Computers & Accessories

ESD USB Hub L

Comparing Hubs X and S to Hub L, it is clear that the design appears to be simplified. Hub L has far more components, and appears to have improved ESD robustness. Close to each connector there is a small bi-directional TVS diode for the USB data signals. These TVS diodes will shunt high voltages to ground or the 5V plane. It is debatably better to shunt the energy to the shield, but Hub L’s inclusion of the TVS diodes is a big step above the other designs.

The other hubs failed to properly treat the ground shield signals separately. Hub L takes an acceptable approach. Ground and shield are capacitively coupled small (100nF) ceramic capacitors near each connector. At one of the connectors, a high resistance (1MΩ) “drain” resistor ties the the ground and shield together. This allows the ground the shield to “float” during short events like ESD strikes, but eventually return to equipotential to prevent ground loop currents in the shield.

On the USB Vbus lines, Hub L inserts a series ferrite bead. These can be effective at mitigating power supply noise and EMI issues, and they can be somewhat beneficial for reducing currents from ESD strikes. Further, on the bottom side Hub L has a large TVS or zener diode. Any ESD energy on the Vbus line would be mostly conducted into the hub’s ground plane. Its arguable whether this energy should be sent to shield instead of ground, but the Hub L design is appropriately protecting the Vbus line from ESD strikes.

This hub has one LED for power indication, and it protrudes through the metal housing. An ESD strike to this LED would likely cause the LED to not light up. Unfortunatley, ithe LED might not light either because it or the power supply was destoryed.

Hub L is the only hub in the roundup that provided protection for the power input. The inputs have rectifying diodes and TVS diodes interconnected between power, return and earth. Any ESD energy on any of the input pins would be shunted to ground and eventually earth. This added substantial cost to the hub, but it is an effective way to protect not just the hub, but all the customer’s equipment that gets connected to it.

Bonus: Acroname S77 USB Hub 2X4

Acroname’s USBHub2x4 draws from over 20 years of industry experience designing and supplying robust electronics for challenging environments. Every external signal is protected against ESD and surge currents with high-speed, low-capacitance TVS diodes. Ferrite beads in series with Vbus further suppress ESD currents and mitigate interference. Shield and ground signals are capacitively coupled and are tied with a high value bleed-off resistor. There are no LEDs protruding beyond or sitting near the metal housing. The USB hub controller and embedded microcontroller also include internal ESD protections. When testing the USB Hub 2×4 device at a certified lab, the device remained functional after being exposed to ±30kV contact-discharge strikes. This thoroughly exceeds the requirements of the IEC-61000-4-2 test standard and pushed the limit of the available test equipment in the lab.

Conclusions

When installing any equipment into a production environment, it is important to consider more than the brand, labeling or marketing bullet points. Even a common product like a USB hub can be the downfall of complex and expensive production environments. Trust your equipment to engineers who design for true industrial environments.

Acroname-designed products are architected with the newest components and design practices that focus on industrial and robustness. Even better: our products are tested by certified test houses to conform to or exceed industry standards and compliance specifications. Acroname’s products are field tested and purchased by customers who depend on them to work reliably in real-world industrial environments. Real-world industrial reliability by design. Always.

Our next article in this series will analyze these same hubs with focus on downstream USB Vbus current limiting. Stay tuned!

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