Disadvantages and problems of electroless capacitor LED light engine

So far, the "light engine" has not yet had a very strict and precise definition, so many readers still don't know what the light engine is. Some people define it as "a light engine" is an integral combination of LED packages (components) or LED arrays (modules), LED drivers, and other brightness, thermal, mechanical, and electrical components. The conventional connector that matches the LED luminaire is directly connected to the branch circuit. The LED luminaire is designed not to use a standard lamp holder." This definition seems to be complete, but it is often confusing because it also includes "other brightness, Thermal, mechanical and electrical components".

In fact, it is very simple to define the light engine: that is, the combination of the LED light source and the LED driving power supply into one module can be called the light engine. More generally, it can be said that any constant current drive source is placed on the LED aluminum substrate to become a light engine.

Some people call the COB LED a light engine, which is obviously wrong! Because it does not have a constant current drive source, there is no "drive", how can it be called "engine"?

The types of power sources can be divided into two categories: DC light engine and AC light engine. However, due to the large number of light engines currently used in indoor lighting, we are paying more attention to the light engine of AC power.

So is the entire LED luminaire also called a light engine? Of course not. Just as the whole car can't be called an engine, the engine is the powertrain of the car, but it doesn't equal the car, and it needs to be added to the car to become a car. Products such as light engines are needed because all lamps have a variety of appearances. Some fixtures can even be called artwork. Take the incandescent lamp, it can be said that it has only one form of the simplest light bulb, at most plus a candle light or a tiny Christmas light. However, its shape can be varied. In particular, the crystal lamp is a variety of arts and crafts, extremely gorgeous. But the LED must also be equipped with an extremely complex constant current drive source to work as an incandescent lamp (it can be illuminated by connecting 220V). If the LED is made into a light engine, it will be brighter when it is connected to 220V like an incandescent lamp (of course, it needs a matching radiator). This on the one hand contributes to standardization and on the other hand greatly facilitates the design of the enclosure (and radiator) designers.

First, the benefits of using a light engine

Many people believe that the main purpose of promoting light engines is to standardize to increase their interchangeability. In fact, this standardization should not only start with the light engine, but should start with the interface between the unified heat sink and the light engine. In the case of an LED bulb, the size of the light engine and the heat sink is basically the same, and specifically relates to the size position of the positioning screw hole and the positioning screw hole of the heat sink. Even with such specific problems, there are still many difficulties. Because it is related to the wattage and the heat dissipation capability of the heat sink. Therefore, the promotion of the light engine can not only focus on the benefits of interchangeability.

It can be said that the biggest benefits of promoting the light engine are as follows.

1. Reduce the R&D expenses of LED lighting manufacturers and shorten the production cycle

Because the optical and electrical specifications of all LED luminaires are set by the light engine manufacturer. Even the corresponding heat sink is recommended. Lamp manufacturers only need to install the light engine inside the radiator. Lamp manufacturers do not even need experienced optical and electronics engineers, nor do they need to purchase expensive optical and electronic equipment. In other words, the light engine can greatly reduce the entry limit of the LED lighting factory, but it will not reduce the quality of the LED lighting. Of course, this requires the manufacturer of the light engine to have a very high level.

2. Promotion of light engine can greatly accelerate the promotion of LED lamps

In recent years, with the ban on incandescent lamps, LED lamps are about to explode. LED lighting factories have sprung up like mushrooms. Although this is an inevitable trend, it also brings a lot of problems, especially the performance of LED lamps and the disorderly price war between various manufacturers. The emergence of LED light engines can solve such problems to a large extent. Because the quality is determined by several big manufacturers that produce light engines, the price is not likely to have much difference. This greatly reduces the internal friction caused by disorderly competition. Help the rapid development of the LED lighting industry.

3. Improve the overall performance of LED lamps

For example, the overall light efficiency (lm/W) is increased, here is the overall light efficiency including power efficiency, of course, the whole lamp should also include the light transmittance of the lampshade (but this is not the company that produces the light engine). Responsible, but responsible for the company that produces the radiator). It can also increase the color temperature, color rendering index and other indicators.

4. Reduced costs

At least the printed board of the constant current driving source can be omitted, and the production cost thereof.

For now, the simplest and most important is the light engine for LED bulbs. Let's take a look at the composition of this light engine.

Second, the light engine of the LED bulb

For a bulb, its composition is very close to standardization. It consists mainly of three major components: LED light source, constant current drive power supply, heat sink and housing (including lamp holder and lamp cover).

So if you take the LED bulb, if you want to make a light engine, you must combine its power board and LED board into one board. It can be seen that the difficulty is very big.

1. Switching power supply is not suitable for light engine

Although the switching power supply has better performance and higher efficiency, the biggest disadvantage is that the number of components is very large, and it is very difficult to integrate with the LED light board. Someone has tried to do this, and the result is very crowded (see Figure 2).

As can be seen from this photo, the entire switching power supply occupies almost 70% of the area, while minimizing the space of the LED. This structure is obviously not feasible because it not only reduces the light-emitting area of ​​the LED, but also blocks part of the light. And the biggest problem is to reduce the heat dissipation area of ​​the LED. It seems likely that there are five 1W LEDs in the picture. If you want to make a 9W or 10W bulb, it is completely impossible to achieve.

2. Linear power supply can reduce component count but is inefficient

In order to reduce the number of components, the easiest way is to use a linear power supply. Usually, the linear power supply has only 5-6 components, which can greatly reduce the occupied area. At present, most of the light engines on the market are linear constant current sources using electroless capacitors. The electroless capacitor solution was proposed by ExClara in the United States in August 2011. Later, 6-7 companies imitated their ideas. A similar chip. Its basic principle is to connect LED strings driven with different constant current values ​​when inputting different values ​​of the AC voltage waveform to obtain a current waveform matching the half sinusoidal waveform after AC rectification, so that power close to 1 can be obtained. Factor. This can be understood from the internal structure diagram and waveform diagram of this chip. Figure 3 is the internal structure of the CL8801 chip of Supertex Corporation of the United States.

Its current and voltage waveforms are shown in Figure 4. It can be seen from the figure that when the AC voltage rises to a certain value, the first switch is turned on, the first string of LEDs is turned on, and the first constant current source is selected; the first switch is continued to rise to a higher value. Disconnected, the second switch is turned on, the first string is turned on with the second string of LEDs at a second current source of higher current, and so on, the last string of on-time is the shortest, but the on-current is the largest. (The last constant current source). This results in a current waveform that is close to the voltage waveform (half sine wave), resulting in the highest power factor. But it also brings a series of shortcomings and problems (see later).

Below are several light engines that use a non-electrolytic linear constant current source.

These are the linear constant current source solutions of electroless capacitors. Daxin's DS6622 uses a bridge stack of varistors, three resistors and one capacitor and two ICs (DS6622). A total of eight components occupy only 10 The area around %. This light engine is a viable solution in terms of footprint. However, its disadvantage is that, like all common linear power supplies, according to the data given by ExClara, the efficiency is 84%-93% when the input mains voltage changes by +/-10%. Moreover, only the mains voltage variation is allowed to be less than 20% and the LED voltage must be within 10% of the design value, otherwise the efficiency will become very low. The power efficiency of the SU1203 is shown in Figure 6.

As can be seen from the figure, the efficiency is about 96% at an input voltage of 200V, and about 80% at an input voltage of 245V. The average is about 88%, which is equivalent to the efficiency at 220V. If the efficiency is 100% at 190V (impossible!), the efficiency at 260V is 70%, then the average is 85%. To sum up, the shortcomings and problems of this electroless capacitor solution are as follows:

1. Inefficient, about 88%.

2. The LED is not a DC drive, but it is switched on and off, so the LED utilization is low, and the high pulse current of the last string of LEDs will also affect its life.

3. The overall luminous efficiency is about 15% lower than that of electrolytic capacitors.

4. The current waveform is close to half a sine wave, so there will be 100Hz flicker. Cannot be used in digital camera photography and security systems.

Third, the combination of power supply and light board will cause serious heat dissipation problems.

Is it a viable light engine solution to use a normal linear constant current source? This is not the case. Although it solves the problem of occupying space, in addition to the disadvantages already mentioned above, there is a drawback that cannot be ignored, that is, the heat dissipation problem. Although no matter where the power is placed, its heat is always dissipated into the space through the heat sink, but when the power supply and the light board are put together, the power consumed by the power supply becomes heat and is directly applied to the light board. It will increase the amount of heat that the lamp board needs to dissipate, which will increase the junction temperature of the LED and reduce its life. Moreover, the biggest disadvantage of linear constant current sources is their extremely low efficiency. So this is an extremely serious problem.

Here is an example to illustrate this problem.

Assume a 10W LED bulb, which means that there are 9W LEDs on the panel. Or it needs 9W power supply. After using the electroless capacitor solution, its efficiency is 84%-93%, and the average value is 88.5%. That is, when it supplies 8.85 W of power to the LED panel, it consumes 11.5% of its own power, which consumes 1.15 W. Then if this power supply is also installed on the light board, it is necessary to increase the heat of the light board, but the light board adds more than 11.5% of the heat! Why?

Because the 8.85W LED is not the 8.85W all turned into heat, but a part of it turned into light. The current LED's luminous efficiency is around 30%. Assuming a luminous efficiency of 40%, then 60% of the energy becomes heat, that is, 5.4W of heat needs to be emitted. Now it has increased by 1.15W and it has become 6.55W. That is to increase the heat by 21.3%. Although the original heat sink also needs to dissipate this part of the heat, it is now added to the LED light board. Because the power supply is placed on the circuit surface of the lamp board, and the circuit surface is transferred to the heat sink through two insulating media, one is the insulating medium of the printed board, and the other is the insulating medium between the aluminum substrate and the heat sink, so The temperature of the circuit surface is different from the temperature of the heat sink, and the LED is placed on the circuit surface of the aluminum substrate. That is to say, since the power is placed on the circuit surface, the heat of the circuit surface of the aluminum substrate is increased by 21.3%, which causes the junction temperature of the LED to increase by 21.3%. How much influence does the increase in junction temperature have on life? See the curve for junction temperature and lifetime given by Cree (Figure 7).

This curve is a sign that the light decays to 70% as the life. If the original LED junction temperature is 85 degrees, its 70% light decay life is 31,000 hours. Now the junction temperature has increased by 21.3%, that is, it has risen to about 100 degrees, and its life has been reduced from 31,000 hours to 15,000 hours, which is 16,000 hours. Therefore, the effect of this temperature rise on life is quite serious. If the original life expectancy can be as high as 30,000 hours, the actual life expectancy is only 15,000 hours. In other words, the light engine of this electroless capacitor solution will seriously reduce the life of the LED.

Fourth, the flicker effect

The 100 Hz flicker effect can be clearly seen when photographing with a digital camera. The left picture of the two photos below (Fig. 8) is illuminated with an electroless capacitor LED. The one on the right is illuminated by an electrolytic capacitor LED.

5. The measured results of the Seoul Light Engine

Seoul Semiconductor Co., Ltd. produced a light engine Acrich2. It can be seen from its circuit diagram that it is also a linear power supply using an electroless capacitor solution. The outline drawing is shown in Figure 9, and the circuit diagram is shown in Figure 10.

We measured its parameters: at 220V, its input power is 4.489W, the total luminous flux is 297.7 lm, the luminous efficiency is only 66.3 lm/W, the color rendering index is 83.1, and the power factor is 0.975. It can be seen that its light effect is very low. And as the input voltage increases, its efficiency is further reduced (see Figure 11a and Figure 11b).

As can be seen from Figure 11a, when the input voltage is increased from 220V to 255V, the input power is increased by a factor of 1.2, while the output lumen is only increased by a factor of 1.1. That is, the light effect will be reduced by 1.1 times. According to Acrich2's data sheet, its power chip has a power dissipation of 1W and an input power of 4.5W, so its power efficiency is only 78%. And this 1W power is added to the LED aluminum substrate, which will definitely increase the heat of the aluminum substrate, increase the LED junction temperature, and thus reduce the life of the LED! Its only advantage is the high power factor, but because its power is less than 5W, whether it is Neither the US Energy Star nor China's national standards require a power factor. So this advantage is superfluous!

Conclusion

At present, almost all LED light engines on the market use electroless capacitors. This solution not only has low efficiency, increases the heat of the lamp board, reduces the life of the LED, but also has low LED utilization and reduced overall light efficiency. 15%, and there is a 100Hz strobe!

Therefore, in order to make the LED light engine a reality, the key technology must have a very efficient constant current driving source. It is possible to put the power supply on the LED aluminum substrate with a highly efficient constant current driving source. Some of the electroless capacitor solutions are not suitable light engine solutions!

Edit: Cedar