Differences between EOS protection schemes of LED

    EOS is the abbreviation of Electrical Over Stress, which refers to all excessive electrical stresses. EOS impact means that when the applied current or voltage of electronic components exceeds the maximum design specification requirements of the components, the performance of the components will be weakened, or even directly damaged.

    LED devices are easy to be damaged by EOS. Sometimes such damage can directly damage the devices to cause failure, and sometimes the failure may occur after EOS occurs for a period of time. The instability of power supply output quality, various over-voltage and over-current noises, and the inrush current phenomenon in hot plug applications are all factors that can cause EOS failure. This EOS phenomenon belongs to short-term overload. In a short time (generally within one second), LED is impacted by peak voltage or current. The energy of this voltage or current exceeds the design rating of LED, thus damaging LED devices.

    After briefly introducing the EOS problem of LED, let‘s take a look at the solution. The protection schemes to protect LED from EOS damage are basically to clamp the transient voltage generated by EOS within the range that the device can withstand. Theoretically, the best protective device to eliminate this transient voltage is the capacitor, which can well absorb the transient voltage. However, in practical applications, due to such factors as the high temperature life and large volume of capacitors, it is really not a better scheme.

    Therefore, most engineers tend to use TVS voltage stabilizing tubes for protection when solving this problem. However, when selecting TVS tubes, they must take into account the maximum power that TVS tubes can withstand. Because of the uncertainty of the transient energy of EOS shocks, in order to fully withstand the transient energy of all EOS shocks, they need to choose more expensive high-power TVS tubes, which is often not desirable for product engineers. At that time, if TVS tubes with relatively small power are selected due to cost problems, the protection function of TVS tubes may be weakened or even damaged if there is a large energy EOS impact in practical applications. 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 clamping voltage of the TVS transistor will also directly affect the effect of EOS impact on subsequent LEDs, because the increase of the voltage applied to both ends of the LED will immediately show the increase of the current flowing through the LED.

    Another solution is to use PolyZen series components of TE Company. This takeoff and landing can effectively reduce EOS interference and improve product reliability. PolyZen product of Circuit Protection Department of TE Company is an independent surface mount device, which integrates a precision zener diode with selectable zener voltage (Vz) and a PolySwitch polymer positive temperature coefficient (PPTC) device. The schematic diagram of the product is shown in Figure 1. PolyZen series is suitable for thin and compact environment with small space. It uses Zener diode with thermal protection function to help protect electronic products from failure caused by voltage transient, reverse bias and incorrect power supply. As shown in Figure 2, the typical application block diagram of Polyzen products shows that 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 turn off the excessive current, thus helping to protect the zener diode and downstream electronic components.

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