Summary :
LED-based indoor and outdoor lighting technology is one of the most dynamically developing electronic research and design target areas. According to recent statements of LEDinside, Europe has a leading position with its 23% share in this sector. The most crucial task of developers – in addition to maximizing the available luminous intensity and efficiency- is achieving the most extended lifetime with the most reliable design, which requires considering external environmental factors that have the most vital and critical influence on durability. Crucial environmental values like ambient temperature, supply current, and voltage should be kept in a specific safety range. This is why to consider the necessity of designing the proper circuit protection.
Transient Surge Threats to Outdoor Devices
It is well known that LED-based systems are supposed to have a longer lifetime than standard lighting systems, but it is only possible to achieve this if the most critical external influences, such as LED chip junction temperature, forward current, and voltage, remain in a safety range. By having more extended durability, the components are subjected to these environmental issues for a more extended period, and for economical operations, the maintenance-free design is a must. To reach the same level of customer trust and satisfaction that conventional lighting systems enjoy, LED systems should incorporate all kinds of safety and protection solutions that meet the meet with requirements of international standards and regulations.
Lightning strikes are traveling electrostatic discharges, usually coming from clouds to the ground, with a magnitude of millions of volts. Up to thousands of volts are applied to copper wires carrying induced current from lightning strikes up to a few miles away. These indirect strikes usually occur in exposed outdoor wires, transmitting surges to devices like streetlights or traffic lights. The Surge Protection Module, upstream of the circuitry, directly faces surge interference from the power line. It diverts or absorbs surge energy, minimizing surge threats to downstream devices like the AC/DC power supply unit in an LED lighting fixture. The initial investment cost and desire for high system reliability are key factors to consider when evaluating the need for installing surge protection devices.
The LED lighting elements are solid-state devices and inherently susceptible to early failure due to surge energy. Depending on where the light fixtures are connected to the utility electrical distribution and geographical location, they can risk a high degree of failure.
Need of Protection
In the case of streetlights, LEDs are sold based on low maintenance and long life. So designers must ensure their designs are well protected to realize the expected savings. However, one problem for LED lighting installations is protecting against transient overvoltage events. Two significant causes of transient overvoltage have long been recognized; system switching transients and transients triggered or excited by lightning discharges (in contrast to direct lightning discharges to the power systems, which are generally destructive and for which economic protection may be challenging to obtain). Median peak currents can range between 30 to 50kA per strike. One particular area of concern is the SMPS front end. Here, the most immediate threat is overvoltage failures. In particular, their pole-mounted outdoor location makes them vulnerable to lighting-induced overvoltages and over currents, two of the most common failure modes at the front end of an SMPS. The other weak point in the lighting circuit is the LED itself. LEDs are fragile solid-state devices, essentially diodes, structured as a P-N junction that emits light when forward-biased. The main LED failure mechanisms are mechanical and thermal involving thermal cycles, thermal shock, and LEDs operating at high temperatures causing wire bonds to age and fail. As the metal oxidizes and becomes brittle over time, the likelihood of an LED failure increases. Electrostatic discharge (ESD) events or surges induced by nearby lightning events are another common cause of LED failure.
Overview of components designed for circuit protection
Some of the famous circuit protection solutions target saving the LED chips or the driver components to provide a maintenance-free, long lifetime; others offer protection against harmful effects caused by possible malfunction of the luminaire. The white LEDS used in general lighting applications are usually containing blue chips that are very sensitive to electrostatic discharges, so it is also advised to use ESD protection. Several producers include tanning diodes in the LED package, but external protection, like parallel PESD devices, could be used when it is not applicable.
As the LED’s lifetime depends on how to remove the generated heat, designers calibrate heat management mainly on self-healing. It is, however, essential to consider the extra heat caused by unexpected environmental issues. It is possible to use a PPTC (Polymer PTC – PolySwitch) device connected in series with the LED chain. It is also advised to connect physically to the heatsink to dissipate excess heat. Under normal circumstances, the PolySwitch, a series element in the circuit, has a low resistance that is much less than the circuit impedance and hardly influences the performance as a response to the self-heating caused by overcurrent events and external heat, the device trips, and its internal structure changes, which increases the resistance quickly and reduces the device’s current to the level that LED chips can safely carry. The LED driver’s output can be protected similarly from overcurrent caused by malfunction of the electronics itself.
Electric fast transients, like high energy spikes coming from the mains, may drastically degrade the lifetime of the whole luminaire. To have helpful protection against overvoltage on the input of the LED driver, one can use a combination of an LVR device and a MOV device. Once a voltage spike appears on the information, the MOV starts to activate, delivering the energy to the ground, which increases the current of the LVR quickly, putting it to a high impedance state and limiting the overcurrent immediately. As seen in the figure, the resistor R1 is not there for protection, to provide further limitation of the current to keep it under the level LVR can withstand even in extreme cases. Instead of MOV, designers can choose silicon-based solutions for LED power supply’s AC input that have no aging effect and are fully resettable, repetitive devices. Protek devices offer a range of transient voltage suppressor diodes designed for 300-440V levels, offering ESD protection up to 8/15 kV and EFT protection with a surge rating of 1,5kW.
In some cases, LED driver inputs are subject to non-transient overvoltage events caused by accidentally connected higher than nominal voltage or reverse polarity power supply. An example can be a LED-based table lamp, which should be energized with an external power supply, and the wrong device is used. In these cases, a semiconductor-based fast clamping device should be used for protection, like the PolyZen device of TE Circuit protection (now Littelfuse). PolyZen combines Zener diode-provided overvoltage protection and a PolySwitch device for current limiting in closed thermal coupling. The PolySwitch device will protect the Zener diode from overheating and burning down by providing effective current limiting once the Zener is activated and its current grows.
As mentioned above, the overvoltage events caused by transients and spikes of the line voltage could be effectively handled using metal oxide varistors (MOV) in LED-based lighting devices. Over-voltage protection devices are not only subjected to short transients, but sometimes they need to take long abnormal overvoltage of minutes or hours in case of faulty mains network or transformer’s neutral cable loss, as seen in the figure.
In these cases, the voltage of the MOV grows above its nominal voltage, and due to the current limiting of the impedance of the load, the fuse may not blow, causing all excess energy concentrated on the MOV, which may cause burning and destroying it.
Lightning strikes, sudden switching of huge inductive or capacitive load, and neutral cable loss can induct such high transient voltages which the varistor cannot handle; the device would suffer from thermal runaway and can blow or burn. Recently, hybrid devices that incorporate adequate overtemperature protection to the clamping overvoltage device are appearing on the market. These devices could be used – among others – for input protection of power supplies of LED-based lighting systems. During normal circumstances, the voltage appearing on the MOV does not exceed the maximum permitted voltage of the device, and even the extra energy generated by handling the fast overvoltage events caused by, e.g., spikes will also be taken. The abnormal cases, such as the neutral cable loss, will result in however long transient overvoltage events, and without adequate overtemperature protection, the MOV would suffer from thermal runaway. The usual solution was using a serial TCO, and an extra overcurrent protection element, like a fuse, to limit the generated transient overcurrent. Instead of using discrete circuit elements, it is better to use an integrated solution. TE Circuit Protection’s (now Littelfuse’s) AC2PRO devices offer hybrid protection, combining a metal oxide varistor and a thermally coupled PPTC device. During abnormal operation, the MOV goes to a low resistance state and starts to conduct current to the ground, which can grow fast to a value, leading to thermal runaway, burning down the MOV, and resulting in fire. The integrated PPTC, however, would also warm up partly because of the thermal coupling with the hot MOV and partly because of the I2R self-heating of the growing current. This warming up will activate the current limiting ability of the PolySwitch, which then would go into a high resistance state and save the MOV from burning down. A further advantage of the device is its reset ability; when the problem is gone, after a specific cooling time, the AC2PRO resets to a normal state and is ready to function normally. Maintenance costs could be minimized as no replacement is needed. The figure shows the operation of the AC2PRO device during a transient event caused by neutral loss. The AC2PRO is currently available in 150 mA, 350 mA, and 700 mA hold current versions, with 400V MOV nominal voltage.
Suggested locations of overvoltage protection components
To protect LED lights as well as possible, it is reasonable to use a protection module that protects in the best way against surges. To achieve this goal, the best way is to use a delta connection.
It is also essential to provide the best location for the MOVs; the bigger size varistor of these surge protection modules should be triggered faster than the smaller varistor inside the LED driver. Therefore its breakdown voltage should be less than the one in the driver, as
large part of the energy should be handled in the larger MOV, and the smaller MOV should only be subjected to the remaining portion that it can still run.
The summary of rules applicable for the installation are:
- The larger MOV1 should handle the central part of the transient energy.
- The selection rule for maximum continuously allowable working voltage (Vm) is: Vm(MOV1) <= Vm(MOV2)
- The maximal clamping voltage (Vc) selection rule is: Vc(MOV1) <= Vc(MOV2)
- It may be an excellent solution to use a serial-connected inductor in combination with the SPD, that MOV1 can handle higher surge energy: V(MOV1) = V(MOV2)*dI/dt
Even in the case of keeping these rules and a huge part of excess energy is handled by the SPD, the LED driver’s input will still be stressed by overvoltage that may cause malfunction or damage. Due to this fact, local protection should cooperate with the SPD. A short and low clamping voltage MOV device should handle the remaining part of the fast transient overvoltage. The clamping voltage on MOV2 should be kept above MOV1’s to maximize I1 and reduce the I2 over-current, ensuring that the remaining short current will not blow the F1 fuse. Enlarging the total resistance of the primary circuit (NTC, EMI filter, NTC, rectifier bridge, PFC module, transformer, transistor) I3 current may be minimized, reducing the risk of LED component damage. To protect the components sensitive to excess voltage that remained after the SPD protection started, we can use fast TVS diodes.