LED Power Supply, 12V and 24V LED Transformer

The major factors to consider when choosing LED power supply for your LED lighting projects

First, the application environment.

• Is it for indoor or outdoor application?
• Does the LED power supply require waterproof or specific IP level?
• Will the LED power supply use conduction cooling or convection cooling?

Answers to these questions can tell the application environment of the LED power supply. The environment determines different types of LED power supplies that are suitable for the requirements of the environment. For example, if you install waterproof LED strip lights in outdoor application or in wet or humid areas, you should choose a waterproof LED power supply with IP rating 65, or 67 better.

The IP rating for LED power supply has two digit codes that indicates the ingress protection(IP) level. It is used to indicate the sealing effectiveness of the power supply enclosures. The more effective the sealing is, the better the enclosures protect against intrusion from solid particles (tools or dust etc) and moisture. For example, IP 65. The first digit 6 means it is dust tight. The second digit 5 means it can resist water jets of 30 kPa pressure at distance of 3 meters. The first digit ranges from 0 to 6. The second is from 0 to 9. The higher the better.

Another environment factor is the temperature. LED power supply operates at the best efficiency withint a certern range of temperature. Power supply generates heat when it is running. Heat built up around the power supply will cause its effiency to decrease. In the worst case, it will cause failure of LED power supply if overheated for an extended period of time. Providing good ventilation for power supply by using heat sink or fans is the best practise. If not availabe, at least make sure don't put LED power supply in a too tiny enclosure.

Second, the input and output voltage of LED power supply.
Most of the LED power supplies have a range of input voltage. This range should include the electricty voltage that is available for your lighting installation. Most residential and commercial properties have 120V power. While industrial places mostly have 277V power. Read the power supply's input voltage range values and make sure either 120V or 277V for your installation falls into that range. For example, an LED power supply with the input voltage range of 100-240V can not be use for the industrial place with 277V power, but it is good for homes with 120V power.

Another important factor is the output voltage of the power supply. The output voltage should exactly match the input voltage of your LED lights. Most LED strip lights operate on 12V or 24V DC. So we choose 12V LED power supply for 12V LED strip lights, 24V LED power supply for 24V LED strip lights.

Third, the wattage needed.
The wattage of the LED power supply should be at least 1.2 times of the total wattage of the LED lights that are going to be hooked to the power supply. The 20 percent more wattage is the cushion used to protect power supply from surge or other incident factors.

For example, you want to use one LED power supply to provide power for five reels of LED strip lights. Each reel of LED strip has a power of 50Watts. What wattage of power supply do you need? Five reels LED strips have total power of 250W. Giving it a 20 percent cushion, it is 250W x 1.2 = 300W. So you need to buy an LED power supply with at least 300 Watts capacity. If the power supply has 400Watts, you still can use it for your 250W project with no harm, but it is more expensive than the 300W power supply.

Fourth, dimmable or non dimmable LED power supply
Often times users want to be able to use a dimmer adjust the brightness level of the LED lights. There are diffent types of LED dimmers, which determine if you need a dimmable or non dimmable LED power supply. PWM dimmer is placed in between power supply and LED lights. It can dim the lights directly. So PWM dimmer requires a non dimmable LED power supply. TRIAC dimmer can not dim the LED lights directly, it regulates the AC side input of the power supply, which connects directly to LED lights such as LED light strip. So the use of this type of dimmer requires a TRIAC dimmable power supply.

Fifth, constant voltage or constant current?
LED power supplies have different types of output: constant voltage or constant current. Constant voltage power supply provides a fixed output voltage with a maximum output current. LED strip lights operate with constant voltage input. So they require constant voltage, typically 12V DC or 24V DC LED power supply. The constant voltage power supply transforms 120-277 VAC to a low voltage direct current. Make sure the circuit of the LED lights does not draw more current than the maxium output current. Otherwise, these LED power supplies will not able to work or maintain a constant voltage.

Certain LED lights require an LED power supply with constant current output. Constant current LED power supply provide a fixed output current that is typically indicated in amps. The voltage output is in a range and will be variable according to the number of LEDs that are used. Make sure the voltage range covers the required voltage of the LED light circuit. As the power supply provides constant current, the LED lights will maintain a constant brightness level. So some commercial LED signs and LED displays use constant current power supply.

Sixth, UL listed or other safety certifications
UL listed LED power supply has better safety protection that those not UL listed. UL stands for Underwriters' Laboratories. UL tests and evaluates components and products and allows those passed the test standards to use certification mark. UL evaluations help ensure the safe use of these products when applied in industrial, commercial and residential applications. UL listed power supplies are requirement for many new and renovation projects. So the UL recognition will help win approval from cities and users. There are also class 2 and class II standards for power supplies, these are also factors that help improve the safety of use and installation.

A quality LED power supply can ensure extended and steady operation with high efficiency for the whole LED lighting system. It plays a vital role in prolonging lifetime of light fixture, saving energy, and lowering maintenance costs.

Due the energy-saving, flexible in design, and long lifetime, LEDs have been fast replacing incandescent and fluorescent lights in indoor and outdoor applications. In order to make full use of the high efficacy and long lifetime of the solid-state lighting devices, you need choose proper power supplies that provide appropriate power for your LEDs. LED Power supply knowledge is further explained in details below to help you make wise decision on selection and purchase.

1. What are LED power supplies?

LED power supplies are power transformers that convert power to designated voltage or current that are suitable for the applied LED products. In general, the input of LED power supplies includes high voltage AC, low voltage DC, high voltage DC, low voltage high frequency AC, etc.

In terms of current output, some LED power supplies are constant current sources that change voltage as the LED forward voltage changes. The core components of the LED power supply include a switch controller, an inductor, a switching component, a feedback resistor, an input filter component, an output filter component, and the like.

According to the requirements of different occasions, there are also an input overvoltage protection circuit, under-voltage protection circuit, LED open circuit protection, over-current protection circuit and so on.

However, LED strip circuits are designed to place LEDs in string, which controls the current flowing through the LEDs with resistors or other current control components. So most LED strips require a constant voltage power supply.

LED power supply was the term that has been used to refer an AC-DC power source that provides a constant voltage output, while LED driver was the term referring to an LED power source that provides a constant current output. Now, the terms LED driver and LED power supply are more and more used interchangeably.

2. What is a quality LED power supply?

2-1. High reliability.
Being able to operate stably for a long time is the basic requirement for lighting products. In particular, the driving power of the LED street lamp is installed at a high altitude, the maintenance is inconvenient, the maintenance cost is also large, and the high reliability can greatly reduce the maintenance cost.

2-2. High efficiency.
LEDs are energy-efficient products that require high efficiency in power supplies. The higher the efficiency of the power supply, the lower its power consumption.

Why do LED power supplies require high efficiency?
High efficiency is not only the overall energy saving requirement of LED lighting systems, but also it is the basis and guarantee of LED transformer for long life and high reliability.

• High efficiency, low loss, low temperature rise, high reliability
  
  For example, if a 110W LED transformer is used, the loss is 5.2W at 95% efficiency and 17.6W at 85% efficiency. The latter is 3.4 times that of the former. Experiments have shown that under the same conditions, the former is 50 to 59 degrees Fahrenheit lower than the latter.
  
  As the power supply temperature decreases, the operating environment of the IC and power semiconductors improves, and the MTBF (mean time between failures) will increase significantly.


• Reduce the operating temperature of the LEDs, delay the light decay, and extend the LEDs lifetime.
  
  For the LED power supply placed in the luminaire, the temperature rise of the power supply directly leads to the temperature rise of the LED luminaire. An increase in the temperature of the LED chip will result in a change in the performance of the light emitting device and an attenuation in the electro-optical conversion efficiency. In severe cases, LEDs may even stop working. Experimental tests have shown that for every 41 degrees Fahrenheit increase in temperature of the LED itself, the luminous flux is reduced by 3%.
  
  LED life and temperature relationship. Heat is the number one killer of LEDs. LEDs operating at high temperatures have a much shorter life expectancy.

2-3. High power factor.

What is PF? Why high PF? What is PFC?
PF is an abbreviation for power factor, which is the ratio of active power(the power does actual work) to apparent power(the power flows to the circuits). PF is only used for AC circuits. There is no PF for DC circuits. PFC is an abbreviation for power factor corrector.

Reactive power is the power consumed by inductors and capacitors in the circuits. A power system requires both active power and reactive power to work properly.

The power factor is the grid's load requirements. The high power factor will greatly reduce the load of the power supply line. For example, in the city at night, the amount of illumination is large, and the similar load is concentrated. The LED power supply with high power factor can reduce the load of the power supply line, reduce the loss, and ensure the stable operation of the lighting products.

Low power factor means low power efficiency. The lower the power factor, the higher the proportion of power consumed in the distribution network. If the low power factor is not corrected, it would require higher reactive power, and the utility facilities must provide much more than the active power.

Higher reactive power results in the need for larger generators, transformers, and transmission lines to compensate for losses. PFC-enabled LED transformers can improve their energy use, reduce losses and reduce harmonics in the grid.

2-4. Driving method.
There are two driving methods currently available for LED transformer. One is a constant voltage source for multiple constant current sources, and each constant current source supplies power to a segment of LEDs separately. In this way, the combination is flexible, one segment of LEDs failure does not affect the operation of other LED channels.

The other is the direct driving method for the constant current supply and the LEDs are operated in series or in parallel. This method has the advantage of lower cost but offset by poor flexibility. It needs to solve the problem of one segment of LEDs failure without affecting the operation of other LEDs.

These two methods coexist. The multiple constant current output method is superior in terms of performance, and more in line with actual needs. It may be the mainstream direction in the future.

2-5. Surge protection.
LED transformer needs to provide surge protection because the ability of LEDs to withstand surges is relatively poor, especially the capacity against reverse voltage. Outdoor LED products, such as LED street lights, due to the start of the grid load and the detection of lightning strikes, various surges come into the grid system.

Some surges will cause LED damage, so that the power supply and lamps need to be frequently replaced. Therefore, good surge protection is also a must for an excellent LED driver.

2-6. Protection function.
LED power supply should also have a series of protection functions, such as overheat protection, short circuit protection, over current and over voltage protection, etc., to ensure that LED lighting products operate safely and stably under various emergencies.

3. How to assure the quality and reliability of LED power supply?

3-1. Standardized professional design process, engineering prototype, small batch trial production.

3-2. Reasonable design margin for components.

3-3. Select high-quality components such as electrolytic capacitors, ICs, power semiconductors, magnetic components, etc.

3-4. Strict quality control and test procedures to perform stress analysis, EVT, SVT, DVT, MTBF, and other experimental verification.

For LED strip light installation, please see following readings that are very helpful：
Use LED aluminum profiles for LED strip lights.
Buying the right LED controllers for LED strips.
LED strip light connectors help installation.