By Maggie Nadjmi, SL Power Electronics
There are two main value propositions for LEDs over traditional light sources; long life expectancy, which saves money on replacement costs, and higher light output for less energy input, to lower electricity bills.
Since an LED is a solid-state component, it must be powered and controlled by an electronic circuit if it is to emit light. Mains-powered lighting therefore requires a driver that performs AC-DC conversion. When mounted in a light fixture, it may be subject to temperatures well over 50°C.
High operating temperatures put stress on components such as electrolytic capacitors found in a driver. If the driver is not designed to handle such stress, it could reduce the lifespan of the fixture, thus undermining one of the two main reasons for a customer to buy it.
This makes it essential to select carefully the power supply to be used in an LED driver. Crucial questions to ask are:
- Does the power supply operate at high power efficiency?
- How well does the power supply cope with high temperatures?
- Are the capacitors in the power supply high-temperature electrolytic types, rated for >5,000 hours of life at 105°C?
The answers to these questions will strongly affect the LED fixture’s ability to offer customers a minimum life expectancy of 50,000 hours.
The question of efficiency in LED lighting is in fact subject to official regulation. In the US, certified residential and commercial lighting fixtures have been able to carry the Energy Star® mark for a number of years; since 2009, specific requirements, updated in 2011, were added for LED lighting products. Not all LED light fixtures can be part of the Energy Star programme: entertainment lighting is excluded from the list of product types that can bear the Energy Star logo.
When designing for Energy Star compliance, lighting OEMs must pay attention to the fixture’s lumens-per-Watt figure. Good performance here will be the result of a combination of a well-designed optical source and a highly efficient driver.
A minimum lifespan of 35,000 hours should also be the target: this requires both the LED and the driver to be of the highest quality and to work reliably throughout this period. These characteristics ensure that the end customer does not have to worry about replacing the fixture from one year to the next.
A lot of the requirements for Energy Star compliance are concerned with the driver. They include:
- A minimum power factor of 0.7 for residential and 0.9 for commercial lighting products
- Transient protection circuitry rated for 2.5kV of input surge based on the ANSI/IEEE C62.41 standard
- Support for an operating temperature of -20°C or lower
- Less than 1s delay from the time the AC input voltage is applied until the light comes on
- No-light stand-by power consumption of less than 0.5W. This value may be increased to 1W if the fixture has intelligence built into it.
If a driver is to offer a power factor of 0.9, the driver must include a Power Factor Correction (PFC) stage, which requires additional components and a more complex design. Inexpensive drivers might claim to meet the power-factor requirements at full power; they might often remain silent about the unit’s PFC performance at lighter loads, when the LEDs are dimmed. In fact, cheap drivers with low-quality PFC circuits will have a much lower power factor than 0.9 at loads below 100%, and this will disqualify them from Energy Star compliance.
In the same way, operation at -20°C is a challenging requirement for the power-system designer; not all commercially available power supplies can achieve such extreme performance.
So careful selection of the LED driver is a crucial step if a lighting OEM is to obtain an Energy Star certification. By choosing a high-quality product from a reputable supplier, such as SL Power’s LB115, shown in Figure 1, the design team can avoid costly extensions of their time to market, and gain the assurance of offering long-lasting LED lighting fixtures suitable for a rapidly changing market.