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. Tyco began researching PPTC materials in 1980 and used it for thermistors in circuit protection. Today, in the context of the increasing use of LED lighting, PPTC will once again show its talents in LED technology.
PPTC full name polymer positive temperature coefficient device (Polymer Positive Temperature Coefficients), 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 returned. Ground resistance state. Not strictly speaking, PPTC is a "self-recoverable fuse." Sounds like a fuse? The difference is that after the fuse is disconnected, there is almost no current in the circuit, and after the PPTC is activated, there is a small holding current in the circuit, and the current is determined by the typical power consumption of the PPTC.
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.
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.
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.
Tyco electronic circuit protection expert Dong Yong pointed out that PPTC itself is hot, coupled with close LED heat, the protection speed is relatively faster, and PPTC products have more packaging forms, 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.
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