LED ceiling lamp and its power supply (IV)

1. Dimming of LED ceiling lights

At present, many well-known LED constant current driver chip companies in the world have spent a lot of effort to develop many chips that can be used with various so-called Triacs for dimming. However, this is also a rather sad and ridiculous thing. Because the thyristor is a product of the 1960s, it is itself a rather old and backward device. It can indeed be used to dim with incandescent lamps, but it will destroy the waveform of the sine wave during the dimming process, thereby causing the power factor of the system to decrease, and it will also generate a large interference signal on the line. When dimming an incandescent lamp, because the brightness of the incandescent lamp is only determined by the effective value of the power supply voltage, it can be dimmed according to the conduction angle of the thyristor, and for the thyristor, the incandescent lamp is an ideal pure resistance load, and it will not have any effect on its work.

But after switching to LED, a series of problems arise. First, LED with rectifier is a capacitive load, which has a great impact on thyristor. It will be unstable at low load unless a resistor is connected in parallel. But it will further reduce the efficiency of the system (increase 1-2W power consumption). In order to make LED also cooperate with thyristor dimming, the power factor of the whole constant current power supply system with rectifier must be increased to look close to pure resistance load. Therefore, many companies have developed active power factor correction chips. The power factor of the whole LED system reaches above 0.9. Many people mistakenly believe that after adopting power factor correction, the power factor of the whole system including thyristor can reach above 0.9. This is a complete misunderstanding. Even if the pure resistance load is connected to thyristor, the power factor will decrease with dimming.

Below is the change in power factor of the entire system (including thyristors) of an LED bulb with power factor correction (up to 0.96) during thyristor dimming (data for incandescent lamps are also included for comparison).

As can be seen from the table, whether it is a power factor corrected LED lamp or an incandescent lamp, the power factor can reach above 0.96 at the beginning. However, as the thyristor dims, its power factor gradually decreases, and when it can no longer be dimmed, the power factor is as low as 0.48 and 0.566. Therefore, as for the entire system, its power factor index does not meet the requirements of the US Energy Star.

There are dozens or even hundreds of thyristor dimmers in the world. Many LED lamps have gone through countless tests and improvements to make them compatible with these thyristors, but in the end, it was a thankless task. Since labor costs are very high abroad, this can be considered a last resort, but in China, other more advanced practices can be adopted.

There are many ways to dim LEDs, and none of these methods have the disadvantages of thyristors. Here are some of the most commonly used methods:

6.1 Using pulse width modulation PWM dimming

LED is a diode, which can realize fast switching. Its switching speed can be as high as microseconds. It is unmatched by any other light-emitting device. Therefore, as long as the power supply is changed to a switching constant current source, its brightness can be changed by changing the pulse width. This method is called pulse width modulation (PWM) dimming method. Figure 15 shows the waveform of this pulse width modulation. If the pulse period is tpwm and the pulse width is ton, then its working ratio D (or aperture ratio) is ton/tpwm. Changing the working ratio of the constant current source pulse can change the brightness of the LED.

The specific method to implement PWM dimming is to connect a MOS switch tube in series in the LED load (Figure 16), and the anode of this LED string is powered by a constant current source.

Then a PWM signal is added to the gate of the MOS tube to quickly switch the string of LEDs. This achieves dimming. There are also many constant current chips that have a PWM interface that can directly receive PWM signals and then output to control the MOS switch tube. So what are the advantages and disadvantages of this PWM dimming method?

1. No color spectrum shift will be produced. Because the LED always works between full amplitude current and 0. 2. High dimming accuracy can be achieved. Because the pulse waveform can be controlled to a very high accuracy, it is easy to achieve an accuracy of one ten-thousandth.

3. It can be combined with digital control technology for control, because any digital signal can be easily converted into a PWM signal.

4. Even if the dimming is within a wide range, flickering will not occur. This is because the working conditions of the constant current source (boost ratio or step-down ratio) will not be changed, and overheating and other problems are even less likely to occur.

The specific method to obtain the PWM signal is to install a PWM generator in the PWM switch and potentiometer on the wall. This PWM generator can be easily formed with a 555 chip (Figure 17).

The indicators of this generator are as follows:

1. Input power: 10-36V, 20mA

2. Output signal: 200Hz PWM signal, 0-100%, 5V (can also be 10V)

3. Control number: can control 5-10 dimmable constant current sources

4. Line length: 10-20m

5. Switch: can directly control the 220V power on or off

Therefore, it needs three wires to connect to the lamp. If you also want to turn the lamp on and off, you can use a switch potentiometer, but in this case, a total of 5 wires need to be connected to the lamp. If you want to use the original wall switch, you have to add three more wires, which is its main disadvantage. Its advantage is that you can use one controller to control 5-10 lamps.

6.2 Segmented switch dimming

In order to utilize the existing wall switch and the two wires in the wall, a Taiwanese company has launched a GM6182 four-stage switch dimming called EZ-Dimming, which is a good solution. It can achieve 4-stage dimming using only the ordinary light switch on the wall. The first switch is full brightness, the second quick switch is 60% brightness, the third quick switch is 40% brightness, and the fourth quick switch is 20% brightness. The advantage of this system is that it can achieve dimming using ordinary wall switches. And its power factor is as high as 0.92 or more. There is no concern about generating interference signals. The disadvantage is that it cannot dim continuously. And the operation is more troublesome. It can actually be designed together with a constant current source (Figure 18).

Although infrared remote control can achieve continuous dimming using PWM, it can also be used for segmented dimming in actual use.

2. Specifications and performance of LED ceiling lamps

The most important performance indicator of an LED lamp is its overall luminous efficacy, which is the ratio of the output luminous flux (expressed in lumens) to the input electrical power, with the unit being lumens per watt (lm/W).

It not only includes the luminous efficiency of the LED itself, but also includes the efficiency of the constant current power supply and the light transmittance of the bulb.

In addition, there are its color temperature, color rendering index, etc.

In order to test the indicators of LED ceiling lamps, we usually put it into a large integrating sphere to measure the total luminous flux it emits and analyze the spectrum it emits.

Now let's take the infrared remote control LED ceiling lamp shown in Figure 7 as an example to test its performance indicators. This is an infrared remote control ceiling lamp using 168 3014 LEDs.

The measured results without adding a mask are as follows.

1. Overall lighting effect

Total lumen number 1383lm, input AC power = 16.57W, so the overall luminous efficiency = 83.46 lm/W

It should be noted that two efficiencies are included, one is the efficiency of the infrared receiver, and the other is the efficiency of the constant current power supply. The efficiency of the constant current power supply was tested separately and the result was 89.5% (Figure 8). The power consumption of the infrared receiver is about 0.5W, so the actual input power should be 16W. If the infrared remote control is not used, its overall luminous efficiency should be 86.43lm/W. If the efficiency of the power supply is taken into account, the power used for the LED is only 14.32W.

This gives 96.5lm/W, which should be the luminous efficiency of the LED itself. If an LED with a luminous efficiency of 120lm/W is used instead, its overall luminous efficiency can reach 107lm/W. Even if the power consumption of the infrared receiver is added, the overall luminous efficiency can exceed 100lm/W.

2. Color temperature: 6171K, which is completely determined by the LED used. If the user requires a lower color temperature, a low color temperature LED can be used instead.

3. Color rendering index: Ra=76.2, which is relatively low. However, the color rendering index currently used is the average of the color rendering indexes of 15 kinds of monochromatic light, which cannot fully represent its color rendering ability. Therefore, the Committee on Information Technology (CIE) does not recommend using CRI to measure the color rendering ability of white light LEDs.

4. If a light shield is added, its overall light effect will certainly decrease. And it will decrease quite a lot. Therefore, it is very important to choose a light shield with high light transmittance. However, if the light transmittance is too high, users will be able to see the light beads, which is also undesirable. Especially if a 1W high-power LED is used, it will be more obvious. It is also very important to use a light shield with high light transmittance when using a low-power LED (such as 3014).

The performance of some foreign high-power ceiling lamps is as follows:

Sharp Ceiling Light

3. Cost and price of ceiling lamps

The biggest problem encountered in promoting LED lamps is the price issue. The most prominent one is that when LED bulbs are used to replace incandescent lamps, the price difference between the two can be as much as 20-50 times. The retail price of ordinary incandescent lamps is about 1-2 yuan, and it has little to do with the wattage, while the price of an LED bulb with the same lumen is about 50 to 100 yuan. For example, an LED bulb that can replace a 60-watt incandescent lamp must be at least 10W, and its price is about 100 yuan or more. However, in LED ceiling lamps, the situation is completely different. The price of popular LED ceiling lamps can be comparable to that of mid-range ordinary ceiling lamps.

Therefore, it can be predicted that the LED ceiling lamp will be the fastest popular LED lamp in Chinese households, which also shows that the market opportunity of LED ceiling lamp is the largest.

4. Ceiling lamp market

Living habits vary from country to country, and the demand for ceiling lamps is also different. Generally speaking, ceiling lamps are almost completely not used in the United States. American homes have never installed lamps in the middle of the room. Many lamps are large vertical lamps that are plugged into the sockets in the corners of the wall with a wall switch to turn on and off a special wall socket. In the bathroom, the lighting is completely dependent on the mirror lamp. So it can be said that ceiling lamps have no market in the United States. Japan is similar to China, and many families use ceiling lamps. They are also called ceiling lamps. Europe prefers gorgeous ceiling lamps, but kitchens and toilets use popular ceiling lamps.

Currently, the market in Japan is developing fastest.

In May 2011, the sales volume of LED ceiling lamps in Japan exceeded that of ordinary ceiling lamps.

The market estimates that Japan has a demand for more than 3 million ceiling lamps per year. If each unit costs RMB 100, the market is more than RMB 300 million per year. However, the current rate of LED introduction is only 20% (by quantity). Toshiba Lighting has the largest market share in Japan's LED ceiling lamps.

In China, ceiling lamps are widely used. The output of ordinary fluorescent lamp ceiling lamps in China is shown in the following figure:

The output in 2010 has exceeded 60 million units, and it is expected that the output will exceed 70 million units in 2011. If the output in 2012 is 80 million units, and one tenth of them are replaced by LED ceiling lamps, the number will reach 8 million units. If each unit costs 100 yuan, the market size will reach 800 million yuan.

The market share of LED ceiling lamps is shown in the figure below:

Among them, LED ceiling lamps have not been listed separately, and may be counted as other LED lamps. It can be seen that LED ceiling lamps still have great market potential.

Popularizing LED ceiling lamps is of great significance to energy conservation and emission reduction

Using LED ceiling lamps to replace ordinary fluorescent ceiling lamps can greatly save electricity and thus reduce carbon dioxide emissions. Generally speaking, every kilowatt-hour of electricity saved can reduce 0.272 kilograms of carbon dioxide. From the measured results, it can be seen that an 8W LED ceiling lamp can replace about 16W ordinary fluorescent ceiling lamp, so it can save 8W of electricity. Assuming that the light is turned on for an average of 3 hours a day, 8760 kWh of electricity can be saved each year, which is equivalent to reducing 2382.72 kilograms of carbon dioxide emissions. Assuming that 8 million LED ceiling lamps can be produced in 2012, a total of 70.08 billion kWh of electricity can be saved (equivalent to 82% of the 84.7 billion kWh of electricity generated by the Three Gorges Hydropower Station in one year). Reduced 19 million tons of carbon dioxide emissions!

According to the latest report from the National Oceanic and Atmospheric Administration (NOAA), the average concentration of carbon dioxide in the atmosphere has increased from about 280ppm (ppm: part per million) before the Industrial Revolution to 389ppm in 2010. China's emissions accounted for about 11% of the world's total in 1992 and 23% in 2008, ranking first in the world. In 2010, global carbon dioxide emissions increased significantly, setting a new record. In 2010, global carbon dioxide emissions were 33.5 billion tons, an increase of 1.88 billion tons, or about 5.9%, over the previous year. China's carbon dioxide emissions in 2010 were about 7.5 billion tons. At the Durban Climate Conference, China pledged to reduce carbon emissions by 15% in 2015 compared to 2010. If it is to reduce 15% in 2015, it will be 1.125 billion tons. If China can use 40 million LED ceiling lamps by 2015, it can reduce carbon dioxide emissions by 0.95 billion tons, accounting for 8.4% of the total commitment. This is an extremely impressive number!