Design and Analysis of High Power LED Lighting Based on PPTC

The biggest design challenges currently encountered by developers of high-power LED lighting solutions are thermal design and overheat protection. They must overcome the heat sensitivity of LED light sources, because excessive heat or improper application will make LED light source performance. discount.

Both theory and practice have proven that the performance and lifetime of LEDs are closely related to the operating temperature of the PN junction of the LED. When the junction temperature of the LED chip rises by 10 °C, the luminous flux will be attenuated by 1%, and the lifetime 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.

Typically for LEDs, approximately 20% of the input power is converted to light and 80% is converted to heat. Depending on a variety of factors, heating may be related to underlying irregularities as well as phonon emissions, seals, materials, and the like. Of the total heat generated by the LED, 90% is transmitted by conduction. To dissipate heat from the LED junction, conduction is the primary channel of heat conduction because convection and radiation account for only about 10% of the total heat transfer.

Clearly, LED light sources require precise power and thermal management systems because most of the power supplied to the LEDs is converted to heat compared to other sources. This heat can affect LED life and color output without proper thermal management. At the same time, since LED drivers are silicon devices, they quickly fail. This means that a fail-safe backup overcurrent protection device must be provided.

So why is the successful high-power LED lighting design inseparable from PPTC? Many people think that the LED driver itself can provide overcurrent and overheat protection. There is no need to add a PPTC device externally because it adds extra cost. But this is the wrong view, many LED drivers are easily damaged by DC voltage and polarity errors. The output of the LED driver can also be damaged or destroyed by a short circuit. Although most LED drivers have built-in safety features, including thermal shutdown and LED open and short detection. However, to protect integrated circuits (ICs) and other sensitive electronic components, additional overcurrent protection devices may be required. Therefore, only PPTC (especially resettable PolySwitch PPTC) is the most suitable solution to prevent overheating damage of high power LEDs.

To illustrate this problem, let's first look at what protection can be provided by a typical LED driver. Typical protection features offered by most high-power LED constant current drivers include thermal shutdown protection, overvoltage/overcurrent comparators, high current MOSFETs, current regulation, and some level of ESD protection. However, its thermal shutdown feature is just a self-protection feature that is difficult to apply to LED overheating design protection. Its overvoltage/overcurrent protection feature requires an external overvoltage protection circuit, which can cause other problems. The failure mode of a high current MOSFET is usually a short circuit, which is a potential unsafe factor. The current regulation characteristics for LED dimming may cause LED overtemperature problems.

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

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