Rectification of AC currents into DC for driving LEDs

As we have mentioned the quantity of light produced by the LED light bulb is proportional to the amount of current flowing through the circuit. Unfortunately, for LEDs, the world’s power supplies provide alternating currents (AC), which means the current follows a sine wave where it goes up and then it goes down. For the LED light bulb this means it would turn on and off in sync with the AC current or your LED light bulb would constantly flicker. We know of only one LED manufacturer Lynk Labs which has developed an AC LED package known as Tesla. The Tesla AC LED is yet to really make it into the market place but we will be keeping an eye on it and offering it to our customers as supply increases and the cost comes down.

To resolve the problem of AC currents the LED light bulb requires some relatively simple (internal or external) electronic circuitry referred to as the constant voltage or constant current power supply or LED driver. The LED driver or power supply circuitry consists of transformers, transistors and capacitors to smooth out the AC current into DC current so that the LED light bulb generates a constant light output.

LED Driver or Power Supply - Internal (Residential) or External (Commercial)

The driver is typically internal to the LED light bulb, usually at the base of the bulb, where it has the potential to create more heat as it is not easily dissipated. This is not usually an issue in residential applications given adequate power supply design and good choice of componentry. Alternatively, in commercial lighting applications or for those LED light bulbs exposed to extended operating hours, the LED driver can be mounted external to the LED light bulb where heat is more freely dissipated and does not add to the heat generated within the LED light bulb itself.

This conversion from AC to DC is relatively efficient, typically 85-90%, but can produce quite a lot of heat depending on the power rating of the device and the efficiency, i.e., if the LED light absorbs more current then the power supply will produce more heat. Because of this reason, MR16 LED light bulbs with built-in power supplies or AC/DC converters that are designed to run on the traditional 12V AC halogen downlight transformer will run cooler if they are coupled with a constant voltage DC power supply. This occurs as the MR16 LED light bulb no longer has to convert the AC current to DC and therefore the built in driver does less work. This is the scenario that most of our residential lighting users have whereby they purchase a 12V MR16 LED bulb and use it in place of their pre-existing dichroic halogen downlight bulbs. Other residential customers use our GU10 LED downlight bulbs which again have an integrated power supply.

Electrolytic and Polyester Metal Film Capacitors in LED Drivers

The main issue with power supply temperatures is the use of electrolytic capacitors in the LED driver or power supply. The electrolytic capacitors are very sensitive to heat such that every 10°C increase in temperature leads to a 50% decrease in the life of the capacitor, such that the life may be reduced from 40,000 hours to 20,000 hours, and so on. To combat this problem, most manufacturers will use long life electrolytic capacitors rated at 2,000 to 8,000 hours or 4,000 to 10,000 hours at 105°C. In extreme conditions we have designs that utilise long life capacitors rated at 125°C.

Fortunately, it is rare for LED light bulb driver temperatures to reach 105°C, unless the LED light has been poorly designed and in some earlier models. In our designs and selected LED lighting our test results show that we typically measure driver temperatures in the range of 40°C to 80°C (at approximately 25°C ambient temperature) depending on configuration and power rating. We consider anywhere from 40°C to 70°C driver or capacitor temperature to be acceptable which provides an estimated capacitor/driver life of approximately 30,000 to 100,000 hours with longer life spans associated with cooler temperatures. Certainly in very hot environments such as a roof space in the summer time which can reach temperatures of 60°C and if  LED light is undergoing prolonged use then we can consider LED driver temperatures will reach over 100°C and more careful thermal engineering must be considered. 

So at a driver temperature of 70 to 80°C, which has the potential to reach 90 to 100°C, with an increase in ambient temperature of 20°C. We would consider such LED drivers unsuitable for commercial applications or those LED light bulbs exposed to extended operating hours. We would generally be wanting to measure baseline driver temperatures for such applications as being between 40°C to 50°C to ensure longegivity in the most extreme environments. 

Therefore, where possible it is important to have some data or knowledge of the LED light bulb driver temperature and the temperature of the environment you wish to use it in. At Lite On LED, where possible we ensure all our LED light bulbs utilise long life capacitors rated at 105°C, and that after extended operating hours (8 hours or more) have driver temperatures of less than 60°C, so that we can ensure the longevity of the LED light power supply and thereby the LED light bulb.

As mentioned another way to increase the life time of the LED light bulb is to use an external power supply, typically a constant voltage or constant current power supply, which is coupled with a LED light bulb which contains no other electronics. As mentioned above, even when coupled with an MR16 LED light bulb, which already contains an internal driver, there will still be a cooling effect.

Selecting the best external power supply

In selecting your external power supply you should always look for a power supply which is rated above that power consumed by the LED light bulb. This will enable the power supply to do less work than what it is rated, thereby reducing the temperature in the power supply and extending the power supply life, in addition to the life of the LED light bulb.

For example, in our testing of the LITE-P15-12V 15W constant voltage driver, if we run the device at near full capacity, ~ 12W, then the capacitor temperature reaches 60°C. In contrast, if we run the device at ~6W, then the capacitor temperature drops to 40°C, thereby significantly extending the life of the capacitor and power supply. This reduction in temperature becomes critical where the LED light bulb is exposed to extended operating hours and potentially harsh environments where ambient temperatures may increase by 20-30°C in a roof or ceiling space or enclosure. However, in most residential applications, the temperature issue is not as crucial.

Shelf Life of Electrolytic Capacitors

Some customers also complain about the shelf life of electrolytic capacitors given figures of around 3 to 5 years, sometimes more. However, with regular use and charging the capacitor undergoes a “self-healing” process so the shelf life parameters are not really relevant unless the LED bulb with an internal driver sits in the cupboard for years. The self healing is a bit like recharging the battery in your car. If you don’t drive your car and let your battery go flat it won’t last as long as if you use it regularly and keep it charged.

As mentioned long life electrolytics are theoretically rated up to 30,000 to 40,000 hours or more at normal operating temperatures so a 20 years + lifespan for residential use is quite feasible given the environment stays cool and quality components are used. Many internal driver applications for residential use also use metallized polyester film capacitors due to the high voltage which are again more tolerant than the electrolytic and are also better at self-healing and depending on model last longer with some specifications of > 300,000 hours. Electrolytic capacitors also permit some level of self-healing such that if they are not used their shelf life is only approximately 5 years. Therefore, running your electronics or LED light intermittently will help it to last longer than if you left it in the cupboard for many years. 

At Lite On LED we carefully select the capacitor types used in our LED lights and test all of our LED lighting designs to ensure that they offer optimal and long lasting performance for your residential or commercial lighting application.

References:

US DOE (2010). Demonstration Assessment of Light-Emitting Diode (LED) Retrofit Lamps. Host Site: InterContinental Hotel, San Francisco, California. Final Report prepared in support of the U.S. DOE Solid-State Lighting Technology Demonstration GATEWAY Program.

April 6, 2013: Ensure long lifetimes from electrolytic capacitors: A case study in LED light bulbs. Mark Fortunato. EDN Network

NIC Components Corp. Extended Lifetime Aluminium Electrolytic Capacitors.

Determination of the shelf life of aluminum electrolytic capacitors. Edward McFaddin, 2002. University of North Texas

Determination of the shelf life of aluminum electrolytic capacitors. Arne Albertsen, Jianghai Europe GmbH

Metallized Polyester Film Capacitors MKT1817 Data Sheet Vishay Roederstein