PTC Overcurrent Protection for Universal Serial Bus Circuit Designs

Positive-temperature-coefficient devices do the job and provide stand-alone compliance with IEC 950 and UL/CSA 1950.

With the surge in computer applications that incorporate plug-and-play or hot-swapping, designing for compliance with safety agency standards has become more critical. A development worthy of examination in this regard is the universal serial bus, a specification many manufacturers are adopting in order to provide a uniform protocol for the addition and configuration of computer peripherals.

The universal serial bus (USB) is based on one port size and a matching connector. It is designed around the concept of a single host with multiple hubs able to provide uniform and simple addition and connection of various peripherals. The bus is designed to sense the addition or removal of a USB-compliant device and then automatically configure accordingly. The goal is to reduce the number of necessary cable connections and configuration steps. The USB is viewed as an inexpensive system for connecting low-data-rate devices. It is designed to accommodate transmission rates up to 12 Mb/sec. USB devices are designed so that several can be powered by a hub through the communication cable without additional power inputs (Figure 1).

Figure 1. A typical USB configuration, with compliant devices powered through a hub.

The PC 98 specification—updated as PC 99—developed by Microsoft, Intel, and a number of computer manufacturers provides guidelines for designing PC systems, buses, and devices. The specification addresses the issue of circuit protection for desktop computers through its power management requirements for individual bus designs. The trend has been underscored by Microsoft's Windows Hardware Design Guide, which promotes plug-and-play and the idea of a sealed PC box, which holds that user access to the inside of the box should not be required.

The combination of hot-swapping with hub-powered devices in USB applications illustrates the particular need for circuit protection. Another concern: the host hub and all self-powered hubs must implement overcurrent protection for safety reasons. Should the cumulative current drawn by a group of downstream ports exceed a preset value, the overcurrent protector removes power from all downstream ports. Overcurrent circuits are used to protect against catastrophic device failures, software errors that lead to devices turning on when the current budget has been exceeded, and user-caused events such as the shorting out of connector pins. This article looks at the need for overcurrent circuit protection in USB port applications and how it might be met.

Overcurrent Circuit Protection

The key to designing for successful compliance is to select the correct components in conjunction with a right-sized power supply. The circuit designer has a choice of technologies when faced with the task of providing overcurrent protection. The options for protection are resettable polymer PTCs (positive-temperature-coefficient devices), fuses, and regulated networks. Regulated networks are commonly referred to as solid-state power-distribution switches or power managers. The traditional fuse and the polymer-based PTC are the most commonly employed solutions.

Fuses are one-time devices. That is, a fuse will provide protection from an overload only once; then it needs to be replaced. The heart of a traditional fuse is a metal element that is heated to its melting point by any excessive current. The circuit current flow decreases to zero as the element melts open, destroying the fuse.

The PTC also reacts to the occurrence of excessive current, but unlike the fuse is a resettable device. It can provide overcurrent circuit protection multiple times. The PTC's conductive polymer increases in resistance when heated by an overload, and this limits the circuit current. When the power source is removed, the device resets itself.

Regulated networks, or power-distribution switches, which combine a number of functions in one package, have also been put forward as resettable solutions for port protection. Typical functions of these devices include overcurrent protection, fault indication, thermal protection, and undervoltage protection. Regulated networks offer very fast current limiting and are typically restricted to operating voltages of less than 7 V. While they do come with a number of useful additional features, they cannot serve as direct replacements for fuses or PTCs in most applications.

The principles of operation for a fuse are generally well understood, but the process by which the current-limiting, resettable PTC provides overcurrent circuit protection is less clear and warrants further discussion.

PTC Overcurrent Protection

PTCs can be used in many USB applications to provide a circuit protection device that is resettable. The polymer-based PTC contains particles of conductive media. The size of the particles and the total amount of conductive media introduced into the polymer material determine the ultimate resistance of the PTC component.

The PTC functions by limiting potentially damaging overcurrent when the current exceeds the specified rating. Heat caused by the overcurrent produces thermal expansion in the polymer material. As the polymer expands, it becomes more resistive and reduces or limits the circuit current to a safe level. The increase in resistance is nonlinear and occurs when the operating current surpassess a trip level. Once the PTC has reached the trip point, its resistance will remain high until the power source is removed. Figure 2 illustrates the resistance-versus-temperature curve of the PTC effect.

Figure 2. Graph of PTC performance with increasing heat caused by overcurrent.

USB Power Management

Different manufacturers use PTCs and fuses differently to achieve USB power management. Some designers employ lower-amperage devices for individual port protection. The individual-port approach provides superior performance in that a single port can be isolated. Manufacturers looking for a lower-cost solution use one higher-amperage device to protect a group of four ports on a bus.

In addition to specifying power management for the USB, UL/CSA 1950 and IEC 950 require that the current through a port be limited to less than 8 A for all devices. The main advantage of a fuse or PTC over a solid-state network in this respect is that they provide stand-alone compliance with IEC 950 and UL/CSA 1950. Clause 2.11 in both IEC 950 and UL/CSA 1950 defines the requirements for computer power output.

The USB standard organization defines a nominal operating voltage of 5 V and a hub operating current of 0.500 A per port. With a typical 230-W ATX-style power supply the +5-V output is rated for 22–25 A. Typical desktop power supplies provide both overcurrent and overvoltage shutoff on the secondary side. Generalizing from several manufacturers' specifications, these supplies are set to shut off at a minimum of 30 A, or at a maximum allowable voltage of 7 V.

Clause 2.11 of the IEC/UL/CSA standards specifies the acceptable maximum output after 60 seconds of operation with any noncapacitive load, including short circuit. The limits are separated into two categories: those for inherently limited power sources (presented in Table 8 in Clause 2.11 of UL 1950) and those for power sources used in conjunction with an overcurrent-protective fuse or breaker (Table 9 in Clause 2.11 of UL 1950).

For a 5-V output voltage, Table 8 of Clause 2.11 specifies that the maximum output should not exceed five times the rated voltage (5 x 5 V = 25 VA). The volt-ampere limit requires a maximum current after 60 seconds of operation of 5 A for a USB port.

If an overcurrent-protective device, fuse or breaker, is used alone, the maximum limit for a 5-V output (see Table 9 in Clause 2.11 of UL 1950) is 50 A. The use of a stand-alone fuse is the least-difficult method of compliance, but that choice sacrifices the resettability desirable in computer applications.

Five options for compliance with the safety standard are available to the designer:

1. The power supply limits the current in compliance with Table 8. This is not probable for USB port protection. Typical short-circuit shutoff for a desktop power supply is 28–30 A, more than five times the allowable current of 5 A.

2. An impedance limits the output in compliance with Table 8. A PTC device rated 2.5 A or less will limit short-circuit current while offering resettability.

3. An overcurrent-protective device is used, and the output is limited in compliance with Table 9. For the USB port, a fuse rated at 5 A or less will interrupt a current in accordance with the specifications in Table 9.

4. A regulating network limits the output in compliance with Table 8, both under normal operating conditions and after any single fault in the regulating network (open or short circuit). This is not probable for USB port protection. Typical short-circuit shutoff for a desktop power supply is 28–30 A, more than five times the allowable current of 5 A. A short-circuit failure for a regulated network causes the circuit to appear as if the regulated network were not present.

5. A regulating network limits the output in compliance with Table 8 under normal operating conditions, and an overcurrent-protective device limits the output in compliance with Table 9 after any single fault in the regulating network (open or short circuit). A regulated network will limit the short-circuit current to less than 5 A under normal operating conditions. For the USB port, a fuse rated at 5 A or less will interrupt a current in accordance with the specifications in Table 9.

To establish USB port protection for a desktop computer, then, the designer has two acceptable alternatives. The three approaches outlined in options 2, 3, and 5 just given provide the end-user with a safe device and achieve compliance with safety agency standards. However, the USB specification requires a resettable mechanism for overcurrent protection. While the fuse solution presented as the third option does meet the safety regulations, the replacement requirement would be a disadvantage for USB port protection.

Figure 3. Self-powered hub configured with individual-port protection.

Figure 4. Self-powered hub configured with multiple-port protection.

Incidentally, both manufacturers of solid-state power switches and Underwriters Laboratories recommend that the use of solid-state devices for port protection be supplemented with a fuse to enable the end device, that is, the computer, to comply with safety agency requirements.

USB Circuit Designs with PTC Overcurrent Protection

USB ports are configured as either self-powered or bus-powered. A self-powered USB hub must supply current up to 500 mA on all of its ports. The self-powered hub does not draw power from the USB stream but may utilize up to 100 mA from upstream devices or hubs to make functionality possible when it is powered down. Bus-powered hubs can draw up to 500 mA from an upstream self-powered connection. Typically, current of 100 mA is available for functions and processors internal to the hub. External ports in a bus-powered hub can supply up to 100 mA per port, with a maximum of four ports per hub.

Voltage-drop calculations for several applications of port protection in USB circuit designs are presented in Table I. The calculations demonstrate the effect of several PTC devices available to provide overcurrent protection. In the design of these ports, consideration must be given to ensuring that the voltage drop does not fall below the minimum of 4.75 V for a self-powered hub port, or 4.40 V for a bus-powered hub port. The upstream voltage supplied to a bus-powered hub is 4.75 V.

Self-powered hub (individual port, see Figure 3):
Power supply		5.000 V
Trace	20 m x 0.5 A	= 0.010 V
Ferrite bead	5 m x 0.5 A	= 0.003 V
LF 1812L150	120 m x 0.5 A	= 0.060 V
Vout		4.933 V
Self-powered hub (multiple port [2], see Figure 4):
Power supply		5.000 V
Trace	(10 m x 1.0 A) + (10 m x 0.5 A)	= 0.015 V
Ferrite bead	5 m x 0.5 A	= 0.003 V
LF 1812L150	120 m x 1.0 A	= 0.120 V
Vout		4.862 V
or
Self-powered hub (multiple port [2], see Figure 4):
Power supply		5.000 V
Trace	(10 m x 1.0 A) + (10 m x 0.5 A)	= 0.015 V
Ferrite bead	5 m x 0.5 A	= 0.003 V
LF 1812L260	50 m x 1.0 A	= 0.050 V
Vout		4.932 V
Self-powered hub (multiple port [3], see Figure 4):
Power supply		5.000 V
Trace	(10 m x 1.5 A) + (10 m x 0.5 A)	= 0.025 V
Ferrite bead	5 m x 0.5 A	= 0.003 V
LF 1812L260	50 m x 1.5 A	= 0.075 V
Vout		4.897 V
Bus-powered hub (individual port, see Figure 5):
Supply voltage		4.750 V
Cable	190 m x 0.5 A	= 0.080 V
Trace	(10 m x 0.4 A) + (10 m x 0.1 A)	= 0.005 V
Ferrite bead	5 m x 0.1 A	= 0.001 V
MOSFET	80 m x 0.4 A	= 0.032 V
LF 1812L150	120 m x 0.1 A	= 0.012 V
Vout		4.620 V
Bus-powered hub (multiple port [2], see Figure 6):
Supply voltage		4.750 V
Cable	190 m x 0.5 A	= 0.080 V
Trace	(10 m x 0.4 A) + (10 m x 0.1 A)	= 0.005 V
Ferrite bead	5 m x 0.1 A	= 0.001 V
MOSFET	80 m x 0.4 A	= 0.032 V
LF 1812L260	120 m x 0.2 A	= 0.010 V
Vout		4.622 V
Bus-powered hub (multiple port [4], see Figure 6):
Supply voltage		4.750 V
Cable	190 m x 0.5 A	= 0.080 V
Trace	(10 m x 0.4 A) + (10 m x 0.1 A)	= 0.005 V
Ferrite bead	5 m x 0.1 A	= 0.001 V
MOSFET	80 m x 0.4 A	= 0.032 V
LF 1812L260	50 m x 0.4 A	= 0.020 V
Vout		4.612 V

Table I. Sample calculations of voltage drop for various applications of port protection in USB circuit designs.

Bus-powered-hub calculations include a resistance budget for the connecting cable. The USB standard specifies that connection cables from host to hub and peripherals have a maximum length of 5 m and a maximum resistance of 190 m. The circuit trace was assumed to be 4 in., with a trace resistance of 5 m/in. A 5-m ferrite bead and two capacitors used for EMI suppression are illustrated in the circuit diagrams. Bus-powered circuits include control logic circuitry (a field-effect transistor with 80-m resistance) which enables software control of bus power and port reset capabilities.

Figure 5. Bus-powered hub configured with individual-port protection.

Figure 6. Bus-powered hub configured with multiple-port protection.

Overcurrent circuit protection scenarios in Figures 3–6 depict individual-port and multiple-port (ganged) protection for self-powered hubs and bus-powered hubs. Individual-port protection offers an advantage over ganged port protection in that if one port fails, the other ports are unaffected. Additionally, the time-to-trip parameter illustrated in Figure 7 allows the design engineer to eliminate false circuit trips due to power-on currents. The calculations show that the voltage drop for overcurrent protection with PTC devices complies with the requirements in the USB specifications.

Figure 7. The time-current curve for calculating PTC time to trip. The time coordinate at the point where the application's overload current intersects the curve is the device average opening time. The device operates properly if the overload current falls to the right of the curve.

Other computer applications for PTC devices are listed in Table II. As an example, the IEEE 1394 "Firewire" is a specification evolving along the same lines as the USB but it is designed to be used for higher-bandwidth peripherals such as CD drives, DVD drives, and desktop video. The specification is designed for transmission rates up to 400 Mb/sec, with eventual implementation of rates up to 3200 Mb/sec. IEEE 1394 calls for an operating current of 1.5 A with voltages of 6–40 V.

Application	Circuit Protection
USB	Overcurrent protection in USB hub or peripheral. Individual or ganged port protection is required to limit current to a maximum of 5 A.
Keyboard and mouse ports	Applications must meet UL 1950/IEC 950 specifications for less than 8 A at the connector. Protects against short circuit arising from a crimped, crushed, or pinched cable or connector.
SCSI interface	Applications must meet UL 1950/IEC 950 specifications for less than 8 A at the connector. Protects against short circuit arising from a crimped, crushed, or pinched cable or connector.
Disk drive	Overcurrent protection of drive motors.
Portable	Overcurrent protection at battery input. Protects electronics against short circuit arising from a crimped, crushed, or pinched cable or connector.

Table II. Computer applications for PTC devices.

Conclusion

The overcurrent circuit protection requirements for USB port applications can be met through a design involving use of a PTC device. PTCs are self-resettable following resolution of an overcurrent condition. The resettable feature of these innovative devices is a convenient alternative to replacement of opened fuses by PC users or technicians. PTC devices exceed the safety requirements of UL 1950 and IEC 950, and allow designers to select a particular device that meets the requirements of the circuit design.

Rick Hapanowicz, PhD, is a marketing engineer for Littelfuse (Des Plaines, IL). He can be reached at rhapanow@littelfuse.com. For more information, go on-line at http://www.usb.org.

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