Comprehensive analysis of LED ceiling lamps

As the heat of home decoration continues to heat up, the changes in ceiling lamps are changing with each passing day. They are no longer confined to the previous single lamps, but are diversified. They absorb the luxury and style of chandeliers and adopt the ceiling-mounted installation method. , to avoid the shortcomings of the room can not be installed with large luxury lighting. In order to let everyone know more about the ceiling lamp, today I will give you a detailed introduction to the knowledge of the ceiling lamp, in order to refer to the design and purchase of the ceiling lamp.

(1) Ordinary ceiling light

1. The shape of the ceiling lamp

Figure 1: Different shapes of ceiling lamps

Although there are many types of ordinary ceiling lamps (Fig. 1), the most common ones are round or square, and the more complex ones have various grotesque crystal ceiling lamps.

2. The lamp of the ceiling lamp

Figure 2: Fluorescent lamp for ceiling lamps

Most of its lamps are T5 fluorescent lamps after bending (Fig. 2). There are many kinds of specifications, mainly the following.

3. Ceiling lamp power

The power of the ceiling lamp is not clearly defined, and the most commonly used power is 10W, 16W, 21W, 28W, 32W, 38W, 40W, and the like.

(2) Problems and disadvantages of ordinary ceiling lamps

1. Low luminous efficiency

The overall lighting efficiency with ballast energy-saving lamps is only 55lm/W, and the highest high-power U-shape is only 80lm/W. And because most of these ceiling lamps are mostly T5-type lamps, they are bent into various shapes. But no matter what shape it is finally bent into, it is 360-degree light, and the upward 180 degrees of light must be reflected back to become useful light, and this reflection process will lose a lot of light. Therefore, at least 70% discount will be applied to the light effect after the lamp is made. That is to say, the above light effect is multiplied by 0.7. Moreover, in actual use, its brightness decays rapidly with time. For example, a Philips 32W ring T5 tricolor lamp YH32RR16 has a color temperature of 6500oK, a luminous efficiency of 67lm/W, and a total lumen of 2150lm. In fact, only 70% of the light can be used. That is only 1500lm.

2. Containing mercury

Mercury has a very low boiling point and can evaporate at normal temperatures. After a waste discarded fluorescent lamp is broken, it immediately emits mercury vapor to the surrounding area, which instantly makes the concentration of mercury in the surrounding air reach 10-20 mg/m3, exceeding the maximum allowable concentration of mercury in the air (0.01 mg/cubic). 1000 to 2000 times the amount of rice, the human body inhaled 2.5 grams of mercury vapor is life-threatening. Moreover, the liquid mercury has a very high permeability and will immediately penetrate into the underground polluted groundwater when it falls to the ground. 1 milligram of mercury is enough to contaminate 5454.5 kilograms of drinking water, making it impossible to meet safe drinking standards. Nowadays, the promotion of energy-saving lamps is actually an irresponsible approach to our children and grandchildren.

3. Low life

Energy-saving lamps are classified according to phosphors: mixed powder, halogen powder, and three primary colors. The life of the halogen powder is 3000-4000 hours; the life of the mixed powder is 4000-6000 hours; the three primary colors are more than 8000 hours.

4. Ballast quality is low

Most of the ballasts used in fluorescent lamps do not give enough power. For example, the authors measured the ballast used in a 32W fluorescent lamp with an output of only 16W, which is only 50% of the rated power. Moreover, the life of all fluorescent lamps is short, mainly because the life of the ballast is very short, and once the fluorescent lamp is damaged, the ballast is usually followed. Almost everyone's experience is that the ballast or lamp should be replaced once a year. Most of the time it is less than a year, and both must be changed.

5. High price

The price of general ceiling lamps ranges from tens to hundreds of dollars. The ceiling lamps of foreign brands should cost more than 600 yuan. As for some ceiling lamps with crystal ornaments, the price is as high as several thousand yuan, and the light source is only But at most it is a 40W shaped fluorescent lamp. Therefore, ceiling lamps are not as cheap as ordinary energy-saving lamps.

(3) Advantages of LED ceiling lamps

1. High luminous efficiency

The luminous efficiency of the LED itself has reached more than 130lm/W, so the overall efficacy of the LED ceiling lamp can reach 100lm/W. And this efficiency is still growing every year. It is estimated that by 2020, the luminous efficiency of LED itself will reach 240lm/W. High light efficiency means energy saving. According to the measured results, an 8W LED can replace a 32W ceiling lamp. The electric power can be saved by more than 4 times.

2. Long life

The life of the LED is 100,000 hours. After the lamp is made, the life is actually recognized as at least 30,000 hours. The high quality can be 50,000 hours. It is 10 times longer than the fluorescent lamp, even the highest life three-color fluorescent lamp. The life expectancy is 10,000 hours, and it is also more than 5 times higher.

3. Does not contain mercury

4. No ultraviolet radiation, no light pollution

5. Lights of various color temperatures can be provided as needed, usually 2550-3700oK, and 4700-7000oK

6. No glass parts, impact, vibration, easy to transport

7. No need for aluminum radiator, low cost and light weight

(4) The composition and heat dissipation of the LED ceiling lamp

Regardless of the gorgeous crystal ceiling lamp, the structure of a common LED ceiling lamp is no different from that of a normal ceiling lamp. They are composed of four parts: the bottom case, the cover, the power supply, and the lamp.

1. LED ceiling lamp lamp beads

Generally, 1W LED lamp beads are used for powers greater than 30W, and low-power LED lamp beads (for example, 3020, 3014, 3528, etc.) are mostly used below 30W. SMD-type low-power LEDs are recommended for heat dissipation and uniformity of illumination.

The beads can be arranged according to their shape. For example, if the shape is circular, the beads are arranged in a circle, and if the shape is square, they are arranged in a square shape. Even so, there are many ways to arrange the lamp bead. The internal structure of a typical LED ceiling lamp is shown in Figure 3.

Figure 3: Internal structure of the LED

In Figure 3, there are two LED lamp beads on the circular printed board. It uses 23 0.4W type 5730 LED lamp beads for a total power of 18W. The middle rectangle is its constant current power supply, and the lower right corner of the box is the photodiode of the remote receiver to receive the infrared control signal of the remote brightness.

What needs to be mentioned is that there are still a large number of low-cost LED ceiling lamps that use low-power straw hat lamps. However, due to the large thermal resistance of the straw hat lamps (450 °C/W), it is difficult to derive heat, resulting in severe light decay and short life. .

2. LED lamp cover

The lampshade usually uses a PC board and its performance is as follows:

(1) Transmittance: The transmittance of PC board is up to 89%, which is comparable to glass. UV-coated panels do not produce yellowing under sunlight, and fogging, poor light transmission, after 10 years, the loss of light transmission is only 6%, the rate of PVC loss is as high as 15%-20%, and the fiberglass is 12 %-20%.

(2) Anti-impact: The impact strength of PC board is 250-300 times that of ordinary glass, 30 times of acrylic sheet of the same thickness, 2-20 times of tempered glass, and there is no crack when it is dropped by 2 meters below 3kg hammer. The name of “not broken glass” and “ring steel”.

(3) UV protection: The PC board is coated with anti-ultraviolet (UV) coating on one side and anti-condensation treatment on the other side, which combines anti-UV, heat insulation and anti-drip function. It blocks UV rays and protects valuable artwork and exhibits from UV damage.

(4) Light weight: The specific gravity is only half of that of glass, saving the cost of transportation, handling, installation and supporting frame.

(5) Flame retardant: The national standard GB50222-95 confirms that the PC board is a non-flammable first grade, namely B1 grade. PC board's own ignition point is 580 degrees Celsius, self-extinguishing from the fire, no toxic gas will be produced when burning, it will not promote the spread of fire.

(6) Flexibility: It can be installed in an arched shape, semi-circular top and window shape according to the design drawing. The minimum bending radius is 175 times the thickness of the plate, and it can also be bent.

(7) Temperature adaptability: PC plate does not appear cold and brittle at -100 °C, does not soften at 135 °C, and its mechanical and mechanical properties do not change significantly in harsh environments.

(8) Weather resistance: The PC board can maintain the stability of various physical indexes in the range of -40 ° C to 120 ° C. The artificial climate aging test was 4000 hours, the yellowing degree was 2, and the light transmittance reduction value was only 0.6%.

3. LED ceiling lamp cooling

Using low-power LED lamp beads, the arrangement is scattered to help dissipate heat. The structural characteristics of the ceiling lamp are also conducive to heat dissipation.

a) The internal air is free to flow without blocking. This is the biggest advantage of ceiling lamps compared to other lamps. The air that is sealed and not circulated is a kind of insulation with good heat insulation performance. In turn, the air that can flow freely is the best heat sink. All the hot objects in the world finally emit heat into the air. Because the bottom plate of the ceiling lamp has many small holes, in the lampshade of the ceiling lamp, the heat generated by the LED lamp bead can be immediately transmitted to the outside through the air.

b) The lamp cover can dissipate heat very well. We know that convection and radiation heat dissipation are mainly related to area and material, while the radiation heat dissipation effect of plastic is very good, and the thickness of the lamp cover is very thin, so the influence of poor heat conduction of plastic can be neglected. This can be illustrated from Figure 4.

Figure 4: Thinner thickness reduces the effect of thermal conductivity

The thickness of the bulb of the ceiling lamp is only 1-2 mm. So although its thermal conductivity is only 0.1-1W/mk, it can quickly transfer heat from one side to the other.

And the radiation capacity of plastic is very strong, see the following table:

Therefore, the bulb can radiate heat well. Because the air inside the ceiling lamp is well circulated, it is easy to bring heat to the bulb. Therefore, the lampshade plays a major role in heat dissipation.

c) Large area: Calculate the formula according to the area of ​​the circle: S = πr2, or S = π (d / 2) 2. For a circular ceiling lamp with a diameter of 45 cm, its bottom plate area is 1590 cm2. Because ceiling lamps are usually installed indoors without excessive room temperature, 40cm2/W of empirical data can be used, which can also dissipate 40W of power. However, since the bottom plate is close to the ceiling, it is not easy to dissipate heat, so the actual heat dissipation effect of the bottom plate is much worse.

Because the lampshade is curved, its area is larger than the circular shape, and its installation is facing down without any blockage, so the heat dissipation effect is better than the bottom plate.

d) The key issue is to make the copper area of ​​the aluminum substrate or printed board on which the LED lamp bead is made large enough.

Experiments have shown that for a ceiling lamp of less than 30 W, a common printed board can be used to dissipate heat well, and in order to utilize freely flowing air. The heat is taken away on both sides of the printed board, so there is no need to heat the aluminum substrate or the printed board to the bottom plate through the thermal conductive glue. Instead, the printed board is raised 5mm away from the bottom plate for air circulation.

e) It is very beneficial to dissipate more ventilation holes on the bottom plate. These vents should be placed on the outer ring to prevent it from being blocked. But it may also bring the possibility of bug penetration.

After taking the above measures, the LED ceiling lamp does not need a heavy finned heat sink to meet the heat dissipation requirements. It can be said that the LED ceiling lamp is the only LED lamp that does not require a special aluminum radiator.

(5) Power supply for LED ceiling lamps

Let us first look at the power of ordinary ceiling lamps.

Ordinary ceiling lamps usually use electronic ballasts, or high frequency transformers. However, the quality of electronic ballasts on the market is relatively poor. For example, I tested a regular ceiling light with Philips' 32W ring tube installed. The measured results are as follows: Pin = 16W, PF = 0.62. So it only uses 50% of the power of the ring tube, and the power factor is also very poor. The author also tested the parameters of a 9W energy-saving lamp, measured Pin = 8.4W, PF = 0.563.

PS: The power factor of both can't meet the requirement of >0.7, but it is still circulating in the market.

The power supply for LED ceiling lamps can be divided into two categories: non-isolated and isolated:

● Non-isolated power supply

1. Non-isolated constant current source using constant current diode

The constant current diode is a linear constant current source whose constant current action can be used to drive the LED. The easiest way is to connect the constant current diode directly to the LED. However, we must pay attention to the selection of the appropriate current and withstand voltage when used in LED driving.

Minimum voltage: Since the constant current diode requires a certain voltage Vk to enter the constant current, the too low power supply voltage cannot work. Usually this Vk is about 5-10V.

Maximum voltage: Since the constant current diode must absorb the change of the power supply voltage, for the same percentage, 220V is twice as large as the 110V variation range. For example, for a range of +10% to -20%, 220V means a range of 22+44=66V. After bridge rectification, this change will increase by 1.2 times to 79.2V. For the 110V power supply, the same range of variation is only equivalent to the 39.6V range. The lower the voltage, the lower the power consumption and the higher the efficiency. Therefore, it can be said that the constant current diode is more suitable for the country with 110V power.

Maximum current: Due to the limited power consumption of the constant current diode, excessive current is not suitable. For example, a 1W LED usually requires 350mA, and a constant current diode is difficult to provide. Even if it can be provided, its power consumption is too large and the overall efficiency is greatly reduced. The most suitable use case for constant current diodes is the AC mains-powered LED lamps. It is most suitable to use a series of small power LEDs in series, that is, high voltage and small current.

Figure 5 is a constant current diode driving source for a ceiling lamp. Its load is 80 3022 series, the total power is 16W. The constant current diode used is also constant current at 60 mA. If the constant current diode at hand is only 30 mA, 2 series parallel connections are required.

Figure 5: Using a constant current diode as the LED driver

Here, the function of the constant current diode is to keep the output current constant when the input mains voltage changes, and of course, the current increase due to the negative temperature coefficient of the LED can be eliminated. However, since the withstand voltage of the constant current diode has a certain limit, the variation of the power supply voltage that it can absorb is also limited. Take the 100V withstand voltage CRD, its operating voltage range is also reduced by its minimum operating voltage of 10V, the available voltage range is only 90V. Used in 220V mains power supply, if the mains change +10%, ~-20%, it is equivalent to 290~211V after rectification, voltage change 79V, within its withstand voltage range. If the LED used is 80, if the forward voltage is 3.3V, then the total voltage is 264V, which is exactly the value after 220V after bridge rectification. At this time, there is no voltage drop on the constant current diode, but at this time it is inoperable and requires at least 10V voltage drop, that is, the voltage after rectification is 274V, and the mains voltage is 228VAC. At that time, the constant current diode has the lowest voltage drop and the lowest power consumption, only 0.03Ax10V=0.3W, and the overall efficiency is up to 96% (of course, the efficiency of the rectifier should be considered, in fact, it will be lower). If the mains is increased to 242VAC, the constant current diode voltage will increase to 26.4V, and its power consumption will also increase to 0.79W, when the efficiency is equal to 91%.

If the mains voltage is lower than 228V, does the constant current diode not work? No, but it is not constant current. At this time, it will reach a new balance point with the LED, that is, the voltage of the two is equal to the rectified voltage of the mains voltage. Because of the nonlinearity of the volt-ampere characteristics of LEDs, it is difficult to express them by formula. In short, when the mains voltage drops, the current in the LED decreases as the mains voltage decreases. Its brightness will also dim. However, at this time, the voltage drop of the constant current diode is not large, so it does not consume a lot of power. So the efficiency is still very high.