Mid-plate GND on USB Type-C Connector - Page 1
Forum section: electronics / general

author: Northy
author_rank: Regular Contributor
timestamp: February 16, 2021, 09:05:28 pm
content: Hi,I've found a USB Type-C connector that I'm interested in using.https://belfuse.com/product/part-details?partn=SS-52400-003https://belfuse.com/resources/drawings/stewartconnector/dr-stw-ss-52400-003.pdfhttps://belfuse.com/resources/datasheets/stewartconnector/ds-stw-usb-type-c-connectors.pdfI've also found one that's identical footprint at Wurth.They have two extra TH pins marked Mid-plate GND. I was unsure if it would be best to connect those pins to Chassis_GND or Digital_GND so I thought I'd ask the manufacturers, the experts right?One replied with Chassis_GND and one replied with Digital_GNDWhen I replied to both saying the other manufacturer had suggested the opposite, both changed their minds, so both still differentThese pins connect to the piece of metal in the 'tongue' of the connector. In my head I think Chassis_GND is the most sensible, but it's going to be a real PITA to connect them to that and not have Chassis_GND and Digital _GND overlap, which isn't a great idea is it?Has anyone used a connector with these pins on it before?G

author: thm_w
author_rank: Super Contributor
timestamp: February 16, 2021, 09:40:06 pm
content: In the drawing you can see it mentioned "ground plates both sides". So its physically going to be connected to the shell.You could grab one, plug in a cable, and test it out.See page 135 here:https://usb.org/sites/default/files/USB%20Type-C%20Spec%20R2.0%20-%20August%202019.pdfand please no discussion of separate chassis/signal ground, its discussed to death:https://www.eevblog.com/forum/projects/usb-shield-ground-connection/

author: Northy
author_rank: Regular Contributor
timestamp: February 17, 2021, 09:22:32 am
content: This goes in a metal box, so my plan is to connect the shell and the two Mid-plate GND pins to chassis GND.But I need to try and do that without it overlapping my Digital GNDThanks,G

author: Northy
author_rank: Regular Contributor
timestamp: March 01, 2021, 09:31:20 am
content: Just getting back to this.So if we agree (and I know some don't) the following:USB Type C shell connects to Chassis GNDMid Plate GND pins connects to Chassis GNDGND pins connect to Digital GNDDigital GND and Chassis GND connect together at some point in the design.I'm struggling to connect the Mid Plate GND pins to Chassis GND without overlapping Digital GND and Chassis GND - which I feel is a bad idea (the overlapping), but is it really?Thanks,G

author: thm_w
author_rank: Super Contributor
timestamp: March 01, 2021, 09:48:31 pm
content: The simple solution mentioned in the thread above is connecting Chassis Gnd to Digital Gnd directly at the connector and only having the one ground plane.If you don't want to or can't do this for various reasons, I would keep them apart and not overlap. While still having a reasonably low impedance path to presumably some metal screw/enclosure connection. Examples:https://blog.upverter.com/2019/11/07/pcb-grounding-techniques-to-do-and-what-not-to-do/https://pcb1001.blogspot.com/2008/09/sata-interface-layout-considerations.html

author: Northy
author_rank: Regular Contributor
timestamp: March 02, 2021, 09:08:37 am
content: I need to keep GND and Chassis_GND separate. They are tied together with a 0R link very similar to your second link at one point only.I've got a solid GND plane on layer 2.I really can't see how to get Chassis_GND to the MID_Plate GND pins without overlapping GND and Chassis_GND,I've already got the back two mechanical mounting pins on islands - just using them for mechanical mounting, not connecting them to the Chassis_GND pour (other than through the connector itself). This part also has a SM pad under the shell to connect to Chassis_GND so I think it will have a good enough connection.G

author: Northy
author_rank: Regular Contributor
timestamp: March 03, 2021, 10:09:17 pm
content: Does anyone have any good ideas on how to connect these Mid GND Pins?Thanks,G

author: PeteH
author_rank: Unknown
timestamp: March 03, 2021, 10:42:24 pm
content: Midplate is a critical return path for your high speed lines... Keeping it separate from the digital ground just causes more problems.Overlap means a bit of capacitance which probably helps reduce the discontinuity but still is not ideal.Needing to keep these separate? By tied at one spot somewhere with a 0R??I don't get it... Either add a bunch of local AC coupling between the two or have a single ground plane for continuity of the reference plane(s).

author: oliviasmithh900
author_rank: Contributor
timestamp: March 04, 2021, 12:32:36 am
content: Quote from: Northy on March 03, 2021, 10:09:17 pmCommon all Grounds .... always worked for me

author: PeteH
author_rank: Unknown
timestamp: March 04, 2021, 12:53:11 am
content: Pretty sure shield ("chassis") and "GND" are tied together in the plug of a type-C cable as well.... So seperate on a PCB does little.

author: Northy
author_rank: Regular Contributor
timestamp: March 04, 2021, 11:09:42 am
content: So what exactly is the Mid Plate GND? Is it same as the connector shield or is it a digital GND signal?G

author: PeteH
author_rank: Unknown
timestamp: March 04, 2021, 11:45:46 am
content: Mid plate is a reference plane for all signals in the connector.... Also provides mechanical support/retention.Usb type C plug ties all of these together (GND, plate, shell)

author: Northy
author_rank: Regular Contributor
timestamp: March 04, 2021, 12:22:26 pm
content: Hi PeteH,What's the thinking between tying them all together in the plug? I expected the cables to be like an HDMI cable where the screen was separate to the GND connections.Thanks,G

author: PeteH
author_rank: Unknown
timestamp: March 04, 2021, 03:19:18 pm
content: Besides it being the right thing to do for EMC... USB specification says to bond them.It would have been nice to see the rational behind that, I could see it being harder to get really good power flow combined with low impedance shield connections / good reference planes in such a small connector.I haven't read through the HDMI spec so I cannot comment on how that connector is "designed" (PCB paddle vs pins/plastic?)Always cost, complexity, performance tradeoffs here... Typically devices (end use market) bond them all anyway, so doing the same on the connector side perhaps simplified the design?

author: Northy
author_rank: Regular Contributor
timestamp: March 04, 2021, 07:42:42 pm
content: Ah, well there it is, many thanks for that, I hadn't seen that in the standard.One of the reasons for separate Chassis GND connection was thinking about ESD zaps during EMC testing. Try are steer the energy into the metal chassis that is connected to Electrical Earth.Oh well, I need to think about this.Thanks,G