by Dan Chandler, Director of Business Development, Tyco Electronics Corporation, Power Components Division/Raychem Circuit Protection Product Line
Protecting an electronic circuit from damage due to excessive current or heat is the primary function of many circuit protection technologies. In the past, this protection took the form of a fuse or fusible link, but in many of today's applications resettable devices such as polymeric positive temperature coefficient (PPTC) devices, ceramic PTC devices, bimetal breakers, and thermostats are the preferred solutions. These devices do not require replacement after a fault event, and allow the circuit to return to the normal operating condition after the power has been removed and/or the overcurrent condition is eliminated. This resettable functionality helps to improve consumer satisfaction and reduce manufacturers' warranty, service, and repair costs.
|Polymeric positive temperature coefficient (PPTC) devices have become the standard solution for resettable overcurrent protection in a wide variety of electronic applications.|
Although often referred to as "resettable fuses", PPTC devices are non-linear thermistors that limit current. Under a fault condition all PTC devices go into a high resistance state, and because normal operation can still result in hazardous voltage being present in parts of the circuit, it is important that the circuit designer understand critical differences between PPTC devices and fuses. Fuses are current interruption devices. When a fuse "blows" the electrical circuit is broken, and the fuse must be replaced. However, once a PPTC device trips, there is a small amount of current flowing through the device. PPTC devices require a low, joule heating leakage current or external heat source in order to maintain their tripped condition. Once the fault condition and/or power are removed, this heat source is eliminated, the device returns to a low resistance status, and does not need to be replaced.
To Fuse or Not to Fuse
Despite the inherent and obvious advantages of resettable devices, there are circumstances where a fuse may be the preferred form of circuit protection. Under conditions where restoration of normal operation poses a potential safety hazard and/or where service on the equipment should be performed after a fault condition has occurred, a fuse or circuit breaker is appropriate. For example, a single-shot fuse, or a manual resettable breaker would be recommended for a garbage disposal because the blades could cause serious harm if the motor were to suddenly resume operation. On the other hand, the resettable PPTC device is a logical solution for protecting computers, networking and telecom equipment, and portable electronics where immediate resumption of operation is desirable.
A failure to understand the precise nature of the PPTC device's resettable functionality may lead to improper use in a circuit. Further confusion may result if the designer is comparing a PPTC to other resettable devices, such as ceramic PTC devices, bimetal thermostats, and push-button breakers. The following table (Figure 1) describes the reset behaviors of several types of overcurrent protection devices.
1Overtemperature and overcurrent protection; 2 Thermal fuses are not designed for overcurrent protection, and generally require large currents to trip.; 3 3Periodically attempts to reset until fault and/or power is removed, or resets to low resistance state when bimetal cools; 4 Automatically resets to low-resistance state once the fault is cleared and power is removed.
Figure 1. Comparison of reset functionality and circuit conditions in fuses and resettable circuit protection devices.
Circuit Design Considerations for PPTC Devices
Some of the critical parameters to consider when designing PPTC devices into a circuit include device hold- and trip-current, the effect of ambient conditions on device performance, device reset time, and leakage current in the tripped state.
Figure 2 illustrates the hold- and trip-current behavior of PPTC devices as a function of temperature. Region A describes the combinations of current and temperature at which the PPTC device will trip and protect the circuit. Region B describes the combinations of current and temperature in which the device will allow for normal operation of the circuit. In region C, it is possible for the device to either trip or remain in the low-resistance state, depending on the individual device resistance and its environment.
Figure 2. Example of hold- and trip-current as a function of temperature.
Since PPTC devices can be thermally activated, any change in the temperature around the device may impact the performance of the device. As the temperature around the device increases, less energy is required to trip the device and thus hold current (IHOLD) decreases. Ceramic as well as polymeric PTC manufacturers provide thermal derating curves and IHOLD versus temperature tables to help the designer select the appropriate device.
The heat transfer environment of the device can significantly impact device performance. In general, by increasing the heat transfer of the device there will be a corresponding increase in power dissipation, time-to-trip and hold-current. The opposite will occur if the heat transfer from the device is decreased. Furthermore, changing the thermal mass around the device will change 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 into a high resistance state. Trip time depends upon the size of the fault current, as shown in Figure 3, as well as the ambient temperature.
Figure 3: Typical time-to-trip (TtT) as a function of fault current for a PPTC device.
A trip event is caused when the rate of heat lost to the environment is less than the rate of heat generated. If the heat generated is greater than the heat lost, the device will increase in temperature. The rate of temperature rise and the total energy required to make the device trip depends on the fault current and heat transfer environment.
Increases in either current, ambient temperature, or both will 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.
Understanding the precise nature of the PPTC device's resettable functionality helps the circuit designer specify and apply the device to achieve the maximum operational benefit. When selecting a PPTC device, designers must consider reset conditions, restoration time, and ambient conditions that can affect the performance of the device. Ultimately, designers must decide what level of protection is required for their applications, and the best way to determine whether or not a specific protection device is appropriate is through a complete system test.
Tyco Electronics Power Components offers detailed application information on the use of PPTC devices at www.tycopowercomponents.com