Successful high
High-power LED lighting solutions, manufacturers are currently experiencing the greatest development challenges is the design of thermal design and thermal protection, they must overcome the strong LED light on the heat sensitivity of the problem, because excessive or improper application of heat will cause the LED light source of the performance of large discount.
Theory and practice has proven, LED and LED performance and lifetime of the PN junction temperature is closely related. When the LED chip junction temperature rise 10 , the flux will decay 1%, LED's life expectancy is reduced by 50% over current, over voltage and over temperature will significantly reduce the emission LED performance and lifetime. So in addition to good thermal design, safe and reliable thermal protection also must be considered.
Usually on the LED, the approximately 20% of the input power is converted to light, 80% is converted to heat. This depends on many factors, fever may be associated with the underlying irregular and phonon emission, sealing, materials and so on. The total heat generated in the LED, 90% through conduction transmission. Results for the dissipation of heat from the LED, heat conduction is the main channel, because all of convection and radiation heat transfer only about 10%.
Clearly, LED light sources require accurate power and thermal management systems, as compared with other light sources to provide electric power to the LED is converted to heat most. Without proper thermal management, which will affect the heat output of LED life and color. Also, because LED drive is silicon devices, they will soon be invalid. This shows that have to be equipped with fail-safe backup overcurrent protection device.
Why, then, that the success of high-power LED lighting design is inseparable from PPTC it? Many people think LED drive itself can provide over-current and thermal protection, no need to add another outside a PPTC device, as this additional cost. But this is wrong, many LED driver is very susceptible to DC voltage and polarity inaccurate. LED driver outputs will be damaged or destroyed due to short circuit. While most LED driver with built-in security features, including thermal shutdown and open and short LED detection. However, to protect the integrated circuit (IC) and other sensitive electronic components, may require additional over-current protection devices. Therefore, only PPTC (in particular, can be reset PolySwitch PPTC) is the power LED to prevent overheating damage to the most appropriate solution.
To illustrate this problem, we first look at a typical LED driver can provide any protection. Most high-power LED constant current driver can provide a typical protection features include: thermal shutdown protection, over voltage / over-current comparator, high-current MOSFET, the current size of the adjustment, a certain level of ESD protection. However, the thermal shutdown feature is just a self-protection function, is difficult to apply to the design of LED overheat protection. The overvoltage / overcurrent protection characteristics of the need for external over-voltage protection circuit, turn power off may cause other problems. High-current MOSFET is usually short-circuit failure mode, which is potentially unsafe. LED dimming to achieve with the current regulation characteristics of LED over-temperature could cause the problem.
PPTC protection of high-power LED, but the idea is different, it has the following four distinct advantages: 1) PPTC the typical failure mode is high impedance, so even in the fault state, LED is safe. 2) PPTC LED can directly measure the temperature, so you can close protection. 3) PPTC can be achieved over-temperature, over current and over-voltage protection function. Very easy to use, requires no external circuitry. Even in the protection of state LED can work. 4) allows designers to achieve more cost-dimming design, without worrying about security issues. Therefore, only the LED drive out with a PPTC device can achieve stronger security, meet the most stringent safety standards and achieve lower costs.
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