Electrolytic capacitors are one of the most important devices in primary and secondary loop filter circuits in switching power supplies. In general, the equivalent circuit of an electrolytic capacitor can be considered as a series connection of an ideal capacitor with a parasitic inductance and an equivalent series resistance, as shown in FIG.
Figure 1 equivalent circuit of electrolytic capacitor
As we all know, switching power supply is the main power source of today's information appliances, making an indelible contribution to the small and lightweight electronic devices. Switching power supplies continue to be smaller, lighter, and more efficient, and are increasingly used in electronic devices, and the penetration rate is increasing. Correspondingly, electrolytic capacitors are required to be small and large in capacity, resistant to ripple current, high frequency and low impedance, high temperature and long life, and more suitable for high density assembly.
Large volume, small volume
Since most of the electrolytic capacitors are wound structures, it is easy to expand the volume, so the capacitance per unit volume is very large, several times to several tens of times larger than other capacitors. However, the acquisition of large capacitance is at the expense of volume expansion. Modern switching power supplies require higher and higher efficiency and smaller and smaller volume. Therefore, it is necessary to find new solutions to obtain large capacitance and small size. Volume of capacitors.
Once an active filter circuit is used on the primary side of the switching power supply, the environment in which the aluminum electrolytic capacitor is used becomes more severe than before:
(1) The high-frequency pulse current is mainly a pulsating current of 20 kHz to 100 kHz, and is greatly increased;
(2) The main switch of the inverter generates heat, which causes the ambient temperature of the aluminum electrolytic capacitor to rise;
(3) The converter uses a booster circuit, so it is required to withstand high voltage aluminum electrolytic capacitors. As a result, the aluminum electrolytic capacitor manufactured by the prior art has to select a larger-sized capacitor because it absorbs a larger pulsating current than ever before. As a result, the power supply is bulky and difficult to use for miniaturized electronic equipment. In order to solve these problems, it is necessary to research and develop a new type of electrolytic capacitor, which is small in size, high in voltage resistance, and allows a large amount of high-frequency pulse current to flow. In addition, this electrolytic capacitor operates in a high temperature environment and has a long working life.
High temperature and long life
In the switching power supply design process, it is inevitable to choose the applicable capacitor. For medium and large-capacity products of 100 μF or more, aluminum electrolytic capacitors are the most widely used because they are inexpensive. However, significant changes have occurred in recent years, and the use of aluminum electrolytic capacitors is increasing. One reason for this change is that the life of aluminum electrolytic capacitors tends to be a weak link in the entire device. The engineer of the power module manufacturer said: "For aluminum electrolytic capacitors, such a component with limited life, if it is not necessary, try not to use it." Because the electrolyte inside the aluminum electrolytic capacitor will evaporate or cause chemical changes, resulting in a decrease in electrostatic capacity or The equivalent series resistance (ESR) increases, and the capacitance performance will definitely deteriorate over time.
The life of an electrolytic capacitor is directly related to the ambient temperature at which the capacitor is operated for a long period of time. The higher the temperature, the shorter the life of the capacitor. Ordinary electrolytic capacitors have been damaged at an ambient temperature of 90 °C. However, there are many types of electrolytic capacitors that have a high operating temperature at an ambient temperature of 90 ° C. When the ratio of the AC current to the rated pulse current of the electrolytic capacitor is 0.5, the lifetime is still 10000 h, but if the temperature rises to 95 ° C When the electrolytic capacitor is damaged. Therefore, when selecting a capacitor, it should be selected according to the specific ambient temperature and other parameter indicators. If the influence of the ambient temperature on the life of the capacitor is neglected, the reliability and stability of the power supply operation will be greatly reduced, and even the equipment will be damaged. And instruments. In general, electrolytic capacitors typically operate at an ambient temperature of 80 ° C and typically meet the 10,000 h life requirement.
On the other hand, the life of an electrolytic capacitor is also related to the AC current and rated pulse current of the capacitor for a long time (generally the test value at an ambient temperature of 85 ° C, but some high temperature resistant electrolytic capacitors are tested at 125 ° C) The ratio of the data) is related. In general, the larger the ratio, the shorter the life of the electrolytic capacitor. When the current flowing through the electrolytic capacitor is 3.8 times the rated current, the electrolytic capacitor is generally damaged. Therefore, the electrolytic capacitor has its safe working area. For the general application, when the ratio of the alternating current to the rated pulse current is 3.0 times or less, the life requirement has been satisfied. The effect of ambient temperature and ripple current on the electrolytic capacitor is shown in Figure 2.
Figure 2 Relationship between the life of an aluminum electrolytic capacitor and temperature and ripple current
High frequency and low impedance
For the small and medium output power switching power supply operating frequency except for a few due to price restrictions still use 20 ~ 40kHz, most are above 50kHz; DC / DC power modules are mostly above 300kHz; high power switching power supply switching frequency is subject to the main switch ( The switching speed limit of IGBT is generally used and is generally 20 to 40 kHz. Although the switching frequency is different, the output rectifying and filtering capacitor of the switching power supply has basically the same function, and the ripple voltage component is filtered out by using the filter capacitor to absorb the switching frequency and the current component of the higher harmonic frequency.
The filter capacitor used at the output of the switching power supply is not the same as the filter capacitor selected in the power frequency circuit. The common electrolytic capacitor used as a filter in the power frequency circuit has a ripple voltage frequency of only 100 Hz, and the charge and discharge time is On the order of milliseconds, in order to obtain a small pulsation coefficient, the required capacitance is as high as several hundred thousand microfarads. Therefore, the general low-frequency common aluminum electrolytic capacitor is manufactured with the objective of increasing the capacitance, the capacitance and loss tangent of the capacitor, and Leakage current is the main parameter to identify its advantages and disadvantages. As an electrolytic capacitor for output filtering in a switching power supply, since most switching power supplies operate in a square wave or rectangular wave state, they contain a rich harmonic voltage and current, and the frequency of the sawtooth voltage. Up to tens of kilohertz or even tens of megahertz, its requirements are different from those of low frequency applications. Capacitance is not the main indicator. The good or bad is its impedance frequency characteristic, as shown in Figure 3.
Figure 3 Impedance frequency characteristics of a 47μF/350V aluminum electrolytic capacitor
As can be seen from the figure, as the frequency increases, the capacitive reactance decreases and the inductive reactance rises. The frequency at which the capacitive reactance is equal to the inductive reactance and cancel each other is the resonant frequency of the aluminum electrolytic capacitor. At this time, the impedance is the lowest, and only the ESR remains. If the ESR is zero, then the impedance is also zero; the frequency continues to rise, the inductive reactance begins to be greater than the capacitive reactance, and when the inductive reactance is close to the ESR, the impedance frequency characteristic begins to rise, being inductive, starting from this frequency. The capacitor is an inductor in time. Due to the manufacturing process, the larger the capacitance, the larger the parasitic inductance, the lower the resonance frequency, and the lower the frequency at which the capacitor is inductive. This requires it to have a low equivalent impedance in the operating frequency band of the switching regulator power supply. At the same time, for the internal power supply, due to the peak noise generated by the semiconductor device starting to work up to hundreds of kilohertz, it can also have good filtering effect. Generally, the ordinary low-frequency ordinary electrolytic capacitor is about 10 kHz, and its impedance begins to show inductivity, which cannot meet the requirements of switching power supply.
The electrolytic capacitor used for the output rectification of the switching power supply requires that the impedance frequency characteristic does not show an upward trend at 300 kHz or even 500 kHz. The ESR of the electrolytic capacitor is low, which can effectively filter the high frequency ripple and spike voltage in the switching regulator. Ordinary electrolytic capacitors begin to show an upward trend after 100 kHz, and the effect of switching power supply output rectification and filtering is relatively poor. The author found in the experiment that the ordinary CDII type 4700μF, 16V electrolytic capacitor, the ripple and spike used for switching power supply output filtering is not lower than the CD03HF type 4700μF, 16V high frequency electrolytic capacitor, and the ordinary electrolytic capacitor temperature rise is relatively high. . When the load is abrupt, the transient response of a conventional electrolytic capacitor is much inferior to that of a high frequency electrolytic capacitor. The switching power supply increases the high frequency of the operating frequency for high efficiency. In particular, the input filtering capacitor of the small high-output switching power supply requires high ripple and the output is low-impedance. In order for the output filter capacitor to be low-impedance at high frequencies, the equivalent series resistance must be reduced.
Ripple current
One of the most important parameters affecting the performance of electrolytic capacitors is the ripple current problem. The influence of ripple current on aluminum electrolytic capacitors is mainly caused by the power consumption in the ESR, which causes the aluminum electrolytic capacitor to generate heat, thereby shortening the service life. It can be seen from the characteristic curve (Fig. 2) that the loss generated by the ripple current on the ESR is proportional to the square of the effective value of the ripple current. Therefore, as the ripple current increases, the hourly life curve is similar to the parabolic function curve. The method of reducing the ripple current can use a larger capacity aluminum electrolytic capacitor. After all, the large-capacity aluminum electrolytic capacitor can withstand a larger ripple current than a small-capacity aluminum electrolytic capacitor; a parallel connection of a plurality of small-capacity aluminum electrolytic capacitors can also be used. A circuit topology with low ripple current can also be selected. In general, the flyback converter produces a relatively large switching current. Table 1 shows the DC current, the rectified filtered ripple current, the switching current, and the total ripple current on the filter capacitor for various switching converter circuit topologies.
Table 1 Rectifier filtered ripple current and switching current of various switching converter circuit topologies
In the case of flat-panel TVs, in order to withstand large currents, it is necessary to further reduce the ESR of the capacitor. The reason is that in digital devices, as the function increases, the current of the circuit tends to become larger and larger. For an image processing circuit that performs MPEG codec work in a liquid crystal television, the current of the power supply circuit in one chip is about 3A in 2006. According to the forecast of relevant people, in order to cope with the full HD (full HD and other requirements to increase the size of the circuit, the current in the chip will increase to about 5A, and will reach 8A ~ 9A around 2008.
If the ESR is small, the amount of decrease in the output voltage of the capacitor is small when a large current flows. The requirement to reduce the ESR with an increase in current may be the main cause of the process of propelling the capacitor replacement. Compared to an ESR with an aluminum electrolytic capacitor of nearly 1 Ω, the ESR of a multilayer ceramic capacitor is small and less than 10 mΩ. The ESR of the conductive polymer capacitor is usually several tens of mΩ, and the ESR is smaller than 10 mΩ. Aluminum electrolytic capacitors are also developing relatively small ESR products, and their ESR is about 1/2 to 1/3 of that of general products.
High reliability
The switching power supply is a DC-regulated power supply with switching control. It is widely used in various communication equipment, household appliances, computers and terminal equipment with its characteristics of small size, light weight and high efficiency. As an input filter and smoothing aluminum electrolytic capacitor, its quality and reliability directly affect the reliability of the switching power supply. Once the aluminum electrolytic capacitor fails, it will cause the switching power supply to fail. The failure modes of aluminum electrolytic capacitors used in switching regulators include breakdown failure, open circuit failure, liquid leakage failure, and electrical parameter tolerance. Among them, the breakdown failure is divided into dielectric breakdown and thermal breakdown. For electrolytic capacitors with high power and high current output switching power supply, the thermal breakdown failure often accounts for a certain proportion; the electrical corrosion causes the aluminum lead strip to break and the capacitor core to dry up. The main failure mode of open circuit failure of aluminum electrolytic capacitors for switching power supply; leakage is a common failure mode of aluminum electrolytic capacitors for switching power supply. Due to the harsh environment and working conditions, leakage often occurs; The most common failure modes of aluminum electrolytic capacitors for voltage power supplies are capacitance reduction, leakage current increase, and loss tangent.
to sum up
Electrolytic capacitors are indispensable in electronic circuits, and with the miniaturization of electronic devices, electrolytic capacitors are increasingly required to have better frequency characteristics, lower ESR, lower impedance, lower ESL, and higher resistance. Pressure performance, lead-free, this is also the future development direction of electrolytic capacitors. Capacitors that are miniaturized and bulk-capable can be achieved by using new dielectric materials such as tantalum and titanium and structural improvements. Low ESR and low ESL can be achieved through the development and optimization of new electrolytes, and the products will move toward higher voltages. In the ever-changing information technology field, capacitors will always be one of the key components. We will continue to develop high-performance capacitors that meet the needs of the information age by applying new technologies and materials.
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