usb interface protection - Page 1
Forum section: electronics / projects

author: snarkysparky
author_rank: Frequent Contributor
timestamp: May 13, 2021, 03:01:52 pm
content: We have a board with USB high speed interface to a PC that we are working on.The board drives many inductive loads. All the loads have freewheeling paths for inductive energy but sometimes we are destroying our FT4222 interface chip whenmany loads are running.I placed steering diodes on D- and D+ to 3.3 and ground. But apparently that isn't enough.I do suspect the local board ground is bouncing quite a lot due to high current loads switching quickly.Any advise on how to harden the design. Optical isolation on USB side would be good but isolators that work at USB 2.0 high speed are $250 as best as i have found.Thanks

author: fcb
author_rank: Super Contributor
timestamp: May 13, 2021, 03:37:04 pm
content: If you need to isolate the USB then I would float the whole USB circuit, not try isolating the USB DP/DM lines. The fact that you are using the FT4222 indicates that you will have quite a few lines to isolate. Can you share what interfaces you are using on the FT4222?Silicon Labs make good isolators, as do TI.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: May 13, 2021, 04:20:32 pm
content: From FT4222 the interface is SPI. single line SPI.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2021, 07:43:20 pm
content: Bouncing ground is certainly a possibility. Have you measured it? Can you show your entire system?Tim

author: snarkysparky
author_rank: Frequent Contributor
timestamp: May 14, 2021, 01:35:11 pm
content: Did a quick sketch of overall system. I believe I need to ground the hardware board common to the machine frame.

author: fcb
author_rank: Super Contributor
timestamp: May 14, 2021, 03:30:03 pm
content: Looks like floating your whole FT4222 circuit would break the loop.Not sure what distances are involved or what your layout looks like, but consider grounding the 24V supply on the machine ground.

author: mjkuwp
author_rank: Unknown
timestamp: May 14, 2021, 05:42:04 pm
content: You could convert the USB to SPI on the PC side as you have already suggested. and use digital isolators to send the signalADM2582ERS-485 in this case is just an electrical hardware layer to send SPI signals.

author: Raj
author_rank: Frequent Contributor
timestamp: May 17, 2021, 03:30:30 am
content: Is it usb to serial? you can get an HC5 or esp32 to make a wireless to serial device. Way cheaper than any other isolation.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: May 17, 2021, 11:55:44 am
content: Raj,My problem is that i need really good speed. Need to transmit 2 Mbyte in about a second. SPI clock is 24 Mhz.I grounded the PCB to the machine frame thereby holding the board ground much closer to the PC ground.But I still managed to kill another FT4222. Their input D+ D- seems very sensitive.I had a USB hub between the board and the PC. This i will remove and make a direct connection.Trying to pick a transient absorber IC but considering this maximum spec for the D+ and D-DC Input Voltage – USBDP and USBDM -0.5 to +3.63 VI can't seem to find anything that will guarantee a clamp voltage this low. Or schottky diode with low enough capacitance ( C < 4pf ) that will clamp this low.As for isolating the downstream side of the IC I am going to have to do a timing analysis to determine in SPI will still work with an isolated optocoupler path given 20ns propagation delays.SS

author: fcb
author_rank: Super Contributor
timestamp: May 17, 2021, 01:28:30 pm
content: We used a really fast coupler from MAXIM (MAX14436FAWE+) a couple of years ago - the attractive feature was very low capacitance between sides (1pF per channel) and blistering speed.It was so good we went against the golden-rule of "no MAXIM".

author: Raj
author_rank: Frequent Contributor
timestamp: May 17, 2021, 04:42:47 pm
content: isolated Ethernet using transformer or similar differential TTL signal?

author: Terry Bites
author_rank: Super Contributor
timestamp: May 20, 2021, 01:28:16 pm
content: Free:This looks like a bad earthing arangement. Commons 1&2 and chassis ground should only meet at one point back at your PSU earthing point. Carefully Bond all the chassis work and run a single bond wire back to the earthing point. Dot count on fixings to provide a low impedance connection between metal parts. Use coper braid, eye crimps and star washers in your bonds.$1 Wind a few turns of your usb lead through a torroid core (blue ferrite) to supress common mode spikes.$ Bit more. USB over LAN Cat5/6 adaptors. Ethernet is isolated by deisgn. Ethernet magnetics are available with kV of isolation.https://www.makeuseof.com/tag/usb-over-ethernet/$ lot more. Use USB to Wi-Fi, eg USB Network Gate$ loads more. USB over fibre, Lindy, Blackbox, Startech. etc.