PPTC and High Power LED Lighting Solution

Manufacturers who develop high power LED lighting solution are currently having the most design challenge, that is heat dissipation design and overheating protection. They must conquer the problem that LED light source is strong sensitive to heat, because too much heat or improper application will make LED light source poor performance.

The theory and practice have proved that the performance and life of LED is closely related with LED PN junction working temperature. When the junction temperature (Tj) of LED chips rises 10 ℃, luminous flux will decay1%, so the life of LED will reduce 50%, thus the overflow, over-voltage and overheat can dramatically reduce the luminous property and use life of LED. So in addition to good thermal design, safe and reliable overheating protection also should be considered.

Generally speaking, to LED, about 20% of the input power is converted to light, while 80% is converted into heat. It depends on a variable of factors. Radiation may relate to the bottom irregular and phonon emission, sealing, material and so on. The total quantity of heat generated in the LED, 90% transmits through the conduction. To dissipate the heat from LED, conduction is the main channel for heat conducting, for thermal convection and radiation accounts for only about 10% of all heat transmission.

Obviously, LED light source needs accurate power and thermal management system. Compared with other light source, most of the electrical energy provided to LED transferred into heat. If there is no proper thermal management, the heat will affect the LED life and color output. At the same time, because the LED drive device belongs to silicon devices, they will soon become invalid. This shows that LED must be equipped with fault security spare over-current protection units.

The successful high power LED lighting design can not leave with PPTC. Many people think the LED drive itself can provide the flow and thermal overload protection, so there is no need to add a PPTC in external devices, because it can add the extra cost. But this is wrong, for many LED drives are easily to be damaged by DC voltage and polarity error. The output of LED drive will also be damaged or destroyed due to short circuit. Although most LED drive contains the built-in security features, including hot shutdown and LED the opening and short circuit testing. But to protect the integrated circuit (IC) and other sensitive electronic components, we may need extra over-current protection device. Therefore, only PPTC (especially reset PolySwitch PPTC) is the most suitable damage solution to prevent overheating of the high power LED.

The typical protection features can be provided by most of the high power LED constant current drive includes: TSD protection, over-voltage/overcurrent comparator, large current MOSFET, current size adjustment, a certain level of ESD protection. But the TSD characteristic is just a self-protection function; it is difficult to be applied to the LED overheating protection design. The overpressure/over-current protection characteristics need external over-voltage protection circuit; shut off the power supply may cause other problems. Large current MOSFET of the failure is usually short circuit; this is the potential safety factor. To realize the current adjust characteristics of LED dimming may cause over-temperature problems.

But the concept of PPTC to protect high power LED is different, and it has the following four unique advantages:

• PPTC failure mode is typical high impedance, so even in fault state, LED is safe.
• LED temperatures can be directly measured by PPTC, so it can have personal protection.
• PPTC can realize over-temperature, overflow and over-voltage protection functions. Very simple to use, and it does not require an external circuit. Even in protection state LED also can work normally.
• It allows the designers to lower costs to realize light dimming design, and don’t have to worry about safety problems. Therefore only with a PPTC device outside LED can realize stronger security and meet the most rigorous safety standards and implementation cost reduction.