Long-life LED Driver Circuit_Part II

The Performance of Electrolytic Capacitors

Ultra long life low voltage aluminum electrolytic capacitors

Adopting new technology to achieve true ultra-long life low voltage aluminum electrolytic capacitors

If low-voltage electrolytic capacitors want to achieve 10,000 hours at a temperature of 105 °C, from the perspective of product quality control and product consistency, electrolytes with high water content should be avoided. Turn to other high-tech.

Mainly include high-quality electrolyte with lower moisture content, new corrosion foil structure and related technologies. Strict production quality management, such as the cleanliness of the production environment, preventing chloride ions from entering the production process (such as chlorine ions exhaled by production operators, etc.), how to prevent moisture from entering the core of electrolytic capacitors, etc.

Through the implementation of the above-mentioned new technologies, new materials and advanced production management measures, the low-voltage aluminum electrolytic capacitors using electrolytes with lower moisture content can fully obtain a life of 105°C / 10,000 hours.

Compared with the low-voltage aluminum electrolytic capacitor with high water content electrolyte, the performance difference of the new low-water content low-voltage aluminum electrolytic capacitor is only slightly larger in ESR. For example, the same 470μF/25V specification, the ESR of RUBYCON’s ZLH series aluminum electrolytic capacitors with high moisture content is only 39mΩ, while the ESR of YXJ series low-voltage aluminum electrolytic capacitors with lower moisture content is 80mΩ. For the 80mΩ ESR of the 470μF/25V specification, there is no problem in application, and there is no need to use 39mΩ.

Problem solving

The problem of early failure caused by hydration reaction of low-voltage and low-resistance aluminum electrolytic capacitors is solved by using low-voltage aluminum electrolytic capacitors with lower moisture content electrolytes. It completely avoids the increase in product price caused by the anti-hydration reaction technology and process. Ensure product consistency and stability. In the series of ultra-long-life low-voltage aluminum electrolytic capacitors, the price of low-water content low-voltage aluminum electrolytic capacitors is lower than that of high-water content low-voltage aluminum electrolytic capacitors.

The 50,000-hour LED drive life completely solves the problem that the life of the LED lamp is significantly lower than the design standard caused by the life of the aluminum electrolytic capacitor. It completely solves the short board of aluminum electrolytic capacitors that affects the life of LED drivers.

Features of Electrolytic Capacitors

Electrolytic capacitors are the cheapest of all types of electrolytic capacitors. Therefore, it is often used in single-phase rectification filtering and DC/DC converters or AC/DC converter output filtering capacitors that require sufficient capacitance.

The LED drive circuit is actually a special case of the switching power supply, that is, a switching power supply that requires voltage limiting and constant current. Since LED lighting requires a heat sink, the LED drive circuit often needs to be made into an isolated converter. For low-power LED drive circuits, in order to reduce costs, flyback switching power supplies are often made.

If the drive circuit and the LED are integrated, the drive circuit will work in a high temperature state, and each component needs to be able to withstand high temperature. LED life is very long, up to tens of thousands of hours, the drive circuit should also meet this requirement. The key element that affects the LED driver circuit is the electrolytic capacitor. Relative to energy-saving lamp capacitors, the volume is small. Small volume requires higher quality aluminum foil and electrolyte, which reflects the improvement of the core technology of aluminum electrolytic capacitor manufacturing.

Characteristics of Capacitors Required for LED Driver Circuits

The places where electrolytic capacitors are needed in the LED driver circuit are:

① Input rectification filter

② Output rectification filter

③ Control IC power bypass

 

1. Input rectification filter

The role of the input rectifier filter capacitor:

① Smooth input rectified voltage

Smoothing the input rectified filter voltage to 20% of the average value (200V after AC 176V rectification, 40V peak-to-peak ripple voltage) requires a capacitance of 1μF/W. With such a large capacitance, only electrolytic capacitors can meet the requirements at low cost.

② Absorb the ripple current generated from the rectifier circuit

When the capacitance of the filter capacitor is selected as 1μF/W, the corresponding filter angular frequency is 15.2 radians/second. It can be seen from the characteristic curve of the rectifier filter capacitor that the current flowing through the capacitor is almost 3 times the average current of the rectified output and 1.5 times the effective value of the AC input current. Or about 7mA/W in terms of output power.

Considering comprehensive factors such as efficiency, it can be approximated as 10mA/W, or it can be approximated as a power frequency ripple current of about 10mA per μF capacitance. In addition, the input rectifier filter capacitor also absorbs the ripple current generated by the LED drive circuit.

③ Absorb the ripple current generated by the LED drive circuit

Generally, the low-power LED drive circuit mostly adopts the flyback converter circuit topology, and the current is discontinuous at the 220V voltage level. In this state, the current drawn by the flyback converter from the rectified output is an intermittent sawtooth wave. Its AC component needs to be absorbed by a filter capacitor. Calculated based on a duty cycle of 0.4, 10mA RMS current is required for every 1W of output power. Taken together, about 15mA rms current is required per μF.

2. Output rectification filter

The output rectifier filter capacitor of the flyback converter will also flow a relatively large current. From the viewpoint of stability, a relatively large capacitance is also required.

This requires that the output rectifier filter capacitor not only needs a relatively large capacitance, but also has a relatively large ripple current bearing capacity. The requirement for the output rectifier filter capacitor with sufficient capacitance is too low will cause the flyback converter to become unstable. Obviously electrolytic capacitors are the best choice. Sufficient ripple current capability requires capacitors with very low ESR. If electrolytic capacitors are used, very low ESR electrolytic capacitors need to be selected.

 

Current flowing through the output filter capacitor

For a normally designed flyback converter, the duty cycle of the output rectifier diode is about 0.5, and the current flowing through the corresponding filter capacitor is 120% of the average output current. That is to say, to output an average current of 1A, the filter capacitor will flow an RMS current of 1.2A.

3. Control IC Power Supply Bypass Capacitor

The control IC power supply bypass capacitor also requires a 50,000-hour lifetime with the LED application state. Among them, the stability of the capacitance is very important, which is related to whether the power supply voltage of the control IC is smooth, so as to reduce the impact on the output of the LED driving circuit as much as possible.

Although the capacitance of this electrolytic capacitor is small, the volume is small, and there is not much ripple current flowing, but it needs a long-term stable capacitance. In particular, the small size is not conducive to the ultra-long life characteristics.

LED Dedicated Long-life Electrolytic Capacitors

ZLH, YXJ series low-voltage electrolytic capacitors and BXA, BXC series high-voltage long-life electrolytic capacitors, electrolytic capacitors with a diameter of 10mm can reach 105 ℃ / 10,000 hours. According to theoretical calculations, it can reach 80,000 hours at 75 °C. Considering other influencing factors, there is no problem in reaching 75°C / 50,000 hours life, which is consistent with the LED life. The last short board affecting the LED driver circuit has been removed, and perhaps it is not necessary to have an LED driver circuit without electrolytic capacitors.

Chinese electrolytic capacitor manufacturers have launched LKF, LKG series high and low voltage LED special long-life electrolytic capacitors. The experimental results compared with the life of RUBYCON are as follows:

Test Conditions for Life Experiment of High Voltage Electrolytic Capacitors Experimental conditions:

Temperature: 125℃,

DC voltage: 351.0VDC,

AC voltage 34.6VAC,

Ripple current: 88mA,

Power supply: 50 Hz.

Experiment purpose: Endurance test to verify life time.

 

The 125°C high temperature life test is performed on the 105°C electrolytic capacitor. Obviously, this is a method to accelerate the experiment.

Conclusion

The life test of domestic 6.8μF/400V /φ10*17 LKG electrolytic capacitors is basically the same as that of Rubycon 6.8μF / 400 10*16 BXA, with a lifespan of 105℃ / 10,000 hours.

The life test of domestic 470μF/25V/φ10*17mm LKG electrolytic capacitors and Rubycon 470μF/25V/φ10*16mm BXA are basically the same, with a lifespan of 105°C / 10,000 hours.

Except for a slightly larger size, domestic long-life electrolytic capacitors can be used in long-life LED drive circuits. Based on years of technology and research and development, domestic manufacturers of aluminum electrolytic capacitors have launched alternative products with the same volume and performance as RUBYCON.

Development of RUBYCON Electrolytic Capacitors

RUBYCON’s ZLH series low-voltage electrolytic capacitors have extremely low ESR and are high-water electrolytic capacitors that require very good hydration-resistant manufacturing technology. Of course the price will be higher. Therefore, RUBYCON newly launched YXJ series low-ESR electrolytic capacitors, which are non-high water-based electrolytic capacitors, which can avoid the difficulty of hydration reaction control caused by high-water-based electrolytes. It also reduces costs. However, the problem is that the ESR is nearly double that of the ZLH series, and it is basically the same as the domestic LKF and LKG series.

RUBYCON’s high pressure BXA series recommends the BXC series due to the high water content. But the loss factor of the BXC series will be slightly larger than the BXA series. This is a compromise between the reliability and high performance of long-life electrolytic capacitors. At the same time, the low-voltage YXJ is also a substitute for ZLH. These all reflect that the high water content is gradually replaced by the conventional water-containing aluminum electrolytic capacitors with new process materials and new manufacturing technologies.

This is an opportunity for electrolytic capacitor manufacturers, which should be seized to develop electrolytic capacitors for driving long-life LED driver circuits.