News | March 26, 1999

Meeting USB Overcurrent Protection Requirements - Part 1

Reliable, resettable over-current protection is a must in the plug-and-play and hot-plug environments.

By Paul Wiener, Raychem Corp.


•Powered Port Protection Architecture
•PolySwitch Devices
•Universal Serial Bus (USB)
•Overcurrent Protection

Universal Serial Bus (USB) technology allows PCs to be built in a more modular fashion and sold like home entertainment systems, for which people buy the specific components they want rather than a pre-configured system. The USB technology allows peripherals, from printers to joysticks, to be seamlessly plugged into a PC, and prompts the computer to reconfigure itself to accept the new devices. The USB bus utilizes a powered port architecture to accomplish these tasks. In the plug-and-play and hot-plug environments, reliable and resettable overcurrent protection is a must.

Powered Port Protection Architecture

USB technology relies on system flexibility and ease of use. The main sources for this flexibility are the plug-and-play and hot-plug capabilities of the computer environment, yet the USB ports must be reliable to pass on data and power, whether to the PC's peripherals or for the functions of the peripherals in the USB system.

Powered ports, therefore, require over-current protection. Underwriters' Laboratories (UL) and the International Electrotechnical Commission (IEC) have established standards for power sources that are used with existing bus systems to provide overcurrent protection. UL1950 and IEC60950 are the primary agency specifications governing the current output of power sources. Powered ports in computers (for example, SCSI, mouse, keyboard, etc.) have required overcurrent protection for several years.

In addition to safety agency standards, industry standards are also driving requirements for reliable products. The Microsoft/Intel PC98 Hardware Design Guide, for example, promotes plug-and-play and the idea of a sealed PC box that does not require access to the inside of the PC box. The sealed box does, however, require resettable overcurrent protection for certain powered ports, such as the SCSI port. The PC98 Hardware Design Guide requires overcurrent protection for terminator power (TERMPWR), citing the positive-temperature-coefficient device as an example of an overcurrent protection device to be used in the circuit.

The USB specification 1.1 has continued the requirement for resettable overcurrent protection, referencing Polymeric PTC's (PolySwitch devices) as an appropriate method. The USB specification also endorses the concepts of plug-and-play and hot swapping, both of which lead to an increase in overcurrent scenarios.


PolySwitch Devices

PolySwitch resettable fuses are polymeric positive temperature coefficient (PPTC) devices appropriate for self-powered and bus-powered Universal Serial Bus (USB) applications conforming to USB requirements. They are suitable for both ganged port protection and individual port protection.

The primary features of a PolySwitch device are:

  • Conformity with USB specification 1.1
  • Compliance with:
    • UL1950/IEC60950 safety requirements
    • PC98 standards
  • Resettable protection
  • UL-recognized safety device
  • Low cost

Functional Description

PolySwitch devices are series elements in a circuit (see Figure 1). The PolySwitch device protects the circuit by going from a low-resistance to a high-resistance state in response to an overcurrent (see Figure 2). Called "tripping" the device, this change from low to high resistance is the result of a rapid increase in the temperature of the device, caused by the generation of heat within the device by I2R heating.

PolySwitch devices are available for a variety of operating currents. Each device is specified by "hold current," the maximum current at which the device will not trip (remain low in resistance) at 20°C. All of the devices have UL/CSA/TÜV safety agency recognition.


Universal Serial Bus (USB)

The goal of USB is to enable devices from different vendors to interoperate in an open architecture. USB features include ease of use for the end user, a wide range of workloads and applications, robustness, synergy with the PC industry, and low-cost implementation. Benefits include self-identifying peripherals, dynamically attachable and reconfigurable peripherals, multiple connections (support for concurrent operation of many devices), support for as many as 127 physical devices, and compatibility with PC plug-and-play architecture.

The Universal Serial Bus connects USB devices with a USB host—each USB system has one USB host. USB devices are classified either as hubs, which provide additional attachment points to the USB, or as functions, which provide capabilities to the system (for example, a digital joystick). Hub devices are then classified as either Bus-Powered Hubs or Self-Powered Hubs.

A Bus-Powered Hub draws all of the power to any internal functions and downstream ports from the USB connector power pins. The hub may draw up to 500 mA from the upstream device. External ports in a Bus-Powered Hub can supply up to 100 mA per port, with a maximum of four external ports.

Self-Powered Hub power for the internal functions and downstream ports does not come from the USB, although the USB interface may draw up to 100 mA from its upstream connection, to allow the interface to function when the remainder of the hub is powered down. The hub must be able to supply up to 500 mA on all of its external downstream ports.

The USB transfers signal and power over a four-wire cable. The differentially driven signaling occurs over two wires. The cable also carries Vbus and GND wires to deliver power to devices. Vbus is nominally +5 V at the source. USB hub and host devices must be protected from damage when a shorted or damaged downstream device is plugged into any USB port. The most common problem is a damaged cable or connector, which often results in a short circuit being applied to a port when the cable is plugged into the host or hub. This will be a fairly "common" occurrence in the lifetime of a PC or a Hub, for example, that is used for joysticks and other game controls. As a result, USB port short-circuit protection must be effective, reliable, and resettable, as well as low in cost.


Overcurrent Protection

The USB specification 1.1 section states that overcurrent protection is required for safety reasons. The hub must have a way to detect the over-current condition and report it to the USB software. The overcurrent trip point should not exceed 5.0 A and must be sufficiently above the maximum allowable port current such that transient currents (e.g. during power up or dynamic attach or reconfiguration) do not trip the over-current protector. The over-current limiting mechanism must be resettable without user mechanical intervention, so single-use fuses cannot be used. Polymeric PTCs are an acceptable method that can be used for over-current limiting.

In summary, USB is a system that supports a plug-and-play environment with port loads of 100 mA and 500 mA at 5 V and with a multitude of devices having several powered ports. A typical desktop computer environment (Figure 3) shows where PolySwitch devices can provide economical and highly reliable protection for powered ports.

Overcurrents can be reported to the USB controller chip by laying a trace from the downstream side of the PolySwitch device to the overcurrent pin on the USB controller chip (see Figure 4).

Click here to see Figure 4.

Under normal conditions, the voltage downstream of the PolySwitch device (VA) will be ~5 V. In the event of a fault, VA will be <1 v.="" this="" change="" in="" voltage="" signals="" a="" fault="" to="" the="" microprocessor="" with="" low-active="" overcurrent="" pins.="">

Design Examples

  • Host/Self-Powered Hub, individual port protection
  • Bus-Powered Hub, individual port protection

Host/Self-Powered Hub

Figure 5 shows a four-port Host/Self-Powered Hub with a PolySwitch miniSMDC110 device or RUSB120 device protecting each port. Individual port protection allows one port to become short-circuited without disabling the other three ports.

Click here to see Figure 5.

Bus-Powered Hub

Figure 6 shows four ports with a PolySwitch miniSMDC075 device or RUSB075 device protecting each port. Individual port protection allows one port to become short-circuited without disabling the other three ports. Included are the optional soft-start components R2 and C1.

Click here to see Figure 6..

End of Part 1

Contributed by: Raychem Corporation, Electronics Division, 300 Constitution Drive, Menlo Park, CA 94025-1164. Tel: 800-227-7040

In part 2 of Meeting USB Overcurrent Protection Requirements, author Paul Wiener will discuss specific design considerations when implementing resettable Polymeric PTC devices and compare them to other overcurrent protection technology solutions.