Find out the five key threats that affect the reliability of LED lighting systems in electronic designs.

By SONJA BROWN,
Director of Product Marketing – Piezo and Protection Devices
EPCOS, a TDK Group Company
www.epcos.com

 

There are several factors that can affect solid
state lighting in electronic design from stress issues, to packaging-related
issues to component-related issues. Design engineers must take these factors into
account. There are five key threats that affect the reliability of LED lighting
systems in electronic designs:

 

  • Electrostatic discharge (ESD) events, including
    lightning
  • Transient
    overcurrent events and surges
  • Current
    and voltage spikes during hot swapping
  • Reverse
    voltage effects
  • Over-temperature protection

 

For
LED lighting systems to be reliable, all of the components and subsystems must
be protected effectively against these dangers, which are encountered during
assembly, maintenance and operation. High-performance
ESD and over-temperature protection can protect against these dangers,
resulting in a longer lifetime, lower maintenance costs and increased
reliability of the LED lighting system. The use of high-quality protection ESD
and over-temperature components provide effective and cost-efficient protection
for LED arrays, their power supplies and control circuits.

 

LED
system overview

An
LED bulb – or more technically, a luminaire – is made up of three basic
subsystems: an LED power supply, a power-input connection to the grid, and an
LED engine. The LED engine can be further broken down into LED arrays, LED
drivers and control units.

 

Over
the past several years, smart LED luminaires have gained popularity due to
features including remote control and maintenance. Because of this, a fourth
subsystem – communication power supplies and interfaces – is also often
integrated into the luminaires. This allows smart networked lighting systems to
be deployed that maximize the efficiency and quality of lighting, and helps
facility managers know the status of each luminaire for maintenance.

 

fapo_Epcos01_ESD_HeatProtectionLEDlightingsystems_nov2016

 

Fig. 1:A basic LED lighting system architecture that showcasing over-voltage
and over-temperature protection devices

 

 

As
the LED lighting system technology has matured, engineers realized they needed
a way to prevent ESD events and high energy surges from affecting the luminaire
and to ensure system reliability. These
surges can cause both immediate failures (often at a junction) and an increased
rate of degradation caused by latent damage. Each of the four subsystems
can be exposed to events that cause this kind of damage. As engineers review
the energy levels of possible events, they can determine what suitable
protection devices are needed.

 

High-energy
surges

 

Voltage-dependent
resistors, also known as varistors, have long been a solution of choice for
overvoltage protection. This is because the electrical resistance varies with
the applied voltage. Engineers should select varistors that are designed to
hold up to the conditions of the end application.

 

Take
for example the protection of power supplies. Metal oxide varistors are
especially well-suited to protect the power supplies of LED lighting systems
from larger energetic surges. The design may specify varistors that feature a
compact design, or that provide protection against big ESD events such as
lightning strikes. And, due to the harsh nature of the elements and cost of
maintenance, LED lighting systems designed for streetlights should meet both
ANSI/IEEE C62.41.2 and the DOE MSSLC Model Specification for LED roadway
luminaires. In these cases, surge arresters combined with varistors offer a
space-saving integrated solution with the most ideal performance.

 

In
the power input connection, a single package consisting of a disk varistor
connected in series with a thermally coupled fuse is best suited to provide the
protection required. In this case, if the varistor overheats, the thermal fuse –
which is encapsulated in a plastic housing – disconnects the varistor from the
power circuit preventing fire, safely shutting down the system.

 

Low-energy
and ESD discharges

 

Transient-voltage-suppression
(TVS) diodes have been used for many years to protect circuits from low-voltage
ESD events below 25 Joules. However, multilayer varistors offer important
advantages over traditional TVS diodes. These include more miniature sizes and
insertion heights, more dependable performance, faster response times, and
better overall operation across wide temperature ranges. In addition, TVS
diodes can effectively be used to meet the absorption requirements in relation
to component size.

 

fapo_Epcos02_ESD_HeatProtectionLEDlightingsystems_nov2016

 

 

Fig. 2: An example of a specialized
or non-standard component use to protect against ESD events

 

 

LED
engines can consist of hundreds of LEDS which are normally series-connected
strings, parallel-connected strings or a combination of both. If one LED fails
in a series-connected string, the entire series will fail. This is because LEDs
in a series-connected string can cause an antenna effect making the array more
sensitive to ESD events. Multilayer varistors can provide protection against
such events.

 

Networks
are often reconfigured, moved, replaced and taken offline for maintenance.
Consequently, hot swapping is a common practice, and hotswapping can cause ESD
events and low-voltage spikes. Multilayer
varistors or TVS diodes that have extremely low parasitic capacitance
are preferred for the ESD protection of data lines for the control of
luminaires. Such diodes will ensure that the devices would remain fully
functional for their specified lifetimes.

 

Diodes vs. varistors

 

Diodes
and varistors are designed to meet unique design challenges for many
applications. These solutions are often ruggedized, have a much smaller
footprint, or are designed to protect against certain types of surge currents.

 

For
example, the EPCOS CeraDiode from TDK is designed to absorb high energy at a
better rate than TVS diodes and has a smaller footprint. 80% of the component
volume of this product is used to absorb the energy ESD events, which is
superior to the 30% of standard TVS diodes – and it has the same performance. Because
of this, non-standard parts should be considered if they can help minimize
space requirements and provide an identical or improved performance. This is
particularly important in applications that require miniaturization, reliable
protection and high performance.

 

Another
example is bidirectional protection against ESD and transient disturbances
provided by multilayer varistors. A typical TVS diode is inherently
unidirectional, making it necessary to design in two diodes. However, varistors
provide bidirectional protection as a single component with the same
protection. In this case, varistors will provide a size and cost advantage to
the design.

 

Other
varistors provide reliable performance at higher temperatures. A typical TVS
diode starts derating at room temperature, which is much lower than many
applications require, making it crucial to select a varistor that can operate reliably
over a very large temperature spectrum, thereby limiting failures caused by
temperature fluctuation.

 

Temperature
protection

 

In most cases, sudden failures in LED lighting
systems are caused by thermal stresses. Because LEDs require a constant current to
deliver an uninterrupted luminance, their temperature must be controlled
precisely within narrow limits. Thermistors, or thermally sensitive resistors,
are an accurate and cost- effective sensor for measuring temperature.

 

A
thermistor is a device whose electrical resistance is controlled by
temperature. There are two types of thermistors:  NTC thermistors and PTC thermistors. In NTC
thermistors, or negative temperature coefficient thermistors, resistance
decreases as temperature increases.  In PTC
thermistors, or positive temperature coefficient thermistors, resistance
increases as temperature increases.

 

fapo_Epcos03_ESD_HeatProtectionLEDlightingsystems_nov2016

 

 

Fig. 3: Temperature-compensated LED driver
without an IC

 

 

NTC
thermistors come in surface mounted design (SMD) packages, and protect the LED
arrays against overheating and help control their temperature profile at peak
lumen efficiency. This is performed by automatically adjusting the current to
the LEDs. Together with intelligent circuits, they enable an effective control
system.

 

In temperature-compensated
LED driver designs without integrated circuits, PTC thermistors can reduce the high
temperatures of the forward current by placing them in series to the LED. In
such a design, most of the LED current flows via the PTC thermistors.

 

Keeping
the lights on

As
LED technology continues to mature, design engineers can increasingly prevent
the high- and low-energy surges and ESD events that affect LED reliability by
simply determining the most suitable protection devices that are required by
the application. In addition, they can also plan for the proper temperature
protection needed in their applications, thereby keeping the lights on in a
very literal way.