize the "ground bounce" phenomena? How big, where and how many would I need? For this, I asked for a calculating formula so I could calculate it from the data supplied.

Decoupling capacitor sizing is usually very imprecise, more is always better, but diminishing returns eventually.
Without the datasheets for power supplies, details of the wiring length and resistance, its a rough guess.

The wiring between arduino and mosfet gates is fairly critical - this should be short, a twisted pair direct to
gate and source of the device. 120 or 150 ohm resistor in series with each Arduino pin will limit current spikes
from the Arduino pins. 10k or so resistors between gate and source will prevent the MOSFETs randomly
switching on at system powerup, which is normally needed.

Thanks everyone for your comments/suggestions

JohnRob:
Are you planning to PWM any of the output modules? Its important because noise generated is occurs when things are changing.

Yes, all but module 4 are planned to use PWM

MarkT:
Any inductive load needs free-wheel diode across it - with a pulse rating adequate for the load current.These current values decide the on-resistances needed for the mosfets, which determine the
heat dissipation (and thus whether heatsinking is needed). Heat dissipation = I^2 R
(So for instance 6A with 20milliohm device dissipates 0.72W, which is borderline for needing
a heatsink).

Understood, I will check the exact resistance of the ones I own (Model IRLB8721PBF) and put some! Is there a rule of thumb for the need of a heat sink for a small MOSFET? >= 0,75 W?

I guess I do not need a ventilator to remove the heat from the heat sink, right? I found these:

MarkT:
If controlling direct from 5V logic the MOSFETS need to be logic-level, this is vital.One for the arduino supply, one for all the mosfet modules is fine - you won’t find a 12V wall wart for
that current level (you omitted module 4’s current requirement BTW, but its over 11A for the others).

Make the supply capable of at least 25% more than the total current rating, never a good idea to
stress things to the limit.

Indeed, the control is from a 5 V logic. Understood! Unfortunately, I still do not know the power of module 3 yet, but I guess it will be limited to the 12 V the adaptor I can find.

MarkT:
Yes supply decoupling of 100µF or more would be good at each module. That will substantially reduce
spikes in supply and ground on switching. 16V rated or 25V rated electrolytics are suitable.Decoupling capacitor sizing is usually very imprecise, more is always better, but diminishing returns eventually.
Without the datasheets for power supplies, details of the wiring length and resistance, its a rough guess.

The wiring between arduino and mosfet gates is fairly critical - this should be short, a twisted pair direct to gate and source of the device. 120 or 150 ohm resistor in series with each Arduino pin will limit current spikes from the Arduino pins. 10k or so resistors between gate and source will prevent the MOSFETs randomly switching on at system powerup, which is normally needed.

I see! How did you come up with the 100µF/module? That way I will know next time what value I need to oversize.

Yes, the 10k resistors with the diode is the configuration I am following (see attached). Thanks! I was not aware of the 120 or 150 ohm resistor in series with each Arduino pin, as I understood the Arduino can only supply 40 mA (low current for what I thought could be dangerous).

An Arduino pin can deliver up to 40 mA peak but it's safer to limit it to 20 mA - so at least a 250Ω resistor for 5V operation between pin and gate.

Gate charge is of the MOSFET you picked is reasonably low; this is an important factor in switching the MOSFET, which is what you do a lot with PWM. The switching will cause extra heat, as the MOSFET will time and again go through partial-open state. The trick is to keep that time as short as possible.

100µF or 220µF is a quite normal value for decoupling. Usually that's enough. That 6A load may need a bit more. It also depends on the wiring and the reaction speed of the power supply, it also has to start working harder.

Because of the inductance of the wires connecting the modules, it's indeed one capacitor per module! Filter at the source.