By NEWMAN CHEN,
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 “P’s” of LED driver selection: power, package and performance.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Selecting the right LED driver package can have a significant impact on driver performance. The following are the other key performance considerations.
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.
Table 1. Performance trade-offs between single-stage and two-stage designs
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.
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.
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 T’s
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 T’s” – 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.