Hello,
short background first:
we are developing a linear (reciprocating) pump motor, and the driver is based on DRV8301. It's currently operational using the DRV8301-69M-KIT EVM.
In the next version of the motor design, it will have a lower winding resistance and a rather high ratio of operating current to stall / inrush current:
rated voltage: 60V (max)
rated current: 12A (typical)
winding resistance (phase-to-phase): 0.67 Ohm
stall current ( = max voltage / resistance): ~90A (!)
I am worried about the inrush currents, especially since they happen very often: it's a linear motor rather than a normal rotary motor, so it changes direction more than 10 times per second. on each direction change, I expect that the current will go up to 90A if not regulated or limited somehow. I don't want to design the power stage to these high current levels, it is unnecessarily expensive for the task, and the repetitive stresses of 90A current aren't good for the reliability of the product.
now to get to the point:
I am considering using the Vds sensing based over-current protection of the DRV8301 for limiting the current, like a chopper circuit. The OC protection will be of course set to current-limit mode rather than latched fault mode. preliminary testing in the lab show it seems to be working, however I have some questions / worries:
1. is it a good idea, generally? it seems the datasheet cares to mention multiple times that the OC protection is not meant for current-regulation. However, if it's merely an issue of accuracy, then I don't mind the 20% threshold spread they warn about - I'm OK with the limit being anywhere between 20A and 30A.
are there reasons other than accuracy of why it's not recommended in the datasheet? and if current-limiting is really not recommended, why does the OC protection even has this mode...?
2. Since at any one time, 2 transistors out of the 6 are conducting, either / both can trigger the OC feature of the DRV8301. Do I have any way to control which does? can I count on both transistors getting turned off, or will it be random? I am worried about the recirculating current path and its implications.
3. how sensitive is the protection circuit to short spikes? is there any way to add filtering (and where)? I think that with good transistors (low Rds_on), my Vds threshold will be quite low, certainly at the lower end of the range the DRV8301 provides, so even small amplitude noises can pass the threshold.
thanks in advance,
Guy.