Feasibility analysis of MLCC completely replacing electrolytic capacitors in LED power supplies

Japan pays attention to the life and comfort of LED lamps. The flicker of LED lamps once made a Japanese city hall official faint. TDK experts replaced the MLCC in LED lighting fixtures with electrolytic capacitors and conducted a flicker test. The test found that when MLCC is used to replace electrolytic capacitors, the same capacity cannot be replaced. Some leading Japanese manufacturers have developed LED lamps that use all MLCCs, and this replacement idea has been widely recognized in Japan.

As we all know, a lot of things happened in Japan this year. The tsunami and the earthquake in Japan caused some problems in the delivery of major Japanese electronic suppliers. Another factor is the power restriction in Japan. Because their production lines cannot operate as expected, there are some hidden dangers in the delivery period of electronic products of Japanese manufacturers. Another factor is the appreciation of the Japanese yen, which indirectly causes the high price of Japanese electronic components, so the cost control pressure on Japanese electronic component manufacturers is very high.

LED lighting is mainly divided into two categories: one is indoor lighting, and the other is outdoor lighting. Indoor lighting is used in public places or large-scale lighting on production lines, and its use environment is very harsh. This use environment also indirectly leads to hidden dangers of life. The second is outdoor lighting, and street lighting is a typical example. Street lighting is used for a long time at external temperatures, especially in the north where the temperature difference is relatively large. LED lighting will be affected by the relatively large temperature difference, and more electrolytic capacitors are used in lamps. Electrolytic capacitors have poor performance at high and low temperatures, which indirectly affects their lifespan. At present, there are still some hidden dangers in the LED lighting industry in terms of lifespan, and the problem of failure in a short time has not been completely solved.

Taking LED indoor lights as an example, since they are used in a sealed environment, the electronic self-heating phenomenon will occur during use, and the temperature of the internal environment is also very high. The current mainstream electrolytic capacitors in LED lighting do not perform very well at high temperatures. Considering this hidden danger, TDK recommends using MLCC to replace electrolytic capacitors to solve the hidden dangers in service life. TDK has been producing MLCC since 1971 and has 4 main factories in Japan, mainly doing the core process of ceramic electrolytic capacitors. Among Japanese component manufacturers, TDK is the only company that places the front process in Japan, which shows that TDK also has very high requirements for quality control. 

Comparison of the characteristics of electrolytic capacitors and MLCCs

Electrolytic capacitors have a great feature, that is, the ESR value is relatively high. If the ESR is relatively high, it will be lost in the form of heat, so the electrolytic capacitor's own heating is relatively serious. In addition, its ripple current is relatively high, which will also cause serious self-heating. ESR plus ripple current, self-heating to a certain extent will accelerate the volatilization rate of the electrolyte, and the capacity will also decay, showing a vicious cycle trend.

In addition, we will use halogen elements in the flux, which will also indirectly cause voltage changes. MLCC, like electrolytic capacitors, also has its disadvantages and advantages. The most important feature of MLCC is its relatively long life. In addition, its performance at high and low temperatures is very consistent, and its characteristics will not be much different under high and low temperatures.

Disadvantages of MLCC:
1. Compared with electrolytic capacitors of the same capacity, the unit price of MLCC is relatively high;
2. MLCC has the characteristic of bias voltage, and its capacity tends to decay with the applied voltage;
3. The capacity range of MLCC is relatively small, and MLCC cannot be as easy as electrolytic capacitors to achieve 2 million or 3 million.

LED lamp flicker caused a Japanese official to faint.

A common phenomenon in the Japanese industry is that people tend to feel tired when working in a room for a long time, and later found that the reason is flicker. If the power supply of the lamp is insufficient, flicker will occur. When it is less than 30%, people tend to feel tired.

At present, LED lighting is more popular. The Japanese government city hall replaced all ordinary energy-saving lamps with LED lighting. Later, an incident occurred. A government official suddenly fainted during work. Later, it was found that the reason was related to the LED flicker phenomenon. LED flicker can cause fatigue. In order to solve this phenomenon, various manufacturers have also made a lot of efforts. There is a circuit called PFC circuit, which is already used by domestic manufacturers. This belongs to the valley filling circuit of PFC circuit. After the first rectification, in the first smoothing stage, if the voltage goes down, the current will be interrupted. If there is an interruption, flicker will occur.

We also did such a test based on the actual situation, replacing the electrolytic capacitor with MLCC to see if its flicker effect can be well solved. First of all, the pink part is the input voltage, the yellow part is the curve of the voltage after valley filling, and the blue part is a performance of the LED current. In the original design, the customer used a 4.7uf electrolytic capacitor here. You can see its current curve. Although it presents a waveform, there is no interruption in the middle, so this design will not have flicker.

We also replaced it with MLCC to see its overall performance. When replacing electrolytic capacitors with MLCC, how to choose voltage and capacity is also a problem that everyone has been considering, so we also did a test, that is, using a smaller model than 4.7uf, 2.2uf, for testing. After the test, it was found that the yellow voltage part would go down. If it goes down, the current flowing over here will be interrupted, so flicker will occur. That is to say, when replacing electrolytic capacitors with MLCC, if a small capacity is used, the flicker phenomenon cannot be solved.

Next, we use a larger capacity, 10uf, for testing. After the test with 10uf, the lower left picture is shown, and it is found that this phenomenon can be solved. From the results, this experiment can prove that if used for this purpose, it is completely possible to replace electrolytic capacitors with MLCC. From this picture, you can see that when replacing electrolytic capacitors with MLCC, you cannot replace them with the same capacity. You have to choose an MLCC with a slightly larger capacity to replace them. If you export, if you export to Japan, Japan is a country that guarantees a lifespan of 40,000 hours. If you use electrolytic capacitors, there is no way to meet their requirement of 40,000 hours for light bulbs, so everyone generally considers using MLCC. Especially for indoor lights like this, the electrolytic capacitor itself is very large, and if it is a small substrate, MLCC has a great advantage. 

In Japan, the idea of ​​replacing electrolytic capacitors with MLCC has been spread.

Now in Japan, the lifespan of electrolytic capacitors has become a big problem. Some cutting-edge LED manufacturers in Japan are also developing bulbs made entirely of MLCC. A certain front-end manufacturer in Japan has developed this kind of power supply, and all the capacitors used in the entire power supply have been replaced with MLCC. (For confidentiality reasons, the picture cannot be shown, it is just an introduction to the idea). In Japan, this idea has been fully spread, and some cutting-edge manufacturers are already using MLCC to replace electrolytic capacitors.

Considering three aspects, the application of MLCC in the field of LED lighting is still possible. In the future, front-end manufacturers may take this route more: considering long life, small substrates, and thin substrates, this kind of substrate can only be pasted with MLCC. The

main MLCCs used in the field of LED lighting are divided into three categories: one is medium-voltage products, products from 100V to 630V; the second is high-temperature corresponding products, corresponding to 150 degrees; the third is stacked capacitors (resistant to mechanical stress, resistant to heat shock 16V-630V). Stacked capacitors are capacitors supported by metal brackets. They have very high performance at high temperatures and are more commonly used in street lamps, such as traffic lights and street lamps.