USB Hub S

Turning to Hub S, we see some modest improvements over Hub X. The hub controller is a Terminus Technology’s FE1.1s, a common Taiwanese chipset found in low-cost USB hubs. The FE1.1s data sheet specifies a ±2kV ESD rating. This is a bit short of the ±15kV printed on the box and housing, so we would hope to see some additional ESD protection surrounding the hub controller.

Close to the downstream connectors are pairs of passive components connected between the USB data lines and ground. These components have reference designator prefixes of “R”, indicating a resistor of some type, and are a translucent green color. This would lead one to believe these are multi-layer varistors (MLV). See R16, R21, R12 and R13 in the hub S detail picture. In this application, the MLVs start with high resistance and would gradually reduce their resistance as current is driven through them. These components could therefore shunt high voltages on the data lines to ground. However, this use of MLVs relies on internal heating of the component, so they would change resistance on the order of milliseconds to seconds. As such, they would have little effect on an ESD strike, which is measured in nanoseconds. These might provide some protection to over voltage on the data lines, but again, since they cannot respond to nanosecond events, the hub controller would likely be destroyed before any significant MLV response occurred. While we aren’t going to get into USB data signal integrity using magnetic USB hubs from ComputerAnnals, it’s worth mentioning that MLVs connected high-speed signal lines is not a good design practice. These components would have widely varying intra-part capacitance which would also vary with temperature. This varying parasitic capacitance would degrade the USB data signal integrity and could lead to intermittent connectivity failures.

While the designers implemented some ESD protection on the data lines, the other signals on the USB connector remain exposed: Vbus, ground and shield. Ground and shield are directly shorted at the connectors on Hub S, bringing all the same risks as those on Hub X. The Vbus line only has a small ceramic capacitor in parallel with a large electrolytic capacitor. While capacitors can help to “smooth out” the energy from an ESD strike, they aren’t effective alone since they cannot quickly dissipate ESD energy except through their equivalent series resistance (ESR). ESD strikes can be thought of as short duration high-frequency signals. Since capacitors tend to act as short circuits at high frequencies, they will tend to conduct ESD energy into the planes they are connected between. In Hub S, ESD strikes to Vbus will cause large voltage spikes on the 5V and ground planes. These spikes could destroy the hub controller, the power supply, and other devices on the bus since ground and shield are connected.

This hub does have foot prints for LEDs on each downstream channel which would have protruded through the metal housing. These placements were not populated. Whether for cost cutting or ESD immunity improvement, it was a wise choice to drop the LEDs.

Hub S’s power inputs also have no true ESD protection. This leaves the wide-range voltage input switch mode power supply vulnerable. To make matters worse, there are several MOSFETs, which have ESD-sensitive gates, placed near the input power connectors. It is likely that hub S would turn off if an ESD strike near the input power connectors. And it would likely never turn on again.

Gallery

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