LED ceiling lamp and its power supply (Part 2)

4.2 LED ceiling lamp shade

The lampshade is usually made of PC board, and its performance is as follows:

(1) Light transmittance: The light transmittance of PC board can reach up to 89%, which is comparable to glass. UV coated board will not turn yellow, fog or have poor light transmittance when exposed to sunlight. After ten years, the light transmittance loss is only 6%, while the loss rate of PVC is as high as 15%-20%, and that of glass fiber is 12%-20%.

(2) Impact resistance: The impact strength of PC board is 250-300 times that of ordinary glass, 30 times that of acrylic board of the same thickness, and 2-20 times that of tempered glass. It will not crack even if it is dropped from two meters with a 3kg hammer. It is known as "unbreakable glass" and "ringing steel".

(3) Anti-ultraviolet: One side of the PC board is coated with an anti-ultraviolet (UV) coating, and the other side is treated with anti-condensation, which combines anti-ultraviolet, heat insulation and anti-drip functions. It can block ultraviolet rays from passing through and is suitable for protecting valuable artworks and exhibits from being damaged by ultraviolet rays.

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

(5) Flame retardant: The national standard GB50222-95 confirms that PC board is flame retardant level 1, that is, B1 level. The ignition point of PC board itself is 580 degrees Celsius. It will extinguish itself after leaving the fire. It will not produce toxic gas when burning and will not help the spread of fire.

(6) Bendability: It can be installed in arch, semicircular top and window shape by cold bending on site according to the design drawing. The minimum bending radius is 175 times the thickness of the plate. It can also be hot bent.

(7) Temperature adaptability: PC boards do not become brittle at -100°C and do not soften at 135°C. Their mechanics and mechanical properties do not change significantly in harsh environments.

(8) Weather resistance: PC board can maintain the stability of various physical indicators in the range of -40℃ to 120℃. After 4000 hours of artificial climate aging test, the yellowing degree is 2 and the transmittance reduction value is only 0.6%.

4.3 Heat dissipation of LED ceiling lamps

Using low-power LED lamp beads and arranging them more dispersedly can help dissipate heat.

The structural characteristics of ceiling lamps are also conducive to heat dissipation.

1. The internal air can flow freely without any obstruction. This is the biggest advantage of ceiling lamps compared to other lamps. Air that is sealed but not circulated is a good insulator. On the contrary, air that can flow freely is the best heat dissipator. All heat-generating objects in the world eventually dissipate heat into the air. Because there are many small holes in the bottom plate of the ceiling lamp, the heat generated by the LED lamp beads in the lampshade of the ceiling lamp can be immediately transferred to the outside through the air.

2. The lampshade can dissipate heat very well. We know that convection and radiation heat dissipation are mainly related to area and material, and the radiation heat dissipation effect of plastic is very good. In addition, the thickness of the lampshade is very thin, so the influence of poor thermal conductivity of plastic can be ignored. 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 1-2mm. So although its thermal conductivity is only 0.1-1W/mk, it can still transfer heat from one side to the other quickly.

Moreover, the radiation capacity of plastic is very strong, as shown in the following table:

Therefore, the bubble shell can radiate heat very well.

Because the air circulation inside the ceiling lamp is very good, it is easy to bring heat to the bulb shell. Therefore, the lampshade plays the main role in heat dissipation.

1. Large area: According to the calculation formula of the area of ​​a circle: S=πr2, or S=π(d/2) 2, for a circular ceiling lamp with a diameter of 45cm, its base area is 1590cm2. Because ceiling lamps are usually installed indoors and there is no excessively high room temperature, the empirical data of 40cm2/W can be used, which can also dissipate 40W of power. However, since the base is close to the ceiling and is not easy to dissipate heat, the actual heat dissipation effect of the base will be much worse.

Because the lampshade is curved, its area is larger than that of a circle, and it is installed downward without any obstruction, so the heat dissipation effect is better than that of the base plate.

2. The key issue is to make the copper foil area of ​​the aluminum substrate or printed circuit board where the LED lamp beads are installed large enough.

Experiments have shown that for ceiling lamps less than 30W, ordinary printed circuit boards can dissipate heat well, and in order to take advantage of the free-flowing air, the heat is taken away from both sides of the printed circuit board. Therefore, there is no need to place the aluminum substrate or printed circuit board close to the bottom plate through thermal conductive adhesive to dissipate heat. Instead, the printed circuit board should be raised 5mm away from the bottom plate to facilitate air circulation.

3. Having more ventilation holes on the bottom plate will greatly improve heat dissipation. These ventilation holes should be made on the outer circle to avoid being blocked. However, it may also cause the possibility of insects getting in.

After taking the above measures, LED ceiling lights do not need heavy finned heat sinks to meet the heat dissipation requirements. It can be said that LED ceiling lights are the only LED lamps that do not require special aluminum heat sinks.

1. Power supply of LED ceiling light

Now let's take a look at the power supply of ordinary ceiling lights.

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, the author tested an ordinary ceiling lamp installed with a Philips 32W ring tube. 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 and measured Pin=8.4W, PF=0.563

The power factor of both products cannot meet the requirement of >0.7, but they are still widely circulated in the market.

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

5.1 Non-isolated constant current source. Since LED ceiling lamps are not as easy to be touched by users as bulb lamps, and since contact heat conduction is not required, its internal structure can easily insulate the aluminum substrate or printed circuit board from the metal base plate, so the use of non-isolated power supply can easily pass CE, UL and other safety certifications. In addition, its installation is usually done by professional electricians, which also reduces the risk of electric shock to users.

We know that LEDs must be driven by constant current sources, otherwise, due to its negative temperature coefficient, the current will rise sharply, causing the junction temperature to rise and the life span to be shortened. Constant current sources are divided into linear and switching types. The advantages of linear constant current sources are that they do not generate electromagnetic interference (EMI), are simple, and have low cost. Its disadvantage is that the efficiency is relatively low.

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

A constant current diode is a linear constant current source, and its constant current function can be used to drive LEDs. The simplest method is to connect the constant current diode directly in series with the LED. However, we must pay attention to selecting the appropriate current and withstand voltage when using it for LED driving.

 Minimum voltage

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

 Maximum voltage

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

 Maximum current

Since the power consumption of constant current diodes is limited, too large current is not suitable. For example, a 1W LED usually requires 350mA, which is difficult for constant current diodes to provide. Even if it can be provided, its power consumption is too large, which greatly reduces the overall efficiency.

 The most suitable application occasion for constant current diodes is AC mains-powered LED lamps, which use many low-power LEDs in series, that is, high voltage and low current.

Figure 4 is a constant current diode driver for ceiling lamps. The load is 80 3022 diodes in series, with a total power of 16W. The constant current diode used is also 60mA. If the constant current diode on hand is only 30mA, two series and parallel connections are required.

Here, the role of the constant current diode is to keep the output current unchanged when the input mains voltage changes, and of course, it can also eliminate the current increase caused by the negative temperature coefficient of the LED. However, since the voltage resistance of the constant current diode is limited, the power supply voltage change it can absorb is also limited. Take the 100V withstand voltage CRD as an example. Its operating voltage range must be subtracted from its minimum operating voltage of 10V, and the available voltage range is only 90V. Used in a 220V mains power supply, if the mains changes by +10%, ~-20%, it is equivalent to 290~211V after rectification, and the voltage changes by 79V, which is within its withstand voltage range. If there are 80 LEDs used, if the forward voltage is 3.3V, then the total voltage is 264V, which is exactly equivalent to the value of 220V after bridge rectification. At this time, there is no voltage drop on the constant current diode, but it cannot work at this time and requires at least a 10V voltage drop, which requires a voltage of 274V after rectification and a mains voltage of 228VAC. At that time, the voltage drop of the constant current diode is the smallest, and the power consumption is also the smallest, only 0.03Ax10V=0.3W, and the overall efficiency can reach up to 96% (of course, the efficiency of the rectifier must also be considered, which is actually lower). If the mains power increases to 242VAC, the voltage of the constant current diode increases to 26.4V, and its power consumption also increases to 0.79W, and the efficiency is equal to 91%.

If the mains voltage is lower than 228V, will the constant current diode not work? No, but it is indeed not constant current. At this time, it and the LED will reach a new balance point, that is, the sum of the voltages of the two is equal to the voltage after rectification of the mains voltage. Because the volt-ampere characteristic of LED is nonlinear, it is difficult to express it with a formula. In short, when the mains voltage decreases, the current in the LED will decrease with the decrease of the mains voltage. Its brightness will also become dimmer. However, at this time, the voltage drop of the constant current diode is not large, so it does not consume much power. So the efficiency is still very high.

It is assumed above that the forward voltage drop of the LED is 3.3V. In fact, even if it is rated at 3.3V, after being powered on for a period of time, the forward voltage drop will drop to 3.1V or even 3.0V due to the increase in junction temperature.

The measured results of an 8W power supply using a constant current diode are shown in the following table:

As can be seen from the table, the maximum efficiency can indeed be very high after using constant current diodes. It is a power supply worth choosing. In order to obtain the highest efficiency at 220V, it seems that more than 90 LEDs should be connected in series.

The parameters of various constant current diodes are shown in the following table: