PPTC Design Considerations for Automotive Circuits

Lisa Jones, Karin Kinsman, and Anthony Cilluffo

New automotive standards for passive components, including PPTC devices, are advancing the acceptance of this technology in the automotive industry.

New vehicle designs rely heavily on electronic circuits and motorized accessories—such as power windows, power seats, sunroof controls, and telematics—to reduce cost, improve reliability, and add functionality. The current move from 14-V systems to 42-V systems is a direct response to the demand for fuel efficiency and even higher power requirements associated with the increasing numbers and complexity of electrical and electronic systems in the automobile platform.

Polymeric positive-temperature-coefficient (PPTC) device technology has been widely applied to overcurrent and overtemperature circuit-protection designs in portable electronics, cell phones, computers, and telecommunications equipment. The Automotive Electronics Council has developed new standards for passive components, including PPTC devices, which is making the use of this technology more attractive in the automotive industry.

Because a vehicle's electrical system represents a large percentage of its cost and weight, the system requires adequate protection against short circuits and overloads. Current limiting can be accomplished by using resistors, fuses, switches, or positive-temperature-coefficient (PTC) devices. Resistors are rarely an acceptable solution because the high-power resistors that are usually required are expensive. One-shot fuses can be used, but they might fatigue, and they must be replaced after a fault event. The limitations of bimetallic switches include cycling and the potential for contacts to weld shut. Ceramic positive-temperature-coefficient (CPTC) devices tend to have high resistance and power-dissipation characteristics. These devices are also relatively large and are vulnerable to cracking as a result of shock or vibration.

In many automotive applications, the preferred solution is a PPTC device, which has low resistance in normal operation and high resistance when exposed to a fault. Electrical shorts or electrically overloaded circuits can cause overcurrent and overtemperature damage. Resettable PPTC devices help prevent such damage to automotive electrical equipment, power distribution systems, signal distribution systems, or electronic components.

Like traditional fuses, PPTC devices limit the flow of dangerously high current during fault conditions. Unlike traditional fuses, PPTC devices reset after the fault is cleared and the power to the circuit is removed. Because a PPTC device does not usually have to be replaced after it trips, and because it is small enough to be mounted directly into a motor or on a circuit board, it can be located inside electronic modules, junction boxes, and power distribution centers. This design architecture allows placement of electronic modules and systems in inaccessible locations. It also enables the use of smaller wires, which can result in smaller wire harnesses and an estimated cable-weight reduction of 40–50%.

PPTC Principle of Operation

PPTC circuit-protection devices are formed from a composite of semicrystalline polymer and conductive particles. At normal temperatures, the conductive particles form low-resistance networks in the polymer. However, if the temperature rises above the device's switching temperature (T_Sw), either from high current through the part or from an increase in the ambient temperature, the crystallites in the polymer melt and become amorphous. The increase in volume during melting of the crystalline phase causes separation of the conductive particles and results in a large nonlinear increase in the resistance of the device.

The resistance typically increases by three or more orders of magnitude, as shown in Figure 1. This increased resistance protects the equipment in the circuit by reducing the amount of current that can flow under the fault condition to a low, steady-state level. The device will remain in its latched (high-resistance) position until the fault is cleared and power to the circuit is removed—at which time the conductive composite cools and recrystallizes, restoring the PPTC to a low-resistance state and the circuit and the affected equipment to normal operating conditions.

Design Considerations for PPTC Devices

Some of the critical parameters to consider when designing PPTC devices into a circuit include device hold current and trip current, the effect of ambient conditions on device performance, device reset time, leakage current in the tripped state, and automatic or manual reset conditions.

Hold and Trip Current. Figure 2 illustrates the hold- and trip-current behavior of PPTC devices as a function of temperature. Region A shows the combinations of current and temperature at which the PPTC device will trip and protect the circuit. Region B shows the combinations of current and temperature at which the device will allow normal operation of the circuit. In region C, it is possible for the device to either trip or to remain in the low-resistance state, depending on the individual device resistance and its environment.

Because PPTC devices can be thermally activated, any change in the temperature around the device could affect the performance of the device. As the temperature around a PPTC device increases, less energy is required to trip the device, and thus its hold current (I_HOLD) decreases. Ceramic as well as polymeric PTC manufacturers provide thermal derating curves and I_HOLD-versus-temperature tables to help designers select devices with the appropriate rating.

Effect of Ambient Conditions on Device Performance. The heat-transfer environment of the device can significantly affect device performance. In general, by increasing the heat transfer of the device, there is a corresponding increase in power dissipation, time-to-trip, and hold current. The opposite occurs if the heat transfer from the device is decreased. Furthermore, changing the thermal mass around the device changes the time-to-trip of the device.

The time-to-trip of a PPTC device is defined as the time needed, from the onset of a fault current, to trip the device. Time-to-trip depends on the size of the fault current and the ambient temperature.

If the heat generated is greater than the heat lost to the environment, the device will increase in temperature, resulting in a trip event. The rate of temperature rise and the total energy required to make a device trip depends on the fault current and heat-transfer environment.

Under normal operating conditions, the heat generated by the device and the heat lost by the device to the environment are in balance:

(1)

where I = current flowing through the device, R = resistance of the device, U = overall heat-transfer coefficient, T = temperature of the device, and T_A = ambient temperature.

Increases in either the current or ambient temperature, or increases in both, cause the device to reach a temperature at which the resistance rapidly increases. This large change in resistance causes a corresponding decrease in the current flowing in the circuit, protecting the circuit from damage.

Figure 2. Example of hold and trip current as a function of temperature.

The hold current is the highest steady-state current that a device will carry for an indefinite period of time without transitioning from the low- to the high-resistance state. Hold current can be fairly accurately defined by the heat-transfer environment. It can be affected by many design choices, such as:

• Placing the device in proximity to a heat-generating source such as a power field-effect transistor (FET), a resistor, or a transformer, resulting in reduced hold current, power dissipation, and time-to-trip.
• Increasing the size of the traces or leads that are in electrical contact with the device, resulting in increased heat transfer and greater hold current, slower time-to-trip, and greater power dissipation
• Attaching the device to a long pair of wires before connecting to the circuit board, increasing the lead length of the device, which results in reduced heat transfer and lowered hold current, power dissipation, and time-to-trip.

A PPTC device's low resistance, fast time-to-trip, and low profile help improve electronics reliability in a small footprint. These devices are compatible with high-volume electronics assembly techniques and are available in surface-mount, radial-leaded, or custom configurations, with a wide range of voltage, current, resistance, and temperature specifications. To select the best device for a specific application, circuit designers should consider the following design criteria:

• Choose the appropriate form factor. Select from radial-leaded, surface-mount, or chip parts. For mounting on circuit boards, a radial-leaded or surface-mount configuration is preferred. Radial-leaded parts are typically wave-soldered to the board. Surface-mount parts are typically reflow-soldered to the board. Chip parts are designed to be held in clips, usually in an electric motor. These parts are often custom designed for specific applications.
• Choose a voltage rating. The voltage rating of a PPTC device should equal or exceed the source voltage in a particular circuit. Also, the expected fault voltage should not be greater than the PPTC voltage rating. When a PPTC device trips, the majority of the circuit voltage appears across the PPTC device because it is the highest-resistance element in the circuit.
• Choose a hold-current rating (at the proper ambient operating temperature). Hold current is defined as the greatest steady-state current the PPTC device can carry without tripping into a high-resistance state (at the specified ambient temperature). Because a PPTC device is a thermal device, the hold current for it decreases with increasing temperature. The actual value of the hold current for a given device and temperature can be obtained from the PPTC device manufacturer. Designers must choose a PPTC device with a hold current at the maximum ambient temperature equal to or greater than the steady-state operating current.
• Check trip time. PPTC device manufacturers can provide accurate time-to-trip curves illustrating how quickly the PPTC device trips at various currents. Designers should determine what fault currents may occur and how quickly the most sensitive system components could be damaged at these currents. A PPTC device should be selected that trips before these sensitive components would be damaged. Many applications experience a start-up surge current from a capacitance or motor. Normally, this in-rush current does not contain enough energy to trip the PPTC device, but designers should confirm performance in their application over the range of expected ambient conditions.
• Check maximum interrupt current. A PPTC normally has a maximum interrupt-current rating, that is, the maximum fault current that the device consistently interrupts while remaining functional.

Applications for Resettable Circuit Protection in Automotive Electronics

The transition to 42-V power and the interim dual-voltage network strategy offers many opportunities for innovation in electrical and electronic system architecture. Decentralization of power distribution, more-complex electronic modules, and smaller, localized wire harnesses are just a few areas of conversion in which resettable circuit protection can play a role.

Wire Harness Protection. Increasing power demands have resulted in complex wire harnesses that add wires, weight, and packaging constraints to automobiles. Each electrical circuit requires adequate protection against short circuits and overloads, and although each load theoretically can be protected with its own dedicated fuse, fuses must be replaced when they blow. This characteristic requires that fuses be mounted in accessible fuse boxes—a requirement that dictates system architecture and forces packaging and system layout compromises.

The conventional solution groups similar circuits together and protects them all with a single fuse. The fuse must be sized to carry the sum of the currents drawn by each of the protected loads; and, to limit risk of damage and fire, the wires feeding from the fuse to each load must be chosen according to the fuse size selected. This design practice often results in oversized wires with high current-carrying capability feeding loads that require relatively low currents. Using heavy-gauge wire also requires use of larger terminals and connectors, which further increases cost, size, and weight. It also increases harness weight, and the weight of the automobile, which has an effect on fuel efficiency.

Because PPTC devices reset when a fault condition clears and power is removed from the circuit, they do not generally require routine replacement or service. Therefore, such devices can be placed inside doors, in switch assemblies, behind instrument panels, in electronic modules, and in other inaccessible areas within the vehicle. As shown in Figure 3, the option of locating circuit-protection devices strategically throughout the vehicle also allows power to be routed via the most direct and efficient route (rather than through a central fuse box), which reduces the number of wires in the harness and allows reduction in their length and weight.

Figure 3. PPTC devices can be used in distributed electronic system architectures to help reduce wire size.

Electronic Control Module Protection. As more and more circuitry is packed into smaller and smaller packages, the width of the copper traces on printed circuit boards (PCBs) is reduced. Because motorized accessories are generally powered from high-amperage circuits, these narrow circuit board traces are susceptible to damage from excessive currents. Printed circuit traces function as wires carrying signals from one point to another. Depending on the cross-sectional area, the traces can carry only a certain amount of current before the heat generated by I²R losses causes them to either melt or become hot enough to delaminate, resulting in damage to the PCB and mounted components.

Electronic module outputs typically require protection from overcurrent situations caused by a short circuit or by the high stall current of motors. Module outputs can also be damaged by failure of some other portion of the system, such as a diode short or loss of a power ground. Because they are one-use devices and must be replaced in the event of a transient fault, fuses are not considered an acceptable solution to these potential problems. Multicomponent circuits used to sense and switch, called smart FETs, are frequently used to address these situations, but such devices require careful design and consume valuable board space. They can also be quite costly.

PPTC circuit-protection devices are gaining acceptance as a practical, cost-effective solution to overcurrent and overtemperature protection of electronic modules. Because they rapidly and effectively limit current to safe levels and are small enough to be mounted directly on the circuit board, each power circuit within the control module can be individually protected with a single device.

Small-Motor Protection. Most automotive actuators are used in applications that require them to move something until it reaches the end of its motion range—to move a seat or close a window, for example. However, because these activities can be manually controlled, the actuator may remain energized after the mechanism reaches its limit of travel. When this condition occurs, the actuator stalls, and its back electromotive force (EMF) falls to zero. Without the back EMF opposing the supply voltage, the actuator's current may rise rapidly to levels typically between two and four times its normal operating value.

Because the actuator's winding is made with very-small-gauge wire, the high stall current causes a rapid rise in temperature. Often within seconds, the temperature may rise sufficiently to permanently damage the enamel varnish used to insulate the wire in the actuator's winding. With the loss of insulating properties, turn-to-turn short circuits may develop throughout the winding, rendering the actuator inoperable and creating a potential for a thermal event (see Figure 4).

Figure 4. To interrupt excessive current, PPTC devices are wired in series with the actuator windings.

When the current or temperature of a winding rises above a certain value, the PPTC device latches into a high-resistance state, limiting current to a low level and preventing damage to the actuator. After the fault and power are removed and the PPTC device cools, the device resets for normal current flow.

Conclusion

PPTC devices provide net cost savings through reduced component count and reduction in wire size. They can help provide protection against short circuits in wire traces and electronic components. The low resistance, relatively fast time-to-trip, and low profile of these devices improve reliability in a small footprint. In addition, these devices provide manufacturing compatibility with high-volume electronics assembly techniques and greater design flexibility through a wide range of product options.

Lisa Jones is automotive market manager, Karin Kinsman is R-Line product manager, and Anthony Cilluffo is automotive sales manager for Tyco Electronics Power Components, Raychem Circuit Protection product line (Menlo Park, CA). The authors can be reached at lajones@tycoelectronics.com or 650-361-2256.