introduction

Electrolytic capacitors are one of the most important components in the primary and secondary 

circuit filtering circuits of switching power supplies. Generally, the equivalent circuit of an electrolytic

 capacitor can be considered as a series connection of an ideal capacitor, parasitic inductance, and

 equivalent series resistance, as shown in Figure 1.

Figure 1 Equivalent circuit of electrolytic capacitor

As is well known, switch mode power supply is the main power source for information appliances

 today, making an indelible contribution to the miniaturization and portability of electronic devices. 

The continuous miniaturization, lightweighting, and high efficiency of switch mode power supplies

 have led to their increasing usage and popularity in electronic devices. Correspondingly, electrolytic 

capacitors are required to be small, high-capacity, ripple resistant, high-frequency, low impedance, 

high-temperature, long-life, and more suitable for high-density assembly.

1. Capacity and volume

Due to the winding structure commonly used in electrolytic capacitors, it is easy to expand the volume,

 resulting in a very large unit volume capacity, several to tens of times larger than other capacitors. 

However, the acquisition of large capacitance comes at the cost of expanding the volume. Modern 

switching power supplies require increasingly high efficiency and smaller volumes. Therefore, it is 

necessary to seek new solutions to obtain capacitors with large capacitance and small volume.

Once an active filter circuit is used on the primary side of a switching power supply, the usage 

environment of aluminum electrolytic capacitors becomes more severe than before:

(1) The high-frequency pulse current mainly consists of pulsating currents ranging from 20 kHz to 

100 kHz, and increases significantly;

(2) The main switch tube of the converter heats up, causing an increase in the surrounding 

temperature of the aluminum electrolytic capacitor;

(3) Transformers often use boost circuits, therefore requiring high voltage resistant aluminum 

electrolytic capacitors. In this way, aluminum electrolytic capacitors manufactured using traditional 

technology have to choose large-sized capacitors due to the need to absorb larger pulsating currents

 than before. As a result, the power supply becomes bulky and difficult to use for miniaturized 

electronic devices. To solve these problems, it is necessary to research and develop a new type of

 electrolytic capacitor that is small in size, resistant to high voltage, and allows a large amount of 

high-frequency pulse current to flow. In addition, this type of electrolytic capacitor needs to have a 

relatively long service life when working in high-temperature environments.

2. Temperature tolerance and lifespan

In the design process of switch mode power supplies, it is inevitable to select suitable capacitors.

 For medium and large capacity products above 100 μ F, aluminum electrolytic capacitors are the most 

widely used due to their low price. However, significant changes have occurred in recent years, with 

an increasing trend towards avoiding the use of aluminum electrolytic capacitors. One reason for this

 change is that the lifespan of aluminum electrolytic capacitors often becomes a weak link in the entire

 equipment. Engineers from power module manufacturers say, "For components with limited lifespan 

such as aluminum electrolytic capacitors, if possible, try not to use them." This is because the 

electrolyte inside aluminum electrolytic capacitors will evaporate or undergo chemical changes, 

resulting in a decrease in electrostatic capacity or an increase in equivalent series resistance (ESR). 

Over time, the performance of the capacitor will definitely deteriorate.

The lifespan of electrolytic capacitors is directly related to the ambient temperature in which they 

operate for a long time. The higher the temperature, the shorter the lifespan of the capacitor. 

Ordinary electrolytic capacitors are already damaged at an ambient temperature of 90 ℃. But now 

there are many types of electrolytic capacitors that work in environments with high temperatures. 

At an ambient temperature of 90 ℃, when the ratio of AC current to rated pulse current through the 

electrolytic capacitor is 0.5, the lifespan is still 10000 hours. However, if the temperature rises to 95 ℃, 

the electrolytic capacitor is already damaged. Therefore, when selecting capacitors, they should be

 chosen based on specific environmental temperature and other parameter indicators. If the impact of

 environmental temperature on the lifespan of capacitors is ignored, the reliability and stability of

 power supply operation will be greatly reduced, and even equipment and instruments may be 

damaged. Generally speaking, electrolytic capacitors can achieve a lifespan of 10000 hours when 

operating at an ambient temperature of 80 ℃.

On the other hand, the lifespan of electrolytic capacitors is also related to the ratio of the AC current 

during long-term operation to the rated pulse current (usually measured at an ambient temperature 

of 85 ℃, but some high-temperature resistant electrolytic capacitors are tested at 125 ℃). Generally 

speaking, the larger the ratio, the shorter the lifespan of the electrolytic capacitor. When the current 

flowing through the electrolytic capacitor is 3.8 times the rated current, the capacitor is usually already 

damaged. So, electrolytic capacitors have their own safe working area. For general applications, when 

the ratio of AC current to rated pulse current is below 3.0 times, the requirements for lifespan have 

been met. The influence of environmental temperature and ripple current on electrolytic capacitors is 

shown in Figure 2.

Figure 2: The relationship between the lifespan, temperature, and ripple current of a certain aluminum

 electrolytic capacitor

3 Frequency characteristics and impedance

For small and medium-sized output power switching power supplies, the operating frequency is 

mostly above 50kHz, except for a few that still use 20-40kHz due to price restrictions; DC/DC power

 modules are mostly above 300kHz; The switching frequency of high-power switching power supplies

 is generally limited to 20-40kHz by the switching speed of the main switch (usually IGBT). Although 

the switching frequencies are different, the output rectification and filtering capacitors of the 

switching power supply have basically the same function, mainly by using the filtering capacitors to

 absorb the current components of the switching frequency and its higher-order harmonics, and filter

 out their ripple voltage components.

The filtering capacitor used at the output end of the switching power supply is different from the 

filtering capacitor selected in the power frequency circuit. The ordinary electrolytic capacitor used for

 filtering in the power frequency circuit has a pulsating voltage frequency of only 100Hz and a 

charging and discharging time on the order of milliseconds. In order to obtain a smaller pulsation

 coefficient, the required capacitance is as high as hundreds of thousands of microfarads. Therefore, 

the main goal of manufacturing ordinary aluminum electrolytic capacitors for low frequencies is to 

increase the capacitance. The capacitance, loss tangent value, and leakage current of capacitors are 

the main parameters to distinguish their advantages and disadvantages. Electrolytic capacitors used 

as output filters in switch mode power supplies, due to the fact that most switch mode power supplies 

operate in a square wave or rectangular wave state, contain extremely rich high-order harmonic 

voltages and currents. The frequency of the sawtooth voltage on them can reach tens of kilohertz, 

even tens of megahertz. Its requirements are different from low-frequency applications, and 

capacitance is not the main indicator. The measure of its quality is its impedance frequency 

characteristics, as shown in Figure 3.

Figure 3 Impedance frequency characteristics of a 47 μ F/350V aluminum electrolytic capacitor

As shown in the figure, with the increase of frequency, the capacitance impedance decreases and the 

inductance impedance increases. The frequency at which the capacitance impedance equals the

 inductance and cancels each other out is the resonant frequency of the aluminum electrolytic

 capacitor. At this time, the impedance is the lowest, leaving only ESR. If ESR is zero, then the

 impedance at this time is also zero; As the frequency continues to rise, the inductive reactance begins

 to exceed the capacitive reactance. When the inductive reactance approaches ESR, the impedance

 frequency characteristics begin to rise and become inductive. From this frequency onwards, the 

capacitor becomes an inductor in time. Due to manufacturing processes, the larger the capacitance,

 the greater the parasitic inductance, and the lower the resonant frequency, resulting in a lower 

inductive frequency of the capacitor. This requires it to have a low equivalent impedance within the

 operating frequency range of the switching power supply. At the same time, for the power supply, 

due to the sharp noise generated by semiconductor devices starting to work, it can also have good 

filtering effect. Generally, ordinary electrolytic capacitors are used at around 10kHz for low frequencies,

 and their impedance begins to show inductance, which cannot meet the requirements of switching 

power supply use.

Electrolytic capacitors used for output rectification of switch mode stabilized power supplies are

 required to have impedance frequency characteristics that do not show an upward trend at 300kHz

 or even 500kHz. Electrolytic capacitors have a low ESR, which can effectively filter out high-frequency 

ripples and peak voltages in switch mode power supplies. However, ordinary electrolytic capacitors

 begin to show an upward trend after 100kHz, and their rectification and filtering effects for switching

 power supply output are relatively poor. The author found in the experiment that the ripple and peak

 of ordinary CDII type 4700 μ F, 16V electrolytic capacitors used for output filtering of switching power 

supplies are not lower than those of CD03HF type 4700 μ F, 16V high-frequency electrolytic capacitors, 

and the temperature rise of ordinary electrolytic capacitors is relatively high. When the load undergoes

 a sudden change, the transient response of ordinary electrolytic capacitors is far inferior to that of 

high-frequency electrolytic capacitors.

Switching power supplies have increased their operating frequency for high efficiency, especially in 

small high output switching power supplies where input filtering capacitors require high ripple and 

low impedance at the output end. To reduce the impedance of output filtering capacitors at high 

frequencies, it is necessary to lower the equivalent series resistance.

4. Ripple current tolerance

One of the most important parameters affecting the performance of electrolytic capacitors is the 

ripple current problem. The main impact of ripple current on aluminum electrolytic capacitors is the 

generation of power consumption on ESR, which causes the capacitors to heat up and shorten their 

service life. From the characteristic curve (Figure 2), it can be seen that the loss generated by ripple 

current on ESR is proportional to the square of the effective value of ripple current. Therefore, as the 

ripple current increases, the hourly life curve resembles a parabolic function curve. The method to 

reduce ripple current can be to use larger capacity aluminum electrolytic capacitors, as the ripple 

current that large capacity aluminum electrolytic capacitors can withstand is greater than that of small

 capacity aluminum electrolytic capacitors; Multiple small capacity aluminum electrolytic capacitors 

can also be connected in parallel, and a circuit topology with low ripple current can be selected. 

Generally speaking, the switching current generated by flyback converters is relatively maximum. 

Table 1 shows the DC current, ripple current of rectification and filtering, switch conversion current,

 and total ripple current on the filtering capacitor for various switch converter circuit topologies.

Table 1 Ripple current and switching current of rectification and filtering for various switch converter 

circuit topologies

For flat screen TVs, in order to withstand high currents, it is necessary to further reduce the ESR of the 

capacitor. The reason is that in digital devices, as the functionality increases, the current of the circuit 

tends to increase. For image processing circuits that perform MPEG encoding and decoding in LCD TVs,

 the current of the power circuit in a single chip was about 3A in 2006. According to predictions from 

relevant sources, after increasing the size of the circuit to meet the requirements of full HD (Full HD), 

the current in the chip will increase to around 5A, and will reach 8A~9A around 2008.

If the ESR is small, the decrease in the output voltage of the capacitor is also small when there is a

 large current flowing. The requirement to reduce ESR with the increase of current may become the 

main reason for promoting the process of capacitor replacement. Compared to the ESR of aluminum 

electrolytic capacitors, which is nearly 1 Ω, the ESR of multi-layer ceramic capacitors is very small, less 

than 10m Ω. The ESR of conductive polymer capacitors is usually several tens of m Ω, and those with 

relatively small ESR are below 10m Ω. Aluminum electrolytic capacitors are also developing products 

with relatively low ESR, which is about 1/2 to 1/3 of that of general products.

5

Reliability level

Switching power supply is a type of DC regulated power supply that uses switch mode control. It is

 widely used in various communication equipment, household appliances, computers, and terminal

 devices due to its small size, lightweight, and high efficiency. As an aluminum electrolytic capacitor 

with input filtering and smoothing functions, its quality and reliability directly affect the reliability of 

the switching power supply. Once the aluminum electrolytic capacitor fails, it will lead to the malfunction 

of the switch voltage regulator power supply.

The failure modes of aluminum electrolytic capacitors used in switch mode stabilized power supplies 

include breakdown failure, open circuit failure, leakage failure, and electrical parameter out of tolerance

 failure. Among them, breakdown failure is divided into dielectric breakdown and thermal breakdown. 

For electrolytic capacitors used in high-power and high current output switching power supplies,

thermal breakdown failure often accounts for a certain proportion; The main failure mode of aluminum

 electrolytic capacitors used in switch mode power supplies is open circuit failure caused by 

electrocorrosion, which leads to the breakage of aluminum lead strips and the drying of capacitor cores

; Leakage is a common failure mode of aluminum electrolytic capacitors used in switch mode power 

supplies. Due to the harsh operating environment and conditions, leakage failure often occurs; The 

most common failure modes of aluminum electrolytic capacitors used in switch mode power supplies 

are reduced capacitance, increased leakage current, and increased loss tangent.

6. Summary

Electrolytic capacitors are essential in electronic circuits, and with the miniaturization of electronic 

devices, there is an increasing demand for electrolytic capacitors to have better frequency characteristics,

 lower ESR, lower impedance, lower ESL, higher voltage resistance, and lead-free. This is also the future 

development direction of electrolytic capacitors. Small and high-capacity capacitors can be achieved 

through the use of new dielectric materials such as niobium and titanium, as well as structural

 improvements. Low ESR and low ESL can be achieved through the development and optimization of

processes and structures for new electrolytes, while the product will develop towards higher voltages. 

In the rapidly developing field of information technology, capacitors will always be one of the key 

components. We will continuously develop high-performance capacitors that meet the needs of the 

information age by applying new technologies and materials.

About Us

Guangdong Fulong Electronic Technology Co., Ltd. is a private high-tech enterprise specializing in the research and development, manufacturing, and sales of aluminum electrolytic capacitors, certified by IATF16949，ISO9001, ISO14001, and ISO45001. The company has a building area of 17000 square meters and more than 50 automated production lines; The total investment is 50 million yuan (RMB), with more than 100 employees, including more than 20 engineering and technical, quality management, and production management personnel. The annual production of capacitors (4 * 5.4~16 * 21.5) is about 400 million pieces; Our products are mainly sold to domestic and international markets, and we strive to promote our company's products globally.

Jiangxi Fulong Electronic Technology Co., Ltd. is a wholly-owned subsidiary of Guangdong Fulong Electronic Technology Co., Ltd. established in Hukou County, Jiangxi Province with a total investment of 500 million yuan. It has 50 fully automated production lines and an annual output of approximately 1.2 billion capacitors (4 * 5.4~16 * 21.5).

Fulong Group has an international professional core technology research and development team for aluminum electrolytic capacitors and a global sales network. Rich experience in the design, research and development, and production management of aluminum electrolytic capacitors. The R&D team follows the technical design theory of aluminum electrolytic capacitors, complies with international and domestic legal and regulatory requirements (RoHs), and adheres to the highest international quality standards

Fulong Group has high-quality and stable raw material supply partners, and key raw materials (such as aluminum foil, electrolytic paper, etc.) are selected from well-known Japanese and Korean companies (KJCC KDKNKK, etc.), ensuring reliable and stable advantages in product quality, environmental protection, safety, and energy conservation.

Fulong Group has a group of highly qualified professional technicians and management personnel who can produce various series of sheet aluminum electrolysis products according to customer requirements. The products include high-frequency low impedance, low leakage, wide temperature, extremely low impedance, non-polar, ultra long life, medium high voltage long life, high reliability and other new types of special requirements for surface mount products; At the same time, the company's products can withstand a soldering temperature of 260 ℃, meeting the lead-free reflow soldering technology required for SMT in the whole machine factory. All technical indicators of the products have reached or exceeded the international level of similar products.

While strengthening management and expanding operational scale, Fulong Group is also committed to improving its competitiveness and customer satisfaction, continuously and steadily increasing customer and social trust, continuously meeting market demand, further optimizing environmental management and expanding social contributions, and developing together with the electronics industry.

Guangdong Follon Electronic Technology Co., Ltd.

Jiangxi Follon Electronic Technology Co., Ltd.

Address: Building 3, South District, Haishan Science and Technology Park, Hukou County, Jiujiang City, Jiangxi Province

www.follon.com

Professional manufacturer of high-quality surface mount electrolytic capacitors