Generally, the equivalent circuit of electrolytic capacitor can be considered as the series connection of ideal capacitor, parasitic inductance and equivalent series resistance, as shown in Figure 1. Korean Sanying electrolytic capacitor is one of the most important devices in the primary and secondary circuit filter circuit of switching power supply.
Figure 1 Equivalent circuit of electrolytic capacitor
As we all know, switching power supply is the main power supply of information appliances today, making an indelible contribution to the miniaturization and portability of electronic equipment. With the continuous miniaturization, lightweight and high efficiency of switching power supply, it is used more and more in electronic equipment, and its popularity is higher and higher. Correspondingly, electrolytic capacitors are required to be small, large capacity, ripple current resistant, high-frequency and low impedance, high temperature and long life, and more suitable for high-density assembly.
Capacitance and volume
Once the active filter circuit is used on the primary side of the switching power supply, the use environment of aluminum electrolytic capacitors becomes more severe than before:
High frequency pulse current is mainly 20 kHz~100kHz pulse current, and it increases greatly;
The main switch tube of the converter heats up, causing the ambient temperature of the aluminum electrolytic capacitor to rise;
Most of the converters use boost circuits, so high voltage resistant aluminum electrolytic capacitors are required.
Sanying electrolytic capacitor is small in size, high voltage resistant, and allows a large amount of high-frequency pulse current to flow. In addition, the working life of the electrolytic capacitor must be relatively long when it works under high temperature.
Bearing temperature and service life
The life of the electrolytic capacitor is directly related to the ambient temperature of the capacitor for long-term operation. The higher the temperature is, the shorter the life of the capacitor is. Ordinary electrolytic capacitors have been damaged when the ambient temperature is 90 ℃. However, there are many kinds of electrolytic capacitors with very high operating environment temperature. When the ambient temperature is 90 ℃ and the ratio of the AC current passing through the electrolytic capacitor to the rated pulse current is 0.5, the service life is still 10,000h. However, if the temperature rises to 95 ℃, the electrolytic capacitor is damaged.
Therefore, capacitors should be selected according to specific ambient temperature and other parameters. If the influence of ambient temperature on capacitor life is ignored, the reliability and stability of power supply will be greatly reduced, and even equipment and instruments will be damaged. Generally speaking, when the ambient temperature is 80 ℃, the electrolytic capacitor can reach the requirement of 10000h life.
On the other hand, the service life of electrolytic capacitors is also related to the ratio of the AC current and rated pulse current of capacitors working for a long time (generally refers to the test value at 85 ℃, but some high temperature resistant electrolytic capacitors are tested at 125 ℃). Generally speaking, the higher the ratio, the shorter the life of the electrolytic capacitor. When the current flowing through the electrolytic capacitor is 3.8 times of the rated current, the electrolytic capacitor is generally damaged. Therefore, electrolytic capacitors have their own safe working area. For general applications, when the ratio of AC current to rated pulse current is less than 3.0 times, the requirements for service life have been met. The influence of ambient temperature and ripple current on electrolytic capacitors is shown in Figure 2.
Fig. 2 The relationship between the life of an aluminum electrolytic capacitor and temperature, ripple current
Frequency characteristics and impedance
The filter capacitor used at the output end of the switching power supply is different from the filter capacitor used in the power frequency circuit. The common electrolytic capacitor used as the filter in the power frequency circuit has a pulse voltage frequency of only 100Hz, and the charge discharge time is in the order of milliseconds. In order to obtain a small pulse coefficient, the capacitance required is as high as hundreds of thousands of micro methods. Therefore, the manufacturing goal of common aluminum electrolytic capacitors for low frequency is to improve the capacitance, Capacitance, tangent value of loss angle and leakage current of capacitor are the main parameters to identify its advantages and disadvantages.
As the electrolytic capacitor used for output filtering in switching regulated power supply, most switching power supplies work in the state of square wave or rectangular wave, and contain extremely rich high-order harmonic voltage and current. The frequency of sawtooth wave voltage on the capacitor is as high as tens of kilohertz, or even tens of megahertz. Its requirements are different from those in low-frequency applications. Capacitance is not the main indicator, but its impedance frequency characteristics are the measure of its quality, as shown in Figure 3.
Figure 3 47 μ Impedance Frequency Characteristics of F/350V Aluminum Electrolytic Capacitors
It can be seen from the figure that with the increase of frequency, the capacitive reactance decreases and the inductive reactance increases. When the capacitive reactance is equal to the inductive reactance and mutually cancels, the frequency 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 is zero; If the frequency continues to rise, the inductive reactance starts to be greater than the capacitive reactance. When the inductive reactance is close to the ESR, the impedance frequency characteristic starts to rise and become inductive. From this frequency, the capacitor is an inductor in time at the frequency above.
Electrolytic capacitors used for output rectification of switching regulated power supply are required to have impedance frequency characteristics that do not show an upward trend at 300 kHz or even 500 kHz. Electrolytic capacitors have low ESR, which can effectively filter high-frequency ripple and peak voltage in switching power supply. However, ordinary electrolytic capacitors began to show an upward trend after 100kHz, and the filtering effect of switching power supply output rectifier is relatively poor.
4700 in common CDII type μ F. 16V electrolytic capacitor, the ripple and peak used for switching power supply output filtering are no better than CD03HF 4700 μ F. The 16V high-frequency electrolytic capacitor is low, and the temperature rise of ordinary electrolytic capacitor is relatively high. When the load is sudden change, the transient response of ordinary electrolytic capacitor is far less than that of high-frequency electrolytic capacitor.
In order to achieve high efficiency, switching power supply improves the high frequency of working frequency, especially in small high output switching power supply, the input filter capacitor requires high ripple, and the output terminal has low impedance. In order to reduce the impedance of output filter capacitor at high frequency, the equivalent series resistance must be reduced.
Ripple current withstand
Ripple current is one of the most important parameters that affect the performance of electrolytic capacitors. The ripple current mainly affects the aluminum electrolytic capacitor by generating power consumption on the ESR to make the aluminum electrolytic capacitor heat up, thereby shortening its service life. From the characteristic curve (Fig. 2), it can be seen that the loss of ripple current on ESR is proportional to the square of the effective value of ripple current. Therefore, with the increase of ripple current, the hourly life curve is similar to a parabolic function curve.
The ripple current can be reduced by using aluminum electrolytic capacitors with large capacity. After all, large capacity aluminum electrolytic capacitors can withstand more ripple current than small capacity aluminum electrolytic capacitors; The parallel connection of several small capacity aluminum electrolytic capacitors can also be used, and the circuit topology with low ripple current can also be used. In general, flyback converter generates the largest switching current. Table 1 shows the DC current, ripple current of rectifier filter, switching current and total ripple current on filter capacitor of various switching converter circuit topologies.
Table 1 Ripple Current and Switching Current of Rectification and Filtering of Various Switching Converter Circuit Topologies
As for flat panel TV, in order to withstand large current, it is necessary to further reduce the ESR of capacitance. The reason is that in digital equipment, with the increase of functions, the current of the circuit has a growing trend. For the image processing circuit for MPEG encoding and decoding in LCD TV, the current of power circuit in a chip was about 3A in 2006.
If the ESR is small, the drop of capacitor output voltage is small when there is a large current flow. With the increase of current, the requirement of reducing ESR may become the main reason to promote the process of capacitor replacement. Compared with the ESR of aluminum electrolytic capacitor of nearly 1 Ω, the ESR of multilayer ceramic capacitor is very small, less than 10 m Ω. The ESR of conductive polymer capacitors is usually tens of m Ω, while the ESR of smaller ones is less than 10m Ω. Aluminum electrolytic capacitors are also developing products with small ESR, which is about 1/2~1/3 of that of general products.
Switching power supply is a kind of DC stabilized voltage power supply with switching control, which is widely used in various communication equipment, household appliances, computers and terminal equipment with the characteristics of small size, light weight and high efficiency. As an input filter and smoothing function, the quality and reliability of aluminum electrolytic capacitor directly affect the reliability of switching power supply. Once the aluminum electrolytic capacitor fails, it will lead to the failure of switching power supply.
The failure modes of aluminum electrolytic capacitors for switching power supply include breakdown failure, open circuit failure, liquid 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 switching power supply with large power and large current output, thermal breakdown failure often accounts for a certain proportion; The main failure mode of open circuit failure of aluminum electrolytic capacitors for switching voltage stabilized power supply is that the aluminum lead strip breaks and the capacitor core dries up due to electric corrosion; Leakage is a common failure mode of aluminum electrolytic capacitors for switching power supply. Due to the harsh operating environment and working conditions, leakage failures often occur; The most common failure modes of aluminum electrolytic capacitors used in switching power supply are capacitance reduction, leakage current increase and loss tangent increase.
Electrolytic capacitors are indispensable in electronic circuits. Moreover, with the miniaturization of electronic equipment, electrolytic capacitors are increasingly required to have better frequency characteristics, lower ESR, lower impedance, lower ESL, higher voltage withstand performance and lead-free, which is also the development direction of electrolytic capacitors in the future. In the rapidly developing field of information technology, capacitors will always be one of the key components. We will continue to meet the needs of the information age to provide high-performance capacitors.