Parallel sense resistors in SMPS design? - Page 1
Forum section: electronics / projects

author: rx8pilot
author_rank: Super Contributor
timestamp: March 17, 2015, 04:14:40 am
content: I have designed a few successful DC-DC converters, but I am still consider myself a relative beginner. A new design I am working on has a TI TPS43060 as a boost controller. 11-17vin to 25Vout at about 3A or so. The specs are kind of irrelevant though. The thing I am wondering about is the webbench reference design and the evaluation board I got have two sense resistors in parallel. I have not seen this and do not know why they do that. For 10mOhms they use two 20mOhm devices.Does anyone know why they would not use a single device?From the datasheet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2015, 04:24:51 am
content: Lots of reasons:- Better valueOnly resistors of a particular size and rating could be found, but not in the required value. So they were connected in parallel to get the correct value and rating.- Lower inductanceWider trace width reduces inductance ~proportionally. Less overshoot on the current sense signal.- Greater power dissipationAgain, resistors of a particular size could be found, but not in the required power ratings. Power resistors are also physically larger, so exhibit more inductance.- Better toleranceIf all the resistors are independent random variables within their tolerance range (say 1%), then N resistors in parallel exhibit sqrt(N) times tighter tolerance. Doesn't matter much for N=2, but up around 5-20, it's not too bad (brings it down to ~0.25%, say).- More flexibilityProbably a big driver on a dev board: if you need some oddball value for your test, you can assemble it from two (maybe more) resistors in combination. The other reasons might not matter at all, it's just convenient.A lot of commercial equipment will do it for the first two reasons, because through hole resistors have higher power ratings and lower cost (or did have lower cost; that's changing more and more these days), but higher inductance. Availability of very small values may also be limited, although if we're talking production, I'd be surprised if one couldn't convince any resistor manufacturer to spin a million of some oddball custom value for peanuts.Tim

author: rx8pilot
author_rank: Super Contributor
timestamp: March 17, 2015, 05:56:21 pm
content: Do you suppose that some of those reasons may be leftover from the days past? I have been using off the shelf current sens resistors from Vishay mainly which are available in 1mOhm increments up to about 20mOhms. The stated inductance is very low already - maybe a few nH of so.Convenience in development I can see. In fact, on my eval board, I may look at a few resistors in single and parallel to measure any differences. It seems that the PCB inductance will generally outweigh the sense resistor parasitic inductance, but I do not no that for sure.

author: Neilm
author_rank: Super Contributor
timestamp: March 17, 2015, 07:33:18 pm
content: Another possibility is that the manufacturer builds there own boards and does not want to have a lot of different resistors in stock.

author: georges80
author_rank: Frequent Contributor
timestamp: March 17, 2015, 08:38:45 pm
content: The one issue with multiple parallel 'sense' resistors is you make kelvin sensing quite difficult.Working with SMT all the time, I will run a heavy trace off each end of the sense resistor that carries the current and then on the 'inside' pad area run two sense leads back to the controller.There are recommendations on the actual land pattern design for the sense resistors by different vendors. There are also sense resistors with separate kelvin connected pads to aid in accurate sensing.With small value sense resistors in the milliohm range, you give up quite a bit of sensing accuracy if you don't take care of the current versus sense paths.'Note also that your schematic is of a eval board and their 'design' criteria is different than a production board...cheers,george.