FOCUS
by
Geoff Archenhold
94 TECHNOLOGY
LED
Cree as well as the typical “cold” in terms of raw lumen output but that is reliable for more than 1 series of LEDs on one or more
luminous flux at 25oC and the with improved lumens per watt million hours in standard operat- output channels using a variety
typical “hot” luminous flux at efficiency as shown in table 2. ing conditions but if the thermal of control techniques to obtain a
60oC measure at the board Reliability of LED systems. management components of desired lighting effect as shown
level. The launch of new standards the LED system are poor it could in Figure 2. In other words it is
such as LM-79-08 entitled only last for a few hundred hours more than just an LED driver
“Electrical and Photometric despite having a great LED! Integrated Circuit.
Incredibly, the “hot” lumen Measurements of Solid-State Therefore, in order to determine The inside of a typical LED driver
output is only slightly poorer Lighting Products” enables the LED systems reliability the system looks very similar to that shown
than the “cold” lumen output lumen depreciation or lumen designer must consider the in Figure 3 with high voltage
eg; approximately 10% less. This maintenance of LEDs to be possible failure modes for each components such as transform-
is unusual as most LED systems measured providing a measure component. ers, capacitors, MOSFETs and
suffer ~30% thermal degrada- of LED reliability. The first work It is generally known that one of inductors.
Table 1: Bridgelux LED Array flux characteristics showing
the weakest parts of an LED sys- There are many types of LED
minimum “cold” lumen output as well the Typical “Hot” tem is the LED driver due to the
IC’s that can be used within the
lumen output for each product. number and types of components LED control system and many of
they contains and so the this ar- them use different power circuit
tion from “cold” lumen data but on LED reliability was undertaken ticle will deal with discussing LED topologies mainly dependant
the Bridgelux team state that by the ASSIST programme at the drivers and their reliability. upon the input and output volt-
they have improved the perfor- Lighting Research Centre in New age requirements of the LED
mance of LED arrays through the York State which proposed a What do we mean by an LED drivers as shown in Figure 4.
introduction of their proprietary lumen maintenance of 70%, cor- driver? Table 3 outlines the advantages
Metal Bond Technology helping responding to a 30% reduction The main issue with defining and disadvantages for each type
to reduce the thermal resistance in initial light output, as the end problems is that of what lan- of driving topology however for
of their LED arrays with figures of useful life for general lighting guage or terminology to start applications that use more than
down to just 0.5°C/W (this which has now been adopted by off with and LED drivers are no two LEDs, and/or there is a large
compares to ~9°C/W for most the relevant standards. exception as a driver means dif- difference between the input
LED emitters) for the 2000 lumen Unfortunately an LED system has ferent things to different people. and output voltages a switch-
arrays. The second advantage many components that need to The definition of LED driver I ing topology is more appropriate
of the arrays is that the system work together in order to enable will refer to in this article is that to use and is a more efficient
reliability and costs are reduced light to be emitted from the LED, which describes a complete LED way of handling varying system
because the LED arrays can be these include: driver subsystem enabling a user circumstances.
attached directly to the heatsink to attach high voltage AC mains Two topologies offer a simple
and so there is one less thermal 1. The LED emitter in and is capable of driving a implementation: the buck topol-
interface required compared to 2. An LED driver (not required if
the current LED arrays created using AC LEDs)
from more than one LED emitter. 3. Mechanical and thermal
The component placement costs management components for
and the MCPCB board are also long operation
taken away from the Bridgelux
system making the products Even if today’s LED emitters can
more cost effective. operate beyond 100,000 hours
This technology improvement the total LED systems reliability
enables the Bridgelux fam- is dependant upon the weakest
ily to compete against a raft of component within the system. For
Figure 2: Simple block diagram of an LED Driver system
conventional lamp technologies example, you could have an LED
ogy and the boost topology. Both
offer high efficiency over a wide
range of electrical parameters.
The two topologies achieve their
effect in different ways, and these
differences affect the efficiency
with which they operate. In the
buck topology, the LED is placed
in series with an inductor. This
means that only a fraction of the
energy has to pass through the
Table 2: Bridgelux arrays now compare favourably with conventional lighting technologies
inductor, the component which
in terms of raw lumen output.
contributes a significant part of
the losses. In this topology, the
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