Ultimately, selection of LED drivers built with higher quality components leads to longer life and better warranty.

By NEWMAN CHEN,
Product Manager,
LinkCom USA,
www.linkcom.com.tw/usa

 

Advances in
technology have resulted in LEDs with exceptional lumen output and years of
reliable operation. All LED lighting systems – regardless of the size and
complexity of the luminaire design – have one basic element in common: a DC
power source. In fact, in many cases, the “weak link” in an LED luminaire
design is the driver. Ultimately, selection of LED drivers built with higher
quality components and conservative design leads to longer life and better
warranty.

 

A typical supplier
catalog of available LED drivers shows the daunting number of variations to
choose from. This article will help you identify the key considerations for
selecting the right driver for a particular application. These are defined as
the three “Ps” of LED driver selection: power, package and performance.

 

Power

For each LED application,
is it necessary to determine the total power required as well as the optimum power
supply configuration – either constant current (CC) or constant voltage (CV). Selecting
the correct power rating for an LED driver requires the simple calculation of
the forward voltage drop across the LEDs multiplied by the forward current.
Arriving at these two numbers requires a bit of analysis.

 

Forward Voltage

The voltage
drop is based on a combination of factors, including the number of LEDs and
arrangement of the LED strings. Figure 1
depicts the three common configurations of LEDs in lighting systems:  series, parallel and series/parallel.

 

LinkCom_LED_Select_fig1

 

 

Figure 1: Common LED string configurations.

 

 

The forward
voltage of the LED array is determined by determining the combined voltage drop
across each element in the LED string. The forward voltage drop across each LED
ranges from 1.8 – 3.3 V. Blue LEDs commonly used in lighting applications are
at the high end of this range.  The total
voltage requirement is calculated as that of an equivalent series string.

 

Forward
Current

 

An essential
element of fixture design is the determination of desired lumen output. The
specifications provided by LED manufacturers will include details of lumen
output vs. current. 

 

Power
Supply Configuration

 

Constant
voltage, or CV, drivers are most commonly used when the same supply will be
used for a variety of different LED arrays. This permits the luminaire
manufacturer to minimize the inventory of drivers. The output voltage must be
sufficient to provide the maximum forward voltage required. Since the voltage
is a constant, current limiting devices are recommended in each LED string (see
Figure 2).  The general purpose of the CV driver does have
the downside of not providing optimum efficiency.

 

 

LinkCom_LED_Select_fig2

 

Figure 2: Current-limiting devices in
each LED string are recommended when using a constant voltage driver.

 

 

To obtain
optimum efficiency, a constant current, or CC, drivers with a narrow output
voltage and current provide the best answer. CC drivers will typically include
an output voltage-limiting circuit.

 

Package

 

The large number
of products found in a LED driver catalog reflects the wide variety of
packaging configurations, including system configuration and location.
Applications include architectural lighting, municipal lighting, industrial and
commercial space illumination, retail display and residential lighting.

 

System Configuration

 

A basic
question you should ask is whether the LED driver will be used to power a
single fixture consisting of multiple devices. Higher power drivers, designed
to operate multiple units would be externally mounted, possibly on its own
heatsink. 

 

In contrast,
drivers built into individual devices would need to be packaged to fit the space
available. For example, a long, narrow driver would be the ideal package for
installation a troffer replacing a ceiling-mounted fluorescent fixture. A
small, cube-shaped device might be the right choice to fit inside a track
lighting fixture.

 

System
Location

 

LED drivers are
found in a wide variety of LED applications in interior and exterior locations
with varying environmental demands. These range from interior wall sconces to
more challenging applications subject to humidity variations (refrigerators) as
well as outdoor locations with wide temperature ranges and/or exposure to
moisture (i.e., outdoor fixtures, high bay lighting).

 

LED drivers
designed for outdoor applications typically have an Ingress Protection (IP)
rating that specifies the level of protection. The IP ratings (Figure 3) are an internationally
recognized standard (IEC international standard EN 60529 (British BS EN
60529:1992, European IEC 60509:1989) and are applied to many everyday devices,
such as electronic/electric consumer goods. 

 

LinkCom_LED_Select_fig3

 

Figure 3: IP ratings for LED drivers
for outdoor use are IP-64 or higher depending on the application.

 

 

For drivers
to be used for landscape lighting and other outdoor applications with a high
probability of occasional immersion in water, an IP67 rating is essential.
Others where exposure to water would be expected but not emersion (e.g., signage),
an IP-64 rating is sufficient.

 

LED drivers
for exterior applications should also be selected based on potential
temperature extremes. Since it is not often known what type of outdoor
environment the drivers will be exposed to, a wide operating temperature range
is recommended.

 

Humidity (and
condensation) must also be taken into consideration, in outdoor as well as some
indoor applications. The solution for these applications is an LED driver where
all the components are fully potted. 

 

For indoor applications, temperature and humidity concerns also must be addressed. No just the driver, but the entire luminaire must be packaged to handle every eventuality. In commercial refrigeration lighting applications, for instance, relative humidity can approach 90%. And in completely sealed lighting fixtures, the internal ambient temperature can easily reach 60°C.
 

 

Performance

 

Selecting the
right LED driver package can have a significant impact on driver performance.
The following are the other key performance considerations.

 

Operational
performance

 

Regulation: The effects of poor regulation in an
LED power supply will be clearly visible in the light output of the LEDs.  Good quality LED drivers will provide output
regulation of ±5% or better.

 

Power Factor: Most LED drivers are based on
switching power supply topologies that, without proper design, can inject a
high degree of harmonic distortion in the ac power. EN61000-3-2 requires that
LED power supplies have a power factor of >0.9. 

 

Ripple / Flicker: The human eye will not detect the
effects of a power supply’s ripple up to 30% in the LED’s light output.
However, in applications where the lighted image is being recorded, much lower
ripple (3%) is required. 

 

LinkCom_SED_Select_Table_LoRes

 

 

Table
1. Performance trade-offs between single-stage and two-stage designs

 

 

Circuit
Protection

 

Over-voltage,
over-current, over-temperature and short-circuit protection are commonly
provided in good quality LED drivers. In addition, the best quality devices
will also provide protection against voltage surges and transients.

 

Agency
Certifications

 

All LED
drivers will require basic agency safety certifications from UL and/or CSA, as
well as FCC or equivalent EMI/RFI certification. In Japan, devices are required
to meet PSE, in Europe, TUV. UL has introduced a series of special listings to
facilitate device interchangeability, including Type TL, Type HL and Class P.

 

Performance
Enhancements

 

LED lighting systems
require significantly less power than alternative sources, but there still are
many applications where control of the light output is indicated. The two most
common are dimming and occupancy sensing.

 

Dimming: LED dimming involves both input and
output control of the driver. On the input side, two types are common – triac
and dc control. On the output, analog control and pulse width modulation (PWM)
techniques are available. Triac dimming input control would be indicated, for
example, when an incandescent fixture is retrofitted with an LED fixture in an
existing circuit. Otherwise, DC control, a linear, 10-V input, is applied as a
control signal. Analog control of the output adjusts the output of a CC driver
over the dimmable range. Most provide control down to 10% output using a 1 – 10-V
control signal. Some dimming outputs move closer to full range dimming but the
practical limit before cut-off is around 8%. Using a constant voltage (CV)
driver, PWM control allows for dimming over a similar range.

 

Occupancy sensing: Occupancy sensing is required in a
many commercial installations, prescribed by California Title 24 and a growing
number of other standards.  This functionality
is being incorporated into the latest versions of LED drivers. Some are
providing an auxiliary output voltage to power the occupancy sensor while
others are being integrated into a complete building management system. Others
are being fitted with wireless control. 

 

Remember
to cross your Ts

 

The
proliferation of LED drivers makes it all the more important to evaluate an LED
driver supplier in the same way one would evaluate the power supply for any
applications where reliability was a priority. During the engineering design
phase, high-reliability drivers utilize conservative design principals, so pay
particular attention to thermal design, EMI performance and efficiency. 

 

After
selecting your driver based on the three P’s (power, package, performance),
make sure you “cross your Ts” – by subjecting the driver to all the necessary
testing. This includes design verification testing (DVT), pre-production pilot
run testing (PVT), environmental testing for sustainable performance in extreme
temperature, and humidity and mechanical testing – including vibration, drop
and force tests to confirm product will not sustain damage during shipment.
Drivers should also undergo volume production, on-going reliability testing
(ORT); accelerated life tests (ALT); destructive physical analysis (DPA); and
mean time to failure (MTBF) analysis. 

 

 Originally published on Power Electronics News.