Design Feature: May 9, 1996

The battery packs in portable electronics can, under certain fault conditions, generate currents great enough to damage the host equipment. Designers must incorporate devices that reliably limit current to safe levels, a task that's complicated by the pressure to reduce the cost of both materials and the assembly, not to mention the size of the device itself.

Faults can arise within the power supply itself or from faults in the host equipment. However, you can address most problems by placing current limiters in the battery pack. These devices also limit the total current delivered to the host equipment and prevent damage due to a short across the power bus.

Users of portable equipment want longer operating time per charge. They also want smaller, lighter equipment. Because the battery pack contributes significantly to size and weight, it's a prime target in the crusade for product shrinkage. Though lithium-ion and future battery technologies promise to power typical portable equipment for hours on end, they also pose potential safety hazards.

Of course, not all faults come from the battery pack itself. One external danger arises when the system gets plugged into the ac mains with an adapter not designed specifically for it (see box, "Design example: adapter mismatch").

Design example: adapter mismatch

Requirement: Battery-powered portable devices must sometimes operate directly from the ac mains, using plug-in adapters. Dangers: Adapters may look identical but differ electrically. Differences include operating voltage (6, 9, and 12V) and plug polarity (some are positive in respect to circuit common; others, negative). Using the wrong adapter can have catastrophic results. Users replace lost adapters with cheaper substitutes, which may not hold to the same standards of output-voltage accuracy and stability. Even if an adapter carries the correct voltage rating, it may supply enough voltage to drive a dangerously high current through the equipment, especially if the equipment draws significantly less current than the adapter's full rated output. Traditional solution: Connect a current limiter between the jack into which the adapter cord plugs and the circuit to which the power is routed. One way is to combine conventional fuses with a photodiode that protects against reverse polarity. The upper portion of the figure shows a simplified diagram of a typical protection circuit. Disadvantages: Conventional fuses must be replaced when they blow. The forward resistance of the photodiode connected in series with a fuse increases power losses and decreases efficiency. Solution: Choose a resettable fuse, such as a polymeric PTC device, and use the photodiode as a voltage clamp by connecting it across the line (see bottom half of the figure).

Another potential problem could occur when the portable system is supplying power to a faulty peripheral, such as a bad hard-disk drive or PC Card fax/modem. A fault in one of these devices can generate enough heat to significantly damage components (see box, "Design example: I/O-port hazards"). Because the power supply must deliver enough current to drive the entire system, the main current limiter may not trip, even when currents in some subsystems are high enough to damage peripherals. To protect both host and peripherals, you may need to use multiple current limiters. Their optimal placement within the system's architecture, however, depends upon the equipment's design and intended use.

To prevent battery damage, your circuit must limit or be prepared to interrupt the flow of current into and out of the battery pack. However, complicating your choice of current limiter are the different problems posed during charging and discharging.

During charging, you need to limit both the rate at which current flows into the battery pack and the total amount of charge delivered. Most battery chargers are tailored to a particular type of battery, with circuits that discontinue the charging process at the appropriate time. But these circuits can fail, allowing the charging process to continue and causing significant damage. Battery packs should have a backup current limiter to prevent this problem.

Providing charge termination with an external current limiter is difficult, because there is no simple, economical, direct method of monitoring charging. You can infer the state of charge, however, through battery temperature, which rises during charging. Thus, you can provide the required protection by equipping a battery pack with a temperature-sensitive current limiter that trips when the battery charge exceeds the accompanying charger's preset cutoff.

Several kinds of current-limiting devices react to external temperature, as well as to their internal temperature rise due to I2R heating. Unfortunately, most react too slowly to provide the required level of protection. An exception is the thermal fuse, which responds to a temperature rise in the same way—and nearly as quickly—as a conventional fuse responds to a current rise.

Discharging dangers

Equipping a battery pack with a thermal fuse provides protection against excessive overcharging but little protection against excessive discharging, which can result from a wide variety of faults within the equipment being powered. The solution is to build one or more current limiters into the battery pack's host equipment.

However, current limiters external to the pack provide no protection against excessive discharge as a result of a short circuit directly across the battery pack's terminals. Even a drained battery can contain significant energy, and a charged spare certainly does. If the user tosses a battery into a briefcase, and the exposed terminals both come into contact with a key, a coin, or even a paper clip, the battery can discharge a very high current, depending on the battery level and the resistance of the short.

To protect against this high current, you can use a bimetallic switch to limit current in conjunction with a thermal fuse. One advantage of bimetallic switches is that they reset themselves after the fault condition clears and, thus, don't have to be replaced after they trip. Fuses, on the other hand, must be replaced when they blow—a very difficult process when the fuse is inside a battery pack.

Bimetallic switches, though widely used, typically suffer from a potentially serious shortcoming: They react to current flowing through their contacts. When the current becomes excessive, the contact mechanism's temperature rises, and the contacts open, interrupting current flow. Eventually, the temperature falls, and the contacts reclose, permitting the current to flow again. The repeated opening and closing of the switch let current flow through the faulty circuit, which can damage both the components in the faulty circuit and the battery itself.

Because overcurrent devices are being forced to fit into ever-shrinking spaces as battery packs shrink, smaller, multifunctional current limiters must protect battery packs in modern portable electronics.

An economical device capable of replacing both the thermal fuse and the overcurrent-limiting device, although still maintaining a low profile, is the positive-temperature-coefficient (PTC) resistor. The PTC resistor limits current by increasing resistance as the current increases. Conversely, as current decreases, so too does the PTC's resistance, causing it to "reset" and thereby eliminating the need to replace the PTC after an overcurrent fault.

The newest PTC devices are based on polymeric technology. Made from a blend of polymers and conductive materials, they operate on an entirely different principle from that of their older counterparts. In their normal state, the conductive materials form conductive chains through the body of the polymeric material. As a result, the resistor's normal resistance is very low—typically, less than 100 mV.

When the current rises above the device's normal trip threshold, the internal temperature rises. The polymeric material expands and changes from a crystalline state to an amorphous state, separating the conductive chains. The resistance quickly increases in value by several decades, reducing the circuit current to very low levels.

Figure 1 shows a characteristic PTC device resistance vs temperature curve. The trickle current is sufficient to keep the internal temperature high, which prevents the conductive chains from reforming. In effect, this process latches the PTC resistor into its tripped state. When the circuit power is removed, the PTC device cools, and the conductive chains reform, lowering the resistance to its normal low value. Figure 2 contrasts the temperature over time for a shorted battery pack protected with a polymeric PTC resistor and with a bimetallic thermostatic switch. Because of the "latching" effect, PTC devices don't experience the same cycling as bimetallic switches. As a result, the temperature of the pack protected by the PTC switch should not reach undesirable levels.

Polymeric PTC devices also feature low thermal mass, which lets them react quickly to overcurrent conditions. Actual trip time depends on the specific formulation, but these devices can trip in seconds. Also, properly designed polymeric PTC devices can be made extremely small and are available in surface-mount packages.

The potential for damage to the battery pack or the host equipment for low-resistance faults and short circuits is real, and the consequences are severe. It is imperative that portable equipment be properly protected against the effects of such faults. Use of polymeric PTC resettable fuses provides the needed protection and avoids the inconvenience and cost of replacing a blown fuse.

Author's biography

Mark Mendenhall is a product engineer in the PolySwitch division of Raychem in Menlo Park, CA. He graduated from the University of California—Berkeley in 1980 with a dual degree in electrical engineering and physics and has held many positions in product development during his 15-year career with Raychem.