PPTC full name polymer positive temperature coefficient device (PolymerPositiveTemperatureCoefficients), its working condition is similar to a switch. When the fault occurs, the PPTC becomes hot and the resistance increases, thus protecting the equipment from damage. After the fault is removed, the PPTC is reset and returns to the ground resistance state. . Not strictly speaking, PPTC is a "self-recoverable fuse." It sounds similar to a fuse. The difference is that there is almost no current in the circuit after the fuse is disconnected. After the PPTC is activated, there is a small holding current in the circuit. The current is determined by the typical power consumption of the PPTC.
Because the PPTC itself is hot, and the LED heat is close to it, the protection speed is relatively faster. In addition, the PPTC product has more package forms, and the package can be customized according to customer requirements, so the flexibility is very strong. However, PPTC is not a standard product. Tyco will work with customers to study the implementation and layout strategies according to the different situations of the system.
Although many drive devices are gradually integrated with overcurrent and overvoltage protection functions, in many cases, especially for overheat protection, discrete component solutions are still needed. Today, in the context of the increasing use of LED lighting, PPTC will once again show its talents in LED technology .
Overcurrent also brings a series of problems. Large currents do not represent high brightness, and sometimes the reduction in operating current results in increased performance and lifetime. For LED lighting applications, the threat of overvoltage comes from many aspects, such as voltage fluctuations, transient switching noise, and induced lightning strikes and ground potential recoil. Overvoltage may cause overheating, PN junction damage, reverse breakdown, and even burnout.
The thermal effect is also a problem that LED lighting cannot avoid. When the junction temperature of the LED chip rises by 10 degrees Celsius, the luminous flux will be attenuated by 1%, and the LED lifetime will be reduced by 50%. In addition to the heat dissipation design, overheat protection is also a factor that must be considered.
For PPTC devices, its failure mode is high impedance and good safety; it can directly measure the temperature of the LED to achieve closed-loop protection. PPTC can achieve overheating, overcurrent and even overvoltage protection without external circuitry and is easy to use. Even in the protected state, the LED lights are not completely extinguished, and it is safer for the engineer to implement the current dimming design.
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