The best solution to prevent high-power LEDs from overheating and damage

Both theory and practice have proved that the performance and life of LEDs are closely related to the operating temperature of the LED's PN junction. When the junction temperature in the LED chip increases by 10°C, the luminous flux will be attenuated by 1%, and the life of the LED will be reduced by 50%. Overcurrent, overvoltage and overheating will significantly reduce the luminous performance and service life of the LED. Therefore, in addition to good heat dissipation design, safe and reliable overheat protection must also be considered.

Usually for LEDs, about 20% of the input power is converted into light and 80% is converted into heat. This depends on a variety of factors. Heat generation may be related to irregularities in the bottom layer and phonon emission, sealing, materials, etc. Of the total heat generated by LEDs, 90% is transmitted by conduction. In order to dissipate the heat from the LED junction, conduction is the main channel of heat conduction, because convection and radiation only account for about 10% of the total heat transfer.

Obviously, LED light sources require precise power and thermal management systems, because compared with other light sources, most of the electrical energy provided to LEDs is converted into heat. Without proper thermal management, this heat will affect LED life and color output. At the same time, since LED driving devices are silicon devices, they will soon fail. This means that a fail-safe backup overcurrent protection device must be equipped.

So, why do you say that successful high-power LED lighting design cannot do without PPTC? Many people think that the LED driver itself can provide over-current and over-heat protection functions, there is no need to add a PPTC device externally, because this will increase Additional cost. But this is a wrong view. Many LED drivers are easily damaged by DC voltage and wrong polarity. The output of the LED driver can also be damaged or destroyed due to a short circuit. Although most LED drivers contain built-in safety features, including thermal shutdown and LED open circuit and short circuit detection. However, to protect integrated circuits (ICs) and other sensitive electronic components, additional overcurrent protection devices may be required. Therefore, only PPTC (especially the resettable PolySwitchPPTC) is the most suitable solution to prevent overheating and damage to high-power LEDs.

The protection features that a typical high-power LED constant current driver can provide include: thermal shutdown protection, overvoltage/overcurrent comparator, high current MOSFET, current size adjustment, and a certain level of ESD protection. However, its thermal shutdown feature is only a self-protection function, and it is difficult to apply to LED overheating design protection. Its overvoltage/overcurrent protection features require an external overvoltage protection circuit, and turning off the power supply may cause other problems. The failure mode of high current MOSFET is usually short circuit, which is a potential unsafe factor. Realizing the current regulation characteristics for LED dimming may cause LED over-temperature problems.

But the concept of PPTC to protect high-power LEDs is different. It has the following 4 unique advantages: 1) The typical failure mode of PPTC is high impedance, so LEDs are safe even in a fault state. 2) PPTC can directly measure the temperature of the LED, so it can be personally protected. 3) PPTC can realize over-temperature, over-current and over-voltage protection functions. It is very simple to use and does not require external circuits. The LED can work normally even in the protected state. 4) Allow designers to achieve dimming design at a lower cost without worrying about safety issues. Therefore, it is only necessary to equip a PPTC device outside the LED driver to achieve stronger security, meet the most stringent safety standards and achieve cost reduction.