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Guarding HB LEDs with PPTC devices

Posted: 17 Nov 2011     Print Version  Bookmark and Share

Keywords:LED  polymeric positive temperature coefficient  Class 2 power 

End-users who pay more for greener lights tend to also expect longer life and improved reliability from that investment. To address these expectations, LED design engineers must consider a wide range of variables which influence the performance and lifetime of their product. From power management to power density to over-voltage and over-temperature protection, the uniqueness of LED technology presents new challenges that are not associated with older technologies.

LED technology has advanced rapidly, with improved chip designs and materials facilitating the development of brighter, more energy-efficient, and longer-lasting light sources that can be used in a wide spectrum of applications. In spite of the technology's growing popularity, it remains a fact that excessive heat or inappropriate applications can dramatically affect LED lifetime and performance.

Overcurrent conditions
LED light output varies with the type of chip, encapsulation, efficiency of individual wafer lots and other variables. LED manufacturers use terms such as high brightness to describe LED intensity. High brightness LED (HB LED) drivers can be either linear or switching current supplies. Linear drivers are most suitable when the supply voltage is slightly greater than the load voltage, and resistors are used to limit the current. Switching supplies are often used because they are more efficient.

Generally, current-sensing resistors provide the feedback to the current regulation controller to monitor the current fed to the HB LEDs. An alternative solution is to use polymeric positive temperature coefficient (PPTC) devices to limit the current through the LEDs.

Figure 1: Typical circuit protection design for HB LED lighting.

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A PPTC device is a series element in a circuit (figure 1). Generally the PPTC device resistance is less than the remainder of the circuit and has little or no influence on normal circuit performance. However, in the event of an over-current condition, the device increases in resistance (trips) and reduces the current in the circuit to a value that can be safely carried by any of the circuit elements. This change results from a rapid rise in the temperature of the device through I2R heating.

The device remains in its tripped or latched position until the fault is cleared. Once power to the circuit is cycled, the PPTC device resets and allows current flow to resume, restoring the circuit to normal operation. While PPTC devices cannot prevent a fault from occurring, they respond quickly, limiting current to a safe level to help prevent collateral damage to downstream components. In addition, their small form factor makes them easy to use in space-constrained applications.

Overtemperature
Unlike traditional lighting, HB LEDs are extremely heat-sensitive and thermal management is an important design consideration. To improve reliability and operating life, the PN junction should not be allowed to reach elevated temperatures. Because PPTC devices are thermally activated, any change in the temperature around the device will affect its performance. As the temperature around the device increases less energy is required to trip the device and thus its hold current value decreases.

How PPTC devices work
Under normal operating conditions the heat generated or lost by the PPTC device is in balance at a relatively low temperature, as shown in Point 1 in figure 2. If the current passing through the device increases while the ambient temperature remains constant, the heat generated by the device also increases. If the increase in current is negligible the generated heat can be lost to the environment and the device will stabilise at a higher temperature, as shown in Point 2 in figure 2.

Figure 2: Typical operating curve for PPTC devices.

If instead of an increase in current, the ambient temperature is raised the device will stabilise at a higher temperature, possibly again at Point 2 on the schematic. Point 2 may also be reached by a combination of current and temperature increases. Further increases in either current or temperature, or a combination of both, will cause the device to reach a temperature where resistance rapidly increases, as shown at Point 3. This is referred to as the lower knee of the curve. Any further increase in current or ambient temperature will cause the device to generate heat at a greater rate than the rate at which heat can be lost to the environment, causing it to heat up rapidly.

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