The ubiquitous Universal Serial Bus (USB) interface is about to undergo another evolution, keeping pace with the ever increasing demand for connectivity bandwidth. USB 3.0, or SuperSpeed USB, is expected to provide a major leap forward in transfer speeds, power management and flexibility.
The USB 3.0 protocol was developed to provide higher data transfer rates and increase power delivery capabilities by supporting higher current levels on each port. It includes new power management features, as well as new cables and connectors that are backward-compatible with USB 2.0 devices. The most significant change is that four additional physical copper data lines have been added in parallel with the existing USB 2.0 bus, as shown in Figure 1. These additional copper data lines are used to transmit high-speed data, but can also transmit ESD (electrostatic discharge) and other harmful voltage transients.
The increased current and power delivery capability of USB 3.0 intensifies the need for new circuit protection solutions. A coordinated circuit protection approach can help protect against damage from over-current, over-voltage and ESD transients in USB 3.0 applications.
Over-current protection
USB 3.0 provides power over two components: a standard host (A-type connector), and a new type of powering device (Powered-B connector). The new SuperSpeed specification increases the amount of current that can flow to a USB device – from 0,5 A to 0,9 A. A new Powered-B connector allows one USB device to charge another device by supplying current up to 1,0 A. Since over-current conditions can affect the power bus, over-current protection is necessary on all power sources (eg, hosts, hubs and Powered-B devices). Over-current protection is also required per UL60950.
Similar to USB 2.0, all types of USB 3.0 hosts must provide power. A single-unit load for USB 3.0 is redefined to be 150 mA, an increase from 100 mA in USB 2.0. A USB 3.0 host must now be able to support up to six units (900 mA) per port. In addition, USB 3.0 hubs may no longer be bus powered. All USB 3.0 hubs must now be capable of supporting up to 900 mA per port. Additionally, over-current protection devices capable of higher current are required for systems that support USB charging and USB 3.0. The USB charger specification utilises the same pinouts as USB 2.0, but allows for even higher current capability (up to 1,5 A per port).
Finally, USB 3.0 defines a new Powered-B connector, whose principal benefit is enhanced portability. The Powered-B connector allows the elimination of USB cables and extra power supplies. Using a Powered-B connector, a USB device can now power another USB device. By providing two extra contacts, the new connector enables a power supplier (with a Powered-B receptacle) to provide up to 5 V ±10%, 1000 mA of current to another device (with a Powered-B plug). For example, a printer can power up a wireless adapter, thus eliminating the need for a wired USB connection.
PolySwitch polymeric positive temperature coefficient (PPTC) devices provide a cost-effective solution for USB over-current protection in all of these applications. PPTC devices can be used for current limiting in USB 3.0 host applications, USB 3.0 hub applications, USB charging applications and USB 3.0 Powered-B applications. As shown in Figure 2, installing a PolySwitch device on the VBUS port of a USB power source limits current in the event of a short circuit, prevents over-current damage caused by a sudden short circuit downstream, and helps achieve UL60950 compliance. Additionally, six Tyco Electronics PESD devices on the USB power source provide low-capacitance ESD protection in a small footprint package.
Over-voltage protection on USB power ports
Although USB defines the power bus, this does not eliminate the risk associated with over-voltage events. Over-voltage events can be caused by a variety of fault conditions, including user error, poorly regulated third-party power supplies, hot disconnect events and inductive transients, to name a few. The interfaces and charging systems may also generate negative voltages, resulting in damage to unprotected peripherals. Although the USB 2.0 power supplies are specified to 5 V ±5%, many high-volume systems using USB 2.0 interfaces are designed to withstand 16 V and even 28 V (continuous).
Because USB 3.0 increases normal operating current and current limits, over-voltage protection devices designed for traditional 0,5 A ports may be inadequate for the new USB 3.0 specification of 0,9 A per port. If a 0,9 A host disconnects, high-voltage inductive spikes can be generated that may negatively affect the devices left on the bus. A well-designed bus will absorb these spikes, thereby protecting devices from exposure to them.
Internal testing by Tyco Electronics has demonstrated that transients caused by hot connect and disconnect, although very short, can exceed levels from 16-24 V. Internal testing has also identified third-party chargers whose open circuit voltage significantly exceeds the 5 V ±5% USB requirements, which can pose a threat to sensitive electronic equipment. Placing over-voltage protection devices, such as PolyZen polymer-protected Zener diode devices on the power inputs of all USB powered devices, specifically on the VBUS port, can help protect against damage caused by over-voltage events.
For USB 3.0 devices, the PolyZen device can be placed on the VBUS of the USB input port, the DPWR port of Powered-B plugs, and the barrel jack power port and VBUS input of USB hub devices. It should be noted that USB 3.0 will not support bus-powered hubs and will only support self-powered hubs. A power source is now needed to power up all ports of the hub in USB 3.0 applications. If a DC power connector is used at the input to the hub, a circuit protection device may be warranted to help protect the hub electronics from damage caused by over-voltage events, from an unregulated or incorrect supply, reverse voltage or voltage transients.
Figure 3 shows how installing a PolyZen device on the VBUS and six PESD devices on a typical USB circuit can help provide a coordinated over-voltage solution.
ESD protection
Over-voltage transients are often the result of ESD and may occur on the power bus as well as the data lines. Although modern ICs are protected up to 2000 V, a human body can easily build up static charge that ranges up to 25 000 V. In I/O port protection applications, a very low-capacitance ESD device with fast clamping and recovery response is required on the data lines.
The existing USB 2.0 protocol allows for data transfer rates of up to 480 Mbps and supports plug-and-play, hot-swappable installation and operation. In comparison, the USB 3.0 specification allows for data transfer rates of up to 5 Gbps, with fall-back support for the lower-speed USB 2.0 specification.
USB 3.0 adds four new pins to the connector to support the new SuperSpeed interface: USB3_TX (differential pair) and USB3_RX (differential pair), as shown in Figure 4.
The SuperSpeed interface of USB 3.0 requires lower-capacitance ESD protection than that of USB 2.0. Adding very low-capacitance PESD devices can help minimise insertion loss in order to meet eye diagram requirements of USB 3.0. With a typical capacitance of 0,2 pF and flat insertion loss to >6 GHz, PESD devices are capable of supporting USB 3.0 application requirements and handling numerous ESD transients.
Compared to most traditional MLV (multilayer varistor) or TVS (transient voltage suppression) diode technologies, PESD devices provide lower capacitance, and their low trigger and low clamping voltage also help protect sensitive electronic components. The devices are applicable for ESD protection on both USB 2.0’s high-speed D+ and D- signal lines and USB 3.0 SuperSpeed signal lines. Adding PESD devices to the circuit protection scheme yields protection levels that meet the specifications of IEC61000-4-2, which specifies 8 kV (typ)/15 kV (max) for contact mode and 15 kV(typ)/25 kV(max) for air discharge mode.
Coordinated circuit protection
A coordinated protection scheme can be used to enhance protection against high-current, high-voltage and ESD transients in USB applications. Figures 5, 6, 7a and 7b illustrate the circuit protection devices that are suitable for USB 2.0, USB 3.0 and Powered-B connector designs.
Device recommendations
PolySwitch over-current protection devices help designers meet the new high-current requirements of the USB 3.0 specification and provide a simple, space-saving current limiting solution. PESD devices provide the low capacitance (typically 0,2 pF) required for high-speed data transmission applications, and are available in the electronics industry’s most popular form factors. PolyZen devices offer designers the simplicity of a traditional clamping diode while obviating the need for significant heatsinking in the event of sustained over-voltage. This single device helps provide protection against damage caused by the use of improper power supplies, voltage transients and other user-generated errors.
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