How to choose the output filter capacitor of switching power supply
The ESR of a capacitor is the equivalent series resistance or impedance of the capacitor.
An ideal capacitor has no resistance. However, in reality, any capacitor has a resistance, and this resistance value is related to the materials and structure that make up the capacitor. In the switching power supply technology before the large-scale application, the common use of linear power supply, power supply circuits are working in the low-frequency DC state, through the filtering and rectification circuit to convert AC into DC. current. In the low-frequency DC power supply, the capacitance of the capacity of the filtering effect plays a decisive role, the series impedance of the capacitor can be ignored. However, the low efficiency of low-frequency power supply The disadvantages of large volume are very obvious.
Due to the progress of electronic technology, in the past two decades, the gradual development of high-frequency switching power supply technology of pulse width modulation, which greatly improves the conversion efficiency of the power supply, but also reduces the volume of the power supply. The volume of the power supply. The higher the operating frequency of the switching power supply, the smaller the size of the power supply. The operating frequency of switching power supply ranges from tens of KHz to several MHz. In the switching power supply, the ESR of the capacitor directly affects the effect of the capacitor, which is more important than the capacity of the capacitor, in fact, the capacitance of the capacitor we are talking about is generally measured at the 120Hz measured value, when the operating frequency increases, the capacitance capacity will be sharply reduced, or even can not play the role of capacitance at all. In general, you should choose capacitors with relatively small ESR. Among the different types of capacitors, electrolytic capacitors usually have the largest ESR, tantalum capacitors the next best, and ceramic capacitors the best. Of course, even Of course, even among electrolytic capacitors, there are ordinary electrolytic capacitors and low-ESR electrolytic capacitors. Low ESR electrolytic capacitors should be used for switching power supply output filtering. In maintenance In the maintenance, if you use ordinary electrolytic capacitors to replace the low ESR electrolytic capacitors, the switching power supply may work for a short time, but the life is certainly not long. Do not get it right, the capacitor will soon
Because the loss is too large and burst or even explode, so the replacement of capacitors should be careful. The same capacity and the same withstand voltage electrolytic capacitors, the size of the often small ESR. The same capacity The same capacity electrolytic capacitors with different withstand voltages, withstand voltage height tends to be small ESR. The same withstand voltage of the same capacity of capacitors, 105 degrees than 85 degrees ESR is smaller. Of course, this is not Absolute, for the same manufacturer of the same series of electrolytic capacitors, basically holds true.
