Circuit Protection Considerations for USB 3.0
The new USB 3.0 – or “SuperSpeed USB” – protocol was developed to provide higher transfer rates, increased maximum bus power and device current draw, and 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 an additional physical bus has been added in parallel with the existing USB 2.0 bus, as shown in Figure 1.
The increased current-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 overcurrent, overvoltage and ESD (electrostatic discharge) transients in USB 3.0 applications.
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 a maximum current rating of 0.5A to 0.9A. The Powered-B connector is able to charge a device by supplying current up to 1A from two additional connectors. Since overcurrent conditions can affect the power bus, overcurrent protection is necessary on all power sources (e.g., hosts and hubs).
The principal benefits of the new Powered-B connector are enhanced portability, convenience and power efficiency. By providing two extra contacts, the new connector enables one type of electronic device to provide up to 1000mA of current to another device. For example, a printer can power-up a wireless adapter, thus eliminating the need for a wired-network connection.
Similar to USB 2.0, all types of USB 3.0 connectors have the ability to provide power. A single-unit load for USB 3.0 is redefined to be 150mA, an increase from 100mA in USB 2.0. An electronic device may draw up to 6 units (900mA max per port from standard and micro connectors).
The standard USB 2.0 connector applications remain the same in USB 3.0. However, whereas the hubs in USB 2.0 can be bus-powered, USB 3.0 will not provide such an option and will only be self-powered. Since a jack port is needed to power-up all connectors of the hub in USB 3.0 applications, a circuit protection device is now needed on the jack to help protect it from damage caused by overvoltage events, which may result from an unregulated supply, reverse voltage and/or voltage transients. Figure 2 shows how installing a PolyZen™ device on the VBUS, and six low-capacitance PESD devices on the device ports, helps provide coordinated overcurrent/overvoltage solution.
Additionally, higher-current-capable overcurrent protection devices are required for systems that support USB charging. Polymeric positive temperature coefficient (PPTC) devices provide a cost-effective solution for USB overcurrent protection. As shown in Figure 3, installing a PolySwitch™ device on the VBUS port helps prevent overcurrent damage caused by a sudden short circuit downstream of the device side, and decreases the power dissipation required in a wide variety of USB applications.
USB overvoltage protection devices designed for traditional 0.5A ports may be inadequate for the new USB specification of 0.9A per port. If a 0.9A host disconnects, high 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.
Multiple interfaces and charging systems can expose portable electronics to damage caused by the use of incorrect chargers, improperly regulated third-party chargers, or by hot connect and disconnect events. Although typical USB power supplies provide regulated lines at 5V +/- 5%, the voltage at these lines may, under certain circumstances, significantly exceed 5.25V. The interfaces and charging systems may also generate negative voltages, resulting in damage to unprotected peripherals.
Internal testing by Tyco Electronics demonstrates that transients caused by hot connect and disconnect, although very short, can exceed levels from 16-24 Volts. Internal testing has also identified third-party chargers whose open circuit voltage exceeds the 5V +/- 5% USB requirements. Placing overvoltage protection devices, such as PolyZen polymer-protected zener diode devices on all USB powered devices, specifically on the VBUS port, can help protect against damage from voltage transients. For USB 3.0 devices, the PolyZen device can be placed on the USB input port, the Powered-B plugs, and the barrel jack power port, where applicable.
Overvoltage 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 2,000V, a human body can easily build up static charge that ranges up to 25,000V. 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 480Mbps 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 5Gbps, 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 minimize insertion loss in order to meet eye diagram requirements of USB 3.0. With a typical capacitance of 0.2pF, and flat insertion loss to >6GHz, PESD devices are capable of 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 helps 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 is 8kV (typ) / 15kV (max) for contact mode and 15kV (typ) / 25kV (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 and 6 illustrate the additional circuit protection devices needed for USB 3.0 designs, as compared with USB 2.0 designs.
PPTC overcurrent protection devices help designers meet the new high-current requirements of the USB 3.0 specification and provide a simple, space-saving solution. PESD devices provide low capacitance (typically 0.2pF) 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 heat sinking. This single device helps provide protection against the use of improper power supplies, as well as protection from damage caused by overcurrent conditions.
About the Author:
Kedar Bhatawadekar, Technical Marketing and Product Application Engineer for Tyco Electronics’ Raychem Circuit Protection Products is responsible for providing business solutions for new applications in a wide range of industries. He earned his B.S. degree in Electrical Engineering from Mumbai University, India and his M.S. degree in Electrical Engineering from San Jose State University.