PPTC overcurrent and overheat protection in LED lighting

PPTC refers to a polymer positive temperature thermistor, which is composed of polymers and conductive particles. After special processing, the conductive particles form a chain-like conductive path in the polymer. When the normal working current passes through (or the component is at normal ambient temperature), the PPTC is in a low resistance state. When an abnormal overcurrent passes through the circuit (or the ambient temperature rises), the heat generated by the large current (or the ambient temperature rises) causes the polymer to expand rapidly, cutting off the conductive path formed by the conductive particles, and the PPTC is in a high resistance state. When the overcurrent (overtemperature state) in the circuit is eliminated, the polymer cools down and the volume returns to normal. The conductive particles re-constitute the conductive path, and the PPTC is in the initial low resistance state. PPTC is small in size, low in cost, and can be used repeatedly, realizing the automatic start and automatic recovery of circuit protection.

In LED lighting, there are different overheat protection methods according to different lamp applications, which are usually divided into two types. One method is to immediately start the shutdown protection when the specified temperature is reached (Figure 4), which is usually used in road traffic lights. The other method is to start the protection when the temperature reaches a certain level and reduce the current drive (Figure 5), which is usually used in indoor lighting and road lighting.

Tyco Electronics has developed a PolySwitch product for LED lighting applications. The product can ensure the normal flow of 350mA current at an ambient temperature of 65°C, and can provide protection when the ambient temperature exceeds 85°C. Through reasonable design and configuration, it can protect LED lamps by shutting down or dimming them.

In the balance between cost and protection scheme, there are usually two protection schemes for overcurrent and overheat protection of giant array lighting or landscape lights that combine series and parallel. One is shunt protection (Figure 6): For multi-string LED systems, a PPTC can be connected in series on each branch for protection. The advantage of this method is that it can achieve accurate protection, and the entire lamp will not stop working due to a failure in a branch, and the protection reliability is good. The second is trunk protection (Figure 7): This protection is to connect a PPTC in series on the main circuit of the lamp to protect the entire lamp. The advantage of this method is low cost and small board area.

In addition, PPTC can be used as a temperature sensor in conjunction with LED drivers to achieve overheat protection for LED array lighting (Figure 8). In large array lighting, due to the large number of LED light strings and the complex structure, the location where overheating failures are prone to occur in actual applications is often not fixed to a specific location, so a single overheating detection device is difficult to provide perfect overheating protection. The low resistance and positive temperature coefficient characteristics of PPTC under normal operation can easily connect several PPTCs in series with the thermal protection detection circuit to achieve multi-point overheating monitoring. Connect multiple PPTCs in series and install them in places where overheating may occur. Due to the low resistance of PPTC, the THRM pin level is 0 and the power circuit works normally. When an overtemperature failure occurs in one or more places of the lamp, one or more PPTCs in the THRM loop are heated, the resistance rises sharply, and the THRM pin level becomes high, thereby starting the overheating protection solution designed by the LED driver to achieve overheating shutdown or overheating reduction drive. This solution is economical and effective for overheating protection of LED surface light sources. Tyco Electronics can provide customers with relevant design and verification services.

Conclusion

Due to its many characteristics, PPTC is very suitable for overcurrent and overheat protection of LED lighting systems. In the design, PTC can be installed on metal core circuit boards or LED heat sinks to achieve the best heat conduction, making PPTC protection more timely and effective. In addition, PPTC products have various packaging forms and can be customized according to customer requirements, which is very flexible. However, there is no standard solution for overheat protection. The specific design must be combined with actual lamps and fully verified.