A brief discussion on the differences between LED EOS protection solutions

As LED technology matures, LED lighting is increasingly used, such as LED flashlights in mobile phones, LED home lamps, LED headlights and taillights in cars, etc. However, you may not be particularly clear about how to effectively extend the life of LEDs. Our topic today revolves around the differences between the solutions for effectively protecting LEDs from electrical overstress (EOS).

EOS is the abbreviation of Electrical Over Stress, which refers to all excessive electrical stress. EOS shock means that when the current or voltage applied to an electronic component exceeds the maximum design specification requirements of the component, the component will experience performance degradation or even direct damage.

LED devices are easily damaged by EOS, which can sometimes directly damage the device and cause failure, while other times failure may occur some time after EOS occurs. Unstable power output quality, various overvoltage and overcurrent noises, and inrush current phenomena in hot-swap applications are all factors that can cause EOS failure. This EOS phenomenon is a short-term overload. In a short period of time (usually within one second), the LED is impacted by a spike voltage or a spike current. The energy of this voltage or current exceeds the design rating of the LED, thereby damaging the LED device.

After briefly introducing the EOS problem of LEDs, let's look at the solution. The basic protection solution for protecting LEDs from EOS damage is to clamp the transient voltage generated by EOS within the range that the device can withstand. In theory, the best protective device to eliminate this transient voltage is a capacitor, which can absorb transient voltage very well. However, in actual applications, due to factors such as the high temperature life and large size of capacitors, it is not a good solution.

Therefore, most engineers tend to use TVS voltage regulators for protection when solving this problem. However, when choosing a TVS tube, the maximum power that the TVS tube can withstand must be considered. Due to the uncertainty of the transient energy of the EOS impact, if you want to fully withstand all the transient energy of the EOS impact, you need to choose a relatively expensive high-power TVS tube, which is often not what product engineers are willing to do. At that time, if a relatively small power TVS tube is selected due to cost issues, if there is a large energy EOS impact in actual application, it may weaken or even damage the protection function of the TVS tube. If the TVS tube is damaged, the subsequent EOS will directly impact the LED, causing damage to the LED. In addition, the accuracy of the TVS tube's clamping voltage will also directly affect the effect of the EOS impact on the subsequent LED, because the increase in voltage applied to the two ends of the LED will immediately manifest as an increase in the current flowing through the LED.

Another solution is to use TE's PolyZen series components, which can effectively reduce EOS intrusion and improve product reliability. TE's PolyZen product from the Circuit Protection Division is an independent surface mount device that integrates a precision Zener diode with a selectable Zener voltage (Vz) and a PolySwitch polymer positive temperature coefficient (PPTC) device. The product schematic is shown in Figure 1. The PolyZen series is suitable for thin and compact environments with limited space. It uses a Zener diode with thermal protection to help protect electronic products from failures caused by voltage transients, reverse bias and incorrect power use. As shown in Figure 2, the typical application block diagram of the Polyzen product, in the fault state, the precision Zener diode can quickly and effectively clamp the voltage and shunt the fault current, while the PolySwitch PPTC component can then quickly shut off the excessive current, thereby helping to protect the Zener diode and downstream electronic components.

Figure 1: Polyzen product schematic

Figure 2: Typical applications of Polyzen products

The typical fault response of PolyZen products is shown in Figure 3. The example shown in the figure is a PolyZen device with an integrated 5.6V Zener voltage. Under a 24V overvoltage condition (Vin), a 10A fault current (Iflt) passes through the system. The precision Zener diode in the PolyZen device can quickly clamp the output voltage (Vout) to around 5.6V, protecting the load circuit. At the same time, the PPTC in the device can quickly act to cut off the current, continuously protecting the Zener diode and the entire circuit for a long time.

Figure 3: Typical failure response of Polyzen products

When PolyZende's products are used in the EOS protection circuit of LEDs, not only can the transient voltage of EOS be quickly clamped to below the safe voltage, but also when the energy of the EOS impact is large, the Zener diode inside the PolyZen device will prompt the PPTC to operate, protecting the Zener diode itself and subsequent circuits. Moreover, in some LED lighting systems that cannot lose lighting due to faults, PPTC is superior to other overcurrent protection devices, because PPTC can still make the LED light up in the protection state, but it will not damage the LED. Moreover, the clamping voltage accuracy of the Zener diode integrated in the PolyZen device is very high, and it can accurately clamp the voltage within a safe range. Therefore, compared with the EOS protection solution of TVS tubes, PolyZen devices have more advantages.

PolyZen series components are integrated overvoltage and overcurrent protection devices with slim profiles and powerful circuit protection functions to help protect sensitive electronic products and avoid expensive product returns and warranty issues caused by overvoltage and overcurrent failures. PolyZen series products provide circuit board designers with convenient overvoltage and overcurrent protection devices, so that they no longer need to spend a lot of time integrating and testing inefficient discrete devices and more expensive IC solutions.

Today, the PolyZen series of products has a rich combination of Zener diodes and PPTCs for practical application selection. Its small size, independent mounting, and multi-functional protection have become an innovative solution with significant performance and price advantages in overcurrent and overvoltage protection applications, surpassing discrete solutions using fuses, Zener diodes, and other passive components.