Theoretical Analysis of Supercapacitor in Rectifier Filter Based on Switching Power Supply

In this paper, the supercapacitor is used for rectification and filtering, and the design and implementation are carried out. The test proves that the supercapacitor can be used for the rectification and filtering of the output end of the power supply, and the filtering effect is excellent. Compared with electrolytic capacitors, it has great advantages.

The filter capacitor plays an important role in the rectification and filtering circuit. The larger the capacitance, the better the filtering effect. Especially in the output of low-voltage rectification (such as 5V, 3.3V or even lower voltage), a large ripple voltage is often generated because the capacitance of the filter capacitor is not large enough. Tests show that when the rectifier filter circuit outputs 1A current, the filter capacitors of 1000, 2200, 3300, 4700 and 10000 microfarads are used respectively. The peak-to-peak values ​​of the ripple voltage are: 6V, 2.8V, 1.9V, 1.1V and 0.6 respectively. V. If a larger filter capacitor is used, the filtering effect will be better. The problem is that large capacitors are bulky. How to solve this problem? People immediately thought of whether supercapacitors could be used for rectification and filtering. This paper will give detailed analysis and test results through theoretical analysis and experiments.

Basic requirements for rectifying and filtering capacitors

In switching power supplies, output rectification filtering has four basic requirements for capacitors: they have sufficient capacitance, meet the required rated voltage, meet the required ESR (equivalent series resistance), and can withstand the corresponding pattern. Wave current value. Only when these four basic requirements are met at the same time, supercapacitors can be used for output rectification filtering of switching power supplies.

The possibility of supercapacitor as a rectifying filter capacitor

For supercapacitors, it can easily reach the Faraday capacitance, so the supercapacitor has enough capacitance as the rectification filter. In the past, the rated voltage of supercapacitors was relatively low, although it could be solved by multiple series connection, but multiple series connection would lead to an increase in ESR, which increased the problem of ESR. The main reason why the supercapacitor can not be used for the rectification and filtering of the output terminal is that the rectification and filtering of the output of the switching power supply requires the filter capacitor to have the lowest ESR, and the ESR of the previous supercapacitor is only a large number. This causes the supercapacitor to heat up when used for output rectification filtering. For example, when five 4.7F supercapacitors are used in series, the ESR is about 300mΩ. At this time, if the 2A output current flows through the supercapacitor, the power is 1.2W, which will cause the supercapacitor to be severely heated. However, the current supercapacitors have reached a very high level. For example, AVX has produced a series of bestcap® supercapacitors, which have both high voltage rating and low ESR characteristics, such as a 90mF/12V bestcap® supercapacitor. The ESR is about 90mΩ, which is much smaller than 300mΩ. It can be seen that the ESP of the bestcap® supercapacitor is much lower than the previous supercapacitor, so that the rated voltage and ESR problems can be solved at the same time; the remaining question is whether the corresponding ripple current can meet the requirements. When selecting the appropriate capacitance (for example, selecting 1000~10000μF per ampere load current), the aluminum electrolytic capacitor basically does not have the ability to withstand the ripple current, and its ESR is relatively low, and the effect is basically no effect on the aluminum electrolytic capacitor. . The ESP of the bestcap® supercapacitor is similar to that of an aluminum electrolytic capacitor, and it has a very good impedance frequency characteristic, so the bestcap® supercapacitor can withstand the corresponding ripple current value. Figure 1 shows the impedance frequency characteristics of a bestcap® supercapacitor. Thus, the bestcap® supercapacitor is suitable for output rectification filtering.

Supercapacitor for rectification filtering

Take the 90mF/12V supercapacitor as an example. The relevant parameters are rated voltage 12V, rated capacitance 90mF, ESR90mΩ, volume 48 & TImes; 30 & TImes; 6.1mm, temperature range -40 ° C ~ +70 ° C.

Using supercapacitor as rectifying filter capacitor

Like the general rectification and filtering circuit, the supercapacitor is used for the rectification and filtering circuit and the output current, and the current waveform of the current filter capacitor is shown in Fig. 2, except that the filter capacitor is replaced by a supercapacitor. Previously, due to the low voltage rating of supercapacitors (2.7V only), several supercapacitors were required in series. For a 9V output regulated power supply (converted output voltage is about 10~12V considering the change of mains voltage), now only one 90mF/12V bestcap® supercapacitor can be used to filter the regulated output of 9V output.