Principle and characteristics of polymer self-recovering fuse
July 16, 2018
The traditional disposable fuse acts as an overcurrent protection element and can only be protected once and needs to be replaced after the fuse. Due to its self-recoverable characteristics, polymer self-resetting fuses (PPTC) can be installed and reused at one time. As a new type of overcurrent protection element, it is gaining more and more applications in line protection.
The polymer self-recovering fuse is composed of a polymer material and conductive particles. Under normal circumstances, the conductive particles form a chain-shaped conductive channel in the polymer matrix material, and the device is maintained in a low-impedance state when an overcurrent (such as a short circuit) occurs in the circuit. The heat generated by the large current causes the polymer material to rapidly expand, cuts off the chain-shaped conductive channel, and the component changes from low impedance to high impedance, blocks the circuit, protects the load, and after the fault is removed, the polymer material automatically shrinks and the conductive channel re- The link, the component itself returns to its original state, thus achieving the purpose of "reuse, no need to replace, once and for all". (The working principle is shown in Figure 1, 2
Self-recovering fuses have no polarity and can be used in series with AC or DC power supplies. Shanghai WAYON's self-resetting fuses are available in a variety of mounting options including light, insert or surface mount.
How to choose
(1) Determine the following data and requirements for the working line:
1. Average operating current (I) and operating voltage (V) (no need to consider peak)
2. The ambient temperature (T) of the component used (possible temperature rise should be taken into account)
3. Component voltage drop / resistance requirements
4. Action time requirements. For example, the protection time requirement of USB60950 for USB3.0 is that the fault current is less than 8A within 5 seconds.
(2) Select a product series according to the V value (V ≤ Vmax), the I value (I ≤ Ih) and the installation method.
(3) Check the temperature-holding current Ih correspondence table of the series products, find the holding current value corresponding to the series of products under the ambient temperature T value, and find one or several models in which the Ih value is greater than or equal to the working current value. This is an alternative model available.
(4) Test the above-mentioned models that meet the requirements in accordance with the requirements of (1) and determine the best model.
Effect of ambient temperature on holding current and trigger current (It)
Both the holding current and the trigger current are variables related to the ambient temperature, which are attenuated as the ambient temperature increases. The attenuation ratio is shown in Figure 3.
Relationship between operating time and ambient temperature and current
The speed of self-recovering fuses varies with ambient temperature and current flow. Higher ambient temperatures or greater currents mean faster operating times. As shown in Figure 4.
Resistance value self-recovery characteristics
After the power-off fault disappears, the component resistance quickly returns to the initial value. Figure 5 shows the recovery characteristics of the self-resetting fuse resistance over time.