LED lighting has revolutionized the lighting industry in terms of electrical efficiency and maintenance with the elimination of lamp replacements. The one weakness LED systems have vs. lamps, is that their driver circuitry is more complex, and drivers are more sensitive to environmental stresses like heat. Most LED drivers will have a rated life, but these numbers can be very misleading. The life expectancy of LED fixtures, and particularly the drivers, can vary widely based on the conditions they operate in.
LED Diodes are Not the Problem
These failures are not likely to be due to the LED diodes themselves, which have proven very reliable. All commercial grade LEDs made today have a published L90 rating. L90 refers to the rate of degradation the LEDs experience at a specified operating temperature. If a fixture has an L90 of 50,000 hours, that means that after 50,000 hours of operation, the light output of the LEDs will have dropped by a maximum of 10%. The L90 of current LED diodes range from 50,000 hours on the low side to as high as 100,000 hours for higher-end LEDs like the Osram diodes we feature in all our fixtures at California Lightworks.
LED Drivers are The Problem
However, the L90 does not tell you the life expectancy of the LED driver. This is where most failures in LED grow-light fixtures occur. Just recently, I saw an LED company publish an expected life rating on their drivers of “50,000 hours”. However, unlike the L90 rating on LED diodes which tells you when the light output has dropped by 10%, the MTBF (Meantime Between Failures) driver tells you when 50% of the drivers will have failed! But these life-cycle tests on drivers are extremely dependent on temperature. I saw one light that gave a life-cycle rating of “50,000 hours @ 70F”. Again, if no temperature is given, 25C (77F) is the likely temp it was tested at. This is fine for most commercial lighting, but NOT for Horticultural lighting. The temperature that grow-lights operate at can be 80 degrees for a vertical fixture two feet from the canopy, or 95 degrees up by the high ceiling of a big indoor grow room, or 120 degrees in the roof of a greenhouse in the direct sun. The life span of an LED driver varies inverse-exponentially with its operating temperature.
Electrolytic Capacitors are the Achilles Heel
All LED driver circuits have a kind of Achilles heel, namely, the electrolytic capacitor. Electrolytic capacitors are power storage devices used for all switch-mode power-supply designs such as LED drivers, as well as computer power supplies, etc. Unfortunately, there is no other type of capacitor that can be substituted in the application that is not dramatically more expensive– like 10-20 times as expensive! So, every driver must use electrolytic capacitors in their design, and electrolytics have a unique aspect to their life expectancy that is quite different from all the other components in the driver circuit.
Most Electronic Components Can Manage High Heat
All the semi-conductor components, like SCRs, transistors, resistors, diodes, even LEDs, have a maximum operating temperature threshold of the component itself, usually around 140C, (284F) and if they stay under that temp, they will last a very long time, easily 50,000 hours or much more. So, whether they are located up on the roof, or in the sun, or on a wall, these components’ life expectancy is quite long and predictable if their internal heat is managed properly. Most LED lights will publish a max operating temperature of 100-110F, but this is pretty loose, since LEDs at the roof of greenhouses routinely experience hotter temperatures than that. Most drivers will also have an internal high-temp override, which turns the driver off if the internal semiconductors get too hot.
Heat Determines Capacitor Failure Rate
But electrolytic capacitors are quite different. The best quality capacitors may have a rating of 50,000 hours at a 70-degree lab-bench operating temperature. However, a 10 degree C increase in operating temperature results in a 50% decrease in the rated lifetime of the electrolytic capacitor! Yes, 50% inverse exponential. That is why those drivers are rated at low temperatures, it is because of the capacitors. That gives the manufacturer an out when the driver does not last 50,000 hours. But there is another problem — the air temperature in a grow room is not the same temperature that the capacitor sees, because it is located inside a sealed enclosure along with other circuitry that is creating heat, normally around 10% of the total power supplied to the fixture. So, a 1000w fixture has 100watts of heat continuously generated by the driver components inside the driver enclosure, and that heat must be continuously removed, or the driver temperature will just keep rising. And the hotter the exterior temperature, the slower the heat is removed.
Location of the Driver is Critical to Preventing Failures
If the driver is integrated into LED fixture, then it is subjected to the heat being produced by the LEDs surrounding the driver in addition to the heat generated from the driver circuitry. A perfect example is certain “Greenhouse” fixtures which sandwich the driver enclosure in between the LED heatsink, and the sun. Not a solid design choice from a heat management point of view. Most LED fixtures today have moved away from fan cooling, which can help remove that heat more efficiently, and instead they rely on passively cooled fixtures and driver enclosures. So, the temperature of the capacitor is going to be much more than room temperature because it is sealed in a small enclosure with 10% of the total power being dissipated inside, even if it is potted with resin.
Grow Room Temperatures Increase the Heat Problem
But now for the real variable in this system– grow room temperature. If the fixture is a vertical fixture and located two feet above the canopy, the temperature outside the driver enclosure is roughly the room temp or ~80 degrees. But if the fixture is located up high in the roof of a greenhouse as mentioned earlier, the air temperature up there could reach over 110 degrees depending on the ventilation. The sun imparts even more heat directly into the driver enclosure through direct infra-red exposure, increasing the driver temperature even more. A driver in direct sun is often too hot to touch. So, if the air temp up high in the greenhouse is 110 degrees, with the sun pounding on it, the temperature the capacitor sees inside the enclosure could easily be 140-180 degrees or more. Fortunately, this is not enough to make the capacitor, or the semiconductors fail, only degrade. But the life expectancy vs temperature curve of electrolytic capacitors is vastly different than the semi-conductor components. As we explained, the electrolytic’s life is inverse exponentially correlated to the operating temperature. So, if that capacitor is rated for 50,000 hours life at 70F, it will last a fraction of that time.
Currently, we are seeing this problem in action. Since the first wave of LED acceptance started 7-8 years ago, we are seeing many of these early LED fixtures beginning to fail. It is not the LED diodes, or semiconductors that are failing, it is those pesky electrolytic capacitors.
Transients and Power Surges
The other major reason drivers fail is due to power surges or what are called transients on the power lines. This is especially problematic in greenhouses where generators and lighting can drive huge power surges that can destroy sensitive components in LED drivers. Normal protective circuits in LED fixtures are not adequate to protect against this threat and this is a major source of LED fixture failure.
The Solution is Remote Centralized Drivers
How do we fix this problem since there is no realistic substitute for the electrolytic capacitor? The most direct answer is centralized drivers. By removing the sensitive driver circuitry from the fixtures located up in the higher temperatures at the roof, (and especially in the direct sun,) and instead mounting the driver (a 10K watt centralized driver in the case of our Megadrive,) on the wall of the grow space or even a cooler adjoining hallway, the operating temperatures of those capacitors can be dramatically reduced. In the case of our Megadrive system, we have also added active fan cooling to better remove the heat of the driver’s dissipation.
What can we expect in terms of driver life improvement from our Megadrive versus an integrated driver in an LED fixture up high in a greenhouse? In the example above, the improvement was twice the projected life expectancy, and that is what you can expect to see in a greenhouse. For indoor spaces, it depends on many factors, but certainly can reach 50% more life.
In addition to mitigating heat, remote drivers can also include industrial level surge protection which will protect the entire group of fixtures from voltage spikes or irregular power sources.
In summary, the most direct way to reduce LED grow light failures and significantly extend fixture life expectancy is to employ a remote driver solution such as the MegaDrive system from California Lightworks. This system not only slashes installation costs but removes sensitive electronics in the LED drivers from the extreme heat of the grow room or greenhouse environment. This is the surest method of extending LED grow light fixture lifespans to match the long life spans of today’s LED diodes.
