Zoltán Kiss- Area Sales Manager - East Europe - Endrich GmbH.
Overcurrent protection with resettable fuses
02 November 2012
Summary :
An overcurrent is a large current, which is high enough to lead to damage of the electrical circuit, by the widely used definition, any current in excess of the rated current of the equipment or the ampacity of the conductor. There are source or load generated abnormalities, which cause current being out of the nominal conditions of operation, and the engineers must think of protecting their circuit when such things may happen.
Source generated overcurrent usually arise from overvoltages caused by abnormal operation of the power supply or the voltage peaks and surges coming from the power line. Power cross can happen, when high voltage accidentally touches the low power circuit (for example power line crosses phone line in a thunderstorm). The duration of surge is shorter, than power cross, most of the cases external events, like lightning increase the voltage in the system. Also transients caused by presence of energy storing elements, like capacitors and inductors could generate overcurrent. In nonlinear circuits even voltage sags – a reduced voltage for a limited time period – can result higher current flowing through. Load generated abnormal circumstances could arise when the failure lowers the total resistance of the load circuit enabling high current flow. Overcurrent may also appear when overload, short circuit or ground fault happens.
Protecting circuits against overcurrent
Growing current flow in a conductor generates larger heat. Excess heat is damaging to electrical components, therefore additional protection devices should be used to protect circuit from excessive current flow These protective devices are designed to keep the flow of current in a circuit at a safe level to prevent the circuit elements from overheating. Circuit protection must determine when a fault condition develops and automatically disconnect the electrical equipment from the voltage source. Slight overcurrents can be allowed to continue for a short time, but as the current magnitude increases, the protection device must open faster. Short circuits must be interrupted instantly.
Conventional fuse – as low resistance resistor – is widely used to cut circuit under excess electrical stress. Its essential component is a metal wire or strip that melts when too much current flows.
Overcurrent protection with PPTC “resettable” devices
Using polymetric positive temperature coefficient (PPTC) overcurrent protection devices (PolySwitches) helps protect against harmful surges as well as overtemperature faults. Likewise traditional fuses these devices also limit the flow of dangerous overcurrent during faults, however reset after the fault disappears and power of the circuit is removed, so no need to replace them, which reduces the cost of repair and maintenance. PolySwitch PPTCs are made of a composite of semi-crystalline polymer and conductive particles, mainly carbon black. At normal circumstances the PolySwitch, which is a series element in the circuit has a resistance that is much less, than the circuit impedance, and hardly has an influence on the performance, due to the conductive particles in the polymer form a conductive low-resistance network.
As a response of the overcurrent event the device trips, increases the resistance quickly and reduces the current of the device to the level that can be safely carried by even the weakest element of the circuit. The physics of this action could be described as a result of a rapid increase in the temperature of the device caused by I2R heating or the increase in the ambient temperature, which forces the crystallites in the polymer to melt and become amorphous, the conductive paths are separated and the resistance grows suddenly.
Looking at the operating curve of the PolySwitch devices, point 1. represents the normal operating conditions, when the heat generated by the device is in balance with the heat loss to the environment. When the current of the device increases, while the ambient temperature is constant or vica versa, the device is warming up, but all generated heat still could be dissipated, therefore the operation is stabilized (point 2). Further increase in current or ambient temperature will force the operating point to be transferred to point 3, where the resistance would rapidly increase after further current or temperature raise. In this section of the curve the heat generated in the deice cannot be dissipated any more, so the internal temperature will rapidly grow, and at this stage very large increase in resistance occurs for a very tiny change of temperature (between point 3 and 4). This is the normal operating region of the device in tripped state, when the current of the circuit is limited to the desired level. No remarkable further increase in resistance will occur after the point 4, as long as the applied voltage is the same, the device stays in tripped – its protective – state. Once the voltage is decreased and the power is removed, the PolySwitch will cool down and reset.
Design considerations and definitions used in PolySwitch terminology
The operating parameters of PolySwitches are important to know for the right selection of such device.
Hold current
The hold current (IH) is the maximum steady-state current that PolySwitch can hold without changing to the high resistance state.
Trip current
The trip current (IT) is the minimum current, where the PolySwitch transits to high resistance state and limits the current at room temperature.
Maximum rated voltage
The maximum rated voltage (Vmax) is the value that can be safely applied to the device during the tripped state.
Maximum rated current
The maximum rated current (Imax) is the maximum fault current, that can flow through the PolySwitch in order to trip the device
Time to trip
The time it takes for the device to trip at a given temperature. Identifying the device’s time-to-trip is important in order to achieve the desired protection capabilities. By definition this parameter is described as the time required to for the voltage drop across the device to rise higher than the 80% of the power source voltage. The time to trip depends significantly on the heat dissipation capability of the part if the value of the fault current is not high enough to warm up the PolySwitch.
It is also important to consider the influence of the ambient temperature to the behavior of the PolySwitch device, as less fault current is required to trip the device if the ambient temperature is higher. The tripping is forced by the I2R self heating and the ambient temperature together.
On the figure describing the hold and trip current dependency of the ambient temperature, there are three operating regions marked. In the first region the PolySwitch device is in the low resistance state, and invisible for the circuit. In the second region it is possible that the device either trips or stays in low resistance state depending on individual device resistance. The third region represents the combinations of current and temperature at which the PolySwitch will definitively trip and protect the circuit.
Very important to mention, that unlike conventional blowing fuses, which - after melting - provide galvanic separation of the circuit from the power source, PolySwitches would still have a remaining small leakage current, which would support the tripped state, represented by the high resistance of the device.
The big advantage of the PolySwitch PPTC devices is the resetting function. Once the voltage is decreased and the power is removed, the PolySwitch will cool down and reset. Cooling down could be a very long process, especially to reach the value of the initial resistance may be not practical to wait for. Therefore when PolySwitch devices are chosen, R1MAX should be taken into consideration when determining hold current, which is the resistance value one hour after the thermal event.
Selecting PolySwitch device for the application
It is mandatory to determine the following parameters of the circuit to protect :
- Maximum ambient temperature
- Maximum operating voltage
- Normal operating current
- Maximum interrupt current
Based on the thermal derating curves/tables the operating temperature value closest to the maximum ambient temperature and the corresponding current value (higher or closest to the operating current) should be taken in account in order to determine the product family.
It has to be verified in the next step by catalog data, if the selected part will be able to handle the circuit’s maximum operating voltage and the maximum interrupt current. Part’s Vmax and Imax should be greater than the corresponding circuit values.
It is also very important to know if the selected PolySwitch will trip fast enough to protect the circuit. For verification, the 20 °C time-to-trip curve of the selected family could be used. If the time is too fast or too slow at expected fault current levels, an alternate device has to be chosen.
It is mandatory to ensure that the application’s temperature range is within the PolySwitch operating temperature range, which is in most of the cases -40°C to 85 or 125°C.
The last verification should be done on physical dimensions, if the space on the PCB makes it possible to use the part.
Special devices
AL-ALCÍM
TE Circuit Protection also designed special combined protective solutions, where PolySwitch devices are packed together with an additional voltage limiting device such as a Zener diode or metal-oxide varistor in a common package. In these devices the activation of voltage limiting protection increases the current flow through the PolySwitch device, while also heats the PPTC as they are thermally coupled together. The trip of the resettable fuse will not only provide overcurrent protection to the circuit behind, but also will protect the voltage limiting device of burning down.
Further design related questions and product support could be requested by the editor of this article.| Share on Facebook | Share on LinkedIn |
References
This article has been published on the following locations:
| # | Media | Link |
|---|---|---|
| 1 | Elektronet 2012/7 | Elektronet : elektronikai informatikai szakfolyóirat, 2012. (21. évf.) 7. sz. 28-30. old. |
| 2 | Elektronet online | Túláramvédelem TE Circuit protection PolySwitch eszközökkel |