Power Switching

Hi @jordanm23,

   I am sorry I cannot give you a 'simple' answer, as you are basically asking a series of questions that forms a complex design chain, which is based on relatively basic principles, but involves a considerable number of features that will be unique to the type of design you are contemplating. Thus, only someone who has built a very similar system will be in a good position to offer answers that come from such experience. 

In the absence of such experience, my suggestion is for you start at a small level, and use that as a prototype to obtain the necessary experience to scale up to the full system later. For example, start with a single motor, single control and single battery. This way you can discover for yourself, what size of PCB you will need, per motor, to dissipate the required amount of heat, etc., bearing in mind the amount of current, etc. you will need to distribute for a complete system.

In terms of using the PCB as heat sink, this depends on many factors, some of which are difficult to determine. However, tools to make a 'first guess' are available. For example, a quick Google brought up:

https://www.heatsinkcalculator.com/pcb-temperature-calculator.html

Please note, I have never used this particular calculator, and hence this is not a recommendation of a particular product. I have no idea how accurate its results will be. However, using it would help to demonstrate some of the decisions that will affect the performance. If I was contemplating doing a project like yours, I would start with one or more such calculations for a single motor design and build that as a PCB design, so that I could see how closely the predicted performance matches reality.

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In terms of "right gate resistor and what gate-drive voltage is best to ensure the transistor switches fully on", the starting point will be the data sheet. e.g. the sheet at https://www.onsemi.com/pdf/datasheet/nvmfs5c628nl-d.pdf

includes the plots:

These show that 4.5V switching voltage can be used to control the device, which may allow certain 5V logic chips to drive it, but the Drain to Source resistance, and hence the amount of heat dissipation required for a given current, will be less if the drive voltage is increased to 10V. Having determined the expected resistance, and knowing the motor current, it is easy to estimate the dissipation, assuming the frequency the gate is switched is very low. (If PWM switching is involved, then the associated disspation must also be considered.) Remember to allow for resistance of the PCB tracks, which will add to total power dissipation.

The gate resistor is often chosen to reduce the chance of oscillation, with the value being a compromise between the oscillation risk and the increased switching time due to the capacitive loading of the gate on the gate driving circuit.

The need for protecting against inductive loads is another study in its right. In some cases, the intrinsic drain to source diode of the MOSFET is sufficient, whilst in other cases, external diodes or snubbers are required. My first impression  would be to add a suitable external device, as a precaution, but obviously this adds weight, space, cost, etc., which may not be acceptable in some situations. Determining this balance will also be aided by prototyping a minimal system, and measuring its electrical properties.

I wish you luck with your project. Best wishes, Dave

@davee Ever thought about having your own channel? You could call it "Sherlock Ohms  Investigates ". (Sorry if that joke's been done a million times, but I'm still relatively new to these spaces). Not sure how you find time to do your own stuff, given the amount of time you spend helping other people.

Hi @mike-parsons,

   Thank you for the suggestion, which I have considered with my tongue in my cheek. 😏 😏  

Applying a massacred perversion of Ohm's Law [ I = V/R ]:

    Current audience expectation = Potential audience appeal / Audience Resistance to Long Sermons

Hence, I calculate a 'channel' of mine would have an expected audience of zero.

I'll leave it to experts like Bill (@dronebot-workshop) who must put in a phenomenal amount of time and effort making such a polished and well-researched result.

As for the original 'Sherlock', from what I have seen on the TV, he was portrayed as an unpleasant, drug addict, smoker, who attempted to play a violin, which is not the kind of reputation I desire to portray.

So for now, whilst thanking you for your kind thoughts, I think I will politely decline your suggestion, and stick to calling myself Dave

    Best wishes, Dave 

Use the MOSFET as a low-side switch. Drive the gate as high as possible. Ten volts gives the lowest on-resistance. Five volts works only for lighter loads. Add a series gate resistor. Use 50–150 ohms typically.
Add a 100k pull-down to ground. Estimate heating using I²R losses. High current needs copper pours or heatsinking. For motors, add a flyback diode. Place it across the load. Snubbers or TVS improve reliability.