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BLDC driver - Many Questions

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uliano:
I've come back to this hobby few weeks ago after a hiatus of maybe 20 years and I'm having a lot of fun.

This weekend I started investigating PWM, hardware side this means MOSFET. After a false start in which breadbord capacitance connection inductance and probably gremlins gave me a hell of ringings I switched to pre-drilled protoyping PCB (I don't know the english term, in italian they are called "thousand holes") and got consistent results.  First I built a simple common source switch(very easy). Then and H-bridge to drive a DC motor (easy and fun).

Question 1) I grok all power MOS have body diodes. Are these enough to act as a freewheel (I think so) or am I supposed to add external part?

Easy successes and lots of fun led my hubris making me greedy: i decided to approach BLDC motors the hard way, i.e. from scratch. My thinking was "a 3 phase bridge is jus one leg more than an H-bridge". Then I realized that maybe I would have liked to look at currents in the windings and inserted small resistor. Then I realized that I had no idea of the magnitude of those currents so, maybe, I had better foresee some kind of variable gain in the sense. Then I realized that it would have been nice to have comparators to look for zero crossings... I mean I had no idea if any of those things would work as expected but that's the point of prototyping. It is not the more up-to date technology: most of the components were in my drawers for more than 20 years!

[attach=1]

(decoupling capacitors 0.1uf and 1uf for TC4422 not shown)

It took me one day, with some undoing and redoing, I did a sketch of the placement of most components but then comparators were added at mid work and some noob mistake (after 20 years you get back to noob again) like not considering front-back view of pinout of one chip :-(.

Building it has been really instructive. As I was progressing I realized that there is an optimal order in connecting wires.  After some hours I started to feel that something went wrong somewhere. Or I made a step too long or I'm not ready for this level of complexity with this prototyping technique) or whatever...

Point is that this object (It didn't work  :-// ) is not debuggable (not by me at least)

[attach=2]
[attach=3]

Question 2) How would you have realized such a prototype? with a different technique or just with a larger board?

It draws current at rest I even found a bug (a missing connection) but to no avail. It is nearly impossible to even see through the mess of cables. I'll keep it for onrment! :-D

Question 3) Is it worth some more time?

Even it was a failure I learned a lot by making it.

Now I know that for this level of complexity I will make a PCB probably most, if not all, in SMD so I have to buy new components. Now it's a good moment to amend errors, add new functions and so on. What I would like to do is a board that lets me to learn about BLDC control, sensored, sensorless, FOC, whatever... for fun. So definitely no fancy components "I do it all" that would fail the learning focus and bring out in hardware as much as possible (just the contrary of real world), as I want to see most signals on the scope. The idea is to stay within 15V limit (4 lipo cells) possibly even less. As for the current, the biggest motor I ordered is this one 4100KV

https://www.aliexpress.com/item/32833212767.html?spm=a2g0s.9042311.0.0.1ec64c4dPMJPJo

which I really don't understand

Question 4) what 80A means? (I hope peak) and

Question 5)  what implication it has on dimensioning the MOSFETs

Most important:

Question 6) That's not a question, it is a suggestions session. Please criticize!  :)

jmelson:

--- Quote from: uliano on October 26, 2021, 03:13:52 pm ---I've come back to this hobby few weeks ago after a hiatus of maybe 20 years and I'm having a lot of fun.


Question 1) I grok all power MOS have body diodes. Are these enough to act as a freewheel (I think so) or am I supposed to add external part?

--- End quote ---
No, the body diodes are VERY slow to turn on.  I was having problems with a brushless drive I made years ago, and crept up toward the current level where the drive blew up.  I was able to see the body diode being forward biased up to 12 V for ten us without starting to conduct.  This ended up blowing the gate driver chip.  I had to add a super-fast diode across the low-side FET to save the driver chip's
high-side driver element.
Jon

uliano:

--- Quote from: jmelson on October 26, 2021, 04:22:15 pm ---I had to add a super-fast diode across the low-side FET to save the driver chip's high-side driver element.

--- End quote ---

So, something like shottky?

Siwastaja:
Body diodes are usually just fine, I have never used parallel diodes in a motor controller MOSFET half or H bridge, ever, including brushed DC and BLDC controllers of different power levels. They turn on just fine, the "problem" is some excess reverse recovery charge compared to schottkys. I guess jmelson's problem has likely been something else or some special case. In DC/DC (synchronous buck) I have used parallel diodes once and saw minor improvements. Even then, the exact choice and placement of the diode was very important, and I remember Tim here commented the improvement might have been just due to the capacitance of the diode acting as a snubber. But you do see these schottkys sometimes in >1MHz DC/DC controller appnotes.

Terry Bites:
The series mosfets need separate non overlapping gate drives. If not then there will be times when top and bottom are on leading to meltdown. You wil probably need to use a bootstrap to get enough drive to the top FET. Have an informative read of the IR2111 dtasheet for instance. The body diode frewheel diodes are ok, but an extenal diode will save you the cost of a new FET.

I would think that 80A is the peak current. Even so it seems very high for a small motor. If the spec is to be belived then on 15V at stall it would draw 1000A!! I cant find an actual data sheet for the motor so the spec is uncertain. Sizing of the FETs depends very much on relaible motor data. As suggested here a curent rating of 3x normal and a voltage rating of at leat 2x battery is a good idea: https://www.diodes.com/assets/Articles/Auto-MOSFET-BLDC2.pdf Then you need to work out how many watts of dissipation there will be in the FETs.  Remember that the  body diodes have thier own power to dissipate in additon to the FETs. How hot will they get?
https://www.re-innovation.co.uk/docs/heatsink-calculations/
Lots to think about.

It can be educational to deconstruct a integrated solution, eg L6234 form ST (Satans Trolls).

Where do the headers go?

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