Did you catch the error?
Most people count sheep to fall asleep.... I count electrons... and then i realized i was missing a few....
The on resistance of the a4988 is .32r(typ) to .43r(max) ; @ 2A : >> this is for each 1/2-bridge, of which there are
2 per phase <<
...each one is driving 2 phases which means I^2R losses are *actually* ~ 5 to 7W !
Temp rise is then:
Utopia : 5W x 32C/W = 164C
Best case scenario: 5W x 65C/W = 325C
Worst case scenario: 6.9W x 115C/W = 791C
The good news is i just found a really nifty little stepper driver IC that's got a typ output RDSon of .105 Ohm, 1/32 micro-stepping and tons of other cool features.
One i especially like is the overtemp warning signal that tells you when its less than 20C away from shutting down from overheating.
And no current sense resistors needed, so no power wasted there.
Another one is the analog Back-EMF output, great for analyzing motor dynamics.
And SPI configuration interface so no jumpers/dip switches necessary..
Plus its much better characterized than the ones Ive seen so far - its *conservatively* rated for 2.2A continuous, 5A @ low duty. These ratings i can trust.
Downside is its 40-50% more expensive and rated 30V, but i think its still a good bargain.
Combined with some better thermal management, i could easily get double or triple the useful *safe* current out of it compared to the $14 a4988 boards.
On the other hand, that extra 40-50% price increase would also cover the 4 dual-mosfets i would need to make an ice-cold 10amp 50V stepper driver, compatible with a4988.
Hard to decide.
Anyone wanna give me their 2 cents ?
Here's a thought - You can get for $10-$20 a super-efficient heatpipe-fan-heatsink for PC CPU's , that can get sub- .5C/W thermal resistance.
What if we had a 4- or 6- channel driver board designed to mate to that ?
With 1C drop from die to slug, and another 2-3 though the board with thermal vias, .5 for solder and thermal gasket...
So maybe 5C/W die to ambient, assuming 40C ambient, run the die at 80C max means safely burning 8W in there.... even 10 or 12 at the outer limits with some margin....
Going backwards from that means... 3W per 1/2-bridge; adjusting max RDSon for temp gives .25 Ohm worst case, .14 typ.
So that's... 3.5 to 4.5 amps continuous; 8 to 10 amps intermittent. Not too shabby! i may just have to try this
Suppose a 4-channel unit like this sold for $100 including heatsink - is that attractive ?