SD540 Motor Controller

[Greg Needel]

Originally Posted by JesseK

I remember these being the cheapest when they came out, but perhaps the SPARKs came out after the SD540's and dove below the price point? Am I remembering this correctly?

We didn't learn about the SD540 until days before the public announcement, at which point our prices had already been set. The SD540 was available to purchase a few weeks prior to the SPARK, but their existence didn't influence our cost.



If I can step back from my obvious SPARK bias a bit, my main issue with the SD540 is that the calibration switch and brake coast mode switch is on the bottom of the unit. I don't understand from a user interface standpoint how this works for teams. Do teams have to lift up their motor controllers while the robot is powered and enabled to calibrate them? Also my team plays with the brake/coast mode all season for different applications and it would be frustrating to me to have to turn over each controller for access to this feature.

[Michael Hill]

Originally Posted by AdamHeard

I just don't see how to get over this huge difference in performance;
http://www.ctr-electronics.com/downl...er-Testing.pdf

The fix for the SD540 power output is probably just a change in the MOSFETs or thermal characteristics. They probably just used MOSFETS with a higher Rds(on). I'm not sure of the actual topography of the SD540s, but I get an equivalent Rds(on) of ~ 17 mOhms with the SD540s. In comparison, the Victor SP has an equivalent of ~ 4 mOhms. When I say I don't know the topography, it'll be an H-bridge, but how many parallel MOSFETS there are on each leg is key. Also, I'm guessing the SRX has an additional current sense resistor over the SP, which would explain why the Victor SP tests better (just my guess). As for the SD540s, I'm betting when they designed it, they trusted the MOSFET data sheet. MOSFET Rds(on) specs in datasheets are some of the biggest lies ever. They'll give you the Rds(on) with a junction temperature of 20 degC on the front page, but when you heat it up to ~100 degC, it can change drastically. Always check the graphs.

[Sperkowsky]

Originally Posted by Anupam Goli

Really? I thought the multi-bank feature sounded pretty cool. Sure the other specs of the controller are less desirable but maybe the next iteration will be better.

Heres the way I see it.

4 Victor SP's or 4 Talon SRX's side by side is 2.75" by 4.74" by 1.185"
A bank of 4 SD540's is 2.7" by 6.1" by 1.2"

So you get a larger footprint with the 'feature' that if one of the controllers in a bank fails you are stuck with a dead controller sitting on your robot.

Now, you could argue that Spark's have a larger footprint which is true. But even with the quantity of 4 they come in cheaper than the bank of 4 SD540's.

and their dimensions in line are not much bigger at
2.74" by 7.5" by 0.868

[Alan Anderson]

Originally Posted by Sperkowsky

4 Victor SP's or 4 Talon SRX's side by side is 2.75" by 4.74" by 1.185"

Did anyone try using such a compact array? I'm worried about how hot the center two are going to get if they are mounted that way.

[Jared]

17 mOhm Rds on is really high for this type of motor controller. At 30 amps, which FRC robots will draw for a few seconds, you're dissipating 15.3 watts in your FETs!

It seems weird to have this so high - you'd almost have to go out of your way to find FETs with 8 mOhm Rds. There are tons of FETs designed for motherboard switching power supplies that do much, much better. It seems more likely there's some other issue, like V_gs dropping on the high side FET, or insufficient gate current, or possibly even thermal issue.

[Chak]

Originally Posted by Alan Anderson

Did anyone try using such a compact array? I'm worried about how hot the center two are going to get if they are mounted that way.

We ran 4 Victor SPs side by side with a footprint of 4.82" * 2.5" * 1.22" with no problems. However, the motor controllers were staggered in a up-down-up-down pattern and raised about 2" above the electronics board, which could've helped with cooling.

[Michael Hill]

Originally Posted by Jared

17 mOhm Rds on is really high for this type of motor controller. At 30 amps, which FRC robots will draw for a few seconds, you're dissipating 15.3 watts in your FETs!

It seems weird to have this so high - you'd almost have to go out of your way to find FETs with 8 mOhm Rds. There are tons of FETs designed for motherboard switching power supplies that do much, much better. It seems more likely there's some other issue, like V_gs dropping on the high side FET, or insufficient gate current, or possibly even thermal issue.

I agree it's too high. Looking at the pictures more closely, it looks like this is the MOSFET: http://www.nxp.com/documents/data_sh...N1R0-30YLD.pdf All looks fine to me (in fact, I think it's the same as, or at least related to, the one used in the Talon SRX as shown in this teardown: http://imgur.com/gallery/2xqoU). The current sense resister seems odd to me. I'm trying to make sense of the marking. The SRX uses a 0.5mOhm sense resistor (M50), but the SD540 has a marking of "50m", which doesn't make any sense to me since the math wouldn't add up (50mOhm). Is there an alternate interpretation of that marking?

Large picture of the SD540 here: https://images-na.ssl-images-amazon....1X6vyE3RyL.jpg

[Jared]

Originally Posted by Michael Hill

I agree it's too high. Looking at the pictures more closely, it looks like this is the MOSFET: http://www.nxp.com/documents/data_sh...N1R0-30YLD.pdf All looks fine to me (in fact, I think it's the same as, or at least related to, the one used in the Talon SRX as shown in this teardown: http://imgur.com/gallery/2xqoU). The current sense resister seems odd to me. I'm trying to make sense of the marking. The SRX uses a 0.5mOhm sense resistor (M50), but the SD540 has a marking of "50m", which doesn't make any sense to me since the math wouldn't add up (50mOhm). Is there an alternate interpretation of that marking?

Large picture of the SD540 here: https://images-na.ssl-images-amazon....1X6vyE3RyL.jpg

I think R50M is 0.5mOhm. It's kind of a weird label, but it's the only thing that makes sense. http://www.tai.com.tw/index.php/en/p...istors/111-rlf

What's a current sense resistor doing here though? I didn't know it needed to measure current.

Also - I just noticed it has 8 FETs (two in parallel everywhere), compared to the Talon's 4. Kind of odd.

[Hitchhiker 42]

Originally Posted by Alan Anderson

Did anyone try using such a compact array? I'm worried about how hot the center two are going to get if they are mounted that way.

Our four Talon SRX's for our drive were mounted side by side very closely and we had no problems with heating. It seems that the SRX's are very good about low heat loss.

[NickE]

Originally Posted by Alan Anderson

Did anyone try using such a compact array? I'm worried about how hot the center two are going to get if they are mounted that way.

We ran 7 Talons in a row with no issues.

[frcguy]

Originally Posted by Hitchhiker 42

Our four Talon SRX's for our drive were mounted side by side very closely and we had no problems with heating. It seems that the SRX's are very good about low heat loss.

Same. We did that and even had them mounted under another plate and there were no issues.

[adciv]

Originally Posted by Jared

What's a current sense resistor doing here though? I didn't know it needed to measure current.

Alternate theory: It's a 0.5 Meg ohm and the purpose is to bleed off the capacitor when not in use.

[Michael Hill]

I'm wondering what the copper thickness of the board is. I see in the SD540 picture that solder is being used to lower the impedance of the traces (the blobs of solder you see on the copper), but it looks a little dodgy in some places (there are some places where no solder is on the traces like next to the nearest MOSFET). Also, if someone has a picture of the other side of the board, that would be cool to see.