I've designed a circuit for a battery operated device which has a USB connector externally exposed for charging and data transfer. It is a non-standard, dockable USB connector with no shield connection available and the entire circuit is housed in a plastic casing with no chassis / protective earth possibility, something like the image below:
For ESD protection,I've followed almost the exact design recommendation provided here: http://www.semtech.com/images/promo/Protecting_USB_Ports_from_ESD_Damage.pdf
I can visualize the current path when Vbus, D+ or D- gets hit by a positive or negative ESD pulse i.e. steering diodes forward conduct for negative pulse or divert to the central TVS for positive pulse, please correct if my understanding is off.
However, I'm not sure what would happen if the exposed GND pin itself receives the zap.
Will a negative ESD strike on GND pin have the same effect as a positive pulse on Vbus, i.e. break-down of the central avalanche TVS leading to clamping?
In case of a positive ESD strike on GND, will the steering diodes and/or the central TVS forward conduct and pass the entire energy (minus 1 diode Vf drop,if that matters at all) to the rest of the circuit, thereby wrecking havoc!? I tried to depict the situation below:
Solutions that I'm considering:
Disconnect Vbus from the central TVS and introduce a stand-alone bidirectional TVS between Vbus and GND with subsequent reverse voltage protection for the rest of the circuit (to tolerate -Vclamp of the bi-dir TVS). It still may not prevent the steering diodes from conducting, plus there are other unidirectional TVS diodes shunting to GND on other exposed IO pins,which may also forward conduct.
Introduce a ferrite bead between the exposed USB GND and circuit GND for whatever meager impedance it may provide!
Any suggestions / insight is welcome, thank you!
Since circuit can draw power from Vbus, series resistors cannot be added in the Vbus-GND loop
Planned test as per IEC 61000-4-2, Level 4 (8/15kV contact/air discharge). Device will be operating on battery power during testing with no USB cable connected,hence all pins would be easily accessible for ESD strikes.