Energizing MOSFET gate with arduino

MarkT:
Yes, this is switch-mode power electronics, its all about such issues. Consider the little box challenge: https://littleboxchallenge.com/

The winners use 35 to 250kHz variable switching frequency and GaNFETs and achieved 2kVA output in
a box size of 14 cubic inches or so. 145W/cubic inch (9kW/litre!) not bad for a power converter.

What the?! Can I assume they all use transformers to step up the voltage or are we talking boost converters? Either way you either have to deal with tremendous bulk in your package (transformers) or tremendous waste (boost converter). How that can be scaled down without compromising on efficiency is a mystery to me.

stuart0: 1. No, never put a large capacitor (or any capacitor other than a that contained in a proper snubber circuit) in parallel with your mosfet.

  1. You don't need DC voltage, only DC current. Allow the motor inductance to filter the current.

  2. You say the mosfet can handle 100 volts. I hope this isn't an absolute maxium rating. Please make sure you stay well away from absolute maximum ratings, as you will definitely get transient over-voltages.

  1. Why? It's in parallel with my mosfet only in the sense that it's in parallel with everything in the main series circuit. How else would I filter a PWM signal (if we ignore the motor for the moment)?

  2. That's true but I don't see the harm, especially in light of the aforementioned inductive bias. Lots of motors have caps built into them for starting or for power factor correction.

  3. It's not the maximum but I think it's 10-20% away from it. The battery is no higher than 100V so unless I create a wicked tank circuit with my capacitor (which I will check), I don't think I will exceed the max.

Gahhhrrrlic: 1. Why? It's in parallel with my mosfet only in the sense that it's in parallel with everything in the main series circuit. How else would I filter a PWM signal (if we ignore the motor for the moment)?

Post a circuit of what you're contemplating. It's good to have a bypass capacitor on your supply, but not in parallel with the mosfet and not in parallel with the load either (actually the same thing in an AC sense)

  1. It's not the maximum but I think it's 10-20% away from it. The battery is no higher than 100V so unless I create a wicked tank circuit with my capacitor (which I will check), I don't think I will exceed the max.

Too close for the sort of current you're switching. You will kill the mosfet very quickly.

Something like this…

Gahhhrrrlic:
Something like this…

That capacitor is in parallel with the supply voltage, not the load or the mosfet. So it’s positioning (powersupply bypass) is ok. It plays absolutely no role in filtering your PWM however.

As for the driver with a 10k pullup. Honestly have you read none of what people have been telling you above?

I don’t want to sound harsh or overly critical (really I don’t), but based on almost everything you’ve written in this thread I think that you are probably well out of your depth on this project.

It's fine. I can tell you right now that I am a different sort than most. I usually do sloppy work (although I encourage myself mentally to at least be safe) but based on past history, 90% of my projects work the first time, even if they break rules in electrical etiquette or don't have the longevity or cleanness that others in the field would desire. I'm rough around the edges so if you're picking on some bad form, I'm sure you've got a valid point. Honestly, I'm not sure what you're talking about with the pull-up so if you wouldn't mind humoring me, I'd be willing to listen and learn. In short, this sort of project isn't outside of my grasp, it just seems that way because I put raw function above form all the time and am generally a disorganized thinker.

Gahhhrrrlic: It's fine. ...

Yeah well it wasn't meant to be a "put down". Your not alone in that the majority of people who post questions on this forum are often out of the depth, and that's why they're asking the questions. It's just that in this case you're controlling around 20 kW of power, and that makes it a little different. :)

Indeed, when you work with lethal amounts of power or at least power that could cause lethal collateral damage (ie. blow up my car) even I tend to do my checks and balances before I flip the switch. To clarify however, I'm well versed in electrical theory so I don't consider this to be "out of my league". I'm just not in any current risk situation since we're talking about a concept rather than me actually building the thing so I'm being careless is all. In retrospect, as for the reason for starting this thread, I could have answered my own question had I put sufficient thought into it. It's not that enigmatic and the data is all available. The real benefit of this discussion is the stuff I had no intention of talking about... subtle nuances like switching noise and filtering techniques; Pieces of wisdom from others more experienced than myself but stuff that I'm perfectly capable of grasping.

Back to the circuit, I realize after looking at it again that it doesn't do what I really wanted it to do. I want to bring the power factor to 0 degrees if possible but I also want to buffer the PWM. Maybe the latter isn't necessary because the armature coils can take it but it might reduce vibration in the motor to smooth it out... just thinking aloud here.

And what was wrong with the driver pullup? I'm probably just not seeing something obvious. Did I invert the PWM or something?

Nevermind. I found it. First, my 2 pullups share the same node so they're effectively redundant. Should just have 1. Second, the high R value and the inherent circuit capacitance makes for an unacceptably large time constant so my fets will heat up too much. At 12V, an R value of 50 ohm should be better. That what I was missing?

EDIT: ...and I inverted the PWM because my pullup configuration is the wrong type. Doesn't matter really, I can invert the PWM in Arduino code but I should consider the best option, not the first one that I think of.

Gahhhrrrlic: Nevermind. I found it. First, my 2 pullups share the same node so they're effectively redundant.

Yes you had a pull up and a pull down, not sure what that is about. However if you've read the replies in this thread you should have realized that you need much lower impedance than that to drive this mosfet. You need a proper "push pull" driver. Both MarkT and allanhurst both explained this at length.

Also your circuit is missing a freewheeling diode, which is a massive omission.

I already bought the freewheel diode in fact. Just forgot to include it in this pizza box sketch. Thanks for the reminder.