Powering a custom 328 PCB with high current components on the board.

I have a working custom Atmega328p PCB prototype(3.3v @ 8mhz). I need to work out a final design and looking for some input. I'm using hardware PWM to pulse an N channel MOSFET at a very low frequency. I also added in a shunt resistor and shunt monitor to sense current. This setup switches loads up to 90a. Everything is working, but I'm powering it with 4 wires from the battery pack connector.

It's about a 1x3 inch board. On one side I have battery + going to Vin which is going to a 3.3v regulator and powering the MCU + components then battery - is going to the GND plane.

On the opposite side of the board I have the MOSFET and shunt resistor with another set of wires from the battery pack. Battery + going to shunt + and battery - going to MOSFET source. The high current pads aren't connected to anything else on the board.

It works great how it is, but I'd really like to eliminate the extra set of wires. My question is, can I just run power to the MCU and components from the shunt + and MOSFET source? Any drawbacks to this?

Could you make a drawing ? You can write it on paper and make a photo of it.

I’m still messing with the design… Here’s an idea of what it looks like, though. Thanks for your input. :slight_smile: I changed some things from my last design, but everything is working great on it so far.

I'm sorry, but with that picture, I can't tell how ground currents will go or how things are connected.

Yes, how about a schematic instead? Much better for circuit analysis.

I can upload some schematics tonight. I think it will be better to show you what I'm trying to do VS how it is now, as it's working just fine like this. How it is now the high current pads from the MOSFET pins and the shunt aren't connected to anything else on the board. I simply added larger pads to accommodate heavy gauge wire.

EDIT:I tried to draw something up. I'm not sure the best way to show you how I'd like to wire it. I'll keep thinking about it.

Hi,
Upload your schematics.
If they are only of the PCB, then draw a schematic with your PCB as a square with the four wires coming out of it and show how it is connected to the real world.

Thanks.....Tom.... :slight_smile:

Here’s how I’d like to wire it. I only added a few components to make it easy to see. I don’t see why this wouldn’t work. See how the 3.3v regulator is fed?

Hi,

Sorry PCB patterns are of little use to circuit analysis.
Upload your schematics.
Then draw a schematic with your PCB as a square with the four wires coming out of it and show how it is connected to the real world.

Thanks.....Tom.... :)

I'm sorry, but I don't understand. I can show you my schematics for that board, but it won't show you anything else other than what the PCB shows. I wish I could explain what I'm trying to do better than that pic of the PCB.

That would be helpful but I think Tom's asking for a schematic showing what else connects to this PCB.

Here’s a schematic of the PCB I showed above. I’m not sure how it helps, but I appreciate the help.

With 90A load, no, don't try to share circuitry, you will probably encounter
problems, you may damage components. Switching large currents fast
generates large induced voltages via electromagnetic induction. If possible
separate the circuits, not bring them closer together.

Add more protection circuitry even. If the MOSFET doesn't already have a
zener from gate to source, add one (15V for standard MOSFETs, 5V6 for
logic level) - though I would not recommend logic level for switching large
currents, use 12V supply and a proper MOSFET driver that can hold
its own against the MOSFET under heavy load.

Thanks! I'll keep the design how it is now as far as the Vin wires go. I'm switching the MOSFET at 66hz. The MOSFETs I will be using have Vgs(th) Max of around 1.6v. At 3.3v Vgs, Rds(on) = 2mohm. I'm running two in parallel. Do you still think a driver is needed? I'm limiting the PWM pin current to about 20ma.

I didn't notice any voltage spikes when I scoped the output at heavy loads, but someone did recommend that I add a schottky diode across the output to protect from spikes. I'll have to look at the datasheet again to see if there's a zener across gate to source as well.

EDIT:Any favorite NMOS low side driver?

N-channel MOSFET? AOI514.
Logic level, low Rds.

I use 32 of them on this board for example, driven by 74HC595.

My frequency is very low @ 66hz. I can see on the scope that the MOSFET I'm using now is fully saturating. It's working as expected. In this situation do you think the driver is really needed?

EDIT:I'm going to go with this one. Someone else linked to it on a different forum.

Can you drive the gate of a 90 amp mosfet directly from an arduino pin ?

With the right MOSFET, gate resistor to limit the current and very low frequency, yes. That’s how I had it and it worked, but I’m using a driver now. Rds(on) should be < 1mohm even at max current now.

I just saw a pic of a guy working on the exact same project, but with a different MCU. He’s powering the 3.3v regulator via the high current input wire from the battery pack. That was my original question for this thread. The high current components and traces are very close to the other components on his board. I wish I could make my board smaller like that, but I’m not convinced that’s ok.

From what you guys are telling me, it’s not ok. I’m chatting with him and he assures me it’s just fine. I might not be explaining what I’m trying to do to you guys correctly. The high current wires are very, very short.