Protection components for LED lighting systems
August 21, 2014
All-round ESD and heat protection
LED lighting systems require high-performance ESD and overtemperature protection if they are to live up to their potential in terms of lifetime, low maintenance costs and overall reliability. EPCOS protection components provide effective and cost-efficient protection for the LED arrays, their power supplies and control circuits.
LEDs make a decisive contribution to reducing the energy consumption of lamps for indoor and outdoor use. Not only do LEDs consume much less power than conventional lighting sources such as fluorescent and HID lamps, they also have a much longer maximum operating life – up to 80,000 hours and even more under ideal conditions.
As a result, the lamps of conventional lighting systems would have to be replaced several times during the normal service life of an LED lamp. This translates into lower maintenance costs and lower overall system operating costs. The key threats to the reliability of LED lighting systems are:
- ESD events, including lightning
- Transient overcurrent events and surges
- Current and voltage spikes during hot swapping
- Reverse voltage effects
Therefore, if LED lighting systems are to be as reliable and long-lived as the LEDs themselves, all of the components and subsystems must be protected effectively against these dangers, which are encountered during assembly, maintenance, and operation.
Architecture of LED lighting systems
An LED luminaire consists of the LED engine (the LED arrays, their LED driver and control units), the LED power supply and the connection to the grid. Companies and facility managers are increasingly deploying smart networked lighting systems in order to maximize the efficiency and quality of lighting, determine the status of luminaire, and enable remote control and maintenance. This means communication power supplies and interfaces can also be integrated in the luminaires.
The energy level of the ESD events and surges, to which each of these subsystems is exposed, determines which protection devices are needed to ensure overall system reliability. Figure 1 shows an overview of the subsystems and their suitable protection devices.
Protection against high energy surges
EPCOS SIOV® varistors have long been a solution of choice for overvoltage protection in general. These metal oxide varistors are especially well-suited for the protection of power supplies of LED lighting systems from larger energetic surges up to 750 J. The new B20* and B25* series of SIOV varistors are protection components that are designed to meet the surge protection requirements up to 10 kV/10 kA in accordance with both ANSI/IEEE C62.41.2 and the DOE MSSLC Model Specification for LED roadway luminaires. These series include three or four SIOV varistors assembled in a single case (Figure 2). The B20* and B25* series are currently available for maximum operating voltages of 320 V, 420 V and 510 V.
TDK is world market leader with its EPCOS surge arresters and varistors that feature compact design and offer the most reliable protection in this stage against very large ESD events such as surges from lightning strikes and other transients. Surge arresters combined with varistors offer a space-saving integrated solution with optimal performance.
The T series of EPCOS ThermoFuse™ varistors, which consist of a disk varistor connected in series with a thermally coupled fuse in a single package, offer special protection in the power input stage. If the varistor overheats, the thermal fuse encapsulated in a special plastic disconnects the varistor from the power circuit and thus prevents the flames and smoke from developing. The T series varistors feature a surge current capability of up to10,000 A and a maximum energy absorption capacity of 440 J for 2 ms.
Protection against low energy ESD
Traditionally, TVS diodes were employed to protect circuits against ESD events below 25 J. ESD protection requirements, however, are becoming more demanding with respect to miniature size and insertion height, reliable performance over a wide temperature range, and fast response times. Multilayer varistors such as the EPCOS CeraDiode® family offer crucial advantages over the traditional TVS diodes and are increasingly becoming the solution of choice for ESD and surge protection in this energy range. CeraDiodes can be used very effectively in order to meet the absorption requirements in relation to component size.
There are many conceivable configurations for the LED engine. It can consist of up to hundreds of LEDs connected in either series or parallel strings or in combinations of both (Figure 3). Series connected strings have the risk that the failure of one LED will cause the entire string to fail, resulting in a significant reduction of light.
Thanks to their extremely low parasitic capacitance, CeraDiodes are also the best choice for the ESD protection of data lines for the control of luminaires. Because it is not unusual for networks to be reconfigured with items being moved, taken out of service or replaced, hot swapping situations are quite common, which can cause ESD events and spikes. Proper ESD protection throughout the total system with CeraDiodes is required to ensure that the devices would remain fully functional for their entire specified lifetimes.
Advantages of CeraDiodes vs. TVS diodes
The CeraDiode features a high energy absorption capability per volume/surface area. Its active volume is almost three times larger than that of a TVS diode, which is only about 30 percent. As a result, LED lighting designers can achieve the same ESD protection to IEC 61000-4-2 and ruggedness against surge currents with a much smaller component (Figure 4). The CeraDiode supports the miniaturization demands of luminaire manufacturers. The EPCOS CeraDiodes in case sizes EIA 0201 and 0402 are well-suited for protecting the LED engine and arrays of LED lighting installations and offer the same protection as TVS diodes that have a much larger footprint. The newest development in case size EIA 01005 features further miniaturization for applications with less space, such as LED flashes.
In view of the many different forms of ESD and transient disturbances, it is usually necessary to provide bidirectional protection. As a varistor, the CeraDiode offers bidirectional protection in a single component, while a TVS diode is inherently unidirectional. Thus, depending on the design requirement it can thus be necessary to design in two diodes for the same protection. This gives the CeraDiode the advantage both in terms of size, cost and placement logistics.
CeraDiodes also compare favorably with respect to their nearly non-derating performance up to 85 °C. The derating of TVS diodes, on the other hand, starts already at room temperature. CeraDiodes thus ensure that the LED electronics can operate reliably over a very large temperature spectrum. This not only helps luminaire designers to minimize the space needed, but it also helps to reduce cost. Specific CeraDiode series even feature non-derating characteristics up to 125 °C or higher. In addition, CeraDiodes offer a fast high response time of <0.5 ns and high pulse absorption capability over a long lifetime.
Effective temperature protection
Because the LEDs require a constant current in order to deliver a constant luminance, their temperature must be controlled precisely within narrow limits. EPCOS NTC and PTC thermistors in SMD packages are designed to protect the LED arrays against overheating and control their temperature profile for the best possible lumen efficiency. This is performed by automatically adjusting the current to the LEDs. Thanks to their narrow tolerance of ±1 % across the board and to their short response time, new miniaturized EPCOS SMD NTC thermistors in case sizes EIA 0402 or 0603 permit the highly precise measurements required for this purpose. Together with intelligent circuits, they then enable the design of effective control systems.
A dedicated LED sample kit available under ordering code B57999V3999J099. EPCOS SMD PTC thermistors as temperature limit sensors represent another simple, reliable and cost-efficient approach for overtemperature protection. Available in case sizes EIA 0402, 0603 and 0805, their wide range of R/T curves with sensing temperatures from 70 °C to 145 °C makes it easy to select the right device. Samples of the complete product range are available in a sample kit (ordering code B59001Z999A99).
In simpler LED driver designs without ICs, PTC thermistors can also be used to reduce the LED forward current at high temperatures by placing them in series to the LED (Figure 5). Leaded or large SMD PTC thermistors (case size EIA 3225 or 4032) are recommended for such applications.
Broad portfolio of components for LED lighting systems
TDK’s complete lineup of protection devices is complemented by a broad spectrum of high-performance TDK and EPCOS film capacitors, MLCCs, inductors, and transformers that enable the design of compact, reliable and long-lived luminaires for indoor and outdoor LED lighting systems.
Table: Portfolio of EPCOS CeraDiodes for LED lighting
|LEDs per string||Ordering code||Case size [EIA]||Max. DC voltage [V]||Breakdown voltage at 1 mA [V]||Max. clamp voltage at 1 A, 8/20 µs [V]|
|2 to 4||B72590D0150H060||0402||15||23||66|
|5 to 6||B72590D0200H060||0402||20||30||80|
|7 to 10||B72590D0360B060||0402||36||45||130|
|11 to 20||B72590D0700B060||0402||70||100||290|
|21 to 26||B72590D0900B060||0402||90||125||350|
|27 to 48||B72590D0201B060||0402||200||230||600|