The relationship between high-power LED lighting design and PPTC

The biggest design challenges currently faced by developers of high-power LED lighting solutions are heat dissipation design and overheat protection. They must overcome the problem that LED light sources are highly sensitive to heat, because excessive heat or improper application will greatly reduce the performance of LED light sources.

Both theory and practice have proven that the performance and life 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 decay 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 a good heat dissipation design, safe and reliable overheat protection must also be considered.

Typically for LEDs, about 20% of the input power is converted to light and 80% is converted to heat. This depends on a variety of factors, and the heating can be related to substrate irregularities and phonon emissions, sealing, materials, etc. Of the total heat generated by an LED, 90% is transferred by conduction. To dissipate the heat from the LED junction, conduction is the main channel for heat transfer, as 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 to other light sources, a greater proportion of the electrical energy supplied to LEDs is converted to heat. Without proper thermal management, this heat can affect LED life and color output. Also, because LED drivers are silicon devices, they fail quickly. This makes fail-safe backup overcurrent protection a must.

So why is a PPTC essential for a successful high-power LED lighting design? Many people think that the LED driver itself can provide overcurrent and overtemperature protection, so there is no need to add an external PPTC device because it will add additional cost. However, this is a 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 contain built-in safety features, including thermal shutdown and LED open and short circuit detection. However, to protect the integrated circuit ( IC ) 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 high-power LEDs from overheating damage.

To illustrate this problem, let's first look at what protection features a typical LED driver can provide. Most high-power LED constant current drivers can provide typical protection features including: 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 is difficult to apply to LED overheating design protection. Its overvoltage/overcurrent protection feature requires an external overvoltage protection circuit, and shutting down the power supply may cause other problems. The failure mode of high-current MOSFET is usually a short circuit, which is a potential unsafe factor. The current regulation feature used to implement LED dimming may cause LED overheating problems.

But the concept of PPTC to protect high-power LEDs is different. It has the following four unique advantages:

1. The typical failure mode of PPTC is high impedance, so the LED is safe even in the fault state;

2. PPTC can directly measure the temperature of LED, so it can provide close protection;

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 protection state;

4. Allow designers to implement dimming design at a lower cost without worrying about safety issues.

Therefore, only a PPTC device is needed outside the LED driver to achieve greater safety, meet the most stringent safety standards and reduce costs.